Original Contribution

Combination pharmacotherapy in the treatment of experimental cardiac arrest^?

Theodoros Xanthos PhD?, Eleni Bassiakou MD, Eleni Koudouna MD, Georgios Rokas MD, Sotirios Goulas MD, Ismene Dontas PhD, Evaggelia Kouskouni PhD,

Despina Perrea PhD, Lila Papadimitriou PhD

Department of Experimental Surgery and Surgical Research, University of Athens, Medical School, 11527 Athens, Greece

Received 5 April 2008; revised 8 May 2008; accepted 13 May 2008

Abstract

Study objectives: Full recovery after cardiopulmonary resuscitation (CPR) is poor. We hypothesized that the coadministration of epinephrine, a ?-blocker such as atenolol, and a calcium sensitizer such as levosimendan during CPR would improve survival and postresuscitation myocardial function.

Methods: Ventricular fibrillation was induced in 60 piglets, which were left untreated for 8 minutes before attempted resuscitation. Animals were randomized into 4 groups (n = 15), to receive epinephrine (group E), epinephrine + atenolol (group E + A), epinephrine + levosimendan (group E + L) and epinephrine + atenolol + levosimendan (group E + A + L) during CPR. electrical defibrillation was attempted 2 minutes after drug administration.

Results: Five animals in group E survived for 48 hours in comparison to 8 animals in groups E + A and E + L and 12 animals in group E + A + L. Postresuscitation cardiac output was significantly better in the animals of group E + A + L. Troponin I remained significantly lower in groups E + A and E + A + L. Serum astroglial protein (S-100) and neuron-specific enolase values in group E + L and E + A + L were statistically lower than those measured in groups E and E + A during the entire observation period. The neurologic alertness score was higher in group E + A + L compared to groups E and E + A. Conclusions: The administration of a drug combination of epinephrine + atenolol + levosimendan, when given during CPR, in a pig model of cardiac arrest, results in improved 48-hour survival and improves postresuscitation cardiac function.

(C) 2009

Introduction

The threshold of coronary perfusion pressure , and therefore Myocardial blood flow, can be restored after prolonged cardiac arrest by administering an adrenergic vasopressor agent, such as epinephrine [1]. The efficacy of

^? This project is cofinanced by Op. Education by ESF (European Social Fund) and National Resources EPEAK II- Pythagoras I.

* Corresponding author. Tel.: +30 2107462500.

E-mail address: [email protected] (T. Xanthos).

epinephrine has been attributed to its ?-adrenergic-mediated vasoconstriction effect in the peripheral circulation. On the other hand, its action on the myocardium has been associated with adverse Cardiac effects that can be minimized by pretreatment with concurrent administration of ?-blocking agents [2,3].

Levosimendan is a positive inotropic drug, but unlike epinephrine, it does not increase myocardial oxygen consumption after its administration [4]. In addition, levosimendan decreases central venous pressure as well as systolic and diastolic pressures in the right atrium [5].

0735-6757/$ - see front matter (C) 2009 doi:10.1016/j.ajem.2008.05.004

Furthermore, reports of improved postresuscitation out- comes make levosimendan a potential drug to use in the pharmacologic treatment of cardiac arrest [6].

Cerebral ischemia that occurs as a result of cardiac arrest leads to brain damage due to the death of neuronal and glial cells. The neurologic outcome and prognosis of patients who have survived cardiac arrest are of major interest [7]. It ranges from subclinical neurocognitive deficits to cata- strophic Neurologic morbidity or death. Several investigators have reported that the increases in serum astroglial protein (S-100) and Neuron-specific enolase are positively correlated with the severity of neurologic damage and the neurologic outcome in patients with brain damage [8].

Results of previous experiments from our group have demonstrated that the coadministration of epinephrine + atenolol and epinephrine + levosimendan significantly improved CPP and return of spontaneous circulation (ROSC) compared to epinephrine alone in an animal model of cardiac arrest [9,10]. Therefore, we conducted a study whose aim was to determine whether the combination of epinephrine, atenolol, and levosimendan would improve survival, postresuscitation myocardial dysfunction, and neurologic alertness in this swine model of cardiac arrest.

