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

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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