A temporally detailed reanalysis of the conditional heart rate response in dog

We used rapid (500 Hz) digital sampling to derive a detailed analysis of the HR conditional response in dog (n = 11) to a 30-sec tone (CS+) followed by a 1/2-sec shock. We found an initial bradycardia at 1.5 +/- 1.0 sec in 6 of 11 dogs from a pre-CS+ control of 85 +/- 26 bpm (mean +/- SD) to 74 +/- 25 bpm (p less than 0.05). In 9 of the 11 dogs there was a subsequent rapid (19.9 +/- 6.7 bpm/sec) increase (+26 +/- 10 bpm) in HR lasting from 1.3 +/- 0.9 to 2.7 +/- 1.0 sec. This was followed by a second, larger (+41 +/- 16, p less than 0.05), but slower (4.0 +/- 2.4 bpm/sec, p less than 0.05) tachycardia which lasted from 4.9 +/- 1.0 to 13.5 +/- 2.6 sec. The peak HR (131 +/- 27 bpm) was attained at 15.0 +/- 3.8 sec, after which HR fell slowly (-2.4 +/- 1.4 bpm/sec) to an average of 92 +/- 29 bpm at 28.6 +/- 1.4 sec (i.e., approximately 1.4 sec prior to shock delivery). This analysis revealed trends in the conditional HR response which were not discernible in individual trials and were obscured in the more traditional analyses of the conditional HR response.