Understanding mechanical ventilation must start with a review of the physiology and mechanics of normal spontaneous breathing. Spontaneous breathing is defined as movement of air into and out of the lungs as a result of work done by an individual’s respiratory muscles. Positive pressure ventilation, on the other hand, is defined as movement of air into the lungs by the application of positive pressure to the airway through an endotracheal tube, tracheostomy tube, or noninvasive mask. Lung Volume The lungs sit inside a chest cavity surrounded by the chest wall. The potential space between the lungs and the chest wall is known as the pleural space. The lungs, composed of elastic tissue, have a tendency to recoil inward, and the chest wall has a tendency to spring outward. If the lungs were removed from the chest cavity and were no longer being influenced by the chest wall or the pleural space, they would collapse like a deflated balloon. Similarly, removing the lungs from the chest cavity would cause the chest wall, no longer being influenced by the lungs or the pleural space, to spring outward. The equilibrium achieved between the lungs’ inward recoil and the
Figure 1.1 Chest wall springing outward and lung recoiling inward. Because of these opposing forces, the pleural space has subatmospheric pressure at the end of expiration.
chest wall’s outward recoil determines lung volume at the end of expiration. As a result of the coupling of the lungs and the chest wall, pressure in the pleural space, known as pleural pressure (Ppl), is less than atmospheric pressure at the end of expiration. This subatmospheric pleural pressure prevents the chest wall from springing outward and the lungs from collapsing inward (Fig. 1.1). Key Concept #1 Balance between lung recoil inward and chest wall recoil outward determines lung volume at end of expiration Transpulmonary Pressure For a given lung volume at equilibrium, the forces pushing the alveolar walls outward must equal the forces pushing the alveolar walls inward. The expanding outward force is alveolar
pressure (Palv). The collapsing inward forces are pleural pressure and lung elastic recoil pressure (Pel). The difference between alveolar pressure and pleural pressure, known as transpulmonary pressure (Ptp), is equal and opposite to lung elastic recoil pressure for a given lung volume (Fig. 1.2). Transpulmonary pressure determines lung volume. Increasing transpulmonary pressure increases the outward distending pressure of the lung, resulting in a larger lung volume. Thus, the lungs can be inflated either by decreasing pleural pressure, as occurs in spontaneous breathing, or by increasing alveolar pressure, as occurs in positive pressure ventilation (Fig. 1.3). Key Concept #2 • To inflate lungs, Ptp must increase • Ptp = Palv− Ppl • To increase Ptp, either decrease Ppl (spontaneous breathing) or increase Palv (positive pressure ventilation) The relationship between the transpulmonary pressure and lung volume is not linear, but rather curvilinear, because as lung volume increases, the lungs become stiffer and less compliant. That is, larger increases in transpulmonary pressure are necessary to achieve the same increase in lung volume at higher lung volume than at lower lung volume. Similarly, increasing transpulmonary pressure by a set amount will lead to a larger increase in lung volume at lower lung volume than at higher lung volume (Fig. 1.4). Spontaneous Breathing Inspiration During spontaneous breathing, inspiration occurs by decreasing pleural pressure, which increases transpulmonary pressure
Figure 1.2 (a) At equilibrium, the sum of the expanding outward forces must equal the sum of the collapsing inward forces at equilibrium. Therefore, alveolar pressure equals the sum of pleural pressure and lung elastic recoil pressure. (b) Transpulmonary pressure is the difference between alveolar pressure and pleural pressure. It is equal and opposite to lung elastic recoil pressure for a given lung volume (Ptp = −Pel). Palv alveolar pressure; Pel lung elastic recoil pressure; Ppl pleural pressure; Ptp transpulmonary pressure
Figure 1.3 Lung inflation occurs either by decreasing pleural pressure (spontaneous breathing) or by increasing alveolar pressure (positive pressure ventilation). In both cases, transpulmonary pressure increases. Palv alveolar pressure; Ppl pleural pressure
(remember Ptp = Palv− Ppl). Under normal conditions, alveolar pressure is equal to atmospheric pressure at the end of expiration. During inspiration, the diaphragm and other inspiratory muscles contract, pushing the abdominal contents downward and the rib cage upward and outward, ultimately increasing intrathoracic volume. Boyle’s law states that, for a fixed
Figure 1.4 Relationship between lung volume and transpulmonary pressure. For a given increase in transpulmonary pressure (ΔP), the resultant increase in lung volume (ΔV) is greater at lower lung volume, where the lung is more compliant, than at higher lung volume.
