Use of Adenosine Antagonists in the Treatment of Bradyarrhythmias and Mechanical Dysfunction Associated With Cardiopulmonary Resuscitation

1. Field of the Invention

The invention relates to the use of adenosine antagonists, either alone or in combination with pharmacologic agents possessing and/or -adrenergic or dopaminergic properties, to treat cardiac rhythm disturbances, mechanical dysfunction and hypotension and to facilitate cardioversion and/or defibrillation in the setting of cardiopulmonary resuscitation.

2. Discussion of the Background

Prolonged cardiopulmonary resuscitation for ventricular fibrillation is associated with the occurrence of asystole or severe bradycardias and profound hemodynamic collapse associated with a severe depression in myocardial contractility (electro-mechanical dissociation) which are usually resistant to therapy. (Rahimtoola S. H., J. Am. Med. Assoc., 247, pages 2485-2890 (1982) and Iseri, L. T., Ann. Int. Med., 88, pages 741-745 (1978).)

Catecholamines and/or para-sympatholytic agents have traditionally been used to treat intractable ventricular fibrillation and post-defibrillation depressions in automaticity, cardiac conduction, and overall hemodynamic collapse. However, these agents are often ineffective, particularly in the setting of prolonged ventricular fibrillation and, as a consequence of their use, may provoke intractable ventricular fibrillation followed by death or severe neurological impairment. See McIntyre, K. M., Lewis, A. J., Eds, Textbook of Advanced Life Support, Vol 9, American Heart Association (1981) and Am. Heart J., 97, 225-228 (1979).

Pharmacological agents are routinely used in the cardiopulmonary resuscitation of patients suffering from cardiac arrest. A review of these pharmacologic agents can be found in Otto C. W., Circ. 74 (supplement IV), IV-80-85 (December 1986). It has been established that the most significant factor in the return of spontaneous circulation during cardiopulmonary resuscitation is the enhancement of .alpha.-adrenergic tone, i.e., an increase in aortic diastolic pressure and coronary perfusion pressure. However, there has been no suggestion that the use of .alpha.-adrenergic agents improve survival relative to the use of epinephrine, a mixed adrenergic agonist with known deleterious effects.

Additionally, there is no clear evidence that epinephrine, the currently recommended pharmacologic agent, can increase the effectiveness of electric shock during the fibrillation.

Lidocaine is the recommended anti-arrhythmic agent for use during cardiac arrest. It is known however that lidocaine can increase the threshhold for defibrillation. Under experimental conditions, the anti-arrhythmic drug, Bretylium, seems to facilitate defibrillation; however, clinical data are less convincing. (Jaffe A. S., Circ. 74 (supplement IV), IV-70-74, December 1986).

The model established by the present inventors confirms that the cardioversion of prolonged hypoxic ventricular fibrillation is accompanied by cardiovascular collapse and depressions in cardiac automaticity, conduction, and contractility. This post-shock hypoxic depression is due, in part, to the collapse in arteriolar tone possibly mediated by the release of endogenous adenosine. In addition, the release of endogenous adenosine from myocardium might selectively dilate coronary resistance vessels promoting hypoperfusion of the subendocardial layer of the heart, the most distal and thus vulnerable cardiac vascular bed. The result would be enhanced ischemic-induced depression in contractility. Endogenous catecholamines are known to be released with vascular collapse and perhaps in response to an electrical shock. The beneficial vasomotor and inotropic effects of endogenous catecholamine release may, in turn, be attenuated by the anti-adrenergic action of endogenous adenosine. The aforementioned potential deleterious effects of endogenous adenosine may be reversed by adenosine antagonism. Thus, adenosine antagonism represents an important new therapy in the amelioration of the overall hemodynamic state during cardiopulmonary resuscitation and following defibrillation and, thus, would be expected to enhance survival of cardiac arrest victims. This concept has never been proposed.

