Temporal Analysis of Sequential Changes in Heart Rate Variability During Non-hypotensive Hypovolemia.

Abstract

Haemorrhage is associated with changes in the cardiac autonomic drive which begins during early stages of mild haemorrhage. The knowledge of chronology of the autonomic changes at smaller timescale during the period of haemorrhage can help identify the primary autonomic parameter which signals the institution of cardiovascular reflex mechanisms. To evaluate the heart rate variability in 2-min sequential segments with one minute overlap during and after the period of mild haemorrhage (450 ml) using blood donation as a model of acute blood loss. 47 male blood donors who had volunteered for blood donation were recruited for the study. Continuous lead II ECG was recorded before the start of the blood donation (5 min), during the period of the blood donation (~ 5-7 min) and after blood donation (5 min). The parasympathetic and sympathetic drive to heart was estimated by measures of heart rate variability in time and frequency domain. A significant decrease in the parameters assessing parasympathetic drive i.e., normalised High frequency (HFn) and NN50 (Number of pairs of adjacent NN intervals differing by more than 50 ms) and pNN50 (NN50 divided by the total number of all NN Intervals) was observed during blood donation at 3-5 min, as compared to baseline. An increase in parameters associated with sympathetic drive i.e., normalised low frequency (LFn) was observed only in the post donation period at 2-3 min. A significant rise in LF/HF ratio which is a marker of sympatho vagal balance was observed at 4-5 min during blood donation when compared to baseline. The initial cardiac autonomic change during mild haemorrhage is withdrawal of parasympathetic drive, followed by an increase in sympathetic tone which occurs much later.