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Digitalis toxicity
Digitalis
toxicity
Development
of cardiac
spontaneously
breathing
arrhythmias
in
vs. artificially
respired
dogs
J. L. Stickney, Ph.D. F. H. Meyers, M.D. San Francisco, Calif.
M
edification of cardiac sympathetic function is known to influence digitalis toxicity in cats,1-3 dogs:-? and humans.* Alterations in acid-base balance have also been found to influence digitalis toxicity.s-13 The effect produced by changes in acid-base balance may be a direct one on digitalis or it may be indirect by way of changes in the responsiveness of some tissues to catecholamines. Acidosis has been shown to decrease sensitivity to sympathomimetics.14-l7 It is known18 that general anesthesia of the type usually employed in animal experiments such as the ones cited above1m3*4e7 often results in significant changes in blood pH and gases. This study was therefore designed to separate the possible role of changes in blood gases induced by respiratory depression from other modifiers of digitalis toxicity, especially the sympathetic nervous system. Specifically, two questions were asked: (1) Is the development of digitalis toxicity the same in spontaneously and artificially respired animals? and (2) Are the respective effects of d,Z-propranolol, and reserpine on digitalis toxicity From
the same in spontaneously and artificially respired animals? Methods
Experiments were performed on male mongrel dogs (8 to 15 kilograms body weight) given morphine (20.0 mg. per kilogram of body weight subcutaneously) followed in one hour by pentobarbital sodium (200.0 mg. intramuscularly). Atropine (1.0 mg. total dose subcutaneously) was administered immediately preceding the morphine sulfate in order to prevent the marked bradycardia which follows morphine administration in dogs. The experiments were begun 90 minutes after the administration of pentobarbital sodium. When utilized, artificial respiration was begun 30 minutes after the administration of pentobarbital sodium. Ouabain was infused intravenously at a rate of 2.5 pg per kilogram of body weight per minute. The solutions were made up for each animal such that the correct dose per minute was contained in a volume of 1 ml. The endpoints of toxicity studied were
the Department of Pharmacology. School of Medicine, University of California Calif. This work was supported by a grant from the Research Committee of the Academic at San Francisco. Received for publication May 10. 1972. Reprint requests to: Dr. J. L. Stickney, Department of Pharmacology, Michigan Mich. 48823.
Vol. 85,No.4,/$. 501-505 April,1973
at San Francisco, Senate.
State
San Francisco.
University
University,
of California
East
American Heart Journal
Lansing.
501
502
.41x. Heart J. A@T’~, 1973
Stickney and Meyers
Table I. Blood gas values of arterial blood samples taken immediately PO, mm.
Hg *
before infusion
of ouabain
SEM
Variables
Control Propranolol-pretreated Reserpine-pretreated
61.0 * 7.1 w* 69.1 * 5.3 (5) 58.8 f 3.7 (4)
115.8
* 8.8 (5) 115.4 * 9.4 (5) 116.0 U)$
*The number in parentheses indicates the number of animals tAI1 values for artificially respired animals are significantly animals. p < 0.01. :Values were not obtained for the second animal in the group
55.3 * 3.7 (5) 51.6 * 3.7 (5) 53.6 * 2.4 (4)
for which blood-gas different from the because
the blood-gas
21.3
* 4.8 6) 17.4 f .70 (5) 21.2 (1)
determinations corresponding analyzer
7.22 * ,029 (5) 7.25 * ,023 (5) 7.21 * .015 (4)
7.50 * .003 (5) 7.64 * .Oll (5) 7.52 (1)
were made. values in spontaneously
respiring
was broken.
Table II. Blood gas values of arterial blood samples taken at death PO, mm.
Hg *
SEM
PCO,
mm.
