We recently utilised a power spectral technique and a cross-sample entropy (CSE) method (Rickman & Moorman 2000) to examine synchrony of the relationship between blood pressure (BP) and renal sympathetic nerve activity (RSNA) during right atrial stretch to mimic plasma volume expansion (Yang et al. 2002). CSE revealed that during the reflex inhibition there was more synchrony between the oscillating signals in the BP and RSNA sequences. In the present study we have used a similar analysis of these signals during a mild haemorrhage which reflexly causes an increase in RSNA in an attempt to maintain BP constant.
The experiments were performed on 10 anaesthetised (urethane 650 mg kg-1, chloralose 50 mg kg-1) Wistar rats. BP was measured from a femoral artery and RSNA from a branch of renal nerve after exposing the left kidney retroperitoneally. A 33 s high frequency (1 kHz) sampling of BP and RSNA was recorded and rectified. The trachea was cannulated and spontaneous respiration maintained. Rectal temperature was maintained at 37 °C by a heating blanket. A femoral vein was cannulated and 1 ml of blood was removed into a pre-heparinised syringe over 1 min and 5 min later slowly reinfused. Data are expressed as means ± S.E.M., and analysed using repeated measures ANOVA. Statistical differences were considered significant when P < 0.05.
Rats were killed by overdose of urethane at the end of experiment.
A coherence measurement from power spectral analysis failed to detect significant changes between baseline and haemorrhage in either averaged coherence over the range 0-10 Hz (0.492 ± 0.01 to 0.491 ± 0.01) or coherence at heart rate frequency (0.94 ± 0.03 to 0.93 ± 0.02). However a non-linear dynamic analysis of the group data using CSE measurements showed that the relationship between BP signals and RSNA time series did change during haemorrhage, from 0.72 ± 0.05 at baseline to 0.79 ± 0.06 (P < 0.05), revealing there was greater asynchrony.
The data suggest that cross-sample entropy calculations characterise the non-linearities underlying cardiovascular control signals and may reveal how homeostatic regulation is achieved by the autonomic nervous system.
We acknowledge the support of The Wellcome Trust.