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

The study explores the adaptation of the multiple inert gas elimination technique to investigate the ventilation-perfusion (VA/Q) relations in conscious, standing horses. It reveals that generally, standstill horses exhibit a good match of ventilation-perfusion but few show anomalies due to insufficient lung perfusion.

Methodology

• The researchers used the multiple inert gas elimination technique, a system known for assessing the balance between the air a person breathes (ventilation) and the blood flow that takes oxygen to the lungs (perfusion).
• The methodology was adjusted to accommodate the subject – conscious, standing horses. This involved an increased rate of inert gas infusion (30 ml/min) for an extended period (60 minutes).
• A nose mask mixing box system was built specifically for collecting expired gas from the horses.
• The subjects were eight adult horses of an average weight of 454 kg and approximately 6.1 years old. These horses were evaluated under resting conditions.

Findings

• Ventilation was discovered to be 65.2 liters/minute, and cardiac output, measured via thermodilution, was 40.2 liters/minute.
• Systemic and pulmonary artery mean pressures were recorded at 114 and 26 mmHg, respectively.
• Three horses presented a unimodal VA/Q distribution, centered around a mean VA/Q of one.
• In contrast, five horses revealed a bimodal distribution with an additional high VA/Q mode, covering 2 to 18 percent of ventilation.
• The mean logarithmic standard deviation of perfusion was 0.41, indicating modest variability.

Interpretation and Conclusion

• No perfusion of ‘low’ VA/Q regions was observed, but minor shunting averaging 1% was recorded. Inert gas dead space averaged 38% of the total ventilation.
• The arterial oxygen tension ranged between 11 to 14 kPa, and the mean arterial CO2 tension was 5.7 kPa.
• Overall, the researchers concluded that a standing horse typically has a well-balanced match between ventilation and perfusion. However, some individuals exhibited high VA/Q regions, which could be due to inadequate perfusion of upper lung regions.