--____-__--
Hg * ---_
pH + SEM
SEM ---
----
Variables Spontaneous respiration Control Propranolol-pretreated Reserpine-pretreated
33.0 * 4.2 w* 44.8 * 10.6 (5) 31.3 * 10.3 (4)
Artijicialt
Spantaneous
respiration
respiration
114.9
* 6.2 (5) 108.3 * 5.2 (5) 98.0 (1X
*The number in parentheses indicates the number of animals TAll values for artificially respired animals are significantly animals. p < 0.01. W&es were not obtained for the second animal in the group
64.0 * 10.6 (5) 61.2 * 5.7 (5) 8.6 64.1 * (4)
Arlijicialt
the dose of ouabain necessary to produce ventricular tachycardia, the lethal dose, and the mode of death. Animals were divided into two primary groups: (1) spontaneously breathing, and (2) artificially respired (300 C.C. room air per kilogram of body weight per minute; 16 strokes per minute; Harvard respiration pump). Within each primary group there were three secondary groups. The first group was the control group. It consisted of animals that received anesthetic agents and ouabain. A second group was pretreated with d&-propranolol. Twenty-five minutes before initiation of the ouabain infusion, the chronotropic response to isoproterenol, 0.25 pg per kilogram of body weight, intravenously,
the blood-gas
respiralicn
16.6 * 4.8 (5) 16.0 * 3.1 (5) 29.6 (1)
for which blood-gas determinations different from the corresponding because
Spontaneous
respiration
analyzer
7.14 * ,049 (5) 7.20 * .040 (5) 7.17 * .047 (4) were values
made. in spontaneously
Artificialt respiration
7.41 5 .042 (5) 7.59 =t ,018 (5) 7.43 (1)
respiring
was broken.
was recorded. When the heart rate returned to control levels, propranolol (2.5 mg. per kilogram of body weight, intravenously) was administered over a five-minute period. Fifteen minutes later, completeness of beta-adrenergic receptor blockade was tested by recording the chronotropic response to a second challenge of isoproterenol. The third group of animals was pretreated with reserpine, 0.1 mg. per kilogram of body weight intramuscularly, 72 and 48 hours before the experiment was begun and thereafter treated as were the animals in the control group. The Lead II electrocardiogram (ECG) was monitored continuously on an oscilloscope and periodic recordings were made
Volrrme Number
85 4
Table III.
Respiration
and digitalis
toxicity
503
Ouabain toxicity Dose of ouabain required to produce ventricular tachycardia Wk. * SEMI
Variables
Lethal dose cf ouabain Wkg. * SEW
Mcde
of death
Sfiontaneous respiration Control Propranolol-pretreated Reserpine-pretreated
*Number tDifference
of animals between
107.8 * 3.1 104.1 * 6.5 116.2 * 9.7
Artijicial respiration 102.8 109.3 106.3
* * *
Spontaneous
Artificial respiration
respiration
7.0 7.3 3.7
143.2 150.0 147.3
* 11.1 f 8.2t * 17.6
that died in ventricular fibrillation cwer the total number these two groups is statistically significant. p < .05.
on a Grass Model 7 polygraph. Blood samples of 2% ml. were taken from a femoral artery by means of a polyethylene catheter. Periodic sampleswere collected in a heparinized glass syringe which was then immediately capped and placed in ice. Within one hour after termination of the experiment Po2, PcoZ, and pH of each sample were determined with a Beckman physiological gas analyzer. The rectal temperature was maintained at 37” C. f 1.0” C. with an electric heating pad when necessary. Student’s T testIs was used to analyze the data for significant differences between groups. A difference was accepted as significant if the p value was less than 0.05. Results
The mean values for PoZ, PcoZ, and pH in the different groups of animals are presented in Tables I and II. Animals that were allowed to respire spontaneously were hypoxic, hypercapnic, and acidotic throughout the experiment. In contrast, artificially respired animals were adequately oxygenatedIs; some animals were slightly hypocapnic. At death, all artificially respired animals had pH values that were greater than those values which are considered to be within the normal range for the awake dog.‘*J” There was a statistically significant difference in all of the blood-gas values of spontaneously respiring animals when compared with those values for corresponding groups of artificially respired animals. The dose of ouabain required to produce
171.9 139.8 161.2
of animals
Spontanecus respiration
* 7.4 * 8.8t * 4.0
5/s* O/5 w
Artificial
respiration 6/6 5/s
o/2
in the group.
ventricular tachycardia, the lethal dose, and the mode of death are summarized in Table III. These indices of digitalis toxicity are the same in control groups of animals whether respiration was spontaneous or artificial. In spontaneously respiring animals, the racemic mixture of propranolol* had no effect on either the dose of ouabain required to produce ventricular tachycardia or on the lethal dose of ouabain. The most frequently occurring mode of death was changed from ventricular fibrillation to cardiac arrest. In artificially respired animals, propranolol did not alter the dose of ouabain required to produce ventricular tachycardia but it decreased the lethal dose (p < 0.02). Ventricular fibrillation remained the most frequent cause of death. Reserpine pretreatment modified neither of the toxic doses of ouabain in spontaneously respiring animals; the most frequently occurring mode of death was cardiac arrest rather than ventricular fibrillation. Although only two reserpine-pretreated animals were respired artificially the results indicate that neither the dose of ouabain necessary to produce ventricular tachycardia nor the lethal dose of ouabain was altered; both animals died in cardiac arrest. Discussion
Spontaneously respiring dogs under general anesthesia (combination morphine and *This
dose of propranolol. significantly decreased proterenol.
