OBJECTIVES: Patients with chronic disorders of consciousness (DOC) may show alterations of autonomic function; however, in this clinical population, no data are available on the specific effects of nociceptive stimuli on cardiac autonomic control. Thus, we aimed at investigating the effects of a noxious stimulation on heart rate variability (HRV) in a population of patients with chronic DOC, taking into account different states of consciousness (vegetative state/unresponsive wakefulness syndrome, VS/UWS and minimally conscious state, MCS). METHODS: We enrolled twenty-four DOC patients (VS/UWS, n鈥?鈥?2 and MCS, n鈥?鈥?2). ECG and respiration were recorded during baseline, immediately after the nociceptive stimulus and, finally, during the recovery period. Linear and nonlinear HRV measures were used to evaluate the cardiac autonomic control. RESULTS: In DOC patients, nonlinear HRV analysis showed that nociceptive stimuli are able to elicit a change of autonomic function characterized by an increased sympathetic and a reduced vagal modulation. A significant reduction of autonomic complexity has also been detected. More interestingly, VS/UWS patients showed a less complex dynamics compared to MCS patients. CONCLUSIONS: Cardiac autonomic responses are able to significantly differentiate the autonomic function between VS/UWS and MCS patients. SIGNIFICANCE: Nonlinear HRV analysis may represent a useful tool to characterize the cardiac autonomic responses to nociceptive stimuli in a chronic DOC population.