Louise Callow and Alissa Scheffer
DEVELOPMENTAL ANATOMY AND PHYSIOLOGY
A. Embryologic Development of the Heart
1. Formation of the Heart Tube. Days 15 to 23 (Table 3.1)
2. Formation of the Heart Loop. Days 23 to 28 (Figure 3.1)
a. Structure of the cardiac wall in the fully developed heart
i. The pericardium
ii. The epicardium
iii. The myocardium
iv. The endocardium
v. Papillary muscles and chordae tendineae
3. Formation of Embryonic Ventricles. Days 22 to 35 (Figure 3.2)
4. Formation of Cardiac Septa. Days 27 to 45
a. Chambers of the fully developed heart
i. The atria
ii. The ventricles
5. Division of the Truncus Arteriosus (TA). Days 32 to 33
6. Formation of Cardiac Valves. Days 34 to 36
a. Cardiac valves in the fully developed heart (Figure 3.3)
i. Atrioventricular (AV) valves
ii. Semilunar valves
7. Formation of the Great Veins. Weeks 4 to 7
B. Genetic Signals to Embryonic Development
1. As more is learned about the human genome, gene control of cardiac embryonic events continues to unfold and is constantly changing, with much more yet to be learned. Genetic control of heart development includes a cascade of signaling molecules that trigger myocardial transcription factors. The transcription factors cause cardiac-specific proteins to be formed. These proteins regulate the growth of muscle types and looping effector genes, resulting in the formation of the normal heart (Pierpont et al., 2007).
2. Key signaling molecules include bone morphogenic proteins (BMPs) and fibroblast growth factors (FGFs). BMPs control cell division, cell death, cell migration, and differentiation. FGF signals proteins that cause angiogenesis (blood vessel formation).
3. Key transcription factors include the following:
a. NKX2-5 transcription factor, which is cardiac specific, is expressed throughout the heart from the earliest stages and controls differentiation from embryo to cardioblast.
b. Endothelin 1 and other factors, which differentiate cardioblasts into Purkinje-type cells.
c. Myocyte-enhancing transcription factor 2 (MEF2) and GATA transcription factors, which cause the cardioblasts to form into the heart tube.
4. Cardiac-specific proteins that result from transcription factor production control the development of the cardiac chambers.
a. Ventricular differentiation is controlled by the MEF2C and Irx4 genes.
b. Formation of the valves is controlled by Smad-6 and NF-ATC transcription factors, calcineurin, and transforming growth factor β (TGF-β).
c. Ventricular differences (right from left) are controlled by d-Hand for the right and NKX2-5 stimulation of e-Hand for the left ventricle (LV).
d. Ufd1 activates d-Hand and Neuropilin 1 activates migration of neural crest cells to form the conotruncus, aorta, and aortic arch.
e. PAX 3 gene signals growth of the aortic outflow tract; Forkhead transcription factor Mfh 1 signals growth of the transverse arch. Gridlock gene signals growth in the area of the coarctation, and FAP2b signals development of the area of the ductus arteriosus.
Formation of the heart tube
Endothelial tubes fuse to the endocardial tube, and the heart begins to beat from a focus in the sinus venosus.
Formation of the heart loop
The initial straight tube now loops. Looping determines the sidedness of the heart and the correct relationship of the heart segments to each other.
Formation of embryonic ventricles
The region of the tube proximal to the fold becomes the embryonic ventricle.
Formation of cardiac septa
The medial walls of the expanding ventricles fuse, forming the major portion of the ventricular septum; an extension of the endocardial cushions and the truncal conus create the membranous septum. Atrial septation and connection of the common pulmonary vein to the left atrium are closely related during atrial septation.
Division of the truncus arteriosus
Outflow tracts divide by fusion of the conotruncal cushions and spiraling follows the course of cushion development.
Formation of the cardiac valves
The mitral valve and tricuspid valve form from endocardial cushions, and the tricuspid valve also forms from the conus septum. The semilunar valves form at the interface of truncal cushions and aorticopulmonary septum.
Formation of the great veins
Three primitive systems, the vitelline, the umbilical, and the cardinal venous systems, form the venous pattern through the process of vasculogenesis.
150C. Systemic Vasculature in the Fully Developed Heart
1. Systemic vessels supply tissues with oxygen and nutrients and remove metabolic wastes. The diameter of the vessels (especially the arterioles) and viscosity of the blood create systemic vascular resistance (SVR). Tissue perfusion is controlled via local chemical reactions and nerves that dilate or constrict blood vessels.
2. Major Components of the Systemic Vasculature
a. An artery is a high-pressure circuit composed of strong, compliant, elastic-walled vessels carrying blood from the heart to the capillary beds. Elastic fibers within the arterial wall enable the wall to stretch during systole and recoil during diastole.
b. Arterioles are the major vessels controlling SVR and arterial pressure. Arterioles are controlled by the autonomic nervous system and by autoregulation. They contain smooth muscle innervated by sympathetic α-adrenergic nerve fibers. Stimulation causes constriction of the vessels; decreased adrenergic discharge dilates the vessels controlling blood distribution to various capillary beds. Arterioles may give rise to metarterioles (precapillaries) or give rise directly to capillaries, where flow is regulated through constriction or dilation.
c. The capillary system allows the exchange of oxygen and carbon dioxide and solutes between blood and tissues and permits fluid volume transfer between plasma and the interstitium. Capillary filtration is related to hydrostatic and osmotic pressures across membranes. Increased hydrostatic pressure leads to movement of fluid from vessel to interstitium via osmosis. Greater capillary osmotic pressure leads to fluid movement from interstitium into vessels. Capillaries lack smooth muscle. Diameter changes are passive because of precapillary and postcapillary resistance. Because of their narrow lumens, capillaries can withstand high internal pressures without rupturing. Laplace’s law states that the tension in the wall of the vessel necessary to balance the distending pressure is lessened as the radius of the blood vessel decreases. Diffusion is the most important process in moving substrates and wastes between the blood and tissues via the capillary system.
d. The venous system stores approximately 65% of the total volume of blood in the circulatory system. Venules receive blood from capillaries and serve as collecting channels and capacitance (storage) vessels. Veins are capacitance vessels that conduct blood to the heart within a low-pressure system surrounded by skeletal muscles. When muscles contract, they compress veins, moving blood toward the heart. Valves in veins prevent retrograde blood flow. Under normal conditions, the venous pump keeps the venous pressure in the lower extremities at 25 mmHg or less. Gravity has profound effects on the erect, mobile individual. Pressure can rise to 90 mmHg in the lower extremities, which results in swelling and a decrease in blood volume because of leakage of fluid from the circulatory system into the interstitium.
