Andersons pediatric cardiology 1119

Hemodynamics and Physiology The consequence of obstruction at the right ventricular outflow necessitates an increase in right ventricular pressure to force blood through the stenosed valve The right ventricular pressure that develops is usually proportional to the degree of obstruction present If stenosis develops early in fetal or neonatal life, right ventricular mass increases by ventricular myocytic hyperplasia, and there is also hyperplasia of the supporting apparatus, such as the capillaries supplying blood to the myocytes, so that the density of capillaries remains normal In this way in neonates, the capacity for the right ventricle to generate high pressures and tolerate moderate stenosis is high Where pulmonary stenosis develops later in life, after the neonatal period, the capacity for hyperplasia is lost, and any increases in ventricular bulk are due to a hypertrophic response There is a compensatory increase in capillary supply, but this does not compensate for the increase in ventricular myocardial mass, and there is a reduction in capillary density Thus the capacity of the ventricle to sustain high pressures and tolerate stenosis is less than in the newborn In the fetus, severe forms of pulmonary stenosis may result in a circulation that resembles pulmonary atresia and right ventricular development may be impaired, resulting in the development of a hypertrophic right ventricle with a hypoplastic cavity When right ventricular dilation is severe, interventricular interaction may occur, such that the left ventricle is constrained within the pericardium, impairing its ability to fill and contributing to the compromised circulation Central cyanosis can occur in neonates when there is a duct-dependent pulmonary circulation or when right ventricular diastolic dysfunction allows right atrial to exceed left atrial pressure, producing a cyanotic shunt across a defect in the atrial septum Natural History Mild pulmonary valve stenosis with a gradient of less than 40 mm Hg that is seen after the first 6 months of life is, in general, a benign condition The severity of the disease may even improve as the child grows,40–44 certainly with a very low incidence of deterioration to the point of requiring an intervention In children with moderate or even severe pulmonary stenosis, right ventricular function seems to be maintained.45 When first seen in infancy, however, even mild pulmonary stenosis can progress and deteriorate.46 Using Doppler echocardiography monitoring, 25% of infants with mild pulmonary stenosis in the neonatal period were shown to develop further significant stenosis,42 and up to half of these patients require intervention.40 Not all of these cases can be explained solely by a postnatal drop in pulmonary vascular resistance Thus it is important to monitor infants carefully, especially when the diagnosis is made in the neonatal period, regardless of their severity of pulmonary stenosis The appearance of the valve in terms of the thickness and mobility of the leaflets is only weakly predictive of future deterioration.42 In patients with deteriorating pulmonary stenosis and intact ventricular septum, right ventricular pressure may, over time, exceed left ventricular pressure The ventricular pressure waveform changes from a broad-based triangular shape with early peak maximal pressure (Fig 42.25) to a tall peaked waveform with the point of maximal pressure delayed to close to the end of systole (Fig 42.26) Compensation for fixed severe stenosis with right ventricular hypertrophy can fail as the patient grows and further demands are made on the ventricle At this stage, the right ventricle may decompensate by dilating, and there is heart failure Further compensatory mechanisms for low cardiac output include increased extraction of oxygen During exercise, even this compensatory mechanism is insufficient, and exercise intolerance is noted, often with the presence of peripheral cyanosis FIG 42.25 Normal low-pressure right ventricular waveform showing a broad-based triangular waveform with an early point of maximal pressure