Hypothermia and the resuscitation of asphyxiated fetal rhesus monkeys

Circulatory responses to asphyxia differ if the asphyxia occurs in utero or ex utero in near-term lambs

Background

A cornerstone of neonatal resuscitation teaching suggests that a rapid vagal-mediated bradycardia is one of the first signs of perinatal compromise. As this understanding is based primarily on fetal studies, we investigated whether the heart rate and blood pressure response to total asphyxia is influenced by whether the animal is in utero or ex utero.

Methods

Fetal sheep were instrumented at ∼139 days of gestation and then asphyxiated by umbilical cord occlusion until mean arterial blood pressure decreased to ∼20 mmHg. Lambs were either completely submerged in amniotic fluid (in utero; n = 8) throughout the asphyxia or were delivered and then remained ex utero (ex utero; n = 8) throughout the asphyxia. Heart rate and arterial blood pressure were continuously recorded.

Results

Heart rate was higher in ex utero lambs than in utero lambs. Heart rates in in utero lambs rapidly decreased, while heart rates in ex utero lambs initially increased following cord occlusion (for ∼1.5 min) before they started to decrease. Mean arterial pressure initially increased then decreased in both groups.

Conclusions

Heart rate response to asphyxia was markedly different depending upon whether the lamb was in utero or ex utero. This indicates that the cardiovascular responses to perinatal asphyxia are significantly influenced by the newborn's local environment. As such, based solely on heart rate, the stage and severity of a perinatal asphyxic event may not be as accurate as previously assumed.

Hypothermia for hypoxic-ischemic encephalopathy

Moderate to severe hypoxic-ischemic injury in newborn infants, manifested as encephalopathy immediately or within hours after birth, is associated with a high risk of either death or a lifetime with disability. In recent multicenter clinical trials, hypothermia initiated within the first 6 postnatal hours has emerged as a therapy that reduces the risk of death or impairment among infants with hypoxic-ischemic encephalopathy. Prior to hypothermia, no therapies directly targeting neonatal encephalopathy secondary to hypoxic-ischemic injury had convincing evidence of efficacy. Hypothermia therapy is now becoming increasingly available at tertiary centers. Despite the deserved enthusiasm for hypothermia, obstetric and neonatology caregivers, as well as society at large, must be reminded that in the clinical trials more than 40% of cooled infants died or survived with impairment. Although hypothermia is an evidence-based therapy, additional discoveries are needed to further improve outcome after HIE. In this article, we briefly present the epidemiology of neonatal encephalopathy due to hypoxic-ischemic injury, describe the rationale for the use of hypothermia therapy for hypoxic-ischemic encephalopathy, and present results of the clinical trials that have demonstrated the efficacy of hypothermia. We also present findings noted during and after these trials that will guide care and direct research for this devastating problem.

Therapeutic hypothermia: applications in pediatric cardiac arrest

There is a rich history for the use of therapeutic hypothermia after cardiac arrest in neonatology and pediatrics. Laboratory reports date back to 1824 in experimental perinatal asphyxia. Similarly, clinical reports in pediatric cold water drowning victims represented key initiating work in the field. The application of therapeutic hypothermia in pediatric drowning victims represented some of the seminal clinical use of this modality in modern neurointensive care. Uncontrolled application (too deep and too long) and unique facets of asphyxial cardiac arrest in children (a very difficult insult to affect any benefit) likely combined to result in abandonment of therapeutic hypothermia in the mid to late 1980s. Important studies in perinatal medicine have built upon the landmark clinical trials in adults, and are once again bringing therapeutic hypothermia into standard care for pediatrics. Although more work is needed, particularly in the use of mild therapeutic hypothermia in children, there is a strong possibility that this important therapy will ultimately have broad applications after cardiac arrest and central nervous system (CNS) insults in the pediatric arena.

Hypothermia in the resuscitation of the neonate: a glance in my rear-view mirror

Recent research indicates that the outcome of perinatal hypoxia-ischaemia can be improved by hypothermia. This was actually first observed about 40 years ago, as discussed in this article. Clinical application of whole-body hypothermia in the asphyxiated neonate showed a positive effect. Reference is made to previous review articles on the subject. A large experimental study indicated that the degree of protection against hypoxic-ischaemic damage, as judged by the conditioned avoidance response, was dependent on the fall in body temperature. The earlier reported resistance against hypothermia in the asphyxiated neonate is discussed and analysed.

CONCLUSION

Experimental and clinical evidence suggest that hypothermia may be of value in resuscitation of the neonate.

The use of hypothermia: a role in the treatment of neonatal asphyxia?