3. Coronary Vasculature in the Fully Developed Heart (Figure 3.4)
a. Arteries branch off the base of the aorta, supplying blood to the conduction system and myocardium.
b. The right coronary artery (RCA) supplies the sinoatrial (SA) node (55% of hearts), the AV node (90% of hearts), the right atrium (RA), right ventricle (RV) muscles, and the inferoposterior wall of the LV. Eighty percent of the time, a branch of the RCA called the posterior descending artery is the terminal portion of the RCA, resulting in a right dominant coronary system. Located in the posterior interventricular groove, it supplies the RV, LV, and the posterior part of the interventricular septum. The RCA gives off a posterior lateral branch that descends from the lateral side of the heart to the apex. It supplies the anteroposterior surface of the RV.
c. The left coronary artery (LCA) branches into the left anterior descending artery (LAD), which supplies the anterior part of the interventricular septum, the anterior wall of the LV, the right bundle branch (RBB), and the anterosuperior division of the left bundle branch (LBB).
d. The circumflex artery, also branching off from the LCA (into major branches of the circumflex artery and one or more obtuse marginal branches, supplies the AV node (10% of hearts), the SA node (45% of hearts), and the posterior surface of the LV via the obtuse marginal branches.
e. Veins return desaturated blood to the heart. They consist of the great cardiac veins, the small cardiac veins (both drain into the coronary sinus, which drains into the RA), and the thebesian vessels (which drain blood into the RA through the atrial wall; Hazinski, 2013).
D. Embryonic, Neonatal, and Pediatric Cardiovascular Physiology
1. Fetal Circulation (Figure 3.5)
a. Gas exchange occurs in the placenta.
b. Umbilical venous blood (the most highly saturated) returns via the umbilical vein from the placenta and accounts for 42% of fetal cardiac output (CO). From the umbilical vein, about half the fetal blood flows through the ductus venosus to the inferior vena cava (IVC) and the other half enters the hepatic portal system.
c. There is a preferential flow of more highly saturated blood through the foramen ovale into the left atrium (LA) and LV to the ascending aorta and to the brain and myocardium.
d. The superior vena cava (SVC) flow is directed to the tricuspid valve and the RV, along with coronary sinus blood. Blood flows from the RV to the pulmonary trunk, and about 8% of RV output perfuses the pulmonary artery (PA). The rest of the RV output flows through the ductus arteriosus to the aorta. Parallel circulation exists in the fetus: the RV pressure equals the LV pressure.
e. Vascular pressure reflects streaming with RA pressure greater than the LA pressure, PA pressure greater than the aortic pressure, and the umbilical vein pressure higher than that of the IVC.
f. The fetal myocardium is less compliant because of the lower ratio of contractile to noncontractile fibers (30% in fetus; 60% in adult).
2. Transitional Circulation
a. Interruption of the umbilical cord creates increased SVR and decreased IVC return to the heart. The primary change in circulation after birth is a shift from gas exchange in the placenta to gas exchange in the lungs.
b. Peripheral vascular resistance (PVR) rapidly decreases in the first 12 to 24 hours of life, and pulmonary blood flow increases. The reduction to normal PVR occurs slowly over 14 to 21 days. This process may be delayed in preterm infants.
c. The ductus arteriosus constricts primarily in response to increased arterial PO2 and is influenced by the loss of placental prostaglandin. The ductus arteriosus in the mature infant is functionally closed 12 to 24 hours after birth, but closure can be reversed with prostaglandin E1 (PGE1). Anatomic closure from fibrosis usually occurs within 2 weeks. The RV ejects all blood into the pulmonary circulation when the ductus arteriosus closes and the PVR decreases. The foramen ovale closes as a result of increased LA pressure from increased pulmonary venous blood return. The ductus venosus closes soon after birth.
d. Rapid loss of the low resistance placental circuit increases the SVR and LV pressure, and LV and RV outputs equalize.
1533. Neonatal and Pediatric Circulation (Figure 3.6)
a. The neonatal myocardium functions at near maximum CO. There is a relatively fixed stroke volume (SV) in the first weeks of life. The larger ratio of noncontractile to contractile muscle fibers disappears after about 1 week.
b. The neonatal myocardium responds to stress by a combination of hyperplasia and hypertrophy. Increased heart rate (HR) produces little change in CO because of a high resting HR in the neonate and newborn. However, because the neonatal myocardium operates high on its CO curve and SV is limited, the CO can be increased by increasing HR more than by increasing contractility. Increased afterload will result in a drop in CO. The neonatal myocardium is extremely sensitive to increases in afterload.
4. Peripheral Blood Vessel Physiology
a. The local mechanism for tissues to control their own blood flow is known as autoregulation. Two major hypotheses exist:
i. Myogenic response hypothesis. As pressure rises, vessels stretch, stimulating the contraction of smooth muscles (feedback mechanism). As pressure decreases, smooth muscles relax.
ii. Metabolic hypothesis. Because of the normal metabolic activity of the tissues, carbon dioxide, potassium, lactate, prostaglandins, and phosphates accumulate and cause vasodilation, which increases the blood flow to the area of activity to flush these waste products away.
154b. There is a delicate balance between the two mechanisms involved in the myogenic response: Myogenic response → vasoconstriction → decrease in blood supply → local increase in metabolites → vasodilation → wastes removed.
c. Autonomic regulation of vessels
i. Sympathetic nervous system fibers secrete norepinephrine at nerve endings, producing vasoconstriction. In arterioles, this mechanism helps regulate blood flow and arterial pressure. In veins, this mechanism helps to vary the amount of blood stored. Venoconstriction causes an increase in venous return to the heart.
d. Stretch receptors. Baroreceptors (pressoreceptors)
i. Receptor sites are located in the aortic arch, carotid sinus, venae cavae, pulmonary arteries, and atria. Sensitive to arterial pressures, the receptor sites are activated by elevated blood pressure (BP) or increased blood volume, resulting in stretching of the arterial walls. The impulse is transmitted from the aortic arch and the carotid sinus to the medulla. Sympathetic action is inhibited and the vagal reflex dominates, resulting in decreased HR and contractility, dilation of the systemic vasculature, and normalized BP.
ii. In response to decreased BP, the vagal tone decreases and the sympathetic system becomes dominant, resulting in increased HR and contractility and arterial and venous constriction and BP elevation to near normal.
e. The vasomotor center in the medulla (also called cardioaccelerator center or cardiac center) consists of the vasoconstrictor and vasodepressor areas.
i. Stimulation of the vasoconstrictor area causes increased HR, SV, and CO and, ultimately, increased arterial BP. Venoconstriction, which decreases stores of blood in the venous system, increases venous return and increases SV.
ii. Inhibition of the vasoconstrictor area stimulates the vasodepressor area, which causes vasodilation. An increase in storage of blood in the venous capacitance system occurs, thereby decreasing SV, CO, and arterial BP.
iii. The vasomotor center works with stretch receptors and chemoreceptors located in the carotid sinus and aortic arch. A rise in BP stimulates the carotid sinus, which inhibits the vasoconstrictor area. This induces vasodilation via stimulation of the vasodepressor area. A fall in oxygen saturation, a rise in carbon dioxide, or a fall in pH stimulates chemoreceptors, which then stimulate the vasoconstrictor center and cause a rise in arterial BP (Hazinski, 2013).
