Identification of constriction in large versus small vessels using the arterial-venous and the double-occlusion techniques in isolated canine lungs.

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 (delta Pa), precapillary (delta Pa'), postcapillary (delta Pv') and downstream venous (delta Pv). Changes in delta Pa and in delta Pv would result from constriction of the large arteries and veins, respectively, while changes in delta Pa' and delta Pv' 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 postcapillary vessels. Elevation of alveolar pressure relative to vascular pressure increased primarily the pressure gradient across the postcapillary vessels (delta Pv'), and to some extent, increased the pressure gradient across the precapillary vessels (delta Pa'). 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.