Ischemic-induced alterations in cardiac sensitivity to digitalis

Concerns about the use of digoxin in acute coronary syndromes

AIMS

The use of digitalis has been plagued by controversy since its initial use. We aimed to determine the relationship between digoxin use and outcomes in hospitalized patients with acute coronary syndromes (ACSs) complicated by heart failure (HF) accounting for sex difference and prior heart diseases.

METHODS AND RESULTS

Of the 25,187 patients presenting with acute HF (Killip class ≥ 2) in the International Survey of Acute Coronary Syndromes (ISACS)-Archives (NCT04008173) registry, 4,722 (18.7%) received digoxin on hospital admission. The main outcome measure was all cause 30-day mortality. Estimates were evaluated by inverse probability of treatment weighting models. Women who received digoxin had a higher rate of death than women who did not receive it (33.8% vs. 29.2%; relative risk [RR] ratio:1.24;95% confidence interval [CI]: 1.12-1.37). Similar odds for mortality with digoxin were observed in men (28.5% vs. 24.9%; RR ratio 1.20; 95% CI:1.10-1.32). Comparable results were obtained in patients with no prior coronary heart disease (RR ratios:1.26; 95% CI: 1.10 to 1.45 in women and RR:1.21; 95% CI: 1.06 to 1.39 in men) and those in sinus rhythm at admission (RR ratios:1.34; 95% CI 1.15 to 1.54 in women and 1.26; 95% CI 1.10 to 1.45 in men).

CONCLUSION

Digoxin therapy is associated with an increased risk of early death among women and men with ACS complicated by HF. This finding highlights the need for re-examination of digoxin use in the clinical setting of ACS.

Elimination, but not accumulation, of metoprolol by rat isolated perfused heart is selectively impaired by hypoxia

1. The influence of hypoxia on the time course of regional myocardial accumulation and elimination of the beta 1-adrenoceptor antagonist metoprolol was investigated by a spontaneously beating rat isolated perfused heart preparation. 2. Myocardial metoprolol content was maximal at 2 min in the left and right ventricles and atria. Neither the extent nor the time of maximal myocardial metoprolol content was significantly influenced by the induction of hypoxia. However, maximal myocardial metoprolol content in both atria and right ventricles was significantly higher than that in the left ventricular samples (P < 0.02; one-factor analysis of variance, 17 d.f.). 3. Elimination of metoprolol (as indicated from residual myocardial metoprolol content at 10 min) was impaired in hypoxic left ventricles (P < 0.01 vs normoxia; unpaired t-test, 10 d.f.) but not in right ventricles or atria. This variation in myocardial metoprolol disposition was not apparent from examination of serial metoprolol concentrations in coronary perfusate. 4. Hypoxia selectively impaired the elimination of metoprolol from the left ventricle, but not the process of drug accumulation, by any region of myocardium. It remains to be determined whether this reflects regional variation in the extent of microcirculatory impairment associated with hypoxia.

The force-interval relationship of the left ventricle: a quantitative description in patients with ischemic heart disease

