Strain specific differences in a cardio-pulmonary resuscitation rat model

Resuscitation 53 (2002) 189
/200 www.elsevier.com/locate/resuscitation

Strain specific differences in a cardio-pulmonary resuscitation rat model U. Ebmeyer a,b,*, G. Keilhoff a, G. Wolf a, W. Ro¨se b a

Institute of Medical Neurobiology, Otto-von-Guericke University Magdeburg, Leipziger Strasse 44, 39120 Magdeburg, Germany b Department of Anaesthesiology and Critical Care Medicine, Otto-von-Guericke University Magdeburg, Leipziger Strasse 44, 39120 Magdeburg, Germany Received 6 June 2001; received in revised form 30 July 2001; accepted 4 January 2002

Abstract An asphyxial cardiac arrest rat model, originally developed for Sprague
/Dawley rats, was transferred to a Wistar rat model. Several strain specific life support adjustments, i.e. ventilator settings, anaesthesia, and drug requirements, were necessary to stabilize the model for Wistar rats. Despite these arrangements numerous resuscitation related variables appeared different. Three groups were evaluated and compared: a temperature monitored Wistar group 1 (n /34), a temperature controlled Wistar group 2 (n /26) and a temperature controlled Sprague
/Dawley group 3 (n /7). Overall, Wistar rats seem to have more sensitive cardiocirculatory system evidenced by a more rapid development of cardiac arrest (164 vs. 201 s), requiring higher adrenaline/epinephrine doses (10 vs. 5 mg/kg) and requiring more time for recovery after resuscitation (i.e. for return of blood pressure and blood gases). Without strict temperature control (as in groups 2/3 rats) group 1 rats went into spontaneous mild to moderate hypothermia during the first 24 h after restoration of spontaneous circulation (ROSC). Spontaneous hypothermia delayed the development of overall visible CA1 neuronal damage 24
/48 h, but did not prevent it; therefore the model seemed to be suitable for future studies. Neuronal damages in the CA1 region in Wistar rats appeared to be more as shrunken cell bodies and pyknotic nuclei before resorption took place, whereas in Sprague
/Dawley rats appeared in the same region. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Resuscitation; Cardiac arrest; Hypothermia; Asphyxia; Outcome; Neurons

1. Introduction Patho-physiological and -biochemical processes during and after resuscitation from cardiac arrest are extremely complex and, thus far, poorly understood [1
/3]. Currently, there is no alternative to the performance of basic research on animals. In clinical as well as in basic science oriented cardiac arrest research rats are the animals most often used. The complexity of the ongoing processes during and after resuscitation from cardiac arrest requires minimization or compensation of genetic and other inter-individual differences as much as possible. Inbred animal strains are therefore used for

* Corresponding author. Tel.: /49-391-67-13523; fax: /49-391-6713501. E-mail address: [email protected] (U. Ebmeyer).

CPR bench research. Otherwise extremely large case numbers would be necessary to compensate individual differences and variations [4,5]; the performance of ‘screening’ or preliminary studies would be almost impossible. The time consuming, resource binding, and very expensive large clinical (multi-center) trials must be reserved for evaluation of the most promising therapeutic approaches [6]. Historic and ongoing studies that have followed the philosophy of going from rat screening studies via large animals and clinical observation and feasibility studies to multicenter-trials are the BrainResuscitation-Clinical-Trial studies (BRCT) I
/III [7
/9], the vasopressin study [10], and the post-resuscitation hypothermia study [11,12]. Unfortunately, the use of inbred animals can generate new specific experimental problems. Deviating or even contradictory results from identical research topics have been reported repeatedly, although comparable experi-

0300-9572/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 3 0 0 - 9 5 7 2 ( 0 2 ) 0 0 0 0 3 - 5

190

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

mental conditions were chosen. Complying with the Utstein guidelines on resuscitation research [13
/17], specifically the recommendations for animal research [18], is mandatory but still no guarantee for exclusion of these problems. To address these problems, several studies have been conducted to demonstrate and explain strain specific differences in rats. The spectrum of contrary observations is wide and covers biochemical, behavioral to sub-cellular and morphological topics. Wistar and Sprague
/Dawley rats are more closer related than, for example, Long-Evans, Fischer or Brown Norway rats; but between the two strains there are still some impressive functional and morphological strain specific differences. Some of these differences seem to be important in ischemia related research. Differences between the two strains can be of physiological nature (such as a variety of baseline conditions) or a different response to a variety of insults or in-vivo manipulations. Examples are reports by Meller et al. concerning different reactions in the cardiovascular
/ respiratory system to hypertension [19] and by Chen et al. concerning different blood pressure and heart rate response after neuropeptide Y injection [20]. Iwasaki et al. describes differences in post-ischemic hippocampal damages after temporary vessel occlusion and hypotension [21]. Other authors report different airway reactions after hypoxia [22], ozone application [23], or inflammation [24]. Some articles are related to behavioral and learning related differences [25,26], pain reaction [27,28], or the sensitivity to seizure inductiblilty [29,30]. Of special interest for ischemia related research reports about strain specific differences in intracellular electrolyte and energy processes [31
/33]. In the early 90s Katz et al. [34] established an asphyxial cardiac arrest (ACA) rat model by modifying, standardizing, and improving a model that was formerly used by Hendickx et al. [35,36]. The model is based on Sprague
/Dawley rats and was primarily used for the observation of resuscitability, post-restoration of spontaneous circulation (ROSC), and functional recovery, as well as the overall semi-quantitative analyses of lightmicroscopically visible neuronal damage [37]. The latter was based on a haematoxylin eosin staining technique. The ACA model has now been adapted by us for mechanism oriented research and for the integration of the longstanding experience in post-ischemia immunohistochemical research techniques. Since most ischemia related research is performed on Wistar rats, and our laboratory traditionally works with Wistar rats it was necessary to transfer the model from Sprague
/Dawley to Wistar (breed Scho¨nwalde, Germany). Adapting the model to Wistar rats should help to make the model more comparable and to enable better integration and comparison of results from others. Here we report results from the transition process to compare between Wistar and Sprague
/Dawley rats in an ACA protocol.

This may help to explain why simple transformation of models or results from one strain to another can be problematic.

