Electrical stability of acutely ischemic myocardium. Influences of heart rate and vagal stimulation

Direct and autonomically mediated effects of oral quinidine on RR/QT relation after an abrupt increase in heart rate

OBJECTIVES

This study evaluates the direct and autonomically mediated effects of oral quinidine on ventricular repolarization in humans.

BACKGROUND

Interactions between quinidine-related vagolytic properties and autonomic modulation on ventricular repolarization are unknown. The relative role of the two components, if present, might improve our understanding of the therapeutic and proarrhythmic mechanisms of quinidine on the ventricular tissue.

METHODS

Rate-related changes in the QT interval were investigated after an abrupt increase in heart rate in 15 patients during atrial pacing. In the control study, the QT interval was measured at six paced cycle lengths (600, 540, 500, 460, 430 and 400 ms) both in the basal state and after autonomic blockade (intravenous propranolol, 0.2 mg/kg, and intravenous atropine, 0.04 mg/kg); oral quinidine was then administered at a daily dosage of 1,200 mg for 3 to 4 days, after which the QT duration was reassessed using the same method in a second study.

RESULTS

During the control study, the mean slope of the regression curve estimating the correlation between pacing cycle length and QT duration was significantly lower after autonomic blockade (0.14 +/- 0.05) than in the basal state (0.27 +/- 0.10, p < 0.05). Quinidine exhibited a prominent but opposite effect on the mean slope of the regression curves in basal conditions (from 0.27 +/- 0.10 to 0.20 +/- 0.07, p < 0.05) and after withdrawal of autonomic modulation (from 0.14 +/- 0.05 to 0.19 +/- 0.05, p < 0.05), thus annulling the differences observed between the two states in the control study.

CONCLUSIONS

A quinidine-induced increase in QT duration as cycle length is prolonged is consistent with a reverse use dependence effect on ventricular repolarization. This effect is not evident in the basal state owing to interaction of quinidine-related vagolytic effect with the autonomic tone. Reverse use dependence and vagolytic activity on ventricular tissue indicate two potentially undesirable effects that could play a role in the lack of efficacy or proarrhythmic effect of quinidine.

Ro 40-5967, a novel calcium channel antagonist, protects against ventricular fibrillation

Ro 40-5967 is a new calcium channel antagonist that binds at the same membrane sites as verapamil, yet has minimal negative inotropic effects. The effects of Ro 40-5967 on the susceptibility to ventricular fibrillation were investigated and compared to diltiazem. Ventricular fibrillation (VF) was induced in 40 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. Twenty-four animals were found to be susceptible to VF and were given the treatments described below. Pretreatment with Ro 40-5967 (n = 17, 1000 micrograms/kg i.v.) significantly (P < 0.001) reduced the incidence of VF (13 of 17 protected) during the exercise plus ischemia test. Diltiazem (n = 8, 1000 micrograms/kg) completely suppressed VF. Lower doses of diltiazem and Ro 40-5967 did not prevent VF. The hemodynamic effects of Ro 40-5967 were also compared to diltiazem and verapamil. Diltiazem and verapamil, but not Ro 40-5967, increased P-R interval in a dose-dependent manner. Even when reflex tachycardia was controlled by beta-adrenoceptor blockade, Ro 40-5967 still exerted only minimal effects on P-R interval. Verapamil, but neither Ro 40-5967 nor diltiazem, provoked a dose-dependent negative inotropic response. All three drugs elicited large increases in coronary blood flow. These data support the hypothesis that calcium entry may play a critical role in the development of malignant arrhythmias during ischemia. Further, Ro 40-5967 can protect against ventricular fibrillation without significant negative inotropic or dromotropic effects.

Prevention of life-threatening arrhythmias by pharmacologic stimulation of the muscarinic receptors with oxotremorine

The potential antiarrhythmic efficacy of pharmacologic parasympathetic activation is still controversial. This study assessed the antiarrhythmic effect of saline solution (n = 9) and of the muscarinic agonist oxotremorine (1.5 micrograms/kg administered intravenously) (n = 17) in a feline animal model in which malignant arrhythmias were reproducibly elicited by the combination of acute myocardial ischemia and left stellate ganglion stimulation. Although saline solution had no effect, oxotremorine significantly decreased heart rate, blood pressure, the incidence of ventricular fibrillation from 47% to 0% (p = 0.004), and the incidence of malignant arrhythmias (either ventricular tachycardia or ventricular fibrillation) from 88% to 12% (p less than 0.001). When reduction in heart rate was prevented by means of atrial pacing (n = 15), the incidence of malignant arrhythmias was still significantly reduced from 87% to 27% (p = 0.001). Arrhythmias were also graded as follows: 0 = no premature ventricular contractions; 1 = 1 to 10 premature ventricular contractions; 2 = 11 to 50 premature ventricular contractions; 3 = ventricular tachycardia; 4 = ventricular fibrillation. Arrhythmia severity was 3.29 +/- 0.16 (SEM) in the control trials and was reduced to 0.76 +/- 0.26 (p less than 0.001) by oxotremorine and to 1.53 +/- 0.34 by oxotremorine and pacing (p = 0.002). Therefore a muscarinic agonist can significantly reduce malignant arrhythmias during acute myocardial ischemia and may represent a novel approach to the prevention of sudden cardiac death.

Comparison of 24-hour parasympathetic activity in endurance-trained and untrained young men

OBJECTIVES

This study compares 24-h parasympathetic activity in aerobically trained and untrained healthy young men.

BACKGROUND

Higher values of parasympathetic nervous system activity are associated with a low mortality rate in patients after myocardial infarction, but it remains uncertain what therapeutic interventions can be used to increase parasympathetic activity. Although it is thought that exercise training can increase parasympathetic activity, studies have reported conflicting results, perhaps because this variable was measured for only brief intervals and usually inferred from changes in reflex responses induced by pharmacologic blockade.

METHODS

Parasympathetic activity was assessed noninvasively from 24-h ECG recordings by calculating high frequency (0.15 to 0.40 Hz) beat to beat heart period variability in eight endurance-trained men (maximal oxygen consumption greater than or equal to 55 ml/kg per min) and eight age-matched (mean = 29 yr) untrained men (maximal oxygen consumption less than or equal to 40 ml/kg per min). The data were analyzed separately for sleeping hours when parasympathetic activity is dominant and also for waking hours.

RESULTS

The geometric mean of high frequency power was greater in the trained than in the untrained men during the day (852 vs. 177 ms2, p less than 0.005), during the night (1,874 vs. 427 ms2, p less than 0.005) and over the entire 24 h (1,165 vs. 276 ms2, p less than 0.001).

CONCLUSIONS

Parasympathetic activity is substantially greater in trained than in untrained men, and this effect is present during both waking and sleeping hours. These data suggest that exercise training may increase parasympathetic activity over the entire day and may therefore prove to be a useful adjunct or alternative to drug therapy in lessening the derangements of autonomic balance found in many cardiovascular diseases.