amount of gas kept at constant temperature, pressure and volume are inversely proportional (pressure = 1/volume). Thus, this increase in intrathoracic volume results in a decrease in intrathoracic pressure and therefore a decrease in pleural pressure. Decreased pleural pressure increases transpulmonary pressure and causes the lungs to inflate. This increase in lung volume, as explained by Boyle’s law, results in a decrease in alveolar pressure, making it lower than atmospheric pressure. Because gas flows from regions of higher pressure to regions of lower pressure, air flows into the lungs until alveolar pressure equals atmospheric pressure. Expiration Quiet expiration is passive. That is, no active contraction of respiratory muscles is required for expiration to occur. The diaphragm and inspiratory muscles relax, the abdominal contents
return to their previous position, and the chest wall recoils, ultimately resulting in a decrease in intrathoracic volume. The decrease in intrathoracic volume results in an increase in intrathoracic pressure and thus an increase in pleural pressure. Increased pleural pressure decreases transpulmonary pressure and causes the lungs to deflate. This decrease in lung volume results in an increase in alveolar pressure, making it higher than atmospheric pressure. Because of this pressure gradient, air flows out of the lungs until alveolar pressure equals atmospheric pressure. Modeling the Respiratory System The flow of air in and out of the lungs can be modeled in a manner similar to an electrical circuit using Ohm’s law, where the voltage (V) across a resistor is equal to the electric current (I) multiplied by the electrical resistance (R). The difference between proximal airway pressure (Pair) measured at the mouth and alveolar pressure (Palv) is analogous to the voltage difference within a circuit. Similarly, flow (Q) and airway resistance (R) in the respiratory system are analogous to the electric current and electrical resistance in the circuit, respectively (Fig. 1.5). The equation for the respiratory system can be rearranged to solve for flow: Q = airalv PP R By convention, flow into the patient (inspiration) is designated as positive, and flow out of the patient (expiration) is designated as negative. Note that when proximal airway pressure equals alveolar pressure, there is no flow present in either direction (Q = 0). Under normal conditions, this scenario occurs twice during the breathing cycle, at the end of expiration and at the end of inspiration.
Figure 1.5 The respiratory system modeled as an electrical circuit. I electric current; Pair proximal airway pressure; Palv alveolar pressure; Q flow; R resistance; V voltage
With spontaneous breathing, proximal airway pressure is equal to atmospheric pressure. During inspiration, the diaphragm and other inspiratory muscles contract, which increases lung volume and decreases alveolar pressure, as previously discussed. This process results in alveolar pressure being less than proximal airway pressure, which remains at atmospheric pressure. Therefore, flow will become a positive value, indicating that air flows into the patient. During expiration, alveolar pressure is higher than proximal airway pressure, which makes flow a negative value, indicating that air flows out of the patient. With positive pressure ventilation, as occurs with mechanical ventilation, the ventilator increases proximal airway pressure during inspiration. This increase in proximal airway pressure relative to alveolar pressure results in a positive value for flow, causing air to flow into the patient. Expiration
with positive pressure ventilation is passive and occurs in a manner similar to that which occurs in spontaneous breathing.The sequence of events for inspiration is different for spontaneous breathing than for positive pressure ventilation. In spontaneous breathing, increased intrathoracic volume leads to decreased alveolar pressure, which leads to air flow-ing into the patient because of the pressure gradient. With positive pressure ventilation, increased proximal airway pres-sure leads to air flowing into the patient, which, because of Boyle’s law, results in an increase in lung volume (Fig. 1.6).
Spontaneous ventilationPositive pressure ventilationInspiratory muscles contractVentilator increases proximalairway pressure↑ Intrathoracic volume↓ Intrathoracic pressure↑ Alveolar pressure↓ Alveolar pressureAir flows into lungs untilalveolar pressureequals atmospheric pressure↓ Pleural pressure↑ Transpulmonary pressure↑ Transpulmonary pressure↑ Lung volume↑ Lung volume
Key Concept #3 • Inspiration with spontaneous breathing: Palv made lower than atmospheric pressure to suck air into lungs • Inspiration with positive pressure ventilation: Pair made higher than atmospheric pressure to push air into lungs