Anesthesia Protocols used to Create Ischemia Reperfusion Myocardial Infarcts in Swine

Abstract

The porcine ischemia-reperfusion model is one of the most commonly used for cardiology research and for testing interventions for myocardial regeneration. In creating ischemic reperfusion injury, the anesthetic protocol is important for assuring hemodynamic stability of the animal during the induction of the experimental lesion and may affect its postoperative survival. This paper reviews the many drugs and anesthetic protocols used in recent studies involving porcine models of ischemia-reperfusion injury. The paper also summarizes the most important characteristics of some commonly used anesthetic drugs. Literature was selected for inclusion in this review if the authors described the anesthetic protocol used and also reported the mortality rate attributed to the creation of the model. This information is an important consideration because the anesthetic protocol can influence hemodynamic stability during the experimental induction of an acute myocardial infarction, thereby impacting the survival rate and affecting the number of animals needed for each study.

In the past 20 y, many animal models of human diseases have been developed for cardiovascular research^19,54,96 to investigate therapies for myocardial regeneration. ^7,27 The porcine model of ischemia-reperfusion is one of the most commonly used models.⁵⁰ The swine (Sus scrofa domestica) heart is anatomically similar to the human heart,^2,41 and the lesion induced by ischemia-reperfusion is representative of the type of injury that occurs in patients who have suffered an acute myocardial infarct.^{16,20,48,86,87}

In swine, the lesion is created by introducing an angioplasty balloon through the femoral artery and occluding the second branch of the anterior descending coronary artery for different periods of time.^15,25,45,60 The use of efficacious and balanced anesthetic protocols during the induction of the lesion promotes hemodynamic stability of the animal, during both the creation of the experimental injury and the postoperative period. An appropriate anesthesia plan is an essential component of the animal use protocol.⁵⁶

Selecting an anesthesia protocol that has minimal influence on infarct size is also crucial. Previous studies postulated that using volatile anesthetic agents such as halogenated gases such as sevoflurane, or opioids such as remifentanil may protect against ischemia-reperfusion injury.^4,18,49,73 Therefore, the use of those agents in this model of cardiac injury remains controversial. The need to create an experimentally adequate infarct size that is compatible with life highlights the importance of selecting an appropriate anesthetic protocol.⁷³ Other studies have described and evaluated the effects of anesthetic drugs, particularly halogenated gases, on the outcomes of various experimental models.^33,69 However, to our knowledge, no reviews specifically compare the impact of different anesthetic protocols on production of porcine ischemia-reperfusion injury.^50,102

The goal of this review is to compare anesthetic protocols reported in the literature and to review and categorize the main characteristics and cardiovascular effects of commonly used drugs in this area of research. To identify original papers relevant to the porcine ischemia-reperfusion model, we searched the Web of Science (WoS) database, using the search terms “myocardial infarct” and reperfus * and porcine or swine model and not human. An additional search was performed with OVID and MEDLINE databases to retrieve other works published by the same authors as well as by other research groups. After screening titles for possible relevance, papers were added to the Mendeley Library. All abstracts were screened, and when deemed relevant, the paper's full text was obtained. References lists were reviewed to identify other relevant studies.

The inclusion criteria are listed in Figure 1. The targeted model of interest is the specific porcine infarction model in which the experimental lesion is induced by occlusion of the left anterior descending coronary artery using an angioplasty balloon. The duration of ischemia in these types of investigations is typically between 40 to 150 min. Papers published between 2008 and 2018 were considered. Study inclusion required providing the mortality rate due to model creation alone, without any experimental treatment or subsequent intervention. The diversity of these studies complicates distinguishing between the percentage of mortality that can be attributed to the anesthetic protocol as opposed to that associated with the experimental treatment or intervention after the infarct has been created.

Figure 1.

Inclusion criteria

We identified a total of 16 research papers that met the inclusion criteria; these are summarized in Table 1. All studies consulted and included in this review had been approved by an ethics and animal welfare committee endorsed by the competent authority of each institution. Each research group used similar but individualized methods of preparation and conditioning of the animals. In general, animals were maintained in stable animal housing with controlled environmental conditions of temperature, light, and humidity. Before each study, they underwent an isolation period during which they received vaccinations and deworming to comply with facility standards. This period also served to acclimatize the animals to the conditions of the center. The research groups used a variety of breeds of pigs, which are included in Table 1. Among the most common breeds are the Large White, Yorkshire, and Göttingen.

