Digitalis (second of four parts)

Acute hemodynamic effects of digoxin alone or in combination with other vasoactive agents in patients with congestive heart failure

Although digitalis preparations have been in use for greater than 200 years, it is only within the last 2 decades that the central hemodynamic and neurohumoral effects occurring over several hours following intravenous administration of digoxin have been investigated in patients with congestive heart failure (CHF). Although digoxin has been shown to stimulate myocardial contractility in tissue preparations, its positive inotropic activity does not consistently translate into improvements in hemodynamic measurements in humans. Digoxin given intravenously results in increased cardiac index and decreased heart rate, left ventricular filling pressure, and right atrial pressure, as well as in acute attenuation of neurohumoral abnormalities, in patients with chronic CHF who have abnormal baseline hemodynamic measurements. Unlike other drugs with positive inotropic activity, however, digoxin does not influence hemodynamics in normal volunteers or in CHF patients in whom hemodynamics have been normalized with other therapies. These differing effects may be related to the drug's diverse peripheral vascular effects in CHF patients in whom vasodilation may occur in comparison with those that occur in normal subjects in whom the peripheral vasoconstrictor effects may prevent the inotropic effects of the drug from being translated into an increase in cardiac output. The hemodynamic effects of digoxin in patients with chronic CHF due primarily to diastolic dysfunction have not been fully investigated. Intravenous digoxin produces hemodynamic effects in patients with CHF associated with acute myocardial infarction, but these changes are small compared with those resulting from the administration of dobutamine. Digoxin does not appear to influence hemodynamic measurements in patients with right ventricular dysfunction unless concomitant left ventricular failure is present. In patients with chronic left ventricular dysfunction, the hemodynamic effects of intravenous digoxin and vasodilators are enhanced when these agents are given in combination.

Efficacy of inotropic support of the failing heart

The clinical literature on the subject of inotropic therapy of heart failure, particularly use of digitalis glycosides, is full of contradictions. Most of this disparity can be accounted for if not reconciled by taking the methodology of the clinical trials into consideration. Because drug interventions may produce subtle effects requiring a subjective determination, the questions being asked in these studies cannot be answered without removing as many sources of bias as possible from the patient management and data analysis. If a study has not been adequately randomized, double-blinded, and placebo-controlled, the clinical findings will be inconclusive at best. Systolic myocardial dysfunction plays a pivotal role in the pathogenesis of CHF in many patients and is a prerequisite for the use of cardiotonic drugs. Although the clinical signs of heart failure may be relieved initially by diuretics and vasodilators, compensation may require the addition of a positive inotrope, particularly in advanced cases. In veterinary medicine, the choice of positive inotrope is limited to digoxin, digitoxin, dobutamine, or amrinone. Digoxin possesses superior pharmacokinetics and is the cardiac glycoside of choice for use in the dog. Dobutamine and amrinone are more potent inotropes, but since they must be administered by continuous intravenous infusion, their use is limited to critical care therapy. At the present time, only digoxin can be administered orally for sustained long-term maintenance therapy. Milrinone, a more potent derivative of amrinone, also offers this option, but it has not been available since its brief trial debut as an investigational drug. None of the nonglycoside alternatives couples the benefits of positive inotropic and negative chronotropic effects. Consequently, digoxin remains the mainstay for chronic inotropic support of the heart. Atrial fibrillation with a rapid ventricular response rate is the prime indication for digoxin. In the last few years, evidence from methodologically sound clinical trials on humans has also restored faith in the efficacy of digoxin for treating heart failure in patients with normal sinus rhythm. From these studies, the profile of a digitalis responsive heart failure patient has emerged. Digoxin is most likely to be efficacious when heart failure is associated with chronic, severe ventricular systolic dysfunction, which has resulted in ventricular dilatation. The most reliable clinical marker is the presence of a third heart sound (gallop rhythm). Although the patients in the worst heart failure generally have the shortest survival time, they may also have the most dramatic short-term clinical benefit. However, once cardiac reserve is exhausted in the terminal stages of failure, cardiotonic stimulation ceases to be effective.(ABSTRACT TRUNCATED AT 400 WORDS)