2. Methods The study was approved by the Animal Care and Use Committees of the State of Saxony-Anhalt and the University of Magdeburg. Animals were treated in compliance with national and local guidelines. Male Sprague
/Dawley and Wistar rats, weighing between 330 and 420 g, were used. Wistar rats were divided into a group with intra-insult temperature monitoring (group 1) and a group with intra-insult temperature control (group 2). Sprague
/Dawley rats were in temperature controlled group 3. In each group some animals were randomly assigned to a sham operated control group (groups 1s, 2s, and 3s). For long term temperature monitoring about half the animals were prepared with implantable telemetric temperature probes (VitalView† ; Mini Mitter Co., Inc., Bend, CO, USA). The minimally invasive procedure was performed under sterile conditions about 1 week prior to the ACA experiments. The small 7/25 mm probes were placed intra-abdominally via a less than 1 cm ‘keyhole incision’. The procedure was performed during a brief period of anaesthesia with 0.5
/1.5% halothane in 50% N2O in O2, animals breathing spontaneously throughout the procedure. All rats were returned to the main holding facility thereafter. This procedure is well-established and considered side effect free [38,39]. General preparation and handling procedures for all animals were identical and have been described previously in detail [34,37]. Briefly, animals had free access to food and water up to 1 h before preparation. Anaesthesia was induced in a chamber with 3% halothane in 50% N2O in O2 and was continued via facemask until tracheal intubation. Intubation was performed using a modified laryngoscope and a 14 gauge venous catheter. Thereafter, rats were placed in the prone position and ventilation controlled (IPPV mode; SAR-830/P, CWE, Inc., Ardmore, PA, USA). To assure adequate sedation and stable cardio-circulatory conditions anaesthesia was maintained and adjusted with 0.5
/1.0% halothane in 50% N2O in O2. Airway pressure, ECG, tympanic and rectal temperature were recorded online (System 1000, CWE, Inc., Ardmore, PA, USA and Windaq-Pro† data acquisition software, DATAQ Instruments, Inc., Akron, OH, USA). For continuous blood pressure monitoring, drug administration, and blood sampling the femoral artery and inferior vena cava were cannulated with a 0.8 mm OD polyethylene catheter via the left femoral vessels. Variables that were monitored and controlled are listed in Table 1. After baseline control and 4 min of

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

191

Table 1 Baseline variables inclusive sham-operated control animals

1

Heart rate (min ) Mean artery pressure (mmHg) T tympanic (8C) Tidal volume (ml/min) Respiratory frequency (min1) Halothane (vol.%) pH (
/) pO2 (mmHg) pCO2 (mmHg) Bicarbonate (mmol/l) Base excess (mmol/l) Glucose (mmol/l) Hematocrite (%)

Group 1, n/34

Group 2, n /26

Group 3, n /7

Total, n/67

P

3329/48 1169/14 36.89/0.7 3759/21 619/2 0.79/0.1 7.399/0.03 2329/39 409/4 24.19/2.1 /0.39/2.7 7.49/2.1 349/4

3439/55 1149/11 37.09/0.2 3619/35 619/2 0.79/0.1 7.389/0.03 2369/45 389/5 23.49/2.1 /1.49/2.5 7.59/2.2 349/4

3469/30 1239/13 37.19/0.3 3259/35 499/9 0.59/0.3 7.409/0.04 2279/30 399/4 24.49/1.5 /0.49/2.0 8.29/1.4 379/4

3369/48 1169/13 36.99/0.6 3659/31 609/5 0.79/0.1 7.399/0.03 2359/42 399/4 24.09/2.1 /0.59/2.6 7.59/2.0 359/4

ns ns ns B/0.001 B/0.001 B/0.001 ns ns ns ns ns ns ns

ventilation with air asphyxia was induced by stopping the ventilator after administration of vecuronium bromide (Norcuron† ). Within a recorded period animals went into cardiac arrest with asystole or pulseless electrical activity and a pulseless mean arterial blood pressure (MAP) of less than 10 mmHg. During asphyxiation the temperature was either only monitored (group 1) or controlled (groups 2 and 3). Immediately before the start of resuscitative procedures animals received intravenously either 10 (groups 1 and 2) or 5 mg/kg (group 3) adrenaline/epinephrine and 1 mmol/kg sodium bicarbonate; control animals received equal amounts of normal saline. After exactly 8 min of asphyxiation resuscitation controlled ventilation 100% O2 and external chest compression (200/min) was started. Return of spontaneously circulation (ROSC) was defined as a pulsatile MAP of at least 60 mmHg. Rats with no ROSC within 5 min of resuscitation were excluded. Arterial blood samples were analyzed during preparation, and after ROSC at fixed time points (baseline and 5, 15, 30, and 60 min post-ROSC). One hour after resuscitation all catheters were removed, cannulated vessels were ligated, and incisions closed. Spontaneously breathing rats were weaned from mechanical ventilation and extubated. They received O2-enriched via a facemask for up to 30 min before they were returned to their cages for recovery and evaluation. Group 1 rats were housed individually in regular rat cages; groups 2 and 3 animals were placed in an incubator with a temperature of 34 8C. The incubator temperature of 34 8C was determined by earlier experiments as being optimal for maintaining normal body temperature. Twenty-four hours postresuscitation all animals were placed individually in regular seized cages with free and easily accessible food and water. Weak or relatively inactive animals were carefully nursed and fed. Overall activity and basic neurological functions were monitored using our previously reported neurological deficit score (NDS) for rats [34,37]. After final neurological evaluation animals

were re-anaesthetized and thoracotomy performed, the left ventricule was cannulated and perfused with normal saline followed by 4% buffered paraformaldehyde in 0.1 M phosphate-buffered saline (PBS), pH 7.4 for fixation. Brains were carefully separated and immersed and fixed for an additional 24 h period at 4 8C. Cryoprotection with 30% dextrose in 0.4% paraformaldehyde was performed before the brains were deep frozen in liquid nitrogen and stored at /80 8C for immunohistochemistry. Twenty micrometer thick slides were prepared using a cryomicrotom at /16 8C. Slides were collected in PBS. For haematoxylin
/eosin staining conventional techniques were used.

