Acute phencyclidine poisoning in the unanesthetized dog: Pathophysiologic profile of acute lethality

Toxicology, 19 (1981) 11--20 © Elsevier/North-Holland Scientific Publishers Ltd.

ACUTE PHENCYCLIDINE POISONING IN THE UNANESTHETIZED DOG: PATHOPHYSIOLOGIC PROFILE OF ACUTE LETHALITY*

ROBERT B. HACKETT**, KEITH W. OBROSKY, RONALD F. BORNE and IRVING W. WATERS

Departments of Pharmacology and Medicinal Chemistry, School of Pharmacy, University of Mississippi, University, MS 38677 (U.S.A.)

(Received May 28th, 1980) (Accepted October 8th, 1980)

SUMMARY

Phencyclidine HC1 was infused intravenously (1.0 mg/kg/min) to unanesthetized mongrel dogs until death. All animals experienced tonic-clonic convulsions (mean convulsive dose: 4.7 + 0.3 mg/kg) which lasted until shortly before death (mean lethal dose: 49.8 + 2.5 mg/kg). Significant increases in heart rate, arterial blood pressures, cardiac output, body temperature, and arterial pCO2 were observed in all animals. Significant reductions from pre~lmg control values were observed in total peripheral resistance, arterial pH, arterial pO2, and respiratory minute volume. Blood lactate, oxygen uptake, and plasma glucose levels rose to values significantly higher than pre-dmg control values then declined during the latter phase of the experiment, glucose levels decreased to final values lower than control. Animals appeared to die of primary respiratory failure, which was exacerbated by hyperthermia, and which resulted in final cardiovascular collapse. INTRODUCTION

In recent years the use of phencyclidine (PCP) among illicit drug users *Supported in part by the Research Institute of Pharmaceutical Sciences, University o f Mississippi, and National Research Service Award 5 T32 6M 07099-04. Presented in part to the Society of Toxicology, March 12, 1979, in New Orleans, LA. **Present Address: Alcon Laboratories, Inc., 6201 South Freeway, P.O. Box 1959, F o r t Worth, TX 76101, U.S.A. Address correspondence to: I.W. Waters, Ph.D., Department of Pharmacology, School of Pharmacy, University of Mississippi, University, MS 38677, U.S.A. Abbreviations: MAP, mean arterial pressure; PCP, phencyclidine; TPR, total peripheral resistance.

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has grown to alarming proportions [1]. The availability of this agent is the cause o f m u c h concern to both clinicians and toxicologists called to attend human overdose, since so little information is available from toxicological investigations. Reports of death from PCP poisonings have also increased dramatically in the last few years [1,2]. Symptoms of human poisoning with PCP include psychosis [3], increases in both systolic and diastolic blood pressures [4], tachycardia [5], cardiac arrhythmias [2] and convulsions [6]. Symptoms preceding PCP fatalities include hypertensive crisis [7], respiratory depression [2], status epilepticus and prolonged coma [8]. Despite the numerous reports of both fatal and near-fatal poisoning with PCP which have appeared in the literature, the cause of death is not clear. To date, no systematic study of PCP toxicity in any laboratory species has been reported. The present study was undertaken to define the pathophysiological profile of acute PCP lethality in conscious dogs. The conscious dog was favored for such work because: (1) in the conscious state, the response to PCP would not be modified by an anesthetic agent; and (2) the size of the animal would permit repeated withdrawal of blood for biochemical analysis as well as simultaneous measurements of both cardiovascular and respiratory parameters. MATERIALS AND METHODS

