I. Definition
A. Closely interrelated terms that collectively refer to the processes by which oxygen (O2) and carbon dioxide (CO2) are transported between atmosphere and tissue via concentration gradients, perfusion, and affinity of hemoglobin for oxygen
B. Oxygenation refers to movement of oxygen into the blood and its transport and delivery to tissue.
1. Diffusion pressures, determined by gradients between the respective partial pressures of alveolar, plasma, and tissue oxygen, allow for movement of oxygen from alveoli to blood to tissue.
2. Hemoglobin and its affinity for oxygen allows for blood transport of far more oxygen than could be dissolved in plasma alone.
3. Perfusion, determined by cardiac output and vascular tone, affects oxygen delivery to tissue capillaries.
C. Ventilation refers to the exchange of extrapulmonary and intra-alveolar gas mixtures.
1. Resupply of oxygen diffused into blood and removal of CO2 diffused from blood allow for maintenance of their respective alveolar-capillary pressure gradients.
2. Patent airway(s), neuromuscular function, and structural integrity of chest wall allow for inspiratory and expiratory air flow.
II. Clinical measurements
A. Exercise tolerance: Ability to perform a normal exercise load (e.g., climb a flight of stairs without stopping) suggests adequate oxygenation and ventilation.
C. Cyanosis
1. Dusky bluish tint from excessive amounts of unsaturated hemoglobin
2. The more central the cyanosis, the greater its severity. For example, cyanosis evident on the face and chest is more severe than cyanosis limited to the fingertips.
D. Arterial blood gases (ABGs)
1. Partial pressures of oxygen and carbon dioxide
2. Oxygen saturation of arterial hemoglobin
3. pH
4. Presence or absence of metabolic imbalance
5. Normal values for arterial blood gas
a. pH = 7.35 to 7.45
b. Partial pressure of oxygen in arterial blood (PaO2) = 80 to 97 mmHg
c. Partial pressure of carbon dioxide in arterial blood (PaCO2) = 35 to 45 mmHg
d. Bicarbonate (HCO3) = 22 to 26 mEq/L
e. Base excess, −3 to +3 mEq/L
f. Saturation of arterial oxygen (SaO2) greater than 98%
6. Partial pressures of arterial oxygen and carbon dioxide and arterial oxygen saturation decline as altitude above sea level increases.
7. Oxygenation
a. Interpret PaO2 in light of fraction of inspired oxygen (FIO2). As a general rule, PaO2 should be 4 to 5 times the percentage of O2 (e.g., the FIO2 of room air is 21%, and a normal PaO2 is 80 to 97 mmHg).
8. Ventilation
a. Ventilation is reciprocally reflected by PaCO2. For example, a rising PaCO2 means that a decrease in ventilation has occurred (and vice versa).
b. Every 10-mm shift in PaCO2 should produce a reciprocal 0.08-shift in pH. A PaCO2 of 50 mm would thus predict a pH of 7.32. Any difference from that predicted shift must be attributed to a concurrent metabolic imbalance or compensation.
III. Limitations
A. Invasiveness
1. Drawing blood for an ABG measurement is often painful for the patient and involves significant risk of injury and compromise of perfusion distal to the site.
2. If frequent measurements are required, as in a critically ill or mechanically ventilated patient, an indwelling arterial catheter should be placed.
C. The ABG sample must be handled with care.
1. The sample must not be allowed to clot and must be heparinized.
2. Excessive amounts of heparin in a small blood sample will alter the pH.
3. Any air bubbles must be expelled from the syringe because oxygen and carbon dioxide in the sample will equilibrate with the air bubble.
4. If more than a few minutes may pass before the sample is measured, it must be chilled to limit the ongoing metabolism of cellular components.
D. The sample may inadvertently be taken from a vein in close proximity to the intended arterial site.
1. The clinician must make an astute analysis for results inconsistent with an arterial sample.
PULSE OXIMETRY
I. Definition
A. Pulse oximetry is a noninvasive, continuous, and relatively inexpensive method for transcutaneous measurement of the degree to which hemoglobin in arterial blood is saturated with oxygen (SaO2).
II. Mechanism
A. Pulse oximetry (SpO2) uses a light-emitting diode to transmit light in the trans-red and near-infrared wavelengths through perfused tissue to a receiving sensor.
B. The device compares light emitted versus light received by the sensor to calculate light absorbed by oxyhemoglobin and thus the saturation of hemoglobin with oxygen.
C. The device can detect a pulsatile fluctuation in saturation and can selectively display the highest measurement as the SpO2 (presumed to reflect the SaO2).
D. The pulse oximetry probe may be applied for intermittent or continuous measurements. It is typically placed on a digit but can be placed on an earlobe, cheek, nose, or toe.
E. The accuracy of the displayed reading may be affected by several factors.
1. Hemoglobin concentration; ideally, hemoglobin level should be greater than 10 mg/dl.
2. Movement of the probe may create artifact, causing the measurement to be rejected. Alternative placement or stabilization with tape may be required.
3. It is recommended but not always necessary that nail polish be removed so an accurate reading is obtained.
IV. Limitations of pulse oximetry
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A. Pulse oximetry is only a presumptive reflection of SaO2 and does not provide information regarding pH, PaCO2, or respiratory rate.
B. Pulse oximetry is confounded by the presence of carbon monoxide.
1. The device will misinterpret carboxyhemoglobin as oxyhemoglobin and thereby provide a false elevation.
2. When carbon monoxide poisoning is suspected, an ABG analysis is a more reliable measurement of oxygenation.
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