Digitalis cardiotoxicity

Clinical manifestations of digitalis toxicity were clearly described by Withering in 1785. One hundred years later, certain digitalis-induced arrhythmias were inscribed on the smoked drum, and shortly thereafter with the introduction of the electrocardiograph, manifestations of digitalis toxicity as recognized today were recorded in animals and human beings. With popularization of the direct-writing electrocardiograph in the late 1940s and the introduction of digitoxin in recommended doses (that in retrospect appear inappropriately high), the documented prevalence of digitalis toxicity increased rapidly. With increased understanding of the interaction of electrolytes and digitalis and perhaps, and more importantly, the widespread use of digoxin in doses derived largely from its inotropic action and, thus, inappropriately low for the management of many of the arrhythmias, the prevalence of digitalis toxicity began to decline again. In addition, the advent of serum level determinations and the widespread acceptance of the concept of "therapeutic" levels which, although frequently falling short of the desired clinical end point, served to preclude digitalis toxicity. With the decline in the incidence of digitalis toxicity consequent to these factors, some of the digitalis-related arrhythmias that were common are now rarely observed. This report focuses on arrhythmias that are highly specific for digitalis toxicity and on those that now are less commonly encountered. The discussion and classification of the arrhythmias are based on their most probable electrophysiologic mechanism.

Binding and structural diversity among high-affinity monoclonal anti-digoxin antibodies

High-affinity monoclonal antibodies specific for the cardiac glycoside digoxin provide a useful system for the study of structure-function relationships between antibody combining site and specific antigenic determinants. Fifteen high-affinity monoclonal anti-digoxin antibodies were produced when spleen cells from A/J mice immunized with digoxin coupled to human serum albumin (Dig-HSA) were fused with the non-secreting murine myeloma Sp2/0 cell line. Each subcloned hybridoma antibody was analyzed for affinity and specificity for structurally related cardiac glycosides by a radioimmunoassay based on the adsorption of free [3H]digoxin to dextran-coated charcoal. All of the anti-digoxin hybridoma proteins demonstrated high affinity constants ranging from 10(9) to 10(12) M-1. Using seven different analogs of digoxin, binding specificities of the monoclonal antibodies were assessed by inhibition radioimmunoassay. The 15 hybridomas produced from fusions involving five mice could be divided into eight sets on the basis of these binding specificities. Certain antibodies exhibit a preference for the aglycone portion of digoxin, while others are more specific for the tridigitoxose sugar moiety of digoxin. Monoclonal antibody H- and L-chains were subjected to N-terminal amino acid sequence analysis. The antibodies may be divided into several sequence homology sets for both H- and L-chains. In most instances, homologous heavy chains are associated with a set of homologous light chains. Homologous partial sequences, however, do not correlate with similar antigenic specificities and affinities for digoxin. Thus the fine specificity for antigen is not dependent on VH- and VL-encoded sequences alone. These data illustrate the broad diversity of the elicited response to a single hapten, even in inbred mice.

Studies on the role of sodium- and potassium-activated adenosine triphosphatase inhibition in the pathogenesis of human hypertension. Changes in vascular and cardiac function following inhibition of the sodium pump in normotensive subjects and effects of calcium entry blockade

