The electrocardiographic features of hypothermia

Hibernation-specific alternative splicing of the mRNA encoding cold-inducible RNA-binding protein in the hearts of hamsters

The hearts of hibernating animals are capable of maintaining constant beating despite a decrease in body temperature to less than 10 °C during hibernation, suggesting that the hearts of hibernators are highly tolerant to a cold temperature. In the present study, we examined the expression pattern of cold-inducible RNA-binding protein (CIRP) in the hearts of hibernating hamsters, since CIRP plays important roles in protection of various types of cells against harmful effects of cold temperature. RT-PCR analysis revealed that CIRP mRNA is constitutively expressed in the heart of a non-hibernating euthermic hamster with several different forms probably due to alternative splicing. The short product contained the complete open reading frame for full-length CIRP. On the other hand, the long product had inserted sequences containing a stop codon, suggesting production of a C-terminal deletion isoform of CIRP. In contrast to non-hibernating hamsters, only the short product was amplified in hibernating animals. Induction of artificial hypothermia in non-hibernating hamsters did not completely mimic the splicing patterns observed in hibernating animals, although a partial shift from long form mRNA to short form was observed. Our results indicate that CIRP expression in the hamster heart is regulated at the level of alternative splicing, which would permit a rapid increment of functional CIRP when entering hibernation.

Out in the cold: the hypothermic heart response

We present an interesting case of a 49-year-old woman with hypothermia and associated Osborn waves (also called J waves) on ECG. She was found on the floor of her home and difficult to arouse. On arrival to the emergency department (ED), her rectal temperature was 87.5°F. ECG showed Osborn waves in diffuse leads. She was intubated in the ED and was started on vasopressor support for hypotension refractory to intravenous fluid boluses. She was transferred to the critical care unit for continued respiratory and cardiovascular support. With active external rewarming her core body temperature continued to improve. Blood pressure also improved and vasopressor was tapered off. She was extubated and was transferred to the medical floor for continued supportive care. Osborn waves on ECG resolved within 12 h of achieving normal range body temperature. The patient was eventually discharged home with medical follow-up.

Assessment of the effects of changes in body temperature on cardiac electrophysiology in anaesthetised guinea pigs

INTRODUCTION

Anaesthetised guinea pigs are commonly used within Safety Pharmacology to evaluate drug effects on cardiac electrophysiology. However, anesthesia compromises the ability to thermoregulate, which can be further challenged when more invasive surgery is required. As anaesthetised animals are often used when screening for cardiotoxicity, thereby influencing go/no-go decisions, we wanted to quantify the impact of small temperature changes on the recorded electrophysiological parameters.

METHODS

Male guinea pigs were anaesthetised by pentobarbital, placed on a pre-heated table and a rectal thermistor inserted for monitoring of body temperature. After intubation animals were vagotomised and β-blocked, and lead II ECG needle electrodes attached. Following thoracotomy an atrial pacing electrode was attached and a suction MAP electrode positioned on the ventricular epicardium. In control animals temperature was kept constant (38.1±0.1°C) over the duration of the experiment. Animals in one group were slowly warmed to 41.9°C by a heating plate and a heating lamp, and in another group slowly cooled to 34.4°C by turning off all heating equipment. MAP duration at 90% repolarisation (MAPD90), AV conduction, ECG and body temperature were recorded during cardiac pacing every 5min up to 50min.

RESULTS

No time-dependent changes were seen in the control group. In contrast, a linear correlation was found between changes in body temperature and MAPD90, AV conduction, QTc and QRS intervals. For each degree temperature fell below 38°C MAPD90 was prolonged by 6.1ms, and for each degree above 38°C MAPD90 was shortened by 5.3ms. Corresponding changes were seen for QTc interval and AV conduction time, while effects on the QRS interval were smaller.

