The effect of nadolol on heart rate and the standard deviation of the RR intervals

The aim of this study was to evaluate the effect of nadolol in modifying the standard deviation of RR intervals (SDRR) and the coefficient of variance (CV) in 47 patients characterized by symptomatic, frequent (>30h−1) and reproducible premature ventricular contractions (PVCs). Sixteen patients had suffered a previous myocardial infarction (Group 1), 22 had various non-ischaemic cardiac diseases (Group 2) and nine had no heart disease (Group 3). All patients underwent two 24 h Holter recordings during the washout period, and a third Holter recording was taken 5 days after the administration of nadolol (80 mg. day−1). The RR cycle length (RR), SDRR and CV were evaluated for each 24 h period, as well as for six daytime (1000–1600h) and six night-time hours (0000–0600h). In all three groups, nadolol was highly effective in lengthening RR. However, the effect on SDRR was different in the three group 24-h SDRR tended to be reduced in all three groups (but the reduction was significant only in Group 1) due to a certain RR homogeneity. On the other hand, daytime SDRR tended to increase (but the increase was significant only in Group 3). CV is concomitantly affected by heart rate and SDRR, therefore it may be more sensitive in evaluating the effect of beta-blockers. The reduction in CV post-nadolol means that the bradycardia-induced effect is associated with an inadequate increase, or even a reduction in SDRR, whereas the absence of any CV clianges (found only in Group 3) is the result of a concomitant increase in both RR cycle length and SDRR It can be concluded that: (1) 24-h SDRR is an inadequate index for verifying the effect of beta-blockers on heart rate variability, and that it is preferable to evaluate CV over shorter period (2) the slowing of heart rate is not always associated with an effort on SDRR; (3) the different effects of nadolol on the three groups may depend on the presence or absence of heart disease.