An endogenous humoral factor which inhibits the sodium- and potassium-activated adenosine triphosphatase (Na-K-ATPase) enzyme in vitro has been incriminated recently of playing a pathogenetic role in experimental and human hypertension. The present study was therefore performed in six healthy volunteers to investigate the hemodynamic consequences of an inhibition of this enzyme by ouabain, a potent and specific inhibitor of Na-K-ATPase. In addition, the role of intracellular calcium as a potential mediator was studied indirectly by the administration of nifedipine, a potent calcium entry blocker with predominant vasodilator properties. Intravenous administration of 8.5 micrograms ouabain/kg body weight inhibited red blood cell (RBC) - Na-K-ATPase by 49% which was accompanied by a significant increase in RBC - ATP and a decrease in intracellular potassium concentrations. This enzyme inhibition resulted in a 24% increase in peripheral vascular resistance. The parallel decrease in cardiac output and heart rate, however, prevented a rise in arterial pressure. This increase in vascular resistance was completely abolished by pretreatment with nifedipine (10 mg orally). In the absence of an effect of nifedipine on Na-K-ATPase, its attenuation of the vasoconstrictor effect of ouabain suggests that the effects of ouabain on the vascular smooth muscle cell are mediated by intracellular calcium. These results demonstrate that inhibition of the Na-K-ATPase enzyme in vivo causes a marked peripheral vasoconstriction. They are also compatible with the concept that an endogenous inhibitor of Na-K-ATPase - in the presence of decreased baroreceptor reflex sensitivity due to blood volume expansion - may play a role in the pathogenesis of human arterial hypertension.

Potassium and the heart

The electrical stability of the heart is more sensitive to the extracellular than to the intracellular potassium concentration. During exercise, extracellular potassium varies rapidly. Catecholamines also modulate the plasma potassium concentration. Hypokalaemia of any cause can precipitate arrhythmias. Ischaemic myocardium loses potassium into the extracellular space within seconds and the cell becomes depolarized. The rise of the extracellular potassium ion concentration accounts for many of the early electrophysiological changes. Abrupt changes of plasma potassium concentration in normal myocardium and a high potassium concentration in ischaemic myocardium can set up electrical forces which initiate arrhythmias. The same phenomenon can account for changes on the electrocardiogram early after the cessation of an exercise test in a patient with ischaemic heart disease. Accumulation of potassium between cells in response to an increase of heart rate is a possible mechanism for false positive exercise tests and Syndrome X.

Hemodynamic effects of combined digoxin and dopamine administration in postoperative patients with cardiac dysfunction

In 10 patients with postoperative cardiac dysfunction which required dopamine for inotropic and hemodynamic support, we observed the cardiovascular effects of short-term digoxin administration. The average dosage of dopamine was 7.45 micrograms/kg per minute and was maintained while the patients were given 1 mg of digoxin over 8 hours. The dosage of dopamine was then tapered over the next 4 hours. We observed a significant increase in the cardiac index (4 hours) and a reduction in the heart rate (8 hours) before the dopamine dosage was reduced. After a reduction in dopamine dosage to 2.28 micrograms/kg per minute, these effects persisted. No significant changes were noted in systemic vascular resistance or pulmonary artery diastolic pressure during digoxin administration. These results indicate that the inotropic effects of dopamine and digoxin are additive when given in combination and that digoxin can be used to significantly reduce the dopamine dosage in patients with postoperative cardiac failure. Thus, the combination of an acute inotropic agent, dopamine, and a chronic inotropic agent, digoxin, appears to be clinically useful in postoperative cardiac dysfunction.

Effect of quinidine on differing sensitivities of Purkinje fibers and myocardium to inhibition of monovalent cation transport by digitalis in dogs

Studies have implicated quinidine in increasing serum digoxin levels, resulting in serious arrhythmias. Arrhythmias caused by digitalis intoxication are thought to originate in Purkinje fibers. Thus, the extent of inhibition of monovalent cation-active transport in Purkinje fibers and myocardium may explain the enhanced toxicity of the combined administration of digoxin and quinidine. Monovalent cation transport was assessed by measuring the uptake of the potassium analog rubidium in samples of myocardium and Purkinje fibers after in vitro exposure to ouabain and after long-term administration of digoxin and quinidine or digoxin alone. A group of dogs received chronic digoxin administration and achieved a steady-stage digoxin administration and achieved a steady-stage digoxin level of 2.1 +/- 0.3 ng/ml. Quinidine administered intravenously caused a 134% increase in the serum digoxin level. The transport in myocardium was unchanged, while it was reduced to 40% of control levels in Purkinje fibers. The difference in sensitivity between Purkinje fibers and myocardium may explain the finding that digitalis-toxic arrhythmias arise in Purkinje fibers and that quinidine, when combined with digitalis, increases the incidence of such arrhythmias.

