Placebo effect and adaptation to noninvasive monitoring of BP

Three 24 h ambulatory monitorings of BP were performed at two-week intervals in 21 untreated hypertensives (mean age 38 +/- 10 yrs, 13 males and 9 females). After the first baseline monitoring, the patients were randomised, according to a cross-over design, to one of the following sequences: no therapy to placebo or placebo to no therapy. At the end of each period, noninvasive ambulatory monitoring was performed. Mean +/- SE 24 h systolic (SBP) and diastolic (DBP) pressures recorded at the first monitoring were 129.2 +/- 3.5 mmHg and 81.7 +/- 2.3 mmHg respectively. At the second and third monitorings, mean 24 h BP differences versus baseline levels were -2.9 +/- 1.8 and -4.7 +/- 1.7 mmHg for SBP, and -2.0 +/- 1.1 and -2.7 +/- 1.5 mmHg for DBP. Both SBP and DBP differences at repeated monitorings were significant by analysis of variance (P less than 0.05). No significant effects on BP of treatment sequence or of placebo administration were found. Analysis of covariance showed a significant relationship between initial 24 h BP and subsequent mean 24 h BP differences (SBP: beta = -0.260, DBP: beta = -0.124). ANOVA performed on waking and sleeping BP separately showed the observed differences to be significant only during waking hours. Regression analysis showed that the decrease in 24 h BP at repeated monitorings was significantly related to the extent of 'white coat'-induced BP increase only for DBP (P = 0.022). For both 24 h SBP and DBP, however, a negative correlation between the alarm reaction to the presence of the physician and 24 h BP decrease at repeated monitorings was observed. It is concluded that noninvasive ambulatory monitoring is subject to adaptative phenomena but not to placebo effect. Factors influencing the defence reactions to manual measurements and to ambulatory monitoring might be partly different.