Table 1.

Studies analyzed, arranged by year of publication

		Drugs administered (dose and route)												Date of occlusion	Mortality
Publication	N,Breed, sex	Ketamine	Midazolam	Atropine	Thiopental	Sufentanil	Pancuronium	Amiodarone	Xylaxine	Isoflurane	Etomidate	Cistracurium	Fentanyl	Acepromazine	Lidocaine	Min	%
Gathier et al., 201828	Large White females, 15	10–15 mg/kg IM	0.7 mg/kg IM	0.5 mg/kg IM	4 mg/kg IV	10 μg/kg/h IV	mg/kg/h IV	300 mg/h IV CRI									25
Baranyai et al., 20175	Large White females, 60	12 mg/kg IM		0.04 mg/kg IM				300 mg/h IV CRI	1 mg/kg IM	2 to 2.5%							12
Collantes et al., 201712	Göttingen females, 6	15 mg/kg IM								3%	5 mg/kg IV	0.03 mg/kg IV	mg/kg/h IV CRI				0
Do et al., 201821	Yorkshire females, 15	30 mg/kg IM		0.04 mg/kg IM													60
Fernández-Jiménez et al., 201526	Large White neutered males, 25	20 mg/kg IM	0.002 mg/kg/h IV CRI					300 mg/h IV CRI	2 mg/kg IM								20
Jansen of Lorkeers et al., 201535	Landrance females, 19	10 mg/kg IM	mg/kg IM + 0.05mg/kg/h IV CRI			2.5 µg/kg/ h IV CRI	0.1 mg/kg/h IV CRI										16
Gomez-Mauricio et al., 201630	Large White females, 14	20 mg/ kg IM + 0.5 mg/kg/h IV CRI															20
Kodstaal et al., 201444	Landrace females, 18	10 mg/kg IM	0.5 mg/kg IM + 0.7 mg/kg/h IV CRI	0.04 mg/kg IM		6 µg/kg/h IV CRI								160 mg / 30 min IV CRI
Saeed et al., 201375	Large White males, 28	20 mg/kg IM								2–5%					0.5mg/kg IM	10mg/ kg IV	33
Uitterdijket al., 201394	Large White, either sex, 14	20mg/kg IM	mg/kg		600 mg IV + 15 mg/kg/h IV CRI												10
Williams et al., 2013100	Yorkshire swine, either sex, 20	33 mg/kg IM								2–4%							26
Mazo et al., 201253	Gottingen minipigs, either sex, 28	15 mg/kg + 2mg /kg azaperone IM								3%	5 mg/kg IV	0.03mg/kg IV	0.05 mg/kg /h IV CRI				28
Ellison et al., 201124	Large White females, 26	20 mg/kg IM		0.05 mg/ kg IM	5 mg/kg IV			50–200 mg IV bolus		1–2%					0.25 mg/kg IM		30
Schuleri et al., 201173	Gottingen minipig females, 22	35 mg/kg IM			Pentobarbital 20–60 mg/kg IV					1–2%							33
Baks et al., 20063	Large White females, 10	20 mg/kg IM	1mg/kg IM		Pentobarbital 12 mg/kg IV					0.6–0.8 %			12.5 µ/kg/h IV CRI				10
Krombach et al., 200545	Large White females, 44	10 mg/kg + Azaperone 4mg/10kg IM		0.05 mg/ 10kg IM	Pentobarbital 10 mg/kg/h IV CRI												23

In relation to the anesthetic protocol, no breed has been specifically associated with a higher mortality. However, a need for higher doses of ketamine and midazolam for sedation in the Yucatan breed as compared with the Yorkshire breed has been reported.⁵¹ Commonly animals undergoing this surgery are kept intubated under general anesthesia throughout the procedure and variables such as heart rate, respiratory rate, invasive blood pressure, pulse oximetry, and capnography are monitored on a scheduled basis. Likewise, animals used in infarct models receive adjuvant medication, usually composed of anticoagulants such as clopidogrel or aspirin, broad-spectrum antibiotics, and antiarrhythmic drugs such as amiodarone.^25,102

Premedication, Sedation, And Analgesia

Ketamine.