2.5 mg. per kilogram the chronotropic
of body response
weight, to iso-
504
Stickney
Am. Heart .I. April, 1373
and Meyers
pentobarbital sodium) were hypoxic, hypercapnic, and acidotic.‘8*20 In spite of this, both the dose of ouabain required to produce ventricular tachycardia and the lethal dose of ouabain in spontaneously respiring dogs were no different from those doses in artificially respired animals. Neither reserpine pretreatment nor d,l-propranolol pretreatment increased either of the toxic doses of ouabain. In the dog, the mode of death produced by digitalis appears to be a more sensitive index of the relationship between catecholamines and digitalis toxicity than either the dose of ouabain required to produce ventricular tachycardia or the lethal dose.7 The presence of cardiac sympathetic function makes it more likely that animals will die in ventricular fibrillation than in cardiac arrest. McLain2r and Gillis have shown that ouabain increases cardiac sympathetic nerve activity. Thus, an amount of norepinephrine sufficient to produce ventricular fibrillation might be “released” by an action of digitalis itself. Artificially respired animals would be expected to die in ventricular fibrillation. On the other hand, under the acidotic conditions which develop during spontaneous breathing in the anesthetized dog, cardiac arrest might be expected to be the most frequently occurring mode of death. Adrenergic receptor function has been found to be decreased during acidosisr4-r7; therefore, norepinephrine released by ouabain would be less active. However, in this study all control animals died in ventricular fibrillation, no matter the type of respiration. Perhaps ventricular fibrillation develops in spontaneously breathing animals because the cardiac actions of catecholamines released reflexly in response to hypoxia and hypercapnia23-25 add to the actions of norepinephrine “released” by ouabain, thus overcoming the effects of any decrease in adrenergic receptor function caused by the acidosis. Propranolol, a beta-adrenergic receptor blocking agent, and reserpine, an agent that depletes norepinephrine from sympathetic nerve endings, both changed the most frequently occurring mode of death from ventricular fibrillation to cardiac arrest in spontaneously breathing animals. Reserpine appeared to have the same effect in
artificially respired animals whereas propranolol did not. This discrepancy may be explained by the effect of hypercapnia and acidosis to decrease adrenergic receptor sensitivity. In spontaneously respiring animals, adrenergic receptor sensitivity is decreased, therefore it is relatively easy for propranolol to competitively block the action of norepinephrine “released” by digitalis. The animals die in cardiac arrest. In artificially respired animals, adrenergic receptor sensitivity is not decreased. Under such conditions it is possible that the dose of propranolol used in this study does not block completely the effects of norepinephrine released by ouabain. The animals die in ventricular fibrillation. Unlike the situation in artificially respired animals pretreated with propranolol, cardiac arrest was the mode of death in artificially respired animals that had been pretreated with reserpine. In the latter situation, there is no norepinephrine available to be “released” by ouabain. Thus the effect, cardiac arrest, is the same whether the animal is breathing spontaneously or whether it is artificially respired. Adrenergic receptor sensitivity is not a factor. The presence or absence of catecholamines can modify digitalis toxicity. Changes in acid-base balance can modify the pharmacological actions of catecholamines and thereby indirectly alter digitalis toxicity. Thus, it is incumbent upon persons studying the relationship between catecholamines and digitalis-induced cardiac arrhythmias to specify the state of acid-base balance in their animals. Any conclusions drawn should include considerations of this parameter. When administering digitalis therapeutically it is incumbent upon the physician to include in his considerations not only the acid-base balance of the patient, but also the presence of drugs which may either increase or decrease sympathetic nervous system function. Changes in either of these two variables might alter the potential for digitalis toxicity to become manifest. Summary
Ouabain toxicity was studied in dogs anesthetized with morphine and pentobarbital. One large group of dogs was allowed to breathe spontaneously, the other
