Dantrolene sodium for treatment of heatstroke victims: Lack of efficacy in a canine model

Dantrolene Sodium for Treatment of Heatstroke Victims: Lack of Efficacy in a Canine Model JAMES T. AMSTERDAM, DMD, MD,* SCOTT A. SYVERUD, MD,* WILLIAM J. BARKER, MD,t GORDON R. BILLS, MD,+ DAVID D. GOLTRA, MD,* JOSEPH C. ARMAO, BA,* JERRIS R. HEDGES, MS, MD*

Dantroiene sodium, a skeletal-muscle relaxant known to be effective for treatment of malignant hyperthermia, was evaluated for efficacy in treatment of heatstroke. Non-exertional heatstroke was induced in 11 dogs by external heating following barbiturate anesthesia. When core temperature reached 43% (109.4”F) heating was discontinued and control animals (n = 6) were ailowed to cool passively in room air. Treatment animals (n = 5) received 5 mg/kg dantroiene sodium intravenously at the start of room-air cooling. Serial temperatures (pulmonary arterial, rectal, cerebral, and subcutaneous), blood chemistry tests (including electrolytes, liver enzymes, and complete blood count), and hemodynamic parameters (including cardiac output, arterial pressure, and urinary output) were followed for 12 hours after induction of heatstroke. Autopsies, including gross and microscopic examination, were performed on ail animals. Dantroiene administration did not significantly affect cooling rates, hemody namic parameters, pathological changes, or clinical outcome. Statistically significant changes in urinary output and serum creatinine observed in the first hours after dantmlene administration can be attributed to the mannitoi vehicle in which the drug was delivered. There were no statistically significant differences in these values at 12 hours. Dantmiene sodium does not appear to enhance passive cooling in treatment of non-exertional canine heatstroke. (Am J Emerg Med 1966;4:399-405)

The primary treatment for heatstroke is to reduce body temperature as quickly as possible. The benefit of rapid cooling has been demonstrated in both canine

From the “Department of Emergency Medicine, University of Cincinnati Medical Center, Cincinnati, the tDepartment of Emergency Medicine, the Fairfax Hospital, Falls Church, Virginia, and the *Department of Pathology and Laboratory Medicine, University of Cincinnati Medical Center, Cincinnati. Manuscript 1966

received

February

11, 1986; accepted

Address reprint requests to Dr. Amsterdam: Emergency Medicine, University of Cincinnati 234 Goodman St., Cincinnati, OH 45267-0769 Key Words: Dantrolene, heatstroke, non-exertional heatstroke. 07356757186

$00.00

+ .25

malignant

February

27,

Department of Medical Center,

hyperthermia,

and human studies.1,2 Conventional methods of cooling have consisted of immersion in an ice bath, packing in ice, cool water sponging with or without fanning, iced lavage, and use of a hypothermia blanket3 In previous canine studies, we have demonstrated the efficacy of iced gastric lavage to achieve rapid cooling and the potential of jet ventilation as an adjunctive cooling technique.4y5 Several case reports have recently appeared in the literature suggesting that dantrolene be used in the treatment of heatstroke.6-8 There have been no controlled studies of dantrolene for this indication. A clinical multicenter trial of dantrolene sodium for human heatstroke is currently in progress.9 Dantrolene sodium, a skeletal muscle relaxant, acts directly on skeletal muscle fibers distal to the neuromuscular junction by the inhibition of the release of calcium ions (Ca”) from the sarcoplasmic reticulum.iO-I3 Muscle spasticity and heat production are limited by the reduced calcium release; therefore, dantrolene is useful in the prophylaxis and treatment of malignant hyperthermia in both humans and swine.14-l6 The clinical picture of patients undergoing malignant hyperthermia and those with heatstroke is very similar.‘jJ’ The questions then arise whether 1) some cases of heatstroke may actually be “stressed induced” cases of malignant hyperthermia$ 2) muscle rigidity may play an important role in the excessive heat production that characterizes exertional heatstroke; and 3) dantrolene administration will reverse the defect in central thermoregulation common to classic heatstroke, exertional heatstroke, and malignant hyperthermia. The present study was designed to evaluate the potential benefit of dantrolene sodium on cooling rate and biochemical changes using an established non-exertional anesthetized canine heatstroke mode1.4,5*18*19A beneficial effect would support the clinical use of the drug in heatstroke and would suggest that dantrolene reverses a pathophysiological mechanism common to heatstroke and malignant hyperthermia victims. 399

