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 sin us heart rate response to exercise were examined as a possible means o f assessing normal and abnormal chronotropic responses. The regression s were plotted for 223 normal subjects and 93 pacemaker patients durin g progressive bicycle exercise. The 93 pacemaker patients consisted of two groups, (a) 46 with symptomatic sick sinus syndrome (daytime sinu s or nodal bradycardia of < 45.min(-1) or atrial asystole > 1.5 s toge ther with other features of the syndrome) and (b) 47 with second- or t hird-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 pr esent at the time of exercise). Examination of the regression lines sh owed them to sufficiently linear in both normal subjects and pacemaker patients to allow calculation of the intercept and slope values. Mult iple regression analysis of these values showed that the AV block grou p did not differ significantly from the normal individuals. However, i n the patients with sick sinus syndrome, the intercept but not the slo pe values were significantly lower than in the normal subjects (P < 0. 01). The patients with sick sinus syndrome appeared to have an abnorma lity of sinus node function, where the basic rate setting is too low a t all levels of exercise, although the acceleration with exercise is n ormal. Among the sick sinus syndrome patients, only four (9%) had inte rcept values lying more than 2 standard deviations below that of the n ormal control group and were deemed to be chronotropically incompetent . Finally, the problems with the various methods used to investigate c hronotropic incompetence are reviewed. All methods involving a mathema tical comparison of patients with controls are limited by the arbitrar y nature of the values chosen to define chronotropic incompetence and the uncertainty concerning the physiological variables involved in hea rt rate control.