The procedures* employed were essentially those described earlier by Catravas et al. [9]. In brief, the animals used in these studies were adult mongrel dogs of either sex with an average body weight of 14 kg (8.2--16.4 kg); no animals used in these studies demonstrated bradycephalic characteristics. Prior to use, animals were housed for a minimum of 5 days in indoor kennels and their general health status were substantiated by a staff veterinarian. On the day of the experiment, each animal was placed in the supine position on a specially constructed operating table and restrained loosely with soft leather binders around the legs; care was taken at all times to assure that the animal experienced only minimal discomfort. Under local anesthesia (1.5% procaine}, polyethylene cannulae were introduced into the right atrium, left ventricle and abdominal aorta via the appropriate femoral vessels. By means of these cannulae measurements of arterial systolic and diastolic pressures, mean right atrial and left ventricular pressures and cardiac output were obtained. Heart rate was recorded from the EKG (Lead II). From these data, mean arterial pressure (MAP), stroke volume and total peripheral resistance (TPR) were calculated. Arterial blood samples were withdrawn for the measurement of pH, pCO2, pO2, blood lactate and plasma glucose levels. Body temperature was monitored by a rectal thermis*The procedures employed in the surgical preparation of the animals used in these experiments conform to the standards for animal care endorsed by the American Physiological Society and were approved by the Animal Welfare Committees of both the School of Pharmacy and the University of Mississippi.

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tor. Control measurements of all parameters were taken then phencyclidine HCI (PCP) was infused via a femoral vein at a dose of 1 mg]kg]min (1.1 ml/min) until death. Measurements were made at 5, 10 and 15 min after beginning drug infusion, then every 15 rain until death. Respiratory responses to PCP were measured in a separate group of animals (n = 5). A tracheal cannula was inserted under local anesthesia and control measurements of oxygen uptake and minute volume were obtained spirometrically. PCP was then administered as to the former group and respiratory functions were measured at sampling times coinciding to the above group until death ensued. Gross necropsies were performed on all animals immediately after death. The presence of massive heart worm (> 10) or h o o k w o r m infestation constituted grounds for rejection of the animal from the experimental protocol. Statistical comparisons were performed between pre-drug control values and each of the subsequent experimental measurements by means of Student's t-test for paired data using an alpha level of 0.05. All calculations were performed with the aid of a DEC-10 c o m p u t e r and the SPSS statistical program package.

RESULTS During the infusion of PCP, the animals exhibited progressively increased m o t o r activity with maximally dilated pupils and finally developed tonicclonic convulsive seizures (mean convulsive onset time: 4.7 -+ 0.3 rain). All animals salivated copiously and appeared to be hallucinating as evidenced b y visual tracking b u t were unresponsive to visual or tactile stimuli. Seizures continued until shortly before death at which time the animals became severely depressed and blood pressures declined rapidly. The mean time to death was 49.8 + 2.4 min. PCP produced an immediate increase in all arterial pressures (Fig. 1). Systolic pressure rose rapidly and reached a peak of 270 -+ 9 m m H g at the 15-min measurement then declined to levels n o t significantly different from pre-drug control pressure just prior to death. Diastolic and mean arterial pressures rose during the initial 10 rain of drug infusion and attained maximum pressures of 140 + 5 a~d 182 -+ 5 m m H g respectively; both pressures then declined to values n o t significantly different from pre-drug control measurements just prior to death. Left ventricular pressure (Table I) paralleled the systolic pressure curve b u t reflected the higher pressures generated in the left ventricle. Approximately 2--3 rain prior to death, all pressures declined rapidly, then fell abruptly to zero. Severe cardiac arrhythmias of ventricular origin were observed during this period. No significant changes were recorded in mean right atrial pressure (Table I). A significant elevation in cardiac o u t p u t (Fig. 1) was observed throughout the first 30 min of PCP infusion with maximum elevation of 512-+ 31 ml/kg/min at the 15-min measurement. Cardiac o u t p u t was n o t significantly

13

210

.•TE '

240

240

RIAL PRESSURES

,

200

200

,.~ 160 s

E E 160

120

M

120

Infusion

D I0

I

i

C

i

10 15

I

I

I

30

45

60

Infusion

600

Time

Time

(rain)

4000 TOTAL PERIPHERAL RESISTANCE

(mini

CARDIAC OUTPUT

36O0

500

~E u

400

~. -0

3200

.E E "~

2500

E 300

200

2400

I

I

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C $ 10 15 Infusion

I

I

30

45

Time

(min)