Volume Nzrmber
85 4
group was respired artificially. The development of digitalis toxicity and the toxic doses of ouabain were found to be the same in spontaneously and in artificially respired dogs. The effect of d,Z-propranolol on digitalis toxicity is not the same in spontaneously and artificially respired animals. In the former, propranolol changed the most frequently occurring mode of death from ventricular fibrillation to cardiac arrest. In the latter, ventricular fibrillation is the most frequently occurring mode of death; the lethal dose is decreased. Reserpine pretreatment produced cardiac arrest in spontaneously breathing and in artificially respired animals. REFERENCES J., Ehrreich, S., and Levitt, B.: Some 1. Roberts, aspects of the cardiac actions of reserpine and pronethalol, Fed. Proc. 24:1421, 1965. J., Ito, R., Reilly, J., and Cairoli, V. J.: 2. Roberts, Influence of reserpine and beta-TM 10 on digitalis-induced ventricular arrhythmia, Circ. Res. 13:149, 1963. J.: On the relationship between auto3. Stickney, nomic nervous system and digitalis toxicity, Fed. Proc. 30284, 1971. C., Aceves, J., and Mendez, R.: The 4. Mendez, anti-adrenergic action of digitalis on the refractory period of the A-V transmission system, J. Pharmacol. Exp. Ther. 131:199, 1961. L., and Nash, C.: Influence of reserpine 5. Boyajy, on arrhythmias, inotropic effects, and myocardial potassium balance induced by digitalis materials, J. Pharmacol. Exp. Ther. 148:193, 1965. L., and Nash, C.: Alteration of ouabain 6. Boyajy, toxicity by cardiac denervation, Toxicol. Appl. Pharmacol. 9:199, 1966. 7. Stickney, J., Lucchesi, B., and Abrams, G.: Pharmacological alteration of the response of the mammalian heart to acetylstrophanthidin, Fed. Proc. 25:621, 1966. 8. Dick, H., McCawley, E. L., and Fisher, W. A.: Reserpine-digitalis toxicity, Arch. Intern. Med. 109503, 1962. P., and Wasserman, F.: Observations 9. Rodensky, on digitalis intoxication, Arch. Intern. Med. 108:171, 1961. 10. Talso, P. J., Remeivchik, A. P., and Cutilletta, A.: Altered myocardial potassium gradients
Respiration
and digitalis
toxicity
50.5
in acute alkalosis and their relationship to acetyl strophanthidin sensitivity in the dog, Circulation 26:794, 1962. 11. Bliss, H., Fishman, W., and Smith, P.: Effect of alterations of blood pH on digitalis toxicity, T. Lab. Clin. Med. 62:53. 1963. D., Robinson,’ M., and Kleiger, R.: 12. Harrison, Role of hypoxia in digitalis toxicity, Circulation .%(Suppl. 111):124, 1966. M. C., Gianelly, R. E., Cutler, S. L., 13. Warren, and Harrison, D. C.: Digitalis toxicity II. The effect of metabolic alkalosis, AM. HEART J. 75:358, 1968. S. M.: Sympatho-adrenal stimulation 14. Tenney, bv carbon dioxide and the inhibitorv effect of carbonic acid on epinephrine response, Am. Jl Physiol. 187:341, 1956. 1.5. Guzman, S., DeLeon, A. C., Jr., West, J., and Bellet, S.: Cardiac effects of isoproterenol, norepinephrine and epinephrine in complete A-V heart block during experimental acidosis and hyperkalemia, Circ. Res. 7:666, 1959. P., Egbert, L., Linde, H., Cooper, D., 16. Sechzer, DriDDs. R.. and Price. H.: Effect of CO, inhalation on’ arterial pressure, ECG, and plasma catecholamines and 17-OH corticosteroids in normal man, J. Appl. Physiol. 15:454, 1960. 17. Zsoter, T. T.: Effect of hypoxia on the vascular response to isoproterenol and norepinephrine, Am HEART I. 77:498. 1969. 18. Mitchell, R. A.: Personal communication. 19. Snedecor, G. W.: Statistical methods, Ames, Iowa. 1956. The Iowa State Collece Press. 20. Mitchell, R. A., Bainton, C. R., anod Edelist, G.: Posthyperventilation apnea in awake dogs during metabolic acidosis and hypoxia, J. Appl. Physiol. 21:1363, 1966. P.: Effects of cardiac glycosides on 21. McLain, spontaneous efferent activity in vagus and sympathetic nerves of cats, Int. J. Neuropharmacol. 8:379, 1969. 22. Gillis, R. A.: Cardiac sympathetic nerve activity: Changes induced by ouabain and propranolol, Science 166:508, 1969. 23. Clowes, G., Jr., Hopkins, A., and Simeone, F.: A comparison of the physiological effects of hypercapnia and hypoxia in the production of cardiac arrest, Ann. Surg. 142:446, 1955. S. M.: Determinants 24. Honig, C. R., and Tenney, of the circulatory response to hypoxia and hypercapnia, AM. HEART J. 53:687, 1957. 25. Goldman, R., and Harrison, D.: The effects of hypoxia and hypercarbia on myocardial catecholamines, J. Pharmacol. Exp. Ther. 174:307, 1970.
