ELECTROCARDIOGRAPHIC CHANGES IN ACCIDENTAL HYPOTHERMIA

[Corrected QT interval during therapeutic hypothermia in hypoxic ischaemic encephalopathy]

INTRODUCTION

Therapeutic hypothermia is the standard treatment for hypoxic ischaemic encephalopathy (HIE), despite not knowing all its effects and complications. Sinus bradycardia is one of the consequences of cooling that has been previously documented in the literature, but little is known about the cardiac electrical activity in these patients.

OBJECTIVE

To determine the corrected QT (QTc) interval in newborns treated with therapeutic hypothermia for HIE.

MATERIAL AND METHODS

A prospective observational study was conducted in all patients treated with hypothermia for HIE that were admitted to our Unit between November 2012 and October 2013. ECGs were performed during hypothermia (every 24h), during the re-warming period (at 34.5°C, 35.5°C, 36.5°C), and on the 7th day of life.

RESULTS

A total of 19 patients were included. A prolonged QTc was observed in all patients during hypothermia, and 84% (n=16) had prolonged QTc in all the ECGs during treatment. In 3 patients, one of the ECGs did not have a prolonged QTc. After re-warming, the QTc interval returned to normal in all patients. No statistically significant differences were seen when the degree of HIE (P=.192) or the use of inotropic support (P=.669) were considered.

CONCLUSIONS

Therapeutic hypothermia applied to asphyxiated newborns with HIE seems to induce a QTc prolongation that resolves when the patient regains physiological temperature.

Heart Rate and Arterial Pressure Changes during Whole-Body Deep Hypothermia

Whole-body deep hypothermia (DH) could be a new therapeutic strategy for asphyxiated newborn. This retrospective study describes how DH modified the heart rate and arterial blood pressure if compared to mild hypothermia (MH). Fourteen in DH and 17 in MH were cooled within the first six hours of life and for the following 72 hours. Hypothermia criteria were gestational age ≥36 weeks; birth weight ≥1800 g; clinical signs of moderate/severe hypoxic-ischemic encephalopathy. Rewarming was obtained in the following 6-12 hours (0.5°C/h) after cooling. Heart rates were the same between the two groups; there was statistically significant difference at the beginning of hypothermia and during rewarming. Three babies in the DH group and 2 in the MH group showed HR < 80 bpm and QTc > 520 ms. Infant submitted to deep hypothermia had not bradycardia or Qtc elongation before cooling and after rewarming. Blood pressure was significantly lower in DH compared to MH during the cooling, and peculiar was the hypotension during rewarming in DH group. Conclusion. The deeper hypothermia is a safe and feasible, only if it is performed by a well-trained team. DH should only be associated with a clinical trial and prospective randomized trials to validate its use.

Changes in the PQRST intervals and heart rate variability associated with rewarming in two newborns undergoing hypothermia therapy

BACKGROUND

Little is known about the effects of hypothermia therapy and subsequent rewarming on the PQRST intervals and heart rate variability (HRV) in term newborns with hypoxic-ischemic encephalopathy (HIE).

OBJECTIVES

This study describes the changes in the PQRST intervals and HRV during rewarming to normal core body temperature of 2 newborns with HIE after hypothermia therapy.

METHODS

Within 6 h after birth, 2 newborns with HIE were cooled to a core body temperature of 33.5 degrees C for 72 h using a cooling blanket, followed by gradual rewarming (0.5 degrees C per hour) until the body temperature reached 36.5 degrees C. Custom instrumentation recorded the electrocardiogram from the leads used for clinical monitoring of vital signs. Generalized linear mixed models were calculated to estimate temperature-related changes in PQRST intervals and HRV.

RESULTS

For every 1 degrees C increase in body temperature, the heart rate increased by 9.2 bpm (95% CI 6.8-11.6), the QTc interval decreased by 21.6 ms (95% CI 17.3-25.9), and low and high frequency HRV decreased by 0.480 dB (95% CI 0.052-0.907) and 0.938 dB (95% CI 0.460-1.416), respectively.

CONCLUSIONS

Hypothermia-induced changes in the electrocardiogram should be monitored carefully in future studies.

The effect of temperature on the QTc interval in the newborn infant receiving extracorporeal membrane oxygenation (ECMO)

OBJECTIVE

To explore the changes in the QTc interval during mild hypothermia in neonates receiving extracorporeal membrane oxygenation (ECMO).

DESIGN

Twenty seven neonates (median gestation 40 weeks; range 33-41 weeks) enrolled in a pilot study of mild hypothermia were studied during the first five days of ECMO. The first group (N=7) were maintained at 37 degrees C throughout the study period. Subsequent groups (N=5) were cooled to 36 degrees C, 35 degrees C and 34 degrees C respectively for twenty four hours and the final group to 34 degrees C for forty eight hours before being rewarmed to 37 degrees C. Using a 24 h digital monitor, the QT and QTc intervals were recorded continuously during the cooling and rewarming period and validated using standard 12 lead electrocardiograms. Patients were carefully assessed clinically and routine biochemistry (including magnesium and calcium) laboratory tests measured pre ECMO and at timed intervals during cooling and rewarming.

