Naunyn-Schmiedeberg's

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Naunyn-Schmiedeberg's Arch Pharmacol (1982) 318:220- 224

Pharmacology

9 Springer-Verlag1982

The Effects of Induced Hypoxia on the Cardiovascular System in Dogs Poisoned with Tetanus Toxin Olusoga A. Sofola and Kayode A. Odusote Department of Physiology and Department of Medicine, College of Medicine, University of Lagos, P.M.B. 12003, Lagos, Nigeria

Summary. Tetanus toxicity was induced in dogs by injecting the toxin subcutaneously in the groin. On developing generalised toxic symptoms, these dogs were characterised by signs of increased sympathetic discharge to the cardiovascular system as evidenced by high basal values of blood pressure, heart rate and LV dP/dt max. Mild to moderate hypoxia induced by ventilation with 10 % 02 in N2 had no appreciable effect on the cardiovascular variables. However, moderate to severe hypoxia induced by ventilation with 7 % 02 in N2 further increased the sympathetic discharge to the heart and blood vessels resulting in increases in heart rate, LVdP/dt max and blood pressure. These responses were abolished by adrenergic blockers. The responses in the tetanus dogs were identical to those seen in dogs without tetanus toxicity. Atropine or moderate lactic acidaemia did not alter the responses to hypoxia. Beta-adrenergic blockers appear to be useful drugs in the control of tetanus patients who show evidence of increased sympathetic activity or who develop hypoxaemia. Key words: Tetanus - Hypoxia - Cardiovascular system Adrenergic blockers

Introduction Overactivity of the sympathetic nervous system during tetanus toxicity as it affects the cardiovascular system has been well described. The cardiovascular effects in man include tachycardia andlabile hypertension (Kerr et al. 1968 ; Corbett et al. 1969; Hollow and Clarke 1975). In addition, noninvasive measurements of cardiac performance has suggested an increase in the inotropic state of the heart during generalised tetanus (Alfred and James 1974). Similarly, studies on experimental animals have shown that administration of tetanus toxin causes tachycardia and an increase in the maximum rate of rise of left ventricular pressure LVdP/dt max (Odusote and Sofola 1976). Patients with generalised tetanus develop hypoxaemia (Femi-Pearse 1974) which is worsened by spasms (Elegbeleye 1978). Systemic hypoxia in normal intact dogs causes an increase in sympathetic nerve activity to the heart and blood vessels resulting in increases in heart rate, myocardial contractility and blood pressure (Downing et al. 1963; Achtel and Downing 1972; Heistad and Abboud 1980). The purpose o f the present investigation is to determine the effects of induced hypoxia on Send offprint requests to O.A. Sofola at the above address

0028-1298/82/0318/0220/$01.00

the cardiovascular system of dogs developing generalised tetanus after toxin administration considering the fact that they already have a high basal sympathetic output to the heart and blood vessels. Furthermore the role of the autonomic components were investigated by means of adrenergic blockers and Atropine.

Methods Experiments were performed on twenty dogs of either sex weighing between 8 and 14 kg. Tetanus was induced in 10 of the dogs by injecting, subcutaneously into the groin, 2 0 0 0 3 0 0 0 M L D . k g -1 tetanus toxin (Swiss Serum Research Institute, Berne, Switzerland). About 3 - 5 days later, the dogs developed generalised tetanus (see Odusote and Sofola 1976). Ten other dogs used as controls had sterile saline injection instead of toxin and will be referred to subsequently as non-tetanus (NT) dogs. The dogs were anaesthetized with sodium pentobarbitone (Sagatal, May & Baker, Dagenham, GB) 3 0 m g - k g -1 given intravenously and anaesthesia maintained with further doses of 3mg. kg-1 about every hour. The trachea was cannulated and the animals were paralysed with gallamine, 3 m g . k g -1 (Flaxedil, May & Baker, Dagenham, GB). They were immediately commenced on intermittent positive pressure ventilation using a respirator (C. F. Palmer Ltd., London, GB). The tidal volume was set at i7 ml- kg- 1 and the rate of the pump at 20 per rain. Blood gases and acid-base balance were monitored frequently using an ABL-2 blood gas analyser (Radiometer, Copenhagen, Denmark). 20 ml molar bicarbonate was added to 200 ml normal saline and infused continuously at about 10 drops per min in order to minimize acidaemia. Rectal temperature, measured with a thermistor probe (Yellow Springs Instruments Inc., OH, USA) was maintained at 38 ~ C by means of a heating lamp under the operating table. A femoral vein was cannulated for the infusion of drugs. A catheter was also inserted into a femoral artery and connected to a Statham strain gauge pressure transducer (Model P23 AC) and then to a Grass polygraph, Model 7D (Grass Instruments, Quincy, MA, USA) for recording arterial blood pressure. In some experiments, right atrial pressure was measured by passing a catheter through the external jugular vein into the atrium. Catheter positions were confirmed postmortem. Left ventricular pressure was measured by passing a catheter-tip pressure transducer model PC380 (Millar Instruments Ltd., Houston, TX, USA) via the right common carotid artery into the left ventricle, while using the