Quantitative descriptions of the mechanical restitution curve as a description of variability in ventricular performance with coupling interval in isolated tissue preparations are widely available. In humans, however, in vivo examination of the force-interval relationship is restricted to test pulse intervals shorter than the sinus cycle length (i.e., incomplete mechanical restitution). The primary objectives in this investigation were therefore to examine this aspect of mechanical restitution in patients with ischemic heart disease and to develop a quantitative description of the phenomenon. Mechanical restitution curves were constructed in 40 patients, most of whom had well-preserved left ventricular (LV) systolic function, undergoing diagnostic cardiac catheterization for the investigation of chest pain, using a single premature test pulse interval during baseline atrial pacing. The mechanical restitution curve, the relationship between LV + dP/dtmax and test pulse interval, was fitted to a rectangular hyperbolic function. From this, the parameter c, the calculated proportional decrease in LV + dP/dtmax at 60% of the resting cycle length, was derived. The mechanical restitution curve-fitting model (involving determination of c) satisfactorily described the force-interval relationship in 37 of the 40 patients studied (as a rectangular hyperbola in 31 and with simple linear regression in 6 patients). The refractory period of the atria/atrioventricular node limited accurate use of the model in the remaining three patients. The parameter c was inversely proportional to both baseline atrial pacing cycle length (P < .001) and LV ejection fraction (P < .02) In patients with normal LV ejection fractions, the derived value of c at a cycle length of 800 ms (c800) was 29.0% baseline LV + dP/dtmax (95% confidence interval, 23.0, 35.0). The presence of hemodynamically significant ischemic heart disease was not a predictor of the parameters of the model. After intravenous injection of the beta-adrenoreceptor antagonist metoprolol in seven patients, there was a significant (P < .05) reduction in both c and LV + dP/dtmax at the baseline atrial pacing cycle length. Thus, the force-interval relationship can be quantitatively studied using incomplete mechanical restitution curves in humans in vivo. This quantitative description probably reflects relative intracellular calcium availability via slow channel activity and can be used to assess effects of cardioactive drugs on frequency-dependent inotropic mechanisms in humans. The predictive value of this mechanical restitution curve model for hemodynamic instability during tachycardia in patients with impaired LV function remains to be determined.

Deleterious effects of digitalis on reperfusion-induced arrhythmias and myocardial injury in ischemic rat hearts: possible involvements of myocardial Na+ and Ca2+ imbalance

Isolated rat hearts were made ischemic for 25 min after an initial recirculating perfusion, followed by 30 min of reperfusion. In some hearts, interventions including administration of ouabain and/or high [K+] in the buffer were performed during the first 10 min of reperfusion. During ischemia, intracellular Na+ (Nai) increased from 15 to 64 mumol/g dry weight (dwt). During reperfusion, Nai declined rapidly (at 10 min of reperfusion: 48 mumol/g dwt, at 30 min: 25 mumol/g dwt) and regular rhythm was recovered within 10 min in hearts without any intervention during reperfusion. 45Ca2+ uptake increased from 0.8 to 7.5 mumol/g dwt after 30 min of reperfusion. Ventricular function recovered by 45%. A 10-min perfusion with 10 or 50 microM of ouabain increased Nai (17 to 21 or 27 mumol/g dwt) with increased left-ventricular (LV) contractile function, but these effects were reversed by combination of high perfusate [K+] (20 mM) in non-ischemic hearts. A 10-min reperfusion with ouabain retarded or stopped the decline in Nai (at 10 min of reperfusion: 54 or 63 mumol/g dwt, at 30 min: 32 or 40 mumol/g dwt). These amounts of ouabain also increased the incidence of ventricular tachyarrhythmias during reperfusion to 30% or 50%, and increased the duration of ventricular fibrillation from 6.5 to 11.5 or 18.0 min. 45Ca2+ uptake reached to 8.8 or 10.0 mumol/g dwt, and function recovered only 35% or 28%. When high perfusate [K+] was combined with ouabain during reperfusion, the retarded decline in Nai, augmented 45Ca2+ uptake, and reduced recovery of function caused by ouabain alone were attenuated. These results suggest that digitalis has toxic effects on reperfused ischemic hearts by inhibition of rapid active outward transport of previously elevated Nai and potentiation of Ca2+ overload.

Effect of hypothermic ischemia and reperfusion on calcium transport by myocardial sarcolemma and sarcoplasmic reticulum

The effects of hypothermic ischemia and reperfusion on sarcolemma and sarcoplasmic reticulum Ca2+ transport were studied in vesicles isolated from rabbit hearts. Hypothermic global ischemia was produced by immersing hearts in saline at 4 degrees C for 3 h. Following hypothermic ischemia, reperfusion was carried out for 40 min using a Langendorff perfusion system for the working heart. Na+,K(+)-ATPase activity of sarcolemmal vesicles (SL), was not depressed by hypothermic ischemia nor by ischemia and reperfusion. The initial rate of Na(+)-Ca2+ exchange in SL vesicles was not depressed, but the maximum amount of Ca2+ uptake was increased both after hypothermic ischemia and after reperfusion. Ca2+ uptake activity of sarcoplasmic reticulum vesicles (SR) isolated from hearts subjected to hypothermic ischemia was slightly lower than that of control, and was further reduced following reperfusion. Ca(2+)-ATPase activity of SR was unaffected by hypothermic ischemia, while it was markedly lowered after reperfusion. Although the phosphoenzyme level in SR vesicles was slightly decreased, the turnover rate was reduced after reperfusion. Reperfusion injury thus took place mainly in SR while SL appeared to be tolerant to ischemia and reperfusion.

