Ventilation-Perfusion Relationships during Anaesthesia and Abdominal Surgery

Nine patients, 61–72 years old, were studied awake and during halothane anaesthesia before and during upper abdominal surgery. Central circulation was evaluated by means of pulmonary artery catheterization and gas exchange by multiple inert gas elimination technique. Awake and supine, a rather wide distribution of _A/ was observed, with regions of low _A/ in five patients and a small shunt (1–3%) in three patients. The overall dispersion of blood flow was log s.d. 0.93. With anaesthesia and mechanical ventilation, cardiac output fell by 30%, whereas pulmonary vascular pressures remained unaltered. The dispersion of pulmonary blood flow against _A/ ratios was further increased with a log s.d. of 1.67. Perfusion of regions with a low _A/ was significantly increased. With surgery, no significant changes were seen in cardiac output or vascular pressures. True shunt appeared or increased in seven patients, ranging from 1.3 to 17%. Perfusion to regions with low _A/ did not change. It is concluded that abdominal surgery interferes with gas exchange, presumably because of a cranial shift of the diaphragm which lowers FRC and thereby promotes airway closure.