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The use of mice and rats as animal models for cardiopulmonary resuscitation research D Papadimitriou, T Xanthos, I Dontas, P Lelovas and D Perrea Lab Anim 2008 42: 265 DOI: 10.1258/la.2007.006035 The online version of this article can be found at: http://lan.sagepub.com/content/42/3/265

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REVIEW ARTICLE

The use of mice and rats as animal models for cardiopulmonary resuscitation research D Papadimitriou, T Xanthos, I Dontas, P Lelovas and D Perrea Department of Experimental Surgery and Surgical Research, University of Athens Medical School, 15B Agiou Thoma Street, 11527 Athens, Greece

Summary Cardiopulmonary resuscitation (CPR) after the induction of cardiac arrest (CA) has been studied in mice and rats. The anatomical and physiological parameters of the cardiopulmonary system of these two species have been defined during experimental studies and are comparable with those of humans. Moreover, these animal models are more ethical to establish and are easier to manipulate, when compared with larger experimental animals. Accordingly, the effects of successful CPR on the function of vital organs, such as the brain, have been investigated because damage to these vital organs is of concern in CA survivors. Furthermore, the efficacy of several drugs, such as adrenaline (epinephrine), vasopressin and nitroglycerin, has been evaluated for use in CA in these small animal models. The purpose of these studies is not only to increase the rate of survival of CA victims, but also to improve their quality of life by reducing damage to their vital organs after CA and during CPR. Keywords Mice; rats; animal model; cardiopulmonary resuscitation

Experimental studies on the mechanisms and therapies of cardiac arrest (CA) have been confined generally to large experimental animals, such as dogs and pigs (Kette et al. 1991, Tang et al. 1997). The amount of resources devoted for research on the mechanisms and therapies has been very limited (Weil et al. 2001). The fact that 700,000 individuals suffer from CA in Europe (Sans et al. 1997), and ,5% of out-of-hospital victims of CA survive (Nolan 2005), justifies the need for augmenting the current research effort (Kaluski et al. 2005). Despite extensive research, the contemporary therapeutic modalities are not efficacious in the treatment of CA. The use of electrical defibrillation and the combination of chest compressions with rescue breathing and intravenous adrenaline (epinephrine) are the only currently recommended therapeutic modalities for treating CA (Handley et al. Correspondence: T Xanthos. Email: [email protected] Accepted 4 September 2007 # Laboratory Animals Ltd

2005). Needless to say, successful resuscitation is not only represented by the return of spontaneous circulation (ROSC), but should also incorporate cerebral resuscitation. Based on the results from clinical and animal studies, only mild hypothermia has been proven thus far to provide cerebral protection during and after CA that is caused by ventricular fibrillation (VF) (Hypothermia after Cardiac Arrest Study Group 2002). Mice and rats have been established as experimental models for conducting research into the mechanisms of CA and the effects of cardiopulmonary resuscitation (CPR), not only because of their low cost but because the clinical situations can be accurately reproduced in these species. Moreover, the haemodynamic measurements during CPR of mice and rats are confirmatory of those established in humans and other mammals, such as swine (Song et al. 2002). Furthermore, these rodent models have the advantage of being subjected to a standardized insult and the resultant haemodynamic and respiratory measurements are pertinent to outcome.

DOI: 10.1258/la.2007.006035. Laboratory Animals (2008) 42, 265 –276

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However, small animal resuscitation models are very sensitive to minimal differences in experimental protocols, where delays between the cessation of precordial chest compression and defibrillation can be fatal (Sato et al. 1997).