How to digitalize and to maintain optimal digoxin levels in congestive heart failure

Due to the narrow therapeutic-to-toxic ratio of digoxin, numerous studies have been done to assess the optimal digoxin level in patients with congestive heart failure. A digoxin level of 0.7-1.5 ng/mL (or 0.9-2.0 nMol/L) is generally considered optimal, but even at these levels toxicity may occur in certain clinical situations such as severe pulmonary disease or when electrolyte or metabolic disturbances are present. The optimal daily maintenance dose of digoxin depends on the preparation given and can be calculated by the equation of Jelliffe, which is largely based on the creatinine clearance of the patient. The daily digoxin dose must also be adjusted to take into consideration disease processes or concomitant drug therapy that can alter the volume of distribution, the biotransformation, or the excretion of the drug.

Reduction in digitalis-associated postinfarction mortality with nadolol in conscious dogs

Previously, we have demonstrated an increased incidence of lethal ischemic arrhythmias in postinfarction dogs with clinically observable serum digoxin concentrations, and a significant reduction in digitalis-related lethal ischemic arrhythmias after subacute left stellectomy. In the present study, the protective actions of acute beta-adrenoceptor blockade with nadolol, 1.0 mg/kg administered intravenously immediately preceding the induction of posterolateral myocardial ischemia, were assessed in conscious dogs with recent, small anterior myocardial infarctions pretreated with digoxin, 0.0125 mg/kg/day intravenously, for 5 to 7 consecutive days (total n = 11). A cohort of postinfarction dogs pretreated with digoxin alone served as a control group (total n = 26). Pre vs postdigoxin electrophysiologic testing indicated reductions in myocardial refractoriness in ventricular noninfarct and infarct zones in both treatment groups, whereas the administration of nadolol tended to reverse the reductions in ventricular refractoriness. Arrhythmia-related deaths in response to posterolateral myocardial ischemia were reduced from 12 of 20 (60%) in the digoxin control group to 2 of 10 (20%) in the digoxin + nadolol group (p = 0.039). Serum digoxin concentrations (1.29 +/- 0.14 ng/ml vs 1.39 +/- 0.24 ng/ml), underlying anterior myocardial infarct size (6.9 +/- 1.5% vs 4.6 +/- 0.9% of left ventricle), and developing posterolateral myocardial infarct size (22.8 +/- 2.5% vs 17.5 +/- 3.6% of left ventricle) did not differ significantly between the digoxin and digoxin + nadolol groups. Acute beta-adrenoceptor blockade with nadolol appears to reduce digitalis-mediated ischemic postinfarction mortality, possibly because of a salutary increase in ventricular refractoriness.

Facilitation of lethal ventricular arrhythmias by therapeutic digoxin in conscious post infarction dogs

The proarrhythmic potential of digoxin, administered in a therapeutic dosage regimen, was evaluated in conscious dogs in the subacute phase of myocardial infarction. In this evaluation, digoxin (0.0125 mg/kg/day intravenously) or vehicle were administered to conscious dogs for periods of 5 to 7 days, commencing 4 to 5 days after anterior myocardial infarction. Before treatment, programmed ventricular stimulation failed to initiate ventricular tachycardia in 26 post infarction dogs. After treatment, programmed stimulation initiated ventricular tachyarrhythmias in only 1 of 13 digoxin-treated dogs (1.36 +/- 0.17 ng/ml serum digoxin) and in 0 of 13 vehicle-treated dogs. However, the incidences of early ventricular fibrilation (4 of 10 digoxin vs 0 of 12 vehicle; p less than 0.05) and of 24-hour mortality (6 of 10 digoxin vs 2 of 12 vehicle; p less than 0.05) occurring in response to the development of posterolateral ischemia in the presence of previous anterior myocardial infarction was significantly greater in digoxin-treated (1.47 +/- 0.19 ng/ml serum digoxin) than in vehicle-treated animals. These findings suggest an enhanced susceptibility toward the development of ischemia-related lethal arrhythmias in the presence of therapeutic digoxin serum concentrations early after myocardial infarction, which is not predicted by programmed ventricular stimulation testing.

