[Vascular conductance and critical closing pressure in the isolated canine lung in situ. Its natural history].

The vascular pressure-flow relationship (P-QL) in West's zone II condition were studied in isolated, in situ, canine, left lower lobe (ISLL) in order to characterize the total resistance in the pulmonary vascular bed (Rp), the normal values for pulmonary vascular conductance (Cv) and for critical closing pressure (PLc). After the basal parameters were obtained, measurements of Cv and PLc were done every 30' in order to know the natural history (NH) of this canine ISLL preparation. The P-QL relationship of the pulmonary vasculature of the ISLL preparation, perfused under classical zone II conditions, can be characterized by a rectilinear segment at high flow, a curvilinear segment at low flow and a pulmonary arterial pressure that exceeds alveolar (PA) pressure at zero flow. This demonstrates the existence of critical closing pressure (PLc) in the pulmonary vascular bed. The mean control Cv and PLc were 38.5 +/- 14 (ml. min)/mmHg and 7.9 +/- 2.2 mmHg respectively; those parameters did not change through the observation of the experiment. PLc was found to be independent of bronchial flow and it was not related to PA when the values for this pressure were less than 5 cm H20. On the contrary, higher levels of PA pressure were significantly related to PLc (r = 0.94, p less than 0.05). We conclude that in this model of ISLL in West's zone II condition it is possible to study the two components of Rp, one given by vessels that determine changes in flow resistance and for the other vessels disclosing critical closure. The values of these components remained stable over 180' of observation.