155E. Neurohormonal Control of the Fully Developed Heart
1. Autonomic Nervous System
a. Sympathetic stimulation initiates the release of norepinephrine. Stimulation of α-adrenergic fiber results in arteriolar vasoconstriction. Stimulation of β-adrenergic (β1) fiber increases SA node discharge, thereby increasing the HR (positive chronotropy); increasing the force of myocardial contraction (positive inotropy); and accelerating AV conduction time (positive dromotropism).
b. Parasympathetic stimulation initiates the release of acetylcholine, which stimulates the action of the right vagus nerve (affecting the SA node) and the left vagus nerve (affecting AV nodal conduction tissue). The rate of SA node discharge is decreased and slows the HR (negative chronotropy). It may slow conduction through AV tissue (negative dromotropism).
2. Natriuretric peptides are produced by the atria, ventricles, and brain and act as regulators of extravascular fluid volume and BP through control of sodium and water by countering the effects of the renin–angiotensin–aldosterone system.
a. Atrial natriuretic peptide (ANP) is produced by the atria in response to atrial-wall tension from increased intravascular volume. Levels in the blood vary significantly with changes in position, exercise, and pacing. ANP reduces sympathetic tone, increases venous capacitance, and shifts intravascular fluid to the extravascular space by increased vascular endothelial permeability. A natural diuresis reduces the extravascular volume by directly affecting the renin–angiotensin–aldosterone receptors in the kidney. ANP reduces peripheral vascular resistance and lowers BP. It lowers the activation of vagal blockers, thus suppressing the reflex tachycardia and vasoconstriction that goes with reduced preload (Colucci & Chen, 2017).
b. Brain natriuretic peptide (BNP) is produced by the ventricles in response to ventricular wall tension and volume expansion. Its effects are similar to those of ANP, producing natriuresis, diuresis, and vasodilation and counteracting the effects of the renin–angiotensin–aldosterone system. BNP also appears to prevent myocardial fibrosis, vascular smooth muscle cell proliferation, and thrombosis. BNP levels in blood are more stable than ANP levels. Both ANP and BNP levels rise rapidly after birth in response to increased LV volume and pressure during transition to extrauterine life. Then levels drop to normal within 2 weeks of life. Gender-related differences are noted after puberty (greater in females). Elevations in BNP are seen in volume overload and ventricular dilation.
c. C-type natriuretic peptide is produced mostly by the central nervous system (CNS), kidneys, and vascular endothelial cells.
3. Chemoreceptors are located in the carotid and aortic bodies and are sensitive to changes in PO2, PCO2, and pH. They affect HR and respiratory rate via stimulation of the vasomotor center in the medulla.
4. Stretch receptors respond to pressure and volume changes. Stretch receptors located in the atria, large veins, and PA produce the Bainbridge reflex. An increase in venous return stretches the receptors. Afferent nerve impulses transmit to the vasomotor center in the medulla. The medulla increases efferent impulses, increasing HR and CO, and enabling the heart to pump out all the blood returned to it.
5. During the respiratory reflex inspiration decreases intrathoracic pressure, increasing venous return to the heart. Inspiration stimulates stretch receptors in the lungs and thorax. Impulses from the stretch receptors inhibit the vasomotor center in the medulla. This inhibition decreases vagal tone, causing an increase in the HR, which allows the heart to pump out the extra blood. This reflex results in “sinus arrhythmia,” which may occur in a normal heart.
F. Variables That Affect CO
1. Cardiac Output = SV × HR. The cardiac index can be used in children because of the potential variation in CO by body size. Cardiac index is equal to CO divided by body surface area (BSA). CO is the amount of blood being pumped by the heart to the tissues and is measured in liters per minute.
2. SV is the amount of ventricular volume pumped during systole. SV is affected by preload, afterload, and contractility. The ejection fraction (the volume ejected vs. volume remaining in the ventricle at the end of systole) is calculated by dividing the SV by the end-diastolic volume and is expressed as a percentage.
3. Preload is the resting force in the myocardium, which is determined by the volume in the ventricles at the end of diastole (left ventricular end-diastolic volume [LVEDV] reflected by left ventricular end-diastolic pressure (LVEDP). Preload can be related to such variables as the volume of blood returned from veins, stretch, and fiber length. An increase in 156preload stretches myocardial muscle fibers, causing more forceful subsequent ventricular contractions, increasing SV and CO. An increase in preload is accomplished by increasing the blood volume returning to the ventricles (Figure 3.9). The Frank–Starling law states that there is a direct relationship between the volume of blood in the heart at the end of diastole and the force of contraction during the next systole. The preload or ventricular filling pressure reflects the initial sarcomere length, which influences the development of myocardial force. Muscle fibers may reach a point of stretch beyond which contraction is no longer enhanced and SV decreases. Increased preload may be related to mitral insufficiency, aortic insufficiency, ventricular septal defect (VSD), atrial septal defect (ASD), patent ductus arteriosus (PDA), fluid overload, and vasoconstrictors. Decreased preload may be related to mitral stenosis (MS), hypovolemia, and vasodilators (Hall, 2016).
4. Afterload is the initial resistance that must be overcome by the ventricles to open the semilunar valves and propel blood into the systemic and pulmonary circulatory systems (Hall, 2016).
a. Afterload is clinically measured as SVR and peripheral vascular resistance (PVR).
b. SVR = mean arterial pressure (MAP) – central venous pressure (CVP)/systemic blood flow.
c. Systemic blood flow is a value measured in resistance units. The resistance units times 80 converts resistance units into dynes per second per square centimeter.
d. Normal SVR = 900 − 1,400 dynes/sec/cm2
e. Factors that increase afterload include fixed anatomic obstructions, peripheral arterial vasoconstriction, systemic hypertension, pulmonary hypertension, polycythemia, and vasoconstrictors. Excessive afterload increases LV or RV stroke work, increases myocardial oxygen demand/consumption, and may result in LV or RV failure.
f. Factors that decrease ventricular afterload include vasodilators and sepsis. This will decrease RV or LV stroke work, potentially decrease myocardial oxygen demand/consumption and thereby improve LV or RV function.
5. Contractility is the strength and efficiency of contraction (force generated). Positive inotropic drugs, sympathetic stimulation, and hypercalcemia can act to increase the contractile state of the myocardium. Factors that can decrease contractility of the myocardium include negative inotropic drugs, hypoxia, hypercapnia, intrinsic depression attributable in part to long-standing congestive heart failure (CHF), parasympathetic stimulation, metabolic acidosis, hypocalcemia, hypoglycemia, hypomagnesemia, hyponatremia, hyperkalemia, the condition of the myocardium, and intrinsic myocardial disease (Hall, 2016).