Ketamine is an antagonist of N-methyl-D aspartate (NMDA) receptors that are traditionally used as a dissociative anesthetic. Ketamine is highly liposoluble and has a large volume of distribution. It is rapidly redistributed to peripheral tissues, with an onset of action less than 1 min after intravenous administration and 5 to 10 min when administered intramuscularly. In swine and other species, this drug has a cumulative effect. As resistance gradually develops after repeated administration, the dose administered on consecutive days must be increased to achieve the same level of sedation.^47,66

Ketamine was used as a sedative for anesthetic premedication in most anesthetic protocols reviewed herein. It can be used at doses of 5 to 30 mg/kg intramuscularly, alone or combined with other drugs, and is commonly combined with benzodiazepines such as diazepam or midazolam.^29,51 These drug combinations prevent or counteract the potential muscle stiffness caused by ketamine; this effect is, however, minimal in the porcine species compared with other species.⁶⁸ Other adverse effects include potential muscle stiffness or hypersalivation; therefore, ketamine is combined with atropine in premedication. However, atropine may increase the oxygen demand of cardiomyocytes, which may lead to an effect that interferes with the subsequent modeling process.⁶⁶

Normally, the sedative and analgesic effects of ketamine appear 10 min after administration and last between 20 and 25 min. Ketamine can also be combined with other sedatives such as α2 agonists (for example xylazine, medetomidine or dexmedetomidine) to extend the duration of its sedative effect to approximately one hour and improve its sedative and analgesic effects in animals that are difficult to manage.^74,95 Ketamine in combination with opioids such as buprenorphine or methadone has been used in pigs, leading to better analgesics effects.^76,78

At high doses above 25 to 30 mg/kg, ketamine can cause tachycardia and sensitize the myocardium to the action of endogenous catecholamines.^9,42 Although this effect has been described in dogs, a study in rats administered a high dose of ketamine revealed good hemodynamic stability and reduced ischemia-reperfusion injury.^46,83 In pigs, a high dose of ketamine appears safer for sedation than are α2 agonists, but it causes more respiratory depression and more prolonged recovery times.^8,42,47,74

Tiletamine-Zolazepam.

Tiletamine-zolazepam is chemically similar to a combination of ketamine and a benzodiazepine, Tiletamine-zolazepam is administered in a 1:1 ratio (250 mg zolazepam, 250 mg tiletamine) at a dose of 20 to 25 mg/kg intramuscularly. The sedative effect appears 5 min after administration and lasts longer than that of ketamine. The initial sedative effect of tiletamine is faster than ketamine; however, it seems to produce greater respiratory depression and leads to longer anesthetic recovery times.¹⁷ In pigs, it can produce an initial state of excitement. Effects at the cardiovascular level are more pronounced than those of ketamine, especially with regard to increasing the heart rate and therefore myocardial oxygen consumption.¹⁷ It has been used in several studies of the porcine model of acute ischemia-reperfusion myocardial infarct, but seems to be associated with higher mortality rates.^32,53

Midazolam.

Midazolam is a short-acting benzodiazepine that exerts its action by stimulating the receptor γ-amino butyric acid (GABA). It is administered at 0.3–0.5 mg/kg intramuscularly or intravenously. Midazolam was found to be used in several studies assessed in this review, either as a sedative or as a continuous infusion.³⁵ However, it can prolong recovery times.²⁰ In some animals, midazolam may lead to a period of excitement if used alone. Therefore, it is administered to previously sedated animals.⁵⁵

Midazolam is mainly used as an adjunct due to its limited effects on the cardiovascular and respiratory systems. It provides a good degree of hypnosis and muscle relaxation, allowing a reduction in the doses of other drugs used at induction and also reducing the necessary minimal alveolar concentration (MAC) of inhalational anesthetics such as sevoflurane or isoflurane by up to 50%.^51,61,62 Good analgesia can be obtained when midazolam is combined with an opioid.^36,70

Fentanyl and Remifentanil.

