Oximetry During One Lung Anaesthesia

Abstract

Anaesthetised patients under controlled ventilation and in the lateral position may show evidence of uneven distribution of ventilation-perfusion ratio as a conse- quence of good ventilation but poor perfusion of the non-dependent lung and poor ventilation but good perfusion of the dependent lung (Nunn 1961). In fact the blood flow distribution was seen to be dependent on gravitational factors (West et al. 1964) while the hyperventilation of the non-dependent lung is determined by two factors:

• 1. The open chest

• 2. The muscle relaxation

The poor ventilation of the dependent lung is determined by the reduction in lung volume produced by general anaesthesia and by the compression caused by abdominal contents and mediastinum (Rheder et al, 1972; Wulff and Aulin 1972). Under these circumstances one lung ventilation will generally result in an increase of alveolar-to-arterial Po₂ difference and in a reduction of arterial oxygen saturation (Brown et al. 1977; Khanom and Branthwaite 1973).