Heart rate variability in patients with diabetes mellitus, ischemic heart disease, and congestive heart failure

The prognosis of patients with heart disease and prediction of sudden cardiac death can be assessed through heart rate variability, an indirect measure of abnormal autonomic control. The authors have evaluated the heart rate variability by 24-hour ambulatory electrocardiographic monitoring in 25 diabetic patients, 19 ischemic heart disease patients, 18 congestive heart failure patients, and 10 normal subjects. Thirteen diabetic patients had autonomic neuropathy and 12 patients did not. Heart rate variability index (mean SD) in patients with diabetes mellitus, ischemic heart disease, and congestive heart failure was significantly lower (34.5 +/- 12.6 ms, 43.7 +/- 15.4 ms, and 34.6 +/- 15.8 ms vs 65.6 +/- 16.7 ms, p less than 0.05) than that of normal subjects. Mean SD was significantly lower in patients with autonomic neuropathy as compared to patients without autonomic neuropathy (26.4 +/- 6.5 ms vs 44.2 +/- 11.0 ms, p less than 0.05) mean SD as compared to survivors: 49 +/- 7 ms in patients with mild ischemic heart disease, 48 +/- 15 ms in patients with severe ischemic heart disease, and 23 +/- 7 ms in patients who died. Similarly, the mean SD in 4 congestive heart failure patients who died was lower significantly (p less than 0.05) than in those who survived (19.0 +/- 5.6 ms vs 40.0 +/- 14.5 ms). Among congestive heart failure patients, clinical improvement by therapy was associated with a significant increase in mean SD. When the mean SD of 30 ms was used as the cutoff point for detection of autonomic dysfunction or patient death, specificity exceeded 90% and sensitivity was 75%.(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic value of baroreflex sensitivity testing after acute myocardial infarction

BACKGROUND

Disturbances of autonomic function are recognised in both the acute and convalescent phases of myocardial infarction. Recent studies have suggested that disordered autonomic function, particularly the loss of protective vagal reflexes, is associated with an increased incidence of arrhythmic deaths. The purpose of this study was to compare the value of differing prognostic indicators with measures of autonomic function and to assess the safety of arterial baroreflex testing early after infarction.

METHODS

As part of a prospective trial of risk stratification in post-infarction patients arterial baroreflex sensitivity, heart rate variability, long term electrocardiographic recordings, exercise stress testing, and ejection fraction were recorded between days 7 and 10 in 122 patients with acute myocardial infarction.

RESULTS

During a one year follow up period there were 10 arrhythmic events. Baroreflex sensitivity was appreciably reduced in these patients suffering arrhythmic events (1.73 SD (1.49) v 7.83 (4.5) ms/mm hg, 95% confidence interval (CI) 4.8 to 7.3, p = 0.0001). Significant correlations were noted with age (r = -0.68, p less than 0.001) but not left ventricular function. When baroreflex sensitivity was adjusted for the effects of age and ventricular function baroreflex sensitivity was still considerably reduced in the arrhythmic group (2.1 v 7.57 ms/mm Hg, p less than 0.0001). Depressed baroreflex sensitivity carried the highest relative risk for arrhythmic events (23.1, 95% CI 7.7 to 69.2) and was superior to other prognostic variables including left ventricular function (10.4, 95% CI 3.3 to 32.6) and heart rate variability (10.1, 95% CI 5.6 to 18.1). No major complications were noted with baroreflex testing and in particular no patients developed ischaemic or arrhythmic symptoms during the procedure.

CONCLUSIONS

Disordered autonomic function as measured by depressed baroreflex sensitivity or reduced heart rate variability was associated with an increase incidence of arrhythmic events in post-infarction patients. Baroreflex testing can be safely performed in the immediate post-infarction period.

Risk stratification for arrhythmic events in postinfarction patients based on heart rate variability, ambulatory electrocardiographic variables and the signal-averaged electrocardiogram

The value of heart rate variability, ambulatory electrocardiographic (ECG) variables and the signal-averaged ECG in the prediction of arrhythmic events (sudden death or life-threatening ventricular arrhythmias) was assessed before hospital discharge in 416 consecutive survivors of acute myocardial infarction. During the follow-up period (range 1 to 1,112 days), there were 24 arrhythmic events and 47 deaths. The initial relation between several prognostic factors and arrhythmic events was explored with use of the Kaplan-Meier product limit estimates of survival function. Impaired heart rate variability less than 20 ms (p less than 0.0000), late potentials (p less than 0.0000), ventricular ectopic beat frequency (p less than 0.0000), repetitive ventricular forms (p less than 0.0000), left ventricular ejection fraction less than 40% (p less than 0.02) and Killip class (p less than 0.02) were identified as significant univariate predictors of arrhythmic events. When these variables were analyzed by using a stepwise Cox regression model, only impaired heart rate variability, followed by late potentials and repetitive ventricular forms remained independent predictors of arrhythmic events. The combination of impaired heart rate variability and late potentials had a sensitivity of 58%, a positive predictive accuracy of 33% and a relative risk of 18.5 for arrhythmic events and was superior to other combinations including those incorporating left ventricular function, exercise ECG, ventricular ectopic beat frequency and repetitive ventricular forms. These results suggest that a simple method of assessment based on heart rate variability and the signal-averaged ECG can select a small subgroup of survivors of myocardial infarction at high risk of future life-threatening arrhythmias and sudden death.

Effect of reflex vagal activation on frequency of ventricular premature complexes

To evaluate the antiarrhythmic effect of reflex-induced vagal activation, phenylephrine was infused in 17 patients with frequent ventricular premature complexes (VPCs). The role of heart rate reduction in suppressing VPCs was explored by pacing the atria at the preinfusion levels. Baroreceptor activation was considered effective when a greater than or equal to 20% decrease in heart rate was observed. Ten patients (59%) achieved the target heart rate decrease (-29 +/- 5%), whereas in 7 (41%) the baroreceptor reflex was considered inadequate. In the former group ("responders"), heart rate decreased from 73 +/- 7 to 52 +/- 6 beats/min (p less than 0.0001). When heart rate was allowed to fluctuate, ectopic activity was completely abolished in 9 of 10 patients; mean number of VPCs decreased from 38 +/- 8 to 0.2 +/- 0.6/100 beats (p less than 0.0001). During pacing, VPCs reappeared but their mean number (22 +/- 10/100 beats) was still significantly reduced compared with control values (p = 0.003). In the "nonresponders," despite adequate blood pressure increases, VPC frequency was not affected. The QT interval lengthened during phenylephrine (392 +/- 17 ms) versus control conditions (372 +/- 18 ms, p = 0.0008) in the responders group, whereas no change was observed in the nonresponders. These results demonstrate that reflex vagal activation markedly reduces VPCs. This effect is only partially rate-dependent; direct and indirect electrophysiologic changes secondary to baroreflex activation are also likely to be involved.