Fentanyl and remifentanil are pure agonists of μ-opioid receptors. They provide an excellent analgesic effect, and due to their rapid onset of action, they can be used as “rescue” analgesics. Fentanyl and remifentanil have minimal effects at the cardiovascular level, but they cause dose-dependent respiratory depression. Therefore, we recommend that they are used with mechanical ventilation or ventilatory support.^13,99

Fentanyl can be administered at a dose of 5 to 10 µg/kg as an intravenous bolus or continuous infusion rate at a dose of 0.005–0.01 mg/kg/h. Although multiple formulations are employed, injectable fentanyl is among those that are mainly used. Transdermal patches are also used in the postoperative period because fentanyl's action commences 24 h after its application.^2,33 Fentanyl and remifentanil potentiate the effects of other drugs such as α 2 agonists and benzodiazepines as they can reduce their doses by 40 to 70 %.⁴⁶ Remifentanil has an ultra-rapid onset of action and a shorter duration than fentanyl, with no cumulative effect, because it is rapidly inactivated by plasma esterases.⁷² Remifentanil is used in continuous infusion at a dose of 0.25–0.3 µg/kg/min without loading dose.^13,34 As an adjuvant drug with inhaled anesthetics, it seems to cause a greater decline in mean arterial pressure and greater bradycardia than benzodiazepines such as midazolam.⁸⁴ In turn, improvement and ischemic conditioning of remifentanil have been postulated in myocardial ischemia cases.^34,64 Its prolonged use at high doses can increase peripheral vascular resistance due to the release of vasopressin. At high doses, it can increase mean arterial pressure and reduce cardiac output.²³ Hyperalgesia has also been postulated at high doses.⁸²

Alpha 2 Agonists.

Alpha 2 agonists act on α-adrenergic receptors. At the central level, they reduce the recapture and release of noradrenaline; at the peripheral level, they bind to the α 2 receptors, causing vasoconstriction.⁹⁷ At low doses, these drugs can be useful as sedatives and analgesics when administered just before the infarct modeling process. Pigs are sensitive to these drugs.⁸⁹ Because of their hemodynamic characteristics, these drugs might be dangerous after infarct creation. However, with adequate monitoring and the use of selective drugs such as dexmedetomidine at low doses of 1 to 2 µg/kg, a good analgesic level can be achieved.⁷⁸ This might be accompanied by a lower heart rate and partial hypotension.^77,97

At the cardiovascular level, an initial phase of peripheral vasoconstriction occurs in which vascular resistance increases. This effect is more pronounced in pulmonary vascular resistance, with elevated blood pressure and increased afterload. These changes lead to a compensatory decrease in the heart rate, ultimately producing a certain degree of bradycardia. After the first 3 to 5 min, either hypertension normalizes or hypotension develops.^17,77,78,97

Xylazine is less selective than other drugs of the same family, such as medetomidine and dexmedetomidine. This family of drugs act by inhibiting the release of noradrenaline at the presynaptic level, leading to sedative, analgesic, and dose-dependent muscle relaxation effects. Dexmedetomidine could reduce MAC and the dose requirements of other anesthetics and provide a moderate analgesia and sedation. These effects have been described in dogs,^63,93 but pigs may show similar effects.^77,78 The effects of xylazine can be reversed with atipamezole.^89,91 At high doses, xylazine can sensitize the myocardium to the action of the endogenous catecholamines, resulting in arrhythmias.³¹

Acepromazine.

Acepromazine is a phenothiazine that has neuroleptic action. It exerts its activity by blocking the dopaminergic receptors as well as the peripheral α-adrenergic receptor, causing peripheral vasodilation and the appearance of relative hypotension. It can be administered at doses of 0.01–0.2 mg/kg. Pigs are commonly treated intramuscularly. Maximal sedation appears at 30 to 40 min after administration, is dose-dependent, and may last 3 to 4 h. Depending on the dose administered and the characteristics of the animal, its action may last for 8 h after administration.⁵¹

Due to peripheral vasodilation, acepromazine also improves the afterload, making it a safe drug for the sedation of patients with moderate myocardial insufficiency. However, its use is discouraged in patients with hypertrophic cardiomyopathy as it can produce marked hypotension because of the heart's inability to compensate this situation with an increase in stroke volume. Acepromazine has a poor analgesic effect that can be compensated by jointly administering opioids.^10,61 In the swine model, acepromazine administered with ketamine was associated with a poor quality of recovery and an increase in the heart rate; a dose of 1 mg/kg was associated with seizures and vomiting in one animal.⁵¹

Induction

Propofol.