221 Table 1. The effects of ventilation with 10 ~ 0 2 on the cardiovascular variables in dogs BP (mm Hg)

LVP (mm Hg)

LV dP/dt max (mm Hg.s -1)

HR (beats 9rain -1)

C

H

C

C

C

Non-tetanus dogs(n=5)

113-L=6

1'15+8

1 2 4 _ + 1 0 127+12

3187_+144 3312_+ 97

1 6 6 _ + 1 8 166_+18

Tetanus dogs(n=6)

131•

131•

132+_ 5 137•

3701•

210•

C H n BP

= = = =

Ventilation with room air Ventilation with 10 ~ 02 in N2 Number of dogs Arterial blood pressure

H

3

H

3723_+148

H

8 208•

8

LVP = left ventricular pressure LV dP/dt max = Maximum rate of rise of left ventricular pressure HR = Heart rate

Values stated are mean _+1 Standard Error of the Means. All changes were not significant (P> 0.1)

pressure tracing as guide. The ventricular pressure was recorded on the polygraph and the output from the pressure amplifier was passed into a differentiator - model 7P20 (Grass Instruments) in order to obtain the rate o f rise of left ventricular pressure - LV dP/dt. The frequency response of the catheter-tip transducer was determined (Ardill et al. 1967) and was flat ( • 5 ~ ) to better than 200 Hz. The output of the differentiator was linear (+_ 5 ~ ) beyond 12,000 m m H g . s - 1. The oxygen tension of the arterial b l o o d was monitored continuously, after heparinisation, by fashioning a femoral arterio-venous loop connected to a roller pump (WatsonM a r l o w Ltd., Falmouth, Cornwall, GB), running at slow speed. The arterial b l o o d passed continuously through an invivo oxygen electrode connected to an Oxytrak oxygen analyser (Critikon, Irvine, CA, USA).

Experimental Procedure. At the end of all cannulations, the measured variables were allowed to remain steady and the oxygen meter reading was stable. With the animals ventilated with r o o m air, control records were taken of blood pressure (BP), left ventricular pressure (LVP), LV dP/dt and in some cases right atrial pressure. The maximum rate of rise of left ventricular pressure - L V d P / d t max was used as the inotropic index of the heart (Wallace et al. 1963; Furnival et al. 1973). Mean BP was also recorded by electronic damping to 0.1 Hz of the phasic pressure recording. The control arterial Po2 was read from the Oxytrak meter. In addition, an arterial blood sample was withdrawn from the femoral artery for blood gas analysis. Records were taken at both slow ( 5 m m . s - t ) and fast (25 ram- s - 1) paper speeds in order to calculate heart rate and LV dP/dt max respectively. Next, the animals were ventilated with low oxygen mixtures containing either :10 ~o O~ in N2 or 7 ~ 02 in N2 for 5 - 10 rain with records taken continuously of the cardiovascular variables and oxygen tension. A n arterial blood sample was also withdrawn at 5 m i n a n d analysed. Six experiments with 1 0 ~ 02 and 8 with 7 ~ 02 were carried out in the tetanus dogs while 5 experiments with 10 ~ O2 and 7 with 7 ~ O2 were performed in non-tetanus dogs. In 2 experiments each, on tetanus and non-tetanus dogs, the response of LV dP/dt max were determined with blood pressure held constant. This was done by connecting a femoral artery to a reservoir of blood connected to air whose pressure was controlled at a constant level. In addition, the effect of beta-adrenergic block with propranolol (Inderal, ICI, Manchester, GB) 1 m g . k g - t , alpha-adrenergic block

with phenoxybenzamine (Dibenyline, Smith, Kline & French, Philadelphia, PA, USA) 2 rag. k g - t and atropine 2 rag. k g - t on the responses to hypoxia were determined. In 3 experiments, on tetanus dogs, 40 ml of 5 ~o lactic acid was infused over 2rain to induce acidaemia. The effects of hypoxia during acidaemia were then determined in these dogs.