Anatomical and physiological similarities and differences between the cardiopulmonary systems of humans, mice and rats It is becoming more and more obvious that it is crucial to have extensive knowledge of the specific anatomical differences and physiological parameters in rodent models of CA for the proper extrapolation of the experimental findings to humans (Webb et al. 1996, Anderson et al. 1998, Waller et al. 2000). Although mice and rats have a fourchamber heart that consists of ventricles and atria and is therefore similar to the human heart, significant differences to the human heart do exist. The heart rate of an adult human is about 60 – 70 beats/min (Wessels & Sedmera 2003), whereas the rates in the mouse and the rat are higher, namely between 500 and 600 beats/min in mice and between 260 and 450 beats/min in rats (Orr 2002). In addition, several cardiopulmonary variables have been defined in mice and rats and are comparable with those measured in humans. Specifically, for anaesthetized mice, the mean aortic pressure ranges between 80 and 100 mmHg, the diastolic aortic pressure ranges between 72 and 90 mmHg, the right atrial pressure ranges between 22 and 8 mmHg, the coronary perfusion pressure (CPP), which is the difference between the minimal aortic diastolic pressure and the right atrial diastolic pressure, is about 83 – 85 mmHg and the endtidal pressure of carbon dioxide (PETCO2) is between 23 and 35 mmHg (Song et al. 2002, Neigh et al. 2004). For anaesthetized rats, the values are similar to those in the anaesthetized mouse: mean aortic pressure 71 – PETCO2 93 mmHg; right atrial pressure 22 to 8 mmHg; CPP 80 –85 mmHg; PETCO2 33 – 40 mmHg (Popp et al. 2007). In humans, the value for the mean aortic Laboratory Animals (2008) 42

D Papadimitriou et al.

pressure is between 70 and 90 mmHg, the right atrial pressure is between 2 and 7 mmHg and the PETCO2 is between 25 and 35 mmHg (Davidson & Bonow 2001). Furthermore, the PETCO2 in all species decreases to almost zero during CA and reaches between 30 and 40% of normal baseline values during precordial compressions. ROSC is heralded by a prominent and progressive increase in end-tidal pCO2, which is coincident with the reappearance of arterial pulsations and a rise in arterial pressure (Gudipati et al. 1988). These observations of the changes in PETCO2 during CPR are indistinguishable from those measured in swine. In addition, the CPP, which is produced by precordial compression when doing CPR, has been shown to be a critical determinant of resuscitability in humans, swine, mice and rats (Ditchey et al. 1982, Halperin et al. 1986, Paradis et al. 1990, Xanthos et al. 2007). The threshold CPP value for successful resuscitation using CPR in the rat and the mouse has been determined to be 20 mmHg (von Planta et al. 1988) and between 16 and 30 mmHg (Song et al. 2002), respectively, whereas in humans it is between 15 and 20 mmHg (Zhong & Dorian 2005). Prolonged failure of myocardial perfusion during CA is followed by global myocardial ischaemic injury and post-resuscitation myocardial dysfunction, both of which account for the fatal progression after successful resuscitation in humans, mice and rats (Tang et al. 1995, Gazmuri et al. 1996, Sun et al. 1999). Because the mouse has a high resting heart rate, the refractory period is correspondingly short (Manoach 1984, Damiano et al. 1990). Therefore, it is difficult to secure re-entrant rhythms with which to sustain VF. Consequently, VF reverts spontaneously to normal sinus rhythm during the initial 1.5 min of electrically-induced CA (Garrey 1914, Wiggers 1940, Winfree 1994), a finding that has not been confirmed in swine and dogs. Therefore, the continuous delivery of a lowintensity electric current is necessary to maintain VF. Moreover, in the setting of VF, the ultimate effect is a failure of the heart to maintain forward blood flow, which includes

Mice and rats as cardiopulmonary resuscitation models

coronary, systemic and pulmonary blood flows. Despite the numerous differences in these parameters from those of humans, the physiological parameters and haemodynamic measurements obtained from mice and rats are well defined during CA and CPR, rendering them useful models to study CA.

