The influence of blood gas changes on hyperthermia-induced seizures in developing rats

Thermoregulation in epilepsy

Several aspects of thermoregulation play a role in epilepsy. Circuitries involved in thermoregulation are affected by seizures and epilepsy, hyperthermia may be both cause and result of seizures, and hypothermia may prevent or abort seizures. Autonomic manifestations of seizures including thermoregulatory disturbances are common in a variety of clinical epilepsy syndromes. Experimental hyperthermia has been studied extensively, predominantly to investigate febrile seizures of childhood. In particular prolonged or complex febrile seizures have been associated with the later development of epilepsy in adulthood and the pathophysiology of how febrile seizures cause epilepsy is of tremendous interest. Febrile seizures represent an opportunity to potentially intervene early in life in susceptible individuals and affect epileptogenesis. The pathophysiologic underpinnings of how hyperthermia induces seizures and how this in turn results in epilepsy are controversial, but likely involve multiple factors. Both glutamatergic and GABAergic neurotransmission is affected, and numerous mutations in genes encoding ion channels have been identified. Cytokines such as interleukin-1β have been implicated in febrile seizures as well as susceptibility to provoked seizures later in life. Hyperthermia is a common feature of generalized convulsive status epilepticus, but may also be seen with nonconvulsive seizures, indicating involvement of thermoregulatory centers.

TRPV1 deletion exacerbates hyperthermic seizures in an age-dependent manner in mice

Febrile seizures (FS) are the most common seizure disorder to affect children. Although there is mounting evidence to support that FS occur when children have fever-induced hyperventilation leading to respiratory alkalosis, the underlying mechanisms of hyperthermia-induced hyperventilation and links to FS remain poorly understood. As transient receptor potential vanilloid-1 (TRPV1) receptors are heat-sensitive, play an important role in adult thermoregulation and modulate respiratory chemoreceptors, we hypothesize that TRPV1 activation is important for hyperthermia-induced hyperventilation leading to respiratory alkalosis and decreased FS thresholds, and consequently, TRPV1 KO mice will be relatively protected from hyperthermic seizures. To test our hypothesis we subjected postnatal (P) day 8-20 TRPV1 KO and C57BL/6 control mice to heated dry air. Seizure threshold temperature, latency and the rate of rise of body temperature during hyperthermia were assessed. At ages where differences in seizure thresholds were identified, head-out plethysmography was used to assess breathing and the rate of expired CO₂ in response to hyperthermia, to determine if the changes in seizure thresholds were related to respiratory alkalosis. Paradoxically, we observed a pro-convulsant effect of TRPV1 deletion (∼4min decrease in seizure latency), and increased ventilation in response to hyperthermia in TRPV1 KO compared to control mice at P20. This pro-convulsant effect of TRPV1 absence was not associated with an increased rate of expired CO₂, however, these mice had a more rapid rise in body temperature following exposure to hyperthermia than controls, and the expected linear relationship between body weight and seizure latency was absent. Based on these findings, we conclude that deletion of the TRPV1 receptor prevents reduction in hyperthermic seizure susceptibility in older mouse pups, via a mechanism that is independent of hyperthermia-induced respiratory alkalosis, but possibly involves impaired development of thermoregulatory mechanisms, although at present the mechanism remain unknown.

Haemodynamic and ventilator management in patients following cardiac arrest

PURPOSE OF REVIEW

The purpose of this study is to review the recent literature describing how to assess and treat postcardiac arrest syndrome associated haemodynamics and manage oxygenation and ventilation derangements.

RECENT FINDINGS

Postcardiac arrest syndrome is a well described entity that includes systemic ischemia-reperfusion response, myocardial dysfunction and neurologic dysfunction. Continued resuscitation in the hours to days following return of spontaneous circulation (ROSC) is important to increase the likelihood of long-term survival and neurological recovery. Post-ROSC hypotension is common and associated with worse outcome. Myocardial dysfunction peaks in the first 24  h following ROSC and in survivors resolves over the next few days. Hyperoxemia (paO₂>300  mmHg) and hypoxemia (paO₂<60  mmHg) are associated with worse outcomes and hyperventilation may exacerbate cerebral ischemic injury by decreasing cerebral oxygenation.

SUMMARY

Patients who are successfully resuscitated from cardiac arrest often have hypotension and myocardial dysfunction. Careful attention to haemodynamic and ventilator management targeting normal blood pressure, normoxemia and normocapnia may help to avoid secondary organ injury and potentially improve outcomes.

Hyperoxemia--too much of a good thing?