6. HR and rhythm can alter CO with excessively high or low HRs. Tachycardia will decrease filling time of the ventricle-limiting CO and coronary perfusion. Bradycardia effects CO through the decreased ability to meet oxygen delivery due to reduced supply from lower HRs (Figure 3.10). Some arrhythmias, primarily heart block, and junctional rhythms decrease synchrony between atrial and ventricular contraction altering adequate emptying and/or filling of the atria or ventricle and can affect CO (Hall, 2016; Hazinski, 2013).
G. Arterial Pressure
Factors that can affect arterial BP include CO, HR, SVR, arterial elasticity, blood volume, blood viscosity, age, BSA, exercise, and anxiety.
1. Pulse pressure is a function of SV and arterial capacitance. This difference between systolic and diastolic BP is expressed in millimeters of mercury (Ps – Pd).
2. MAP is the average pressure in the aorta based on the volume of blood in the arterial system and the elastic properties of the arterial walls. It is calculated by MAP = systolic BP + (2 × diastolic BP)/3.
3. Regulation of arterial pressure is also under the control of the renin–angiotensin–aldosterone system and involves renin, a protease secreted by the kidney that converts angiotensin I to angiotensin II. Release of renin from the kidney is stimulated by stretch receptors in juxtaglomerular cells that are sensitive to changes in BP. Decreased BP, a rise in sympathetic output, or a fall in sodium concentration results in increased renin secretion. Increased BP results in decreased renin secretion. Angiotensin II is the most potent vasoconstrictor known, producing arteriolar constriction and an increase in systolic and diastolic pressures.
4. Other mechanisms include capillary fluid shift mechanisms, local control mechanisms, and the renal-fluid volume process. With a rise in arterial pressure, the kidneys excrete more fluid, causing a reduction in extracellular fluid and blood volume; this reduces circulating blood volume and potentially CO, leading to normalization of arterial pressure. With a fall in arterial pressure, the kidneys retain fluid, causing increased intravascular volume and, it is hoped, increased CO, which may result in normalization of arterial pressure.
ANATOMY OF THE CARDIAC CONDUCTION SYSTEM
A. SA Node (Figure 3.11)
The SA node is the pacemaker of the heart because it possesses the fastest rate of automaticity (spontaneous generation of impulses).
B. Internodal Atrial Pathways
Internodal atrial pathways, which consist of the anterior tract (Bachmann’s), middle tract (Wenckebach’s), and posterior tract (Thorel’s), conduct impulses from the SA node through the RA to the AV node.
C. Bachmann’s Bundle
Bachmann’s bundle conducts impulses from the SA node to the LA.
D. AV Node
The AV node (AV junction) delays impulse transmission between atria and ventricles, allowing time for ventricular filling following atrial contraction and before ventricular systole. The AV node controls the number of impulses (if the atrial rate becomes excessive) reaching the ventricles, thereby having some control over HR.
E. Bundle of His
The bundle of His is composed of thick fibers arising from the AV node that travel over the crest of the ventricular septum on its right side to the bundle-branch system.
F. Bundle-Branch System
The bundle-branch system is composed of pathways that arise from the bundle of His. The RBB is a direct continuation of the bundle of His that transmits impulses down the right side of the interventricular septum toward the RV myocardium. The bundle divides into three parts (anterior, lateral, and posterior), dividing further to become parts of the Purkinje system. The LBB separates into the left posterior fascicle (which transmits impulses over the posterior and inferior endocardial surfaces of the LV) and the left anterior fascicle (which transmits impulses to the anterior and superior endocardial surfaces of the LV).
G. Purkinje System
The Purkinje system arises from the distal portion of the bundle branches and transmits impulses into the subendocardial layers of both ventricles. It provides for depolarization (from endocardium to epicardium) followed by ventricular contraction and ejection of blood from the ventricles.
A. Properties of the Myocardial Conduction System
1. Automaticity is the ability to generate impulses spontaneously. The cardiac muscle is the only muscle in the body with this property.
2. Rhythmicity is the regularity of impulse generation. These impulses should follow the normal conduction system previously outlined.
3. Conductivity is the ability to transmit impulses. This is assuming the conduction system of the heart 159remains intact and is not blocked or altered by medication, trauma, hypoxia, or congenital disease processes.
4. Excitability is the ability to respond to stimulation. This includes both the parasympathetic and sympathetic nervous system simulation.
B. Excitation–Contractile Process of Cardiac Muscle
1. The sodium and unbound calcium ion concentrations are greater outside the cell, and the potassium ion concentration is greater inside the cell. The resting membrane potential (RMP) for myocardial muscle fibers is −80 to −90 mV.
2. Depolarization can result from chemical, electrical, or mechanical stimulation. The stimulus reduces the RMP to a less negative value (depolarization). The threshold potential is the voltage level, where an action potential (AP) is produced. For all cardiac tissue except the SA and AV nodes, the threshold potential is −60 to −70 mV. For the SA and AV nodes, the threshold potential is −30 to −40 mV. Reaching the threshold causes changes in the membrane. The permeability of the cell membrane is altered, opening specialized channels in the membrane, which permits the passage of sodium and calcium ions into the cell. The AP is the graphic representation of this change (Figure 3.12).
3. The AP produced during depolarization is transmitted to the interior of the cell via T tubules, which transmit the AP to all myofibrils. Calcium is stored in the lateral sacs of the sarcoplasmic reticulum and is released during the AP. Calcium enters the interior of the cell, causing an interaction between actin and myosin filaments through a complex interaction with enzymes. Actin filaments move progressively inward on myosin filaments as successive electrochemical interactions take place (interdigitation). The result is shortening of sarcomeres and then of muscle fibers and thus myocardial contraction. Relaxation of muscle fibers occurs when free calcium is pumped back into the sarcoplasmic reticulum.
1604. The “gate” theory proposes fast channels of the membrane specific for sodium may be controlled by the following:
a. The activation gate opens the fast channels as the RMP becomes less negative, allowing a rapid influx of sodium into the cell, which causes depolarization (phase 0 of the AP).
b. The deactivation gate closes the channels impeding the influx of sodium into the cell. Closure of the gates is complete by phase 1.
5. A return to the RMP results from an inward current of calcium and potassium ions that diffuse out of the cell.
a. Phase 2, the plateau phase, occurs as calcium flows in and potassium flows out.
b. Phase 3, the rapid depolarization phase, occurs as the calcium channels close and potassium rapidly moves out of the cell.
c. Phase 4, the resting phase, of the sodium–potassium pump regulates the concentration of cations in the cell. This pump, found in the cell membrane, actively pumps excess sodium out of the cell and pumps in the potassium.
i. Unlike other cells of the heart that require another stimulus to depolarize them once they have been repolarized, the SA and AV nodes spontaneously depolarize (generate impulses) in phase 4. This spontaneous depolarization is due to the steady influx of sodium and the efflux of potassium ions, raising the nodal tissues back to the threshold potential and initiating an AP. This phenomenon is known as automaticity.
