DISTRIBUTION OF INSPIRED GAS TO EACH LUNG IN THE ANESTHETIZED HORSE AND INFLUENCE OF BODY SHAPE

The distribution of inspired gas to each lung, time constants of the l ungs and parameters of gas exchange were studied in 2 groups of horses (mean bwt 606 kg), anaesthetised using thiopentone and chloral hydrat e and breathing room air. One group (n=4) had a downward curved abdomi nal contour (round-bellied) and the other group (n=4) had an upward cu rved abdominal contour (flat-bellied). An equal distribution of inspir ed gas between the lungs existed in both groups in dorsal recumbency. Flat-bellied horses maintained this equal distribution in lateral recu mbency whereas in round-bellied horses an uneven distribution of tidal volume (V-T) developed. The percentage of (V-T) distributed to the de pendent lung was 23% and 38% for left and right lateral recumbency res pectively. The distribution of VT agreed with the ratio of time consta nts of the lungs in flat-bellied horses but differed markedly from thi s ratio in round-bellied horses suggesting that, in the latter, factor s other than compliance and resistance play a role in distribution of ventilation. Round bellied horses had a lower PaO2 and a larger (A-a)P aO2 than flat-bellied horses in all body positions. The results are co mpatible with the known hypothesis that pressure exerted by abdominal contents on the dependent lung and diaphragm is an important factor in ventilation/perfusion mismatch of the anaesthetised horse.