Minor Changes in Core Temperature Prior to Cardiac Arrest Influence Outcomes

Abstract

Aim:

To investigate whether slight variations in core temperature prior to cardiac arrest (CA) influence short-term outcomes and mitochondrial functions.

Methods and Materials:

Three groups of New Zealand White rabbits (n = 12/group) were submitted to 15 minutes of CA at 38°C (T-38 group), 39°C (T-39), or 40°C (T 40) and 120 minutes of reperfusion. A Sham-operated group (n = 6) underwent only surgery. Restoration of spontaneous circulation (ROSC), survival, hemodynamics, and pupillary reactivity were recorded. Animals surviving to the end of the observation period were euthanized to assess fresh brain and heart mitochondrial functions (permeability transition and oxidative phosphorylation). Markers of brain and heart damages were also measured.

Results:

The duration of asphyxia required to induce CA was significantly lower in the T-40 group when compared to the T-38 group (P < .05). The rate of ROSC was >80% in all groups (P = nonsignificant [ns]). Survival significantly differed among the T-38, T-39, and T-40 groups: 10 (83%) of 12, 7 (58%) of 12, and 4 (33%) of 12, respectively (log-rank test, P = .027). At the end of the protocol, none of the animals in the T-40 group had pupillary reflexes compared to 8 (67%) of 12 in the T-38 group (P < .05). Troponin and protein S100B were significantly higher in the T-40 versus T-38 group (P < .05). Cardiac arrest significantly impaired both inner mitochondrial membrane integrity and oxidative phosphorylation in all groups. Brain mitochondria disorders were significantly more severe in the T-40 group compared to the T-38 group (P < .05).

Conclusion:

Small changes in body temperature prior to asphyxial CA significantly influence brain mitochondrial functions and short-term outcomes in rabbits.

Introduction

More than 500 000 unexpected cardiac arrests (CAs) occur annually in the United States and almost as many in Europe. ^1,2 Prognosis remains poor with less than 10% of CA victims discharged from the hospital. ^{1,3 –5} For decades, it has been known that an unwitnessed CA, an initial rhythm categorized as not shockable (ie, asystole and pulseless activity), and a long delay between collapse and cardiopulmonary resuscitation (CPR) are the most important factors associated with death or irreversible neurologic damage. ⁶ Recently, postresuscitation fever has also been identified as an independent risk factor for poor outcome. ^7,8

The consequence of a resuscitated CA is the interruption, followed by the restoration, of blood flow to all organs, which represents a global ischemia–reperfusion (I/R) insult. In this setting, a growing body of evidence suggests that mitochondrial dysfunctions trigger I/R injury, particularly in both the brain and the heart. ^{9 –13} We, as well as others, have previously reported that the opening of a calcium-sensitive megachannel located in the inner membrane of the mitochondria, called the mitochondrial permeability transition pore (mPTP), is a key event in the pathophysiology of post-CA syndrome. ^10,12,13 Consistent evidence in models other than CA has shown that small changes in temperature during I/R strongly influence the extent of injuries in both the brain and the heart. ^{14 –20} In experimental CA, decreasing temperature during CA (ie, before restoration of spontaneous circulation [ROSC]) should be highly protective. ²¹ The general consensus is that temperature alters resistance to ischemia by modulating cellular metabolism. ^22,23 However, it has also recently been shown that a small temperature decrease during I/R insults limits brain and cardiac damage by preventing both mPTP opening and oxidative phosphorylation impairment. ^11,20 The aim of the present study was to investigate how small changes in body temperature prior to CA may influence both mitochondrial functions and outcomes.

Methods and Materials

The investigation conformed to French laws and the revised “Guide for the Care and Use of Laboratory Animals” by the Institute of Laboratory Animal Resources, Commission on Life Sciences, National Research Council (National Academy Press, Washington, DC, 1996). All experiments were approved by the Lyon I Claude Bernard University Committee for Animal Research.

Animal Preparation

Male New Zealand White rabbits (2.5-3.5 kg) were anesthetized by intramuscular injection of xylazine (5 mg/kg) and ketamine (50 mg/kg), as described previously. ^10,13 An intravenous (iv) injection infusion of a mixture of xylazine (25 µg/kg/min) and ketamine (50 µg/kg/min) was then maintained throughout the experiment. A tracheotomy was performed, and rabbits underwent mechanical ventilation with 30% oxygen.

