Prognostic Value of Ambulatory Blood Pressures
With the advent of ambulatory blood pressure recording capability, many new empirical questions could be considered regarding the enhanced prediction of risk for cardiovascular disease associated with chronically elevated blood pressures. As with home blood pressures, it could be hypothesized that ambulatory blood pressure measures would predict target organ pathology and associated risk for cardiovascular disease better than clinic-derived indices of blood pressure.
This is indeed the case. Almost all investigations that have compared clinic derived and ambulatory measures of blood pressure with various indicators of target organ pathology (left ventricular hypertrophy, for example) have found ambulatory measures of blood pressure to predict target organ damage better than clinic-derived measures (see Appel and Stason, 1993; Mancia et al., 1996; Verdecchia et al., 1999; White, 1990; 1999). In a review of studies relating degree of target organ pathology with both ambulatory and clinic-derived measures of blood pressure, Verdecchia et al. reported mean correlations (weighted for sample size differences among studies) between left ventricular mass and ambulatory blood pressures to be .50 and .44 for SBP and DBP, respectively.
The corresponding mean correlations between left ventricular mass and clinic blood pressures were .35 and .32 for SBP and DBP, respectively, showing quite clearly that ambulatory methods of blood pressure assessment were more closely linked with degree of target organ pathology than clinic methods. Furthermore, reductions in left ventricular hypertrophy associated with pharmacologic treatment of high blood pressure have been shown to be associated with reductions in ambulatory, but not clinic, blood pressure measurements (Mancia et al., 1997).
In addition to predicting target organ pathology better than clinic-derived measures of blood pressure, ambulatory measures of blood pressure have been shown to be better predictors of subsequent cardiovascular and cerebrovascular events (Clement et al., 2003; Perloff, Sokolow, and Cowan, 1983; Perloff et al., 1989).
In a review of several prospective trials relating ambulatory blood pressures to actual cardiovascular disease endpoints, White (1999) reports a stronger relation between ambulatory blood pressure and cardiovascular and cerebrovascular disease endpoints than between clinic-derived measures of blood pressure and disease endpoints in “nearly every study performed during the past decade” (p.S20).
Although it is often assumed that the superiority of ambulatory over clinic blood pressure measures is related to the importance of stressors that patients encounter on a daily basis in determining the overall risk for target organ pathology, it is possible that these findings simply reflect the improved reliability of measurement that occurs with ambulatory methods due to the increased frequency of measurement.
Kamarck and colleagues (2002) conducted a unique study to examine this alternative hypothesis. In this study, measures of carotid atherosclerosis were compared with blood pressures obtained in the clinic, during an ambulatory monitoring period, and during an extended clinic session of comparable length. Because the same number of measurements and the same blood pressure measurement instrumentation were used during both the extended clinic session and ambulatory monitoring periods, these measurement confounds could be controlled.
Results of this study demonstrated the superiority of ambulatory over clinic measures in predicting carotid atherosclerosis, providing support for the initial hypothesis that predicting target organ pathology involves consideration of the daily lifestyles of hyper tensive patients.
It is quite clear that ambulatory blood pressure measurements provide information regarding risk for cardiovascular disease that cannot be obtained from simply measuring blood pressures in the clinic (Pickering et al., 2005). Ambulatory methods of blood pressure assessment provide a broader assessment of blood pressure status during various phases of daily life including the night, when patients are presumably exhibiting true resting baseline blood pressure levels.
Based upon observations that blood pressure declines during nighttime ambulatory recording, several investigators have given serious attention to examining the relation between this nocturnal decrease and risk for cardiovascular disease.