Vagal stimulation and prevention of sudden death in conscious dogs with a healed myocardial infarction

The interest for the antifibrillatory effect of vagal stimulation has been largely limited by the fact that this concept seemed restricted to acute experiments in anesthetized animals. To explore the potentially protective role of vagal stimulation in conscious animals we developed a chronically implantable device to be placed around the cervical right vagus. An anterior myocardial infarction was produced in 161 dogs; 1 month later an exercise stress test was performed on the 105 survivors. Toward the end of the test the circumflex coronary artery was occluded for 2 minutes. Fifty-nine (56%) dogs developed ventricular fibrillation and, before this test was repeated, were assigned either to a control group (n = 24) or to be instrumented with the vagal device (n = 35). Five dogs were excluded because of electrode malfunction. Compared with the heart rate level attained after 30 seconds of occlusion during exercise in the control test, vagal stimulation led to a decrease of approximately 75 beats/min (from 255 +/- 33 to 170 +/- 36 beats/min, p less than 0.001). In the control group 22 (92%) of 24 dogs developed ventricular fibrillation during the second exercise and ischemia test. By contrast, during vagal stimulation ventricular fibrillation occurred in only 3 (10%) of the 30 dogs tested and recurred in 26 (87%) during an additional exercise and ischemia test in the control condition (p less than 0.001 versus the vagal stimulation test; internal control analysis). Combined analysis of the tests performed in the control condition showed that ventricular fibrillation was reproducible in 48 (89%) of the 54 dogs tested. The protective effect of vagal stimulation was also significant in the group comparison analysis and even after exclusion of those four dogs in which ventricular fibrillation was not reproducible (92% versus 11.5%, control versus vagal stimulation, p less than 0.001). When heart rate was kept constant by atrial pacing, the vagally mediated protection was still significant (p = 0.015) as five (55%) of nine dogs survived the test. This study shows that vagal stimulation, performed shortly after the onset of an acute ischemic episode in conscious animals with a healed myocardial infarction, can effectively prevent ventricular fibrillation. This striking result seems to depend on multiple mechanisms having a synergistic action. The decrease in heart rate is an important but not always essential protective mechanism. The electrophysiological effects secondary to the vagally mediated antagonism of the sympathetic activity on the heart are likely to play a major role.

Effect of atenolol and diltiazem on heart period variability in normal persons

Several time and frequency domain measures of heart period variability are reduced 1 to 2 weeks after myocardial infarction, and a reduced standard deviation of normal RR intervals over a 24 h period (SDNN) is associated with increased mortality. The predictive accuracy of heart period variability may be reduced by drugs used to treat patients after myocardial infarction. Accordingly, a randomized, three period, placebo-controlled, crossover (Latin square) design was used to determine the effect of atenolol and diltiazem on time and frequency measures of heart period variability calculated from 24 h continuous electrocardiographic recordings during treatment with atenolol, diltiazem and placebo in 18 normal volunteers. During atenolol treatment, the 24 h average normal RR (NN) interval increased 24% (p less than 0.001). The three measures of tonic vagal activity were significantly increased (p less than 0.001) during atenolol treatment: percent of successive normal RR intervals greater than 50 ms = 69%, root mean square successive difference of normal RR intervals = 61% and high frequency power in the heart period power spectrum = 84%. Low frequency power also increased 45% (p less than 0.01), indicating that this variable also is an indicator of tonic vagal activity over 24 h. Diltiazem had no significant effect on the 24 h average NN interval or on any measure of heart period variability. The decreased mortality rate after myocardial infarction associated with beta-adrenergic blocker but not calcium channel blocker therapy may be attributed in part to an increase in vagal tone caused by beta-blockers.

Sympathetic--parasympathetic interaction and sudden death

The effects of the autonomic nervous system on malignant arrhythmias, particularly in the setting of ischemic heart disease, have been widely investigated and described. Specifically, it has been shown that while sympathetic hyperactivity is arrhythmogenic, an increased vagal activity often exerts a beneficial effect. New insights on the relationship between autonomic activity and sudden cardiac death have been obtained in conscious dogs in which a healed myocardial infarction, acute myocardial ischemia, and exercise are combined. In this chronic animal model it was shown that myocardial infarction reduces baroreflex sensitivity and heart rate variability (markers of vagal reflex and tonic activity to the heart) and that a depressed baroreflex sensitivity or a reduced heart rate variability after myocardial infarction indicate an increased risk for ventricular fibrillation. The validity of these experimental observations was confirmed in clinical studies in patients with a myocardial infarction. The protective effect of vagal activity was further confirmed in two experimental studies in which muscarinic stimulation, both electrically and pharmacologically induced, was able to prevent ventricular fibrillation during acute myocardial ischemia. These observations have led to new research directions. At the experimental level, the effect of Gi proteins activity blockade by pertussis toxin on the cardiac response to vagal activation is currently evaluated in conscious dogs. At the clinical level, the prognostic value after myocardial infarction of baroreflex sensitivity and of heart rate variability will be tested in a large, multicenter, prospective study.

Autonomic pathophysiology in heart failure patients. Sympathetic-cholinergic interrelations

We conducted this study in an effort to characterize and understand vagal abnormalities in heart failure patients whose sympathetic activity is known. We measured sympathetic (peroneal nerve muscle sympathetic recordings and antecubital vein plasma norepinephrine levels) and vagal (R-R intervals and their standard deviations) activities in eight heart failure patients and eight age-matched healthy volunteers, before and after parasympathomimetic and parasympatholytic intravenous doses of atropine sulfate. At rest, sympathetic and parasympathetic outflows were related reciprocally: heart failure patients had high sympathetic and low parasympathetic outflows, and healthy subjects had low sympathetic and high parasympathetic outflows. Low dose atropine, which is known to increase the activity of central vagal-cardiac motoneurons, significantly increased R-R intervals in healthy subjects, but did not alter R-R intervals in heart failure patients. Thus, our data document reciprocal supranormal sympathetic and subnormal parasympathetic outflows in heart failure patients and suggest that these abnormalities result in part from abnormalities within the central nervous system.