Results

Blood Gas Changes The average values of arterial Po2, Pco2 and p H in the dogs ventilated with r o o m air were 93.8 _+ 3 . 4 m m H g , 38.6 _+ 2 . 1 m m Hg and 7.38 • 0.02 units respectively. During ventilation with 10 ~ 02, the corresponding values were 46.1 _+ 3.1 m m H g , 37.9 • 2 . 3 m m H g and 7.38 + 0.02 units and with 7 ~ O2, they were 38.3 • 3.5 m m H g , 37.1 • 1 . 9 m m H g and 7.39 • 0.02 units respectively.

Cardiovascular Variables in Non-Tetanus and Tetanus Dogs Analysis of the overall results in the 10 dogs in each group showed that dogs with generalised tetanus had a higher control heart rate, LV dP/dt max and blood pressure than the non-tetanus dogs. In the tetanus dogs the mean heart rate was 204 beats per min compared with 146 ( P < 0.001) in nontetanus dogs; BP was 1 2 3 m m H g as against 1 0 8 m m H g ( P < 0.01) and L V d P / d t max was 3651 m m Hg. s-1 against 3 0 8 5 m m H g . s -~ ( P < 0.01).

Cardiovascular Responses to Ventilation with 10 ~ 0 2 The average responses o f b l o 0 d pressure, heart rate, LVP, and L V d P / d t max in both non-tetanus and tetanus dogs are summarised in Table 1. In both groups hypoxaemia induced by ventilation with 10 ~ 02 in N2 had no significant effects on the variables measured ( P > 0.1).

Effects of Ventilation with 7 %

0 2

In the tetanus dogs, ventilation with 7 ~ O2 in N2 resulted in an increase in all the measured cardiovascular variables. The responses started within 1 - 2 min, reached a peak in 3 - 4 min and persisted for up to 10rain. However, the responses at 5 rain only were analysed. There were significant increases in heart rate from 199 +_ 11 to 211 + 11 b e a t s . r a i n -1

222 Table 2. Effects of ventilation with 7 ~ 02 on the cardiovascular system of dogs BP (mm Hg)

LVP (mm Hg)

LV dP/dt max (mm Hg. s- 1)

HR (beats. min- 1)

C

C

C

C

H

H

H

H

Non-tetanus d o g s ( n = 7 ) p

105• 120_+8 < 0.001

123• 148_+10 < 0.001

2971_+120 3271_+120 < 0.001

145_+ 9 162_+11 < 0.005

Tetanus dogs()~=8) p

118• 137• < 0.001

129_+7 151_+ 9 < 0.001

3610_+ 63 4037_+ 93 < 0.001

199_+11 211• < 0.025

C H

= Ventilation with room air = Ventilation with 7 ~ 02 in N2

SEM = Standard Error of the Mean P = Calculated from paired, t-test

Other abbreviations as in Table I

Fig. 1. Records showing the effects of ventilation with 7 ~ 02 in N 2 on some cardiovascular variables. The increases in left ventricular pressure (LVP) and its derivative (dP/dt) and also in arterial blood pressure (BP) returned to control levels again on returning to ventilation with room air. Right atrial pressure (Atrial press) decreased slightly

(P < 0.025), L V d P / d t m a x f r o m 3610 • 63 to 4037 • 93 m m H g s - ~ (P < 0.001). There were also significant increases in b l o o d pressure ( P < 0 . 0 0 1 ) and L V P ( P < 0 . 0 0 1 ) - see Table 2. There was r e d u c t i o n in the right atrial pressure f r o m an average control value o f 6 . 2 m m H g to 5 . 8 m m H g during h y p o x i a in the 3 dogs in which this was determined. O n c h a n g i n g to ventilation with r o o m air, the variables r e t u r n e d to control values within 2 rain. A n e x a m p l e o f such a response is s h o w n in Fig. l. The responses o b s e r v e d in the tetanus dogs were identical to those seen in the n o n - t e t a n u s dogs (Table 2). The changes in BP, L V d P / d t max, L V P a n d heart rate in response to h y p o x i a seen in the tetanus dogs were n o t significantly

different f r o m those observed in the n o n - t e t a n u s ( P > 0.1 in all cases using Student's t-test).

dogs

Effect of Atropine, Propranolol and Phenoxybenzamine I n t r a v e n o u s injection o f a t r o p i n e (2 m g . k g - 1 ) in 3 tetanus dogs caused a slight increase in heart rate on average o f 6 beats, m i n - 1 f r o m m e a n control value o f 210 beats, m i n - 1. In 3 n o n - t e t a n u s dogs, the average increase in heart rate on injection o f a t r o p i n e was 12 beats, m i n - 1 f r o m m e a n control o f 148 beats, m i n - l. The injection of atropine, however, had no significant effect on the c a r d i o v a s c u l a r responses to hypoxia. In 2 tetanus dogs, with the b l o o d pressure held constant, ventilation with 7 ~ 0 2 in N2 resulted in an average increase