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MATERIALS AND METHODS General Preparation Eleven adult mongrel dogs weighing 16-22 kg were studied. The animals were anesthetized with 30 mg/kg pentobarbital, and their coats were shaved with animal shears. Animals were intubated with a standard cuffed endotracheal tube connected to a volume cycled ventilator (Harvard Pump, Model 607), which was set at a rate of 16 breaths per minute and a tidal volume of 25 ml/kg and adjusted to maintain a Pko, of 30 to 40 mm Hg. Room air was used for ventilation. Cardiac activity was monitored on a chart recorder (Hewlett Packard, Model 7758A) via limb leads. Hemodynamic monitoring consisted of a 15-cm arterial catheter placed into the left femoral artery via cutdown and a pediatric thermodilution pulmonary artery catheter (American Edwards, Model 93-132-5Fr), which was passed through a venous introducer (6 French, Cordis) via cut down in the right external jugular vein and advanced into the proximal pulmonary artery under fluoroscopic guidance. Arterial and pulmonary artery pressures were monitored on the chart recorder using high-frequency responsive pressure transducers (Statham P23 1D). Dextrose 5% in lactated Ringers’ solution was administered at a rate of 2 mlikglhr using a continuous infusion pump (IVAC model #630). A left femoral venous line was placed in treatment animals (n = 5) for delivery of dantrolene sodium. A urinary catheter was placed for monitoring of hourly urinary output. Temperatures were monitored in the rectum, pulmonary artery, subcutaneous tissue of the chest wall, and cerebral cortex. Pulmonary artery temperature was determined from the thermodilution catheter. Brain and subcutaneous temperatures were monitored using modified thermodilution catheters (American Edwards, Model 93A-131-7Fr). The thermistors were prepared and calibrated as previously described,4 and were consistent to within 0.03”C. All temperatures were read from the “patient temperature” mode of the cardiac output computer (American Edwards, Model 9520A). Uniform placement of the subcutaneous, rectal, and cerebral thermistors was as previously described, and intracerebral position of the tip of the brain thermistor was confirmed at autopsy.4 Heatstroke was induced by external heating through a circulating water blanket (Blanketrol, Cincinnati Subzero Products) using a technique developed by Bynum and used by us in previous studies.4J8,19 Room temperature and relative humidity were controlled at 20°C and 65%, respectively. To prevent burns, animals were wrapped in thin corrugated cardboard prior to wrapping in the thermal blanket; temperature between the skin and cardboard was 400

