| Lung volume increases above end-expiratory volume on inspiration. During spontaneous inspiration, intrathoracic pressure decreases, but on positive-pressure ventilation, inspiration occurs by an increase in intrathoracic pressure. The distribution of ventilation and perfusion in the lung varies with these pressure and volume changes.
Alterations in the distribution of ventilation
After induction of anaesthesia in the supine position, non-dependent parts of the lung are more compliant than the dependent zones. Positive-pressure ventilation preferentially ventilates these non-dependent zones. This phenomenon becomes further accentuated if the lungs have increased density as in ARDS or left ventricular failure.
Alterations in distribution of perfusion
Below a mean intrathoracic pressure of 30 cm H2O, distribution of perfusion remains normal during mechanical ventilation. In non-dependent areas of lung with relatively lower capillary pressure, blood vessels are susceptible to compression as mean intrathoracic pressure increases above 30 cm H2O. Compression of the alveolar capillaries in these preferentially ventilated zones reduces blood flow and worsens V/Q mismatch and Vd/Vt ratios. This can be further exaggerated by PEEP.
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