Differentiation of autonomic nervous activity in different stages of coma displayed by power spectrum analysis of heart rate variability.

The analysis of heart rate (HR) variability offers a noninvasive method to investigate autonomic nervous system activity in comatose patients. We analyzed three components of the HR variability in a group of comatose patients: the low-frequency band (LF), representing mainly sympathetic influence, the mid-frequency band (MF), representing sympathetic and parasympathetic influence, and the high-frequency band (HF), representing the parasympathetic influence. A value for sympathovagal balance was defined as LF/HF and MF/HF ratio. Moreover, the skin conductance level (SCL) and the skin conductance resistance (SCR) variability were recorded. The patient group consisted of 22 patients with traumatic brain injuries. Coma depth was assessed by the Glacow Coma Scale and artifact-free HR, SCL, and SCR were measured 75 times in the patient group. The results documented a significant gain in sympathetic nervous system activity corresponding with the state of emerging from coma. This gain was most pronounced in the HF component of the HR and in the sympathovagal balance between LF/HF. The findings in SCL and SCR variability endorsed this result. It is concluded that emerging from coma is accompanied by an increasing influence of the sympathetic nervous system on HR control. This leads to a change in the sympathovagal balance, i.e., a reintegration of parasympathetic and sympathetic activity.