Influence of Traumatic Brain Injury by Fluid Percussion on Heart Rate Variability in the Acute Phase of Damage in Rats

Traumatic brain injury (TBI) is a condition that changes the autonomic system, modulating the heart rate variability (HRV) at all levels of brain lesions. Although fluid percussion injury (FPI) model can reproduce all degrees of severity of clinical TBI, there is still a lack of comprehensive analysis of linear and non-linear HRV metrics following FPI. The present study sought to assess the influence of the FPI model on time-domain (HR, mean NN, SD1, SD2, SDNN, RMSSD, and SD1/SD2) and frequency-domain (LF, HF, and LF/HF). A non-invasive electrocardiogram recording was used in anesthetized and awake male Wistar rats, both before and for three days after moderate FPI. Although a decrease in the SD2 occurred in the anesthetized state, an increase in HFnu led to a reduction in HR during baseline evaluations. Post-TBI analyses revealed that neither the sham nor the TBI groups exhibited HR alterations under the influence of isoflurane; however, both groups showed a decrease in parasympathetic activity (RMSSD, SD1, and HFnu). Under isoflurane anesthesia, only the TBI group exhibited changes in LFnu, HFnu, and LF/HF metrics for three days. In contrast, awake animals experienced an increase in HR for three days post-injury, with a critical period at 24 hours when SD2, LFnu, HFnu, and LF/HF were altered. With few exceptions, the sham group did not exhibit significant differences in the awake state. Therefore, the effects of isoflurane predominate over TBI effects in both time- and frequency-domain metrics, while FPI in awake animals indicates a critical period of altered specific metrics at 24 hours post-injury.