Vascular Resistance - Calculation of Resistance

Calculation of Resistance

The basic tenet of calculating resistance is that flow is equal to driving pressure divided by resistance.

The systemic vascular resistance can therefore be calculated in units of dyn·s·cm−5 as

where mean arterial pressure is 2/3 of diastolic blood pressure plus 1/3 of systolic blood pressure.

The pulmonary vascular resistance can therefore be calculated in units of dyn·s·cm−5 as

where the pressures are measured in units of millimetres of mercury (mmHg) and the cardiac output is measured in units of litres per minute (L/min). The pulmonary artery wedged pressure (also called pulmonary artery occlusion pressure or PAOP) is a measurement in which one of the pulmonary arteries is occluded, and the pressure downstream from the occlusion is measured in order to approximately sample the left atrial pressure. Therefore the numerator of the above equation is the pressure difference between the input to the pulmonary blood circuit (where the heart's right ventricle connects to the pulmonary trunk) and the output of the circuit (which is the input to the left atrium of the heart). The above equation contains a numerical constant to compensate for the units used, but is conceptually equivalent to the following:

where R is the pulmonary vascular resistance (fluid resistance), ΔP is the pressure difference across the pulmonary circuit, and Q is the rate of blood flow through it.

As an example: If Systolic pressure: 120 mmHg, Diastolic pressure: 80 mmHg, Right atrial mean pressure: 3 mmHg, Cardiac output: 5 l/min, Then Mean Arterial Pressure would be : (2 Diastolic pressure + Systolic pressure)/3 = 93.3 mmHg, and Systemic vascular resistance: (93 - 3) / 5 =18 Wood Units. or Systemic vascular resistance: 18 x 80 = 1440 dyn·s/cm5 These values are in the normal limits.