It is known that endogenous adenosine can depress the electrical conduction through the atrioventricular (A-V) node, and that adenosine antagonism can reverse this phenomenom. U.S. Pat. No. 4,364,922 discloses a method of treating atrioventricular conduction block using adenosine antagonists. However, A-V conduction disturbances play only a small role in the overall constellation of factors which characterize bradyasystolic arrest. The more predominant factors noted in the clinical sector include profound bradycardia associated with junctional and ventricular escape rhythms, and hemodynamic depression secondary to vascular collapse and severe inotropic dysfunction (see Iseri, L. T. et al, loc cit). The use of adenosine antagonism to reverse these phenomena in the setting of prolonged cardiopulmonary resuscitation has never been proposed.

Accordingly, there exists a need for a more effective pharmacologic method of treating the bradyarrhythmias associated with prolonged cardiopulmonary resuscitation. In addition, there exists a further need for a method of treating these arrhythmias which does not evolve into intractible ventricular fibrillation.

In addition to the aforementioned, the inventors have found that adenosine antagonism lowers the threshold current required for defibrillation and thereby increases the effectiveness of electric shock therapy. Thus, an intravenously administered form of adenosine antagonism may be important in reducing the duration of ventricular fibrillation, a prognostic factor known to enhance survival in the setting of cardiac arrest and resuscitation. In addition, an orally administered and longer-acting form of adenosine antagonism may be important in lowering the energy and current requirements of shock therapy administered by implanted electrodes, increase the efficacy of electric shock, and reduce the energy load on devices thus extending the device's battery life and battery replacement schedule. Adenosine antagonism may also lower the threshold current for cardiac pacing via endocardial and epicardial electrode placement or via transthoracic pacing electrodes. In addition to intravenous and oral forms, a local sustained release form of adenosine antagonist may be incorporated into the electrode tips of endocardial and epicardial leads for the purpose of lowering pacing thresholds.

At present there are no claims in the medical literature divulging the use of adenosine antagonism to lower energy and current requirements for cardioversion, defibrillation, and cardiac pacing. Kralios et al (Am Heart J: 105(4): 580-586) infused exogenous adenosine into canine coronary arteries and demonstrated a reduction in the threshold current necessary to induce ventricular fibrillation. However, the authors conclude "physiologic or pharmacologic coronary vasodilation without evidence of concomitant myocardial hypoxia" may contribute to the electrical instability resulting in ventricular fibrillation. No claim was made that the release of endogenous adenosine during the conditions of anoxia or hypoxia mediated such electrical instability or that antagonism of endogenously released adenosine would promote ventricular defibrillation. Ruffy et al (J. Am. Coll. Cardiology 9(2): 142A, 1987) showed that aminophylline (10 mg/Kg IV) lowered defibrillation threshold in conscious dogs. Aminophylline is a relatively weak adenosine antagonist with well established effects on phosphodiesterase inhibition. The authors did not correlate effects on defibrillation threshold with serum levels of aminophylline. At a dose of 10 mg/Kg, the authors could not and did not claim that the effect of aminophylline was mediated via adenosine antagonism.

Additionally, the inventors have demonstrated in a pentobarbital anesthetized dog that 8-phenylsulfonyltheophylline (8-PST) (5 mg/Kg IV) lowered the peak threshold current for defibrillation of VF lasting 1 minute from 16.6A to 13.1A with a rise to 15.1A during a wash-out period. In contrast to aminophylline, 8-PST is highly specific for competitive adenosine antagonism and causes no significant inhibition of phosphodiesterase activity (Clemo, S. H. F. and L. Belardinelli, 1985). A comparative study of antagonism by alkylxanthines on the negative dromotropic effect of adenosine and hypoxia in isolated guinea-pig hearts is disclosed in Current Clinical Practice Series No. 19: Anti-asthma Xanthines and Adenosine, K. E. Anderson and C. Kong, Princeton, Sydney, Tokyo, pp. 417-422; and F. W. Smellie, C. W. Davis, J. W. Daly, and J. N. Wells, 1979. Alkylxanthine inhibition of adenosine-elicited accumulation of cyclic AMP in brain slices and of brain phosphodiesterase activity is disclosed in Life Sci. 24:2475-2482. Thus, we claim that the observed effect is mediated by adenosine antagonism and is consistent with our other experimental observations. Since no convincing clinical data exist establishing the efficacy of presently available pharmacologic agents in reducing threshold currents for defibrillation, we propose that there exists a need for such an agent.