Effect of digitalis treatment on survival after acute myocardial infarction

To determine whether treatment with digitalis is associated with decreased survival after acute myocardial infarction (AMI), data from 504 patients who were enrolled in a postinfarction natural history study were analyzed. At the time of discharge, 229 patients (45%) were taking digitalis. After 3 years of follow-up, the cumulative survival rate for patients discharged on a regimen of digitalis was 66%, compared with 87% for those not treated (p less than 0.001). Univariate analysis showed that statistically significant differences existed between the 2 groups with respect to age, previous AMI, left ventricular failure in the coronary care unit, atrial fibrillation in the coronary care unit, peak creatine kinase levels, enlarged heart and pulmonary vascular congestion on the discharge chest x-ray, ventricular arrhythmias and treatment with diuretic, antiarrhythmic and beta-blocking drugs. Survival analysis using Cox's regression model showed that the association between digitalis and decreased survival was of borderline significance after adjustment for atrial fibrillation and left ventricular failure. Serum digoxin concentration was measured in 83% of the patients who took digitalis. Survival was inversely and significantly related to serum digoxin, i.e., the higher the serum digoxin concentration, the lower the long-term survival rate. After adjusting for atrial fibrillation and left ventricular failure, serum digoxin was not significantly related to survival. Taken together with the results of 3 other large, nonrandomized studies of digitalis treatment after AMI, this study suggests that digitalis treatment may have adverse effects on survival during follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced tolerance to digitalis-induced arrhythmias caused by coronary-flow alterations in isolated perfused heart of guinea pigs

The ligation of a main branch of coronary vein elevates the sensitivity of isolated, perfused (Langendorff) preparations of guinea-pig heart to arrhythmogenic actions of digoxin. Retrograde perfusion studies indicate that the presence of an area with a high extracellular K+ concentration, adjacent to an area with normal K+ concentration and exposed to the glycoside, predisposes the heart to digitalis-induced toxicity. This model may be useful in studying the mechanism by which acute myocardial infarction decreases the therapeutic index of cardiac glycosides.

Effects of myocardial ischemia on cardiac contractility and ion transport in working and Langendorff rabbit heart

In order to define the ischemic myocardial injury in terms of the possible alteration of cardiac membrane properties, Rb+ uptake and 3H-ouabain binding to cardiac tissue were compared in normal and ischemic rabbit hearts. In isolated Langendorff or working rabbit hearts, a global ischemia was produced by reducing the coronary flow of oxygenated Krebs-Henseleit solution for various time intervals. The normal or ischemic tissues were subjected to study the uptake of Rb+ and ouabain bindings. In Langendorff hearts there was little or no significant difference in active uptake of Rb+ in normal and ischemic heart. However, in the working heart, there was a significant decrease in Rb+ uptake in the ischemic hearts. As the case of Rb+ uptake ouabain binding was significantly reduced in ischemic working heart in comparison to working normal heart preparations at high concentrations of ouabain, the present study suggests the importance of cardiac contractility for the cellular energy metabolism and membrane transport activity.