While avoiding hypoxemia has long been a goal of critical care practitioners, less attention has been paid to the potential for excessive oxygenation. Interest has mounted recently in understanding the clinical effects of hyperoxemia during critical illness, in particular its impact following cardiac arrest. In this issue of Critical Care, Dell’Anna and colleagues review available animal and human data evaluating the impact of hyperoxemia after cardiac arrest. They conclude that while hyperoxemia during cardiopulmonary resuscitation is probably desirable, it should probably be avoided during post-resuscitation care. These conclusions are in line with two broader themes in contemporary critical care: that less may be more; and that it is time to look beyond simply preventing short-term mortality towards longer-term outcomes.

How much oxygen in adult cardiac arrest?

Although experimental studies have suggested that a high arterial oxygen pressure (PaO₂) might aggravate post-anoxic brain injury, clinical studies in patients resuscitated from cardiac arrest (CA) have given conflicting results. Some studies found that a PaO₂ of more than 300 mm Hg (hyperoxemia) was an independent predictor of poor outcome, but others reported no association between blood oxygenation and neurological recovery in this setting. In this article, we review the potential mechanisms of oxygen toxicity after CA, animal data available in this field, and key human studies dealing with the impact of oxygen management in CA patients, highlighting some potential confounders and limitations and indicating future areas of research in this field. From the currently available literature, high oxygen concentrations during cardiopulmonary resuscitation seem preferable, whereas hyperoxemia should be avoided in the post-CA care. A specific threshold for oxygen toxicity has not yet been identified. The mechanisms of oxygen toxicity after CA, such as seizure development, reactive oxygen species production, and the development of organ dysfunction, need to be further evaluated in prospective studies.

Association between hypocapnia and febrile seizures

The purpose of this study is to determine whether hyperthermia-induced hyperventilation with subsequent hypocapnia is relevant to febrile seizures in children. This is only the second study to measure pCO2 and pH values in children with febrile seizures. This prospective case-control study enrolled 18 children who presented with febrile seizures and 18 children who presented with a febrile illness without seizures. Venous blood gas analyses were measured both from the febrile seizure and control group. There was no significant difference in mean blood pH between the febrile seizure and control groups but blood pCO2 was significantly lower in the febrile seizure group. Patients with complex febrile seizures exhibited significantly lower pCO2 levels within 1 hour of seizure onset than patients with simplex febrile seizures. These data indicate that febrile seizures may be associated with hyperventilation and that the ensuing hypocapnia may contribute to the development of febrile seizures.

Origins of temporal lobe epilepsy: febrile seizures and febrile status epilepticus

Temporal lobe epilepsy (TLE) and hippocampal sclerosis (HS) commonly arise following early-life long seizures, and especially febrile status epilepticus (FSE). However, there are major gaps in our knowledge regarding the causal relationships of FSE, TLE, HS and cognitive disturbances that hamper diagnosis, biomarker development and prevention. The critical questions include: What is the true probability of developing TLE after FSE? Are there predictive markers for those at risk? A fundamental question is whether FSE is simply a marker of individuals who are destined to develop TLE, or if FSE contributes to the risk of developing TLE. If FSE does contribute to epileptogenesis, then does this happen only in the setting of a predisposed brain? These questions are addressed within this review, using information gleaned over the past two decades from clinical studies as well as animal models.

Atypical febrile seizures, mesial temporal lobe epilepsy, and dual pathology

Febrile seizures occurring in the neonatal period, especially when prolonged, are thought to be involved in the later development of mesial temporal lobe epilepsy (mTLE) in children. The presence of an often undetected, underlying cortical malformation has also been reported to be implicated in the epileptogenesis process following febrile seizures. This paper highlights some of the various animal models of febrile seizures and of cortical malformation and portrays a two-hit model that efficiently mimics these two insults and leads to spontaneous recurrent seizures in adult rats. Potential mechanisms are further proposed to explain how these two insults may each, or together, contribute to network hyperexcitability and epileptogenesis. Finally the clinical relevance of the two-hit model is briefly discussed in light of a therapeutic and preventive approach to mTLE.

Febrile seizures and temporal lobe epileptogenesis

Febrile seizures (FS) are a common neurological disorder that affects children. Simple FS are thought to be benign but experimental and clinical evidence support that the risk of developing epilepsy after FS increases if the FS are prolonged and the brain is abnormal. In addition, prolonged FS (PFS) have many deleterious long-term effects characterized mainly in the hippocampus but may involve the whole brain and that prompt abortive treatment of PFS may prevent some of the adverse effects. This review focuses on some of the key factors involved in the generation of FS, factors leading to PFS and potential mechanisms and functional correlates leading to temporal lobe epilepsy (TLE).

