The incidental finding of high systolic pulmonary artery pressure on echocardiography is common. What we should do about it varies according to clinical presentation, comorbidities, and results of other tests, including assessment of the right ventricle. Thus, the optimal approach ranges from no further investigation to right heart catheterization and, in some cases, referral to a pulmonary hypertension center.
THE TWO MEASUREMENTS COMPARED
Although it raises concern, the finding of high systolic pulmonary artery pressure is not enough to diagnose pulmonary hypertension. In fact, several other conditions are associated with high systolic pulmonary artery pressure on echocardiography (Table 1). The diagnosis must be confirmed with right heart catheterization.1
Echocardiography provides an estimate of the systolic pulmonary artery pressure that is calculated from other values, whereas right heart catheterization gives a direct measurement of the mean pulmonary artery pressure, which is necessary for diagnosing pulmonary hypertension. The two values are correlated, but the differences are noteworthy.
WHAT IS PULMONARY HYPERTENSION?
Pulmonary hypertension is defined by a resting mean pulmonary artery pressure 25 mm Hg or greater during right heart catheterization.1 The large number of conditions associated with pulmonary hypertension can be divided into five groups2:
- Group 1, pulmonary artery hypertension
- Group 2, pulmonary hypertension associated with left heart disease
- Group 3, pulmonary hypertension due to chronic lung disease or hypoxia
- Group 4, chronic thromboembolic pulmonary hypertension
- Group 5, pulmonary hypertension due to unclear multifactorial mechanisms.2
Pulmonary artery hypertension (group 1) is a syndrome characterized by a restricted flow of small pulmonary arteries that can be idiopathic, heritable, or induced by anorexigens, connective tissue disease, congenital heart disease, portal hypertension, human immunodeficiency virus (HIV), or schistosomiasis.2,3 In spite of significant advances in therapy in the last 3 decades, pulmonary artery hypertension continues to lead to right heart failure and death,4 and the diagnosis has adverse prognostic implications. Therefore, it is essential to be attentive when reviewing the echocardiogram, since an elevated systolic pulmonary artery pressure may be an important clue to pulmonary hypertension.
ESTIMATED PRESSURE: HOW HIGH IS TOO HIGH?
There is no consensus on the optimal cutoff of echocardiographic systolic pulmonary artery pressure to trigger a further evaluation for pulmonary hypertension.
A retrospective evaluation of nearly 16,000 normal echocardiograms found that the 95% upper limit for systolic pulmonary artery pressure was 37 mm Hg.5
European guidelines6 propose that pulmonary hypertension is unlikely if the estimated systolic pulmonary artery pressure is 36 mm Hg or lower, possible if it is 37 to 50 mm Hg, and likely if it is higher than 50 mm Hg.6
The 2009 consensus document of the American College of Cardiology Foundation and American Heart Association3 recommends a systolic pulmonary artery pressure greater than 40 mm Hg as the threshold to suggest further evaluation in a patient with unexplained dyspnea.
Converting the systolic pulmonary artery pressure to the mean pressure
Although not validated to use with echocardiography, the most accurate estimate of mean pulmonary artery pressure was shown in one study7 to be obtained with the equation:
0.61 × systolic pulmonary artery pressure
+ 2 mm Hg
Using this formula, a systolic pulmonary artery pressure of 37 mm Hg would correspond to a mean pulmonary artery pressure of 24.6 mm Hg. A systolic pulmonary artery pressure of 40 mm Hg would correspond to a mean pulmonary artery pressure of 26.4 mm Hg.
Estimated systolic pulmonary artery pressure depends on several variables
Systolic pulmonary artery pressure is estimated using the simplified Bernoulli equation8:
4 × tricuspid regurgitation jet velocity2 (m/s)
+ right atrial pressure (mm Hg)
Tricuspid regurgitation is present in over 75% of the normal population. The regurgitation velocity across the tricuspid valve must be measured to estimate the pressure gradient between the right ventricle and the right atrium. The right atrial pressure is estimated from the diameter of the inferior vena cava and the degree of inspiratory collapse with the sniff test. As the right atrial pressure increases, the inferior vena cava dilates and inspiratory collapse decreases.8 If there is no gradient across the right ventricular outflow tract or pulmonary valve, the right ventricular systolic pressure is equal to the systolic pulmonary artery pressure.
Since tricuspid regurgitation velocity is squared and then multiplied by 4, small deviations of this measurement lead to markedly different systolic pulmonary artery pressure values. To avoid this problem, the tricuspid regurgitation velocity needs to be looked at in multiple echocardiographic views to find the best alignment with the flow and an adequate envelope.