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To assess the additional diagnostic precision conferred by ambulatory blood pressure monitoring on clinic blood pressure measurement in evaluating the severity of isolated systolic hypertension.The association between left ventricular size as determined by ECG voltages [R-wave voltages in lead V5 (RV5) and S-wave voltages in lead V1, (SV1,)] and blood pressure as assessed by clinic measurements and ambulatory blood pressure monitoring was studied in 97 elderly patients included in the placebo run-in phase of the Syst-Eur trial. The additional diagnostic precision conferred by ambulatory monitoring on clinic blood pressure measurements was assessed by relating the residual ambulatory blood pressure level to the ECG-left ventricular size. The residual ambulatory blood pressure level was calculated by subtracting the predicted ambulatory blood pressure level for each patient (using the linear regression equation relating both techniques for the group) from the observed ambulatory blood pressure.Clinic systolic blood pressure was on average 20mmHg higher (P<0.001) than daytime ambulatory blood pressure while diastolic blood pressure was similar with both techniques. The sum of SV1 + RV5 was significantly related to clinic systolic pressure (r=0.25), and 24-h (systolic, r=0.37; diastolic, r=0.29), daytime (systolic, r=0.30; diastolic, r=0.19) and night-time (systolic, r=0.33; diastolic, r=0.28) ambulatory blood pressure levels. These findings were not affected by adjustment for gender, age and the body mass index. The sum of SV1 + RV5 was significantly related to the residual 24-h (systolic, r=0.30; diastolic, r=0.31), daytime systolic (r=0.20) and night-time (systolic, r=0.31; diastolic, r=0.29) ambulatory blood pressure monitoring levels.Ambulatory blood pressure monitoring adds to the diagnostic precision of clinic blood pressure measurement in assessing the severity of hypertension in this population. The ongoing side project on ambulatory blood pressure monitoring in the Syst-Eur study should establish whether these findings hold true for morbidity and mortality.