toxicity
Development
of cardiac
spontaneously
breathing
arrhythmias
in
vs. artificially
respired
dogs
J. L. Stickney, Ph.D. F. H. Meyers, M.D. San Francisco, Calif.
M
edification of cardiac sympathetic function is known to influence digitalis toxicity in cats,1-3 dogs:-? and humans.* Alterations in acid-base balance have also been found to influence digitalis toxicity.s-13 The effect produced by changes in acid-base balance may be a direct one on digitalis or it may be indirect by way of changes in the responsiveness of some tissues to catecholamines. Acidosis has been shown to decrease sensitivity to sympathomimetics.14-l7 It is known18 that general anesthesia of the type usually employed in animal experiments such as the ones cited above1m3*4e7 often results in significant changes in blood pH and gases. This study was therefore designed to separate the possible role of changes in blood gases induced by respiratory depression from other modifiers of digitalis toxicity, especially the sympathetic nervous system. Specifically, two questions were asked: (1) Is the development of digitalis toxicity the same in spontaneously and artificially respired animals? and (2) Are the respective effects of d,Z-propranolol, and reserpine on digitalis toxicity From
the same in spontaneously and artificially respired animals? Methods
Experiments were performed on male mongrel dogs (8 to 15 kilograms body weight) given morphine (20.0 mg. per kilogram of body weight subcutaneously) followed in one hour by pentobarbital sodium (200.0 mg. intramuscularly). Atropine (1.0 mg. total dose subcutaneously) was administered immediately preceding the morphine sulfate in order to prevent the marked bradycardia which follows morphine administration in dogs. The experiments were begun 90 minutes after the administration of pentobarbital sodium. When utilized, artificial respiration was begun 30 minutes after the administration of pentobarbital sodium. Ouabain was infused intravenously at a rate of 2.5 pg per kilogram of body weight per minute. The solutions were made up for each animal such that the correct dose per minute was contained in a volume of 1 ml. The endpoints of toxicity studied were
the Department of Pharmacology. School of Medicine, University of California Calif. This work was supported by a grant from the Research Committee of the Academic at San Francisco. Received for publication May 10. 1972. Reprint requests to: Dr. J. L. Stickney, Department of Pharmacology, Michigan Mich. 48823.
Vol. 85,No.4,/$. 501-505 April,1973
at San Francisco, Senate.
State
San Francisco.
University
University,
of California
East
American Heart Journal
Lansing.
501
502
.41x. Heart J. A@T’~, 1973
Stickney and Meyers
Table I. Blood gas values of arterial blood samples taken immediately PO, mm.
Hg *
before infusion
of ouabain
SEM
Variables
Control Propranolol-pretreated Reserpine-pretreated
61.0 * 7.1 w* 69.1 * 5.3 (5) 58.8 f 3.7 (4)
115.8
* 8.8 (5) 115.4 * 9.4 (5) 116.0 U)$
*The number in parentheses indicates the number of animals tAI1 values for artificially respired animals are significantly animals. p < 0.01. :Values were not obtained for the second animal in the group
55.3 * 3.7 (5) 51.6 * 3.7 (5) 53.6 * 2.4 (4)
for which blood-gas different from the because
the blood-gas
21.3
* 4.8 6) 17.4 f .70 (5) 21.2 (1)
determinations corresponding analyzer
7.22 * ,029 (5) 7.25 * ,023 (5) 7.21 * .015 (4)
7.50 * .003 (5) 7.64 * .Oll (5) 7.52 (1)
were made. values in spontaneously
respiring
was broken.
Table II. Blood gas values of arterial blood samples taken at death PO, mm.
Hg *
SEM
PCO,
mm.