Methods

Ethical approval for the investigation and the experi- mental procedures in accordance with Greek legislation was given by the General Directorate of Veterinary Services. The experimental protocol has been previously described [9,10]. Sixty healthy Landrace/Large-White piglets of both sexes aged 10 to 15 weeks and whose average weight was 19 +- 2 kg were used in the study. The animals were randomized before any procedure with the use of a sealed envelope, the contents of which provided for randomization of the animals into 4 different groups of 15 animals each: group E (epinephrine), group E + A (epinephrine + atenolol), group E + L (epinephrine + levosimendan), and group E + A + L (epinephrine + atenolol + levosimendan). The study was blinded as to the medication used. Only the principal investigator knew the medications received by animals. The preparation of the medication was done by the principal investigator who did not take any other part in the experiments. Data were analyzed by a specialist not informed about the medications used in each group.

Briefly, initial sedation in each animal was achieved with Intramuscular ketamine 10 mg/kg, midazolam 0.5 mg/kg, and atropine 0.05 mg/kg. Propofol anesthesia 2.5 mg/kg was also delivered as an Intravenous bolus, via the lateral auricular vein. While spontaneously breathing, but anesthetized, the pigs were intubated with a 5.0 tracheal tube. Propofol 1 mg/kg, cis-atracurium (0.15 mg/kg), and fentanyl 4 ug/kg were then administered intravenously to reach the desired depth of anesthesia, Muscle relaxation, and analgesia. Once this depth

was reached, 0.2 mg/kg/min of propofol and 2 ug/kg/min of cis-atracurium were given intravenously to maintain the anesthesia level. Additional doses of fentanyl (1 ug/kg) were administered when the heart rate exceeded 120 beats per minute and/or the systolic blood pressure exceeded 120 mm Hg. Animals were ventilated by a volume controlled ventilator (VentiPac Sims pneuPac, Luton, UK) with a total tidal volume of 15 mL/kg, supplying FiO2 = 30%. End-tidal PCO2 (PETCO2) was monitored with a side-stream infrared carbon dioxide analyzer (Nihon Kohden Corp, Bergamo, Italy). The respiratory frequency was adjusted to maintain PETCO2 between 35 and 40 mm Hg. Electrocardiogram was recorded continuously, using leads II and V₅. Pulse oximetry (SpO2) (Vet/Ox Plus 4700, Heska, Heska, USA) was also used as a means of peripheral Tissue oxygenation and was monitored continuously. The Pulse oximeter was placed on the tongue of the anesthetized animal.

For measurement of left ventricular function, a 5.5/7.5- MHz biplane Doppler transesophageal echocardiographic transducer with 4-way flexure was used (model 21366A, Hewlett-Packard Co, Medical Products Group, NY, USA), as previously described [11]. The cardiac output was calculated as the product of time-velocity integral of Doppler trans- aortic flow, the diameter of the aortic valve, and heart rate. All measurements were performed by the same investigator (TX), with significant experience in performing transeso- phageal echocardiographic measurements.

The left internal jugular vein and left carotid artery were dissected. For measurement of the aortic pressure, a normal saline-filled (model 6523, USCI CR, Bart Inc, Papapostolou, Athens, Greece) arterial catheter was inserted into the aorta via the left carotid artery. Mean arterial pressure was determined by the electronic integration of the aortic blood pressure waveform. A Swan-Ganz catheter (Opticath 5.5 F, 75 cm, Abbott, Ethicon Mersilk, Athens, Greece) was inserted into the right atrium via the left jugular vein for continuous measurement of right atrial pressure and cardiac output by thermodilution as previously described [12]. Coronary perfusion pressure was calculated as the difference between decompression diastolic aortic and time-coincident right atrial pressure measured at the end of each minute of precordial compression [10].

The Right internal jugular vein was also surgically prepared. After allowing the animals to stabilize for 60 minutes, blood was drawn from this vessel for measurements of baseline serum levels of troponin-I, S-100 protein, and NSE. Lactate was measured with a Blood gas analyzer (Nova Biomedical pHOx plus C, Waltham, Massachusetts, USA). A 5F pacemaker catheter (Pacel; 100 cm, St Jude Medical, Ladakis, Athens, Greece) was then inserted into the right ventricle, through the exposed right jugular vein, and used to induce ventricular fibrillation (VF), as previously described [9]. When VF was induced, mechanical ventilation ceased and animals were left untreated for 8 minutes. Animals in group E (n = 15) were treated with 0.02 mg/kg epinephrine + 20 mL 5% dextrose/water (D/W) + 10 mL 5% D/W as