Propofol is a hypnotic agent that exerts its action on GABA A receptors. Due to its high lipid solubility, it has an ultra-fast effect with a short duration of 3 to 5 min. Propofol is mainly used for anesthetic induction prior to intubation at doses of 4 to 6mg/kg intravenously (IV) without previous sedation, or 1 to 3 mg/kg depending on the sedation used. Administration of propofol with a benzodiazepine and an opioid is a common combination.^22,59

Propofol can be used for short-term sedation in the form of an intraoperative rescue bolus or continuous infusion for the maintenance of total intravenous anesthesia protocols at a dose of 0.1–0.5mg/kg/min. As has been described in other species, propofol produces a severe depression in breathing, causing periods of apnea mainly after rapid administration as a bolus or at high doses.⁶⁵ Thus, the use of mechanical ventilation is recommended.⁵⁸

In pediatric human patients, the so-called propofol infusion syndrome has been described after continued administration for periods that exceed 48 h at doses greater than 4 mg/kg/h.³⁷ Acidosis, refractory bradycardia, myocardial failure, rhabdomyolysis, asystole, and death are also associated with this syndrome. Although this syndrome might be induced by prolonged and repeated administration, it has not been described in swine.^11,88

At the cardiovascular level, hypotension results from a reduction in peripheral vascular resistance and blockade of the baroreceptors, preventing the increase in compensatory heart rate during the first 2 min after administration. This can cause mild or moderate bradycardia, resulting in a decrease in oxygen consumption and myocardial blood flow. The use of propofol in cardiac surgery has been questioned due to its depressant effect on vascular resistance and myocardial contractility.^11,43 However, greater hemodynamic stability has been observed compared with induction or maintenance with thiopental.⁸⁰ Propofol has been used as principal anesthetic for creation of acute myocardial infarcts to avoid the possible cardioprotective effect of halogenated gases.^46,85

Thiopental.

Thiopental is a thiobarbiturate that acts on the GABA receptors. It is primarily used for anesthetic induction prior to endotracheal intubation due to its rapid action at doses of 15 to 20 mg/kg in animals without premedication and 7 to 12 mg/kg in previously sedated animals, or as a rescue hypnotic bolus. It is administered intravenously because of the irritation and muscular necrosis caused by its alkaline pH. Thiopental also results in dose-dependent respiratory depression.¹⁷ As it has a poor analgesic effect, combination with opioids or benzodiazepines is recommended.¹⁶ It is stored in the fatty tissue and has a cumulative dose-dependent effect, which discourage its use for total intravenous anesthesia. However, some published articles have reported the use of a similar agent such as pentobarbital in acute myocardial infarct model.^{3,23,27,42,81,94} Thiopental is less expensive than agents such as propofol or halogenated gases. In the United States, this drug is a controlled substance, which complicates obtaining and using it.

At the cardiovascular level, thiopental and drugs from the same group (e.g., pentobarbital) decrease arterial pressure,⁹⁸ producing compensatory tachycardia, which increases myocardial oxygen consumption. As a compensatory effect, it also decreases coronary vascular resistance and increases myocardial blood flow, potentially leading to ventricular arrhythmias.⁴³ In swine used for cardiology research, thiopental may interact with drugs used as premedication (for example, amiodarone), resulting in refractory hypotension.⁶⁵

Maintenance

Halogenated Gases.

Isoflurane and Sevoflurane.

Isoflurane and sevoflurane are fluorinated ethers that are administered in a volatile form. They provide greater safety and control of the anesthetic plane than drugs used in total intravenous anesthesia because of the ability to rapidly change their concentration in blood and their faster transfer to the central nervous system.

The blood concentration of these drugs depends on blood/gas solubility, cardiac output, and the difference in the drug's partial pressure between the alveoli and blood. The blood/gas solubility is a measure of the velocity of induction, recovery, and changes in the levels of anesthesia. Therefore, a lower coefficient leads to a higher rate of drug concentration change. The isoflurane coefficient at 37 °C is 1.4 compared with that a coefficient of 0.63 to 0.69 for sevoflurane. This means that the necessary amount of the latter drug is greater than that of the former, leading to a higher economic cost. However, the degree of control of the anesthetic level is better with sevoflurane because its concentration in blood can be varied faster, making it a relatively safer anesthetic.³³

Administering these drugs to animals that have not been previously sedated can cause agitation, nervousness, apnea, and hypoxemia. Furthermore, high concentrations of isoflurane irritate the airways and produce cough. Although induction with sevoflurane is smoother and less irritating, it is still only recommended for use in depressed animals or when vascular access is not available.

