Causes of gas exchange impairment during general anaesthesia

This review describes the distribution of ventilation and blood flow in the anaesthetized subject, during spontaneous breathing and after muscle paralysis. Within minutes after induction of anaesthesia, the diaphragm is shifted cranially (supine position), functional residual capacity is reduced and collapse of dependent lung regions can be seen by means of computed tomography. These changes occur whether anaesthesia is intravenous (barbiturate) or inhalational (halothane) and whether ventilation is spontaneous or mechanical. Ventilation is subsequently reduced in dependent lung regions, whereas blood flow is preferentially distributed to the lower lung regions. This causes a ventilation/perfusion mismatch, the hall-mark of which is shunt. Additional factors such as airway closure and release of hypoxic pulmonary vasoconstriction may contribute to the gas exchange disturbance. The major features of the lung function impairment are already present during spontaneous breathing in the anaesthetized subject, and muscle paralysis adds only little to the disturbance.