C. Refractoriness of Heart Muscle
1. Absolute refractory period encompasses phases 0, 1, 2, and part of 3 of the APs. During this period of time, the cell cannot respond to another stimulus and produce an AP.
2. Relative refractory period (the latter part of phase 3) is a period when a strong stimulus can cause depolarization.
3. Supernormal period occurs at the end of phase 3. During this time, a very weak stimulus that would not normally elicit an AP can evoke a response and cause depolarization.
4. The vulnerable period is the point at the very beginning of the relative refractory period. A stimulus at this time (which corresponds to the peak of the T wave on the ECG) can result in myocardial electrical chaos. The R-on-T phenomenon occurs when a premature QRS complex interrupts the previous beat’s T wave, which may result in ventricular tachycardia (VT) or ventricular fibrillation (VF).
D. Physiologic Response to Depolarization and Repolarization
1. Electrical depolarization of the atria is represented by the P wave on the ECG. Following atrial depolarization, the pressure in the atria rises higher than the diastolic pressure in the ventricles, forcing blood from the atria into the resting ventricles.
2. Electrical depolarization of the ventricles occurs, producing the QRS complex on the ECG. Isometric or isovolumetric contraction is the first phase of ventricular contraction (systole). Ventricular pressure rises while ventricular volume remains stable because the semilunar valves have not yet opened. The increased pressure in the ventricles closes the AV valves. As ventricular pressure exceeds great vessel pressure, the semilunar valves open. Blood from the ventricles is rapidly ejected into the great vessels. As the outflow of blood from the ventricles decreases, the pressure in the ventricles also decreases, falling below the pressure in the great vessels. This causes a backflow of blood from the great vessels to the ventricles, which closes the semilunar valves. The dicrotic notch on the arterial pressure tracing represents closure of the aortic valve.
3. Repolarization of the ventricles occurs during mechanical systole and produces the T wave on the ECG. Atrial repolarization is not seen on the ECG as it occurs during ventricular depolarization.
4. Isometric or isovolumetric relaxation occurs when ventricular pressure falls rapidly following semilunar valve closure. Intraventricular volume remains static before the AV valves open. A “v” wave is produced on the atrial pressure curve during isometric relaxation related to blood flow into the atria from the pulmonic and systemic circuits. As ventricular pressure remains lower than atrial pressure, the AV valves open to initiate the rapid-filling phase (Figure 3.13).
1. Gestational and Birth History (Fillipps & Bucciarelli, 2015)
a. Maternal and paternal congenital heart disease (CHD) history, family heart disease history, acquired and CHD history are important to screen for specific genetic and familial transmission of specific congenital or acquired cardiac disease.
b. Maternal health history, serologies, exposures and infections, medications (prescribed and illicit), gestational diseases, and alcohol exposure outline potential environmental factors known to cause congenital cardiac disease. They may also guide maternal and infant care during and immediately after delivery. Gaining knowledge of these exposures and appropriate treatment regimens is essential to providing care for the neonate with heart disease and potential need for early operative intervention.
c. Birth history, Apgar scores, screening saturations, birth weight, and gestational age guide care surrounding potential birth trauma with end-organ complications and additional risks associated with cardiopulmonary bypass (CPB) that influence decision making regarding interventions if needed.
d. If the infant had a fetal diagnosis of CHD, a conformational postnatal echocardiogram (echo) and electrocardiogram will be performed shortly after delivery. Assessment for further intervention to prevent compromise to the newborn’s cardiovascular stability can be performed at the time of delivery.
2. General History (Bates, 2017)
a. The chief complaint is the patient’s or parents’ description of why they are seeking evaluation.
b. History of present illness will determine the onset, description, course, and duration of the specific symptom complex. Evaluate exacerbations and remissions of signs and symptoms, including the following:
162i. Feeding pattern. Note the duration, frequency, associated distress, volume taken, if stopping to rest or breathe during eating, and caloric supplementation required (poor feeding/weight gain is the earliest sign of CHF).
ii. Fatigue. Note whether the child tires while feeding or playing.
iii. Edema. Assess for presence of orbital and sacral edema.
iv. Diaphoresis. Note the location, degree, and precipitating factors.
v. Dyspnea, tachypnea. Note whether these occur with or without activity.
vi. Color. Obtain information if available regarding the oxygen saturation with or without activity; skin color, and saturation in each extremity (differential cyanosis). Acrocyanosis may be present in distal portions of the extremities, around the mouth and nail beds and can be a normal variant in newborns that increases when crying. Cyanosis is always abnormal. Mottling or pallor can be a sign of poor CO.
vii. Squatting. This occurs in children with cyanotic lesions (specifically, tetralogy of Fallot) or when repairs are delayed or the defect is undiagnosed (generally children without previous medical care).
viii. Growth. Graph against normal limits for height (>2 years)/length (<2 years), weight, and head circumference. With CHF, weight will fall below normal limits before height does. Head circumference is preserved until CHF is long-standing.
ix. Frequency of infections. Particularly note respiratory illnesses but include any past history of sepsis or bacteremia and strep throat.
x. Syncope. Is there an associated prodrome, is there associated dizziness, is there a particular time of occurrence or stimulating factor?
xi. Capillary refill is normally 3 to 4 seconds and may be prolonged due to cardiovascular compromise.
xii. Clubbing of the nailbeds indicates long-standing arterial desaturation.
xiii. Palpitations. Are they with or without chest pain, are they felt in the neck or throat, do they start rapidly, or are they gradual in onset?
c. Past history and family history include all previous illnesses, injuries, and family history of similar disease (i.e., history of CHD and early onset coronary artery disease).
i. Obtain prenatal and perinatal history, including in vitro fertilization, prenatal care, and fetal diagnosis.
ii. Family history should include evaluation of inheritance risk of single gene chromosomal disorders or multifactorial syndromes associated with CHD (Tables 3.2–3.4). Determination of a family history of sudden or unexplained death at a young age is important information in that it may represent a potentially inherited cardiac disease.
iii. Explore environmental exposures, include drug teratogens, external radiation exposure, smoking, maternal systemic diseases, and infectious exposure of the fetus (Table 3.5).
iv. There is a slightly higher risk (2.5%–16%) of CHD if one parent or sibling has CHD, especially the mother.
d. Psychosocial history includes the use of nonprescription drugs or alcohol during pregnancy or taken by the child, daily living patterns, relationships with significant others, recreational habits, educational level of the child and parents, and developmental level of the child. Potential exposure of the child to abuse in the home or by a caregiver should be explored.
e. The medication history should include all medications prescribed or obtained over the counter, including herbal remedies, dosages, and reason for use.
