Enhanced venous compliance sensibility at physiological ranges of pressure: experimental validation of a biphasic model

Recent non-invasive studies in humans have shown the relationship between venous compliance and pressure within the physiological range to be best described by a biphasic model. This model sets a breakpoint pressure that splits the pressure range in two different linear compliance phases. Six ovine internal jugular vein segments were studied in-vitro under pulsatile pressure and flow conditions achieved by a mock circulation loop. Instantaneous pressure (Konigsberg) and diameter (sonomicrometry) signals were assessed in a wide range (0 to 60 mmHg) of intravascular pressure. For each segment the low pressure range was set between 0 mmHg and the breakpoint pressure and the high pressure range from the breakpoint pressure to 60 mmHg. The mean venous negative slope δC/δP for low pressures was significantly steeper than for high pressures (p<0.01). The calculated r for the linear trend lines resulted r =0.96±0.03 (p<0.01) for low pressures and r =0.79±0.07 (p<0.05) for high pressures, validating the biphasic model in-vitro. The δC/δP would describe a compliance change following a pressure variation and might be adopted as a compliance sensibility index.