Effects of discontinuing maintenance digoxin therapy in patients with ischemic heart disease and congestive heart failure in sinus rhythm

To evaluate the importance of oral maintenance digoxin therapy in chronic congestive heart failure (CHF), 24 patients in sinus rhythm on maintenance digoxin for documented CHF were studied prospectively on and off the drug. The average duration of therapy was 39 months (range 2 to 180). All 24 patients had documented coronary artery disease (CAD): 22 were in New York Heart Association functional class III and 2 in class II. Twenty-one patients (88%) were receiving diuretic or vasodilator therapy, or both, before digoxin discontinuance. At 1 month off digoxin and with no increase in doses of other medications excepting minor increases in antianginal therapy in 2 patients, no difference was observed in the group as a whole in symptoms, resting heart rate, arterial blood pressure, physical findings, weight, cardiothoracic ratio, radiographic signs of pulmonary congestion, radionuclide left ventricular ejection fraction (LVEF), duration of symptom-limited treadmill exercise (14 patients), or CHF score, compared with evaluation during maintenance digoxin therapy. Similar results were obtained in a subgroup of 9 patients with a resting LVEF less than 0.35 (0.27 +/- 0.02; mean +/- standard error of the mean). Six patients had a decrease and 5 patients an increase in LVEF of greater than or equal to 0.05 units after cessation of digoxin. Off digoxin, the CHF score increased by only 1 point in 2 patients, but also decreased in 2 patients. Thus, in this study population comprised of patients with CAD with documented CHF, most of whom were receiving diuretics or vasodilators, or both, digoxin withdrawal had no adverse clinical or hemodynamic effects.

Effects of oral digoxin on ventricular ectopy and its relation to left ventricular function

The ventricular antiarrhythmic properties of oral digoxin were examined in 13 patients with chronic ventricular ectopy using serial 24-hour electrocardiographic monitoring. Mean premature ventricular complex frequency (per 1,000 normal beats) decreased from 56 +/- 47 during the placebo period to 40 +/- 27 (p = not significant [NS]) and 25 +/- 17 (p less than 0.05) during daily administration of digoxin, 0.25 and 0.375 mg. Digoxin had no significant effect on the qualitative occurrence of complex ventricular arrhythmia patterns (multiformity, bigeminy, couplets, ventricular tachycardia). Radionuclide left ventricular (LV) ejection fraction was measured during the placebo period. Seven patients had normal (ejection fraction greater than 50%) and 6 abnormal global LV performance. In the normal group, the mean premature ventricular complex frequency decreased from 69 +/- 58 to 20 +/- 18 (p less than 0.05) and the mean couplet frequency decreased from 0.59 +/- 0.85 to 0.07 +/- 0.06 (p less than 0.04) during the placebo and 0.375 mg digoxin dosing periods, respectively. In contrast, no significant changes in either variable occurred after digoxin in subjects with depressed LV function. This study indicates that oral digoxin is moderately effective in suppressing premature ventricular complexes, and that its effects are greatest in patients with normal overall LV performance.

