STUDIES ON DIGITALIS. IV. OBSERVATIONS IN MAN ON THE EFFECTS OF DIGITALIS PREPARATIONS ON THE CONTRACTILITY OF THE NON-FAILING HEART AND ON TOTAL VASCULAR RESISTANCE

Present status of digitalis treatment of acute myocardial infarction

The available experimental and clinical data still leave many unanswered questions concerning the role of digitalis therapy after acute myocardial infarction. It seems clear that little is to be gained by glycoside administration to patients with uncomplicated infarctions who do not have cardiomegaly. The precise role of digitalis therapy in the treatment of cardiogenic shock remains undefined on the basis of the available data. However, until evidence to the contrary appears, its use should be continued on the basis of experience with experimental cardiogenic shock and the presumption that this state, when observed clinically, is a form of extreme left ventricular failure.

Digitalis appears to be indicated in the treatment of atrial fibrillation with rapid ventricular rate complicating acute myocardial infarction. Other supraventricular arrhythmias, such as atrial flutter and ectopic atrial tachycardia, frequently require larger doses of the glycoside, and other means of therapy, such as antiarrhythmic agents or electrical conversion, may be preferable in such instances.

Digitalis is commonly utilized early in the treatment of congestive heart failure and cardiomegaly complicating acute myocardial infarction, despite the paucity of experimental and clinical evidence in support of such therapy. Whether the digitalis glycosides should continue to be used routinely in such patients is an important subject for clinical investigation.

Regional hemodynamic effects of a digitalis glycoside in the conscious dog with and without experimental heart failure

The effects of intravenous ouabain (G-strophanthin), 20 µg/kg, on the superior mesenteric, renal, and external iliac beds were studied over a 30-minute period in eight normal conscious dogs before, and in five of them after, cholinergic blockade and in six conscious dogs with heart failure produced by tricuspid avulsion and progressive pulmonary stenosis. Blood flows were measured with Doppler ultrasonic and electromagnetic flowmeters. In normal dogs, prior to cholinergic blockade, ouabain caused early increases in mesenteric, renal, and external iliac resistances but later, between 15 and 30 minutes, mesenteric resistance decreased below the control level while renal and external iliac resistances remained elevated. After cholinergic blockade, the ouabain-induced alterations in renal and iliac resistance were unaltered, but the later decline in resistance in the mesenteric bed was reversed, resulting in sustained constriction. The late decline in mesenteric resistance also was not observed in the conscious animal after the injection of ouabain, 5 µg/kg, directly into the mesenteric artery or in the majority of dogs given ouabain intravenously after having been anesthetized with pentobarbital sodium. In the conscious dogs with heart failure, ouabain resulted in increased renal and iliac resistances initially, and in vasodilatation later while mesenteric resistance fell initially and remained below control for the entire 30 minutes. Thus, in the normal conscious dog, ouabain causes vasoconstriction in the regional vascular beds, apparently by its direct vascular action, and, in addition, a cholinergically mediated vasodilatation in the mesenteric bed. In the conscious dog with heart failure, ouabain increases blood flow to all beds, reducing the compensatory vasoconstriction of the low output state.

Effects of cardiac depression and of anesthesia on the myocardial action of a cardiac glycoside

The effects of ouabain (G-strophanthin) 20 mug/kg, on left ventricular (LV) pressure (P), diameter (D), velocity of contraction (dD/dt), and dP/dt were studied in conscious dogs instrumented with ultrasonic diameter gauges and miniature pressure gauges. The effects of ouabain were compared on separate occasions in the same dogs after cardiac depression with propranolol, 3.0 mg/kg, and also after general anesthesia with Na pentobarbital, 30 mg/kg. Maximal pressor effects were observed in the first 10 min, but maximal effects on the contractile state occurred at 30 min after ouabain. At this time, in conscious dogs, ouabain had increased LV isolength systolic pressure by 5%, LV isolength velocity by only 9%, and LV (dP/dt)/P by 21%, while end systolic diameter (ESD) decreased slightly and end diastolic diameter (EDD) and heart rate (HR) were unchanged. After anesthesia, ouabain increased LV systolic pressure by 8%, velocity 32%, (dP/dt)/P by 47%, and ESD decreased by 1.2 mm while EDD rose slightly and HR fell by 26 beats/min. Returning HR to control with atrial pacing decreased EDD 0.9 mm below control. After cardiac depression with propranolol, ouabain caused responses similar to those observed in the anesthetized dogs. Thus, the cardiac glycoside was found to exert only minor inotropic effects on the nonfailing heart of conscious dogs but far more striking inotropic responses in the anesthetized state.

