Pressure-Flow Relationship and Longitudinal Distribution of Pulmonary Vascular Resistance in Heartworm-Infected Dogs

Abstract

ABSTRACT. The pressure-flow relationships and the longitudinal distributions of pulmonary vascular resistance in normal and heartworminfected (HWI) dogs were compared in an isolated, blood perfused preparation. The pulmonary circulation was partitioned into pulmonary arterial, middle, and venous segment based on the concept of a five element lumped model. The pulmonary arterial pressure-flow relationships were found to be non-linear and convex to the pressure axis in both normal and HWI lungs. The pressure-flow relationships of the pulmonary arterial and venous segment were linear and these slopes in the HWI lungs were significantly higher than the normal lungs. The pressure gradient of the middle segment was increased as flow increased at lower flow range, however, it was not increased during higher perfusion range in both lungs. At higher flow, the pressure gradient of the middle segment in the HWI lungs was significantly higher than the normal lungs. These results suggest that the ohmic resistance was almost equal to the sum of the two slopes of the pressure-flow relationships of the pulmonary arterial and venous segment because the pressure gradient of the middle segment was not altered as flow increased during higher perfusion rate. Because the slopes of the pressure-flow relationships of the pulmonary arterial and venous segment were increased with heartworm infection, the ohmic resistance of HWI lungs would be higher than normal lungs. The intercept pressure on the pressure axis of the linear portion of the pulmonary arterial pressure-flow relationship, a critical closing pressure, was regarded as pressure gradient of the middle segment during higher perfusing rate because the intercept pressures of pressure-flow relationships of pulmonary arterial and venous segment were almost equal to zero. Therefore, the critical closing pressure of HWI lungs would be higher than normal lungs. The pulmonary hypertension of filariasis appears to be due to an increase in ohmic resistance and elevated critical closing pressure. KEY WORDS: canine, filariasis, pressure-flow relationship, pulmonary vascular resistance, vascular occlusion technique.J. Vet. Med. Sci. 65(9): 965-970, 2003 In normal dogs, an increase in pulmonary blood flow is associated with a decrease in pulmonary vascular resistance without a marked elevation in pulmonary arterial pressure [14,15]. The reduction in pulmonary vascular resistance is generally considered to be due to capillary recruitment and/ or dilatation [6,19]. In dogs with heartworm disease, however, the reduction in pulmonary vascular resistance that accompanies an increase in cardiac output is smaller in magnitude than normal dogs [12,13]. The capillary bed in heartworm-infected (HWI) dogs may not be able to recruit and/or dilate efficiently with the increase in cardiac output [14,15]. The behaviors of the pulmonary vasculature in HWI dogs in response to increases in cardiac output have not been well studied.In this study, pressure-flow curves at different perfusing flows were obtaine...

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