3. Results 3.1. Haemodynamic variables In all groups baseline values were similar and within normal ranges (Table 1). Preparation and verification of standard physiological variables required typically less than 1 h (429/11 min). After baseline recording and FiO2 reduction to 21% asphyxiation was initiated. Shortly thereafter mean arterial blood pressures (MAP) peaked at 1479/13 (groups 1/2) or 1569/15 mmHg before circulatory depression occurred. Wistar rats went into cardiac arrest within 1649/24 s whereas Sprague
/Dawley rats within 2019/10 s maintained circulation for significantly longer (P /0.001; Table 2). By the end of the 8-min asphyxiation period all rats were in cardiac arrest. At that time more than 90% of groups 1 and 2 animals were in asystole while about half of all Sprague
/Dawley rats still had some electrical activity in form of pulseless electrical activity. However, the form of electrical activity did not influence resuscitability. ROSC was achieved in 90% of all animals. Sixty Wistar (34 group 1 and 26 group 2) and seven Sprague
/Dawley rats could be used for further evaluations. As shown in

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

192 Table 2 Arrest related time intervals

Time to arrest (s) ROSC time

Group 1

Group 2

Group 3

Total

P

1659/23 (105
/205) 37.59/16.7 (22
/90)

1639/25 (111
/198) 58.69/58.0 (22
/260)

2019/10 (185
/215) 23.19/1.9 (21
/26)

1689/26 (105
/215) 44.29/39.6 (21
/260)

0.001 0.033

Ranges in brackets.

Fig. 1 blood pressure values were measured according to the protocol and averaged at fixed time points as well as at consistently reproducible extremes post-ROSC. Averaged baseline MAP was slightly lower in groups 1/2 (1159/12, 1149/11 mmHg) than in group 3 (1289/9 mmHg); P /0.011. The somewhat higher MAP in Sprague
/Dawley rats was an almost consistent trend throughout the experiment. During cardiac arrest MAP dropped in all animals to a static blood pressure of less than 10 mmHg. Immediately after resuscitation MAP peak values rose in all animals above 150 mmHg or an average of 60% above baseline (1st MAP peak ) directly followed by a 1st MAP low . The 1st MAP low was the only time when the average blood pressure in group 3 (729/8 mmHg) was less than 799/17% than in groups 1 and 2 (929/29, 809/15 mmHg). While group 1 animals maintained the best relative blood pressure 799/17% at that time point (compared to baseline), in Sprague
/ Dawley rats it dropped to only 569/16%. At 5 min post-ROSC all blood pressure curves climbed again toward the 2nd MAP peak . This was 158% of the preinsult level similar in all groups. In terms of absolute numbers the 2nd MAP peak was higher in group 3 (2009/9 mmHg) than in groups 1 (1819/10 mmHg) and 2 (1779/11 mmHg); P B/0.001. Thereafter, blood pressure values fell and reached the lowest values during the second half of the 1 h observation and intensive care period. While Wistar rats had averaged low volumes in the 60s (group 1: 679/17; group 2: 619/16 mmHg) retained Sprague
/Dawley rats of group 3 had higher MAP at 879/24 mmHg. Critical low blood pressure

levels (MAPB/50 mmHg) that required additional volume resuscitation were seen in about one-fifth of all Wistar but not in Sprague
/Dawley rats. Sixty minutes after ROSC the final measurements were performed before catheters were removed and wounds could be closed. At that time MAP’s were at 869/20 mmHg in group 1, 799/20 mmHg in group 2, and 1139/21 mmHg in group 3 (P /0.001). These pressures corresponded to 759/18, 709/19 and 889/18% of the pre-insult levels. Pre-insult heart rates were at 3409/52 heart beats/min without any group differences. Five minutes after ROSC heart rates increased temporarily by 49, 41, and 22% to 4859/18, 4729/39, and 4369/12 heart beats/min. The observed tachycardia was more pronounced in groups 1 and 2 than group 3 (P B/0.001); P /0.035. Fifteen and 30 min after ROSC heart rates returned to normal and were only slightly above baseline frequencies. At 60 min heart rates were about 20 heart beats/min below baseline in groups 1 and 2. 3.2. Blood gas values As expected, all animals went into pronounced acidosis during ischaemia and early reperfusion (Table 3). At 5 min after ROSC pH values were similar in all groups at an average low of 7.099/0.07. Ten minutes later (15 min post-ROSC) pH values in Wistar rats rose but were still acidotic. The pH in Sprague
/Dawley rats recovered significantly faster and reached normal ranges within less than 30 min. During the 1-h post-ROSC intensive care period pH values returned in all groups to

Fig. 1. Arterial blood pressures over time (k, group 1; ^, group 2; I, group 3).

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

193

Table 3 Post-resuscitation blood gas variables Time (min)

Group 1

Group 2

Group 3

P

Respiratory frequency (min)

5 60

619/2 629/3

629/2 629/3

479/10 499/11

pH (
/)

5 15 30 60

7.109/0.06 7.249/0.05 7.339/0.05 7.419/0.04

7.099/0.08 7.229/0.06 7.289/0.05 7.389/0.04

7.119/0.04 7.329/0.07 7.369/0.09 7.419/0.02

ns 0.001 0.001 0.015

pO2 (mmHg)

5 15 30 60

2049/50 2039/103 3219/93 3729/73

2299/51 2759/82 3289/69 3699/74

2459/30 3179/143 3599/130 3819/70

ns 0.004 ns ns

pCO2 (mmHg)

5 15 30 60

659/12 539/10 479/9 419/6

609/11 479/8 459/7 389/8

669/7 439/11 439/10 419/5

ns 0.011 ns ns

Bicarbonate (mmol/l)

5 15 30 60

16.79/8.6 20.19/1.7 23.59/2.6 25.79/2.7

14.29/2.1 18.29/2.9 20.39/2.7 22.39/5.2

16.69/0.6 21.49/1.2 23.99/1.1 26.19/1.0

ns B/0.001 B/0.001 0.002

Base excess (mmol/l)