--____-__--
Hg * ---_
pH + SEM
SEM ---
----
Variables Spontaneous respiration Control Propranolol-pretreated Reserpine-pretreated
33.0 * 4.2 w* 44.8 * 10.6 (5) 31.3 * 10.3 (4)
Artijicialt
Spantaneous
respiration
respiration
114.9
* 6.2 (5) 108.3 * 5.2 (5) 98.0 (1X
*The number in parentheses indicates the number of animals TAll values for artificially respired animals are significantly animals. p < 0.01. W&es were not obtained for the second animal in the group
64.0 * 10.6 (5) 61.2 * 5.7 (5) 8.6 64.1 * (4)
Arlijicialt
the dose of ouabain necessary to produce ventricular tachycardia, the lethal dose, and the mode of death. Animals were divided into two primary groups: (1) spontaneously breathing, and (2) artificially respired (300 C.C. room air per kilogram of body weight per minute; 16 strokes per minute; Harvard respiration pump). Within each primary group there were three secondary groups. The first group was the control group. It consisted of animals that received anesthetic agents and ouabain. A second group was pretreated with d&-propranolol. Twenty-five minutes before initiation of the ouabain infusion, the chronotropic response to isoproterenol, 0.25 pg per kilogram of body weight, intravenously,
the blood-gas
respiralicn
16.6 * 4.8 (5) 16.0 * 3.1 (5) 29.6 (1)
for which blood-gas determinations different from the corresponding because
Spontaneous
respiration
analyzer
7.14 * ,049 (5) 7.20 * .040 (5) 7.17 * .047 (4) were values
made. in spontaneously
Artificialt respiration
7.41 5 .042 (5) 7.59 =t ,018 (5) 7.43 (1)
respiring
was broken.
was recorded. When the heart rate returned to control levels, propranolol (2.5 mg. per kilogram of body weight, intravenously) was administered over a five-minute period. Fifteen minutes later, completeness of beta-adrenergic receptor blockade was tested by recording the chronotropic response to a second challenge of isoproterenol. The third group of animals was pretreated with reserpine, 0.1 mg. per kilogram of body weight intramuscularly, 72 and 48 hours before the experiment was begun and thereafter treated as were the animals in the control group. The Lead II electrocardiogram (ECG) was monitored continuously on an oscilloscope and periodic recordings were made
Volrrme Number
85 4
Table III.
Respiration
and digitalis
toxicity
503
Ouabain toxicity Dose of ouabain required to produce ventricular tachycardia Wk. * SEMI
Variables
Lethal dose cf ouabain Wkg. * SEW
Mcde
of death
Sfiontaneous respiration Control Propranolol-pretreated Reserpine-pretreated
*Number tDifference
of animals between
107.8 * 3.1 104.1 * 6.5 116.2 * 9.7
Artijicial respiration 102.8 109.3 106.3
* * *
Spontaneous
Artificial respiration
respiration
7.0 7.3 3.7
143.2 150.0 147.3
* 11.1 f 8.2t * 17.6
that died in ventricular fibrillation cwer the total number these two groups is statistically significant. p < .05.
on a Grass Model 7 polygraph. Blood samples of 2% ml. were taken from a femoral artery by means of a polyethylene catheter. Periodic sampleswere collected in a heparinized glass syringe which was then immediately capped and placed in ice. Within one hour after termination of the experiment Po2, PcoZ, and pH of each sample were determined with a Beckman physiological gas analyzer. The rectal temperature was maintained at 37” C. f 1.0” C. with an electric heating pad when necessary. Student’s T testIs was used to analyze the data for significant differences between groups. A difference was accepted as significant if the p value was less than 0.05. Results
The mean values for PoZ, PcoZ, and pH in the different groups of animals are presented in Tables I and II. Animals that were allowed to respire spontaneously were hypoxic, hypercapnic, and acidotic throughout the experiment. In contrast, artificially respired animals were adequately oxygenatedIs; some animals were slightly hypocapnic. At death, all artificially respired animals had pH values that were greater than those values which are considered to be within the normal range for the awake dog.‘*J” There was a statistically significant difference in all of the blood-gas values of spontaneously respiring animals when compared with those values for corresponding groups of artificially respired animals. The dose of ouabain required to produce
171.9 139.8 161.2
of animals
Spontanecus respiration
* 7.4 * 8.8t * 4.0
5/s* O/5 w
Artificial
respiration 6/6 5/s
o/2
in the group.
ventricular tachycardia, the lethal dose, and the mode of death are summarized in Table III. These indices of digitalis toxicity are the same in control groups of animals whether respiration was spontaneous or artificial. In spontaneously respiring animals, the racemic mixture of propranolol* had no effect on either the dose of ouabain required to produce ventricular tachycardia or on the lethal dose of ouabain. The most frequently occurring mode of death was changed from ventricular fibrillation to cardiac arrest. In artificially respired animals, propranolol did not alter the dose of ouabain required to produce ventricular tachycardia but it decreased the lethal dose (p < 0.02). Ventricular fibrillation remained the most frequent cause of death. Reserpine pretreatment modified neither of the toxic doses of ouabain in spontaneously respiring animals; the most frequently occurring mode of death was cardiac arrest rather than ventricular fibrillation. Although only two reserpine-pretreated animals were respired artificially the results indicate that neither the dose of ouabain necessary to produce ventricular tachycardia nor the lethal dose of ouabain was altered; both animals died in cardiac arrest. Discussion
Spontaneously respiring dogs under general anesthesia (combination morphine and *This
dose of propranolol. significantly decreased proterenol.