DISCUSSION

The data highlights the importance of carefully controlling body temperature when performing electrophysiological recordings in laboratory animals. A change by a single degree can affect electrophysiological parameters by 5-10%, thus increasing the risk for a false positive or negative interpretation of cardiotoxicity.

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.

Myocardial damage after prolonged accidental hypothermia: a case report

INTRODUCTION

A case of cardiac toxicity due to prolonged hypothermia is reported.

CASE PRESENTATION

A 42-year-old woman of Caucasian origin presented with hypothermia after an accident. She developed atrial fibrillation and heart failure with minor electrocardiographic changes, which mimicked ischaemia. She recovered completely after one month of treatment for heart failure.

CONCLUSION

Prolonged hypothermia, which mimicks ischaemia, may cause arrhythmias and heart failure.

Central A1-receptor activation associated with onset of torpor protects the heart against low temperature in the Syrian hamster

Body temperature drops dramatically during hibernation, but the heart retains the ability to contract and is resistant to induction of arrhythmia. Although adaptive changes in the heart prior to hibernation may be involved in the cold-resistant property, it remains unclear whether these changes are sufficient for maintaining cardiac pulsatility under an extreme hypothermic condition. We forcibly induced hypothermia in Syrian hamsters by pentobarbital anesthesia combined with cooling of the animals. This allows reproduction of a hypothermic condition in the absence of possible hibernation-specific reactions. Unlike hypothermia in natural hibernation, the forced induction of hypothermia caused atrioventricular block. Furthermore, J-waves, which are typically observed during hypothermia in nonhibernators, were recorded on an ECG. The origin of the J-wave seemed to be related to irreversible injury of the myocardium, because J-waves remained after recovery of body temperature. An abnormal ECG was also found when hypothermia was induced in hamsters that were well adapted to a cold and darkened environment or hamsters that had already experienced hibernation. These results suggest that acclimatization prior to hibernation does not have a crucial effect at least on acquisition of cardiac resistance to low temperature. In contrast, an abnormal ECG was not observed in the case of hypothermia induced by central administration of an adenosine A1-receptor agonist and subsequent cooling, confirming the importance of the adenosine system for inducing hibernation. Our results suggest that some specific mechanisms, which may be driven by a central adenosine system, operate for maintaining the proper cardiac pulsatility under extreme hypothermia.

Bradyarrhythmia in an anaesthetised, elderly, hypertensive cat

A 14-year-old neutered male domestic shorthaired cat was presented to the University Veterinary Centre Sydney for evaluation and treatment of dental disease. This cat developed an unusual bradyarrhythmia under anaesthesia. The possible causes and treatment of the dysrythmia are discussed.

The effect of cerebral hypothermia on white and grey matter injury induced by severe hypoxia in preterm fetal sheep

Prolonged, moderate cerebral hypothermia is consistently neuroprotective after experimental hypoxia-ischaemia; however, it has not been tested in the preterm brain. Preterm (0.7 gestation) fetal sheep received complete umbilical cord occlusion for 25 min followed by cerebral hypothermia (fetal extradural temperature reduced from 39.4 +/- 0.3 to 29.5 +/- 2.6 degrees C) from 90 min to 70 h after the end of occlusion or sham cooling. Occlusion led to severe acidosis and profound hypotension, which recovered rapidly after release of occlusion. After 3 days recovery the EEG spectral frequency, but not total intensity, was increased in the hypothermia-occlusion group compared with normothermia-occlusion. Hypothermia was associated with a significant overall reduction in loss of immature oligodendrocytes in the periventricular white matter (P < 0.001), and neuronal loss in the hippocampus and basal ganglia (P < 0.001), with suppression of activated caspase-3 and microglia (isolectin-B4 positive). Proliferation was significantly reduced in periventricular white matter after occlusion (P < 0.05), but not improved after hypothermia. In conclusion, delayed, prolonged head cooling after a profound hypoxic insult in the preterm fetus was associated with a significant reduction in loss of neurons and immature oligodendroglia, with evidence of EEG and haemodynamic improvement after 3 days recovery, but also with a persisting reduction in proliferation of cells in the periventricular region. Further studies are required to evaluate the long-term impact of cooling on brain growth and maturation.