5 H September

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maintained at less than 48°C. When the rectal temperature reached 43.o”C, the heating blanket and cardboard were removed, and the animal was placed in a supine position. This moment was designated as “time zero” (T,). The dogs in the control group (n = 6) were allowed to cool passively in the supine position. Animals in the treatment group were given 5 mg/kg of dantrolene sodium reconstituted in a solution of 750 mg/kg of mannitol, 15 ml/kg of sterile water, and enough sodium hydroxide to yield a pH of 9.5. This was given as an intravenous bolus over 5 minutes beginning at T,. After T,, the dogs were monitored until mean arterial pressure fell below 25 mmHg or until 12 hours had elapsed, at which time final blood levels were drawn and the animal was killed with intravenous pentobarbital. All animals were maintained on the Harvard ventilator for the first two hours of cooling and then were allowed to breathe spontaneously. Cardiac outputs were determined using 5-ml roomtemperature saline injections through the thermodilution catheter. Outputs were measured at the beginning of the experiment, at T,, and 30, 60, and 120 minutes after T,, and at the completion of the experiment. Temperatures were recorded at ten-minute intervals during heating, five-minute intervals for the first two hours of cooling, at lS-minute intervals until pulmonary artery temperature reached 37”C, and at hourly intervals for the remainder of the experiment. Anesthesia adequate to suppress spontaneous limb movement was maintained throughout the experiment with 30-mg increments of pentobarbital. Pathological Analysis Post-mortem examinations were performed on all animals within 12 hours of death. All major organs, including the brain, were grossly examined and weighed. Tissue samples were fixed in 10% formalin, prepared, and examined in the manner previously described.4 Qualitative comparison of changes observed in each of the two groups was made by systematic evaluation of the paraffin sections and gross findings. Laboratory Analysis Serial laboratory studies, including complete blood counts, electrolytes, blood urea nitrogen, creatinine, calcium, phosphorus, hepatic enzymes, cardiac enzymes with fractionation, prothrombin time. and urinalysis were obtained from the femoral arterial line prior to heating, at T,, at 30,60, and 120 minutes after T,, and just prior to death of the animal. Serial arterial blood gases were obtained at baseline (T,) and at 30, 60, and 120 minutes after T,. Arterial blood gas analysis was performed on an Instrumentation Laboratory System Analyzer (Model 1303), and values were

AMSTERDAM

mathematically corrected for animal temperature. Creatinine kinase isoenzyme analysis was performed using agarose electrophoresis scanned on a densitometer (Quick Scan, Helena Laboratories) to yield a graphic plot of MM, MB, and BB isoenzyme band amounts. The percentage of total creatine phosphokinase (CPK) made up by each isoenzyme was calculated using an established method.20~2’ Data Analysis Cooling rates for each animal were determined by linear regression analysis of all data points from ten mintues after To to 120 minutes after T,. This interval was chosen because temperatures tended to plateau in the.control group and then fall after ten minutes. Cooling rates were corrected for the body surface area (BSA) according to the method described by Cowgill.** One control animal was used only for calculation of cooling rates; hemodynamic and laboratory parameters were not measured for this animal. This control animal was killed at 150 minutes after To; mean arterial pressure was 80 mm Hg at this time. Statistical Analysis Measured parameters in the two groups were compared for statistically significant differences using the two-tailed Student’s t-test, with P < 0.05denoting significance. If the variances differed significantly between the groups (F-test of variances), a BehrensFischer t-test was used. When a significant difference in mean values for the parameters was detected, the data were re-analyzed to determine whether the change from baseline values was also significantly different between the groups. When differences between parameters measured in the same group of animals were analyzed, the two-tailed paired Student’s t-test was used. RESULTS Survival Three animals in each group maintained a mean arterial pressure >25 mm Hg for a full 12 hours after heating. Two animals in each group did not and were killed when mean arterial pressure fell below this level. This occurred at six and 11 hours after heating for the two dantrolene-treated animals, and at seven and 11 hours after heating for the two controls. Cooling Rates The five dantrolene-treated animals had a mean cooling rate (determined over the first two hours of cooling) that was smaller although not statistically significantly different (P > 0.05) than the mean cooling

TABLE1.