Accordingly, one object of the present invention is to provide a method of treating post-resuscitation bradyarrhythmias, hemodynamic collapse, and electro-mechanical dissociation associated with prolonged cardiopulmonary resuscitation.

Another object of the invention is to provide a method of treating asystole and bradyarrhythmias associated with hypoxic or anoxic depression of automaticity, conduction, contractility and vasomotor tone.

A further object of the invention is to promote the efficacy of cardiopulmonary resuscitation itself by altering the potential deleterious vasomotor effect of endogenously released adenosine. The invention thus promotes the maintenance of aortic diastolic pressure and coronary perfusion pressure and reverses harmful hypoperfusion of the subendocardium mediated by an adenosine-induced coronary "steal" phenomenon.

A further object of the invention is to provide a pharmacologic method for lowering the threshold current and energy for defibrillation thus enhancing the efficacy of defibrillation. One mechanism whereby this is accomplished is by favorably altering peripheral and cardiac vasomotor tone.

Another object of the invention is to provide a pharmacologic method for lowering the current and energy requirements for cardiac pacing (via transthoracic, epicardial, or endocardial electrodes) in the setting of resuscitation.

A further object of the invention is to provide a method of treating post-resuscitation asystole and bradyarrhythmias using a pharmacologic agent which lowers the threshhold for defibrillation.

A further object of the invention is to provide a method of treating of post-resuscitation arrhythmias with adenosine antagonists.

These and other objects of the present invention which will become apparent from the following detailed description have been achieved by the present method, comprising this step of:

administering to a human or animal during or after cardiopulmonary resuscitation an amount of an adenosine antagonist sufficient to alleviate post-resuscitation bradyarrhythmias and hemodynamic collapse.

Note these aforementioned objects and methods are proposed for potential use either alone or in combination with .alpha.- and/or .beta.-adrenergic or dopaminergic agents with the possibility of lowering doses required and avoiding potential dose-dependent side-effects while maintaining desired efficacy.

The present invention has succeeded in providing a method for the treatment of post-resuscitation bradyarrhythmias and hemodynamic collapse associated with prolonged cardiopulmonary resuscitation from ventricular fibrillation and/or cardiac arrest. The inventors have discovered that endogenous adenosine not only depresses the atrioventricular (AV) nodal conduction but also is associated with the post-shock hypoxic depression of automaticity, contractility and hypotension of heart muscle. It has also been discovered that adenosine antagonism can reverse such depression and restore normal electrophysiologic and mechanical function in a short period of time. Thus, the invention involves the use of an adenosine antagonist, either alone or in combination with pharamacologic agents possessing .alpha.- and/or .beta.-adrenergic or dopaminergic properties, to treat cardiac rhythm disturbances, mechanical dysfunction and hypotension and to facilitate cardioversion and/or defibrillation during and/or after cardiopulmonary resuscitation.

By "adenosine antagonists" is meant any agent which acts, by whatever mechanism to reduce the effect or the interstial concentration of adenosine in myocardial tissue. An antagonist of adenosine may be a competitive inhibitor or a substance that reduces the concentration of adenosine by destroying adenosine or that causes its destruction by altering metabolic pathways normally present in cells or extracellular fluid. An irreversal inhibitor is not suitable for the present invention since the action of adenosine performing its inherent regulatory functions must not be permenantly impaired. Examples of known adenosine antagonists are the xanthines, methylxanthines (e.g., 8-(p-sulfophenyl)theophylline) and the novel non-xanthine adenosine antagonists (e.g., imidazopyrimidine, pyrazolopyridine, etazolate, pyrazoloquinoline, and triazoloquinazoline (Pflugers Archiv 407: S31, 1986).