Experimental studies in mice and rats The description of the experimental studies in mouse and rat models of CA and their results are summarized in Tables 1 and 2. Vital organ damage in experimentallyinduced CA Global cerebral ischaemia following CA is a major concern for human CA victims because it results in neuronal death that occurs predominantly in ‘watershed’ regions of the brain. The hippocampus is one of the main regions of the brain that is damaged predominantly during CA (Bottiger et al. 1999, Sadowski et al. 1999, Kofler et al. 2004). However, a decrease in the hippocampal volume is not significantly correlated with memory impairment in human CA survivors (Grubb et al. 2000). The poor neurological outcome of CA in humans, demonstrated by moderate to severe cognitive deficits, learning difficulties, changes in emotional and social behaviours and depression (Nunes et al. 2003, Reich et al. 1983), is probably due to the use of non-efficacious drugs at the time of CA. At the time of writing, we are not aware of any known pharmacological agent that has been shown to improve the neurological outcome in human CA survivors, despite promising results from animal studies (Jastremski et al. 1989, Roine et al. 1993). In addition to the studies on global cerebral ischaemia, the results of many investigations of focal ischaemia in which various vesselocclusion techniques, such as bilateral common carotid artery occlusion and/or three or four vessel occlusion in the absence or presence of systemic hypotension, have been described in mice (Ginsberg & Busto 1989, Traystman 2003). In contrast to global

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cerebral ischaemia models, which produce behavioural deficits, in which there are histopathological signs of injury, and which therefore reflect the clinical situation, these vessel occlusion techniques do not accurately reflect the clinical situation in humans. More specifically, vessel occlusion does not cause a complete cessation of circulation, due to the presence of collateral arteries. The often-observed asymmetrical injury and the strain-related differences in these models (Fujii et al. 1997) are caused by different degrees of hypoplasia of the posterior collateral artery. Furthermore, blood flow to hindbrain regions, such as the brainstem and cerebellum, is maintained in these vascular occlusion models (Sheng et al. 1999). Another disadvantage of vessel occlusion techniques is that compressing the chest during CPR and impaired postischaemic cardiac function create a period of low blood flow that in itself is capable of causing more neuronal damage by aggravating the no-reflow phenomenon (Bo¨ttiger et al. 1997). The resultant brain ischaemia is, thus, isolated in vessel occlusion techniques, whereas the whole body is affected in human CA. Thus, the clinical situation is much more complex than most global ischaemia occlusion models reflect. Furthermore, the many intra- and post-ischaemic factors that might influence neurological outcome, as well as the efficacy of various therapeutic modalities, have been overlooked when using the focal ischaemia models. To study the neurological effects of global cerebral ischaemia, the most widely used rodents are gerbils, mice and rats (Ginsberg & Busto 1989, Traystman 2003, Popp et al. 2007). In a global cerebral ischaemia mouse model, Kofler and colleagues proved that the severity of the brain injury was dependent upon the duration of CA and the intra-ischaemic brain temperature (Kofler et al. 2004). In order to increase survival, it was found that reducing body temperature to 278C, to cause intra-ischaemic whole-body hypothermia, improved neurological recovery from CA. In addition, induction of post-ischaemic mild hypothermia (target temperature 32 –348C measured in the human urinary bladder) has been shown Laboratory Animals (2008) 42

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(4) To standardize a method of CPR in rats (von Planta et al. 1988)

(1) To study whether the haemodynamic and respiratory changes during cardiopulmonary resuscitation (CPR) in mice are comparable with those of large mammals (Song et al. 2002) (2) To characterize spatial learning and memory deficits elicited by cardiac arrest (CA) with CPR (CA/CPR) in mice (Neigh et al. 2004) (3) To characterize histopathological and behavioural features of CA (Kofler et al. 2004) KCl