RESULTS

The mean difference between the continuous digital and 12 lead ECG values for QTc was -13.3 ms. During the first 24 h of cooling, the mean (95th centile) values for the digitally measured QTc interval at 37 degrees C=431(506) milliseconds (ms); 36 degrees C=459(521) ms; 35 degrees C=445(516) ms; 34 degrees C=465(531) ms; 34 degrees C for 48 h=466(521) ms. During this period overall QTc increased by 3.12 ms (95% confidence intervals 6.17 to 0.84; p=0.04) for each degree fall in body temperature. During rewarming, there was no significant relationship between QTc and temperature change. No serious arrhythmias were during cooling. Using univariate analysis, no relationship was found between QTc and electrolytes, heart rate and blood pressure.

CONCLUSIONS

QTc shows significant variability in individuals, and only a small proportion of this can be explained by rectal temperature. Mild hypothermia was not associated with serious cardiac arrhythmias.

Effects of season and temperature acclimation on electrocardiogram and heart rate of toads

Electrocardiogram (ECG) was recorded from unrestrained toads of two species, one resistant (Bufo viridis) and the other sensitive (B. regularis) to low temperatures. Although the temperature-sensitive species could not survive at temperatures below 7-8 degrees C, heart rate is linearly and similarly related to ambient temperature in the two species. In B. viridis, the cold resistant species, heart rate in winter was 25% lower than in summer, and the dependency of heart rate on temperature was reduced by 50% in toads acclimated to 10 degrees C in winter. It was not possible to disclose any effect of acclimation on heart rate in summer, due to the large variation in the recorded values. The QRS amplitude in the ECG was considerably reduced at low temperature only in B. viridis. It is concluded that differences in cardiac activity cannot account for the distinct difference in thermal relations of the two species, and that it should reside at other regulatory levels.

The Osborn wave in hypothermia

A patient is reported who presented with fatal hypothermia. The electrocardiographic changes of a sinus bradycardia, prolonged QT interval and Osborn waves were documented and correlated with body temperature. The possible genesis of these electrocardiographic changes is reviewed in this article.

The cardiac rhythm in accidental hypothermia

An analysis of the ECGs from 60 patients with accidental hypothermia reveals several facts concerning rhythmicity. Echocardiograms were useful to determine the precise rhythm in cases without discernible P waves. Atrial fibrillation (AF) was unusual in mild hypothermia (greater than 32.0 degrees C). AF was often observed in moderate (32.0-26.0 degrees C) and moderately deep (less than 26.0 degrees C) hypothermia. However, about half of the cases with moderately deep hypothermia remained in sinus, atrial, or junctional rhythm. AF was usually converted to sinus rhythm without any antiarrhythmic agents soon after normothermia was restored. There was no significant difference in the mortality rate between the AF and non-AF groups. Therefore, no aggressive treatment for this arrhythmia seems to be necessary.

The J wave in accidental hypothermia

Electrocardiograms of 50 patients with accidental hypothermia were reviewed with regard to the J wave with the following results: (1) J waves were observed in 40 of 50 cases. (2) J waves were recorded most frequently in leads II or V6 (34 of 40 cases, 85%). However, in deep hypothermia, the J wave was often most prominent in leads V3 or V4. (3) The size of the J wave appeared to be related to body temperature. Below 30 degrees C, large J waves were often observed; above 30 degrees C, J waves were usually smaller. (4) J waves were not distinctive in the cases with clockwise rotation. (5) The appearance and the size of the J waves seemed not to be associated with the arterial pH. (6) The J wave decreased in size along with rise of the body temperature. However, a small J wave persisted in many cases even after normothermia was restored. It was also difficult to distinguish these small J waves from small notches at the QRS-ST junction which are sometimes observed in normal individuals not subjected to hypothermia.

Evolutionary changes in the electrocardiogram of severe progressive hypothermia

A patient is reported who developed progressive hypothermia during therapy for adult respiratory distress syndrome. Electrocardiographic changes (sinus bradycardia, prolonged PR interval, prolonged QTc interval, "Osborn waves") were documented and correlated with body temperature. The significance of these changes is discussed and the relationship between the degree of hypothermia and the presence of "Osborn waves" is noted.

Effect of cold isotonic glucose infusion on A-V nodal conduction

His bundle electrograms were performed in eight patients with organic heart disease. Recordings were made at various rates utilizing right atrial pacing. A solution of 100 cc of iced cold 5% glucose and water was infused through a cardiac catheter placed at the level of the tricuspid valve. The P-A, A-H, H-Q and H-S intervals were determined before and immediately after the cold water infusion. A significant prolongation of the A-H interval occurred with negligible effects on the P-A, H-Q and H-S intervals. At the atrial pacing rate of 100/min the average A-H interval increased from the control value of 116 to 125 msec after the infusion (P less than 0.02); at the pacing rate of 140/min, the A-H interval increased from 147 to 158 msec (P less than 0.01). This represents an impairment in conduction through the atrioventricular node.

Accidental hypothermia

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Accidental exposure to freezing: clinical and laboratory observations during convalescence from near-fatal hypothermia

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