Preferred examples of methylxanthines are 1,3,7-trimethylxanthine (caffeine); 3,7-dimethylxanthine (theobromine); 1,3-dimethylxanthine (theophylline); aminophylline; and the xanthine derivatives disclosed in the specification of U.S. Pat. No. 4,364,922 incorporated herein by reference. The method of synthesizing the xanthine is not critical and can be performed by any known method of synthesizing these compounds, for example, the method disclosed in U.S. Pat. No. 4,364,922.

The method of the present invention relates to the enhancement of the efficacy of cardiopulmonary resuscitation and to the treatment of post-resuscitation asystole, bradyarrhythmias, electro-mechanical dissociation, and hemodynamic collapse. The method also relates to the lowering of energy and current requirements for defibrillation, cardioversion, and cardiac pacing in the setting of resuscitation.

The adenosine antagonists used in the present method may be used alone or in combination with pharmacologic agents possessing .alpha.- and/or .beta.-adrenergic or dopaminergic properties. Preferred examples of .alpha.-adrenergic agents are epinephrine, norepinephrine, phenylephrine, metaraminol, and methoxamine. Preferred examples of .beta.-adrenergic agents are epinephrine, norepinephrine, and isoproterenol. Preferred examples of dopaminergic agents are dopamine and dobutamine.

Dosages of the adenosine antagonists for treating post-resuscitation cardiac arrhythmias fall within the range of 0.1-20 mg/kg. An effective dose may be recognized by the alleviation of bradycardia and reversal of hemodynamic collapse.

Standard procedures for administration of adenosine antagonists such as theophylline and aminophylline at effective dosage levels are well established and are well known to those skilled in the art. For example, the recommended therapeutic range for plasma levels of theophylline for patients with reversible obstruction of the airways is from 10-20 .mu.g/ml.

Similar plasma levels are suggested above for the treatment of the resuscitation and post-resuscitation state. These plasma levels may be established by standard methods of administration, including but not limited to intravenous injection, oral injestion via tablets, capsules, or liquids, suppository implantation, intramuscular injection, inhalation and the local release from implanted electrodes. Any of these methods, which are able to provide the proper plasma level are suitable for the present invention. The locally released preparations would not require systemic concentrations or effects to obtain the desired action of optimizing electrode efficacy. The preferred method of adminstration is intravenous injection for resuscitation and post-resuscitation states. Intravenous administration of the adenosine antagonists and/or .alpha.-adrenergic or .beta.-adrenergic or dopaminergic agents may consist of a single injection, a loading dose followed by continuous administration of the lower level maintenance dose, injection spaced over a period of time, continuous injection of a low level maintenance dose, injection spaced over a period of time, continuous injection of a low level maintenance dose, or other types of administration that are suitable for the particular needs of the individual human or animal being treated. Dosages of theophylline and aminophylline required for specific plasma levels are well known to those in the art, as shown in the article "Rational Intravenous Dosages of Aminophylline" by Mitenko and Ogilvie, New England J. Med., 289, pages 600-603 (1973). For example, to achieve a theophylline plasma level of 10 .mu.g/ml, theophylline is administered in an initial loading dose of 5-6 mg/kg followed by a continuous maintenance dose of 0.90 mg/kg/hr.

Administration of these amounts is sufficient for achieving and maintaining a plasma level of 10 .mu.g/ml for any method in which theophylline or a derivative is absorbed into the blood stream without being destroyed. Nonlimiting examples include intravenous injection, absorption by the large intestine from suppositories, absorption by the small intestine from capsules that release theophylline or other adenosine antagonists in the intestine after passing through the stomach, or absorption through the lungs. Methods that require the adenosine antagonists to pass through the stomach may be subject to destruction of the antagonists and accordingly must be either protected in a form that is not destroyed in the stomach or administered in a large dose so that the amount reaching the blood stream is sufficient to achieve the desired effective level.

When the adenosine antagonists is administered with an .alpha.-adrenergic or .beta.-adrenergic or dopaminergic agent, the relative ratio of these two components should be in the range of 0.01:1.0 to about 1.0:0.01.

