MECHANISMS OF BREATHING
The periodic exchange of alveolar gas with the fresh gas from the upper airway reoxy-genates desaturated blood and eliminates CO2. This exchange is brought about by small cyclic pressure gradients established within the airways. During spontaneous ventilation, these gradients are second-ary to variations in intrathoracic pressure; during mechanical ventilation, they are produced by inter-mittent positive pressure in the upper airway.
Normal pressure variations during spontaneous breathing are shown in Figure 23–3. The pressure within alveoli is always greater than the surrounding (intrathoracic) pressure unless the alveoli are col-lapsed. Alveolar pressure is normally atmospheric (zero for reference) at end-inspiration and end-expiration. By convention in pulmonary physiology, pleural pressure is used as a measure of intrathoracic pressure. Although it may not be entirely correct to refer to the pressure in a potential space, the concept allows the calculation of transpulmonary pressure.
Transpulmonary pressure, or Ptranspulmonary, is then defined as follows:
Ptranspulmonary = Palveolar −Pintrapleural
At end-expiration, intrapleural pressure nor-mally averages about –5 cm H2O, and because alveo-lar pressure is 0 (no flow), transpulmonary pressure
is +5 cm H2O.
Diaphragmatic and intercostal muscle acti-vation during inspiration expands the chest and decreases intrapleural pressure from –5 cm H 2O to –8 or –9 cm H2O. As a result, alveolar pressure also decreases (between –3 and –4 cm H 2O), and an alveolar–upper airway gradient is established; gas flows from the upper airway into alveoli. At end-inspiration (when gas inflow has ceased), alveolar pressure returns to zero, but intrapleural pressure remains decreased; the new transpulmonary pres-sure (5 cm H2O) sustains lung expansion.
During expiration, diaphragmatic relaxation returns intrapleural pressure to –5 cm H2O. Now the transpulmonary pressure does not support the new lung volume, and the elastic recoil of the lung causes a reversal of the previous alveolar–upper airway gradient; gas flows out of alveoli, and original lung volume is restored.
Most forms of mechanical ventilation intermittently apply positive airway pressure at the upper airway. During inspiration, gas flows into alveoli until alveo-lar pressure reaches that in the upper airway. During the expiratory phase of the ventilator, the positive airway pressure is removed or decreased; the gradi-ent reverses, allowing gas flow out of alveoli.