Oxygenation During One-Lung Anaesthesia
Arterial oxygenation is regularly impaired during one-lung anaesthesia, that is ventilation only in the dependent lung with the patient in the lateral position. This has often been attributed to persisting blood flow through the nondependent lung and efforts have been avocated to minimize this blood flow. However, success has been limited, indicating additional causes of gas exchange impairment, residing in the dependent lung. Recently, prompt collapse of dependent lung regions has been demonstrated on induction of general anaesthesia. The collapsed area has been named compression atelectasis to indicate that it is not caused by closure of airways and slow resorption of gas but rather to reduction of thoracic volume. The atelectasis causes shunt which may be the only or the major cause of gas exchange impairment during anaesthesia. Positive end-expiratory pressure applied to the nondependent lung forces blood flow to the dependent lung and may increase perfusion of the atelectatic region. PEEP to the dependent lung to counter lung collapse will force blood flow to the nondependent, nonventilated lung. Thus, measures to improve arterial oxygenation during one-lung anaesthesia should be directed towards both lungs, reducing blood flow through the upper, nonventilated lung and countering collapse in the dependent lung. The way to achieve this remains to be found.
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