Mice

Alternating current

KCl

Mice

Rats

Alternating current

Method of cardiac arrest

Mice

Animal model

After 4 min of ventricular fibrillation

10, 12 or 14 min after induction of CA

7 min 45 s into the arrest period, the mouse was ventilated on 100% oxygen

After 4 min of untreated cardiac arrest

Time of basic life support

Experimental studies of changes in vital organs after cardiac arrest

Aim of the study

Table 1

10, 12, or 14 min after induction of CA

At 8 min following injection of KCl, 8 mg of adrenaline in 0.5 mL saline

Time of drugs administration

A dissociation between functional and histological outcome was found, emphasizing the importance of combining both outcome measures for evaluation of neuroprotective strategies This model of CPR demonstrated large venoarterial and pCO2 gradients associated with reduced pulmonary excretion of CO2 during the low-flow state. Mean aortic pressure, coronary perfusion pressure, and end-tidal CO2 during chest compression were predictive of successful resuscitation

CA and CPR in a mouse model results in selective memory impairment such that acquisition, but not retention, of a spatial task is inhibited

In contrast to the results found in large mammals, cardiac arrest in both the rat and the mouse is, therefore, more often associated with pulseless electrical activity of the heart or asystole

Conclusions

268 D Papadimitriou et al.

(1) To examine the effectiveness of the delayed combination of nitroglycerin with vasopressin in cardiopulmonary resuscitation (CPR) in a 6 min asphyxia rat model (Kono et al. 2002b) (2) To compare the effects of vasopressin and adrenaline with mesenteric ischaemia, as determined by intestinal mucosal tonometer pCO2 during the post-resuscitation period (Studer et al. 2002) (3) To examine if a non-selective [beta]-adrenoceptor anatagonist agent improves the outcome of CPR, in comparison with adrenaline (Huang et al. 2004) (4) To examine whether the ß-adrenergic effects of adrenaline, when administered during CPR, adversely affect post-resuscitation myocardial function when compared with those of a selective a-adrenoceptor agonist, phenylephrine, and when adrenaline was combined with a short-acting ß1-adrenoceptor antagonist (Tang et al. 1995) (5) To assess possible benefits regarding neurological recovery, in a cardiac arrest model in rats, where vasopressin versus adrenaline and the combination of both drugs was tested against a placebo (Popp et al. 2007)

Rats

Rats

Electrical fibrillation with alternating current

Current

Alternating current

Rats

Rats

Suffocation, induced by obstructing the tracheal tube

Method of cardiac arrest

Rats

Animal model

Experimental studies of drugs

Aim of the study

Table 2

4 (group 1) and 8 (group 2) min after ventricular fibrillation

4 min after ventricular fibrillation

6 min after suffocation

Time of basic life support (BLS)

After 7 min of global ischaemia

4 min after BLS

9 min after ventricular fibrillation

Vasopressin started just before BLS (V-Gr) and nitroglycerin (VN-Gr) was administered 45 s after vasopressin

Time of drugs administration

Administration of vasopressin during CPR does not improve behavioural and cerebral histopathological outcome, compared with the use of adrenaline or the combination of both vasopressors, after cardiac arrest in this cardiac arrest rat model

Adrenaline, when administered during CPR under the experimental conditions, significantly increases the severity of post-resuscitation myocardial dysfunction in consequence of its ß1-adrenergic actions

The [beta]-adrenergic effect of adrenaline significantly increased the severity of post-resuscitation myocardial dysfunction

The administration of vasopressin instead of adrenaline for CPR tends to be associated with lower resuscitation success, but less mesenteric ischaemia

The delayed combination of nitroglycerin after the administration of vasopressin was more effective than vasopressin alone (increased survival rate)

