Identification of Constriction in Large versus Small Vessels using the Arterial-Venous and the Double-Occlusion Techniques in Isolated Canine Lungs

Abstract

By applying the arterial and venous occlusion technique and the double-occlusion technique in the isolated perfused dog left lower lobe, the total pressure gradient across the pulmonary vasculature can be partitioned into four segments: upstream arterial (ΔPa), precapillary (ΔPa´), postcapillary (ΔPv´) and downstream venous (ΔPv). Changes in ΔPa and in ΔPv would result from constriction of the large arteries and veins, respectively, while changes in ΔPa´ and ΔPv´ would result from constriction of the smallest muscular arteries and veins. The results revealed that at the dosages used, serotonin and angiotensin II constricted the large arteries only. Histamine constricted the large and small veins, whereas norepinephrine constricted the large arteries and large veins. Hypoxia constricted primarily the precapillary vessels, and slightly the post-capillary vessels. Elevation of alveolar pressure relative to vascular pressure increased primarily the pressure gradient across the postcapillary vessels (ΔPv´), and to some extent, increased the pressure gradient across the precapillary vessels (ΔPa´)·Thus the two techniques when applied together in the isolated lung can differentiate between constriction in large versus small vessels. Caution is recommended when extrapolating these results to other animal species, because the site of vasoconstriction may be different for a given substance

Capillary pressure estimates from arterial and venous occlusion in intact dog lung

“…It differs from the venous occlusion pressure-time profile with a rapidly ascending phase followed by a slow quasilinear rise in pressure [15], which is observed in isolated lung using constant flow perfusion mode, because all the pulmonary veins are occluded. The arterial postocclusion pressure profiles observed in the intact animal are similar to those observed in isolated lung, whether perfused at constant flow [6][7][8] or at constant pressure [9,14].…”

“…However, the data from isolated lung experiments show that Pc,ao overestimates Pc as estimated either from venous occlusion (Pc,vo) or from double occlusion (Pc,do). In isolated lung preparations [7,[9][10][11], Pc,ao is usually above Pc,do, which is itself above Pc,vo. When the zerotime extrapolation of the slow segment technique is used, the Pc,ao -Pc,vo fractional difference in isolated dog lung ranges 20-30% [6,9].…”

“…These different estimates give different values of Pc in isolated lung preparations [7,[9][10][11]. Pc,ao is above Pc,do, which is itself above Pc,vo.…”

“…The first technique equates Pc to the zerotime extrapolation of the exponential fit of the slowly decaying segment [2,6]. The second technique equates Pc to the inflection point between the rapidly and slowly decaying segments [2,3,7,8].…”

“…Pc,ao will not be a good estimate of Pc in man if the middle fraction of the Pa-Pv difference is as large as observed in isolated dog lung preparations [7,[9][10][11]. In the intact dog lung, Pc,vo was previously compared to Pc,ao in only one study, the study of HOLLOWAY et al [3].…”