Echocardiography

Echocardiographic assessment of pulmonary hypertension

Echocardiography is the screening method of choice for suspected pulmonary hypertension. Echocardiography has two roles: one, to screen for causes of increased right-sided pressures, such as atrial and ventricular shunts, congenital abnormalities, valvular diseases, reduced LV function and more; two, to assess qualitatively and quantitatively the severity of pulmonary hypertension.

Transthoracic echocardiography is sufficient for initial screening and assessment of hemodynamic severity in many cases. Transesophageal echocardiography is recommended for a more precise identification (or exclusion) of structural abnormalities as cause of pulmonary hypertension, e. g. anomalous pulmonary veins.

M-mode and 2D-echocardiography should be used for exact measurements of all cardiac chambers: right atrial and right ventricular dimensions, right ventricular hypertrophy, and right ventricular function should be assessed. LV dimension and shape of the intraventricular septum and LV function have to be examined. This allows an echocardiographic differentiation of RV pressure overload vs. volume overload, if right sided heart chambers are enlarged (1). Dimensions of the pulmonary artery and degree of pulmonary insufficiency should be measured. In addition, dimensions and compressibility of the inferior vena cava should be assessed.

M-mode and 2D-echocardiography features of pulmonary hypertension are as follows:

1. Diminished or absent atrial wave (a-wave) of the pulmonary valve.
2. Midsystolic closure or notching of the pulmonary valve: "w" of pulmonic valve during systole.
3. Enlargement of right-sided heart chambers.
4. Flattened interventricular septum (D-shape) and paradoxical movement of the septum during systole.


Doppler Echocardiography
In the presence of tricuspid regurgitation and pulmonary insufficiency Doppler Echocardiography allows a rough estimation of systolic and diastolic pulmonary artery pressure. Measurement of tricuspid regurgitant jet velocity by continuous wave Doppler echocardiography gives an estimation of right ventricular systolic pressure, which in the absence of pulmonic stenosis, is equivalent to systolic pulmonary artery pressure. The end-diastolic velocity of pulmonary regurgitation reflects the end-diastolic pressure gradient between pulmonary artery and right ventricle. Adding right atrial pressure to the gradient allows assessment of systolic and diastolic pulmonary artery pressure.


Systolic pulmonary artery pressure
The tricuspid pressure gradient is derived from a modified Bernoulli equation that describes the relationship between a pressure gradient and peak flow velocity.

Transtricuspid pressure gradient (DP)
= 4 x peak tricuspid regurgitation velocity² (4 x v²TR)

The addition of estimated right atrial pressure (RAP) yields right ventricular systolic pressure, and hence systolic pulmonary artery pressure (PAPsyst.).

PAPsyst. = 4 x v²TR + RAP

RA pressure can be estimated clinically by measuring jugular venous pressure or by the respiratory motion of the inferior vena cava seen on 2D echocardiography. A decrease of the diameter of the inferior vena cava by 50 % or more usually reflects an RA pressure of less than 10 mmHg. Conversely, a respiratory decrease of less than 50 % signifies an RA pressure greater than 10 mmHg (2).

An example of a simultaneous recording of TR velocity an invasive pulmonary artery pressure is  shown in the figure. Invasive measured systolic artery pressure is 65 mmHg, tricuspid regurgitation pressure gradient is 57 mmHg. Therefore, in this example RA pressure would be 8 mmHg.

Echocardiographic assessment of pulmonary artery systolic pressure has several limitations:
The estimation of right atrial pressure is often a source of error.

Even if right atrial pressure is directly measured, slight systematic underestimation of PAP syst. or on the other hand false positive diagnosis of pulmonary hypertension might be possible with the tricuspid regurgitant method (3, 4).

In the echocardiography report we recommend to indicate the tricuspid regurgitant pressure gradient and the estimated systolic pulmonary artery pressure. The assumed RA pressure should be indicated together with the method, that was used for its estimation, e. g. height of jugular vein pressure.


