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

The effect of therapeutic hypothermia on heart rate variability

OBJECTIVE

Heart rate variability (HRV) reflects integrity of the autonomic nervous system. However, no study has investigated the impact of therapeutic hypothermia (TH) on HRV measures in infants with hypoxic-ischemic encephalopathy (HIE). In this study, we evaluate the influence of temperature on measures of HRV for a group of infants with favorable outcomes, as compared with a control group of infants with unfavorable outcomes.

STUDY DESIGN

Term-born infants with moderate-severe HIE underwent standard TH treatment and prospective electroencephalography (EEG) and electrocardiogram (ECG) recording. Infants with favorable outcome (no seizures, normal/mild EEG scores at 96 h, no magnetic resonance imaging brain injury and normal neurodevelopmental scores at 18 to 24 months) were matched on gestational age, sex and worst encephalopathy score to a group of infants with unfavorable outcomes. Time- and frequency-domain HRV measures were calculated from 60 min of ECG data obtained at three time points: 24 h (hypothermia), 48 h (hypothermia) and 96 h (normothermia). The effect of time and temperature were evaluated using repeated-measures analysis of variance.

RESULTS

Sixteen infants were included (8 favorable, 8 unfavorable). For both groups of infants, an increase in the HR, RR and HF power was associated with an increase in temperature, but was not associated with any other HRV measure. In contrast, measures of HRV increased over time, as encephalopathy decreased, for infants with favorable outcomes (reflecting increased cortical-autonomic integration), but not for those with unfavorable outcomes.

CONCLUSIONS

In general, the effect of hypothermia on measures of HRV is limited to changes in heart rate (bradycardia) and respiratory rate, as opposed to changes in true variability. This supports the hypothesis that persistent changes in HRV are driven by the underlying brain injury and not by the process of TH.

Strategies for therapeutic hypometabothermia

Although therapeutic hypothermia and metabolic suppression have shown robust neuroprotection in experimental brain ischemia, systemic complications have limited their use in treating acute stroke patients. The core temperature and basic metabolic rate are tightly regulated and maintained in a very stable level in mammals. Simply lowering body temperature or metabolic rate is actually a brutal therapy that may cause more systemic as well as regional problems other than providing protection. These problems are commonly seen in hypothermia and barbiturate coma. The main innovative concept of this review is to propose thermogenically optimal and synergistic reduction of core temperature and metabolic rate in therapeutic hypometabothermia using novel and clinically practical approaches. When metabolism and body temperature are reduced in a systematically synergistic manner, the outcome will be maximal protection and safe recovery, which happen in natural process, such as in hibernation, daily torpor and estivation.

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

The consequences of acute hypothermia include impaired cardiovascular performance, ultimately leading to circulatory collapse. We examined the extent to which this results from intrinsic limitations to cardiac performance or physiological dysregulation/autonomic imbalance, and whether chronic cold exposure could ameliorate the impaired function. Wistar rats were held at a 12 h:12 h light:dark (L:D) photoperiod and room temperature (21°C; euthermic controls), or exposed to a simulated onset of winter in an environmental chamber by progressive acclimation to 1 h:23 h L:D and 4°C over 4 weeks. In vivo, acute cold exposure (core temperature, Tb=25°C) resulted in hypotension (approximately –20%) due to low cardiac output (approximately –30%) accompanying a bradycardia (approximately –50%). Cold acclimation (CA) induced only partial compensation for this challenge, including increased coronary flow at Tb=37°C (but not at Tb=25°C), maintenance of ventricular capillarity and altered sympathovagal balance (increased low:high frequency in power spectral analysis, PSA), suggesting physiological responses alone were insufficient to maintain cardiovascular performance. However, PSA showed maintenance of cardiorespiratory coupling on acute cold exposure in both groups. Ex vivo cardiac performance revealed no change in intrinsic heart rate, but a mechanical impairment of cardiac function at low temperatures following CA. While CA involved an increased capacity for β-oxidation, there was a paradoxical reduction in developed pressure as a result of adrenergic down-regulation. These data suggest that integrated plasticity is the key to cardiovascular accommodation of chronic exposure to a cold environment, but with the potential for improvement by intervention, for example with agents such as non-catecholamine inotropes.

Core temperature variation is associated with heart rate variability independent of cardiac index: a study of 278 trauma patients

PURPOSE

The purpose of this study is to determine if temperature extremes are associated with reduced heart rate variability (HRV) and "cardiac uncoupling."