Pathogenesis of rheumatoid arthritis: a vascular hypothesis

Past research into the pathogenesis of RA has generally concerned itself with established inflammation. The present review summarizes alterations in microvascular anatomy and function which occur during the hypoxic state, in various experimental and disease conditions. It further shows that tissue hypoxia is a common finding in RA and that the microvascular alterations of RA are similar to those produced by experimental hypoxia. The available data suggest that microcirculatory compromise, concomitant with an increase in metabolic needs of synovial tissue, may initiate tissue injury via anoxia and acidosis, resulting in hydrolytic enzyme release, increased vascular permeability and acceleration of inflammatory processes. It is further believed that the microcirculatory abnormality may be generalized, accounting for the systemic manifestations often seen in RA. Factors effecting arteriolar blood flow obstruction are reviewed to identify areas for future investigation in RA and other disorders involving microvasculopathy. The multitude of longknown and newly recognized factors predisposing to vasospasm and vasodilatation have been outlined as a guide to possible mechanisms which may be operative in RA. An attempt has been made to gather and synthesize the available data in the hope that it may stimulate other investigators to pursue more definitive research into specific areas which may show early microvascular abnormalities in the pathophysiology of RA. Identification of factors operative early in the pathogenesis of RA, before it becomes self-perpetuating, may well be a step in the direction of preventing the ravages of this disease, or providing insight to more effective control.

Acute and chronic digitalis toxicity: presentation and treatment

Although digitalis toxicity is commonly encountered, it can easily be mistaken because of its many manifestations. Recognition of toxicity depends on an awareness of the difference between an acute digitalis overdose and chronic digitalis toxicity. The gastrointestinal, central nervous, and cardiac conduction systems are most commonly involved. Ventricular ectopy and arrhythmias are common with chronic toxicity, while disturbances of sinus impulse formation, A-V blocks, and nodal arrhythmias are seen with both acute and chronic digitalis toxicity. Treatment is mainly supportive, except in acute poisoning, in which case attempts must be made to clear as much drug as possible from the gastrointestinal tract.

Digitalis-associated cardiac mortality after myocardial infarction

The effect of digitalis therapy on 4-month posthospital cardiac mortality was investigated in 812 patients who survived the hospital phase of acute myocardial infraction. A stepwise multiple logistic regression analysis was used to identify variables associated with increased mortality and to adjust for differences in confounding variables between digitalis and nondigitalis patients. The major 4-month mortality (10 of 26 patients [38.5%]) occurred in digitalis-treated patients with congestive heart failure in the coronary care unit and complex ventricular premature depolarizations (VPDs) on the predischarge Holter recording. Logistic analyses that controlled for confounding variables indicated that digitalis use contributed to the increased mortality rate in this high-risk subset. The predicted mortality difference due to digitalis in patients with congestive heart failure and complex VPDs, adjusted for relevant nondigitalis risk factor variables, was 30% (90% confidence interval 18-42%). This retrospective study suggests that digitalis use increases the early posthospital mortality of myocardial infarction patients with combined electrical and mechanical dysfunction.

Digoxin disposition in obesity: clinical pharmacokinetic investigation

Digoxin pharmacokinetics were studied in 16 obese (mean +/- SD weight, 100.2 +/- 36.8 kg) and 13 control (64.6 +/- 10.5 kg) subjects. All subjects had normal renal function and no other coexisting disease. After administration of 0.75 mg digoxin by intravenous infusion, multiple plasma samples obtained over the 96 hours following infusion were analyzed for digoxin concentration by radioimmunoassay. Pharmacokinetic parameters were determined by weighted iterative nonlinear least squares regression analysis. Elimination half-life (t 1/2) was not different between obese and control groups (35.6 +/- 10.5 vs 41.2 +/- 16.7 hours). Absolute volume of distribution (Vd) also was not different (981 +/- 301 vs 937 +/- 397 liters), nor was total clearance of digoxin (328 +/- 82 vs 278 +/- 87 ml/min). Elimination t 1/2 was significantly negatively correlated with clearance among all subjects (r = -0.46; p less than 0.01). Using percent ideal body weight (IBW) as a measure of obesity, no correlation was found between percent IBW and Vd (r = 0.03). Thus digoxin is similarly distributed into IBW in obese and normal weight subjects, and there is no significant distribution of digoxin into excess body weight over IBV. In addition, there is no difference in total metabolic clearance or elimination half-life between obese and control subjects. Digoxin loading and maintenance dosage should be calculated on the basis of IBW, which reflects lean body mass, rather than TBW, which reflects adipose tissue weight in addition to lean body mass.