ET AL n DANTROLENE

SODIUM

IN HEATSTROKE

Cooling Rates of Various Tissues* Controls (n = 6)

Cooling Rates (“C/min/m* Brain Pulmonarv Arterv Rectal _ _ Subcutaneous Mean BSAt fm2) Mean Weight (kg)

BSA) 0.0351 0.0386 0.0358 0.0594 0.83 20.7

f 2 2 ? i 2

0.0071 0.0082 0.0077 0.0173 0.043 1.4

Dantrolene In = 5) 0.0314 0.0322 0.0358 0.0473 0.75 18.0

-r- 0.0104 * 0.0126 +- 0.0169 2 0.0141 2 0.058 2 1.6

* Values are mean * SD. t Body surface area.

rate determined for the control animals (Table 1). Because of the small number of animals used in this study, the possibility of failure to detect a statistically significant difference where one truly existed was considered. Assuming that treatment with dantrolene would double rectal cooling rate, the probability of failing to detect a statistically significant difference in rectal cooling rate using two groups of five animals each is less than 10%. For both groups, the cooling rate obtained from a subcutaneous temperature probe was greater than the central cooling rate obtained from a rectal temperature probe (P < 0.05) (Fig. 1). Typical temperature cooling curves for a control animal are shown in Figure 2. laboratory Findings Analysis of laboratory data was performed for five animals in each group. One control animal was used for cooling rate data collection only. Although a number of hematologic, urinary, and serum chemistry changes were noted during the heating and subsequent cooling period, the two groups were comparable in their response. There were no statistically significant differences found between groups in leukocyte count, erythrocyte count, hemoglobin, hematocrit, platelet count, or protime; each group demonstrated leukocytosis, hemoconcentration, a decrease in pfatelet count compared with baseline, and an elevation in the prothrombin time as a result of heatstroke. The two groups also did not show a statistically significant difference in mean arterial blood gas parameters; each demonstrated a significant acidosis at 120 minutes of cooling as a result of heatstroke. No statistically significant differences were found between the two groups regarding the following serum chemistries: blood urea nitrogen, sodium, potassium, chloride, carbon dioxide content, phosphorous, calcium, total protein, albumin, alkaline phosphatase, creatine kinase, and hepatic exzymes (SGOT, SGPT, LDH). Changes in creatine kinase levels are shown graphitally in Figure 3. Creatine kinase isoenzyme values are shown in Table 2. Urine protein level, pH, and 401

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45

0.1000

Cooling Rates 0.0800

44

0

Controls

m

Dantrolene

T

“E 2

0.0800

A

rnY

3

LT

0.0400

-COOLING__)

-HEATING

l--r

43

Cooling Curves Control Animal +5

42

0-O

O-O Skin Rectal

D.-O Brain ti

41

Pulmonary Artery

9

Rectal

FIGURE

Central Venous

40

39

Skin

Brain

36

1 (above). Cooling rates for control and dantrolene-

treated groups.

37

FIGURE 2 (righr). Typical cooling curves recorded during study. Central venous temperature is actually mixed venous temperature as measured in the proximal pulmonary artery.

I

I

I

-60

0

60

hl

I

120

160

Minutes After Heating

275

2500 !z ;

2000

2 5 z

1500-

-

W Controls A-A Treatment

250

i

with Dantrolene

-

5 z 5 : r%

1000

-

500 -

1

13

0

30

60

120

11

”

,

12Hrs

Time (mid

FIGURE 3 (above). Creatine kinase levels after induction of-heatstroke. p indicates baseline level. Shaded area represents period of external heating. FIGURE 4 (right). Hemodynamic changes during and after heatstroke induction. p indicates post-anesthesia, pre-external heating values.

