Ventilation‐perfusion relationships in the anaesthetised horse

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Abstract
Ventilation-perfusion relationships were studied by the multiple inert gas elimination technique in seven horses while they were conscious and udirng inhalation anaesthesia with halothane. A geneerally good match between ventilation and perfusion was found in the conscious, standing horse. During anaesthesia a huge shunt developed, ie perfusion of completely unventilated lung regions, both in dorsal and left lateral recumbency and whether the horse was breathing spontaneously or mechanically ventilated. The shunt was significantly greater and the arterial oxygen tension (PaO2) significantly lower in dorsal than in left lateral recumbency. Little or not perfusion of low .ovrhdot.VA/.ovrhdot.Q regions was observed during anaesthesia, whether ventilation was spontaneous or mechanical. Positive end-expiratory pressure (PEEP) did not significantly improve PaO2 or reduce the shunt. Selective mechanical ventilation of dependent lung regions with PEEP reduced the shunt markedly, an effect that was not achieved by conventional mechanical ventilation with general PEEP. The findings seem compatible with alveolar collapse during anaesthesia, causing shunt, whereas the absence of clearly low .ovrhdot.VA/.ovrhdot.Q regions questions the role of airway colsure as the major disturbance of gas exchange.