I

60

'5 1'0 1'5 Infusion

3'0 Time

4is

6{)

(rain)

Fig. 1. Effect o f a lethal infusion o f phencyclidine HCI (1.0 m g / k g / m i n , i.v.) on systolic (S), diastolic (D), mean arterial pressure (M), cardiac output, heart rate, and total peripheral resistance in the unanesthetized dog. Twenty-two animals were used in this study, however, the number o f animals is variable with the parameter tested. Each point represents the mean -* 1 S.E. o f at least 6 animals. *P < 0.05 compared to pre-drug control.

different from pre-drug control values after the 30-rain measurement. Similarly, heart rate increased rapidly and remained significantly elevated over pre-drug control values during the first 30 rain. A m a x i m u m rate of 220-+ 6 bpm was observed at the 15-rain measurement (Fig. 1). Total peripheral resistance demonstrated a declining trend and was significantly lower than pre-dmg control measurements at the 15-rain measurement (Fig. 1). No significant changes were observed in stroke volume (Table I). Changes in respiratory minute volume and oxygen uptake are shown in Fig. 2. Minute volume declined sharply at the 5-rain measurement, returned to within pre-drug control values, and then declined significantly at the 30-rain measurement. Oxygen uptake, on the other hand, increased linearly for the first 10 rain of PCP infusion and remained significantly greater than pre-dmg control values through the 30-rain measurement. It was not possible

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TABLE I E F F E C T O F A L E T H A L I N F U S I O N O F PCP O N P A R A M E T E R S IN T H E U N A N E S T H E T I Z E D D O G a

SEVERAL

PHYSIOLOGICAL

Parameters

Control

5 min

10 m i n

15 m i n

30 m i n

45 m i n

60 r a i n

Left ventricular pressure

193 ± 13

297 ±15 b

304 ± l0 b

305 ± 15 b

259 _* 15 b

216 ± 8

180 ±4

4.7 ~ 1.6

4.6 ± 1.7

9.0 ± 3.1

7.2 ± 3.0

24 ~2 101 ± 5 32.3 + 9.0

24 ±4 119 -+ 7 38.6 ± 7.5

23 ± 3 135 ± 8b 48.5 *- 9.5

24 ± 3 139 ± 125 54.4 ± 7.6 b

(mmHg) M e a n r i g h t atrial pressure (mmHg) Stroke volume (ml) Plasma glucose (mg/dl) Blood lactate (mg/dl)

6.3 ± 2.3

3.3 ~ 3.9

8.0 ± 5.0

26 ~4 129 ± 15 40.1 ~- 5.9

26 ±4 80 ± 95 28.9 ~ 5.9

23 ± 2

a PCP i n f u s e d i.v. at 1.0 m g k g - ' m i n - l ; values r e p r e s e n t t h e m e a n ~ S.E. o f 5 a n i m a l s . b S i g n i f i c a n t l y d i f f e r e n t f r o m p r e - d r u g c o n t r o l (P < 0 . 0 5 ) .

to obtain accurate assessments of either respiratory rate or tidal volume because of the severity of the seizures in these animals. Despite the increase in oxygen uptake, arterial pO2 declined significantly from a pre-drug control value of 81 + 1 to 32 + 2 mmHg at the 60-min measurement (Fig. 3). Arterial pCO2 rose significantly during the same RESPIRATORY MINUTE VOLUME

34

KE

30 26

E 22 -~. E

i

Infusion Time (rain)

i

i

J

Infusion Time (min)

Fig. 2. E f f e c t o f a l e t h a l i n f u s i o n o f p h e n c y c l i d i n e HCI (1.0 m g / k g / m i n , i.v.) o n respirat o r y m i n u t e v o l u m e a n d o x y g e n u p t a k e in t h e u n a n e s t h e t i z e d dog. E a c h p o i n t r e p r e s e n t s t h e m e a n -+ 1 S.E. o f 5 a n i m a l s . *P < 0 . 0 5 c o m p a r e d t o p r e - d r u g c o n t r o l .