meantSD~~6

0 3060901201H2, Controls

1 a 3 0 1 8 b-s 1 0 0 30609012i);’ Dantrolene

Time in Minutes After Heating

402

AMSTERDAM

erythrocyte count were not statistically significantly different between the two groups. Statistically significant differences between the two groups were found for only a few of the parameters. Mean creatinine levels were significantly lower (P < 0.05) for the dantrolene group at 30 and 60 minutes following the start of cooling (1.0 ? 0.2 and 1.0 ? 0.2 for the dantrolene group versus 1.4 _t 0.3 and 1.5 + 0.3 for the control group, respectively) although subsequent levels were not significantly different. The final mean urine specific gravity was significantly higher (P < 0.05) for the dantrolene group (1.043 + 0.0) than the control group (1.015 rt 0.007). Mean urinary production rate was statistically significantly greater (P < 0.05) for the dantrolene group (44 + 32 ml/hr versus 9 + 9 ml/hr) at 120 minutes following the start of cooling, although the urinary production rate was similar for the two groups at the time of final measurement (12 ? 7 ml/hr for the dantrolene group versus 21 ? 21 ml/hr for the controls). Hemodynamic Data Hemodynamic changes with time are shown graphically for both groups in Figure 4. With the exception of a larger central venous pressure measurement at 60 minutes after the start of cooling for the dantroiene group, no statistically significant differences were found between the groups. Pathological Findings At post-mortem examination, identical pathological findings were observed in both groups of animals. Observed abnormalities included generalized vascular congestion, pulmonary edema, central lobular necrosis of the liver, and myonecrosis of skeletal muscle. These findings are similar to those previously described in dogs and humans exposed to extreme or prolonged hyperthermia. 18*23 No differences in degree, type, or distribution of pathological abnormalities were observed between the two groups. DISCUSSION Heatstroke is characterized by exposure to an excessive heat load resulting in a marked increase in body temperature, central nervous system dysfunction, hemodynamic instability, and tissue damage.23 Excessive heat load can result from increased internal heat production, increased external heat absorption, decreased heat dissipation, or any combination thereof. In classic heatstroke, defective heat loss mechanisms predominate. 24 In exertional heatstroke, excessive heat production predominates.25 In both types, heat production exceeds heat dissipation capabilities.

TABLE 2.

ET AL n DANTROLENE

Creatine

SODIUM

Kinase (CK) lsoenzyme

Baseline

Peak Heating

IN HEATSTROKE

Levels’

After 120 Min Cooling

Final

Control (n = 5) CK-MM (U/l) 144 k 41 CK-BB (U/I) 110 ‘_ 48

46 2 32 134 1?:91

532 ? 388 303 + 262

554 f 456 1,072 + 762

Dantrolene (n = 5) CK-MM (U/I) 105 2 66 CK-BB (U/I) 93 ” 53

46 i 18 71 2 19

1,090 + 651 321 2 193

382 f 315 1,151 + 737

l

All values are mean

+- SD.

Malignant hyperthermia is a familial disorder characterized by muscle rigidity and hyperpyrexia in response to various anesthetic agents.z6 Excess heat production caused by the hypermetabolic state of skeletal muscle is presumed to be the origin of the hyperpyrexia. Muscle rigidity is also a common clinical observation in cases of human heatstroke.23*27 The similar clinical, laboratory, and pathological characteristics of heatstroke and malignant hyperthermia have led some to hypothesize a single pathophysiological mechanism for both disorders.” A central disorder of thermoregulation in response to excessive heat load (from any source) is hypothesized. Supporting this possibility are the observations that patients with known susceptibility to malignant hyperthermia are more prone to develop heatstroke,i7 and that patients who have recovered from heatstroke subsequently have abnormal physiological responses to heat stress.** There are three theoretical rationales for the use of dantrolene sodium in human heatstroke: 1) some cases of heatstroke may actually be “stressed induced” cases of malignant hyperthermia$ 2) muscle rigidity may play an important role in the excessive heat production that characterizes exertional heatstroke; and 3) dantrolene administration might reverse the defect in central thermoregulation common to classic heatstroke, exertional heatstroke, and malignant hyperthermia. This study used an anesthetized, externally heated canine model to test for dantrolene efficacy. This model resembles classic heatstroke in that exertional heat production is not the major factor in the excessive heat load. Furthermore, muscle rigidity is not a characteristic of this model, although the clinical course, laboratory test alterations, and pathological changes observed closely mimic those observed in exertional heatstroke and malignant hyperthermia.‘p2 The model used in this study differed from that developed by Bynumi8 in that the dogs were orotracheally intubated and had controlled ventilations during the heatstroke period. This variation eliminated physiological panting responses in the dog, which employ a counter-current exchange cooling mechanism that 403