0 Points		10 Points	20 Points
Posture	Lying on	Attempts to stand	Stands
	each side		normally
Gait	None	Ataxic	Normal
Stimuli	No	Response to painful	Response to
	response	stimuli only	all stimuli
Pupils	Mydriasis	Anisocoria	Normal
Convulsions	Generalized	Clonic-tonic	None

placebo. Animals in group E + A (n = 15) were treated with

Table 1 Neurologic alertness scores

0.02 mg/kg epinephrine + 0.05 mg/kg atenolol in 20 mL 5% D/W + 10 mL 5% D/W as placebo. Animals in group E + L (n = 15) were treated with 0.02 mg/kg epinephrine +20 mL 5% D/W as placebo +0.012 mg/kg levosimendan in 10 mL 5% D/W and animals in group E + A + L (n = 15) were treated with 0.02 mg/kg epinephrine + 0.05 mg/kg atenolol in 20 mL dilution 5% D/W + 0.012 mg/kg levosimendan in 10 mL 5% D/W. All drug regimens were given bolus via the left internal jugular vein.

Resuscitation procedures were initiated with ventilation with inspired 100% oxygen and precordial compression using a mechanical chest compressor (Thumper, Michigan Instruments, Michigan, USA) for 2 minutes. After 2 minutes of precordial compression, defibrillation with 4 J/kg mono- phasic waveform shock (Porta Pak/90, Medical Research Laboratories Inc, Ohio, USA) was attempted. In case of failure to convert to a cardiac rhythm that was compatible with pulse, precordial compression was resumed for 2 minutes before delivery of a second shock. The sequence of precordial compression followed by a single shock of 4 J/ kg was repeated until ROSC or asystole. Return of spontaneous circulation was defined as an organized cardiac rhythm and mean arterial pressure of more than 60 mm Hg. Those animals, in which spontaneous circulation was restored, were monitored for 4 hours, while anesthesia was maintained. Blood was drawn at 1 hour, 2 hours, and 4 hours

Table 2 Hemodynamics, SpO2, and the serum levels of lactate, troponin-I, S-100 protein, and NSE in the 4 experimental groups

for determination of the serum levels of troponin I, lactate, S-100 protein, and NSE. After collection, the blood samples were kept in a refrigerator at 4?C, until required. After 4 hours of postresuscitation monitoring, the intravenous infusions of cis-atracurium and propofol were discontinued. All catheters were removed using a surgical technique as previously described [13]. The ventilator was switched off and the animals were ventilated by manual squeezing of the reservoir bag (FiO2 100%). Atropine

0.2 mg/kg followed by neostigmine 0.05 mg/kg were

administered after the first spontaneous swallowing reflex was detected. The animals were extubated after adequate inspiration depth was ascertained and the SpO2 measurement was more than 97%. Vital signs were monitored throughout recovery. After the appearance of the righting reflex, each animal was returned to its enclosure. Each parameter of neurologic alertness (Table 1) of the surviving animals was scored 48 hours after ROSC. After the final measurements were completed, the animals were euthanatized by a fatal overdose of thiopental.

For measurements of serum levels of troponin-I, S-100 protein, and NSE, each blood sample was centrifuged at 3000 rpm for 10 minutes and the serum was collected and stored in liquid nitrogen at -70?C, until required. The serum levels of troponin I were determined using a commercial assay (Abbott Axysm System, Abbott Diagnostics, Athens, Greece) [14]. The serum levels of S-100 protein and NSE were determined by an immuneluminometric assay (LIAI- SON Sangtec 100, Sangtec Medical, Bromma, Sweden). All assays were performed in a blinded manner.

The total number of ventricular ectopic beats was counted over an interval of 30 minutes after successful resuscitation as previously described [15].

