Effects of different insulin infusion rates on heart rate variability in lean and obese subjects

The low-frequency to high-frequency ratio (LF/HF ratio) is an index of card iac sympathovagal balance. We hypothesized that insulin might also stimulat e the LF/HF ratio. Thus, 15 lean and 15 obese subjects were studied. Each s ubject underwent sequential hyperinsulinemic clamps (insulin infusion rate 0.50, 1, and 2 mU/kg . min) while the heart rate was recorded by the Holter technique continuously. Indirect calorimetry allowed determination of the respiratory quotient (Rq) and substrate oxidation. The leg blood flow (LBF) , leg vascular resistance (LVR), and plasma norepinephrine concentration we re also measured. In seven lean subjects, hyperinsulinemic clamps were repe ated along with propranolol infusion (0.1 mg . kg(-1) as an intravenous bol us dose followed by continuous intravenous infusion of 0.5 mg . kg(-1). min (-1) throughout the study). Lean subjects had better insulin action than ob ese subjects. Insulin infusion was associated with an increase of the Delta LF/HF ratio in both lean (P < .001 for time-dependent changes) and obese ( P < .02 for time-dependent changes) subjects; however, the extent of insuli n-mediated stimulation of the LF/HF ratio was greater in lean versus obese subjects. Insulin infusion did not significantly affect HF values in both g roups. Independently of gender, body fat, changes in the plasma norepinephr ine concentration, LBF, and LVR, the Delta LF/HF ratio at the end of the fa stest insulin infusion (0.8 +/- 0.2 v 0.3 +/- 0.2, P < .04) was still great er in lean versus obese subjects. The Delta LF/HF ratio was also more stimu lated during insulin versus insulin + propranolol infusion in lean subjects . In conclusion, insulin stimulates the LF/HF ratio in both lean and obese subjects and thus produces a shift in the cardiac autonomic nervous system activity toward sympathetic predominance. Copyright (C) 1999 by W.B. Saunde rs Company.