Conclusions

Mice and rats as cardiopulmonary resuscitation models 269


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to reduce the extent of brain damage after global ischaemia (Hypothermia after Cardiac Arrest Study Group 2002, Sanders 2006). There is also experimental evidence that demonstrates that hypothermia must be maintained for at least 1 or 2 h to be effective (Welsh & Harris 1991). These researchers showed that a brief period of hypothermia, of ,30 min, was unlikely to exert a neuroprotective effect. Despite neuronal preservation, the demonstration of dissociation between functional and histological outcomes confirms the results of numerous other studies in which electrophysiological or behavioural abnormalities were described (Himori et al. 1990, Hori & Carpenter 1994, Dowden & Corbett 1999). This dissociation emphasizes also the importance of combining measures of functional and histological outcomes when evaluating neuroprotective strategies (Corbett & Nurse 1998). In addition, injuries in the hippocampus have been described in mice and rats in CA and CPR experiments. Specifically, these mice were used to characterize the spatial learning and memory deficits that were elicited by CA and CPR. The results of such investigations demonstrated that CA and CPR can impede with the acquisition of new spatial memory tasks without impacting upon the retention of previously learned spatial memory tasks (Neigh et al. 2004). The mice that were subjected to CA and CPR in the current study (Neigh et al. 2004) exhibited a reduction in the total number of dendritic spines in the pyramidal cells in the CA1 region of the hippocampus. A similar reduction in the number of dendritic spines following CA and CPR has been described in the rat cortex (Akulinin et al. 1997). A decrease in dendritic spine density has been observed also in several chronic and debilitating human diseases that include alcohol abuse, epilepsy and Alzheimer’s disease (Fiala et al. 2002). Pharmacological studies in experimental CA Adrenaline (epinephrine) has been the preferred biogenic amine for the treatment of human CA for .30 years (Otto & Yakaitis Laboratory Animals (2008) 42


1984, Robinson et al. 1989) because its a-adrenergic-mediated vasopressor action increases the CPP (Michael et al. 1984). Its b-adrenergic-mediated actions, however, are deleterious for the fibrillating myocardium (Ditchey & Lindenfeld 1988). The results of studies conducted in rats have established that b-adrenergic-mediated positive inotropic actions provoke disproportionate increases in myocardial oxygen consumption and, thereby, increase the severity of myocardial ischaemia (Livesay et al. 1978, Niemann et al. 1986, Wright et al. 1986, Ditchey & Lindenfeld 1988, Halperin & Guerci 1990, Tang et al. 1995, Huang et al. 2004). In these experimental studies, it was found that adrenaline increases also the severity of postresuscitation myocardial dysfunction, with a consequent reduction in the duration of postresuscitation survival, when compared with the results obtained following treatment with the selective a-adrenoceptor agonist phenylephrine, or a combination of adrenaline and a b-adrenoceptor antagonist. Moreover, adrenaline-treated rats require a larger number of electrical countershocks to convert VF to a cardiac rhythm that is compatible with a pulse (Tang et al. 1995). This result is consistent with those described in previously published reports (Niemann et al. 1986, Wright et al. 1986). Furthermore, Ditchey et al. (1994) observed that pretreating dogs with the non-selective b-adrenoceptor antagonist propranolol could reduce the severity of myocardial injury during CPR, without compromising the likelihood of successful defibrillation or the restoration of spontaneous circulation and the post-resuscitation left ventricular function. In their study, they also found that the non-selective b-adrenoceptor antagonist increased CPP during CPR. This result suggests that blocking of b-adrenoceptors can cause vasoconstriction by allowing unopposed a-adrenergic stimulation of adrenergic receptors in all resistance vessels to occur. Therefore, it may be that the use of nonselective b-adrenergic blockade improves the balance between myocardial oxygen requirements and oxygen demands by increasing oxygen delivery to the heart. This finding has recently been confirmed in a rat model of CA


