Changes in blood flow distribution in equine lungs induced by anaesthesia

The distribution of blood flow to the lungs was measured with labelled microspheres injected into horses before and during anaesthesia. Anaesthesia was induced with glycerol guaiacolate ether and ketamine, and maintained with the spontaneous breathing of halothane in oxygen. In a preliminary group of six horses, flow distribution was observed during anaesthesia in sternal, dorsal and right lateral recumbency. In two other groups, each of four horses, the flow distribution with time during either sternal or right lateral recumbency was observed. A small, constant proportion of the 15 micron diameter microspheres bypassed the lungs. No increase in shunting with anaesthesia was detected. Blood flow distributed predominantly cranioventrally in the conscious horse in concordance with the expected effects of hydrostatic forces within the lung. In the anaesthetized horse a stable distribution rapidly developed whereby the flow was directed preferentially caudodorsally and evenly divided between the left and right lungs regardless of whether the horse was on its back, side or sternum. We conclude that an unidentified factor overrides gravitational effects on perfusion in the anaesthetized horse. If under anaesthesia, ventilation were distributed according to gravity, hypoxia would arise from this disassociation of ventilation from perfusion. The hypoxia would be most intense in dorsal recumbency, and least intense in sternal recumbency. This was reflected in the degree of hypoxaemia observed. A small but significant change in flow distribution with the phase of the respiratory cycle was detected in the conscious, standing horse.