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does not occur in man1*J9 Increased survival rates have been observed in conscious dogs with heatstroke that are able to pant versus unconscious dogs that cannot.30 Elimination of this physiological response in our dogs may account for the observation of brain temperatures that were not significantly different from pulmonary artery core temperatures. Rectal cooling rates observed in the control group were slower than those reported by Bynum (0.03Wmin versus O.O6”C/ min).18 This may be explained by the absence of evaporative heat loss and cavernous sinus countercurrent exchange heat loss in our study. Dantrolene sodium administration did not affect cooling rates, hemodynamic parameters, or pathological findings in this study. The dose of dantrolene administered (5 mg/kg) was chosen based on a barbiturate-anesthetized canine study that showed that maximal inhibition of skeletal muscle twitch contraction strength was reached when one half of a 10 mg/kg dose of dantrolene sodium had been administered.31 No effect on cardiac or smooth muscle was observed at this dosage level. In another barbiturate-anesthetized canine study, an intravenous dose of 10 mg/kg dantrolene sodium did not significantly alter cardiac output, central venous pressure, or mean arterial pressure when compared with controls given the same volume of saline solution.32 This dose did not have an appreciable effect on respiratory function. The 5 mg/kg dose used in this study is effective in reversing the muscle rigidity of malignant hyperthermia in swine33 and is approximately twice the mg/kg dose found in a multicenter trial to be effective for human malignant hyperthermia.16 The only laboratory values that were significantly different between dantrolene and control groups were serum creatinine, urinary volume, and urine specific gravity. These differences may be explained by the vehicle in which the dantrolene was administered. The dantrolene preparation used in this study is the same as that available for clinical use in humans. Because of the low solubility of dantrolene in normal physiological solutions, it must be reconstituted in sterile water rendered alkaline by the addition of sodium hydroxide and hyperosmotic by the addition of mannitol. The resulting dose of mannitol administered with the dantrolene was considerable (750 mg/kg). The osmotic diuresis induced by the mannitol may fully account for the increased urinary production and decreased creatinine observed in the dantrolene group during the first hours of cooling. The higher urine specific gravity observed in the dantrolene group at 12 hours is probably the result of intravascular volume contraction and increased serum osmolarity induced by osmotic diuresis. The hemodynamic, hematological, enzymatic, and electrolytic changes observed after heating were sim404

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ilar in both groups and are consistent with heatstroke as observed in dogs and humans.lJ These changes have been reported and discussed elsewhere.1J8,23+27 Of note is that dantrolene administration did not affect calcium or phosphorous levels. This is consistent with the proposed mechanism of action of the drug in inhibiting intracellular release of calcium from the sarcoplasmic reticulum of skeletal muscle cells1*~i3 Extracellular calcium ion concentrations (and therefore serum calcium) would not be expected to change. Creatine kinase isoenzyme analysis revealed statistically significant elevation of both the MM and BB fractions after heatstroke induction (Table 2). The absence of an MB fraction elevation does not infer a lack of myocardial damage, because canine myocardium contains a very low proportion of MB isoenzyme (<2%), and clinical myocardial infarction induced by left anterior descending coronary artery ligation in dogs may not be accompanied by a large MB fraction elevation.34 Creatine kinase MM fraction activity is found in the canine heart and intestine as well as skeletal muscle. The BB fraction is normally present in dog sera and can be isolated from spleen and intestine as well as brain.34 The isoenzyme elevations observed in this study are consistent with thermal damage to multiple organs, including brain, heart, and skeletal muscle. Pathological findings confirmed multi-organ cellular damage. lsoenzyme fractions were not statistically significantly different between the dantrolene and control groups. The lack of efficacy of dantrolene administration observed in this study implies that the drug, at the dosage level administered, does not reverse a theoretical central disorder of thermoregulation. Such a central thermoregulation disorder may not exist. Heatstroke may simply be a manifestation of exceeding the normal physiological capacity of the body to deal with a heat load. As is the case with congestive heart failure, this overload may occur in a wide range of clinical settings including extreme exertion in normal adults, mild exposure to hot humid environments in the elderly or debilitated, or in unusual diseased states such as thyroid storm or malignant hyperthermia. The similar clinical, laboratory, and pathological changes observed in malignant hyperthermia, classic heatstroke, and exertional heatstroke may be because of their common mechanism of injury (i.e., thermal damage to the cell), rather than because of a common muscular or central thermoregulatory disorder. Based on the results of this limited canine study, one would not expect dantrolene administration to be effective in patients with “classic” heatstroke or on patients with exertional heatstroke where muscle rigidity was not a significant factor. At best, dantrolene administration could only decrease heat production. Treatment attention must still be focused on cooling