Subcellular [3H]digoxin distribution after temporary myocardial ischemia in dogs

Following a 90-min coronary occlusion and 2 h reperfusion in 11 dogs, total tissue and subcellular distributions of [3H]digoxin in non-ischemic and various ischemic tissues were measured. In the non-ischemic tissue, [3H]digoxin in the crude homogenate, sediments obtained from 1000 X g, 10000 X g and 100000 X g centrifugations, and final supernatant fraction were 0.70 +/- 0.05, 0.79 +/- 0.05, 0.64 +/- 0.04, 3.87 +/- 0.34 and 0.19 +/- 0.02 ng/mg protein, respectively. As in studies with total tissue [3H]digoxin uptake, a reciprocal correlation was observed in reduction of digoxin binding in the crude homogenates and the 1000 X g sediments with increasing severity of ischemic injury estimated from the loss of nitro-blue-tetrazolium (NBT) stain. A 20% and 80% loss of NBT stain was associated with a 13.3% and 63.5% decrease in digoxin binding, respectively. In contrast, digoxin binding in the 10000 X g sediments increased progressively with the severity of ischemia. No significant change was observed in the final supernatant fraction. Digoxin binding in the 100000 X g sediments, which generally represent specific binding and which are associated with the pharmacologic effects, was not altered in tissues with a loss of NBT stain up to 50%. In fact, a loss of 80% NBT was associated with only a 33.9% decrease in digoxin binding. Thus, it appears that measurement of total tissue digoxin uptake does not provide an accurate measure of the effects of acute ischemia on specific digoxin binding. The ability of the peri- and moderately ischemic tissues (with less than 50% loss of NBT stain) to specifically bind digitalis was not altered after temporary myocardial ischemia.

Biphasic effects of acetylstrophanthidin on automaticity in guinea pig ventricular muscle

The effects of acetylstrophanthidin (AS) on depolarization-induced automaticity and contractility of guinea pig papillary muscle were studied in a single sucrose gap chamber with microelectrode and current-clamp techniques. The concentration used, 1.4-1.8 microM, never induced automaticity in preparations at their normal resting potential. Twenty min after superfusion with AS, action potential duration (APD) was prolonged and the force of contraction increased. These were associated with an increase in slope of phase 4 depolarization and an increase in the membrane resistance (Rm) of muscles depolarized with small constant current pulses. With longer (50-80 min) periods of AS superfusion, APD became shorter, Rm decreased to less than predrug values, and in depolarized preparations, the slope of phase 4 decreased. Contractility remained unchanged throughout this second phase. All of these effects were fully reversible upon 60 min of superfusion with AS-free Tyrode solution. We suggest that the biphasic effects of AS on the automaticity of depolarized ventricular muscle cells are caused by an initial decrease followed by a later increase in transmembrane potassium conductance.

Is ouabain-sensitive rubidium or potassium uptake a measure of sodium pump activity in isolated cardiac muscle?

(1) The significance of the specific (ouabain-sensitive) 86Rb+ or 42K+ uptake by cardiac muscle preparations which are not 'sodium-loaded' was studied. (2) In left atrial preparations of guinea-pig heart, resting 86Rb+ uptake was relatively low. It was markedly increased by electrical stimulation. This stimulated uptake was further enhanced by isoproterenol and inhibited by verapamil. (3) In rat atria, the resting 86Rb+ uptake was somewhat higher than in guinea-pig atria, and the increase in uptake caused by electrical stimulation was smaller. In guinea-pig right ventricular papillary muscle, the resting uptake was highest among those tissues studied, and the response to electrical stimulation was smallest. In the latter tissue, verapamil produced only a minimal inhibition of the specific 86Rb+ uptake. (4) The effect of the frequency of electrical stimulation of 86Rb+ uptake paralleled its influence on the force of contraction, suggesting the involvement of intracellular sodium in both events. (5) In both left atrial and right papillary muscle preparations of guinea-pig heart, specific 42K+ uptake observed with 5.8 mM K+ was relatively high, and was increased only slightly by electrical stimulation. This electrical stimulation, however, increased ouabain-induced inhibition of 42K+ uptake, suggesting that the stimulation increases the amount of Na+ available to the sodium pump. (6) When the K+ concentration was 1 mM, the resting 42K+ uptake was low, and could be enhanced by electrical stimulation. (7) Thus, in cardiac muscle preparations which are not sodium loaded, the specific 86Rb+ or 42K+ uptake can be used to estimate the rate of sodium influx, which is equivalent to the rate of sodium efflux under steady-state conditions, provided that neither Rb+ nor K+ is in excess compared to the Na+ available to the pump. If Rb+ or K+ is in excess, its specific uptake may not reflect changes in transmembrane Na+ movement.