I. Inadequate intrinsic respiratory capacity to prevent or compensate for severe hypoxia and/or hypercarbia due to the following:
A. Neuromuscular (NM) depression or failure
1. Drugs
a. Opioids
b. Sedatives
c. NM blockers
2. Trauma
a. Spinal cord injury (SCI)
b. Phrenic nerve injury
3. Disease
a. Guillain-Barré syndrome
b. Amyotrophic lateral sclerosis (ALS)
c. Myasthenia gravis
d. Shock
4. Exhaustion
a. Status asthmaticus
b. Sustained severe work of breathing (WOB)
5. Sustained apnea of any cause
B. Persistent hypoxia (partial pressure of oxygen in arterial blood [PaO2] less than 60) and/or hypercarbia (partial pressure of carbon dioxide in arterial blood [PaCO2] greater than 50) refractory to noninvasive supplemental oxygen and/or airway maintenance (suction, position)
1. Diffusion defects
a. Aspiration
c. Acute respiratory distress syndrome (ARDS)
d. Chronic obstructive pulmonary disease (COPD)
e. Pneumonia
2. Ventilation defects
a. COPD
b. Pickwickian syndrome
c. Flail chest
d. Pneumothorax
e. Atelectasis
3. Perfusion defects
a. Shock
b. Pulmonary embolus
c. Malignant arrhythmias
GENERAL PRINCIPLES OF VENTILATION
I. Inspiratory air flow occurs as the result of a pressure gradient in which extrapulmonary pressure is greater than intrapulmonary pressure.
A. This can result from lowering intrapulmonary pressure to below extrapulmonary pressure or by raising extrapulmonary pressure to above intrapulmonary pressure.
1. Normal human inspiration occurs when chest volume is expanded by contraction of the diaphragm and elevation of the ribs, creating a negative intrapulmonary pressure and drawing air into the lungs. (“People suck to breathe.”)
2. Mechanical ventilation creates positive extrapulmonary pressure generated by a device that forces air into the lungs. (“Ventilators blow to breathe.”)
a. Positive-pressure ventilation requires a sealed airway, most commonly attained by an endotracheal tube (ETT) with an inflatable cuff.
II. Expiratory air flow occurs as the result of a pressure gradient in which intrapulmonary pressure is raised to above extrapulmonary pressure.
A. Normal expiration, whether inspiration is spontaneous or mechanical, relies on passive elastic recoil of lung tissue and of chest wall muscles.
1. Chest wall and abdominal musculature can actively augment passive elastic recoil.
2. Mechanical positive-pressure ventilators cannot create an expiratory pressure gradient.
VARIABLES FOR MECHANICAL VENTILATORS
II. Rate is the number of mechanical breaths delivered each minute.
A. A mechanical breath cycle may be defined by a TV target or by a pressure target.
B. Set ventilator rates will be adjusted to achieve pH and PaCO2 goals.
III. Fraction of inspired oxygen (FIO2) is the decimal value produced by dividing partial pressure of oxygen (PO2) by total pressure of the mixture.
A. FIO2 is most correctly expressed by a decimal value rather than by a percentage.
1. FIO2 of room air is approximately 0.21.
2. FIO2 of 100% oxygen is 1.
B. FIO2 settings on ventilators typically range from 0.35 to 1, depending on patient requirements.
1. FIO2 that exceeds 0.5 for longer than 24 hours may result in oxygen toxicity.
IV. Inspiratory cycles vary as follows:
A. Volume cycled—pressure limited (volume targeted)
1. A preset TV is delivered unless a set pressure limit is reached, terminating the cycle.
a. Ensures that TV is not determined by compliance but has higher risk of barotrauma
B. Pressure cycled—volume limited (pressure targeted)
1. A preset pressure is delivered unless a volume limit is reached, terminating the cycle.
a. Allows more natural TV with less risk of barotrauma
b. Rate may have to be adjusted to compensate for variable tidal volumes.
V. Positive end-expiratory pressure (PEEP) prevents return of intrapulmonary pressure to equal extrapulmonary pressure at the end of expiration.
A. Effect is that of an incomplete expiration: increased functional residual volume (FRV) remains.
1. Result of increased FRV is twofold.
a. Greater number of alveoli opened for gas exchange throughout ventilatory cycle
B. Effect of increased alveolar ventilation is twofold.
1. Increased PaO2 without increase in FIO2
2. Reduced atelectasis
C. Risks
1. Barotrauma due to hyperdistention and high intrapulmonary pressures
2. Impedance of central venous return, resulting in lower cardiac output
a. Most significant with low central venous pressure (CVP) or right ventricular (RV) diastolic dysfunction
b. Increased intracranial pressure
D. Typical PEEP settings range from 5 to 10 cm H2O.
1. “Higher” PEEP up to 10 to 20 cm H2O may be used with low-compliance conditions such as ARDS.
VI. Continuous positive airway pressure (CPAP) is functionally equivalent to PEEP.
A. CPAP is effectively PEEP without mechanically delivered inspirations (Rate = 0).
B. Patient must have independent inspiratory capability.
C. Same risks and benefits as PEEP
VII. Pressure support reflects an augmentation of flow rate during spontaneous inspiration.
A. Result is to overcome resistance to flow through ventilator circuit (valves, corrugated tubing, narrow lumina), thus reducing spontaneous inspiratory effort (WOB).
B. At higher levels of pressure, can provide full ventilatory support
VIII. Alarms alert the clinician to unacceptable deviations from various critical ventilator variables.
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A. High-pressure alarms when proximal airway pressures exceed set limits
1. Secretion accumulation
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