A 16G catheter was inserted into the right jugular vein for administration of drugs and fluids. Another 16G catheter was inserted in the right internal carotid artery to measure aortic blood pressure. A left thoracotomy was performed in the fourth left intercostal space to expose the heart. A 15-minute stabilization period was then observed before experimentation.

Cardiac Arrest and Resuscitation

Rabbits were paralyzed with an iv injection of pancuronium bromide (50 µg/kg). As previously reported in our well-validated model of rabbit CA, primary asphyxial CA was induced by the withdrawal of mechanical ventilation after curarization. ^10,13 After 15 minutes of untreated CA, resuscitation was started with the resumption of mechanical ventilation with 100% oxygen, open chest cardiac massage with dissecting forceps (≈200 bpm), and iv administration of epinephrine (20 µg/kg) every 3 to 5 minutes until ROSC. ¹⁰ All heart massages were performed by the same investigator.

Restoration of spontaneous circulation was defined as the return of an organized cardiac rhythm for at least 2 minutes, with mean arterial pressure (MAP) >15 mm Hg and end-tidal CO₂ (EtCO₂) >15 mm Hg. Resuscitative efforts were stopped, and the animal was declared dead if the EtCO₂ was less than 10 mm Hg for more than 5 minutes or in the absence of ROSC after 30 minutes of CPR.

Experimental Protocol

After the stabilization period, animals were randomly assigned to 1 of the 4 following groups: Sham-operated group (surgery without CA), T-38, T-39, or T-40 groups. These groups correspond to the body temperatures at which the CA was performed (38°C, 39°C, and 40°C, respectively) and which were stabilized by means of a heating lamp or ice packs, as appropriate. In all CA groups, CPR was followed by 120 minutes of reperfusion. Normothermia was defined in the rabbits as a physiologic esophageal temperature of 39°C. ²⁴

Measures

Heart rate (HR) and MAP were continuously monitored on a multichannel recorder (Model 4000; Gould Inc, Cleveland, Ohio) and saved on a personal computer-based data acquisition system supported by IOX software (IOX 1.567; Emka Technologies, France). Esophageal temperature was also continuously monitored (Monitor 1165A; Hewlett Packard, Louisville, Kentucky). The EtCO₂, used as a surrogate marker of cardiac output, was also monitored. ²⁵

The duration of asphyxia before CA, the occurrence of ROSC, the duration of heart massage before ROSC, the epinephrine dose required to obtain the first ROSC episode, and the duration of survival were all recorded. Pupillary reactivity to light, a rough functional estimate of cerebral damage after CA, was defined as present when pupils constricted more than 1 mm. ²⁶

Preparation of Brain and Heart Isolated Mitochondria

Animals surviving to the end of the observation period (ie, 120 minutes after CA) were euthanized, and fresh mitochondria were isolated from the brain and heart tissue. As previously described, mitochondria from the left ventricular anterior wall and the left side of the cerebral cortex were isolated by differential centrifugation in cold isolation buffer containing 70 mmol/L sucrose, 210 mmol/L mannitol, and 1 mmol/L EGTA in Tris/HCL, pH 7.4 and resuspended in the same buffer devoid of calcium chelating agent. ^{10,13,27 –29} Mitochondria were kept over ice prior to experiments.

Calcium Retention Capacity and oxidative Phosphorylation

The calcium retention capacity (CRC), which is an index of the resistance of the mPTP to opening following matrix Ca²⁺ accumulation, was determined on fresh mitochondria using the calcium-sensitive probe calcium green-5N (Molecular Probes, Eugene, Oregon), as described previously. ^10,13,27 Oxygen consumption was determined by the polarographic method of Chance and Williams using a Clark oxygen electrode. State 3 (adenosine diphosphate [ADP] stimulated), state 4 (ADP limited), and respiratory control index (RCI: state 3/state 4) were determined by oxygraphy (Oroboros Oxygraph, Paar, Austria), as described previously. ^10,13,27,30 Electron donors to complex I (glutamate 5 mmol/L and malate 5 mmol/L) were used for both CRC and respiration assays.