AMSTERDAM

techniques, since rapid cooling is clearly the single most important factor in improving survival.1,2,4 Dantrolene administration could be efficacious in increasing the cooling rate of exertional heatstroke victims with muscle rigidity or in patients with stressinduced malignant hyperthermia.6y8 This study did not test dantrolene in an exertional model. Further testing of dantrolene in a canine exertional heatstroke model, perhaps in conjunction with other cooling modalities, would seem in order.

13.

14.

15. 16. 17.

CONCLUSION In an anesthetized, non-exertional canine heatstroke model, dantrolene sodium administration did not alter cooling rates, survival, laboratory changes, hemodynamic parameters, or pathological changes when compared with passively cooled controls. Dantrolene does not appear to be efficacious for heatstroke syndromes in which muscle rigidity is not a significant factor. Further testing of dantrolene in an exertional heatstroke model and systematic experimental comparison of various cooling techniques would seem in order. The authors thank American Edwards Laboratories for supplying the cardiac output equipment and catheters, and Norwich Eaton Pharmaceuticals for supplying the dantrolene sodium (Dantriuma) used in this study. In addition, the authors thank Mrs. Marjorie Gabel for her useful experimental advice.

18. 19.

20.

21.

22.

23.

24.

REFERENCES 1. Shibolet S, Coil R, Gilat T. Heatstroke: Its clinical picture and mechanism in 36 cases. J Med 1967;36:525-548. 2. Shapiro Y, Rosenthal T, Sohar E. Experimental heatstroke: A model in dogs. Arch Intern Med 1973;131:688-692. 3. Tintinalli JE. Heatstroke. JACEP 1976;5:525-528. 4. Syverud SA, Barker WJ, Amsterdam JT, et al. Iced gastric lavage for treatment of heatstroke: Efficacy in a canine model. Ann Emerg Med 1985;14:424-432. 5. Barker WJ, Amsterdam JT, Syverud SA, et al. High frequency jet ventilation cooling in a canine hyperthermia model. Ann Emerg Med 1986;15:680-684. 6. Gronert GA, Thompson R, Onofrio B. Human malignant hyperthermia: Awake episodes and correction by dantrolene. Anesth Analg 1980;59:377-378. 7. Lydiatt J, Hill GE. Treatment of heatstroke with dantrolene (letter to the editor). JAMA 1981;248:41-42. 8. Denborough MA. Heatstroke and malignant hyperpyrexia. Med J Aust 1982;1:204-205. 9. Macalad FV. Study of efficacy of dantrolene sodium in treatment of patients with heatstroke (clinical project protocol). Norwich, Connecticut: Norwich Eaton Pharmaceuticals, 1983. 10. Ellis KO, Carpenter JF. Studies on the mechanism of action of dantrolene sodium, a skeletal muscle relaxant. NauynSchmiedebergs Arch Pharmacol 1972;275:83-93. 11. Anderson IL, Jones EW. Dantrolene sodium in porcine malignant hyperthermia: Studies on isolated muscle strips. In Aldrete JS, Britt BA (eds). Second International Symposium on Malignant Hyperthermia. New York: Grune and Stratton, 1978:509-534. 12. Desmedt JE, Hainaut K. Inhibition of the intracellular re-

