Pressure Measurements in the Lesser Circulation of the Newborn Rabbit

Abstract

An attempt is described to investigate whether some of the physiological responses of the pulmonary vascular bed in anæsthetized rabbits aged 0–31 days differ from those observed in other species, and it is shown that accurate measurements of phasic right ventricular pressure can be made in small animals with high heart rates using standard equipment. A constant relationship of resting peak right ventricular to mean pulmonary arterial pressure is demonstrated in the resting state, but there are indications that the quantitative relationship changes with the circumstances. The variation with age of systemic and peak right ventricular pressure is shown; the latter changes are different from those described in other species and together with results obtained during hypoxia and asphyxia provide indirect evidence of ductus arteriosus closure on day 3. Pulmonary vascular pressor responses comparable to those of other species are never seen in the first 3 weeks and the fall in systemic pressure in response to asphyxia in very young animals is not due to coincident intense pulmonary vasoconstriction but could be explained by hypoxic dilatation of a patent ductus. A systemic response to hypoxia opposite to that to asphyxia is observed in rabbits older than 4 days. There is no evidence that intravenous infusion of acetylcholine in well‐oxygenated or hypoxic newborn rabbits has any direct effect on the pulmonary vascular bed.

Hypoxia and pulmonary arterial pressure in the rabbit

“…The present data together with Dennis's (1968) report indicate that the new-born isolated perfused rabbit lung has a poorly developed pressor response to hypoxia, and the magnitude of this response increases with post-natal age. It is possible that in the rabbit a strong hypoxic pulmonary pressor mechanism is not present in foetal life.…”

“…In three rabbits 2-5 pg acetylcholine caused a mean increase in pulmonary arterial pressure of 2 mm Hg from an initial value of 16 + 1 mm Hg, while Dennis (1968) found that in asphyxiated and hypoxic new-born rabbits the right ventricular systolic pressure showed a small increase irrespective of the direction of the change in systemic arterial pressure. She found that the magnitude of this hypoxic right ventricular pressor response increased with post-natal age.…”

“…In the new-born rabbit, hypoxia causes an increase in peak right ventricular pressure (Dennis, 1968); its effect on pulmonary arterial pressure in the adult is uncertain. Dirken & Heemstra (1948a, b, c) used measurements of arterial oxygen saturation to suggest that there might have been a pulmonary vasoconstrictor response to hypoxia, which reached a peak in 8 hr.…”