Ventilation-perfusion relationships in the standing horse: an inert gas elimination study

The multiple inert gas elimination technique was adapted for use in the conscious standing horse. The modifications included increased infusion rate of the inert gases (30 ml/min), extended infusion time (60 mins) in order to reach steady state, and construction of a nose mask mixing box system for collection of expired gas. Eight adult horses with a mean weight of 454 kg and a mean age of 6.1 years were studied while standing under resting conditions. Ventilation was 65.2 liters/min and cardiac output measured by thermodilution 40.2 liters/min. Systemic and pulmonary artery mean pressures were 114 and 26 mmHg, respectively. Three horses showed a unimodal VA/Q distribution centred upon a mean VA/Q of one. Five horses had a bimodal distribution with an additional high VA/Q mode comprising 2 to 18 per cent of ventilation. The mean logarithmic standard deviation of perfusion was 0.41. There was no perfusion of 'low' VA/Q regions, but minor shunting on an average of 1 per cent was noted. Inert gas dead space (minus apparatus dead space) averaged 38 per cent of total ventilation. Arterial oxygen tension varied from 11 to 14 kPa and the mean arterial CO2 tension was 5.7 kPa. It is concluded that the standing horse in general has a good match between ventilation and perfusion but that some individuals display high VA/Q regions, possibly explained by poor perfusion of upper lung regions.