5 15 30 60

/12.19/1.5 /5.49/2.2 /1.39/3.2 1.29/3.2

/13.79/3.2 /8.19/4.0 /5.39/3.5 /2.09/4.1

/10.19/0.7 /3.59/2.4 /1.19/1.0 1.49/1.5

0.007 B/0.001 B/0.001 0.002

baseline levels. Base excess values followed the same pattern but were more pronounced, returning in groups 1 and 3 animals close to normal values within 15 min, and reached pre-insult levels shortly thereafter. Group 2 normothermic Wistar rats recovered to normal, but not baseline levels, within the 1-h observation period. Five minutes post-ROSC arterial carbon dioxide tensions were moderately elevated with the smallest increase in group 2 animals (609/11 mmol/l) followed by group 1 (659/12 mmol/l) and group 3 (669/7 mmol/l). Final pCO2 values were normal with averaged 409/7 mmol/l. Baseline arterial pO2 was 2359/42 mmHg (FiO2 /0.5) without any group differences. After resuscitation and ongoing ventilation with a FiO2 /1.0 pO2 values rose in all groups to above baseline levels within 30 min. While the averaged pO2 values in groups 1 and 3 increased steadily over time it continued to stay below baseline level in group 2 for 15 min (P /0.004). The delayed arterial pO2 recovery appears to be caused by mild-tomoderate pulmonary oedema in four group 2 animals during the first 20 min post-ROSC. All animals were treated successfully by airway suction and increasing positive-end-expiratory-pressure (PEEP) to 5 cm H2O. By the end of the 1 h intensive care and monitoring phase average arterial pO2 values were at 3729/72 mmHg (FiO2 /1.0) in all groups. At baseline there were no significant group differences in serum glucose 7.89/2.0 mmol/l. Glucose levels increased about 1.5 times during early reperfusion with a

B/0.001 B/0.001

peak of 12.29/3.1 mmol/l at 5 min post-ROSC. Thereafter, glucose values decreased spontaneously. While in Sprague
/Dawley rats serum glucose returned to normoglycemic conditions, it decreased further in both Wistar groups. Some group 1 and 2 animals went into hypoglycemia (lowest measure values: 1.6 mmol/l) and had to be treated with supplemental glucose intravenously. By the end of the 60 min control period serum levels were still significantly lower in Wistar rats (group 1: 5.09/1.2 mmol/l, group 2: 4.79/1.4 mmol/l) than in group 3 Sprague
/Dawley rats (6.99/1.0 mmol/l); P B/ 0.001. Compared to baseline all 60-min glucose levels remained significantly decreased. Overall haematocrit values maintained stable. Individual temporary changes were caused by mild insults related to blood cell concentration (interstitial volume depletion) or some mild treatment (related haemodilution). 3.3. Temperature Pre-insult temperature monitoring showed different 24 h base cycles between Wistar and Sprague
/Dawley rats. Sprague
/Dawley rats had average core temperatures of 36.99/0.12 8C (slightly lower in day time and slightly higher at night). Wistar rats averaged core temperatures of 37.49/0.43 8C were measured. The 0.5 8C higher temperature in Wistar rats was mainly caused by the 38 8C (almost 1.0 8C higher) core

194

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

Fig. 2. Normal temperature cycles in Wistar (black) and Sprague
/Dawley (gray) rats.

temperature at night time (Fig. 2). During preparation, the insult and the 1st hour post-ROSC, temperature was controlled was according to protocol. After weaning, extubation, and a short period of stabilization and oxygen supplementation animals were returned to the animal holding area. Rats in group 1 had to retain and maintain body temperature on their own (housed in regular cages at room temperature) whereas groups 2 and 3 animals were warmed in an incubator at 34 8C air temperature for 24 h. All cages were prepared with easily accessible water and food. During the first 24 h after ROSC physiological temperature regulation was disturbed in animals or all groups (Fig. 3). Core temperatures in group 1 decreased to levels of moderate hypothermia at around 32 8C for a period of about 8 h. The low temperatures were in a range that must be considered secondarily neuroprotective [40]. Temperatures recovered and continued to stay at almost 35 8C between 8 and 15 h post-ROSC. Thereafter, core temperatures dropped again by an average of 18. Twenty-four hours after ROSC core temperatures in

group 1 animals returned to 35 8C with a tendency to recover further. In incubator housed animals of groups 2 and 3 temperatures were in the normal to only mildly hypothermic range with lowest values in the mid 30s at about 8 h post-ROSC. Soon thereafter temperatures in these animals recovered to 37 8C and were stable for the remainder of the first post-resuscitation day. Secondary temperature drop seemed to be caused in all groups by a lack of motor activity. Animals that did not survive the first 24 h after ROSC demonstrated a complete failure of autonomous temperature regulation. 3.4. Post-resuscitation findings After resuscitation and stabilization rats had free access to water and food. Animals that were unable to feed and drink spontaneous-supply were nursed and fed with a glucose-electrolyte solution at least four times a day. Every 24 h an overall and neurological follow-up was performed. By the end of the observation period all animals could be successfully perfusion-fixed. Brains

Fig. 3. Core temperatures during the first 24 h post-resuscitation: (A) regular temperatures (see Fig. 2); (B) temperature controlled animals (groups 2/3); (C) temperature monitored rats (group 1); (D) not surviving rat (dead 23:55 at 25.5 8C).

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

195

Fig. 4. CA1 regions in overview. Cont, control animals without insult; 8 h
/7 days survival time after 8 min of insult.

were further fixed and handled as described above. General necropsies were performed in all animals. Resuscitation related side effects and complications were seen in about a fifth of all animals. These artifacts were mild pulmonary atelectasis, mild myocardial haematoma, and macrohaematuria. In two animals that did not survive the scheduled observation period free intraabdominal blood was found. In both cases it could not be determined if the bleeding was from an injured parenchymal organ or an injured vessel; haemorrhagic shock was considered to be the cause of death. Only brains of properly euthanized animals were processed further. Representative sections of haematoxylin eosin stained CA1 regions are shown in Figs. 4 and 5.

3.5. Histology Histological evaluation of brains from sham operated control animals of all groups showed similar results. Across all non-asphyxiated animals the average proportion of damaged or dead CA1 neurons was less than 5%. No group or strain specific differences were found. Eight minutes of asphyxiation caused major to subtotal neuronal damage within the CA1 region of all strains and groups. In groups 1 and 2 */both from the same strain but with different temperature control */damaged neurons appeared similar and were characterized by shrunken cell bodies and pyknotic nuclei. The proportion of visibly damaged neurons was relatively constant

196

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

Fig. 5. CA1 regions in detail. Cont, control animals without insult; 8 h
/7 days survival time after 8 min of insult.

but the pattern of cellular disintegration and induction of resorptive processes was significantly different. Spontaneous hypothermia during insult and post-ROSC recovery caused a 24
/48 h delay in the development of the light microscopically visible picture but was unable to prevent the damage. While at 168 h (1 week) the hippocampal CA1 region in group 1 brains was still clearly visible and revealed numerous dead cell bodies, the same region in group 2 animals showed an almost complete resorption of the damaged neurons and only minor remaining cell debris. The morphological picture but not the degree of damage was distinct different in group 3 animals. Cellular damages in Sprague
/Dawley rat brains were characterized mainly by oedema, infarction, and cell lysis and less by shrunken neurons and

pyknotic nuclei. One week after resuscitation, there were only very few vital CA1 neurons left. At that time the resorptive processes were almost as advanced as in the other normothermic controlled group 2 animals. Overall by the end of the maximal survival time of 168 h nearly all neurons (969/5%) of the CA1 regions in groups 1, 2, and 3 animals were either visibly damaged or in some stage of resorption; independently of strain specific or temperature modulated differences.