15

9o

Z450

~

ARTERIAL BLOOD

80 70 60

E E

50 40

GASES

7.350 pCO~

7.250

30

PO 2

20

7.150 i

I

I

I

I

Infusion Time (mini

Infusion Time (mini

43

BODY

TEMPERATURE

41 U 40

39

38

'

'

'

'

C 3 10 15

'

'

'

~0

43

60

Infusion Time (rain)

Fig. 3. Effect of a lethal infusion of phencyclidine HCI (1.0 mg/kg/min, i.v.) on arterial pCO2, pO2, pH, and body temperature in the urh~nesthetized dog. 'I~enty-two animals were used in this study, however, the number of animals is variable with the parameter tested. Each point represents the mean ± 1 S.E. of at least 6 animals. * P < 0.05 compared to pre-drug control.

time interval from a control value of 30 -+ 1 to 56 -+ 3 mmHg at the 60-min measurement (Fig. 3). Arterial pH declined sharply from a control value of 7.431 -+ 0.014 to 7.250 -+ 0.023 at the 10-min measurement; this decline continued until a critically acidemic level (pH 7.181) was reached at the time of the last sample (Fig. 3). Body temperature increased linearly during the first 30 min of the PCP infusion then remained at approximately 41.9°C until death (Fig. 3). During the early phases of the infusion, both blood lactate and plasma glucose levels increased significantly above pre-drug control levels then declined during the latter phases of drug infusion; glucose levels declined to final values significantly lower than control (Table I). Post-mortem examination revealed occasional subendocardial punctate hemorrhages in the left and right ventricles and occasional epicardial punctate hemorrhages. All animals died in diastole. Diffuse pulmonary hemorrhage was present bilaterally on the surface of the lungs; moderate pulmonary congestion was also present. Hepatic, renal, mesenteric and intestinal tissues

16

appeared normal in size and displayed mild to moderate congestion. The spleen was frequently found to be contracted. Microscopic post~mortem examinations were n o t performed. DISCUSSION

The powerful hypertensive, inotropic and chronotropic responses observed in this study were typical of the reactions to a potent sympathomimetic agent. Clinical reports of PCP poisoning describe similar symptoms which are interpreted to result from intense sympathetic stimulation; i.e.,hypertension [7], tachycardia [5], and cardiac arrhythmias [2]. In support of the sympathomimetic origins of these reactions, experimental studies have demonstrated that the pressor response induced by PCP in cats and rats was abolished by a-adrenergic blocking agents [10]. Furthermore, O'DonneU and WanstaU [11] postulated that PCP inhibits the neuronal reuptake of norepinephrine. Hence, a peripheral sympathomimetic action of PCP could account for the cardiovascular effects of the drug observed in this study. Alternatively, these cardiovascular responses could also result from central actions of PCP. Reports indicate that the drug inhibits central neuronal reuptake of norepinephrine [12] and stimulates its release as well [13]. Despite the magnitude of the pressor response, it is unlikely that hypertension per se was the direct cause of the fataloutcome in these PCP poisoned animals. Waters et al. [14] in the same animal model demonstrated that neuromuscular blockade effectively protected animals against a potentially lethal dose of cocaine, yet arterial pressures in those experiments were similar in magnitude to the pressures reported here. The cardiac arrhythmias observed in this study undoubtedly contributed to the declining trend in the cardiovascular parameters seen during the latter phases of these experiments; such arrhythmias have been reported in clinical instances of PCP poisoning [2,15]; in vitro studies have demonstrated that PCP possesses myocardial depressant activity [10,15]. Cardiovascular functions in all animals declined toward the end of the experiment; we believe that the decline in these parameters was secondary to respiratory depression since respiratory failure was observed to occur prior to cardiovascular collapse. The sharp decline in respiratory minute volume at the 5-rain sample period corresponded to the period of most severe convulsions in these animals, thus early respiratory embarrassment can be attributed partly to severe muscular activity. The severity of the convulsions had lessened at the 10-minute sampling period and respiratory minute volume returned to pre-drug control values. At the 30-rain interval apparently primary respiratory depression was present as evidenced by the decrease in the minute volume. The fall in arterial PO2 and concomitant rise in arterial pCO2 suggests an insensitivity of central control systems to these stimuli. The mechanism for the respiratory depression remains obscure but m a y be due in part to interaction of PCP with centralcholinergicsites.Vincent et al. [16] 17

