Heart rate variability and electrocardiogram waveform as predictors of morbidity during hypothermia and rewarming in rats

Abstract

This study examined electrocardiogram (ECG) waveform, heart rate (HR), mean blood pressure (BP), and HR variability as potential autonomic signatures of hypothermia and rewarming. Adult male Sprague-Dawley rats had telemetry transmitters surgically implanted, and 2 weeks were allowed for recovery prior to induction of hypothermia. Rats were lightly anesthetized (sodium pentobarbital, 35 mg/kg i.p.) and placed in a coil of copper tubing through which temperature-controlled water was circulated. Animals were cooled to a core temperature (Tc) of 20 degrees C, maintained there for 30 min, and then rewarmed. Data (Tc, BP, HR from ECG, and 10-s strips of ECG waveforms) were collected every 5 min throughout hypothermia and rewarming. Both HR and BP declined after initial increases with the drop in HR starting at a higher Tc than the drop in BP (29.6 +/- 2.4 degrees C vs. 27.1 +/- 3.3 degrees C, p < 0.05). Animals that were not successfully rewarmed exhibited a significant (p < 0.05) increase in the normalized standard deviation of interbeat intervals (IBI) throughout cooling compared with animals that were successfully rewarmed. The T wave of the ECG increased in amplitude and area with decreasing Tc. T-wave amplitude and IBI variability show potential as predictors of survival in hypothermic victims.

Cold-impaired cardiac performance in rats is only partially overcome by cold acclimation

“…A reflex bradycardia is usually associated with greater vagal tone and increased HRV, but vagotomy prior to acute cooling was without significant effect on heart rate in vivo, or elements of the cardiac cycle identified from the ECG, confirming the findings of low vagal tone in anaesthetised rats (Fewell et al, 2007;Matthew et al, 2002). However, vagotomy enhanced the prolongation of QRS and S-T intervals, implicating a vagal contribution to ventricular repolarisation during a hypothermic challenge.…”

Heart rate variability in rodents: uses and caveats in toxicological studies

“…There were no corresponding changes in the power spectral density, suggesting that the elevation in HRV may have occurred without significant ANS input. Another rodent study showed that the survivability of hypothermic rats during rewarming was decreased, as the observed increase in normalized HRV was indicative of an adverse outcome [118].…”

A model of deep experimental hypothermia and rewarming in rat

“…In that model, hypothermia severely decreased flow to most organs that was partially restored upon rewarming. Moreover, in a non-invasive hypothermic model a mortality rate of 21% when rewarming from 20 C was observed (Mathew et al, 2002b). In our model the use of ventilatory support, which stabilise acid-base status, improved survival from recovery.…”

“…This fact also implies that blood fails in distributing the heat through the body. The effect of temperature gradients in different vascular regions may result in a disassociation between metabolism and flow (Enomoto et al, 1996;Nakamura et al, 2003) resulting in cardiac failure due to circulatory collapse (Mathew et al, 2002b) could explain mortality rate during the earlier stage of fast rewarming. Although previous reports demonstrate that hypothermic mice could be quickly rewarmed by radiofrequency radiation, such a high rewarming rate would produce protein denaturation when techniques others than microwave rewarming are used (Gordon, 1982).…”