4. Discussion We have demonstrated the successful adaptation of the ACA model from a Sprague
/Dawley to a Wistar

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

strain. The transformation has required some model modifications and discovered several strain specific physiological differences. Some of these differences appear to be potentially influential on clinical and morphological outcome. In general, Sprague
/Dawley rats seem to have a more ‘robust’ cardio-vascular systems as seen by the longer ‘time to cardiac arrest’ period, the shorter ROSC time*/even with a lower adrenaline epinephrine requirement */and the more stable immediate post-resuscitation phase. The latter was demonstrated by higher primary and secondary blood pressure peaks, which are important in regard to the prevention of no-reflow areas, and by later higher general blood pressure level [41,42]. It is well known that a blood pressure elevated above normal (spontaneously or physiological) immediately after ROSC is beneficial for outcome [43
/46]. Elevated heart rates on the other side */as seen initially in groups 1 and 2*/may increase myocardial oxygen consumption and the risk of secondary arrhythmias. The latter was seen in both Wistar rat groups and might be caused by the higher adrenaline/epinephrine requirement [47
/49]. Attempting to reduce the initial adrenaline/epinephrine dose in Wistar rats to the same level as used in Sprague
/Dawley rats caused delayed or ineffective ROSC. On the contrary, trying to apply higher doses of adrenaline/epinephrine in Sprague
/Dawley rats (like the 10 mg/kg in Wistar rats) increased the risk of pulmonary oedemas. The lower 1st MAP low seen in group 3 is probably caused by the lower adrenaline/epinephrine dose; it was the only time point where groups 1 and 2 average MAP’s were higher than the blood pressure in group 3. Therefore, the strain specific optimized adrenaline/epinephrine doses were used as evaluated in specifically performed experiments beforehand. In spite of the higher catecholamine use some group 1 and 2 animals did require considerably longer resuscitation times before a spontaneous circulation was restored. Whereas the shortest time to ROSC was similar across all groups (21
/22 s) the upper ranges were significantly different (90/260/26 s). The longer intervals to ROSC seemed to be caused by differences in the vitality of the ischaemic myocardium. It appears that in some cases */particularly in the normothermic Wistar rats */myocardial conditioning is required before ROSC could be established. Furthermore, the hypothetical higher myocardial sensitivity in Wistar rats would explain the loss of resuscitability seen in Wistar rats after 10 min of asphyxiation that is possible in Sprague
/ Dawley rats. This may also explain the tendency for less stable blood pressure in groups 1 and 2 in the second half of the initial post-resuscitation hour and may also be a cause for animal losses during the first 24 h. Surprisingly, the longer cardiac arrest time in groups l and 2 had only marginal effects on the degree of the initial acidosis and therefore does not explain the slower

197

arterial blood gas recovery thereafter. Strain specific differences in peripheral reperfusion and microcirculation could be a possible explanation for the delayed recovery. The differences between groups 1 and 2 may well be caused by the protective mild hypothermia. Temperature differences also seem to be an explanation for the delayed pO2 recovery in group 1 animals. Termination of the experiments by perfusion fixation of deeply anaesthetized animals in combination with cryoprotection and /80 8C preservation worked sufficiently in all animals; as did the standard HE staining and slide preparation. As expected, 8 min of asphyxiation followed by cardio-pulmonary resuscitation with a standard advanced life support protocol caused major neuronal damage in the particularly vulnerable hippocampal CA1 region. The insult is severe enough to generate early demonstrable damages and seems to be therefore a useful tool for evaluation and screening studies. Differences in the dynamics of morphological changes between groups 1 and 2 underlined again the importance of strict temperature control throughout the insult and the 1st day of survival. Spontaneously developed mild post-resuscitative hypothermia delayed light-microscopically visible neuronal damages within the hippocampal CA1 region by 24
/48 h. The neuroprotective effect of hypothermia has been known for many years [50
/53]. Comparable observations in animal survival during the 1st days post-ROSC were recently published by Hachimi-Idrissi et al. [46]. Using essentially the same model they demonstrated a better longtime survival rate in rats after controlled hypothermia (34 8C) and induced hypertension (MAP 140 mmHg) for 1 h immediately post-ROSC. Confirming our observations, secondary death occurred in the experiments by Hachimi-Idrissi et al. only within the first 7
/8 days post-ROSC; the timeframe of our evaluations. The therapeutic and, more so, the time gain effects of actively induced mild to moderate post-resuscitative hypothermia, is so well established that this intervention is currently in an advanced phase of clinical evaluation [54
/58]. However, recently the effect of spontaneous hypothermia during the first 24 h post-ROSC was widely underestimated [40]. Since, in this study, spontaneously developed hypothermia only delayed but did not prevent neuronal damage, it must be considered that under the conditions used spontaneously hypothermia cannot be seen as an independent treatment but more so as a potential interventional tool and as a study influencing co-factor. The fact that the final extent of histological damage was identical is an indicator of the severity of the insult applied. The loss of adequate temperature control seems to be caused either by postischaemic central nervous system dysregulation, by a lack of motoractivity or, more reasonably, by a combination of both. Therefore, it must be stressed that