MATERIALS AND METHODS

This was a retrospective, observational cohort study performed on 278 trauma intensive care unit admissions that had continuous HR, cardiac index (CI), and core temperature data from "thermodilution" Swan-Ganz catheter. Dense (captured second-by-second) physiologic data were divided into 5-minute intervals (N = 136 133; 11 344 hours of data). Mean CI, mean temperature, and integer HR SD were computed for each interval. Critically low HRV was defined as HR SD from 0.3 to 0.6 beats per minute. Temperature extremes were defined as less than 36°C or greater than 39°C.

RESULTS

Low HRV and CI vary with temperature. Temperature extremes are associated with increased risk for critically low HRV (odds ratio, >1.8). Cardiac index increases with temperature until hyperthermia (>40°C). At temperature extremes, changes in CI were not explained solely by changes in HR.

CONCLUSIONS

The conclusions of this study are (1) temperature extremes are associated with low HRV, potentially reflecting cardiac autonomic dysfunction; (2) CI increases with temperature; and (3) HRV provides additional physiologic information unobtainable via current invasive cardiac monitoring and current vital signs.

Deep hypothermia impact on acid-base parameters and liver antioxidant status in an in vivo rat model

Although clinical hypothermia is used for reducing postischemic damage, injurious effects have also been reported. To determine whether hypoxia and oxidative stress are induced by systemic deep hypothermia, we used an in vivo rat model keeping the arterial Pco2 constant. Animals were divided into 4 groups: sham, 2 h deep hypothermia (21 degrees C), 1 h posthypothermia (rewarmed to 37 degrees C after 2 h deep hypothermia), and 3 h normothermia. Blood gases, portal vein blood flow, arterial pressure, and heart rate were monitored throughout the experiment. Liver enzyme antioxidant activity was also examined. The hemodynamic parameters decreased drastically during hypothermia, but were fully restored after rewarming. No changes in hepatic antioxidant activity (catalase, glutathione peroxidase, and superoxide dismutase) were observed. The redox level in liver (GSH/GSSG ratio) was preserved in hypothermia but decreased when animals were rewarmed. ALT did not increase and no evidence of tissue hypoxia was detected in liver regarding the restricted flow during hypothermia. With the described protocol, deep hypothermia is regarded as an experimental safe model.

Piperazine protects the rat heart against sudden cardiac death from barium chloride-induced ventricular fibrillation

Fifteen of 20 Wistar albino rats were treated with various doses of the anthelmintic agent piperazine citrate (15, 30, and 60 mg/kg body weight). All 20 rats were subsequently given barium chloride, 20 mg/kg. The 5 rats (25%) that did not receive piperazine citrate developed ventricular fibrillations after barium chloride was administered to them, via one of the external jugular veins, and died shortly thereafter. The remaining 75% of the rats were fully protected by all the doses of piperazine citrate employed for the study. Barium chloride did not produce any dysrhythmic phenomenon in the piperazine-protected rats. Conversely, sinus rhythm was maintained in the electrocardiogram of all the rats, with every P wave followed by a normal QRS-T complex. This may portend a novel application for an old drug.

Time course of effect of piperazine citrate on the electrocardiogram of the rat

The acute electrocardiographic (ECG) changes induced by 4 different doses of piperazine citrate (15, 30, 60, and 100 mg/kg) were determined in the anesthetized Wistar rat. A dose-dependent reduction in heart rate occurred, from 9.03%+/-2.97% at a dose of 15 mg/kg to 30.84%+/-3.4% at 100 mg/kg body weight. The P-R interval showed dose-dependent increases over values at equilibration, increasing from 10.69% +/-2.82% at 15 mg/kg to 24.79%+/-2.71% at 100 mg/kg. Similarly, the Q-T interval corrected for heart rate (Q-Tc) showed dose-dependent increases, from 4.7%+/-1.89% at 15 mg/kg to 29.40%+/-6.09% at 100 mg/kg. In comparison with values for controls, all these changes except those associated with 15 mg/kg were statistically significant (P<0.05). Piperazine did not have any effect on the duration of the QRS complex except at 100 mg/kg, the dose at which marked widening occurred in 3 of the 7 rats. Dysrhythmic phenomena, including various forms of atrioventricular (AV) block, sometimes with idioventricular rhythms, were also evident in the 3 rats. Severe bradycardia from sino-atrial depression and AV block was also observed. At this concentration (100 mg/kg), 3 of 7 rats died of complete heart block within 30 minutes of drug administration. It was concluded that piperazine citrate at the suggested antiarrhythmic dose (15 mg/kg intravenously) and even at four times that dose was associated with no ECG abnormality suggestive of cardiotoxicity. However, at 100 mg/kg very serious ECG aberrations can occur, with severe heart block and death.