(Huang et al. 2004). It would, therefore, be appropriate to re-evaluate the use of adrenaline as the drug of first choice for CPR and the use of b-adrenoceptor antagonists during CPR in order to minimize post-resuscitation myocardial dysfunction. Furthermore, it is also appropriate to explore the efficacy of new therapeutic agents in the pharmacological treatment of CA. These drawbacks of adrenaline have led to the use of another vasopressor drug, vasopressin. Vasopressin is an endogenous pressor peptide whose effects have been studied in animal models of CA and which has been used successfully in CPR in humans (Prengel A et al. 1996, Lindner et al. 1997). Theoretically, vasopressin is a desirable vasopressor for use in cases of CA and in CPR because it causes selective vasoconstriction of resistance vessels in non-vital tissues and, at the same time it preserves blood flow to vital organs, such as the heart and brain (Mayr et al. 2001). On the other hand, the unwanted cardiovascular effects of vasopressin, such as transient ischaemia, transmural myocardial injury without infarction, acute myocardial infarction and ventricular arrhythmias including VF, have been described in humans since 1947 (Ruskin 1947, Sirinek et al. 1989). Vasopressin is able to increase systemic vascular resistance by direct stimulation of the peripheral V1 receptor (Ericsson 1971). In addition, it is better able to maintain CPP because it decreases myocardial blood flow and is a more potent vasoconstrictor than adrenaline. Furthermore, it has been demonstrated in human forearm vessels (Suzuki et al. 1989) and isolated human mesenteric arteries (Martinez et al. 1994) that vasopressin may have a biphasic action. This biphasic action is characterized by an initial potent vasoconstriction, which is mediated by stimulation of V1 receptors and is then followed by vasodilation, which is mediated by stimulation of V2 receptors. In clinical practice, successful resuscitation in refractory human CA using vasopressin has been reported (Lindner et al. 1996). Two randomized clinical studies have documented higher immediate and 24 h survival in patients who were resuscitated

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after out-of-hospital VF when vasopressin was used instead of adrenaline (Lindner et al. 1997). No differences between the effects of vasopressin and adrenaline on the rates of survival following in-hospital CA have been documented (Stiell et al. 2001), and this finding demonstrates that vasopressin is not always more efficacious than adrenaline. It has been reported that the use of vasopressin or adrenaline does not affect patient survival. Despite the results from animal experiments that suggest improved outcomes in experimental CA with the use of either adrenaline or vasopressin, the results of clinical trials of the use of these two agents have failed to show that either is more efficacious than the other because of their many adverse cardiovascular effects (Zhong & Dorian 2005). A similar conclusion to that reached in the above-mentioned human study was obtained from a study involving VF in rats (Kono et al. 2002a). In this study, the investigators reported that vasopressin was as effective as adrenaline in the treatment of experimental CA. Popp et al. (2007) reported recently that the administration of vasopressin during CPR improved immediate survival when compared with the use of adrenaline in CA and CPR rats. In another rat study, Studer et al. (2002) reported that successful immediate survival following resuscitation tended to be higher in adrenaline-treated rats when compared with that in vasopressintreated rats. In this rat study, all the resuscitated rats showed evidence of depressed cardiovascular function, as indicated by a decreased oxyhaemoglobin saturation in right atrial blood and an increased right atrial versus arterial pCO2 gradient. Successfully resuscitated vasopressin-treated rats differed from the adrenaline-treated rats in having a higher right atrial oxyhaemoglobin saturation, a lower intestinal tonometric pCO2, a decreased tonometric – arterial pCO2 gradient and a trend towards a smaller right atrial– arterial pCO2 gradient, as well as lower serum lactate levels. The reason for these differences between systemic oxygenation and mesenteric ischaemia is that adrenaline (1) induces more myocardial damage because of its a-adrenergic and Laboratory Animals (2008) 42