198

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

temperature control is mandatory not only during the insult and early post-ROSC monitoring, but also for compensating autoregulatoric dysfunctions. Unexpected were the enormous differences in the primary type of visible damage. Whereas damaged neurons of Wistar rat groups 1 and 2 were characterized by shrunken neuronal cell bodies and pyknotic nuclei, the damaged neurons of Sprague
/Dawley rats showed oedematous and late lytic cell bodies without the massive chromatin aggregation. The morphological picture seen in group 1 and 2 animals was similar to these generally characterized by apoptotic cell eliminations, whereas the neuronal changes seen in group 3 animals appear to be more of a necrotic character. The question arising is: do post-ischemic
/post-resuscitative cell death processes in Wistar and Sprague
/Dawley rats differ in the main underlying mechanism? Apoptosis is an energy requiring and generally actively controlled process of programmed cell death; necrosis is a strictly pathological process generating accidental cell death. Whereas the latter is considered therapy-resistant, apoptotic processes are considered at least partially therapy sensitive. Considering that the different morphological pictures seen in Wistar and Sprague
/Dawley rats are really caused by dominantly apoptotic as opposed to necrotic processes, the influence of a selected rat strain on the effect of treatment oriented research would be enormous. Therefore, further studies are needed to specifically distinguish between apoptotic and necrotic cell death in Wistar and Sprague
/Dawley rats. In conclusion, the model of ACA developed primarily for the Sprague
/Dawley rat was successfully transferred into a Wistar rat model. Several physiological parameters had to be adjusted. Clinical as well as overall hippocampal CA1 morphological evaluation discovered strain specific post-resuscitative differences. Both rat strains can be used for resuscitation research. However, simple comparison or applying results from one strain to another however can be faulty. The distinction between apoptotic and necrotic neuronal cell death requires further studies since the underlying mechanism will influence further treatment oriented studies critically.

Acknowledgements This study was possible through grant support from the State of Saxony-Anhalt (FKZ 2768A/0087H), a grant from the Laerdal Foundation for Acute Medicine, and by Support from the Hoffman La Roche AG.

References [1] Ebmeyer U, Abramson N, Katz LM. Current research directions in cerebral resuscitation after cardiac arrest. Curr Opin Crit Care 1995;1:182
/8. [2] Safar P. Cerebral resuscitation after cardiac arrest: research initiatives and future directions. Ann Emerg Med 1993;22:324
/ 49. [3] Safar P. Resuscitation medicine research: quo vadis. Ann Emerg Med 1996;27:542
/52. [4] Abramson NS, Safar P, Kelsey SF, Detre KM. Clinical trials and cerebral resuscitation research. Ann Emerg Med 1984;13:868
/72. [5] Safar P. Ethical dilemmas in resuscitation medicine. Minerva Anestesiol 1994;60:603
/6. [6] Abramson NS, Kelsey SF, Safar P, Sutton-Tyrrell K. Simpson’s paradox and clinical trials: what you find is not necessarily what you prove. Ann Emerg Med 1992;21:1480
/2 (see comments). [7] Brain Resuscitation Clinical Trial I Study Group. A randomized clinical study of cardiopulmonary
/cerebral resuscitation: design, methods, and patient characteristics. Am J Emerg Med 1986;4:72
/86. [8] Brain Resuscitation Clinical Trial I Study Group. Randomized clinical study of thiopental loading in comatose survivors of cardiac arrest. N Engl J Med 1986;314:397
/403. [9] Brain Resuscitation Clinical Trial II Study Group. A randomized clinical study of a calcium-entry blocker (lidoflazine) in the treatment of comatose survivors of cardiac arrest. N Engl J Med 1991;324:1225
/31. [10] Wenzel V, Lindner KH, Prengel AW, Maier C, Voelckel W, Lurie KG, et al. Vasopressin improves vital organ blood flow after prolonged cardiac arrest with postcountershock pulseless electrical activity in pigs. Crit Care Med 1999;27:486
/92 (see comments). [11] Sterz F, Zeiner A, Kurkciyan I, Janata K, Mullner M, Domanovits H, et al. Mild resuscitative hypothermia and outcome after cardiopulmonary resuscitation. J Neurosurg Anesthesiol 1996;8:88
/96. [12] Baumgartner WA, Walinsky PL, Salazar JD, Tseng EE, Brock MV, Doty JR, et al. Assessing the impact of cerebral injury after cardiac surgery: will determining the mechanism reduce this injury? Ann Thorac Surg 1999;67:1871
/3. [13] Suominen P, Olkkola KT, Voipio V, Korpela R, Palo R, Rasanen J. Utstein style reporting of in-hospital pediatric cardiopulmonary resuscitation. Resuscitation 2000;45:17
/25. [14] Dick WF, Baskett PJ. Recommendations for uniform reporting of data following major trauma */the Utstein style. A report of a working party of the International Trauma Anaesthesia and Critical Care Society (ITACCS). Resuscitation 1999;42:81
/100. [15] Cummins RO, Chamberlain D, Hazinski MF, Nadkarni V, Kloeck W, Kramer E, et al. Recommended guidelines for reviewing, reporting, and conducting research on in-hospital resuscitation: the in-hospital ‘Utstein style’. American Heart Association. Ann Emerg Med 1997;29:650
/79. [16] Zaritsky A, Nadkarni V, Hazinski MF, Foltin G, Quan L, Wright J, et al. Recommended guidelines for uniform reporting of pediatric advanced life support: the pediatric Utstein style. American Academy of Pediatrics, American Heart Association and the European Resuscitation Council. Ann Emerg Med 1995;26:487
/503. [17] Cummins RO, Chamberlain DA, Abramson NS, Allen M, Baskett PJ, Becker L, et al. Recommended guidelines for uniform reporting of data from out-of-hospital cardiac arrest: the Utstein Style. A statement for health professionals from a task force of the American Heart Association, the European Resuscitation Council, the Heart and Stroke Foundation of Canada, and the Australian Resuscitation Council. Circulation 1991;84:960
/75.

