Resistance across a vascular bed cannot be measured directly, but can be calculated based on Ohm's law, on the equation: resistance equals beginning pressure minus ending pressure divided by flow. In this equation, one must use mean pressures. Flow is expressed in liters per minute. The resultant is expressed as resistance units.
Pulmonary vascular resistance
Pulmonary artery mean pressure, beginning pressure is 50 mmHg and pulmonary vein mean pressure, end pressure, is 5 mmHg. Pulmonary blood flow is 4.5 liters/minute, reflecting the shunt of 1.5 liter/minute across the ventricular septal defect. Pulmonary vascular resistance, therefore, is fifty minus five divided by 4.5, or ten units. This is significantly elevated over normal.
Systemic vascular resistance
In this patient, aortic mean pressure, beginning pressure, is 80 mmHg and right atrial mean pressure, ending pressure is 5 mmHg. Systemic blood flow measured at 3 L/minute. Systemic vascular resistance, therefore, is eighty minus five, divided by 3, or 25 resistance units.
Pulmonary/systemic vascular resistance ratio
The ratio of pulmonary to systemic vascular resistance, therefore, is ten divided by 25 or 0.4. Normal pulmonary to systemic resistance ratio is less than 0.1. A ratio that exceeds 0.3 in a patient with a left-to-right shunt suggests disease of the pulmonary arterioles, a process that can be progressive, even if the left-to-right shunt is surgically closed. High pulmonary arteriolar resistance approaching a ratio of 1.0 may contraindicate surgical closure of the defect.