Heart rate variability after acute head injury in children

Alteration in heart rate variability (HRV) has been used to assess autonomic nervous system dysfunction in children after acute brain injury. We hypothesized that altered HRV is related to cerebral perfusion pressure in children with acute traumatic brain injury. In this study, we have quantified HRV by power spectral analysis of heart rate and examined its relationship to cerebral perfusion pressure. Methods: Admission criteria included documented traumatic brain injury occurring within eight hours of hospital admission, GCS of ≤ 8 and placement of an ICP monitor. Exclusion criteria included recent cardiopulmonary arrest, β-adrenergic receptor antagonist and/or atropine use within the previous 24 hours, history of pre-existing neurologic injury or disease, and moderate dose catecholamine infusions (dopamine > 5 mcg/kg/min or epinephrine > 0.1 mcg/kg/min). Patients were followed until discontinuation of ICP monitoring, death or withdrawal of life support. The ECG signal was sampled at 128 Hz after passing through a 50 Hz filter and edited for motion artifact, dysrhythmias and ectopic beats. Power spectral analysis gave hourly LF/HF ratios. Patients receiving pentobarbital were analyzed separately since pentobarbital is a well described suppressant of heart rate variability. Results: The LF/HF ratio in the nonpentobarbital group (n=11) decreased significantly below a CPP of 40 mm Hg (3.34 ± 3.71 vs. 0.71 ± 0.84, p<0.001). The pentobarbital group (n=4) did not exhibit a statistically significant difference above or below this 40 mm Hg threshold (0.25 ± 0.34 vs. 0.21 ± 0.35, p=0.29). Discussion: Abnormal HRV has been associated with poor outcome and altered cerebral perfusion. Our results show that in children with traumatic brain injury, the LF/HF ratio drops markedly below a CPP of 40 mm Hg. We speculate that these children may have more favorable outcomes if their CPPs are kept greater than 40 mm Hg. Further studies are warranted to investigate the role of HRV in the active management of traumatic brain injury in children.