The pathophysiology of these conditions is determined by one of four general hemodynamic principles according to type of lesion: (1) communication at ventricular or great vessel level, (2) communication at atrial level, (3) obstructions, and (4) valvar regurgitation.

In addition, pulmonary hypertension leads to characteristic clinical and laboratory findings.

The first principle concerns conditions with a communication between the great vessels (e.g. PDA ) or between the ventricles (e.g. VSD).

When the size of the defect or communication approaches or exceeds the diameter of the aortic root (nonpressure-restrictive defects), the systolic pressures in the ventricles and great vessels are equal. Pressures on the right side of the heart are elevated to systemic levels.

Normally, systemic vascular resistance rises slowly with age, whereas the pulmonary vascular resistance shows a sharp fall in the neonates and a more gradual decline in infancy. This fall in pulmonary vascular resistance is partially related to regression of the thick-walled pulmonary arterioles of the fetal period to the adult pattern of pulmonary arterioles, which have a wide lumen.

Pulmonary vascular resistance falls in all infants following birth, but in infants with a large communication the fall in pulmonary vascular resistance may not be as great but still profoundly affects the patient.

In a patient with a large communication, the systolic pressure of the pulmonary artery (P) remains constant as it is determined largely by the systemic arterial pressure. Therefore, according to the equation P = R_P × Q_P, as the pulmonary vascular resistance (R_P) falls in infancy, pulmonary blood flow (Q_P) increases. If some factor, such as the development of pulmonary vascular disease, increases pulmonary vascular resistance, the pulmonary blood flow decreases, but the pulmonary arterial pressure remains constant.

In defects or communications smaller than the diameter of the aortic root (pressure-restrictive defects), the relative systemic and pulmonary vascular resistances determine the direction of blood flow through the communication, as in large defects; but the size of the defects does not allow pressure equilibration. Therefore, a systolic pressure difference exists across the communication.

The impedance to blood flow through a small defect is a major determining factor governing the magnitude of the blood flow through it. Therefore, if pulmonary and systemic resistances are normal and the aortic and left ventricular systolic pressures are higher than the pulmonary arterial and right ventricular systolic pressures, respectively, then the shunt in these small-sized communications is from the aorta to the pulmonary artery, or from the left ventricle to the right ventricle.

In these conditions, the sizes of the left atrium and left ventricle are enlarged proportionally to the volume of pulmonary blood flow and the right ventricle is hypertrophied to the level of pulmonary artery pressure. Echocardiography is very helpful in identifying the diagnosis and showing the size of the communication. The hemodynamics are accessible by measuring the left ventricular dimensions, which increase as the volume of pulmonary blood flow increases. The left atrial size also increases but cannot be measured as clearly. Right ventricular pressure can be assessed from the velocity of the jet through the tricuspid valve according to the simplified Bernoulli equation PG = V² × 4, where PG is the pressure gradient and V is the velocity of the jet through the tricuspid valve.

Communication at the atrial level

The second hemodynamic principle governs shunts that occur at the atrial level. Most atrial communications leading to signs and symptoms are large, hence atrial pressures are equal. Therefore, pressure differences cannot be the primary determinant of blood flow through the atrial communication.

Compliance describes the volume change per unit pressure change. At any given pressure, the more compliant the ventricle, the greater is the volume that it can receive.

Ventricular compliance depends on the thickness of the ventricular wall and on factors, such as fibrosis, that alter the stiffness of the ventricle. Usually a thinner ventricular wall means that the ventricle is more compliant.