• 1. Statistical analysis

Data are expressed as mean +- SD for continuous variables and as percentages for categorical data. The

Group E		Group E + A	Group E + L	Group E + A + L
Heart rate (beats/min)	85.1 +- 8.2	88.2 +- 6.1	87.1 +- 5.7	84.9 +- 9.1
systolic aortic pressure (mm Hg)	110.4 +- 10.3	108.7 +- 12.1	107.9 +- 12.9	112.5 +- 7.8
Diastolic aortic pressure (mm Hg)	75.5 +- 8.9	72.8 +- 10.0	74.7 +- 9.8	78.1 +- 6.8
Right atrium systolic pressure (mm Hg)	12.3 +- 1.3	12.7 +- 1.1	11.8 +- 1.8	12.6 +- 1.0
Right atrium diastolic pressure (mm Hg)	8.5 +- 0.9	7.9 +- 1.0	8.3 +- 0.8	8.2 +- 0.7
CPP (mm Hg)	67.7 +- 9.2	65.1 +- 10.2	66.8 +- 12.1	60.6 +- 12.2
SpO2 (%)	99.3 +- 0.2	98.7 +- 0.7	99.1 +- 0.4	99.2 +- 0.2
Cardiac output (thermodilution) (L/min)	5.8 +- 0.5	5.7 +- 0.4	5.7 +- 0.3	5.8 +- 0.3
Cardiac output (echocardiography) (L/min)	5.6 +- 0.5	5.5 +- 0.4	5.6 +- 0.3	5.7 +- 0.3
Troponin I (ng/mL)	0.2 +- 0.1	0.1 +- 0.1	0.1 +- 0.1	0.1 +- 0.1
arterial blood lactate (mmol/L)	1.5 +- 0.4	1.7 +- 0.4	1.6 +- 0.2	1.4 +- 0.3
NSE (ng/L)	0.2 +- 0.1	0.3 +- 0.1	0.2 +- 0.1	0.2 +- 0.2
S-100 protein (ng/L)	0.5 +- 0.2	0.4 +- 0.3	0.3 +- 0.2	0.3 +- 0.2

Fig. 1 Coronary perfusion pressure values during VF and CPR. BL indicates Baseline. *P b .05, group E vs group E + A, group E + L and group E + A + L.

normality of the distribution of the data was tested using Kolmogorov-Smirnov statistics with a Lilliefors signifi- cance level. Comparisons of continuous variables were analyzed using a Student t test and Mann-Whitney nonparametric test, as appropriate. The false discovery rate was set at 5% for multiple comparisons. The categorical variables were analyzed using Fisher exact test. Paired t test and Wilcoxon signed rank sum test were used for the comparison of different time measurements of parameters for each group. Differences were considered statistically significant if the null hypothesis could be rejected with more than 95% confidence (P b .05). The SPSS computer software program was used for all analyses

(SPSS, v 13.00, SPSS Inc, Chicago, Ill).

Results

Baseline measurements in hemodynamics, blood gas tensions, and the serum levels of troponin-I, S-100 protein, and NSE were not different from each other in the 4 groups of animals (Table 2). At the end of the eighth minute of VF, the values of CPP were indistinguishable between the 4 groups. At the end of the first cycle of chest compression, and just before the first defibrillation attempt, CPP increased significantly in groups E + A, E + L, and E + A + L compared to group E (Fig. 1).

Spontaneous circulation was restored in 8 animals in group E, 10 animals in group E + A, 11 animals in group E + L, and all 15 animals in group E + A + L (P b .05 group E vs group E + A + L). The number of premature ventricular ectopies was significantly greater in group E and group E + L animals, compared to that in animals of group E + A and group E + A + L. The incidence of ventricular dysrhythmias

was approximately equal in group E and group E + L animals. This result contrasted with what was observed in the animals from groups E + A and E + A + L, where the ?- adrenoceptor effects of epinephrine were blocked by the ?- adrenoceptor antagonist, atenolol (Table 3).

In group E + A + L animals, arterial Blood lactate levels were significantly lower than those measured in groups E, E

+ A, and E + L animals and remained significantly lower 2 hours postresuscitation. Arterial blood lactate levels returned to normal in all 4 groups of animals, 4 hours postresuscitation. More striking were the differences in serum Troponin I levels between the 4 groups of animals. More specifically, serum Troponin-I levels in groups E and E + L animals were significantly higher than those in groups E + A and E + A + L animals at 2 hours postresuscitation. These differences were further exarcerbated at 4 hours postresuscitation. The time-dependent changes in serum troponin I and arterial blood lactate levels in 4 groups are shown in Fig. 2.

The serum levels of S-100 protein and NSE increased significantly in all Resuscitated animals during the post-

Table 3 Incidence of postresuscitation dysrhythmias

Group E

Premature ventricular contractions

65 +- 20 ?

