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Abstract & Commentary
Synopsis: The expected upper limit of pulmonary artery systolic pressure may include 40 mm Hg in older or obese subjects, and these findings support the use of age and body mass index corrected values in defining the normal range of pulmonary artery systolic pressure estimates by Doppler echocardiography.
Source: McQuillan BM, et al. Circulation. 2001;104: 2797-2802.
The factors that affect pulmonary artery pressure in apparent normal individuals are unclear. Previous small studies have suggested that age, obesity, and systemic blood pressure may be important. Thus, McQuillan and coworkers at the Massachusetts General Hospital reviewed their echo laboratory database of 102,818 echocardiographically normal subjects over the last decade. They selected 15,596 with less than moderate tricuspid regurgitation (TR) and mild or less aortic and mitral regurgitation, which represented 15% of the total number of subjects available—8914 women and 6682 men. The final study population included the 10,719 subjects with TR, which was 69% of those meeting the initial selection criteria. The detection of TR was influenced by the time period during the decade in which the study was done. TR was detected in 48% in 1990 vs. 80% in 1999, but there was no change in the average TR jet velocity. Using a central venous pressure mean of 10 mm Hg, pulmonary artery systolic pressure (PASP) was estimated as the sum of the TR jet gradient plus the assumed central venous pressure of 10. The average PASP was 28 mm Hg (95% confidence interval [CI] = 19-38). The PASP value varied somewhat with the reason for the study, from a high of 30 mm Hg (CI, 17-44) in those referred to evaluate pulmonary pressure, to a low of 26 mm Hg (CI, 19-34) in those with suspected aortic valve disease. Certain factors were found to be predictors of PASP: age, < 20 years, PASP = 27 mm Hg, age > 60 years; PASP = 32 mm Hg; and body mass index (BMI), < 20; PASP = 27 mm Hg, BMI > 35, PASP = 31 mm Hg. Also predictive were measures of left ventricular hypertrophy, left atrial size, aortic size, and left ventricular ejection fraction (direct correlate). PASP values > 30 mm Hg were observed in 28% of these echo normals with TR, and the most robust predictor of this level of PASP was advanced age (odds ratio, 2.7). McQuillan et al concluded that the expected upper limit of PASP may include 40 mm Hg in older or obese subjects, and these findings support the use of age and BMI corrected values in defining the normal range of PASP estimates by Doppler echocardiography.
It is clear from these data that using the cardiac catheterization derived normal upper limit of PASP of 30 mm Hg for Doppler echo estimates is inappropriate and would misclassify many normal subjects as having mild pulmonary hypertension. Such a misclassification could have serious consequences for the patient with regard to establishing other diagnoses, such as pulmonary vascular disease; directing further evaluation, such as pulmonary angiography; and suggesting treatment, such as calcium blockers. There are several problems with comparison to a cath gold standard. Catheter pressure is usually measured in the pulmonary artery by fluid filled systems with variable frequency response. Doppler pressure is measured as the peak instantaneous pressure at the tricuspid valve. The estimation of right atrial pressure to complete the estimate of PASP by Doppler is problematic. Clearly in many normal subjects it is not 10 mm Hg, but this assumed pressure worked well empirically in several small studies compared to catheterization pressure measurements. Others have proposed estimating right atrial pressure by observing the size and amount of inspiratory collapse of the inferior vena cava. A popular technique is to estimate right atrial pressure in increments of 5 mm Hg using the caval observations. Some have suggested that the jet gradient alone be used since in normals right atrial pressure is near zero. This would bring Doppler estimates in line with the cath upper limit of normal but would not be useful for estimating PASP in those with high right atrial pressures. So we are stuck with an imperfect system where normal PASP can be up to 40 mm Hg in some individuals. Since age and BMI accounted for the largest proportion of the deviation in normal values, McQuillan et al’s suggestion that they be used to normalize the values is a good one. Unfortunately, the formula for this correction has not been provided. For the present, we must realize that older, obese subjects and those with left ventricular hypertrophy may have values up to 40 mm Hg without having clinically important pulmonary hypertension.
Dr. Crawford is Professor of Medicine, Mayo Medical School, Consultant, Mayo Clinic in Scottsdale, AZ.