Measuring transient systemic conductance and capacitance responses using a modified Brooksby-Donald technique in...

The Brooksby-Donald approach uses two flow probes to measure the inflow and outflow from an organ or vascular bed; the difference in flow can be time-integrated to assess changes in venous capacitance.

To measure changes in subdiaphragmatic venous capacitance and arterial conductance in acutely instrumented rabbits, and to document the acute vascular responses to intravenous injections of pharmacological agents.

In artificially ventilated, fentanyl-anesthetized New Zealand female rabbits, ultrasonic flow probes were attached to the descending thoracic aorta and inferior vena cava to measure subdiaphragmatic inflow and outflow. Systemic arterial and venous pressures were measured. Conductance was calculated as aortic flow divided by the difference between systemic arterial and venous pressures. Changes in capacitance were assessed by integrating flow differences. Nitroglycerin (NG), isoproterenol hydrochloride, phenylephrine hydrochloride (PE) and vasopressin (VP) were administered intravenously in stepwise injections, and transient conductance and capacitance responses were determined.

NG significantly increased conductance and capacitance, while isoproterenol hydrochloride had significant effects only on conductance. Both VP and PE significantly decreased conductance and capacitance.

The modified Brooksby-Donald approach used in the present study was validated by the observed concordant increases in conductance and capacitance caused by NG, and decreases caused by VP and PE. This approach may be useful to characterize specific comparative conductance-capacitance properties of various vasoactive agents.