The importance and problems of sterilisation of anaesthetic equipment

At one litre above the functional residual capacity (FRC), pulmonary blood flow is distributed preferentially to the dependent parts of the lungs. However, the distribution of inspired gas is known to be influenced by many factors which may adversely influence the regional distribution of ventilation/perfusion ratios, thereby increasing the alveolar/ arterial Po2 difference. Under normal circumstances, when a patient breathes above FRC, compliance is greater in the lower part of the lungs, which therefore receive the greater part of the inspired gas during slow breathing. This conveniently matches the enhanced circulation enjoyed by this area. However, it now appears that certain factors may divert gas away from the dependent part of the lungs. Firstly, a fast inspiration will be distributed more in accord with the regional time constants than with the regional compliances. The upper part of the lung has lower compliance but also lower airway resistance thus giving a shorter regional time constant. Therefore fast inspirations deflect gas away from the best perfused areas of lung, and this may explain the high dead space and low dynamic compliance which have been observed. Another factor which may deflect inspired gas away from the dependent parts of the lung is airway closure. It has now been shown that air passages in the most dependent parts of the lungs may become closed at reduced lung volume. When this occurs, the sequestered gas equilibrates with the mixed venous blood and the flow through these areas constitutes a shunt causing a fall in arterial Poz. Many factors may cause ventilation to take place at reduced lung volume during anaesthesia. Furthermore, it has been clearly established that airway closure occurs at a progressively larger lung volume as age advances. Also, the areas of closure will be greater with forced expirations, the Trendelenberg position and any additional factor which reduces the lung volume. Herein lies a probable explanation of the effect of age upon arterial Poz, and possibly the reduction of arterial Po2 in many clinical situations.