The pharmaceutical compositions of the present invention may contain one or more adenosine antagonists as well as one or more .alpha.-adrenergic and/or .beta.-adrenergic or dopaminergic agents. The pharmaceutical preparations may be prepared in any of the customary methods well known in the art.

The adenosine antagonists may be admixed with any pharmaceutically acceptable carrier or carriers, such as water, ethanol, inert solids or any other carrier customarily used for the type of administration in question.

The method of the present invention may be used in the treatment of humans and in the practice of veterinary medicine on warm blooded mammals. Examples of mammals which may be treated include cats, dogs, horses, etc.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

Mongrel dogs weighing 20 to 26 Kg were anesthetized with intravenous sodium pentobarbital (25-30 mg/Kg), intubated, and mechanically ventilated at a rate sufficient to maintain PaCO.sub.2 at 35 to 45 mmHg and pH between 7.35 and 7.45. The right femoral artery was cannulated for determination of arterial blood pressure. ECG lead II was continuously monitored. VF was induced by low voltage alternating current through a 6 Fr bipolar electrode catheter introduced into the right carotid artery and advanced into the left ventricle. The cessation of ventilation and clamping of the endotracheal tube preceded each induction of VF. Defibrillatory shocks of damped sinusoidal waveforms were administered with a Physio Control Lifepak 6 defibrillator via an apparatus which maintained balance and constant pressure of thoracic electrodes (50 newtons). A 6 Fr angiographic catheter was introduced into the right external jugular vein and advanced to the right atrium for purposes of drug administration. Rapid bolus infusions of 8-PST (5 mg/Kg.sup.-1) were administered intravenously.

Prior to the experiments designed to elucidate the efficacy of adenosine antagonism, the threshold current for defibrillation was determined by delivering sequential incremental DC shocks for VF lasting less than 45 seconds. No depression of automaticity, conduction, or hemodynamic state was noted FIG. 1). The current was measured in a storage oscilloscope for the waveform generated across a current sensitive resistor in series with a voltage divider network with known applied voltage. During subsequent experimentation, all shocks were administered with a peak current of 35 Amps (A) on the first two shocks and at 40 to 50 A on all subsequent shocks.

Experiments were divided into two groups of dogs; that is, Group A control dogs (no pre-treatment) and Group B dogs were pre-treated with 8-PST. In both groups, closed chest massage was not performed and DC shocks were initiated following 2 minutes of VF. In Group A, following defibrillation 8-PST was infused after marked bradycardia and hemodynamic collapse had developed. In a subset of dogs, ventilation was performed prior to 8-PST infusion. In Group B, the dogs were pre-treated with 8-PST (5 mg/Kg IV) 2 to 5 minutes before the induction of VF. Following defibrillation, if hemodynamic collapse persisted, ventilation without closed chest massage and/or a subsequent infusion of 8-PST was administered. Ventilation was routinely initiated following hemodynamic recovery.

Statistical analysis of variables was based on the Student's T-distribution for paired and unpaired data (15). Significant difference was considered for p<0.05. All values were expressed as the mean .+-. standard error of the mean (SEM).

Dogs pre-treated with 8-PST (Group B) required significantly fewer countershocks (1.3.+-.0.2; n=8) versus Group A (2.5.+-.0.6; n=9), p<0.05) even though the pretest ventricular defibrillation threshold (VDT) for VF less than 45 seconds for Group A (23.2.+-.1.2 A) Group B (25.5.+-.2.1 A) and were similar. No dog in Group B required more than two shocks of 35 A to defibrillate. The duration of VF was significantly different for the two groups (157.+-.25 secs for Group A and 125.+-.3 secs for Group B; p<0.05).