Effects of a digitalis glycoside on coronary and systemic dynamics in conscious dogs

The effects of ouabain (G-strophanthin), 20 µg/kg were compared in 12 conscious dogs with Doppler flow transducers on the ascending aorta and left circumflex coronary artery and pressure gauges in the aorta, and in 9 of these dogs after general anesthesia with Na pentobarbital. In conscious dogs ouabain caused an initial bradycardia, but heart rate returned almost to control at 15 to 30 minutes, while arterial pressure rose and remained elevated. Cardiac output and coronary blood flow decreased initially, returned to control by 5 minutes and then remained constant. Systemic, mean, and late diastolic coronary resistances were elevated within 1 minute and remained elevated for 30 minutes. After anesthesia, ouabain caused similar increases in arterial pressure and slightly greater increases in systemic resistance, but the bradycardia and reduction of cardiac output were more profound and sustained. In the anesthetized state, coronary resistance rose when heart rate was allowed to slow after ouabain but was not elevated when heart rate was returned to control. Thus, in the conscious state, ouabain caused a distinct elevation in coronary and systemic resistances with no change in cardiac output, while in the anesthetized state ouabain reduced cardiac output and when heart rate was controlled, did not alter coronary resistance.

Effects of acute digitalization on the pulmonary blood volume in patients with heart disease

Basic hemodynamic data and the pulmonary blood volume (PBV) were determined in 23 patients before and after intravenous administration of acetyl strophanthidin during right and left heart catheterization. These patients were divided into two groups depending upon whether or not digitalization produced a significant fall in left ventricular end-diastolic pressure (LV D ). Group I consisted of 11 patients who manifested a significant decrease in LV D , while in group II no change was noted after digitalization. In both groups, a significant rise in LV stroke volume, stroke work, and maximal rate of rise of LV systolic pressure (dp/dt) was recorded. In group I, a significant decrease in the pulmonary vascular distending pressure (P d ) was accompanied by a decrease in PBV. In contrast, no change in P d or PBV was noted in the patients in group II. Changes in pulmonary vascular resistance (PVR) were inconsistent. It is concluded that: (1) in patients of group I the concordant decrease in P d and PBV represented a passive effect on the pulmonary vascular bed mediated by improved LV performance and decrease in LV D ; (2) in patients of group II, despite evidence of significant positive inotropic effects on the left ventricle, the failure of LV D to fall prevented a decrease in P d and PBV; (3) no evidence of active vasomotor effects of acetyl strophanthidin in the lung could be demonstrated. Changes in PVR were capricious and not apparently useful in assessment of the presence or absence of pulmonary vasomotor effects induced by digitalis. The results of this study differ from the findings of other investigators in cardiac patients and experimental animals. Some of the differences may be attributed to different experimental methods and to species variation in vascular reactivity.

Also, the findings in this report contrast with those previously reported for isoproterenol and aminophylline, agents that produce both positive inotropic effects on the heart and active vasomotion in the pulmonary circulation.

The role of inotropic variation in ventricular function during atrial fibrillation

A series of experiments were performed upon intact anesthetized dogs to determine the relevance of a variety of hemodynamic variables to the irregular ventricular performance associated with atrial fibrillation. During experimentally induced atrial fibrillation central aortic pulse pressure was measured in relation to the duration of the preceding diastolic interval, the relative degree of cycle-length change, the magnitude of the preceding aortic end-diastolic pressure, the rate of ventricular tension development (at a fixed diastolic tension), and to ventricular end-diastolic pressure. While all of the latter variables bore a significant relation to the chosen parameters of ventricular function, the most linear correlation lay with the rate of ventricular tension development. It has been suggested, as a consequence, that the irregular ventricular performance observed during atrial fibrillation under these experimental conditions, may be more directly related to variation in the inotropic state of the ventricular myocardium than to an expression of the Frank-Starling concept, resulting from variable ventricular filling. The lability of the inotropic or contractile state has in turn been attributed to abrupt cycle-length change effecting inotropic alteration analogous to postextrasystolic potentiation of contractility and, at rapid rates, effecting an alternation of the contractile state.

New concepts of digitalis

The data presented in this review have substantiated the following ideas:* There is a direct relationship between digitalis dosage and myocardial contractile force. A small dose of digitalis produces a small increase of the contractile force of the heart and a large dose increases the force of the heart's contraction by a much greater amount, whether the heart is normal, has a poor reserve, or is in frank failure.* Digitalis is of benefit to the patient with cardiovascular disease whether his heart is in failure or not.