reported that PCP binds competitively to central muscarinic receptors. Additionally, Pinchasi et al. [17] reported that PCP demonstrated anticholinesterase activity in mouse brain in vitro. One of the most striking effects o f the administration of acetylcholinesterase inhibitors is respiratory failure [ 18 ]. An additional contribution to the rise in arterial CO2 tension could arise as the result of a generalized increase in metabolism; both in vivo and in vitro studies have demonstrated that PCP induces an increase in oxygen consumption [6,19,20] thus by implication an increase in CO2 production. Although oxygen uptake has declined to within pre-drug control limits by the 45-min sampling period, the oxygen demands of the animals were presumably very high due in part to the elevation in b o d y temperature of almost 3°C. The fall in the arterial pO2 of nearly 35 m m H g reflects the inability of the respiratory system to keep pace with the oxygen demands. The mechanism for the initial hyperglycemia was not established in this study; the hypoglycemic effect of PCP observed in the latter part of these experiments has also been reported in the rhesus m o n k e y [21]. The rapid fall in arterial pH observed in these animals is most likely due to the combination of respiratory acidosis and lactic acidemia. All PCP-treated animals experienced severe clonic-tonic convulsions which lasted until shortly before death. As a result of this muscular activity, arterial lactate levels increased to nearly double the control values; during the final phase of the experiment, arterial pH was approximately 7.2. This reduction in pH could contribute significantly to the lethal effect of the drug since acidosis is known to decrease cardiovascular tone [22,23]. The present study, however, cannot accurately determine the contribution of acidemia to the lethal outcome. The severe increase in body temperature which occurred during the administration of PCP can almost certainly be linked to the intense muscular activity which occurred during the convulsive episodes. Increases in b o d y temperature have been reported following the administration of other psychostimulants such as amphetamine or cocaine [9,24] and ephedrine [25]. Waters et al. [14] demonstrated that the rise in b o d y temperature induced by cocaine was abolished by neuromuscular blockade. Furthermore, these authors reported that pancuronium-treated animals survived a dose of cocaine which was lethal to naive animals. The importance of hyperthermia as a contributor to the lethal process was further emphasized in subsequent studies by this author [14]. Dogs which were exposed to low ambient temperatures ( - 5 ° C ) failed to exhibit a hyperthermic response to cocaine and all animals survived a potentially lethal dose o f the drug. Hyperthermia must be considered as a factor contributing to the lethal o u t c o m e of PCP poisoning in the conscious dog. Marple et al. [26] reported that swine died when rectal temperature was increased to 43°C over a period of 1.5 h. These authors also reported significant decreases in arterial pH and pO~ during this time period. Similarly, Hubbard et al. [27] observed 95--100% mortality when core temperatures

18

of rats reached 42°C. Exposure of mongrel dogs to ambient heat resulted in 100% mortality when rectal temperatures increased to 43°C [28]. The lethal temperature observed by these authors are similar to the rectal temperatures observed at the time of death in the PCP-treated dogs in this study (41.9 + 0.3°C). Hubbard et al. [27] have demonstrated in rats that exertion-induced hyperthermia produced death at a lower core temperature than hyperthermia alone. It seems reasonable, therefore, that the degree of physical exertion associated with the severe convulsions observed in this study would likewise lower the lethal core temperatures in dogs. The primary objective of this study was to establish the pathophysiological profile of acute PCP poisoning in the conscious dog. From our observations, acute PCP lethality in this animal model is a result of primary respiratory failure which was exacerbated by hyperthermia and which resulted in final cardiovascular collapse. Additional studies are underway in our laboratory to elucidate the pharmacologic-toxicologic mechanism(s) of PCP lethality. These studies should provide an experimental basis for the rational choice of antidotal treatment in clinical instances of acute PCP poisoning. ACKNOWLEDGEMENTS