223

Fig.2. Cardiovascular responses to hypoxia (7% 02 in N2) before and after combined alpha- and beta-adrenergic block. The increases in the cardiovascular variables during hypoxia before the block (leftpanel) became depressed afterwards (rightpanel)

in LVdP/dt max from 3500 to 3900mm Hg.s -1 and heart rate from 177 to 192 beats, min- 1 These cardiac responses to hypoxia were completely abolished by intravenous propranolol (1 mg.kg-1). However, in these dogs, without blood pressure being controlled, a pressor response was still observed after beta-adrenergic block; the blood pressure increasing on average from 100 to 123mmHg. Intravenous injection of phenoxybenzamine following the propranolol, however, abolished the pressor response to hypoxia, such that in 1 dog, the mean blood pressure even decreased from 60 to 55 mm Hg (see Fig. 2).

Effect of Lactic Acid In 4 tetanus dogs, 40 ml of a 5 % solution of lactic acid was infused into a femoral vein over a period of 2min. This reduced the pH of the arterial blood on average from 7.37 to 7.27. This induced acidaemia, however, had no effect on the responses to hypoxia.

Discussion

The results of the present experiments have shown that moderate to severe hypoxaemia, induced by ventilation with 7 % 02 in N2, results in increases in the blood pressure, heart rate and the maximum rate of rise of left ventricular pressure (LVdP/dt max) in dogs with generalised tetanus toxicity. These responses were abolished by adrenergic blocking drugs

indicating that they are due to increased sympathetic discharge to the heart and blood vessels. This increase is observed in spite of the high sympathetic tone already present in the tetanus dogs as reported earlier (Odusote and Sofola 1976) and also seen in the present series of experiments. The changes in the cardiovascular parameters observed in the tetanus dogs were comparable to those in the non-tetanus dogs. Thus, the increase in the sympathetic excitation seen during hypoxia in normal dogs (Downing et al. 1963; Achtel and Downing 1972; Kransey and Koehler 1977) also occurs 9in dogs with generalised tetanus following poisoning with tetanus toxin. The deduction from these observations is that although the tetanus dogs do have a high sympathetic discharge at rest, it is still possible for this to be increased further by hypoxia, suggesting that the increased basal sympathetic tone in these dogs is not maximal. The response of heart rate and LV dP/dt max to hypoxia were abolished by beta-adrenergic block with propranolol but not by atropine. Beta-adrenergic block did not affect appreciably the pressor response to hypoxia in the tetanus dogs but this was abolished by phenoxybenzamine - an alpha blocker. This suggests that the rise in blood pressure seen in these dogs during hypoxia is contributed to mainly by the arterioles. After combined beta- and alpha-adrenergic block, the response to hypoxia after an initial period was depressant resulting in a slight fall in blood pressure and LV dP/dt max (see Fig. 2). This suggests that hypoxia directly depresses the heart and blood vessels, an observation which is

224 similar to p r e v i o u s reports in n o r m a l dogs ( D e t a r and B o h r 1968; P i r z a d a et al. 1975; H e l l s t r a n d et al. 1977). In n o r m a l dogs, it has been s h o w n that m o d e r a t e degrees o f a c i d a e m i a do n o t affect the responses o f the c a r d i o v a s c u l a r system to sympathetic stimulation (Linden and N o r m a n 1969). O u r o b s e r v a t i o n in tetanus dogs with a p a t h o l o g i c a l increase in sympathetic discharge agrees with this. Patients with tetanus develop h y p o x a e m i a a n d m o d e r a t e a c i d a e m i a (Femi-Pearse 1974; Elegbeleye 1978) and the results o f the present investigation indicate that the hypo x a e m i a seen in tetanus patients m i g h t exercebate the increased sympathetic tone r e p o r t e d in these patients. This can therefore lead to increased cardiac l o a d which m a y predispose to cardiac failure and m a n y patients with tetanus have been observed to develop peripheral o e d e m a ( E d m o n d s o n and Flowers 1979). The use o f p r o p r a n o l o l has been f o u n d to be beneficial in the therapy for tetanus ( C h e a h et al. 1972; E d m o n d s o n and Flowers 1979). O u r present results will justify the c o n t i n u a l use o f beta-adrenergic blockers in tetanus patients w h o s h o w signs o f sympathetic overactivity whether or not this is a g g r a v a t e d by h y p o x a e m i a . The corollary is that h y p o x a e m i a should be excluded in any tetanus patient with clinical evidence o f s y m p a t h e t i c overactivity. In conclusion, m o d e r a t e to severe h y p o x a e m i a induced in dogs with tetanus toxicity, by ventilation with 7 ~o 0 2 in N2 exarcebates the increased sympathetic discharge to the heart and b l o o d vessels, effects which are m i n i m i z e d by adrenergic blockers.