Potentiation of adrenergic venomotor responses in dogs by cardiac glycosides

Acetylstrophanthidin, 0.3 to 10 µg/ml, caused contraction of helical strips of the dog cutaneous vein in an organ bath at 37°C. A latent period of 12 to 60 minutes preceded the contraction. During this time, the vein exhibited a progressive increase in response to transmural electric stimulation and added norepinephrine. As the dose of acetylstrophanthidin was increased, the latent period shortened and the increased response during the latent period was potentiated. When the contraction caused by 10 µg/ml was maximal, no additional contraction occurred with electric stimulation; the response to electric stimulation was only partially restored by the repeated washing out of the drug, despite relaxation of the strip to almost base-line tension. Since transmural electric stimulation causes contraction by release of tissue catecholamines, these experiments demonstrate that digitalis glycosides sensitize the cutaneous veins of the dog to adrenergic stimulation. This was confirmed in dogs anesthetized with pentobarbital. Intravenous injections of acetylstrophanthidin and digoxin, in doses too small (12.5 to 15.5 µ/kg) to cause direct constriction of the lateral saphenous vein, potentiated the constriction of this vein caused by electric stimulation of the lumbar sympathetic chain.

Role of acetylstrophanthidin in augmenting myocardial oxygen consumption. Relation of increased O-2 consumption to changes in velocity of contraction

A polarographic method was used to determine the effects of acetylstrophanthidin on myocardial oxygen consumption (MV · O 2 ) of 19 cat papillary muscles contracting under both afterloaded and isometric conditions. Under afterloaded conditions, acetylstrophanthidin shifted the force-velocity relation to the right and produced increments in both the extent and velocity of shortening at constant levels of developed force. These changes in myocardial mechanical behavior after strophanthidin were always associated with an increased MV · O 2 . Since both the extent and velocity of shortening increased following augmentation of the contractile state with glycoside, consideration of the individual effect of these variables in myocardial mechanical behavior on MV · O 2 was precluded. Therefore, experiments were performed on isometrically contracting muscle to evaluate the effect of changes in the velocity of contraction on MV · O 2 independent of changes in contractile element shortening. Comparison of the MV · O 2 of isometric contractions at equal levels of developed tension (and thus equivalent amounts of internal contractile element shortening) was accomplished by decreasing initial muscle length after augmentation of the contractile state by acetylstrophanthidin. Under these conditions, augmentation of the contractile state, characterized by an increase in the velocity of contraction, was associated with an increased MV · O 2 . Thus it is concluded that the effect of acetylstrophanthidin is to increase MV · O 2 at a constant load and that this augmentation of MV · O 2 can be related to the change in contractile state of the muscle.

Effects of ouabain on force of contraction, oxygen consumption, and metabolism of free fatty acids in the perfused rabbit heart

Isolated rabbit hearts were perfused with a suspension of red cells which assured adequate oxygenation. Ouabain added to the perfusate (2.8 x 10 -7 M final concentration) promptly increased the force of contraction and oxygen consumption of the hearts. These events appeared to occur simultaneously. The extraction ratio of palmitate-1- 14 C added to the perfusate (0.31) was not altered by ouabain. However, ouabain increased the oxidation of the extracted palmitate from an average of 34% to an average of 71% at the time intervals studied.

Studies on digitalis. XVI. Effects on myocardial oxygen consumption

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Studies on digitalis. XV. Effects of cardiac glycosides on myocardial force-velocity relations in the nonfailing human heart

The effects of ouabain (0.01 mg/kg) on ventricular force-velocity relations were studied in six patients who had previously undergone corrective cardiac operations. The technique employed consisted of exposing cineradiograms at 30 frames per second and measuring the velocity of movement of roentgen-opaque markers that had been sutured to the external surfaces of the ventricles while simultaneously recording intraventricular pressures. A beat-to-beat analysis of the ventricular force-velocity relation was then accomplished by relating the velocity of marker movement and intraventricular pressure at constant ventricular dimensions. It was observed that ouabain always augmented myocardial contractility as reflected in the force-velocity relation. Velocity of shortening increased an average of 77±5 ( sem )% while intraventricular pressure rose by an average of 23±6%. Despite this improvement in contractility, no consistent changes in cardiac output were observed. Analogous changes in force-velocity curves were obtained when a cardiac glycoside was added to isolated papillary muscles removed from normal cats. It is concluded that the fundamental action of digitalis glycosides is to augment the contractile state of the heart, whether normal or failing, but that in the absence of heart failure this improvement is not translated into an increase in cardiac output.