Ventilation-perfusion relationships in the anaesthetised horse.

This research studies the relationship between ventilation and blood flow in anesthetized horses, finding that while the match is generally good in conscious horses, a shunt (or bypass of oxygen transfer) develops during anesthesia which is not significantly improved by positive end-expiratory pressure. Selective mechanical ventilation, however, does result in a marked reduction.

Objective of Research

• The primary objective of this study is to assess the match between ventilation (air flow) and perfusion (blood flow) in horses. The researchers are particularly interested in how anesthesia with halothane affects these relationships, both when the horses are breathing naturally and when they are ventilated mechanically. Additionally, the study aims to gauge the effectiveness of applying positive end-expiratory pressure (PEEP) for improving these conditions.

Methodology and Experimentation

• The researchers used the multiple inert gas elimination technique to investigate ventilation-perfusion relationships in seven horses. Experiments were conducted when the horses stood conscious and during inhalation anesthesia.
• Different positional situations of the horse during anesthesia were tested, including dorsal (back) and left lateral (side) recumbency. The response of the horses breathing spontaneously as well as with mechanical ventilation was recorded.

Key Findings

• In a horse’s normal state, there was generally a good match observed between its ventilation and perfusion.
• However, during anesthesia, a major shunt developed. In simpler terms, a portion of the lung experienced blood flowing through it without any corresponding ventilation.
• This shunt was noticed to be significantly larger and the arterial oxygen tension (PaO2) substantially lower when the horse was in a dorsally recumbent state than when it was recumbent on its left lateral side.
• Applying PEEP didn’t bring any considerable improvements to PaO2 or reduce the shunt.
• Selective mechanical ventilation, which involved focusing on dependent lung regions combined with PEEP, resulted in a striking reduction in the shunt. This impact wasn’t observed with conventional mechanical ventilation utilizing general PEEP.

Conclusions

• The observations made led researchers to believe that the shunting could be primarily due to collapse of alveoli during anesthesia.
• Interestingly, an absence of regions with clear low ventilation-perfusion (VA/Q) values called into question whether closure of the airway is the main cause of disturbance in the gas exchange.