Ventricular vulnerability in diabetes and myocardial norepinephrine release

Previously the authors have observed a reduction of the ventricular fibrillation threshold (VFT) in a mild diabetic model. This investigation examines the role of more severe hyperglycemia in altering the ventricular fibrillation threshold and how the sympathetic nervous system modulates the response. Alloxan diabetes was induced in eight male mongrel dogs 3-5 years of age (Group 2), for comparison with matched controls (Group 1). Hemoglobin A1c rose from 2.9 +/- .4-7.8 +/- .3% and body weight was maintained with daily insulin. After 1 year, anesthesia was induced with chloralose and an electrode catheter placed at the right ventricular apex. VFT was 41.7 +/- 1.8 ma in Group 1 and 27.8 +/- 2.1 ma in the diabetics of Group 2 (p less than .001). There was significantly greater decline of VFT in response to epinephrine infusion in Group 2. The threshold in diabetics rose to normal levels after infusion of the beta-blocking agent, esmolol. Subsequently, the response of the cardiac sympathetic system was assessed during ventricular pacing at 200 beats/minute. Serial paired blood samples were taken from catheters in the aorta and coronary sinus for catecholamine assay by HPLC. Both groups had similar coronary blood flow responses by the thermal method, as well as changes in arterial pressure. While no change occurred in Group 1, a progressive rise of norepinephrine (NE) concentration was observed in coronary venous effluent of Group 2 (p less than .01). The basal arterial-coronary sinus difference was-123 +/- 52 pg/ml, which rose during pacing in Group 2 to a peak of -376 +/- 9.3 pg/ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired parasympathetic control of heart rate after myocardial infarction

We measured the variation in heart rate during deep breathing, a sensitive non-invasive measure of cardiac parasympathetic activity, in 95 patients 3 weeks after myocardial infarction and in 40 asymptomatic healthy controls. The variation in rate was significantly lower (11.6 +/- 6.1 vs 17.6 +/- 7.3 beats/min, P less than 0.001) in patients with myocardial infarction than in controls. Forty-nine patients (52%) and 5 controls (13%) were considered to have diminished (less than or equal to 10 beats/min) variation of rate. The diminution in this variation was not related to the type or location of myocardial infarction, to maximum release of CK-MB or to cardiovascular medication. Our results suggest that impairment of vagal control of heart rate is common after myocardial infarction. The impairment cannot be predicted by any specific feature of the disease.

Termination of ventricular tachycardia by carotid sinus stimulation

A 56-year-old white male developed sustained ventricular tachycardia which was terminated by carotid sinus stimulation. Intracardiac electrophysiologic study reproduced the tachycardia which was repeatedly terminated by carotid massage. This report, together with others reviewed here, suggests that tachycardia termination by simple carotid stimulation does not always prove a supraventricular origin.

Electrophysiologic alterations produced by hypoxia in the canine heart

The purpose of this study was to test whether hypoxia caused simultaneous stimulation of sympathetic and vagal cardiac nerves. To do this, we determined in chloralose-anesthetized, open-chest dogs, changes in sinus cycle length (SCL), atrioventricular nodal conduction time (AH interval), and effective refractory periods (ERP) of the right atrium (RA) and left ventricle (LV) during hypoxia induced by either hypoventilation or by breathing 10% oxygen in nitrogen. We found that hypoventilation decreased arterial pH (mean +/- SE: 7.365 +/- 0.011 versus 7.244 +/- 0.013, p less than 0.001) and Po2 (105.2 +/- 3.6 versus 50.5 +/- 2.0 mm Hg, p less than 0.001), and increased Pco2 (24.4 +/- 0.7 versus 41.9 +/- 1.1 mm Hg, p less than 0.001). SCL (373 +/- 19 versus 485 +/- 35 msec, p less than 0.001), AH interval (92 +/- 4 versus 111 +/- 6 msec, p less than 0.005), and LVERP (159.8 +/- 3.2 versus 162.0 +/- 3.3, p less than 0.05) lengthened, while RAERP shortened (137.7 +/- 2.6 versus 128.8 +/- 3.5, p less than 0.001). After bilateral vagotomy, these electrophysiologic changes were attenuated, and transection of ansae subclaviae following vagotomy did not affect hypoventilation-induced changes that were present after vagotomy alone. In contrast, breathing 10% oxygen decreased Po2 (104.2 +/- 3.3 versus 48.1 +/- 1.7, p less than 0.001) but did not change pH and Pco2. SCL lengthened slightly (444 +/- 15 versus 463 +/- 17, p less than 0.001), but other measured electrophysiologic parameters were not affected. The norepinephrine concentration in the coronary sinus blood increased by 109% to 121% during hypoxia induced by both methods. Hypoventilatory hypoxia blunted sympathetic-induced shortening of LVERP and potentiated vagally-induced SCL lengthening. Neither type of hypoxia affected induction of atrial and ventricular tachyarrhythmias by programmed electrical stimulation. We conclude that the electrophysiologic response during hypoventilatory hypoxia is due primarily to heightened vagal tone. Norepinephrine concentration in the coronary sinus blood more than doubled, but did not alter cardiac electrophysiologic properties, possibly because of simultaneous increase in vagal tone, or possibly due to the release of adenosine or other factors that impaired cardiac electrophysiologic response to elevated norepinephrine levels.

Antiadrenergic effects of adenosine on His-Purkinje automaticity. Evidence for accentuated antagonism

The effects of adenosine on the human His-Purkinje system (HPS) were studied in nine patients with complete atrioventricular (AV) block. Adenosine had minimal effect on the control HPS cycle length, but in the presence of isoproterenol increased it from 906 +/- 183 to 1,449 +/- 350 ms, P less than 0.001. Aminophylline, a competitive adenosine antagonist, completely abolished this antiadrenergic effect of adenosine. In isolated guinea pig hearts with surgically induced AV block, isoproterenol decreased the HPS rate by 36%, whereas in the presence of 1,3-dipropyl-8-phenyl-xanthine, a potent adenosine antagonist, the HPS rate decreased by 48% and was associated with an increased release of adenosine. Therefore, by blocking the effects of adenosine at the receptor level, the physiologic negative feedback mechanism by which adenosine antagonizes the effects of catecholamines was uncoupled. The results of this study indicate that adenosine's effects on the human HPS are primarily antiadrenergic and are thus consistent with the concept of accentuated antagonism. These effects of adenosine may serve as a counterregulatory metabolic response that improves the O2 supply-demand ratio perturbed by enhanced sympathetic tone. Some catecholamine-mediated ventricular arrhythmias that occur during ischemia or enhanced adrenergic stress may be due to an imbalance in this negative feedback system.

Correlation of heart rate variability with clinical and angiographic variables and late mortality after coronary angiography

Decreased heart rate (HR) variability is associated with increased mortality after myocardial infarction, but the prognostic value of HR variability in patients without recent myocardial infarction and its correlation with other clinical and angiographic data have not previously been reported. In the present study, detailed clinical assessments and 24-hour ambulatory electrocardiograms were performed prospectively on 100 patients undergoing elective coronary angiography. HR variability was inversely correlated with HR (r = -0.38, p = 0.001), diabetes mellitus (r = -0.22, p = 0.025) and digoxin use (r = -0.29, p = 0.004), but not with left ventricular ejection fraction, extent of coronary artery disease or other clinical, electrocardiographic or angiographic variables. All patients were followed for 1 year. Major clinical events after initial discharge occurred in 10 patients and included 6 deaths and 4 coronary bypass operations. Left ventricular ejection fraction was the only variable that correlated with the occurrence of a clinical event (p = 0.002). Decreased HR variability and ejection fraction were the best predictors of mortality (both p less than 0.01), and the contribution of HR variability to mortality was independent of ejection fraction, extent of coronary artery disease and other variables. Furthermore, 11 patients with HR variability less than 50 ms had an 18-fold increase in mortality compared with patients with HR variability greater than 50 ms (36 vs 2%, p = 0.001). Thus, decreased HR variability is a potent independent predictor of mortality in the 12 months following elective coronary angiography in patients without recent myocardial infarction.

