Anaesthesia: Lung Imaging and Gas Exchange Abnormalities

A fall in functional residual capacity (FRC) by anaesthesia promotes airway closure. It impedes ventilation, primarily in dependent lung regions and causes atelectasis. The decrease or elimination of ventilation results in regions with low ventilation-perfusion ratios (V_A/Q) combined with those with intrapulmonary shunt. They occur early during induction of anaesthesia and may remain for a couple of days postoperatively. Both contribute to an oxygenation impairment but can be prevented by recruitment manoeuvres, followed by positive end-expiratory pressure (PEEP). Low V_A/Q areas increase with age but shunt (caused by the atelectasis) has a peak at 45-50 years and then decreases with further rise in age. Shunt increases with increasing body mass index (BMI) whereas areas with low V_A/Q do not. These findings suggest different approaches in ventilatory support during anaesthesia in the young to middle-aged and the elderly patient, and in the lean and over-weight patient.