Segmental and total microvascular resistances during hemorrhagic hypotension in rabbit omentum

Single input-output microvascular modules in the rabbit omentum were studied to quantitate total modular resistance (R(T)) and the changes in resistance of successive serial segments (R(seg)) during hemorrhagic hypotension (55 mmHg for 1 h). R(T) was calculated from the pressure drop between input and output vessels and the total flow through the module. Changes in R(seg) were estimated from alterations in single microvessel hindrance (1/diam⁴) for selected microvessels within a module together with a correction for flow redistribution within each segment derived from changes in the proportion of total flow. Mean R(T) increased to 2.1 times control within the first 10 min of systemic hypotension and gradually declined over 1 h. Response of R(seg) varied in different generations of microvessels (arterioles and venules subdivided by size). R(seg) for arterioles and venules <30 μm, but not that for arterioles and venules >30 μm, showed comparable or greater changes than R(T). Calculated differences between R(T) and the summed R(seg) of measured segments suggest that the unmeasured R(seg) in capillaries and blood rheological parameters may have significantly influenced changes in R(T) during hemorrhagic hypotension.