Successive-beat analysis of cardiovascular orienting and conditional responses

We have shown that prominent changes in heart rate (HR) and blood pressure (BP) occur in dogs during classical defensive conditioning. These changes are usually analyzed as average HR and BP during control periods versus averages during conditional stimulus (CS) and unconditional stimulus (US) periods. A new method has been developed for evaluation of these functions, viz., analysis across many trials of successive beats beginning at onset of CS and working backward (control period) and forward (CS period). Nine awake dogs were continuously studied: six with HR and intra-arterial BP measurements and three with HR measurements only. A drop in HR occurs 1 to 2 beats after CS onset (often noted as a “dropped” beat on individual trials). In some dogs, on the third to fifth beat HR reaches its maximum during the CS period, thereafter falling slightly, but still above baseline; other dogs show progressively increasing HR throughout the whole CS period. At US onset (foreleg shock) HR usually rises rapidly to a peak greater than that at CS maximum. BP follows HR changes fairly closely, diastolic better than systolic. These results indicate that I) this method can reveal transients previously unsuspected, 2) a prominent initial sudden bradycardia followed by tachycardia commonly occurs in conditioning in dogs, 3) the latent period of prominent HR increase is usually quite short (1.3 to 1.6 secs.), and 4) BP follows HR directly during the early part of the CS.