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.