Influence of Pulmonary Arterial and Left Atrial Pressures on Pulmonary Vascular Resistance

Abstract

A method has been developed by which flow to each lung, as well as pulmonary artery and left atrial pressures can be measured and varied at will. Vascular resistances were determined over a wide range of these variables. Both pressures have a marked effect on vascular resistances; the higher either pressure, the lower the resistances. This effect is most marked at low levels of pressure and flow. The absolute levels of the pressures, by affecting vascular distension, are major determinants of pulmonary vascular resistance.T HE existence of vasomotor activity in the pulmonary vascular bed is well established, though the degree and significance of such activity have been much debated. Recent interest has focused on the influence of hypoxia and drugs on pulmonary vascular reactions, and on changes of pulmonary vascular resistance associated with mitral stenosis and certain types of congenital heart disease. Confusion exists in this subject because of insufficient knowledge of the mechanical factors which influence resistance. Changes of vasomotor tone can be inferred only if the background of mechanical factors is sufficiently known. Since the pulmonary vascular bed is a highly distensible system, one would expect that changes of luminal pressure might have a large effect on resistance through changes of caliber of the system. Some information on this effect has been provided by the work of

Major differences in the pulmonary circulation between birds and mammals

“…This conclusion is consistent with previous measurements in dogs by Gilbert et al (1958) and West and Dollery (1965). Borst et al (1956) reported a critical closing pressure of 7 mmHg in dog lungs but this was probably an artifact caused by the positive alveolar pressure that was being used to ventilate the lung (West and Dollery 1965). Fig.…”

“…In fact on the average there was a slight increase in PVR as pulmonary arterial pressure was raised, while there was a slight decrease as pulmonary venous pressure was raised, but in both cases the changes were very small and not statistically significant. This is very different behavior from that seen in the mammalian lung where both increases in pulmonary arterial pressure or pulmonary venous pressure result in large falls in PVR (Haddy and Campbell 1953;Borst et al 1956;Lloyd and Wright 1960;Roos et al 1961). The mechanism of the fall in PVR is believed to be mainly the recruitment and distension of pulmonary capillaries that occurs when the pressure inside them is increased, and this behavior has been demonstrated histologically (Glazier et al 1969).…”

“…For example, in the study by Borst et al (1956) an increase in left atrial pressure from 8.2 to 11.7 mmHg when pulmonary arterial pressure was kept constant at 25.1 mmHg reduced the pulmonary vascular resistance by about 22%. Similar results have been found by other investigators (Haddy and Campbell 1953;Lloyd and Wright 1960;Roos et al 1961).…”

“…For example, Borst et al (1956) measured the pulmonary vascular resistance of one separately perfused lung in anesthetized dogs when the pulmonary arterial pressure was increased from 12 to 17 mmHg and the left atrial pressure was held constant at 5.9-6.2 mmHg. The result was a fall in pulmonary vascular resistance from 0.025 to 0.015 mmHg.min.ml −1 , that is a decrease of about 40%.…”

Pulmonary Hypertension Caused by Pulmonary Venous Hypertension

“…The fact that many patients have normal PVR may be partly due to passive distention of pulmonary resistance vessels due to pulmonary venous hypertension. 41,42 Considerable note has been made of PH "out of proportion" to the magnitude of LA pressure (i.e., a transpulmonary gradient >12 mmHg). 40,43 Some observers even suggest that this disproportionate increase in PAP may be due to factors independent of the PVH.…”