Effects of rapid digitalization on total and regional myocardial performance in patients with coronary artery disease

In order to evaluate the effects of rapid digitalization on LV volumes, ejection fraction, and asynergy, 21 patients without heart failure were studied with a combination of hemodynamic and angiographic techniques before and after administration of intravenous ouabain (0.007 mg./Kg.). Seven patients had no CAD and served as normal (control) subjects (Group I), while 14 patients had extensive coronary disease (Group II). All pre-ouabain parameters were within the normal limits in Group I. After ouabain infusion, all indices of LV contractility: dP/dt, VCF, and ejection fraction rose significantly in the normal group, while LV filling pressure and end-diastolic volume remained unchanged. The baseline hemodynamic and volumetric values for Group II patients corresponded closely to their normal (Group I) counterparts, and exhibited similar changes after ouabain administration. Eight patients in Group II also had regional disorders of LV contractility, delineated by 23 abnormal hemiaxes of shortening. After ouabain, 15 out of 23 asynergic segments (65 per cent) improved, seven remained unchanged, and one worsened. It is therefore concluded that rapid digitalization not only enhances LV performance in normal subjects and in patients with CAD, but can also markedly reduce the extent of LV asynergy.

Reversal of advanced digitoxin toxicity and modification of pharmacokinetics by specific antibodies and Fab fragments

The effects of Fab fragments of high-affinity specific antibodies have been studied in a canine experimental model of lethal digitoxin toxicity. Selected antiserum from sheep immunized and boosted with a digoxin-serum albumin conjugate contained antibodies that cross-reacted with digitoxin with an average intrinsic association constant of 1.4 x 10(10) M(-1) as determined by equilibrium dialysis. Rapid second-order association kinetics (k(f) = 3.7 x 10(6) M(-1) per s) and slow dissociation kinetics (k(r) = 1.9 x 10(-4) per s) were documented for the antibody-digitoxin complex. Eight dogs given 0.5 mg/kg digitoxin intravenously developed ventricular tachycardia after 23+/-4 (SEM) min. Control nonspecific Fab fragments were then given. All animals died an average of 101+/-36 min after digitoxin administration. Another eight dogs given the same digitoxin dose similarly developed ventricular tachycardia after 28+/-3 min. This group then received a molar equivalent dose of specific Fab fragments intravenously over 3 min, followed by a 30-min infusion of one-third of the initial dose. All dogs survived. Conducted sinus beats reappeared 18+/-4 min after initial Fab infusion, and stable normal sinus rhythm was present at 54+/-16 min. Plasma total digitoxin concentrations increased threefold during the hour after initial Fab infusion, while plasma free digitoxin concentration decreased to less than 0.1 ng/ml. Effects on digitoxin pharmacokinetics of these Fab fragments and the antibody population from which they were derived were further investigated in a primate species. Unlike common laboratory animals previously studied, the rhesus monkey was found to have a prolonged elimination half-life, estimated at 135 and 118 h by radioimmunoassay and [(3)H]digitoxin measurements, respectively, similar to man and thus providing a clinically relevant experimental model. Intravenous administration of 2 mol of specific Fab fragments per mole of digitoxin 6 h after 0.2 mg of digitoxin produced a rapid 4.3-fold increase in plasma total digitoxin concentration followed by a rapid fall (t((1/2)) 4 h) accompanied by a 14-fold enhancement of urinary digitoxin excretion over control values during the 6-h period after Fab was given. Analytical studies were consistent with increased excretion of native digitoxin rather than metabolites, and the glycoside was found in equilibrium dialysis studies to be excreted in the urine in Fab-bound form. Administration of 2 mol of specific antibody binding sites per mole of digitoxin as intact IgG caused a greater and more prolonged increase in plasma total digitoxin concentration, peaking 13-fold above control levels. In contrast to the effects of Fab, however, specific IgG reduced the rate of urinary digitoxin excretion substantially below control values. We conclude that Fab fragments of antibodies with high affinity for digitoxin are capable of rapid reversal of advanced, otherwise lethal digitoxin toxicity, and are capable of reducing the plasma half-life and accelerating urinary excretion of digitoxin.

