Respiratory induced heart rate variability (rHRV) was analysed in mechanically ventilated patients during two levels of sedation and brain death. Our aim was to determine whether rHRV can distinguish between different levels of sedation and especially between brain death and sedated patients.
In this study 30 critically ill and 23 brain death patients were included and four respiratory rates of 15, 12, 8 and 6 breaths per minute, each lasting 5 min were used. Two sedation levels, basal and deep, were performed in the critically ill patients. Heart rate and blood pressure changes induced by ventilation were subsequently detected and analysed.
Significant differences were found in rHRV and rHRV adjusted for tidal volume (rHRV/VT) between critically ill and brain death patients during slow breathing at 6 or 8 breaths per minute. The rHRV at 6 breaths per minute was below 15 ms in all brain death subjects except one. The rHRV/VT was lower than 25 ms/l at both 6 and 8 breaths per minute in all brain death patients and simultaneously at 75% of non-brain death patients was higher (specificity 1, sensitivity 0.24). Differences in rHRV and rHRV/VTs between basal and deep sedation were not significant.
The main clinical benefit of the study is the finding that rHRV and rHRV/VT during 6 and 8 breaths per minute can differentiate between critically ill and brain death patients. An rHRV/VT exceeding 25 ms/l reliably excludes brain death.