Early post-convulsive prolongation of QT time in children

AIM

An important differential diagnosis of seizures in childhood is the long QT syndrome. Childhood epilepsy occurs about 400 times more often than long QT syndrome. We had observed children with slight post-convulsive prolongation of QT time more often than the reported incidence of long QT syndrome. We therefore conducted a prospective study to define the characteristics of post-convulsive prolongation of QT time in children.

METHODS

We investigated 30 consecutive infants and children (3 mo to 14 y) within 2 h after seizures. A follow-up ECG was obtained 1-9 d later. We also obtained ECGs from 30 healthy age- and gender-matched controls. We calculated the QT interval corrected for heart rate (QTc) by Bazett's formula in leads II, V5, V6, QT dispersion and the number of notched T waves.

RESULTS

We found a QTc interval of more than 440 ms in one or more leads in the first ECG in seven of 30 infants and children compared to 1 of 30 in the follow-up ECG (p=0.0003) and two of 30 in the healthy controls (p=0.14). Average QTc was higher for all leads in the first ECG. This was statistically significant in leads II (414 vs 402 ms, p=0.008), V5 (416 vs 404 ms, p=0.002) and V6 (415 vs 399 ms, p=0.001). Compared to healthy controls, QT dispersion was slightly larger in the early post-convulsive ECG (36 vs 31 ms, p=0.03). Notched T waves occurred more frequently in the early compared to the late post-convulsive ECGs (p=0.009).

CONCLUSION

Slight to moderate post-convulsive prolongation of the QT interval is not rare but transient in paediatric patients.

The Clinical Value of the ECG in Noncardiac Conditions

The ECG is an indispensable tool in the ICU for the detection and diagnosis of heart disease. ECG abnormalities however can be present in a wide variety of noncardiac conditions, complicating the differential diagnosis with primary cardiac pathology. This overview discusses the ECG abnormalities and their pathophysiologic basis in the most frequently encountered noncardiac conditions, such as electrolyte abnormalities, pulmonary embolism, CNS diseases, esophageal disorders, hypothermia, and drug-related and other conditions. Knowledge of the characteristic ECG changes may provide early clues to the presence of these disorders, the prompt recognition of which can be life saving.

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

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.

The importance of brain temperature in patients after severe head injury: relationship to intracranial pressure, cerebral perfusion pressure, cerebral blood flow, and outcome

Brain temperature was continuously measured in 58 patients after severe head injury and compared to rectal temperature, intracranial pressure, cerebral blood flow, and outcome after 3 months. The temperature difference between brain and rectal temperature was also calculated. Mild hypothermia (34-36 degrees C) was also used to treat uncontrollable intracranial pressure (ICP) above 20 mm Hg when other methods failed. Brain and rectal temperature were strongly correlated (r = 0.866; p < 0.001). Four groups were identified. The mean brain temperature ranged from 36.9 +/- 0.4 degrees C in the normothermic group to 38.2 +/- 0.5 degrees C in the hyperthermic group, 35.3 +/- 0.5 degrees C in the mild therapeutic hypothermia group, and 34.3 +/- 1.5 degrees C in the hypothermia group without active cooling. The mean DeltaT(br-rect) was positive for patients with a T(br) above 36.0 degrees C (0.0 +/- 0.5 degrees C) and negative for patients during mild therapeutic hypothermia (-0.2 +/- 0.6 degrees C) and also in those with a brain temperature below 36 degrees C without active cooling (0.8 +/- -1.4 degrees C) - the spontaneous hypothermic group. The cerebral perfusion pressure (CPP) was increased significantly by active cooling compared to the normothermic and hyperthermic groups. The mean cerebral blood flow (CBF) in patients with a brain temperature between 36.0 degrees C and 37.5 degrees C was 37.8 +/- 14.0 mL/100 g/min. The lowest CBF was measured in patients with a brain temperature <36.0 degrees C and a negative brain-rectal temperature difference (17.1 +/- 14.0 mL/100 g/min). A positive trend for improved outcome was seen in patients with mild hypothermia. Simultaneous monitoring of brain and rectal temperature provides important diagnostic and prognostic information to guide the treatment of patients after severe head injury (SHI) and the wide differentials that can develop between the brain and core temperature, especially during rapid cooling, strongly supports the use of brain temperature measurement if therapeutic hypothermia is considered for head injury care.