Febrile seizures: mechanisms and relationship to epilepsy

Studies of febrile seizures have been driven by two major enigmas: first, how these most common of human seizures are generated by fever has not been known. Second, epidemiological studies have linked prolonged febrile seizures with the development of temporal lobe epilepsy, yet whether long or recurrent febrile seizures cause temporal lobe epilepsy has remained unresolved. To investigate these questions, a model of prolonged (complex) febrile seizures was developed in immature rats and mice, permitting mechanistic examination of the potential causal relationships of fever and seizures, and of febrile seizures and limbic epilepsy. Although the model relied on hyperthermia, it was discovered that the hyperthermia-induced secretion of endogenous fever mediators including interleukin-1beta, which contributed to the generation of these 'febrile' seizures. In addition, prolonged experimental febrile seizures provoked epilepsy in a third of the animals. Investigations of the mechanisms of this epileptogenesis demonstrated that expression of specific ion (HCN) channels and of endocannabinoid signaling, may be involved. These may provide novel drug targets for intervention in the epileptogenic process.

Fever, febrile seizures and epilepsy

Seizures induced by fever (febrile seizures) are the most common type of pathological brain activity in infants and children. These febrile seizures and their potential contribution to the mechanisms of limbic (temporal lobe) epilepsy have been a topic of major clinical and scientific interest. Key questions include the mechanisms by which fever generates seizures, the effects of long febrile seizures on neuronal function and the potential contribution of these seizures to epilepsy. This review builds on recent advances derived from animal models and summarizes our current knowledge of the mechanisms underlying febrile seizures and of changes in neuronal gene expression and function that facilitate the enduring effects of prolonged febrile seizures on neuronal and network excitability. The review also discusses the relevance of these findings to the general mechanisms of epileptogenesis during development and points out gaps in our knowledge, including the relationship of animal models to human febrile seizures and epilepsy.

Chemical and biological investigations of a toxic plant from Central Africa, Magnistipula butayei subsp. montana

Magnistipula butayei subsp. montana (Chrysobalanaceae) is known, in the Great Lakes Region, to possess toxicological properties. In this paper, we investigated the acute toxicity (dose levels 50-1600 mg/kg) of its aqueous extract, administered orally to adult Wistar rats. This study demonstrated that the freeze-dried aqueous extract (5%, w/w) possesses high toxicity. The extract caused hypothermia, neurological disorders, including extensor reflex of maximal convulsive induced-seizures at about 2 h after the administered dose, and death occurred (LD50=370 mg/kg) in a dose dependent manner. Blood parameter evaluation revealed slight variations, but these might not have clinical relevance. Histological examination of internal organs (lungs, liver, heart and kidneys) did not reveal any abnormality in the treated group compared to the control. Therefore, it can be concluded that Magnistipula butayei subsp. montana aqueous extract, given orally, is toxic and that its target is the central nervous system. General phytochemical screening revealed that the plant did not contain significant amounts of products known to be toxic, such as alkaloids or cardioactive glycosides, but only catechic tannins, amino acids, saponins and other aphrogen principles in the three parts of the species (fruit, leave and bark).

Pharmacological biotin supplementation maintains biotin status and function in rats administered dietary carbamazepine

Biotin status and function are decreased during oral carbamazepine (CBZ) administration in both humans and rats, but it is not known whether biotin supplementation can prevent these decreases. To test the effectiveness of pharmacologic biotin supplementation during CBZ administration, 55 rats were randomly divided into 4 groups (0.06 mg biotin/kg diet +/- 3.75 g CBZ/kg diet and 6.0 mg biotin/kg diet +/- 3.75 g CBZ/kg diet). CBZ and biotin-supplemented diets began on d 5 and 26, respectively, and continued through d 68. Rats (n = 5/group) were killed on d 5, 26, 47 or 68. CBZ reduced serum and liver free biotin (P < 0.05), whereas biotin supplementation during CBZ administration maintained biotin status. CBZ also decreased specific activities and abundance of biotinylated pyruvate and acetyl CoA carboxylases (PC and ACC, P < 0.05) in brain and liver, whereas biotin supplementation prevented these decreases for ACC. Specific activity of PC was maintained upon biotin supplementation, but the abundance of biotinylated PC remained significantly decreased. Brain and serum lactate were elevated after 68 d of CBZ treatment and were reduced to control lactate concentrations upon biotin supplementation (P < 0.05). Conversion of lactate to pyruvate and simultaneous generation of NADH during biotin supplementation could explain how increases in PC activity occur without changes in the abundance of biotinylated PC because we found NADH to be an activator of PC activity in vitro. These results support the use of biotin supplementation as a concurrent strategy during CBZ administration to help maintain biotin status, function of biotin-dependent enzymes and decrease CBZ-induced lactate accumulation.

The effect of GABAergic system activity on hyperthermia-induced seizures in rats

Developing rats were given GABA antagonists and agonist before electrical seizure discharges were induced by heating the brain of a rat with infra-red rays. The thresholds for the GABA antagonist groups were significantly lower than that for the control, and the threshold for the GABA agonist group was significantly higher than that for the control. These results support the hypothesis that reduced GABAergic system activity underlies febrile seizures.