Noninvasive assessment of left ventricular performance in patients with chronic obstructive pulmonary disease

In patients with chronic obstructive pulmonary disease (COPD), the clinical differentiation between dyspnea due to left ventricular dysfunction and that due to pulmonary events is difficult. Invasive techniques have been the only reliable diagnostic approach. To assess the potential value of noninvasive techniques in this context, 27 patients with COPD and with clinically suspected left ventricular dysfunction were studied by echocardiography, radionuclide angiography, and right cardiac catheterization. In 20 (74%), adequate echocardiogram were obtained. Of these 20 patients, 17 had normal pulmonary arterial wedge pressures at rest and during submaximal handgrip exercise. Sixteen of these 17 had normal left ventricular performance by all three echocardiographic criteria used; in one patient, two criteria were not interpretable, but the third was normal. Results of radionuclide studies were normal in 15 patients, borderline in one, and not measurable in one. Of the three patients with abnormal wedge pressures, at least one echocardiographic criterion was abnormal in all. Radionuclide data were abnormal in two and not measurable in one. We conclude that left ventricular dysfunction is infrequently present in patients with COPD in whom such dysfunction is clinically suspected, that the two noninvasive techniques described here can be applied successfully to a high percentage of patients with COPD, and that the agreement among echocardiographic, radionuclide, and wedge pressure data is excellent.

A comparative study of the homogeneous enzyme immunoassay (EMIT) and two radioimmunoassays (RIA's) for digoxin

Our experience with the determination of digoxin in plasma or serum using a homogeneous enzyme immunoassay technique (EMIT) is reported. The day-to-day precision of the EMIT digoxin assay was investigated with different series of calibrators. Coefficients of variation varied from 10 to 25 percent in the range of 0.65--7.0 ng digoxin/ml. The accuracy was established by determining the mean recovery (96 percen) of spiked serum samples (0.0--6.0 ng digoxin/ml). The cross reactivity of structure related compounds: digitoxin, spironolactone (Aldactone) and prednisone were investigated. Amniotic fluid, umbilical cord serum and serum of pregnant patients were examined for false positive reaction. Serum samples of 111 patients from two hospitals, who were treated with digoxin, were analysed by EMIT and 3H-radioimmunoassay (RIA); 38 of these samples were also determined by 125I-RIA. A good correlation was found between EMIT assay and these techniques (r =0.90 and 0.91, respectively).

Effects of digitalis on resting and isometric exercise myocardial perfusion in patients with coronary artery disease and left ventricular dysfunction

Digitalis has been shown to improve the impaired ventricular function associated with coronary artery disease as well as to increase myocardial oxygen consumption and produce coronary vasoconstriction. To elucidate the net result of these contrasting effects, six patients with coronary artery disease and left ventricular ejection fractions less than 0.50 had 1.0 mCi thallium-201 injected intravenously at rest and during three minutes of 33% of maximal handgrip, off and on 0.25 mg daily maintenance digoxin. Thallium-201 scintigram images were taken 30 minutes later and were computer processed with orthogonal linearly interpolated background subtraction and maximal count density equalization. Processed images were visually graded on a 0, 1, or 2 scale for 18 sectors--nine from the AP projections and nine from the 40 degrees left anterior oblique projections. A score resulting from the summation of the 18 sector grades was made for each study, the maximum score being 36. Off digitalis, patients performing handgrip exercise decreased their scintigram scores from 25.7 +/- 1.5 (mean +/- SEM) to 23.0 +/- 1.0, P less than 0.05. When patients were on maintenance digoxin, scores did not change significantly during handgrip exercise. Post exercise scores were significantly higher on digoxin than off (P less than 0.05), whereas, resting scores were unaffected by digoxin. These data suggest that myocardial perfusion, as measured by thallium-201 uptake, is improved in patients on digitalis who have coronary artery disease and left ventricular dysfunction.

