Applied respiratory physiology

Anaesthesia has many effects on respiratory physiology, the knowledge of which is relevant to clinical practice. Anaesthesia causes decreased muscle tone in the upper airway, which can lead to airway obstruction. Pulmonary hypoventilation occurs in the spontaneously breathing patient. There is a progressive decrease in the ventilatory response to CO2 with increasing concentration of volatile agents, and even low doses of volatile agents have a profound effect on the ventilatory response to hypoxia. Functional residual capacity (FRC) is significantly reduced in the anaesthetized patient. Airway closure occurs when closing capacity exceeds FRC; with a reduced FRC this is more likely to happen especially in older patients or patients with coexisting lung pathology when closing capacity may be increased. The resulting atelectasis will affect oxygenation. Respiratory system compliance reduces very early during anaesthesia and there is little difference between the paralysed and spontaneously breathing patient. Alveolar dead space is decreased due to impairment of V/Q matching. During anaesthesia, venous admixture accounts for 10% of cardiac output due to increased shunt and changes in V/Q scatter. During anaesthesia and surgery, patient position, type of surgery, smoking and obesity all have specific effects on respiratory physiology. Exercise physiology parameters such as anaerobic threshold have a role as a measure of cardiorespiratory fitness such as in cardiopulmonary exercise testing (CPX). CPX is increasingly used in risk stratification in patients undergoing major surgery. Anaerobic threshold is the point at which oxygen delivery mechanisms can no longer match the oxygen demand required in exercise.