Autonomic mechanisms and sudden death. New insights from analysis of baroreceptor reflexes in conscious dogs with and without a myocardial infarction

We have suggested that among conscious dogs with a healed anterior wall myocardial infarction (MI) a depressed baroreflex sensitivity (BRS) carries a higher risk of developing ventricular fibrillation during a brief ischemic episode associated with an exercise stress test. The clinical and pathophysiological implications of our previous findings prompted the present study, which addressed three major questions: 1) Is, indeed, analysis of BRS after MI a specific and sensitive marker for sudden death-risk stratification? 2) Does MI modify BRS? 3) Does analysis of BRS before MI provide information about outcome during ischemic episodes occurring after MI? An anterior MI was produced in 301 dogs, and 4 weeks later, a 2-minute circumflex coronary artery occlusion beginning during the last minute of an exercise stress test could be performed in 192 animals. Ventricular fibrillation occurred in 106 (55%) dogs (susceptible to sudden death), whereas 86 (45%) dogs (resistant to sudden death) survived. BRS was assessed by the phenylephrine method and was expressed by the regression line relating RR intervals to blood-pressure changes. BRS was significantly lower among susceptible than among resistant dogs (9.1 +/- 6.0 vs. 17.7 +/- 6.5 msec/mm Hg, p less than 0.0001). The risk for sudden death increased from 20% (15 of 73 dogs) for a BRS greater than 15 msec/mm Hg to 91% (62 of 68 dogs) for a BRS less than 9 msec/mm Hg (p less than 0.001). An internal control study in 55 animals showed that BRS was reduced 4 weeks after MI compared with control conditions (13.5 +/- 6.7 vs. 17.8 +/- 6.6 msec/mm Hg, p less than 0.001) and that a reduction occurred in 73% of animals. Susceptible dogs and those that spontaneously died after MI had a lower BRS even before the MI (16.2 +/- 5.9 vs. 22.2 +/- 6.2 msec/mm Hg, p less than 0.001). The risk for sudden death after MI increased from 35% (nine of 26 dogs) for a BRS before MI greater than 20 msec/mm Hg to 85% (17 of 20 dogs) for a BRS before MI less than 14 msec/mm Hg (p less than 0.001). This study demonstrates that the presence of a reduced BRS is associated with a greater susceptibility to ventricular fibrillation during subsequent ischemic episodes. In the majority of dogs, BRS is reduced after an MI. The results in 192 conscious dogs with a healed MI indicate that analysis of BRS is a powerful tool for risk stratification not only after, but even before, the occurrence of an MI.

Ventricular fibrillation threshold during acute ischemia in hypertrophied rat hearts

Ventricular fibrillation threshold was significantly lower in hypertrophied hearts than in normal hearts. Ischemia produced by coronary occlusion reduced fibrillation threshold in both normal and hypertrophied hearts, but the maximum reduction in fibrillation threshold was observed earlier in hypertrophied hearts.

Components of heart rate variability measured during healing of acute myocardial infarction

A high degree of heart rate (HR) variability is found in persons with normal hearts, whereas low HR variability can be found in patients with severe coronary artery disease, congestive heart failure and diabetic neuropathy. Two weeks after acute myocardial infarction, low HR variability predicted reduced long-term survival even after adjusting for clinical risk indicators, left ventricular ejection fraction, HR and ventricular arrhythmias. The present study elucidated the causes of differences in HR and HR variability between patients with low and high HR variability. In a matched-pair study, 10 patients with low HR variability (24-hour standard deviation of N-N intervals less than 50 ms) were randomly selected. For each of these 10 patients, a control patient with high HR variability (24-hour standard deviation of N-N intervals greater than or equal to 100 ms), matched for age, left ventricular ejection fraction and rales in the coronary care unit was selected. Patients who were taking either digitalis or beta-adrenergic blocking drugs were excluded. Analysis of 24-hour electrocardiograms showed that for the low HR variability group compared with the high: (1) the daytime and nighttime average HR was faster; (2) the difference between daytime and nighttime HR was less; (3) the proportion of differences greater than 50 ms between successive N-N intervals was smaller; and (4) the number of HR "spikes" per day (increase in HR greater than or equal to 10 beats/min, lasting from 3 to 15 minutes) was less.(ABSTRACT TRUNCATED AT 250 WORDS)

Devices external pulse generator: a reliable temporary pacemaker?

Two cases of malfunctioning APC (American Pacemaker Company) Devices pacemakers are reported. Two patients with sick sinus syndrome were temporarily paced using APC Devices pacemaker models EC 4542 and EV 4543 respectively, which showed transient increase (53% and 83% of the preset rate, respectively) in pacing rate. The sudden increase of the pacing rate might be the pacemaker's design to switch to fixed-rate pacing at nominally 25% higher than the selected rate in the presence of an excessive level of electrical interference. However, the increased rate was much faster than the interference rate. Great caution should be paid when APC Devices pacemaker is to be used.

Protective effect of vagal stimulation on reperfusion arrhythmias in cats

The role of the autonomic nervous system in modulating reperfusion arrhythmias is still unclear. Experiments with sympathetic denervation or alpha- and beta-adrenergic blocking agents have provided mixed results, while the effect of parasympathetic activation has not been investigated extensively. The effect of bilateral vagotomy and of vagal stimulation was studied, with and without attendant bradycardia, on the incidence of reperfusion arrhythmias in alpha-chloralose anesthetized cats. The left anterior descending coronary artery was occluded for 20 minutes, followed by reperfusion in 105 animals. The incidence and severity of reperfusion arrhythmias was compared in 1) neurally intact animals (heart rate 208 +/- 24 beats/min), 2) animals with acute bilateral vagotomy (heart rate 233 +/- 25 beats/min), 3) animals with vagal stimulation adjusted to maintain heart rate at 90-100 beats/min, and 4) animals with vagal stimulation + ventricular pacing to maintain heart rate at prestimulation values. All the neurally intact and vagotomized animals developed complex reperfusion arrhythmias, but these arrhythmias occurred in only 60 and 72%, respectively, of the animals with vagal stimulation and vagal stimulation + pacing (p less than 0.005 vs. neurally intact and p less than 0.02 vs. vagotomy). The incidence of ventricular fibrillation was similar in neurally intact (62%) and vagotomized (58%) animals; it was strikingly lower (7%, p less than 0.01) in animals with vagal stimulation when heart rate was allowed to decrease, and it was 48% when heart rate was kept constant during vagal stimulation. A selective protection from sustained (greater than 30 seconds duration) ventricular tachycardia was observed in animals with vagal stimulation independent of heart rate changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Molsidomine prevents post-ischaemic ventricular fibrillation in dogs