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200 [18] Idris AH, Becker LB, Ornato JP, Hedges JR, Bircher NG, Chandra NC, et al. Utstein-style guidelines for uniform reporting of laboratory CPR research: a statement for health care professionals from a Task Force of the American Heart Association, the American College of Emergency Physicians, the American College of Cardiology, the European Resuscitation Council, the Heart and Stroke Foundation of Canada, the Institute of Critical Care Medicine, the Safar Center for Resuscitation Research and the Society for Academic Emergency Medicine. Ann Emerg Med 1996;28:527
/41. [19] Meller ST, Lewis SJ, Brody MJ, Gebhart GF. Nociceptive afferent vagal input is enhanced after transection of the aortic depressor nerve. Hypertension 1990;15:797
/802. [20] Chen X, Henderson K, Beinfeld MC, Westfall TC. Alterations in blood pressure of normotensive and hypertensive rats following intrathecal injections of neuropeptide Y. J Cardiovasc Pharmacol 1988;12:473
/8. [21] Iwasaki H, Ohmachi Y, Kume E, Krieglstein J. Strain differences in vulnerability of hippocampal neurons to transient cerebral ischaemia in the rat. Int J Exp Pathol 1995;76:171
/8. [22] Bochnowicz S, Osborn RR, Luttmann MA, Louden C, Hart T, Hay DW, et al. Differences in time-related cardiopulmonary responses to hypoxia in three rat strains. Clin Exp Hypertens 2000;22:471
/92. [23] Depuydt P, Joos GF, Pauwels RA. Ambient ozone concentrations induce airway hyperresponsiveness in some rat strains. Eur Respir J 1999;14:125
/31. [24] Matsubara S, Nakata A, Kikuchi M, Kikkawa H, Ikezawa K, Naito K. Strain-related differences in Sephadex bead-induced airway hyperresponsiveness and inflammation in rats. Inflamm Res 1997;46:299
/305. [25] Andrews JS, Jansen JH, Linders S, Princen A, Broekkamp CL. Performance of four different rat strains in the autoshaping, twoobject discrimination, and swim maze tests of learning and memory. Physiol Behav 1995;57:785
/90. [26] Onaivi ES, Maguire PA, Tsai NF, Davies MF, Loew GH. Comparison of behavioral and central BDZ binding profile in three rat lines. Pharmacol Biochem Behav 1992;43:825
/31. [27] Yoon YW, Lee DH, Lee BH, Chung K, Chung JM. Different strains and substrains of rats show different levels of neuropathic pain behaviors. Exp Brain Res 1999;129:167
/71. [28] Woolfolk DR, Holtzman SG. Rat strain differences in the potentiation of morphine-induced analgesia by stress. Pharmacol Biochem Behav 1995;51:699
/703. [29] Manahan VD. Long-term depression in freely moving rats is dependent upon strain variation, induction protocol and behavioral state. Cereb Cortex 2000;10:482
/7. [30] Loscher W, Cramer S, Ebert U. Differences in kindling development in seven outbred and inbred rat strains. Exp Neurol 1998;154:551
/9. [31] Berdanier CD, Thomson AR. Comparative studies on mitochondrial respiration in four strains of rats (Rattus norvegicus). Comp Biochem Physiol [B] 1986;85:531
/5. [32] Lau YT, Tsai CJ, Tseng AH. Comparison of sodium transport processes of human and rat erythrocytes in hypertension. J Formos Med Assoc 1992;91:674
/9. [33] Honda H, Shibuya T, Salafsky B. Brain synaptosomal Ca2/ uptake: comparison of Sprague
/Dawley, Wistar
/Kyoto and spontaneously hypertensive rats. Comp Biochem Physiol B 1990;95:555
/8. [34] Katz L, Ebmeyer U, Safar P, Radovsky A, Neumar R. Outcome model of asphyxial cardiac arrest in rats. J Cereb Blood Flow Metab 1995;15:1032
/9. [35] Hendrickx HH, Rao GR, Safar P, Gisvold SE. Asphyxia, cardiac arrest and resuscitation in rats. I. Short term recovery. Resuscitation 1984;12:97
/116.

199

[36] Hendrickx HH, Safar P, Miller A. Asphyxia, cardiac arrest and resuscitation in rats. II. Long term behavioral changes. Resuscitation 1984;12:117
/28. [37] Radovsky A, Katz L, Ebmeyer U, Safar P. Ischemic neurons in rat brains after 6, 8, or 10 minutes of transient hypoxic ischemia. Toxicol Pathol 1997;25:500
/5. [38] Chamberlain D, Cummins RO. Recommended guidelines for uniform reporting of data from out-of-hospital cardiac arrest: the ‘Utstein style’. European Resuscitation Council, American Heart Association, Heart and Stroke Foundation of Canada and Australian Resuscitation Council. Eur J Anaesthesiol 1992;9:245
/56. [39] Faber JM, Akers TK, Belknap JK, Aasen GH. Commonalities between gas anesthetics (nitrous oxide, nitrogen and/or argon) and ethanol intoxication in hot and cold selection line mice. Biomed Sci Instrum 1991;27:127
/30. [40] Hickey RW, Ferimer H, Alexander HL, Garman RH, Callaway CW, Hicks S, et al. Delayed, spontaneous hypothermia reduces neuronal damage after asphyxial cardiac arrest in rats. Crit Care Med 2000;28:3511
/6. [41] Bottiger BW, Krumnikl JJ, Gass P, Schmitz B, Motsch J, Martin E. The cerebral ‘no-reflow’ phenomenon after cardiac arrest in rats */influence of low-flow reperfusion. Resuscitation 1997;34:79
/87. [42] Fischer M, Hossmann KA. No-reflow after cardiac arrest. Intensive Care Med 1995;21:132
/41. [43] Shaffner DH, Eleff SM, Brambrink AM, Sugimoto H, Izuta M, Koehler RC, et al. Effect of arrest time and cerebral perfusion pressure during cardiopulmonary resuscitation on cerebral blood flow, metabolism, adenosine triphosphate recovery, and pH in dogs. Crit Care Med 1999;27:1335
/42. [44] Ebmeyer U, Safar P, Radovsky A, Xiao F, Capone A, Tanigawa K, et al. Thiopental combination treatments for cerebral resuscitation after prolonged cardiac arrest in dogs. Exploratory outcome study. Resuscitation 2000;45:119
/31. [45] Leonov Y, Sterz F, Safar P, Johnson DW, Tisherman SA, Oku K. Hypertension with hemodilution prevents multifocal cerebral hypoperfusion after cardiac arrest in dogs. Stroke 1992; 23:45
/53. [46] Hachimi-Idrissi S, Corne L, Huyghens L. The effect of mild hypothermia and induced hypertension on long term survival rate and neurological outcome after asphyxial cardiac arrest in rats. Resuscitation 2001;49:73
/82. [47] Karns JL. Epinephrine-induced potentially lethal arrhythmia during arthroscopic shoulder surgery: a case report. AANA J 1999;67:419
/21. [48] Berg RA, Otto CW, Kern KB, Hilwig RW, Sanders AB, Henry CP, et al. A randomized, blinded trial of high-dose epinephrine versus standard-dose epinephrine in a swine model of pediatric asphyxial cardiac arrest. Crit Care Med 1996;24: 1695
/700. [49] Berg RA, Otto CW, Kern KB, Sanders AB, Hilwig RW, Hansen KK, et al. High-dose epinephrine results in greater early mortality after resuscitation from prolonged cardiac arrest in pigs: a prospective, randomized study. Crit Care Med 1994;22:282
/90. [50] Dietrich WD, Alonso O, Busto R, Globus MY, Ginsberg MD. Post-traumatic brain hypothermia reduces histopathological damage following concussive brain injury in the rat. Acta Neuropathol (Berl) 1994;87:250
/8. [51] Sterz F, Safar P, Tisherman S, Radovsky A, Kuboyama K, Oku K. Mild hypothermic cardiopulmonary resuscitation improves outcome after prolonged cardiac arrest in dogs. Crit Care Med 1991;19:379
/89 (see comments). [52] Safar P, Klain M, Tisherman S. Selective brain cooling after cardiac arrest. Crit Care Med 1996;24:911
/4 (editorial; comment).

