PITFALLS IN THE ASSESSMENT OF VASCULAR-RESISTANCE

Vascular resistance (R) to flow is calculated as the ratio of perfusio n pressure between two points (P-1-P-2) to flow rate (ii). In high-pre ssure circuits (e.g., systemic), it is justifiable to use arterial pre ssure (P-1) to represent perfusion pressure. It is not permissible to calculate resistance without measuring Q unless a comparison is being made in the resistance of different segments of a vascular bed; Q bein g constant, R: (P-1-P-2). In a low-pressure circuit (lower vertebrates , pulmonary) it is imperative to measure P-2 to quantify the perfusion pressure. In dosed circuits with a vertical orientation the effect of gravity on both the arterial and the venous pressures must be conside red. In such circuits the perfusion pressure is due to viscous resista nce only, excluding the gravitational pressure between the two points.