Forty anaesthetized dogs were subjected to left circumflex coronary artery ligation followed by reperfusion. Molsidomine was randomly administered to 20 dogs (50 micrograms kg-1 as an i.v. bolus - 15 min prior to coronary occlusion - followed by an infusion of 0.05 micrograms kg-1 min-1. Standard electrocardiographic leads 2 and 3 were continuously recorded to measure ST segment and delta R% changes and to document both the number of ventricular premature beats and the onset of ventricular fibrillation; aortic pressure and cardiac output were measured; thromboxane B2 plasma levels, platelet aggregation produced by ADP, and molsidomine plasma levels were determined before and at 10, 30 and 75 min after the start of the drug protocol. Molsidomine protected the treated animals from early (10 min) post-ischaemic ventricular fibrillation (0 of 20 vs 6 of 20, P = 0.0202), reduced the incidence of overall post-occlusion ventricular fibrillation (3 of 20 vs 10 of 20, P = 0.0407) and improved the total survival rate (P = 0.0067). In molsidomine treated dogs: mean aortic pressure and the rate-pressure product were lowered 10 min after the start of the drug; immediate post-occlusion (3 min) ST segment changes (0.82 +/- 0.52 vs 1.52 +/- 0.78 mV, P less than 0.025) and delta R% changes (37 +/- 50 vs 90 +/- 84%, P less than 0.025) were less marked; the number of ventricular premature beats was lowered and finally, a progressive decline of platelet aggregation produced by ADP was achieved after 75 min of drug infusion. These results were obtained in the presence of mean plasma levels of molsidomine ranging from 20 to 28 ng ml-1. The time-action curve of the antifibrillatory effect of molsidomine parallels those at the level of post-ischaemic electrocardiographic changes.

Left ventricular dysfunction and altered autonomic activity: a possible link to sudden cardiac death

There is now a growing body of clinical evidence that suggests a strong association between left ventricular dysfunction and sudden cardiac death in patients recovering from myocardial infarction. The mechanisms underlying this association remain to be determined. Alterations within the autonomic nervous system may represent one factor that links an impairment in cardiac function to an increased mortality. Since ventricular dysfunction would tend to reduce stroke volume, an increased sympathetic and/or decreased parasympathetic efferent activity may compensate for this fall in stroke volume by increasing heart rate and/or the force of contraction (inotropic state) in an attempt to maintain a more normal cardiac output. Similar changes in autonomic activity are, in fact, known to increase the vulnerability to ventricular fibrillation. Therefore, I propose that myocardial infarction induces changes in cardiac function which in turn elicits autonomic efferent changes. As a consequence of these compensatory reflex changes the heart becomes less electrically stable and thereby more prone to lethal arrhythmias.

The circumflex coronary artery occlusion canine model of sudden death: methodology and electrocardiographic observations

This study deals with a left circumflex coronary artery occlusion-reperfusion canine model of sudden death using multiple electrocardiographic leads to define a subgroup of animals at high risk of ventricular fibrillation. Occlusion was followed by ventricular fibrillation in 15 of 30 animals (50%). In the 15 dogs surviving 60 min postocclusion, reperfusion gave rise to ten cases of ventricular fibrillation (66.7%). Thus, the total incidence of occlusion-reperfusion ventricular fibrillation was 25 of 30 (83.3%). Electrocardiographic delta R% changes greater than or equal to 25% in leads 2 and 3 at both 3 and 5 min postocclusion predicted the occurrence of postocclusion ventricular fibrillation with 80% specificity and 56% sensitivity. However, ST segment elevation greater than or equal to 0.5 mV in the same leads at the same times predicted postocclusion ventricular fibrillation with 67% specificity and 100% sensitivity. When only dogs with ST segment elevation greater than or equal to 0.5 mV in leads 2 and 3 at both 3 and 5 min postocclusion were considered, ventricular fibrillation postocclusion was seen in 15 of 20 dogs (75%) and ventricular fibrillation after reperfusion occurred in four of the remaining five animals (80%). Thus, in the subgroup of animals presenting with ST segment elevation greater than or equal to 0.5 mV in leads 2 and 3 at both 3 and 5 min postocclusion, the overall incidence of occlusion-reperfusion ventricular fibrillation was 19 of 20 (95%). These data may be useful in studies aimed at testing the effectiveness of drugs or other interventions in a canine model of sudden death.

Electrophysiological mechanisms underlying rate-dependent changes of refractoriness in normal and segmentally depressed canine Purkinje fibers. The characteristics of post-repolarization refractoriness

Tissues from diseased hearts are known to exhibit post-repolarization refractoriness and rate-dependent changes of the refractory period that are often inconsistent with changes in action potential duration. To examine the electrophysiological mechanisms responsible for such rate-dependent changes of the refractory period, a narrow inexcitable zone was created by superfusing the central segments of Purkinje fibers with an "ion-free" isotonic sucrose solution. The degree of conduction impairment could be finely regulated by varying the resistance of the extracellular shunt pathway. At intermediate or low levels of block, the refractory period remained unchanged or decreased, respectively, as the rate was increased. At relatively high levels of block, however, we observed marked increases of the refractory period in response to increases in the stimulation rate. The disparity of refractoriness between normally conducting fibers and fibers exhibiting discontinuous conduction characteristics and post-repolarization refractoriness increased dramatically as a function of increasing stimulation rate. With the aid of current clamp techniques, we demonstrate that the differential behavior is due to the interplay between rate-dependent changes in the restitution of excitability at the site beyond the depressed zone secondary to changes in passive and active membrane properties and in the intensity of local circuit current provided to that site by activity generated in the segment proximal to the zone of block. Our data suggest that rate-dependent changes of refractoriness in Purkinje tissue are principally governed by attendant changes in membrane resistance.