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

Heart rate variability (HRV) is a measure of cardiac pacing dynamics that has recently garnered a great deal of interest in environmental health studies. While the use of these measures has become popular, much uncertainty remains in the interpretation of results, both in terms of human and animal research. In humans, HRV endpoints, specifically chronic alterations in baseline HRV patterns, have been reasonably well characterized as prognostic indicators of adverse outcomes for a variety of diseases. However, such information is lacking for reversible HRV changes that may be induced by short-term exposures to environmental toxicants. Furthermore, there are minimal substantive data, either acute or chronic, regarding the pathological interpretation or prognostic value of toxicant-induced changes in HRV in rodents. The present report summarizes the physiological and clinical aspects of HRV, the methodological processes for obtaining these endpoints, and previous human and animal studies in the field of environmental health. Furthermore, we include a discussion of important caveats and recommendations for the interpretation of HRV data in animal research.

Aging alters regulation of visceral sympathetic nerve responses to acute hypothermia

Hypothermia produced by acute cooling prominently alters sympathetic nerve outflow. Skin sympathoexcitatory responses to skin cooling are attenuated in aged compared with young subjects, suggesting that advancing age influences sympathetic nerve responsiveness to hypothermia. However, regulation of skin sympathetic nerve discharge (SND) is only one component of the complex sympathetic nerve response profile to hypothermia. Whether aging alters the responsiveness of sympathetic nerves innervating other targets during acute cooling is not known. In the present study, using multifiber recordings of splenic, renal, and adrenal sympathetic nerve activity, we tested the hypothesis that hypothermia-induced changes in visceral SND would be attenuated in middle-aged and aged compared with young Fischer 344 (F344) rats. Colonic temperature (Tc) was progressively reduced from 38°C to 31°C in young (3 to 6 mo), middle-aged (12 mo), and aged (24 mo) baroreceptor-innervated and sinoaortic-denervated (SAD), urethane-chloralose anesthetized, F344 rats. The following observations were made. 1) Progressive hypothermia significantly ( P < 0.05) reduced splenic, renal, and adrenal SND in young baroreceptor-innervated F344 rats. 2) Reductions in splenic, renal, and adrenal SND to progressive hypothermia were less consistently observed and, when observed, were generally attenuated in baroreceptor-innervated middle-aged and aged compared with young F344 rats. 3) Differences in splenic, renal, and adrenal SND responses to reduced Tc were observed in SAD young, middle-aged, and aged F344 rats, suggesting that age-associated attenuations in SND responses to acute cooling are not the result of age-dependent modifications in arterial baroreflex regulation of SND. These findings demonstrate that advancing chronological age alters the regulation of visceral SND responses to progressive hypothermia, modifications that may contribute to the inability of aged individuals to adequately respond to acute bouts of hypothermia.

Perioperative normothermia depends on intraoperative warming procedure, extent of the surgical intervention and age of the experimental animal

The maintenance of a physiological body temperature during and early after surgical interventions in experimental animals such as rodents is often neglected. Therefore the positive influence of an adequate use of warming blankets (WB) on the rectal body temperature in rats was investigated during two different surgical interventions, with a special focus on possible differences between young adult (2.5+/-0.14 months) and adult animals (9.3+/-0.13 months). Anesthesia was induced with isoflurane short inhalation and maintained with ketamine and domitor intramuscularly. Animals were divided into ten groups according to (a) the age of the animals, (b) the temperature of the WB and (c) the kind of surgical intervention (either an intravenous [i.v.] cannulation of the right external jugular vein or an intra-aortal implantation of a telemetric transmitter or both). Results clearly show that the surface temperature of the WB has a major impact on the perioperative thermoregulation. The rectal body temperature of animals operated on a cooler WB dramatically decreased depending on the age of the rat and also on the extent of the surgical intervention. The opening of the abdominal cavity in older rats resulted in a severe hypothermia: they lost 5.6 degrees C compared to 3.2 degrees C in the young adult rats. The implantation of the i.v. catheter had no serious effect on the thermoregulation. In conclusion, the results clearly show that an adequate perioperative warming system positively influences the postoperative outcome in young adult and most notably in adult rats and thus enables early postoperative experiments without effects on measured parameters.

Hypothermia and localized cold injuries

Hypothermia and localized cold injuries are largely preventable with proper preparation for activities in cold environments. Proficient field management is crucial to the final outcome in terms of function and viability because proper care is vital to preventing exacerbation of the initial exposure and injury. Rapid rewarming is optimal when further cold exposure can be avoided reliably. Repetitive freeze-thaw cycles are associated with increased morbidity and tissue loss caused by progressive microvascular injury and thrombosis. The subsequent care is largely supportive and consists of wound care and physical and hydrotherapy to promote optimal functional recovery.