B. Physical Examination
a. Assess the general appearance.
i. Note size for age (height, weight, and head circumference graphed against normal limits), activity level, level of consciousness, and physical characteristics of chromosomal defects (genetic phenotypes, i.e., Down syndrome).
b. Assess the skin and mucous membranes.
i. Note pallor, cyanosis, or mottling. Skin color is influenced by vasoconstriction/vasodilation. Cyanosis is evident if saturation is less than 85%, which is equivalent to 5 g of reduced hemoglobin per 100 mL of blood. The degree of visible cyanosis is dependent on total hemoglobin and its saturation. Respiratory cyanosis decreases with crying (improved respiratory effort) and oxygen. In cardiac disease, cyanosis increases with crying (increased resistance to pulmonary blood flow and shunting) and remains unchanged with oxygen administration. Acrocyanosis (cyanosis of the extremities) is normal in the newborn with vasomotor instability. Note the distribution of cyanosis over the body. Peripheral cyanosis (extremities, perioral [around the mouth]) may represent hypothermia or decreased flow, whereas central cyanosis (mucous membranes) indicates reduced hemoglobin saturation. Chronic cyanosis stimulates erythropoiesis and polycythemia, which cause increased blood viscosity and an increased risk of spontaneous cerebrovascular accidents, brain abscess, thrombocytopenia with short platelet survival, reduced platelet aggregation with hemorrhagic abnormalities (which may cause operative bleeding), and vascular sheer stress producing increased PVR, even in the face of decreased pulmonary blood flow.
Truncus, TOF, VSD, PDA
VSD, ASD, TOF
VSD, PDA, ASD, TGV
PS, AS, coarct, PDA, TGV, TOF, HLHS
VSD, TGV, TOF
Ebstein’s anomaly, tricuspid atresia, ASD
TOF, TGV, DORV, truncus, VSD
PDA, PS, AS, coarct, VSD, ASD
Myocarditis, fetal heart block
Myocarditis, nonimmune hydrops fetalis with high output heart failure
TGV, VSD, hypertrophic cardiomyopathy
Congenital complete heart block
TOF, VSD, ASD
CHB, PVCs, cardiomyopathy
AS, aortic stenosis; ASD, atrial septal defect; CHB, complete heart block; CMV, cytomegalovirus; coarct, coarctation of the aorta; DORV, double-outlet right ventricle; HLHS, hypoplastic left heart syndrome; PDA, patent ductus arteriosus; PS, pulmonary stenosis; PVC, premature ventricular contraction; SVT, supraventricular tachycardia; TA, truncus arteriosus; TGV, transposition of the great vessels; TOF, tetralogy of Fallot; VSD, ventricular septal defect.
ii. Note edema, which is more common in the periorbital and sacral areas of infants.
iii. Note the patient’s temperature. Skin temperature is influenced by the environment and by CO and assists in describing the level of decreased perfusion (i.e., cold to knee, cold to midthigh).
iv. Note the presence of diaphoresis at rest and/or with crying/exercise/feeding.
c. Observe the extremities
i. Note clubbing of nail beds indicated by a flattened angle of the nail base to 180 or more degrees (normal is about 160 degrees). Clubbing develops after decreased oxygen saturation persists longer than 6 months (Figure 3.14).
ii. Compare both sides for equal growth, particularly length, in children requiring multiple catheterization procedures or with a history of arterial occlusions from monitoring lines.
iii. Note the color of the nailbeds, palms, and soles of feet.
d. Observe the chest and precordium for visible pulsations.
167i. An active precordium with heaves or thrusts over the precordium is noted in volume overload such as left-to-right shunts or aortic or mitral insufficiency.
ii. Note the shape, contour, and symmetry of the chest, Harrison’s groove in older children, and the visible point of maximal intensity of cardiac impulse (point of maximal impulse [PMI]).
e. Observe the neck for jugular venous distention.
i. Note the PMI, normally found at the fifth left intercostal space (LICS), medial to the midclavicular line after 7 years or at the fourth LICS before 7 years. Lateral displacement of the PMI away from the left sternal border (LSB) indicates elevated diaphragm or left ventricular hypertrophy (LVH). Medial displacement toward the sternum indicates right ventricular hypertrophy (RVH) or an abnormally small LV.
ii. Seven areas should be palpated (Figure 3.15), including the supraclavicular, aortic, pulmonary, tricuspid, mitral, epigastric, and ectopic areas.
iii. Thrills. Use the ball portion of the palm of the hand to palpate for thrills (feels like a vibration or a cat purring).
1681) In the aortic area, thrills indicate aortic stenosis (AS).
2) In the pulmonic area with radiation to the left side of the neck, thrills indicate pulmonic stenosis.
3) In the apical area during systole, thrill indicates mitral regurgitation (MR) and during diastole, MS.
4) If felt in the suprasternal notch area, thrill may indicate AS, pulmonic stenosis, or a PDA.
5) If felt in the intercostal spaces (ICSs), thrill may indicate coarctation of the aorta with collateral circulation.
6) If felt in the mid to lower LSB, a VSD may be indicated.
iv. Lifts are pulsations noted under the palm of the hand. Pulsation in the pulmonic area indicates MS or hypertension. Lifts in the tricuspid area may indicate a VSD, elevated RV pressure, pulmonary stenosis (PS), pulmonary hypertension, or an ASD.
v. Friction rubs are similar to the sensation of rubbing two pieces of material together. Pericardial friction rubs can be heard continuously throughout the cardiac cycle. Pleural friction rubs occur only during respiration.
b. Peripheral pulses
i. These are rated on a scale of 0 to 4, with 0 = absent; 1+ = palpable but thready, easily obliterated; 2+ = normal; 3+ = full; 4+ = full and bounding.
ii. Common arterial sites for palpation include the carotid, brachial, radial, femoral, popliteal, dorsalis pedis, and posterior tibialis.
iii. Obtain simultaneous assessments of upper and lower extremity pulses and evaluate their strength, equality, and intensity as an indication of coarctation of the aorta. In the presence of a coarctation without a widely patent PDA, the lower extremity pulses are weak and the upper extremity pulses are strong and full.
iv. Strong, bounding pulses are found in a PDA, aortic regurgitation, AV fistulas, and TA. Waterhammer pulses may be found with aortic insufficiency or a PDA secondary to low diastolic pressure. Delayed, weak pulses are found in cardiac tamponade, AS, MS, CHF, shock, and hypoplastic left heart syndrome (HLHS). Pulsus alternans (alternating pulse waves, every other beat weaker than the preceding beat) indicates a weak heart muscle, as seen in severe hypertension or LV failure.
c. Capillary filling time is evaluated by compressing the extremity with moderate pressure and noting the time required for the blanched area to reperfuse. Normal capillary filling time is less than 3 seconds with the extremity at the level of the heart.