Arrhythmia susceptibility and myocardial composition in diabetes. Influence of physical conditioning

Abnormal myocardial composition in diabetes mellitus has been described, but the effects on ventricular vulnerability have not been defined. We have assessed the susceptibility to arrhythmias in a canine model after 1 yr of mild diabetes induced by alloxan. Since physical conditioning can affect metabolic abnormalities in diabetes, this intervention has also been evaluated. Group 1 served as controls and groups 3 and 4 were diabetic. Animals in the latter group as well as nondiabetic controls of group 2 were exercised on a treadmill for the last 8 mo of the experiment. After 1 yr, anesthesia was induced with chloralose for vulnerability studies. The ventricular fibrillation threshold of 24.4 +/- 1.9 mA in group 3 was significantly less than in normals (45.1 +/- 2.2). Spontaneous arrhythmias were also more prevalent in diabetics during acute ischemia (group 3-A). Increased ventricular vulnerability after epinephrine infusion was present in the sedentary diabetes despite normal ventricular function responsiveness. In a superfused preparation of myocardium, resting membrane potential and action potential amplitude were normal in diabetics, and beta-adrenergic stimulation shortened repolarization more than in controls. Myocardial collagen concentrations, which included an interfibrillar distribution on morphologic examination, were increased in group 3. In the trained diabetics of group 4 the basal vulnerability thresholds and responses to epinephrine were normal. While myocardial collagen levels were normal, cholesterol and triglyceride increments persisted. Thus, in mild experimental diabetes, enhanced susceptibility to arrhythmias exists; this susceptibility may be based on a combination of nonhomogenous collagen accumulation affecting local conduction and increased electrophysiologic sensitivity to catecholamines.

Cardiac effects of thoracic epidural morphine caused by increased vagal activity in the dog

This study was carried out in order to investigate possible side-effects of thoracic epidural morphine on cardiac electrophysiology, haemodynamics and metabolism. In pentobarbital-anaesthetized dogs, intracardiac conduction times were determined by His bundle electrography, and refractoriness by programmed electrical stimulation; monophasic action potential recordings were obtained from the right ventricle by the suction electrode technique. Cardiac output, left ventricular and aortic blood pressures were measured, as well as plasma concentrations of morphine, free fatty acids, glycerol, glucose and lactate. Thoracic epidural morphine (0.12 mg X kg-1) reduced spontaneous heart rate, prolonged atrioventricular nodal conduction time and refractoriness, and reduced left ventricular dP/dt max. Bilateral vagotomy reversed these effects. Intra-atrial, His Purkinje and intraventricular conduction times, atrial and ventricular refractoriness and action potential duration, stroke volume and mean aortic blood pressure, as well as the metabolic variables, were not significantly influenced by thoracic epidural morphine with or without vagotomy. Peak plasma morphine levels of 12-25 ng X ml-1 were measured 10 min after morphine injection. In conclusion, this study demonstrates depressive side-effects of epidural morphine on cardiac function, mediated by an increased vagal activity.

Prophylactic lidocaine in the early phase of suspected myocardial infarction

Four hundred two patients with suspected myocardial infarction seen within 6 hours of the onset of symptoms entered a double-blind randomized trial of lidocaine vs placebo. During the 1 hour after administration of the drug the incidence of ventricular fibrillation or sustained ventricular tachycardia among the 204 patients with acute myocardial infarction was low, 1.5%. Lidocaine, given in a 300 mg dose intramuscularly followed by 100 mg intravenously, did not prevent sustained ventricular tachycardia, although there was a significant reduction in the number of patients with warning arrhythmias between 15 and 45 minutes after the administration of lidocaine (p less than 0.05). The average plasma lidocaine level 10 minutes after administration for patients without a myocardial infarction was significantly higher than that for patients with an acute infarction. The mean plasma lidocaine level of patients on beta-blocking agents was no different from that in patients not on beta blocking agents. During the 1-hour study period, the incidence of central nervous system side effects was significantly greater in the lidocaine group, hypotension occurred in 11 patients, nine of whom had received lidocaine, and four patients died from asystole, three of whom had had lidocaine. We cannot advocate the administration of lidocaine prophylactically in the early hours of suspected myocardial infarction.

Carbachol and dibutyryl cyclic GMP on the vulnerability to ventricular fibrillation in rat isolated hearts

The hypothesis that elevation of intracellular guanosine 3':5' cyclic monophosphate (cyclic GMP) concentrations may increase electrical stability of the myocardium was examined by determination of ventricular fibrillation thresholds (VFT) on isolated perfused hearts of the rat. Hearts were paced to circumvent any complicating effects of bradycardia. Using this system, carbachol produced a concentration-related reduction in VFT. The reduction in VFT produced by carbachol was not significantly modified by a high concentration of atenolol (10(-5)M), indicating that the increased vulnerability to ventricular fibrillation was not an indirect consequence of catecholamine release from intramyocardial stores. Atropine (10(-6)M) blocked the carbachol-induced reduction in VFT. At the concentrations of carbachol used to reduce VFT, myocardial cyclic GMP concentrations were also elevated. The dibutyryl analogue of cyclic GMP (10(-4)M) mimicked the effect of carbachol in reducing VFT. Carbachol potentiated the adrenaline (3 X 10(-7)M)-induced reduction in VFT.

Ventricular fibrillation

Ventricular fibrillation is the most common mechanism of sudden unexpected cardiac death in persons with asymptomatic or symptomatic coronary artery disease. The electrophysiologic mechanisms reviewed in this article include: automaticity of pacemaker fibers, transformation of nonpacemaker into pacemaker fibers, "injury" currents and reentry. Some of the conditions facilitating ventricular fibrillation include bradycardia, long QT syndrome, electrocution, electrolyte imbalance, drugs, sympathetic stimulation and myocardial ischemia. Electrophysiologic studies during acute myocardial ischemia suggest that the earliest activity at the onset of arrhythmia may originate at the sites of the surviving Purkinje fibers or at the epicardial rim. Reentrant arrhythmias arising in ischemic myocardium are attributed to nonhomogeneous distribution of local hyperkalemia and acidosis.

The long-term increase of baseline and reflexly augmented levels of human vagal-cardiac nervous activity induced by scopolamine

We tested the hypothesis that transdermal scopolamine increases vagal-cardiac nervous outflow over the long term in 16 healthy young men. Twenty-four hours after application of one scopolamine patch, the average RR interval was increased by 13% and the average standard deviation of the RR interval (taken as an index of the level of vagal-cardiac nervous activity) was increased by 31%. Baroreceptor-cardiac reflex responsiveness (as reflected by prolongation of RR interval provoked by graded neck suction) also was increased substantially. These findings suggest that vagal-cardiac nervous activity can be augmented pharmacologically in man on a long-term basis. Since vagal outflow influences cardiac electrical properties in an important way, these findings may have therapeutic implications.

Prognostic significance of field response in out-of-hospital ventricular fibrillation

We reviewed 94 cases of prehospital ventricular fibrillation (VF) to determine aspects of field response that predicted outcome. Only one of 37 patients (3 percent) failing to achieve rhythms other than VF or asystole after the first two defibrillations survived to hospital discharge compared to nine of 57 (16 percent) achieving organized rhythms by this point (p less than 0.05). None of 56 patients failing to achieve pulses prior to transport survived to hospital discharge compared to ten of 38 achieving field pulses (p less than 0.01). However, survival to discharge was not significantly different between patients who developed pulses immediately with their rhythms (5 of 17, 29 percent) and those who were defibrillated into pulseless rhythms but later developed pulses in the field (five of 21, 24 percent). Thus, for prehospital VF, the best field response identifies potential survivors prior to hospital arrival. In addition, the frequent occurrence and potentially favorable outcome of an initially pulseless rhythm necessitates reevaluation of current therapy.