2.5 mg. per kilogram the chronotropic
of body response
weight, to iso-
504
Stickney
Am. Heart .I. April, 1373
and Meyers
pentobarbital sodium) were hypoxic, hypercapnic, and acidotic.‘8*20 In spite of this, both the dose of ouabain required to produce ventricular tachycardia and the lethal dose of ouabain in spontaneously respiring dogs were no different from those doses in artificially respired animals. Neither reserpine pretreatment nor d,l-propranolol pretreatment increased either of the toxic doses of ouabain. In the dog, the mode of death produced by digitalis appears to be a more sensitive index of the relationship between catecholamines and digitalis toxicity than either the dose of ouabain required to produce ventricular tachycardia or the lethal dose.7 The presence of cardiac sympathetic function makes it more likely that animals will die in ventricular fibrillation than in cardiac arrest. McLain2r and Gillis have shown that ouabain increases cardiac sympathetic nerve activity. Thus, an amount of norepinephrine sufficient to produce ventricular fibrillation might be “released” by an action of digitalis itself. Artificially respired animals would be expected to die in ventricular fibrillation. On the other hand, under the acidotic conditions which develop during spontaneous breathing in the anesthetized dog, cardiac arrest might be expected to be the most frequently occurring mode of death. Adrenergic receptor function has been found to be decreased during acidosisr4-r7; therefore, norepinephrine released by ouabain would be less active. However, in this study all control animals died in ventricular fibrillation, no matter the type of respiration. Perhaps ventricular fibrillation develops in spontaneously breathing animals because the cardiac actions of catecholamines released reflexly in response to hypoxia and hypercapnia23-25 add to the actions of norepinephrine “released” by ouabain, thus overcoming the effects of any decrease in adrenergic receptor function caused by the acidosis. Propranolol, a beta-adrenergic receptor blocking agent, and reserpine, an agent that depletes norepinephrine from sympathetic nerve endings, both changed the most frequently occurring mode of death from ventricular fibrillation to cardiac arrest in spontaneously breathing animals. Reserpine appeared to have the same effect in
artificially respired animals whereas propranolol did not. This discrepancy may be explained by the effect of hypercapnia and acidosis to decrease adrenergic receptor sensitivity. In spontaneously respiring animals, adrenergic receptor sensitivity is decreased, therefore it is relatively easy for propranolol to competitively block the action of norepinephrine “released” by digitalis. The animals die in cardiac arrest. In artificially respired animals, adrenergic receptor sensitivity is not decreased. Under such conditions it is possible that the dose of propranolol used in this study does not block completely the effects of norepinephrine released by ouabain. The animals die in ventricular fibrillation. Unlike the situation in artificially respired animals pretreated with propranolol, cardiac arrest was the mode of death in artificially respired animals that had been pretreated with reserpine. In the latter situation, there is no norepinephrine available to be “released” by ouabain. Thus the effect, cardiac arrest, is the same whether the animal is breathing spontaneously or whether it is artificially respired. Adrenergic receptor sensitivity is not a factor. The presence or absence of catecholamines can modify digitalis toxicity. Changes in acid-base balance can modify the pharmacological actions of catecholamines and thereby indirectly alter digitalis toxicity. Thus, it is incumbent upon persons studying the relationship between catecholamines and digitalis-induced cardiac arrhythmias to specify the state of acid-base balance in their animals. Any conclusions drawn should include considerations of this parameter. When administering digitalis therapeutically it is incumbent upon the physician to include in his considerations not only the acid-base balance of the patient, but also the presence of drugs which may either increase or decrease sympathetic nervous system function. Changes in either of these two variables might alter the potential for digitalis toxicity to become manifest. Summary
Ouabain toxicity was studied in dogs anesthetized with morphine and pentobarbital. One large group of dogs was allowed to breathe spontaneously, the other
