Normal respiratory physiology and the effects of mechanical ventilation
Oxygenation is by diffusion of oxygen across the alveolar capillary membrane. Its prime determinants are inspired oxygen concentration and mean airway pressure. The mean airway pressure is modified by changes in peak airway pressure, positive end-expiratory pressure (PEEP) and the inspiratory:expiratory time ratio (I:E). Removal of carbon dioxide requires movement of air in and out of the lung. Its prime determinant is minute volume whatever the changes made to the ventilator. Arterial carbon dioxide (PaCO2) is regulated at about 5.3 kPa. A normal tidal volume (Figure 6) is 350–500 ml or 5–7 ml/kg in an adult and the normal respiratory rate is 10–15 breaths/minute and depends on the requirement for carbon dioxide elimination. The total volume of the respiratory system or total lung capacity when expanded by voluntary effort is 5–6 litres in the average adult. The total alveolar volume can be divided into that which can be measured at the lips (vital capacity) and that which remains in the lungs after maximal expiration (residual volume). The functional residual capacity is the combination of expiratory reserve volume and residual volume (Figure 6).
Resistance is the pressure difference between the beginning and end of a tube divided by the flow of gas volume per unit time. In the lung, it is the difference between atmospheric pressure and alveolar pressure. The tracheobronchial tree is a dynamic system of cartilaginous structures that can distend or narrow, depending on the forces exerted on them. The smaller more distal airways are kept open by the balance of the intrapleural pressures and the elastic recoil of the lung. During inspiration, elongation of the elastic pulmonary fibres increases the elastic retraction pressure and as the bronchioles are stretched by the stronger radial pull the bronchial resistance decreases. With expiration, the elastic recoil of the lung increases, the bronchioles become narrower and the flow resistance increases. These changes in the flow characteristics during the respiratory phases explain why the expiratory phase is slightly longer than the inspiratory phase. Dynamic compression of the small airways occurs when the intrapleural pressures increase to about 40 cmH2O during forced expiration. When the intrapleural pressures are considerably greater than the intraluminal pressures, narrowing or closure of the bronchioles occurs (Figure 7). If airway closure is premature, the closing volume encroaches on the functional residual capacity (FRC).
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