Biochemical Markers of Cardiac and Brain Damages

At the end of the protocol, troponine Ic was measured at an off-site reference laboratory. Blood level of protein S100B, a marker of neuronal lysis, was also determined after 120 minutes of reperfusion, by using an ELISA test, according to the manufacturer’s instructions (KA0037; Abnova, Jhongli-city, Taiwan). ³¹

Statistical Analysis

Results were analyzed with the Graphpad Prism 6 software (GraphPad Software, La Jolla, California). Data were expressed as mean ± standard error of the mean or number (%). Comparisons of categorical variables were performed using 2-sided Fisher exact test. Normality of continuous data was assessed using the Kolmogorov-smirnov test, and continuous variables were compared using 1-way analysis of variance (ANOVA) or by the Kruskal-Wallis test, as appropriate. Comparisons between time-based measurements within each group were performed with 2-way ANOVA with repeated measures on 1 factor. The effect of different temperatures on survival was compared using Kaplan-Meier survival curves (long-rank test). Statistical significance was defined as a value of P < .05.

Results

Forty-two rabbits were used in this study (Sham: n = 6, T-38: n = 12, T-39: n = 12, and T-40: n = 12). The baseline temperature of the animals was 38.8°C ± 0.2°C. As intended, temperature at the onset of CA was significantly different (P < .05) among the 3 experimental groups (Figure 1).

Figure 1.

Temperature and hemodynamics. Temperature (panel A), end-tidal CO₂ (EtCO₂, panel B), heart rate (panel C), and mean arterial pressure (MAP, panel D) were plotted according to time at baseline and following restoration of spontaneous circulation. T-38, T-39, and T-40 indicate the groups of rabbits with a core temperature at 38°C, 39°C, and 40°C prior to cardiac arrest (CA), respectively (n = 12/group). *P < .05 versus T-38 group.

Cardiac Arrest, Resuscitation, and Outcomes

As presented in Table 1, the duration of asphyxia before CA was significantly lower in the T-40 group (P < .05 compared to T-39 and T-38). The rate of ROSC was >80% in the 3 groups (P = ns). Among survivors, the dose of epinephrine and the CPR time to ROSC did not differ significantly (P = ns). Ventricular fibrillation during resuscitation was recorded in 0, 1, and 3 rabbits in the T-38, T-39, and T-40 groups, respectively (P = ns). Although 10 (83%) of the 12 rabbits survived after CA in the T 38 group, only 7 (58%) of 12 and 4 (33%) of 12 survived in the T-39 and T-40 groups, respectively (P = .027; Figure 2). Pupillary reactivity was absent 120 minutes after CA in all rabbits at the higher temperature (ie, T-40), whereas it was present in 8 (67%) of the 12 rabbits in the T-38 group at the end of the protocol (P < .05).

Table 1.

Cardiopulmonary Resuscitation.

	Asphyxia, s	Heart massage, s	Epinephrine, µg/kg	ROSC, n (%)
T-38	416 ± 14	130 ± 22	27 ± 5	12/12 (100)
T-39	372 ± 9	211 ± 68	31 ± 5	11/12 (92)
T-40	319 ± 10a,b	242 ± 57	39 ± 4	10/12 (83)

Abbreviation: ROSC, Restoration of spontaneous circulation.

^a P < .05 versus T-38 group.

^b P < .05 versus T-39 group.

Figure 2.

Survival. Kaplan-Meier survival curves in the different experimental groups submitted to 15 minutes of asphyxial cardiac arrest (CA). Survival was higher when core temperature was lower at the onset of CA (P < .05 by log-rank test). T-38, T-39, and T-40 indicate the groups of rabbits with a core temperature at 38°C, 39°C, and 40°C prior to CA, respectively (n = 12/group).

Hemodynamics

As shown in Figure 1, there was no difference in baseline hemodynamics between the CA groups, with the exception of HR, which was significantly lower in the T-38 group (P < .05 compared to the T-39 and T-40 groups). Hemodynamics did not change significantly over time in the Sham group (data not shown). The MAP was severely impaired in all groups after CA (P < .05 compared to baseline; Figure 1). In the first minute of heart massage, MAP reached 20 ± 2 mm Hg (P = ns among groups). When compared to the T-38 group, MAP was significantly lower 5 minutes after CA in the T-40 group (P < .05). A similar trend was observed during the rest of the follow-up. A significant decrease in EtCO₂ after CA was observed in the T-40 group (Figure 1).