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group was respired artificially. The development of digitalis toxicity and the toxic doses of ouabain were found to be the same in spontaneously and in artificially respired dogs. The effect of d,Z-propranolol on digitalis toxicity is not the same in spontaneously and artificially respired animals. In the former, propranolol changed the most frequently occurring mode of death from ventricular fibrillation to cardiac arrest. In the latter, ventricular fibrillation is the most frequently occurring mode of death; the lethal dose is decreased. Reserpine pretreatment produced cardiac arrest in spontaneously breathing and in artificially respired animals. REFERENCES J., Ehrreich, S., and Levitt, B.: Some 1. Roberts, aspects of the cardiac actions of reserpine and pronethalol, Fed. Proc. 24:1421, 1965. J., Ito, R., Reilly, J., and Cairoli, V. J.: 2. Roberts, Influence of reserpine and beta-TM 10 on digitalis-induced ventricular arrhythmia, Circ. Res. 13:149, 1963. J.: On the relationship between auto3. Stickney, nomic nervous system and digitalis toxicity, Fed. Proc. 30284, 1971. C., Aceves, J., and Mendez, R.: The 4. Mendez, anti-adrenergic action of digitalis on the refractory period of the A-V transmission system, J. Pharmacol. Exp. Ther. 131:199, 1961. L., and Nash, C.: Influence of reserpine 5. Boyajy, on arrhythmias, inotropic effects, and myocardial potassium balance induced by digitalis materials, J. Pharmacol. Exp. Ther. 148:193, 1965. L., and Nash, C.: Alteration of ouabain 6. Boyajy, toxicity by cardiac denervation, Toxicol. Appl. Pharmacol. 9:199, 1966. 7. Stickney, J., Lucchesi, B., and Abrams, G.: Pharmacological alteration of the response of the mammalian heart to acetylstrophanthidin, Fed. Proc. 25:621, 1966. 8. Dick, H., McCawley, E. L., and Fisher, W. A.: Reserpine-digitalis toxicity, Arch. Intern. Med. 109503, 1962. P., and Wasserman, F.: Observations 9. Rodensky, on digitalis intoxication, Arch. Intern. Med. 108:171, 1961. 10. Talso, P. J., Remeivchik, A. P., and Cutilletta, A.: Altered myocardial potassium gradients
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in acute alkalosis and their relationship to acetyl strophanthidin sensitivity in the dog, Circulation 26:794, 1962. 11. Bliss, H., Fishman, W., and Smith, P.: Effect of alterations of blood pH on digitalis toxicity, T. Lab. Clin. Med. 62:53. 1963. D., Robinson,’ M., and Kleiger, R.: 12. Harrison, Role of hypoxia in digitalis toxicity, Circulation .%(Suppl. 111):124, 1966. M. C., Gianelly, R. E., Cutler, S. L., 13. Warren, and Harrison, D. C.: Digitalis toxicity II. The effect of metabolic alkalosis, AM. HEART J. 75:358, 1968. S. M.: Sympatho-adrenal stimulation 14. Tenney, bv carbon dioxide and the inhibitorv effect of carbonic acid on epinephrine response, Am. Jl Physiol. 187:341, 1956. 1.5. Guzman, S., DeLeon, A. C., Jr., West, J., and Bellet, S.: Cardiac effects of isoproterenol, norepinephrine and epinephrine in complete A-V heart block during experimental acidosis and hyperkalemia, Circ. Res. 7:666, 1959. P., Egbert, L., Linde, H., Cooper, D., 16. Sechzer, DriDDs. R.. and Price. H.: Effect of CO, inhalation on’ arterial pressure, ECG, and plasma catecholamines and 17-OH corticosteroids in normal man, J. Appl. Physiol. 15:454, 1960. 17. Zsoter, T. T.: Effect of hypoxia on the vascular response to isoproterenol and norepinephrine, Am HEART I. 77:498. 1969. 18. Mitchell, R. A.: Personal communication. 19. Snedecor, G. W.: Statistical methods, Ames, Iowa. 1956. The Iowa State Collece Press. 20. Mitchell, R. A., Bainton, C. R., anod Edelist, G.: Posthyperventilation apnea in awake dogs during metabolic acidosis and hypoxia, J. Appl. Physiol. 21:1363, 1966. P.: Effects of cardiac glycosides on 21. McLain, spontaneous efferent activity in vagus and sympathetic nerves of cats, Int. J. Neuropharmacol. 8:379, 1969. 22. Gillis, R. A.: Cardiac sympathetic nerve activity: Changes induced by ouabain and propranolol, Science 166:508, 1969. 23. Clowes, G., Jr., Hopkins, A., and Simeone, F.: A comparison of the physiological effects of hypercapnia and hypoxia in the production of cardiac arrest, Ann. Surg. 142:446, 1955. S. M.: Determinants 24. Honig, C. R., and Tenney, of the circulatory response to hypoxia and hypercapnia, AM. HEART J. 53:687, 1957. 25. Goldman, R., and Harrison, D.: The effects of hypoxia and hypercarbia on myocardial catecholamines, J. Pharmacol. Exp. Ther. 174:307, 1970.