200

U. Ebmeyer et al. / Resuscitation 53 (2002) 189
/200

[53] Holzer M, Behringer W, Schorkhuber W, Zeiner A, Sterz F, Laggner AN, et al. Mild hypothermia and outcome after CPR. Hypothermia for Cardiac Arrest (HACA) Study Group. Acta Anaesthesiol Scand Suppl 1997;111:55
/8. [54] Clifton GL, Choi SC, Miller ER, Levin HS, Smith KR, Jr., Muizelaar JP, et al. Intercenter variance in clinical trials of head trauma */experience of the National Acute Brain Injury Study: Hypothermia. J Neurosurg 2001;95:751
/5. [55] Eisenburger P, Sterz F, Holzer M, Zeiner A, Scheinecker W, Havel C, et al. Therapeutic hypothermia after cardiac arrest. Curr Opin Crit Care 2001;7:184
/8.

[56] Marion DW, Obrist WD, Carlier PM, Penrod LE, Darby JM. The use of moderate therapeutic hypothermia for patients with severe head injuries: a preliminary report. J Neurosurg 1993;79:354
/62. [57] Marion DW. Therapeutic moderate hypothermia and fever. Curr Pharm Des 2001;7:1533
/6. [58] Zeiner A, Holzer M, Sterz F, Behringer W, Schorkhuber W, Mullner M, et al. Mild resuscitative hypothermia to improve neurological outcome after cardiac arrest. A clinical feasibility trial. Hypothermia After Cardiac Arrest (HACA) Study Group. Stroke 2000;31:86
/94.

Portuguese Abstract and Keywords Um modelo de paragem cardı´aca por asfixia em ratos, inicialmente desenvolvido para ratos Sprague
/Dawley, foi transferido para um modelo com ratos Wistar. Foram necessa´rios va´rios ajustamentos nos protocolos de suporte de vida especı´ficos para a linhagem, como os paraˆmetros de ventilac¸a˜o, de anestesia e as necessidades em medicamento para estabilizar o modelo para ratos Wistar. Apesar destes ajustes, muitas das varia´veis relacionadas com a reanimac¸a˜o permaneceram diferentes. Compararam-se e avaliaramse treˆs grupos: grupo 1 Wistar com monitorizac¸a˜o de temperatura (n /34), grupo 2 Wistar com temperatura controlada (n /26) e um grupo 3 Sprague
/Dawley com temperatura controlada (n /7). Globalmente, os ratos Wistar parece terem um sistema cardiocirculato´rio mais sensı´vel, evidenciado pelo desenvolvimento mais ra´pido de paragem cardı´aca (164 vs 201 s), necessitando de doses maiores de adrenalina/epinefrina (10 vs 5 mg/kg) e de mais tempo para recuperar apo´s reanimac¸a˜o (i.e. para o retorno de pressa˜o arterial e normalizac¸a˜o dos gases do sangue). Quando na˜o ha´ um controle apertado de temperatura (como nos ratos dos grupos 2 e 3) evoluem para uma hipotermia ligeira ou moderada nas 24 h que se seguem a` restaurac¸a˜o da circulac¸a˜o espontaˆnea. A hipotermia espontaˆnea atrasou 24
/48 h o desenvolvimento de lesa˜o global neuronal CA1 visı´vel, mas na˜o impediu; por conseguinte, o modelo parece adequado para estudos futuros. As leso˜es neuronais na regia˜o CA1 dos ratos Wistar parecem provocar enrugamento dos corpos celulares e nu´cleos picno´ticos antes da reabsorc¸a˜o enquanto nos ratos Sprague
/Dawley aparecem na mesma regia˜o. Palavras chave : Ressuscitac¸a˜o; Paragem cardı´aca; Hipotermia; Asfixia; Progno´stico; Neuro´nios

Spanish Abstract and Keywords Se transfirio´ a un modelo en ratas Wistar, un modelo de paro cardı´aco por asfixia, originalmente disen˜ado para ratas Sprague
/ Dawley. Fueron necesarios una serie de ajustes de soporte vital especı´ficos para la raza, por ejemplo programaciones de ventilador, anestesia, requerimientos de drogas para estabilizar el modelo para las ratas Wistar. A ¯ pesar de estos arreglos, muchas variables relacionadas con la resucitacio´n fueron diferentes. Se evaluaron y compararon tres grupos: El grupo Wistar 1 con monitorizacio´n de la temperatura (n /34), el grupo Wistar 2 (n /26) con temperatura controlada y el grupo Sprague
/Dawley 3 (n /7). De todas, las ratas Wistar parecen tener un sistema cardiocirculatorio ma´s sensible evidenciado por un desarrollo mas rapido del paro cardı´aco (164 vs 201 s), requiriendo mas altas dosis de adrenalina/epinefrina (10 vs 5 mg/kg ) y requirieron ma´s tiempo para recuperarse despue´s de la resucitacio´n (por ejemplo para retornar la presio´n arterial y gases arteriales).Sin control estricto de la temperatura (como en las ratas de los grupos 2/3) las ratas del grupo 1 fueron a hipotermia leve a moderada en las primeras 24 horas post retorno a circulacio´n esponta´nea (ROSC). La hipotermia esponta´nea demoro´ 24
/48 h el desarrollo del dan˜o total visible de las neuronas CAI, pero no lo previno; por ello el modelo parecio´ adecuado para futuros estudios. Los dan˜os neuronales en la regio´n CAI en las ratas Wistar parecieron ser mayores (more) ya que (as) antes de la reabsorcio´n se vieron los cuerpos celulares encogidos y los nu´cleos picno´ticos, mientras que en las Sprague
/Dawley aparecieron en la misma regio´n. Palabras clave : Resucitacio´n; Paro cardı´aco; Hipotermia; Asfixia; Resultado; Neuronas