Acknowledgement. We wish to acknowledge the technical assistance of Mr. Albert Adjenu and secretarial assistance of Messrs. F. O. Oderinde and C. Owunna. We are also indebted to the Biomedical Communication Department for the illustrations and especially Prof. Dr. R. Germanier of Swiss Serum and Vaccine Institute, Berne, for the Tetanus toxin used in this study. The study was carried out from the research grant by the College of Medicine, University of Lagos to one of us (O.A.S.).

References

Achtel RA, Downing SE (1972) Ventricular responses to hypoxaemia following chemoreceptor denervation and adrenalectomy. Am Heart J 84:377-386 Alfred EE, James OF (1974) Measurement of cardiac pre-ejection period as a guide to the therapy of sympathetic overactivity in tetanus. Anaesthesiol Intensiv Care 2: 251 - 255 Ardill BL, Fentem PH, Wellard MJ (1967) An electromagnetic pressure generator for testing the frequency response of transducers and catheter systems. J Physiol (London) 192 : 19 P - 21 P

Cheah PS, Mah PK, Feng PH (1972) Severe tetanus successfully treated with high dose of diazepam (Valium) and propranolol. A case report. Singapore Med J 13 : 163-165 Corbett JL, Kerr JH, Prys-Roberts C, Crampton Smith A, Spalding JMK (1969) Cardiovascular disturbances in severe tetanus due to overactivity of the sympathetic nervous system. Anaesthesia 24:198-212 Detar R, Bohr DF (1968) Oxygen and vascular smooth muscle contraction. Am J Physiol 214:241-244 Downing SE, Talner NS, Gardner TH (1966) Influences of hypoxaemia and acidaemia on left ventricular function. Am J Physio1210:1327-1334 Edmondson IRA, Flowers MW (1979) Intensive care in tetanus management, complications and mortality in 100 cases. Br Med J 1 : 1401 1404

Elegbeleye OO (1978) Effect of muscle spasms on blood gases and acidbase status in tetanus patients. Eur J Clin Invest 8:423-424 Femi-Pearse D (1974) Blood gas tensions, acid-base status and spirometry in tetanus. Am Rev Resp Dis 110:390--394 FurnivaI CM, Linden RS, Snow HM (1970) Inotropic changes in the left ventricle: the effect of changes in heart rate, aortic pressure and enddiastolic pressure. J Physiol (London) 211:359-387 Heistad DD, Abboud FM (1980) Circulatory adjustments to hypoxia. Circulation 61:463 - 470 Hellstrand F, Johansson B, Norberg K (1977) Mechanical, electrical and biochemical effects of hypoxia and substrate removal on spontaneously active vascular smooth muscle. Acta Physiol Scand 100:69-83 Hollow VM, Clarke GM (1975) Autonomic manifestations of tetanus. Anaesthesiol Intensiv Care 3:142-147 Kerr Jtt, Corbett JL, Prys-Roberts C, Crampton Smith A, Spalding JMK (1968) Involvement of the sympathetic nervous system in tetanus. Lancet II: 236 - 241 Kransey JA, Koehler RC (1977) Influence of arterial hypoxia on cardiac and coronary dynamics in the conscious sino-aortic denervated dog. J Appl Physiol 43 : 1012-1018 Linden RJ, Norman J (1969) The effect of acidaemia on the response to stimulation of autonomic nerves to the heart. J Physiol (London) 200: 51 - 71 Odusote KA, Sofola OA (1976) Haemodynamic changes during experimental tetanus toxicity in dogs. Naunyn-Schmiedeberg's Arch Pharmacol 295:159 - 164 Pirzada FA, Hood WB, Messer JV, Bing OHL (1975) Effects of hypoxia; cyanide, and ischaemia on myocardial contraction: observations in isolated muscle and intact heart. Cardiovasc Res 9:38-46 Wallace AG, Skinner NS, Mitchell JH (1963) Haemodynamic determinants of the maximal rate of rise of left ventricular pressure. Am J Physiol 205 : 30 - 36

Received March 26/Accepted September 23, 1981