Several experimental and clinical studies have described the cardioprotective properties of halogenated gases,¹⁸ suggesting protective properties or improvement of conditioning in myocardial acute ischemia. These properties are attributed to several mechanisms.⁹² They help to preserve the reserves of ATP in the cardiomyocytes, decrease the number of free radicals, improve the afterload, and decrease the heart rate, ultimately reducing oxygen demand of the myocardium and its metabolism.^47,64 On the other hand, a lower incidence of arrhythmias is observed relative to when an intravenous anesthetic is employed.^7,42 This has been explained by some authors⁸⁵ as the ability of halogenated gases to maintain calcium homeostasis, causing an antiarrhythmic effect similar to drugs such as verapamil.

In the porcine model of acute myocardial infarction of the ischemia reperfusion type, sevoflurane maintenance produced better hemodynamic stability than did isoflurane, based on mean arterial pressure.⁷⁰ Frequency of ventricular fibrillation was 81% in swine anesthetized with isoflurane compared with 52% in pigs anesthetized with sevoflurane⁷⁰ The survival rate in this study was 96% in the sevoflurane group compared with 75% in the isoflurane group.⁷⁰

A cause of anesthetic mortality to consider in pigs is malignant hyperthermia. This condition is a recessive monogenic inherited disease characterized by a neuromuscular disorder with a unique autosomal locus, initially called the halothane gene HAL and currently called the ryanodine receptor gene Ryr1. It is characterized by the disturbance of calcium homeostasis in skeletal muscle. The most common form of malignant hyperthermia can be triggered by volatile anesthetic agents and can be fatal if not quickly treated.⁷¹ However, most vendors that provide research swine guarantee that the animals are free of the gene that predisposes to this anesthetic complication.

Antiarrhythmic adjuvants

When creating myocardial infarcts in pigs, antiarrhythmics are usually used to prevent ventricular arrhythmias, with lidocaine and amiodarone the most commonly used drugs. When administered together, these drugs seem to have a synergistic effect. As both decrease the heart rate, their use facilitates the performance of catheterization. Also, oxygenation of myocytes during diastole is greater and does not decrease cardiac output.

Lidocaine.

Lidocaine is a local amide-type anesthetic that blocks voltage-dependent sodium channels and when present in large concentrations, blocks potassium channels. Lidocaine has an antiarrhythmic effect at the ventricular level at a dose of 2 mg/kg and at a continuous infusion of 200 µg-1 mg/kg/h.⁵²

In continuous infusion, lidocaine was found to decrease MAC, thereby decreasing the need for inhalational anesthetics.⁶⁸ Lidocaine exerts anti-inflammatory and analgesic effects in species such as dogs, cats, and horses. However, despite having an antiarrhythmic effect in pigs, lidocaine does not appear to decrease the MAC of halogenated anesthetics and its analgesic effect is much lower.⁶⁸

Amiodarone.

Amiodarone is a class III antiarrhythmic that exerts its effect on sodium and potassium channels, prolonging the action potential and refractory period and slowing the intracardiac conduction of the action potential.^57,67 Because it has far fewer adverse effects than are reported for other antiarrhythmic agents,^38,39 it is used to treat ventricular arrhythmias of unknown origin or patients who are refractory to other treatments. A dose of 5 mg/kg administered as a slow intravenous bolus can be administered in continuous infusion at 150 to 300 mg/h.

In a swine model of ventricular fibrillation, administration of amiodarone with adrenaline did not improve defibrillation efficacy and caused hypotensive effects.³⁹ However, the study reported minor arrhythmic effects during the recovery period after ventricular fibrillation.³⁹ The administration of continuous infusion of amiodarone in swine at 24 h after the ischemic event is described in the literature.⁷⁹ The effect of amiodarone in an intravenous bolus lasts approximately 15 min.⁵⁷

Extravasation of injectable drug formulation produces tissue necrosis while oral administration of high doses before the procedure can lead to greater hypotension and predispose patients to arrhythmias during the induction period. This occurs despite the absence of adverse effects in a porcine model of ischemic cardiomyopathy after long-term amiodarone administration.^38,39,79,101

Discussion

This review revealed the scarcity of articles that specifically report the anesthesia-related mortality that occurs in association with creating ischemia-reperfusion myocardial infarcts in pigs. The articles that were identified showed high heterogeneity in their anesthesia protocols. In many studies, animals that die during the model development process are replaced and therefore are not counted as experimental deaths. In these studies, the mortality rate is determined only at the end of the study.

