Distribution of inspired gas to each lung in the anaesthetised horse and influence of body shape.

This study examined the effects of body shape on gas distribution in the lungs of anaesthetised horses. It found that gas distribution varies between ’round-bellied’ and ‘flat-bellied’ horses, which might be crucial in understanding the ventilation/perfusion mismatch in anaesthetised horses.

Objective and Methodology

• The study aimed to examine how the shape of a horse’s body influences the distribution of inhaled gas to each lung and the parameters of gas exchange. This was done by inducing anaesthesia in two groups of horses and observed their breathing patterns under different body postures.
• The researchers selected horses with an average body weight (bwt) of 606 kg. The horses were divided into two groups determined by their body shape: those with a downward curved abdominal contour (round-bellied) and those with an upward curved abdominal contour (flat-bellied).
• The anaesthesia was achieved by using thiopentone and chloral hydrate, and the horses were allowed to breathe room air during the observation period.

Key Findings

• The study found that both body shapes displayed an equal distribution of inhaled gas between the lungs when examined in a dorsal recumbency (lying on their back). However, when placed in a lateral recumbency (lying on their side), the flat-bellied horses maintained the equal distribution while round-bellied horses displayed an uneven distribution of the tidal volume of the breathing cycle.
• This difference was traced partially to the time constants of the lungs in flat-bellied horses but varied significantly in round-bellied horses. It suggests that other factors than compliance (the lungs’ ability to expand) and resistance (airway resistance to airflow) are at play in the round-bellied horses.
• The study also found that round-bellied horses had a lower partial pressure of oxygen in the arterial blood (PaO2) and hence, a larger (A-a)PaO2, the difference between the alveolar and arterial oxygen partial pressure, indicating less efficient gas exchange compared to flat-bellied horses.

Conclusion and Implications

• The results align with the hypothesis that the pressure exerted by the abdominal contents on the dependent lung and diaphragm is a significant factor in the ventilation/perfusion mismatch of the anaesthetised horse. This mismatch describes a condition where the amount of air reaching the lungs does not match the amount of blood flow in the capillaries, resulting in inefficient oxygen and carbon dioxide exchange.
• This study emphasizes the significance of considering the body shape of horses during anaesthesia, as it directly impacts the distribution of inspired gas, and subsequently, the overall gas exchange efficiency.