Diastolic pulmonary artery pressure
CV-Doppler flow velocity allows estimation of pressure gradient at end-diastole. At end-diastole RV pressure = RA pressure (see figure 2). The sum of pulmonary artery pressure gradient and right atrial pressure does indicate diastolic pulmonary artery pressure (PAPdiast.).

Pulmonary artery pressure gradient
= 4 x end-diastolic velocity of pulmonary regurgitation² (PREDV²)

PAPdiast = 4 x PREDV² + RAP

Mean pulmonary artery pressure
The early diastolic peak pulmonary regurgitant velocity allows estimation of mean PAP (5). The peak diastolic pressure gradient between pulmonary artery and right ventricle approximates the mean pulmonary artery pressure (PAP).

Mean PAP = 4 x Peak PR velocity²

Mean pulmonary artery pressure can also be estimated by examining the Doppler flow velocity in the RV outflow tract. RV outflow tract velocity declines as pulmonary artery pressure increases. Measuring right ventricular outflow acceleration time permits calculation of mean PAP (6, 7).

Doppler Echocardiography allows also to measure time intervals of right ventricular contraction. By comparing right ventricular ejection time and tricuspid regurgitant time one can measure the isovolumic contraction and relaxation time of the right ventricle. This provides an estimate of the contractility of the right ventricle and has been shown to correlate well with prognosis in primary pulmonary hypertension (8).


Mitral and hepatic vein velocity patterns
In patients with pulmonary hypertension secondary to poor LV function mitral inflow velocity patterns usually have a restrictive pattern (increased E velocity, decreased A velocity, E/A > 1,5 and decreased deceleration time). A normal mitral inflow velocity pattern in patients with pulmonary hypertension usually indicates a pulmonary process as cause of pulmonary hypertension. Severe cases of pulmonary hypertension result in a shift of the interventricular septum and may thereby induce diastolic dysfunction of the left ventricle and an abnormal filling pattern (9).
 
Hepatic vein velocity has a characteristic pattern in patients with pulmonary hypertension. Increased diastolic pressure and decreased compliance of the right ventricle results in a prominent atrial flow reversal in the hepatic vein.


References:

1. Louie EK, Lin SS, Reynertson SI, Brundage BH, Levitsky S, Rich S. Pressure and volume loading of the right ventricle have opposite effects on left ventricular ejection fraction. Circulation 1995;92:819-24.

2. Kircher BJ, Himelman RB, Schiller NB. Noninvasive estimation of right atrial pressure from the inspiratory collapse of the inferior vena cava. Am J Cardiol 1990;66:493-6.

3. Brecker SJ, Gibbs JS, Fox KM, Yacoub MH, Gibson DG. Comparison of Doppler derived haemodynamic variables and simultaneous high fidelity pressure measurements in severe pulmonary hypertension. Br Heart J 1994;72:384-9.

4. Vachiery JL, Brimioulle S, Crasset V, Naeije R. False-positive diagnosis of pulmonary hypertension by Doppler echocardiography. Eur Respir J 1998;12:1476-8.

5. Masuyama T, Kodama K, Kitabatake A, Sato H, Nanto S, Inoue M. Continuous-wave Doppler echocardiographic detection of pulmonary regurgitation and its application to noninvasive estimation of pulmonary artery pressure. Circulation 1986;74:484-92.

6. Kitabatake A, Inoue M, Asao M, Masuyama T, Tanouchi J, Morita T, et al. Noninvasive evaluation of pulmonary hypertension by a pulsed Doppler technique. Circulation 1983;68:302-9.

7. Chan KL, Currie PJ, Seward JB, Hagler DJ, Mair DD, Tajik AJ. Comparison of three Doppler ultrasound methods in the prediction of pulmonary artery pressure. J Am Coll Cardiol 1987;9:549-54.

8. Tei C, Dujardin KS, Hodge DO, Bailey KR, McGoon MD, Tajik AJ, et al. Doppler echocardiographic index for assessment of global right ventricular function. J Am Soc Echocardiogr 1996;9:838-47.

9. Schena M, Clini E, Errera D, Quadri A. Echo-Doppler evaluation of left ventricular impairment in chronic cor pulmonale. Chest 1996;109:1446-51.

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