Ambulatory blood pressure and clinical trials.

Ambulatory blood pressure monitoring essentially estimates the average value of systolic and diastolic pressure from readings that fluctuate with mental and physical activity throughout the day. The improved reproducibility of blood pressure measurement achieved in this way increases the precision of clinical trials and/or reduces the number of subjects needed in a trial. Thus it is possible to detect a 5 mmHg diastolic difference between two treatments (with a statistical power of 0.9) using less than 20 subjects in a crossover trial. Ambulatory monitoring has its limitations. Since it is an averaging process, the ability to follow the effect of drugs over short periods of time is strictly limited. To overcome this handicap it is necessary to reduce the spontaneous variability of blood pressure, which can be done by limiting mental and physical activity while the drug action is being monitored. This can be achieved in the laboratory but not in freely moving subjects.