In Group B, the post-defibrillation ventricular cycle length (583.+-.47 msec; n=8) was only slightly different from the pre-8-PST (395.+-.29 msec; n=8) and pre-VF (397.+-.29 msec; n=8) cycle lengths. While transient AV block was seen immediately post-defibrillation, 6 of 8 animals from Group B exhibited sinus rhythm within 10 seconds following defibrillation. In contrast, severe bradycardia was noted post-defibrillation in Group A animals with 5 of 9 animals exhibiting high grade AV block, idioventricular and idionodal rhythms. The mean post-defibrillation ventricular cycle length for Group A was 2428.+-.516 msec, n=9. Subsequent infusions of 8-PST, however, restored sinus rhythm in 4 of these 5 animals and reversed bradycardia in 7 of 9 animals shortening the post-defibrillation cycle length to 940.+-.63 msecs. (FIG. 1.)

No animal from Group A exhibited hemodynamic recovery. The post-defibrillation pre-intervention blood pressure in Group A equalled 22.+-.3/17.+-.2 mmHg, n=9. Immediately after defibrillation, ventilation had no effect on blood pressure and failed to promote hemodynamic recovery. (FIG. 2.)

Most notably, pre-treatment with 8-PST (Group B) significantly improved the post-defibrillation hemodynamic state with 6 of 8 animals exhibiting complete recovery. Two of six required no interventions. (FIG. 3.) Two others recovered following the onset of ventilation while two required in addition a second bolus infusion of 8-PST. Prior to VF, 8-PST infusion had had no significant effect on blood pressure or heart rate.

The 6 animals with complete hemodynamic recovery displayed a marked overshoot in blood pressure (227.+-.14/108.+-.12 mmHg) which later reduced to 141.+-.5/97.+-.5 mmHg, approximating the pre-VF values (156.+-.11/101.+-.3 mmHg). In the 4 animals requiring ventilation and/or a second infusion of 8-PST, the post-defibrillation pre-intervention blood pressure equalled 53.+-.6/26.+-.4 mmHg. In the two animals which failed to recover despite intervention, the post-defibrillation, pre-intervention blood pressure averaged 28/24 mmHg. Of interest, no animal from Group B refibrillated after prior conversion of VF.

Only two animals in Group B required more than one countershock (i.e., 2 shocks) to defibrillate in 3 episodes of VF. The mean duration of VF in these animals was 137.+-.4 secs., and the mean post-defibrillation, pre-intervention blood pressure was 105.+-.57/51.+-.20. Both recovered. Of the 3 animals from Group A which required 2 countershocks to defibrillate, the mean duration of VF was comparable (146.+-.4 secs), but the mean post-defibrillation, pre-intervention blood pressure measured significantly less (23.+-.2/19.+-.3 mmHg; p<0.05).

Two of nine animals from Group A, and 6 of 8 animals from Group B required one shock to defibrillate and thus had comparable VF durations (120 secs). The mean post-defibrillation, pre-intervention blood pressures for Groups A and B were significantly different (36.+-.3/25.+-.0 mmHg versus 81.0.+-.35/43.+-.18 mmHg respectively, p<0.05).

Two of nine animals from Group A, and 6 of 8 animals from Group B required one shock to defibrillate and thus had comparable VF durations (120 secs). The mean post-defibrillation, pre-intervention blood pressures for Groups A and B were significantly different (36.+-.3/25.+-.0 mmHg versus 81.0.+-.35/43.+-.18 mmHg respectively, p<0.05).

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

FIG. 1: Effect of intravenous 8-PST (5 mg/Kg) infusion on heart rate and rhythm (Group A). Panel A: Sinus arrest and junctional rhythm (cycle length=2880 msecs) prior to 8-PST infusion. Panel B: Sinus rhythm (cycle length=1240 msecs) at 15 secs. after rapid 8-PST infusion.

FIG. 2: Lack of effect of ventilation alone following defibrillation. Following defibrillation (3rd shock at 44 A), idioventricular rhythm (cycle length=4360 msecs; blood pressure=28/26 mmHg) appears. Ventilation alone had no effect on heart rate or blood pressure.

FIG. 3: Effect of 8-PST (5 mg/Kg) pre-treatment on post-defibrillation blood pressure. After defibrillation (35A) at 2 minutes, 2:1 AV block appears transiently, followed by sinus rhythm and a marked overshoot in blood pressure. Ventilation has not been re-initiated.