Beta-adrenergic blockade may prolong life in post-infarction patients in part by increasing vagal cardiac inhibition

Beta-adrenergic blocking drugs prolong lives of post-infarction patients primarily by preventing sudden cardiac death. The mechanisms responsible for this beneficial effect are not understood clearly, since beta-blockers, in doses used in most clinical trials, are only weakly effective against stable ventricular arrhythmias. Arrhythmias during myocardial ischemia may differ from arrhythmias in other clinical settings in that they depend importantly upon autonomic neural factors, including the balance between levels of sympathetic cardiac stimulation and parasympathetic cardiac inhibition. Beta-blockers reduce sympathetic cardiac stimulation, and they may influence this balance favorably in another important way: a well documented, but not generally appreciated property of beta-blocking drugs is that they also increase levels of vagal cardiac inhibition. I propose that beta-blockade prevents arrhythmic deaths in post-infarction patients in part by increasing levels of vagal cardiac inhibition.

Prognostic implications of complicated ventricular arrhythmias early after hospital discharge in acute myocardial infarction: a serial ambulatory electrocardiography study

To assess the prevalence and prognostic implications of complicated ventricular ectopic depolarizations (VEDs) after hospital discharge in patients with acute myocardial infarction (AMI), we obtained serial 24-hour Holter recordings in 85 patients during the first 6 weeks after AMI. Recordings were obtained during two coronary care unit time intervals, two hospital ward time intervals, and during four weekly time intervals after discharge. Complicated VEDs were defined as unifocal VEDs greater than or equal to 10/1000 beats for 24 hours, multiform VEDs, pairs, or ventricular tachycardia. At 1 year follow-up, there were nine cardiac deaths (six sudden deaths and three deaths from recurrent AMI). The mean left ventricular ejection fraction at discharge in the cardiac death patients was 29 +/- 12% (sudden death patients 24 +/- 11% and AMI death patients 40 +/- 6%) compared to 49 +/- 13% in the survivors (p less than 0.001). Patients with complicated VEDs at discharge (2 weeks after AMI) or during the first 4 weeks after discharge (3 to 6 weeks after AMI) were significantly more likely to have sudden death at follow-up compared to patients without complicated VEDs. Of the six sudden death patients, four (66%) had complicated VEDs at discharge compared to 18 of 68 survivors (26%) (p less than 0.05). One of three patients who died of recurrent AMI had complicated VEDs. No Holter data were obtained at hospital discharge in eight of the survivors.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of cardiac cholinergic activation on the induction and maintenance of ventricular fibrillation

The influence of cardiac cholinergic activation was studied in rats and cats on the induction and maintenance of ventricular fibrillation (VF). Acetylcholine (ACH 2-25 micrograms/kg), in doses which did not cause bradycardia or hypotension, induced appearance of spontaneous VF (duration 2-60 sec.) in 9/20 rats which have a high sympathetic autoregulation and in 3/6 cats only, 20-40 secs after the latter had been given adrenaline. ACh (10-45 micrograms/kg) and methacholine (10-40 micrograms/kg) also significantly prolonged the fibrillatory period induced electrically in cats and rats with and without atrial or ventricular pacing. The induction or prolongation of VF did not occur when higher doses of ACh (50-100 micrograms/kg) were given to rats. The influence of moderate amounts of cholinergic agents on the heart may be due to localised effects resulting in asynchronous activity. Alternatively, they may produce a discharge of multiple ectopic pacemakers or a disturbance in impulse conduction. Higher doses of ACh depress the S-A and ventricular ectopic activity node thereby decreasing the probability of inducing VF. It is concluded that under conditions of raised cardiac adrenergic activity, a moderate increase in cholinergic influence can both induce and prolong VF. The relevance of these findings to the "sudden infant death" syndrome is discussed.

Relationship between infarct size and incidence of severe ventricular arrhythmias in a double-blind trial with metoprolol in acute myocardial infarction

In 585 patients having an acute myocardial infarction for the first time the relationship was investigated between estimated infarct size and the incidence of ventricular fibrillation and treated ventricular tachycardia during hospitalization. The size of the infarct was estimated from analyses of heat stable lactate dehydrogenase (LD) (EC 1.1.1.27.) in serum collected every 12 hr for 48-108 hr. All patients participated in a double-blind comparison of the beta 1-selective blocker metoprolol with placebo in suspected acute myocardial infarction. A correlation was observed between the enzymatically estimated infarct size and the incidence of ventricular fibrillation and treated ventricular tachycardia in patients on placebo (P less than 0.001), while this could not be demonstrated in patients on the beta-blocker (P greater than 0.2). In placebo treated patients there was a correlation between the maximum heat stable LD activity and early ventricular fibrillation (P = 0.034), late ventricular fibrillation (P less than 0.001), primary ventricular fibrillation (P = 0.002) as well as secondary ventricular fibrillation (P = 0.034). It is concluded that there seems to be a relatively strong correlation between the final size of the infarction and the occurrence of severe ventricular arrhythmias. Treatment with beta-blockade appeared to disturb this correlation.

The mechanism of dysrhythmias in variant angina pectoris: occlusive versus reperfusion

Thirty-six patients with variant angina pectoris (VAP) were analyzed to investigate whether the mechanism underlying dysrhythmia is related to coronary occlusion or reperfusion. Fifteen of the 36 patients demonstrated dysrhythmias (42%). Twelve of 15 patients (80%) experienced dysrhythmia prior to the acme of ST-segment elevation (occlusive dysrhythmia), and those of tachyarrhythmia type were characterized by the presence of ventricular premature beats initially isolated, increasing in frequency, and preceding the more malignant forms of dysrhythmias, such as ventricular tachycardia or ventricular fibrillation. The occlusive dysrhythmias included ventricular dysrhythmia (ventricular premature beats, ventricular tachycardia, slow ventricular tachycardia, ventricular fibrillation) in eight patients and conduction abnormalities (second- and third-degree AV block, left posterior fascicular block) in four patients. Thirteen episodes of VAP were fully recorded electrocardiographically. The average time to onset of dysrhythmia, after the beginning of ST-segment elevation, was 4.94 minutes +/- 1.52. The duration of the episodes without dysrhythmia was 0.86 minute +/- 0.53. The "reperfusion dysrhythmia" occurred in three patients (20%) and was characterized by the appearance of isolated couplets of ventricular premature beats, ventricular tachycardia, or ventricular fibrillation without prodromal ectopic activity. The dysrhythmia occurred in one patient during the resolution of ST-segment elevation and in two patients within seconds of ST-segment normalization. We conclude that the occlusive related dysrhythmias are the most important mechanism in VAP. They are dependent on the duration of the ischemic episode.