Andersons pediatric cardiology 491

FIG 19.51 Tricuspid regurgitant jet used to measure right ventricular systolic pressure The peak velocity is 2.18 m/s This corresponds to a peak gradient of 19 mm Hg Adding a right atrial pressure of 5 mm Hg gives a right ventricular systolic pressure of 24 mm Hg, which is normal If a ventricular septal defect is present, it is often possible to interrogate the flow across the defect By measuring the peak systolic velocity, the peak instantaneous pressure difference between the left and right ventricles can then be calculated To estimate right ventricular systolic pressure, left ventricular systolic pressure is assumed to be equal to systemic systolic arterial blood pressure Subtracting the pressure gradient across the defect from the estimated left ventricular systolic pressure provides an estimate of the right ventricular systolic pressure This again is based on several assumptions, which can make the method unreliable First, the assumption that arterial pressure is equal to left ventricular systolic pressure is only true if there is no left ventricular outflow tract obstruction Second, if there is a significant delay in the buildup of pressure during systole in both ventricles, as in the presence of right bundle branch block, the peak instantaneous gradient measured during systole overestimates the real gradient, potentially missing significant pulmonary hypertension Third, it can be difficult to align the cursor with the jet of the defect, potentially underestimating the gradient It is also possible that obstruction may be present across the right ventricular outflow tract If the arterial duct is patent, the direction and the velocity of flow measured across the duct can be used to estimate the pulmonary arterial pressure When the shunt is right to left, or bidirectional, this indicates suprasystemic or systemic pressures in the pulmonary arteries respectively When the shunt is left to right, a Doppler measurement of the flow can be obtained It can be difficult to align the cursor with the direction of the flow and obtain a reliable Doppler tracing, but in most patients it is possible to obtain a good Doppler tracing When the duct becomes more restrictive, the flow will become continuous It can then become difficult to define the peak gradient In general, it is better to use the mean gradient Subtraction of this mean ductal gradient from the mean systemic arterial blood pressure gives the mean pulmonary arterial pressure The majority of patients have a variable degree of pulmonary regurgitation, from which it is possible to obtain a reliable pulsed wave or continuous wave Doppler signal This can usually be obtained from the parasternal short-axis or long-axis views or sometimes from the subcostal windows When a good tracing is obtained, two different measurements can be performed Of these, the peak diastolic gradient between the pulmonary artery and the right ventricle has been shown to correlate highly with the mean pulmonary arterial pressure This relationship is coincidental and very fortuitous for the clinical echocardiographer Because the pressure gradient reflects the pressure difference between the pulmonary artery and the right ventricle at that time, right ventricular diastolic pressure should be added to the measurement (Fig 19.52) An estimated right atrial pressure is generally used as an estimate In addition, pulmonary end-diastolic pressure can be estimated based on the pulmonary regurgitant jet by measuring the end-diastolic velocity, P The gradient between the pulmonary arteries and the right ventricle is then provided using the equation FIG 19.52 Pulsed wave Doppler from the pulmonary regurgitant jet obtained from a parasternal short-axis view The maximal velocity is measured to be 1.58 m/s, which corresponds to a gradient of 10 mm Hg Adding a mean right atrial pressure of 5 mm Hg predicts a mean pulmonary arterial pressure of approximately 15 mm Hg where PADP is the pulmonary arterial end-diastolic pressure, V is the enddiastolic velocity, and RVED is the right ventricular end-diastolic pressure Again, right atrial pressures are used as an estimate for right ventricular enddiastolic pressure, but this is not always a good estimate When pulmonary regurgitation becomes severe, there might be significant shortening of the regurgitant jet due to fast equalization between pulmonary and right ventricular diastolic pressures related to the large regurgitant volume Continuity Equation A second important hemodynamic principle, which has been used in a number of Doppler applications, is the continuity equation This is based on the principle of conservation of mass within the cardiovascular system, which predicts that the flow will be equal at various points within the circulation The flow across all four valves within the heart will be equal in the absence of intracardiac shunts or valvar regurgitation Flow across a fixed orifice is equal to the product of the cross-sectional area of the orifice and the velocity of flow To calculate velocity through an orifice, it is necessary to calculate the mean velocity across this area This can be done by measuring the time velocity integral, which is calculated electronically by tracing the Doppler velocity signal The stroke volume is the product of the cross-sectional area and the time velocity interval The continuity equation has been used in different echocardiographic applications The first is the calculation of stroke volume and cardiac output This can be performed by obtaining a Doppler signal in the left ventricular outflow tract, usually from an apical four-chamber view, and measuring the time velocity interval If the diameter of the left ventricular outflow tract is measured, usually from a parasternal long-axis view, the cross-sectional area can be calculated as π(D/2)2, which is equal to 0.785D2 Then stroke volume is the product of 0.785, the square of the diameter, and the time velocity interval Multiplying this value