In the studies we included (Table 1), the average mortality rates associated with the creation of the model ranged from 10% to 60%. Furthermore, nearly all studies used female swine. Hormonal influences on the evolution of acute myocardial infarction in human clinical cases has been described^14,102 and may be present in the porcine model.

The mortality rate of the model can be influenced by anesthetic reactions such as malignant hyperthermia, which is more frequent in swine than other species but is nonetheless rare. Another possible cause of mortality in this model that is not directly associated with anesthesia protocol is the individual variability of coronary anatomy, specifically the existence and distribution of collateral branches of the left anterior descending coronary artery. The duration of the ischemic period may also be related to the mortality rate. This relationship could indirectly be due to a longer duration of anesthesia, which is associated with a higher risk of complications, another factor that can influence the mortality rate in the period after the creation of the defect is the stress that animals may experience during subsequent handling, as pigs are easily stressed.¹⁶

The main limitation of this study is the difficulty of carrying out an exhaustive review in light of the great variety of studies that use ischemia-reperfusion injury and the difficulty of finding publications that report mortality rates during the creation of the infarct, prior to carrying out experimental procedures or other interventions. Furthermore, studies that include the mortality rate associated with creation of the infarction vary widely in the anesthetic protocols used.

Studies included in this review and those consulted in the references used ketamine or one of its derivatives, such as tiletamine, in combination with benzodiazepine as a sedative in anesthetic premedication. In the literature review, the authors found that tiletamine displays more acute cardiovascular effects than those observed after ketamine administration;²⁸ a higher mortality rate has been reported when tiletamine is used for preanesthetic sedation.^32,17 On the other hand, mortality rates vary among groups that use different antiarrhythmics such as amiodarone and lidocaine prior to the creation of the model and during the procedure. In addition, the mortality rate also varies among studies that use inhaled isoflurane or total intravenous anesthesia for anesthetic maintenance. Among the studies selected in this review, 3 had a slightly higher mortality (33%); these studies used tiletamine-zolazepam, pentobarbital or thiopental.^76,82,89 Pentobarbital depresses myocardial contractility and accentuates hypotensive effects, and the combination tiletamine zolazepam seems to cause more hemodynamic instability than similar combinations such as ketamine midazolam. However, the studies we reviewed did not show an increase in mortality rate due to induction time of myocardial ischemia. A possible future project could be a multicenter comparison of centers around the world that use different anesthetic protocols to create infarcts and verify the mortality rate associated with each anesthetic protocol.

A recommended balanced anesthetic protocol could include premedication with 10 to 30 mg/kg ketamine in combination with an opioid and benzodiazepine to obtain an adequate sedation plane.⁸ Propofol might provide stable cardiovascular induction. After the induction period, a low dose of a selective α2 agonist such as dexmedetomidine (0.5 to 1 µg/kg) may be useful to decrease heart rate and compensate for the relative hypotension induced by propofol. However, the animal should be monitored for bradycardia. Although maintenance with sevoflurane could increase cardiovascular stability, further studies are needed to determine the effect of these drugs on infarct size. In addition, the halogenated dose may be reduced with a continuous infusion of midazolam, causing a more neutral effect at the level of the myocardium and a lower cardioprotective effect. Continuous infusion of antiarrhythmics such as lidocaine or amiodarone can also effectively prevent the occurrence of ventricular arrhythmias.

Conclusions

The choice and design of an adequate anesthetic protocol can provide hemodynamic stability in the creation of the acute myocardial infarction model, as reflected by a higher survival rate that ultimately contributes to reducing the number of animals used. More reviews and studies comparing different anesthetic agents that are specifically used to construct this type of model could be useful in the area of cardiovascular research, especially with regard to identifying drugs that influence amount of damage and to balance achieving the desired infarct size with maintaining a low mortality rate.