THE INFLUENCE OF PREMEDICATION ON HEART-RATE-VARIABILITY

Analysis of heart rate variability has been used to study the effects of midazolam, morphine and clonidine on the autonomic nervous system, when administered to patients for premedication. Ninety-five patients were studied 60 min before and 60 min after premedication. Normal sali ne (n = 25), midazolam 0.08 mg.kg(-1) (n = 24), morphine 0.15 mg.kg(-1 ) (n = 23), or clonidine 2 mu g.kg(-1) (n = 23) were administered intr amuscularly by random allocation. A Holter device was connected to the patient during the study period. Using power spectral analysis the lo w-frequency and high-frequency components were calculated from the Hol ter recordings. These are markers for sympathetic and parasympathetic activity respectively; the low-to high-frequency ratio was also calcul ated, a ratio of >1 signifying sympathetic dominance. A significant re duction was noticed in both low-frequency and high-frequency power in the three premedicated groups, whereas no changes were observed in the normal saline group. In the case of midazolam, both the low and high frequencies were decreased but the low- to high-frequency ratio did no t change significantly. Morphine and clonidine depressed the low-frequ ency component more than the high-frequency component and the low- to high-frequency ratio was decreased, suggesting parasympathetic dominan ce. We conclude that heart rate variability may be a useful tool for i nvestigating the effect of drugs on the autonomic nervous system.