Natural variability of pulmonary haemodynamics.

In the interpretation of the effect of a drug or intervention in a single patient, it is desirable to know the spontaneous fluctuations of the variable under study. A few studies have determined pulmonary haemodynamic variability in acute and short-term intervals. It seems reasonable to adopt a confidence limit of 6 mmHg or 22% for PPA changes to be significant. The long-term evolution is difficult to determine because of the unavoidable patient selection for repeat examinations several years apart, and because of the necessary treatment. In slight or moderate pulmonary hypertension the changes in PPA with time seem to be of small magnitude: less than 1 mmHg per year.