Biochemical Markers of Cardiac and Brain Damage

The T-40 group displayed a significant (P < .05) increase in troponin Ic (43 ± 12 μg/L) when compared to the T-38 group (13 ± 3 μg/L) and Sham (1 ± 1 μg/L; n = 4-6/group). Blood level of protein S100B was significantly (P < .05) higher in the T-40 group (1201 ± 155 pg/L) than in the T-38 group and Sham groups (483 ± 15 and 263 ± 22 pg/L, respectively; n = 4-6/group).

Mitochondrial Functions

As depicted in Figure 3, the amount of Ca²⁺ required to open the mPTP of mitochondria isolated from both brain and heart (ie, the CRC) was significantly decreased after resuscitated CA in all groups when compared to the Shams (P < .05). In addition, the susceptibility to mPTP opening was significantly increased in brain mitochondria from the T-40 group when compared to the T-38 group (P < .05; Figure 3). A similar trend was observed in the heart (P = not significant; Figure 3). As shown in Table 2, ADP-stimulated respiration was significantly decreased after CA in both brain and heart mitochondria from the T-39 and the T-40 group when compared to the Shams (P < .05). These anomalies were all the more significant in brain mitochondria from the T-40 group (Table 2).

Figure 3.

Calcium retention capacity. Panel A: calcium retention capacity (CRC), an indicator of mitochondrial permeability transition, was significantly reduced in brain mitochondria isolated from all CA groups (T-38, T-39, and T-40 indicate the groups of rabbits with a core temperature at 38°C, 39°C, and 40°C prior to CA, respectively; n = 4-7/group). The higher the core temperature prior to cardiac arrest, the lower the CRC in brain mitochondria. Panel B: CRC was also significantly decreased in mitochondria isolated from hearts after resuscitated CA but no significant difference was observed between CA groups (n = 4-7/group). *P < .05 versus Sham; †P < .05 versus T-38 group. CA indicates cardiac arrest.

Table 2.

Oxidative Phosphorylation.^a

	State 3	State 4	RCI
Heart
Sham	93 ± 10	13 ± 2	6.8 ± 0.3
T-38	65 ± 8	9 ± 2	7.0 ± 1.0
T-39	44 ± 4b	8 ± 1	5.9 ± 0.5
T-40	57 ± 6b	11 ± 3	5.3 ± 1.0
Brain
Sham	42 ± 2	5 ± 1	8.3 ± 0.6
T-38	27 ± 1b	5 ± 1	6.6 ± 1.2
T-39	21 ± 3b	5 ± 1	4.5 ± 0.7b
T-40	9 ± 2b,c,d	4 ± 0	2.2 ± 0.3b,c

Abbreviation: RCI, respiratory control index.

^aState 3 and state 4 are expressed as nanograms-atoms of oxygen/min/mg proteins; RCI (state 3/state 4; n = 4-7/group).

^b P < .05 versus Sham group.

^c P < .05 versus T-38 group.

^d P < .05 versus T-39 group.

Discussion

This study demonstrates that small changes in body temperature prior to CA (ie, close to physiological values) significantly alter brain mitochondrial functions and short-term outcomes in rabbits. Most deaths that occur after successfully resuscitated CA are linked to the post-CA syndrome that encompasses cardiovascular failure and anoxoischemic brain damage. ³² In the present study, we showed that minor changes in body temperature (as small as 1°C-2°C) prior to CA have a significant impact on hemodynamics, survival, heart damage, and neurological outcomes. Our results are in agreement with previous studies demonstrating that variations in the same magnitude in per ischemic temperatures critically determine the extent of I/R injury in brain, heart, kidney, and liver. ^{14 –20,33,34} It has been shown experimentally that changes in temperature during coronary occlusion, within a physiologic range, strongly affect myocardial infarct size and, consequently, the risk of heart failure. ^{17 –19} Using stroke models, it has also been previously reported that small perischemic changes in temperature strongly alter neurological outcomes. ^{14 –16,35} For example, Wass et al showed that the neurological function score was subnormal when core temperature was decreased by only 1°C during transient global cerebral ischemia, whereas a 1°C increase leads to irreversible neurologic deficits. ¹⁶ To date, it has only been shown, in experimental CA that mild therapeutic hypothermia (induced early after resuscitation) improves both hemodynamics and the neurologic status. ^36,37 Our work extends this demonstration of the role of temperature to the period before CA. Indeed, we have found that perischemic temperature influences a clinically relevant brain stem reflex (ie, pupillary reactivity), the degree of neuronal lysis as assessed by protein S-100B, as well as the severity of the postresuscitation cardiovascular failure. Our experimental results are also consistent with clinical studies. Zeiner et al reported that an increase in temperature by only 1°C following CA doubled the risk of an unfavorable neurologic recovery in humans. ⁷ In the same way, Nielsen et al recently demonstrated that lowering the body temperature by 1°C after out-of-hospital CA was as effective as aiming for a temperature of 32°C to 34°C to improve neurological outcomes. ³⁸

