Interchangeability of ambulatory and office blood pressure: limitations of reproducibility and agreement

[Reproducibility of arterial pressure measured in the ELSA-Brasil with 24-hour pressure monitoring]

OBJECTIVE

To determine the reproducibility of casual arterial pressure measurement and to confirm pressure diagnosis by monitoring of participants in the ELSA-Brasil (Estudo Longitudinal de Saúde do Adulto - Brazilian Longitudinal Study for Adult Health).

METHODS

Casual blood pressure was measured with an oscilometric device. A sub-sample of participants (N = 255) from Espírito Santo state (Southeastern Brazil) was reevaluated using the same methodology following one to ten weeks and, in addition, underwent arterial blood pressure monitoring. Diagnosis of hypertension used cut off points of 140/90 mmHg for casual pressure and 130/80 mmHg for arterial blood pressure monitoring. White coat hypertension was defined as the presence of hypertension in casual blood pressure and normal arterial blood pressure monitoring, and converse findings characterized masked hypertension.

RESULTS

Data are from 230 participants that on the two occasions were free from antihypertensive medication (N1 = 153) or under the same antihypertensive regimen (N2 = 77). Normotension was confirmed by arterial blood pressure monitoring in 120 out of 134 participants of the N1 group. In N2, blood pressure control was confirmed by arterial blood pressure monitoring in 43 of 54 participants with controlled hypertension per casual blood pressure. Overall diagnostic concordance between casual blood pressure and arterial blood pressure monitoring was 78% (kappa = 0.44). In the N1 group, six subjects (4%) presented white coat hypertension, and 23 subjects (25%) presented with masked hypertension.

CONCLUSIONS

Diagnostic concordance between casual blood pressure and arterial blood pressure monitoring was moderate. The rigorous standardization of casual blood pressure measurement adopted in the ELSA-Brasil study was able to reduce white coat hypertension. The high frequency of masked hypertension may suggest that pressure values obtained by arterial blood pressure monitoring indicate an elevated degree of stress at work.

Master blood pressure variation in office, by self, and in sleep recordings

Variation inherent to each session in any recording situation has to be mastered to obtain informative mean values and differences. Variation due to time trend and unsteady consecutive readings versus inter-arm variations in blood pressure (BP) in the operator-initiated, self-initiated, and sleep recordings were studied by oscillometry. They were small in operator-initiated recordings, increased in self-recordings, and were paramount in sleep. A passive supine patient and simultaneous recordings with cuffs placed in equal horizontal positions are essential conditions for the estimation of a real mean inter-arm difference. The relative variation of pulse pressure (PP), compared with other components, was largest within each situation. Most of the systolic BP (SBP)/diastolic BP (DBP) variation in sleep was due to unequal vertical cuff distances to the heart level. Only sleep PP was independent of the vertical position of cuffs. A plot of inter-arm differences can visualize systematic errors in sequences of sleep SBP/DBP. As is well known, every doubling of variation (SD) requires quadrupling the number of independent recordings in order to retain the same size of confidence interval (CI) for a mean value.

Patient-specific differences between blood pressure estimated from 24 h ambulatory measurements and serial office self-recordings

The long-term true blood pressure, estimated in terms of the repeatable 24 h ambulatory mean (24 hAmb) pressure, is probably best related to progressive cardiovascular deterioration and is vital for clinical decision-making. Serial self-measurements (SELF) under quiet conditions may reflect this level, but with an annoying uncertainty in individual patients. This uncertainty is characterized by analyzing differences between the SELF readings of seated/supine patients on their own and the reference 24 hAmb values based on 72 recordings. From at-random replicated sessions of 59 subjects, the sources of uncertainty can be separated into three components: a systematic (non-random) variable difference in level from person to person, a mean systematic difference in level between methods, and random variability determining repeatability between sessions. An unstable alerting reaction distorts about six initial self-recordings and increases the random variability. The following 7-12 or 13-24 'steady-state' values show a comparable random variability but reflect the 24 hAmb level only when compared as the average of all patients. The standard deviation for repeatability is about 50-100% higher for SELF than for 24 hAmb. The patient-specific difference between methods contributes more than +/-10 mmHg to the uncertainty interval of the SELF results. These patient-specific differences recur at repeat sessions and thus cannot be reduced by averaging the results of sequential SELF sessions. In contrast, two 24 hAmb results provide a 95% uncertainty interval of +/-5 mm Hg. Thus, the averaged results of multiple SELF sessions can be used to detect major changes in blood pressure but are, as a result of patient-specific differences, too unreliable when the value to be estimated (the true value) is the expected long-term value of 24 hAmb results.