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lease of calcium by dantrolene in barnacle giant muscle fibres. J Physiol 1977;265:565-585. Moulds RFW. A comparison of the effects of sodium thiocyanate and dantrolene sodium on an isolated mammalian skeletal muscle. Br J Pharmacol 1977;59:129-133. Pinder RM, Brogden RN, Speight TM, et al. Dantrolene sodium: A review of its pharmacological properties. Drugs 1977;13:3-23. Gronert GA. Malignant hyperthermia. Anesthesiology 1980:53:395-423. Kolb ME, Horne ML, Martz R. Dantrolene in human malignant hyperthermia. Anesthesiology 1982;56:254-262. Jardon OM. Physiologic stress, heatstroke, malignant hyperthermia-A perspective. Military Med 1982;147:8-14. Bynum G, Patton J, Bowers W, et al. An anesthetized dog heatstroke model. J Appl Physiol 1977;43:292-296. Bynum G, Patton J, Bowers W, et al. Peritoneal lavage cooling in an ‘anesthetized dog heatstroke model. Aviat Space Environ Med 1978;49:779-784. Roe CR, Limbird LE, Wagner GS, et al. Combined isoenzyme analysis in the diagnosis of myocardial injury: Application of electrophoretic methods for detection and quantitation of the creatine phosphokinase MB isoenzyme. J Lab Clin Med 1978;80:577-590. Roberts R, Henry PD, Witteeveen SA, et al. Quantification of serum creatinine phosphokinase isoenzyme activity. Am J Cardiol 1974;33:650-654. Cowgill GR, Drabkin DL. Determination of a formula for the surface area of the dog together with a consideration of formulae available for other species. Am J Physiol 1927;81:36-61. Malamud N. Havmaker W. Custer RP. Heatstroke: A clinicopathologjc study of 125 fatal cases, Military Surgeon 1946;99:397-431. Sprung CL. Hemodynamic alterations of heatstroke in the elderly. Chest 1979;75:362-366. Shibolet S. Heatstroke: A review. Aviat Space Environ Med 1976;47:280-301. Nelson TE, Flewellen EH. The malignant hyperthermia syndrome. N Engl J Med 1983;309:416-418. Ferris EB, Blankenhorn MA, Robinson HW, et al. Heatstroke: Clinical and chemical observations on 44 cases. J Clin Invest 1938;17:249-262. Robinson S, Wiley SL, Myhre LG, et al. Temperature regulation of men following heatstroke. Isr J Med Sci 1976;12:786-795. Baker MA. A brain cooling system in mammals. Sci Am 1979;240:130-139. Magazanik A, Epstein Y, Udassin R, et al. Tap water, an efficient method for cooling heatstroke victims-A model in dogs. Aviat Space Environ Med 1980;51:864-866. Ellis KO, Butterfield JL, Wessels FL, et al. A comparison of skeletal, cardiac, and smooth muscle action of dantrolene sodium-A skeletal muscle relaxant. Arch Int Pharmacodyn 1976;224:118-132. Ellis RH, Simpson P, Tatham P, et al. The cardiovascular effects of dantrolene sodium in dogs. Anesthesia 1975;30:318-322. Harrison GG. Control of malignant hyperpyrexic syndrome in MHS swine by dantrolene sodium. Br J Anaesth 1975;47:62-65. Roberts R, Sobel BE. Effect of selected drugs and myocardial infarction on the disappearance of creatine kinase from the circulation in conscious dogs. Cardiovasc Res 1977;11:103-112. Rapaport E. The fractional disappearance rate of the separate isoenzymes of creatine phosphokinase in the dog. Cardiovasc Res 1975;9:473-477. 40.5