Numerous theories have been postulated to explain the effect of temperature changes on I/R injury. ^22,23 It is presumed that the beneficial effects of lowering the per-ischemic temperature are mostly due to a slowing of cell metabolism. ^22,23 The reverse is true for hyperthermia. However, our current understanding is that the alteration in metabolic rate is only one of the many mechanisms underlying the effects of temperature. Thus, temperature may modulate cell signalling pathways (eg, survival kinases), excitotoxicity, blood–brain barrier permeability, inflammation, oxidative stress, and ionic fluxes, among others. ^22,23,39 Regarding these last 2 mechanisms, reactive oxygen generation and mitochondrial calcium load are of particular interest because they are major regulators of mPTP opening. ^40,41 Under normal conditions, the mitochondrial inner membrane is impermeable to almost all metabolites and ions, and the mPTP is in a closed conformation. Under some stress conditions, such as global I/R, the combination of a burst of radical oxygen species and a calcium overload may open mPTP. This results in the collapse of mitochondrial membrane potential (ΔΨm), uncoupling of the respiratory chain, matrix swelling, and efflux of calcium and proapoptotic factors, which may lead to cellular dysfunctions and/or cell death. ^38,39 Involvement of the mPTP in the pathophysiology of post-CA syndrome is supported by evidence that pharmacological mPTP inhibition at the time of resuscitation prevents both cardiovascular failure and brain damage. ^10,12,13 The present study confirms that resuscitated CA increases susceptibility to mPTP opening and impairs mitochondrial respiration. Although we observed a clear decrease in both CRC and RCI in brain mitochondria when the temperature rose, we did not observe any significant difference in mPTP opening and mitochondrial respiration in the heart. We can therefore speculate that brain mitochondria are more susceptible to small changes in temperature. Another feasible explanation is that the rabbits with the most severely injured heart mitochondria did not survive to the end of the experimental protocol. To the best of our knowledge, such in vivo effects on brain mitochondrial functions from small changes in perischemic temperature have not been previously reported. Until now, similar alterations in mitochondrial functions have only been observed in 1 study where temperature manipulation (ie, hypothermia) was applied after CA. ¹¹ In summary, it is reasonable to assume that small changes in temperature, including during the pre-CA period, may influence the severity of the post-CA syndrome by affecting mitochondrial functions.

The majority of patients who receive CPR do not survive to leave hospital. ^{1,3 –5} Consequently, identification of factors influencing resuscitation and outcomes, especially if they are modifiable, is still crucial for the improved management of CA. Several preexisting conditions, which significantly influence outcomes, such as age, sex, and underlying disease, have been identified. ^{1,3 –5} Unfortunately, most of them are not modifiable. Here, we report a new experimental finding that core temperature prior to CA (an easily modifiable factor) strongly influences outcomes. In clinical practice, this question is of importance since slight alterations in core temperature, as well as unexpected CA, are both common occurrences in hospitalized patients. ^{2 –5,42} Previous clinical studies have also reported that underlying conditions, known to be associated with fever, such as sepsis, stroke, or malignancies, are predictive of bad outcomes when present before CA. ^4,5 Unfortunately, the potential independent role of temperature was not evaluated in these studies. In the present work, we chose an asphyxia-induced nonshockable CA model to test our hypothesis because asystole, frequently precipitated by hypoxia, is the first monitored arrhythmia in a majority of in-hospital CA. ^{3 –5} It seems likely that our experimental data can be transferred to humans. If this is the case, the knowledge of the core temperature prior to an in hospital CA might be useful to predict the outcomes in this setting. Moreover, it would encourage the better control of core temperature in patients at high risk of CA.

Conclusion

In summary, the present study provides evidence that small change in body temperature at the onset of CA results in a profound effect on mitochondrial functions and short-term outcomes in rabbits. Clinical studies are now needed to test whether our results can be translated to humans.