The chronotropic response of the sinus node to exercise: A new method of analysis and a study of pacemaker patients

In this study, the intercepts and slopes of the regression lines of sinus heart rate response to exercise were examined as a possible means of assessing normal and abnormal chronotropic responses. The regressions were plotted for 223 normal subjects and 93 pacemaker patients during progressive bicycle exercise. The 93 pacemaker patients consisted of two groups, (a) 46 with symptomatic sick sinus syndrome (daytime sinus or nodal bradycardia of < 45.min^-1 or atrial asystole > 1.5 s together with other features of the syndrome) and (b) 47 with second- or third-degree AV block (in whom a one-to-one AV relationship was either restored by dual chamber pacing or in whom normal AV conduction was present at the time of exercise). Examination of the regression lines showed them to be sufficiently linear in both normal subjects and pacemaker patients to allow calculation of the intercept and slope values. Multiple regression analysis of these values showed that the AV block group did not differ significantly from the normal individuals. However, in the patients with sick sinus syndrome, the intercept but not the slope values were significantly lower than in the normal subjects (P < 0.01). The patients with sick sinus syndrome appeared to have an abnormality of sinus node function, where the basic rate setting is too low at all levels of exercise, although the acceleration with exercise is normal. Among the sick sinus syndrome patients, only four (9%) had intercept values lying more than 2 standard deviations below that of the normal control group and were deemed to be chronotropically incompetent. Finally, the problems with the various methods used to investigate chronotropic incompetence are reviewed. All methods involving a mathematical comparison of patients with controls are limited by the arbitrary nature of the values chosen to define chronotropic incompetence and the uncertainty concerning the physiological variables involved in heart rate control.