Drug Effects on Heart Rate and Heart Rate Variability during a Prolonged Reaction Task†

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Abstract
The effects of an amphetamine and a barbiturate on heart rate were investigated during long-term performance. Subjects worked for three hours in a serial reaction test, which included blocks with variable or constant interstimulus intervals (ISI( Besides the intorbeat interval (IBI), derived from the successive K-peaks of the ECG, the variability of IBI was scored in three ways. Each of these four scores increased as a function of time-on-task, indicating a gradually decreasing activation level during the three hour session Amphetamine had an activating effect, decreasing both IBI and IBI variability; the barbiturate effect on the other hand was paradoxical: this drug tended to increase IBI variability, but to decrease TBI The IBI changes between constant and variable blocks were neglectable after amphetamine, while these changes were pronounced after barbiturate treatment. IBI variability was reduced during blocks with variable ISIs, where mental effort was assumed to be maximal. This reduction in variability was larger for amphetamine and tended to be smaller for barbiturate as compared to the placebo condition.