The electrocardiogram in hypothermia

OBJECTIVE

Hypothermia is known to adversely affect the electrocardiogram (ECG) in many cases. This study set out to determine the incidence of defined cardiac dysrhythmias, J waves, and conduction abnormalities in urban hypothermia.

METHODS

A prospective, multicenter study was carried out to determine the incidence of defined cardiac rhythms in patients suffering from accidental urban hypothermia. The ECGs were independently analyzed by 2 of the authors and placed into 1 of 6 rhythm categories.

RESULTS

Seventy-three ECGs were analyzed. Normal sinus rhythm was the most common rhythm (41%). Overall mortality was 36% (26/73). J waves occurred in 36% of survivors and 38% of non-survivors and were, therefore, not prognostic. Shivering artifact was present in 66% of survivors and 38% of nonsurvivors. Although there was no statistically significant association between J waves and survival (P = .21), the presence of shivering artifact was associated with survival in severe hypothermia (P = .047). Atrial fibrillation and junctional bradycardia were both associated with high mortality.

CONCLUSIONS

This study confirms that the ECG is abnormal in the majority of patients suffering from accidental hypothermia. J waves do not appear to be independently prognostic in hypothermia. The results suggest that the inability to mount a shivering response may be associated with a poorer outcome; this finding requires further study.

Accidental hypothermia

Individuals at extremes of age and those who have certain underlying medical conditions are at greatest risk for hypothermia. Hypothermia may occur during any season of the year and in any climate. Prompt recognition of hypothermia and early institution of the rewarming techniques are imperative for a successful outcome with minimal complications. Several rewarming techniques are available and the decision to use any of them depends on the degree of hypothermia, the condition of the patient, and the rewarming rate possible with the technique chosen.

Environment-dependent sports emergencies

As more individuals participate in sports and outdoor activities, the frequency of environmentally related illnesses will increase. Participants in sporting events of long duration and those requiring particularly inclement weather and adverse conditions are especially prone to developing injury. Hypothermia, heat-related illnesses, and high-altitude illnesses are multisystem emergencies that require immediate, specific therapeutic maneuvers. Physicians must be able to recognize the signs and symptoms of these medical emergencies and institute definitive care. Rapid core rewarming, airway control, and prolonged cardiopulmonary resuscitation are the key factors in managing the hypothermic patient. Adequate cooling and volume resuscitation provide the basis for treating the hyperthermic patient. The patient with high altitude-related illness should be returned to a lower elevation and given supplemental oxygen. Specific intervention depends on the patient's presentation. Environmental illnesses cause severe morbidity and mortality and are frequently within the control of the sport participant. Thus physicians must educate their patients on basic preventive measures if they are going to participate in outdoor sporting activities.

Prolonged resuscitation in acute deep hypothermia

A case report is described with successful outcome of prolonged cardiopulmonary resuscitation in a 30-year-old man suffering from acute deep hypothermia. His lowest temperature recorded was 23 degrees C. Continuous external cardiac massage was required for a total of 4.5 h whilst rewarming was instituted. The patient eventually left hospital with no permanent sequelae. A review of hypothermia follows, emphasising some important management principals and pitfalls.