Group Group Group

E + A E + L E + A + L

32 +- 17 57 +- 24 ? 30 +- 14

Salvos 15 +- 5 ?

Episodes of 18 +- 3 ?

ventricular tachycardia

4 +- 3 12 +- 6 ? 3 +- 3

2 +- 1 16 +- 2 ? 3 +- 1

* P b .05, group E and group E + L vs group E + A and group E + A + L.

Fig. 2 A, Troponin I fluctuation for the observation period. *P b .05, group E and group E + L vs group E + A and group E + A + L. B, Arterial blood lactate fluctuation. *P b .05, group E + A and group E + L vs group E; **P b .05, group E + A + L vs group E + A and group E + L.

resuscitation period. However, the serum levels of S-100 protein and NSE in groups E + L and E + A + L animals were significantly lower than those measured in groups E and E + A animals, during the entire postresuscitation period (Fig. 3). There was no difference in the echocardiographic and thermodilution determinations of cardiac output. The cardiac output of successfully resuscitated animals in all groups was significantly reduced 10 minutes postresuscitation. However, relatively rapid recovery over the ensuing 4 hours to near baseline values was observed in group E + A + L animals, as

shown in Fig. 4.

At the end of the experiment (after the 48 hours observation period), there were 5 surviving animals in group E, 8 survivors in groups E + A and E + L, and 12 survivors in group E + A + L.

Neurologic alertness was significantly better (P b.05) in group E + A + L animals compared to those in groups E and E + A but not in group E + L animals (Fig. 5).

Discussion

Epinephrine was formally incorporated into the Amer- ican Heart Association guidelines for resuscitation in 1973. Unfortunately, epinephrine has adverse effects, which are deleterious to the myocardium [2]. The severity of myocardial Ischemic injury is thus amplified and accounts, at least in part, for postresuscitation myocardial dysfunction [3].

Fig. 3 A, NSE fluctuation. B, S-100 fluctuation. *P b .05, group E and group E + A vs group E + L and group E + A + L.

The results of earlier studies have shown that the administration of ?-adrenergic blocking agents before inducing cardiac arrest minimized myocardial injury and improved postresuscitation survival in animal models [2,3]. However, studies in which ?-adrenoceptor blockers were administered during CPR are limited. The ?-blocker esmolol, administered immediately after defibrillation, but before CPR, improved ROSC and 4-hour survival after prolonged VF in animal models [2,3].

Levosimendan is a calcium sensitizer that binds to the N- terminal domain of cardiac troponin in a calcium-dependent manner [16]. Furthermore, levosimendan enhances contrac- tility, without increasing myocardial oxygen demand [17]. Levosimendan also causes vasodilatation, and it does not increase the occurrence of dysrhythmias [18]. Toller et al

[19] reported that the presence of levosimendan increased the potency of epinephrine and the presence of epinephrine increased the potency of levosimendan, concerning myocar- dial contractility. Krummikl et al [20] reported that levosimendan stabilized hemodynamics of a cardiac arrest patient who was treated with epinephrine. Such findings have lead to the notion of using levosimendan, which does not have any arrhythmogenic activity, to enhance myocardial contractility, without increasing myocardial oxygen con- sumption, and thereby increase the recovery of cardiac output in individuals experiencing cardiac arrest. Huang et al

[21] compared the postresuscitation effects of levosimendan to dobutamine in a pig model of VF and closed-chest resuscitation and found that levosimendan improved post- resuscitation myocardial dysfunction. This is in accordance

Fig. 4 A, Echocardiographic cardiac output. B, Thermodilution cardiac output. *P b.05, group E + A and group E + L vs group E. **P b.05, group E + A + L vs group E + A and group E + L.

Fig. 5 Neurologic alertness score in the 4 groups. The bold black horizontal line indicates the median and the “whiskers” (vertical lines) the interquartile range.

with our findings where the addition of levosimendan improved cardiac output.

Cardiac arrest produces a global metabolically locked-in state of the cerebral circulation, that is, there is an absence of substrate delivery or product removal [7]. brain ischemia is an outcome whose underlying cause is multifactorial and can occur as a result of spatial and temporal impairment in cerebral blood flow (CBF). Furthermore, a close relationship exists between the degree of CBF reduction and the development of histologic injury [22]. Cerebral blood flow reduction to less than 40% of normal for 30 minutes leads to edema development upon reperfusion [23].