Perinatal asphyxia remains one of the most devastating neurologic processes. Although the understanding of the pathophysiology after perinatal asphyxia is extensive, there are few therapeutic interventions available to prevent or even mitigate the devastating process that unfolds after injury. The search for a safe and efficacious therapy has prompted scientists and clinicians to consider various promising therapies. One such therapy is therapeutic hypothermia. On the basis of adult, pediatric, and animal research, there is increasing evidence to suggest that therapeutic hypothermia may be an effective intervention to lessen the secondary neuronal injury that ensues after a hypoxic-ischemic insult. In this article the historic and modern-day uses of therapeutic hypothermia are first reviewed. The pathophysiology of neonatal asphyxia is examined next, with emphasis on the changes that occur when therapeutic hypothermia is implemented. Potential side-effects of the therapy in the neonate and the debate over systemic vs selective hypothermia are discussed. Lastly, although hypothermia as a potential treatment modality for neonates with hypoxic-ischemic encephalopathy is supported by numerous studies, the need for well-designed multicenter trials with detailed patient entry criteria and therapeutic conditions is emphasized.

Ventricular fibrillation in a swine model of acute pediatric asphyxial cardiac arrest

STUDY OBJECTIVE

To determine cardiac rhythms in a swine model of acute pediatric asphyxial cardiac arrest.

DESIGN

Prospective electrocardiographic evaluation of 36 piglets.

SETTING

University hospital laboratory.

INTERVENTION

Piglets were acutely asphyxiated by endotracheal tube clamping until 10 min after loss of aortic pulsations. Resuscitative efforts were then provided.

RESULTS

None of the animals had ventricular fibrillation (VF) when loss of aortic pulsations occurred (11 +/- 2 min after clamping). Fourteen of the 36 piglets exhibited VF during the asphyxial insult. VF converted to asystole in four piglets prior to resuscitation. Immediately prior to resuscitation, VF occurred in 10 piglets, asystole in 19 piglets, and bradyarrhythmias in seven piglets.

CONCLUSION

VF occurs frequently in this piglet model of prolonged asphyxial cardiac arrest, consistent with recent observations in pediatric prehospital cardiac arrests. VF occurred late in the asphyxial process.

Regional cerebral blood flow following hypothermic circulatory arrest in newborn dogs

A model of hypothermic circulatory arrest has been developed in newborn dogs which simulates the procedure used for the operative repair of congenital cardiac defects in human infants. Hypothermic circulatory arrest for 1.0 h causes no brain damage, whereas cardiac arrest for 1.75 h results in damage of the cerebral cortex, basal ganglia and to a lesser extent the claustrum and amygdaloid nucleus. In the present study, we determined regional cerebral blood flow (rCBF) during 24 h of recovery from hypothermic circulatory arrest. Newborn nitrous oxide anesthetized and artificially ventilated dogs were cooled to 20 degrees C and subjected to cardiac arrest by the i.v. injection of KCl for either 1.0 or 1.75 h. Thereafter, animals were resuscitated, rewarmed to 37 degrees C, and rCBF measured with [14C]iodoantipyrine at either 2 or 18 h of recovery. Control animals were rendered hypothermic to 20 degrees C without cardiac arrest for 1.0 or 1.75 h prior to rewarming. No alterations in CBF at either 2 or 18 h of recovery were present in any of 16 analyzed structures in animals previously subjected to hypothermic circulatory arrest compared to controls rendered hypothermic alone. A direct linear correlation existed between mean arterial blood pressure and blood flow within frontal, parietal and occipital cortex, occipital white matter, hypothalamus and cerebellar vermis in puppies arrested for 1.75 h and recovered for 2 h, suggesting a loss of CBF autoregulation at this interval. No such association between blood pressure and CBF was apparent at 18 h of recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothermia enhances protective effect of MK-801 against hypoxic/ischemic brain damage in infant rats

Accumulating evidence suggests that the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor may play an important role in hypoxic/ischemic (H/I) brain damage. Accordingly, it has been shown that the NMDA antagonist, MK-801, partially protects the infant rat brain against H/I damage. Here we show that reducing the body temperature of the infant rat also confers partial protection against H/I brain damage and that mild hypothermia plus MK-801 treatment provides total protection against such damage. Relevance of these findings to the prevention of perinatal brain damage in humans is discussed.

Failure of hypothermia as treatment for asphyxiated newborn rabbits

Cooling is known to prolong survival in newborn animals when used before the onset of asphyxia. It has therefore been advocated as a treatment for birth asphyxia in humans. Since it is not possible to cool a human baby before the onset of birth asphyxia, experiments were designed to test the effect of cooling after asphyxia had already started. Newborn rabbits were asphyxiated in 100% nitrogen and were cooled either quickly (drop of 1 degree C in 45 s) or slowly (drop of 1 degree C in 2 min) at varying intervals after asphyxia had started. When compared with controls, there was an increase in survival only when fast cooling was used early in asphyxia. This fast rate of cooling is impossible to obtain in a human baby weighing from 30 to 60 times more than a newborn rabbit. Further litters of rabbits were asphyxiated in utero. After deliver they were placed in environmental temperatures of either 37 degrees C, 20 degrees C, or 0 degrees C and observed for spontaneous recovery. The animals who were cooled survived less often than those kept at 37 degrees C. The results of these experiments suggest that hypothermia has little to offer in the treatment of birth asphyxia in humans.

Scientific basis for current perinatal care

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