d. The liver is palpated starting in the lower abdomen and pressing upward at the right costal margin until the liver edge is palpated. The liver edge of an infant is normally 3 cm below the costal margin. The liver edge of a 1-year-old is at 2 cm below the margin; the liver edge of a 4- to 5-year-old child is at 1 cm below. By adolescence, the edge is either not palpable or is at the costal margin (Duderstadt, 2014).
a. HR and rhythm are assessed together by auscultating the apical area. Asses the regularity and rate for expected limitations for age and activity status. HR should be counted for at least 60 seconds to be most accurate. The regularity of the rhythm will be assessed over the same 60-second time frame.
b. To appropriately obtain an accurate BP, a cuff bladder that is at least two thirds the circumference and two thirds the length of the extremity is used. Obtain four extremity BP readings during the initial assessment to rule out coarctation of the aorta. Thigh pressure is equal to upper extremity pressure until a child is 1 year old; after that, the thigh pressure may be higher. Upper extremity pressures are higher in patients with a coarctation and no ductus arteriosus. If the upper extremity pulses are lower than the lower extremity pulses, a reverse coarctation may be present. This can be seen with ductal stents that obstruct antegrade aortic flow.
c. Note the pulse pressure. Low diastolic pressure increases the pulse pressure and may indicate a PDA, a large systemic to PA shunt, PA to aorta collaterals, or aortic regurgitation. Decreased pulse pressure may indicate AS, vasoconstriction, low CO, or cardiac tamponade. A narrowed pulse pressure with both low systolic and higher diastolic BP may indicate cardiac tamponade.
169d. Pulsus paradoxus is an exaggeration of the normal physiologic response to inspiration. Usually on inspiration there is a fall of less than 10 mmHg in arterial systolic pressure. In pulsus paradoxus, the drop exceeds 10 mmHg during normal inspiratory effort. It can also be found in pericardial effusion, hypovolemia, pericardial tamponade, significant asthma, shock, or hypovolemia.
e. Heart sounds (Figure 3.16)
i. S1 represents closure of the mitral and tricuspid valves and the beginning of systole. The mitral component of S1 is loudest at the apex. The tricuspid component of S1 is loudest at the fifth ICS to the left of the sternum. S1 is louder than S2 at the apex. S1 is increased in intensity from MS, anemia, fever, exercise, and hyperthyroidism. S1 is decreased in intensity from first-degree AV block, MR, shock, cardiomyopathy, hypothyroidism, and left bundle-branch block (LBBB). A split S1 denotes separation of mitral and tricuspid sounds and is normally heard in the tricuspid area. If audible at the anterior axillary line, it is more likely an aortic ejection click (Park, 2016).
ii. S2 represents closure of the aortic and pulmonic valves at the beginning of diastole. The aortic component of S2 is loudest at the second right intercostal space (RICS). The pulmonic component of S2 is loudest at the second LICS. S2 is heard best in the aortic and pulmonic areas. Increased intensity may be normal or may indicate hypertension or coarctation. Decreased intensity (best heard in the aortic area) indicates AS. Decreased intensity (best heard in the pulmonic area) indicates pulmonic stenosis or tricuspid atresia. A split S2 (best heard in the pulmonic area during inspiration) is physiologically related to the increased venous return to the RV, thus delaying closure of the pulmonary valve, and may be normal. A fixed split of S2 represents delayed closure of the pulmonic valve from increased pulmonary blood flow through the RV as in ASD and total anomalous pulmonary venous return (TAPVR). A widely split S2 may occur with delayed activation of the RV from right bundle-branch block (RBBB), LV pacing, or ectopic beats. A single S2 may be heard in tetralogy of Fallot, pulmonary atresia or stenosis, and transposition of the great vessels (TGV; Park, 2016).
f. Extra heart sounds (Park, 2016)
i. S3 is caused by the rapid entry of blood into the ventricles. S3 is best heard at the apex with the bell and may be a normal finding in children. S3 sounds like “Ken-tuc-ky.” A loud S3, or ventricular gallop, is a pathologic finding caused by resistance to ventricular filling related to increased volume load or decreased compliance. It occurs in MR, CHF, tricuspid insufficiency, left-to-right shunts, and anemia. It is likely the result of tensing of the chordae tendinae with rapid filling of the ventricle (Figure 3.16).
ii. S4 is produced by atrial contraction, is best heard at the apex, and is almost never a normal finding in children. It is a dull, low-pitched heart sound occurring late in diastole from contraction of the atria as they force the final bolus of blood into stiff ventricles prior to ventricular systole. It sounds like “Ten-nes-see.” It can indicate AS, pulmonic stenosis, hypertension, heart failure (HF), and anemia.
iii. Clicks occur in mid- to late systole and are loudest over the mitral or tricuspid auscultation region. They are the result of systolic prolapse of the mitral or tricuspid valve accompanied by valve regurgitation. They can also be noted in the pulmonary or aortic region due to stenosis from a bicuspid valve.
iv. Snaps occur during diastole shortly after the S2. They are the result of opening of the mitral or tricuspid valves. The more severe the stenosis, the shorter the interval between aortic closure and the opening snap.
170g. Murmurs are heard because of turbulent flow through an abnormal opening or obstructed area. The following are evaluated (Park, 2016):
i. Timing of systolic murmurs
1) Systolic ejection murmurs are heard between S1 and S2 (early, mid, or late). The intensity increases and then decreases known as crescendo–descendo. Holosystolic murmurs that are blowing and higher pitched and are heard throughout systole without change in intensity correlate with valve regurgitation and VSDs, and are the result of ventricular pressure exceeding atrial pressure at the onset of systole with immediate flow back across the regurgitant valve or the result of LV pressure exceeding RV pressure prior to the opening of the aortic valve in VSD. Generally, the smaller the VSD, the louder the murmur because of the increased turbulence. Midsystolic ejection murmurs begin after S1 in mid- to late systole and may be accompanied by a midsystolic ejection click. They are usually crescendo—decrescendo (Figure 3.17).
ii. Intensity of systolic murmurs is based on a scale:
1) Grade I. Barely audible (not heard in all positions)
2) Grade II. Just easily audible (not heard in all positions)
3) Grade III. Heard well in all positions
4) Grade IV. Heard well, a palpable thrill
5) Grade V. Louder, can be heard with stethoscope partly off chest
6) Grade VI. Heard with stethoscope off chest
iii. Timing of diastolic murmurs
1) Early decrescendo murmurs are high pitched and blowing and are often the result of regurgitant flow through a valve.