The large cerebral vessels in most animals contain predominately ?₁-adrenergic receptors, which mediate vasoconstriction after their stimulation by adrenergic agonists. Although only recently investigated, the adreno- ceptors of the cerebral microvasculature (intraparenchymal vessels) are predominately of the ?₁ subtype. These ?₁- adrenergic receptors mediate cerebral vasodilatation after

their stimulation by adrenergic agonists and have been demonstrated in the cerebral microvasculature of pigs, cats, and rats [24]. Thus, it is assumed that stimulation by mixed adrenergic agonists, such as epinephrine, would cause vasoconstriction in large cerebral blood vessels but also cause vasorelaxation of the cerebral microcirculation in the pigs [25]. Although the administration of atenolol would be expected to adversely affect CBF, in our study, the results did not confirm this hypothesis, as exhibited by the biomarkers of cerebral ischemia and the neurologic alertness scores. More specifically, the serum levels of NSE and S-100 protein and the neurologic alertness scores of animals in the epinephrine group were not statistically significant different from those in the group E + A.

A pronounced peripheral vasodilator effect of levosimen- dan has been described in previous studies [5]. Therefore, the administration of levosimendan would cause vasodilatation and thereby restore blood flow to the underperfused cerebral parenchyma [10]. This was also indicated by the measure- ments of NSE and S-100 in this pig model of cardiac arrest and CPR, where statistically significant difference was noted in both markers in the groups where levosimendan was added. The cerebral ischemia markers were not affected significantly by the coadministration of atenolol in the E + A + L group.

Neuron-specific enolase is a dimeric intracellular enzyme of glucose metabolism and occurs as ??-enolase in neurons and as ??-enolase in neuroendocrine cells and in small lung cancer cells [7]. Several authors have shown that the serum levels of NSE are elevated in different cerebral diseases. Furthermore, it has been reported that its serum levels are correlated with the extent of cerebral damage and, in some studies, with the prognosis of the patients [7].

S-100 protein is a dimeric intracellular calcium-binding protein. The heterodimer S-100a (??-form) occurs in melanocytes and glia cells, and the homodimer S-100b (??-form) is found in glia and Schwann cells. Furthermore, S-100 protein has been implicated in neuronal differentiation and proliferation [7].

The concomitant use of levosimendan and atenolol could be of potential value, as the effect of levosimendan in conjunction with ?-blockers, minimized postresuscitation myocardial dysfunction after successful resuscitation from cardiac arrest [26]. Previous data had shown that atenolol did not alter the positive inotropic effect of levosimendan in isolated guinea pig papillary muscle [27]. Furthermore, recent results from a clinical study indicate that concomitant use of ?-adrenoceptor blockers did not influence the Hemodynamic responses of levosimendan [28].

The results of this study have demonstrated that atenolol in combination with epinephrine is superior to epinephrine alone regarding postresuscitation myocardial dysfunction in this animal model of cardiac arrest. However, this combination does not minimize cerebral damage, as exhibited by the elevated serum levels of the biomarkers of cerebral ischemia and the low neurologic alertness

score. The administration of levosimendan and epinephrine was better than epinephrine alone regarding postresuscita- tion myocardial function and significantly decreased the elevated serum levels of the biomarkers of cerebral ischemia. However, this combination does not appear to protect myocardial cells as exhibited by the elevated serum values of troponin I in this model. Therefore, we suggest that the coadministration of epinephrine, atenolol, and levosimendan is beneficial for postresuscitation myocardial function and 48-hour survival after VF because it combines the positive effects of ?-adrenoceptor blocker and levosimendan.

It is recognized that this study had limitations. The interactions of the Anesthetic agents were not assessed. Furthermore, the experiment was performed on apparently healthy pigs, whereas most of the victims of cardiac arrest have underlying pathologies. Moreover, vasopressin, which could be of potential use in CPR, was not used. Within these limitations, we conclude that the suggested therapeutic combination of epinephrine, atenolol, and levosimendan was more efficacious than epinephrine, epinephrine + atenolol, and epinephrine + levosimendan in 48-hour survival and postresuscitation cardiac function in this model of cardiac arrest.

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