Changes in the lungs of the new‐born lamb

Abstract

In a previous series of experiments it was shown that artificial positive pressure ventilation of the lungs of foetal lambs caused an immediate fall of pulmonary and systemic arterial blood pressures (Ardran, Dawes, Prichard, Reynolds & Wyatt, 1952). In three lambs the systemic arterial pressure recovered, while the pulmonary arterial pressure continued to fall towards the level commonly found in adult animals. The observations of Reynolds, Ardran & Prichard (1953) on pulmonary circulation time in these newly delivered lambs also showed that on ventilation of the lungs pulmonary blood flow increased, on an average fourfold. These experiments led to the conclusion that ventilation of the lungs for the first time after birth leads to a decrease of pulmonary vascular resistance. This is an idea which has often been discussed during the last three hundred years but has not been tested by direct experiment. In order to obtain a direct estimate of pulmonary vascular resistance it is necessary to measure the quantity of blood flowing through a pulmonary artery per minute and also the pulmonary arterial and left atrial pressures. This paper gives an account of such measurements, from which it is concluded that positive pressure ventilation of the foetal lungs after birth causes an immediate and dramatic decrease of pulmonary vascular resistance, which must be one of the principal factors in the neonatal changes in the circulation. A brief report of some of these observations has already been given (Dawes, Mott, Widdicombe & Wyatt, 1952). METHODSTwenty Welsh ewes were used, whose duration of pregnancy was known, and three HampshireClun Forest ewes (the maternal grandam of the lamb being Clun Forest) whose date of tupping was not known, but which were in a very advanced state of pregnancy. The weight of the Welsh ewes ranged from 22-33 kg and that of the Hampshire-Clun Forest ewes from 70-85 kg. They were anaesthetized by slow injection of 'dial-urethane' (diallyl-barbituric acid 0-1 g, urethane 0-4 g/ml.) into the external jugular vein. At first a dose of 0-5 ml./kg was used, but further experience showed that 0-2-0-35 ml./kg was sufficient to keep the ewe and lamb lightly anaesthetized.

The effects of hypoxia in the new‐born lamb before and after denervation of the carotid chemoreceptors*

“…The results, shown in Table 1, make it clear that, by 12 hr, pulmonary artery pressure was less than the high values recorded by Dawes, Mott, Widdicombe & Wyatt (1953) in the mature sheep foetus but, by the eighth day, they were still higher than the low levels reported by Reeves, Grover & Grover (1963) in the lamb a few weeks old. These intermediate values are similar in range to those given by Adams & Lind (1957), Eldridge & Hultgren (1955) and by , in the new-born infant of comparable age.…”

“…However, their calculations, using the Fick quotient, would, in the presence of a left-to-right shunt, overestimate the pulmonary A-V difference and underestimate the true pulmonary flow. This could be more than a trifling error since Dawes et al (1953) had previously shown that normally the left-to-right shunt could contribute up to 50 % of pulmonary flow. If, in their series, the sequence was that found in this study, i.e.…”

Ventilation and oxygenation induce endothelial nitric oxide synthase gene expression in the lungs of fetal lambs.

“…After birth, with initiation of ventilation by the lungs, and the subsequent increase in pulmonary and systemic arterial blood oxygen tensions, pulmonary vascular resistance decreases and pulmonary blood flow increases by 8-10-fold to match systemic blood flow (1)(2)(3)(4). This process is regulated by a complex and incompletely understood interplay between mechanical and metabolic factors (5).…”

“…This process is regulated by a complex and incompletely understood interplay between mechanical and metabolic factors (5). For example, the replacement of fluid with gas in the alveoli changes alveolar surface tension, which unkinks the small pulmonary arteries, and causes an immediate decrease in pulmonary vascular resistance and increase in pulmonary blood flow (2,5). There is also release of vasoactive substances, such as prostacyclin (PGI 2 ), which decrease pulmonary vascular resistance and increase pulmonary blood flow (6,7).…”

“…There is also release of vasoactive substances, such as prostacyclin (PGI 2 ), which decrease pulmonary vascular resistance and increase pulmonary blood flow (6,7). Both rhythmic distension of the fetal lamb lung and ventilation without oxygenation produce partial pulmonary vasodilatation (2).…”