We are pleased to acknowledge the technical assistance of Cheryl Weakley and Calvin Collins. We are also grateful to Mrs. Rita Ewing for her excellent secretarial assistance. REFERENCES 1 R.C. Petersen and R.C. Stillman, Phencyclidine: A review. NIDA Bull. May 1978, Department of Health, Education and Welfare, Rockville, 1978. 2 C.B. Liden, F.H. Lovejoy and C.E. Costello, J. Am. Med. Assoc., 234 (1975) 513. 3 J.M. Rainey, Jr. and M.K. Crowder, Am. J. Psychol., 132 (1975) 1076. 4 R.S. Burns and S.E. Lerner, Clin. Toxicol., 12 (1978) 463. 5 H. Neubauer, D.M. Sundland and S. Gershon, Int. J. Neuropsychiatry, 2 (1966) 216. 6 E.F. Domino, Int. Rev. Neurobiol., 6 (1964) 303. 7 J.W. Eastman and S.N. Cohen, J. Am. Med. Assoc., 231 (1975) 1270. 8 G.F. Kessler, L2¢1. Demers, C. Berlin and R.W. Brennan, N. Engl. J. Med., 291 (1974) 979. 9 J.D. Catravas, I.W. Waters, J.P. Hickenbottom and W.M. Davis, J. Pharmacol. Exp. Ther., 202 (1977) 230. 10 K.F. Ilett, B. Jarrott, S.R. O'Donnell and J.C. Wanstall, Br. J. Pharmacol, Chemother., 28 (1966) 73. 11 S.R. O'Donnell and J.C. Wanstall, J. Pharm. Pharmacol., 20 (1968) 125. 12 R.E. Garey and R.G. Heath, Life Sci., 18 (1976) 1105. 13 R.J. Hitzeman, H.H. Loh and E.F. Domino, Arch. Int. Pharmacodyn. Ther., 202 (1973) 252. 14 I.W. Waters, J.D. Catravas and M.A. Waiz, The Pharmacologist, 20 (1978) 165. 15 G. Chen, C.R. Ensor, D. Russel and B. Bonner, J. Pharmacol. Exp. Ther., 127 (1959) 241. 16 J.P. Vincent, D. Cavey, J.M. Kamenka, P. Geneste and M. Lazdunski, Brain Res., 152 (1978) 176.

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17 18 19 20 21 22 23 24 25 26 27 28

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I. Pinchssi, S. Maayani and M. Sokolovsky, Biochem. Pharmacol., 26 (1977) 1671. H.L. Borison, Pharmacol. Ther, B., 3 (1977)211. H. Lees, Biochem. Pharmacol., 11 (1962) 1115. H. Lees, Biochem. Pharmacol., 17 (1968) 845. R.D. Hemm and N.W. Johnson, Toxicol. Appl. Pharmacol., 43 (1978) 279. G.D. Ford, W.H. Cline, Jr. and W.W. Fleming, Am. J. Physiol. 215 (1968) 1123. G.W. Jelks and T.E. Emerson, Jr., Proc. Soc. Exp. Biol. Med., 146 (1974) 59. E.G. Zalis, G. Kaplan, G.D. Lunberg and R.A. Knutson, Proc. Soc. Exp. Biol. Med., 118 (1965) 557. D.I. Peterson and M.G. Hardinge, J. Pharm. Pharmacol., 19 (1967) 810. D.N. Marple, D.J. Jones, C.W. Alliston and J.C. Forrest, J. Anita. Sci., 39 (1974) 79. R.W. Hubbard, W.D. Bowers, W.T. Matthew, F.C. Curtis, R.E.L. Criss, G.M. Sheldon and J.R. Ratteree, J. Appl. Physiol., 42 (1977) 809. Y. Shapiro, T. Rosenthal and E. Sohar, Arch. Int. Med., 131 (1973) 688.