2) Mid- to late “rumbling” diastolic murmurs result from turbulent flow across a stenotic valve. They occur less commonly from increased flow across the mitral or tricuspid valve. This murmur becomes louder at the end of diastole, when atrial contraction accelerates flow across the stenotic valve (Figure 3.18).
iv. Intensity of diastolic murmurs is based on a 4-point scale:
1) Grade I. Barely audible
2) Grade II. Faint but immediately audible
3) Grade III. Easily heard
4) Grade IV. Very loud
v. Continuous murmurs are heard throughout the cardiac cycle without an audible break between systole and diastole. They are the result of blood flow from an area of high pressure to low pressure, most often caused by a PDA or systemic to pulmonary shunt (Figure 3.18).
vi. To-and-fro murmurs are distinguishable from continuous murmurs in that they do not go past S2. They have a discrete systolic and diastolic component. They are commonly noted in patients with RV–PA valved conduits or Sano shunts.
vii. Location of heart murmurs
1) Apical area (with some extension up to the pulmonic area): Murmurs of mitral insufficiency or stenosis, subaortic stenosis, aortic insufficiency, aortic ejection click of AS, and click or late systolic murmur of mitral valve prolapse
2) Tricuspid area (with some extension up to the pulmonic area): Murmurs of tricuspid insufficiency or stenosis, pulmonary insufficiency, VSD, and aortic insufficiency
3) Aortic area. Murmurs of AS or insufficiency
4) Pulmonic area. Murmurs of PS or insufficiency, ASD, pulmonary ejection click, and PDA
5) Note areas of the murmur’s radiation specifically to other areas of chest, axilla, or back
viii. Pitch and quality of the murmur
1) High-pitched murmurs are heard with the diaphragm of the stethoscope and low-pitched murmurs are heard with the bell of the stethoscope.
2) The quality of a murmur is described as blowing, rumbling, harsh, or musical (Duderstadt, 2014).
C. Noninvasive Diagnostic Studies
1. Laboratory studies to be ordered may include electrolytes with renal function, hepatic function tests and total and direct bilirubin, complete blood count (CBC), lipid profile, calcium (total and ionized), magnesium, BNP levels, troponin levels, lactate levels, and a clotting profile (prothrombin time [PT], partial thromboplastin time [PTT], activated clotting time [ACT], bleeding time, and platelet count.
2. Pulse oximetry uses changes in infrared light to evaluate the level of saturated hemoglobin, 172providing an indirect measurement of oxygen saturation (normal 96%–100%). Pulse oximetry can be used to evaluate or trend cyanosis or to assess tolerance of procedures (suctioning, sedation). Pulse oximetry is performed routinely prior to discharge in all newborns to assess for potential cardiac disease (Mahle, Newburger, et al., 2014).
3. Chest Radiograph. This evaluates heart size by estimation of the cardiothoracic ratio, which is determined by cardiac width compared with the thoracic width of the chest (Herring, 2012). The normal cardiothoracic ratio is 50% or less. A large thymus in infants may be mistaken for cardiomegaly. Cardiac borders are evaluated on an anterioposterior film (Figure 3.19). The right border indicates the RA. The left lower border indicates the LV; the LA blends into the ventricular shadow unless it is significantly enlarged. The first convexity above the apex is the PA. The second convexity above the apex is the aortic arch. Specific defects can be identified from abnormal borders. A boot-shaped heart can indicate tetralogy of Fallot related to RV hypertrophy with apex upturned. A convex shoulder of the aorta is seen in transposition of the great arteries (looks like an egg with a narrow superior mediastinum). Increased pulmonary vascularity is indicated by arteries that appear enlarged and extend into the lateral third of the lung field as seen in ASD, VSD, PDA, TAPVR, truncus, transposition, and AV canal (Figure 3.20). Decreased pulmonary vascularity is noted when the hilum appears small, lung fields are empty and devoid of vessels, and the x-ray image appears dark/black. This may be noted in tetralogy of Fallot, tricuspid atresia, Ebstein anomaly, severe pulmonary hypertension, and transposition with PS (Figure 3.21).
a. Lung fields/lung health are assessed by looking for atelectasis, effusion, pneumothorax, pneumonia, and volume loss/expansion. The diaphragm location and margin of diaphragm should be sharp.
b. Diaphragms should be located at symmetrical positions on each side of the chest. Elevation 173of the diaphragm on one side suggests paralysis of diaphragm or subpulmonic effusion.
c. Assessment of any invasive monitoring lines or tubes should be performed on every x-ray obtained when these are present. Endotracheal tube (ETT), nasogastric or transpyloric tube, chest tubes, intracardiac lines, pacemaker leads (temporary or permanent), stents, coils, artificial valves, ventricular assistive device (VAD) and extracorporeal membrane oxygenation (ECMO) cannulas, central venous line (CVL), umbilical artery catheter (UAC), and umbilical venous catheter (UVC) should be evaluated. A comparison to location on the previous x-ray should be performed.
d. Abnormal bony structures may be associated with syndromes or from injury during delivery and should be noted and recorded.
4. Electrocardiogram. ECG provides a noninvasive examination of HR, rhythm, conduction abnormalities, and provides clues to diagnosis of certain cardiomyopathies, congenital heart defects, and hypertension. It can detect electrolyte imbalances, including hypocalcemia (prolong ST segment), hyperkalemia (shortens ST segment), hypokalemia (flat T waves, PR interval is prolonged), and hyperkalemia (tall, peaked T waves; Park & Guntheroth, 2006).
a. Horizontal lines are a measurement of time. Each small block equals 0.04 second, and each larger dark block equals 0.2 second, with use of standard paper speed of 25 mm/sec.
b. Vertical lines represent a measurement of voltage with each small block equal to 0.1 mV or 1 mm and each larger block equal to 0.5 mV or 5 mm (if gain of ECG is set to standard of 1 mV = 10 mm).
5. ECG Waves and Intervals (Figure 3.22)
a. The P wave represents atrial depolarization. It is measured from the beginning of the P wave 174to the end of the wave, when it returns to the baseline. Usually, it is less than 0.08 second. The normal amplitude is less than 2.5 mm (3 mm in the neonate), and it is usually gently rounded with all waves having the same appearance. Right atrial enlargement is noted by tall, peaked P waves greater than 2.5 mm. Left atrial enlargement (LAE) demonstrates a wide, notched P wave.
b. The PR interval represents atrial depolarization and conduction through the AV node. It is measured from the beginning of the P wave to the beginning of the QRS complex. The normal PR interval is 0.12 to 0.20 second (shorter in younger children with faster HRs). A prolonged PR interval is an indication of first-degree heart block (Table 3.6).
c. The QRS interval represents ventricular depolarization. It is measured from the beginning of the QRS to the end of the QRS. Normal duration is 0.06 to 0.10 second. Prolonged QRS duration may indicate interventricular conduction delay. Although commonly called the QRS complex, the first initial downward deflection is labeled Q, the first upward deflection is labeled R, the first downward deflection after the R wave is labeled S, and any other deflections are labeled with an accent to indicate “prime,” such as rSR′. The size of the wave is indicated by a capital or small letter; waves over 5 mm are in capitals.
d. The T wave represents ventricular repolarization and should be in the same direction as the QRS complex. The T wave may change configuration with hypokalemia (flattened) or hyperkalemia (peaked).