Natural History of Sinus Node Chronotropy in Paced Patients

The natural history of chronotropic incompetence is not clear. To assess this, we evaluated corrected sinus node recovery time (cSNRT) and sinus node chronotropy at rest and during exercise in two groups of syncopal patients with sinus node disease. Group A comprised patients with resting bradycardia but normal cSNRT and group B had resting bradycardia and prolonged cSNRT (> 1000 ms). An additional two groups (C and D) were studied. Group C comprised patients with complete AV (CAVB) and no evidence of sinus node disease and group D were asymptomatic controls of similar age. At diagnosis, patients with symptomatic bradycardia but normal cSNRT and no evidence of carotid sinus syndrome (group A) had resting bradycardia and impaired peak heart rate (PHR-I) on exercise compared to controls (P < 0.001 and P < 0.05, respectively), but no reduction in exercise duration. At follow-up group A patients demonstrated an increase in resting rate that was significantly slower than the controls (P < 0.01). Peak heart rate (PHR-II) also remained significantly slower (P < 0.05). There was no difference in exercise duration between groups A and D at follow-up. Group B was further subdivided according to follow-up findings of preservation of atrial activity in seven patients (group B-1) and junctional rhythm without any atrial activity in four patients (group B-2). Retrospective analysis showed no significant difference in resting heart rate at initial examination but group B-2 showed a significantly lower peak heart rate on exercise compared with B-1 (P < 0.01). Follow-up exercise tests revealed reduced exercise capacity in B-2 patients when compared with B-1 (P < 0.05) and both B-1 and B-2 had significantly reduced exercise capacity when compared with control groups C and D (P < 0.001). Group C patients had an initial sinus node chronotropic response to exercise, which was not different from control but significantly better (P < 0.01) than group B. At follow-up, the mean peak sinus rate of group C patients was unchanged, while there was an insignificant prolongation of cSNRT. Thus, patients with resting bradycardia, blunted peak heart rate response to exercise, and markedly prolonged CSNRT are those most likely to show chronotropic incompetence over the long-term and should be considered for rate responsive dual (or single) chamber pacing systems.