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positive inotropic actions and (2) increases the severity of post-resuscitation myocardial dysfunction in rats to a greater extent than vasopressin (Tang et al. 1995). Reduced myocardial dysfunction in the vasopressintreated rats would have resulted in (1) a higher pCO2, as indicated by a higher right atrial-haemoglobin oxygen saturation, (2) reduced regional mesenteric ischaemia, as indicated by a smaller pCO2 gap and (3) lower serum lactate levels when compared with those measured in the adrenaline-treated rats. Vasopressin also causes a decreased systemic oxygen uptake, associated with a baroreceptor reflex-mediated withdrawal of sympathetic tone (Liard 1989). Furthermore, the effects of vasopressin on the function of vital organs, such as the brain, heart and intestines, but not its effects on survival, have also been studied in experimental animals, such as pigs (Lindner et al. 1995) and rats (Studer et al. 2002, Popp et al. 2007). These results demonstrate that vasopressin can be used to bring about ROSC, but can also provide vital organ protection. The effects of vasopressin on mesenteric function have also been studied during CA and CPR in rats (Studer et al. 2002). In some animal models of CA, the use of vasopressin caused less mesenteric ischaemia than the use of adrenaline. This result differs from that observed in humans with stable cardiovascular function, where exogenously administered vasopressin induces splanchnic vasoconstriction (Iwao et al. 1996). Furthermore, the absolute or relative increases in mesenteric perfusion can be explained by vasopressin having a biphasic action in the vasculature. In patients with cardiovascular shock, vasopressin can induce an even more pronounced mesenteric vasoconstriction because the vasoconstriction may be potentiated by the action of endogenous angiotensin (Reilly et al. 1992, Reilly & Bulkley 1993). Unfortunately, the limitation of these animal studies is that the experimental animals are free of any cardiovascular disease. Accordingly, it still remains to be determined whether the observed effects of vasopressin on Laboratory Animals (2008) 42


mesenteric perfusion during the postresuscitation period can be extrapolated to CA patients with atherosclerosis. Due to the numerous adverse cardiovascular effects of vasopressin and adrenaline and their ineffectiveness in successful CPR, their use in conjunction with the potent vasodilator nitroglycerin has been evaluated in CA in porcine (Lurie et al. 2002) and rat models of CA (Kono et al. 2002b) in an effort to reduce the incidence of these adverse effects. Nitroglycerin is known to increase cardiac output, while, at the same time, it maintains CPP and mean blood flow in animal models of CA (Bache 1978, Brazzamono et al. 1984, Colley & Sivarajan 1984, Zito et al. 1985, Brazzamono et al. 1988, Wenzel et al. 1998) and in humans (Miller et al. 1977, Groszmann et al. 1982). In addition, the drug causes systemic vasodilation, especially in coronary blood vessels. A decrease in systemic vascular resistance might reduce the CPP and mean blood flow and could affect the outcome of CPR. It has been reported that nitroglycerin increased mean blood flow in the ischaemic area of the heart and decreased the resistance of the collateral vascular system (Bache 1978). In a rat study undertaken by Kono et al. (2002b), a delay in the administration of nitroglycerin after the administration of vasopressin was more effective than vasopressin alone regarding resuscitation outcomes. The significantly greater resuscitation rates observed in this study might be due to the above-mentioned pharmacological properties of nitroglycerin. In this study, the arterial pH and bicarbonate levels after the resuscitation phase were low and a reduced survival rate might have been expected because of severe acidosis. A low survival rate was not found, however, because nitroglycerin caused vasodilation and accelerated the washout of anaerobic metabolites that were produced during hypoxia. The increased resuscitation and survival rate was due to adequate maintenance of the systemic circulation. Nitroglycerin can cause hypoxic pulmonary vasoconstriction and impair pulmonary


oxygenation of the blood (Miller 2000, Youngberg 2000). These drug-induced aberrations were not observed in the Kono study. Because no severe nitroglycerin-induced side-effects, such as hypoxia or hypotension, were observed after the resuscitation phase in this rat model, the combined use of nitroglycerin and vasopressin may increase the survival rate of human CA victims when compared with that obtained following the administration of adrenaline alone, as has been previously described in pig models (Lurie et al. 2002).

Conclusion CA is an emergency situation and CPR is used in order to increase the survival rate of its victims. In cases of CA, it is important to use established therapeutic modalities. Mice and rats can be used as good animal models of CA and for studying the effects of CPR due to their anatomical and physiological similarities to humans. In addition, there are also ethical advantages in using these animals when compared with the use of other animal species, despite their sensitivity to minimal differences in the experimental protocols. The efficacy of new drugs and therapeutic modalities has been studied in these animal models of CA. The purpose of such studies is to increase the survival rate and quality of life of CA victims and to evaluate new drugs and therapeutic modalities before their introduction into clinical practice.

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