Paradoxical behavior of serum digoxin concentrations in an anuric neonate

Serum digoxin values were determined in a newborn infant with severe heart failure and renal failure. The half-life of digoxin in the serum appeared to change, possibly the result of prolonged distribution and/or absorption owing to circulatory insufficiency, or to the accumulation of cross-reacting metabolites of digoxin in the serum. No clinical toxicity was apparent, and no cardiac arrhythmia was observed. The need for monitoring serum digoxin concentration and clinical effect in newborn infants is emphasized.

Comparative effects of lidocaine and procainamide on acutely impaired hemodynamics

Controversy exists regarding the relative safety of intravenously administered lidocaine and procainamide to patients with acutely impaired hemodynamics. Accordingly, their effects were studied in 15 such patients, 14 with acute myocardial infarction and one with cardiomyopathy and severe congestive heart failure. All had elevated levels of pulmonary capillary wedge pressure (greater than 15 mm Hg) and/or low cardiac index (less than 2.5 liters/min/m2). Patients were given lidocaine, a 100 mg bolus followed by a 3 mg/min infusion and, after at least a 30 minute recovery period, procainamide, a 100 mg bolus over 2 minutes followed by a 20 mg/min infusion for 20 to 25 minutes. Hemodynamic measurements were compared early and late in the infusion of each drug. Small, clinically insignificant differences were observed in the hemodynamic responses to the drugs, and no clinically significant deterioration occurred with either. Conventional therapeutic doses of intravenous procainamide can be administered by this regimen, to patients with acute myocardial infarction complicated by cardiac failure or low cardiac output, without producing deleterious hemodynamic effects.

The effect of intravenous digoxin on the occurrence of ventricular tachyarrhythmias in acute myocardial infarction in man

Patients with acute myocardial infarction were allocated to two groups according to a double blind-system of radomization. The patients (n = 18) in one of the groups received digoxin intravenously as an injection of 0.01 mg. per kilogram of body weight during 10 minutes. The patients in the other group (n = 15) received saline and served as controls. A continuous ECG record was obtained from each patient during 1 hour preceding the administration of digoxin or saline and was continued for 3 hours following the injection. No antiarrhythmic treatment was given during the time of the study. Based on the continuous ECG, calculations were made of the relative incidence of patients with different types of ventricular tachyarrhythmias during the period of observation as well as the percentage of arrhythmia-containing 1 minute intervals observed during this period. There was no statistical difference between the incidence of ventricular tachyarrhythmias in the two groups in the 1 hour period preceding drug injection. The administration of digoxin and saline did not change the incidence of ventricular tachyarrhythmias and there was also no statistically significant difference between the two groups as regards the incidence of patients showing different types of ventricular tachyarrhythmias during the 3 hour period following drug administration, Considering the 1-minute intervals, those without any ventricular premature contractions were less in the digoxin group (92 per cent) than in the saline group (88 per cent; p less than 0.001). Serum levels of digoxin at the end of the observation period were well above what is considered the minimum therapeutic level and in three patients the level approached or reached the toxic range. In these three patients there was still no increased incidence of ventricular tachyarrhythmias. It is concluded that patients with acute myocardial infarction complicated by incipient left ventricular failure do not show an increased sensitivity to an ordinary dose of digoxin as measured by the occurrence of ventricular tachyarrhythmia.

Effects of ischemia and coronary reperfusion on myocardial digoxin uptake

The effects of coronary reperfusion on the uptake of digoxin by ischemic myocardium were studied in 17 open chest dogs undergoing anterior wall infarction produced by snaring confluent branches of the left coronary arterial system. Epicardial electrograms delineated ischemic, border and nonischemic zones. The hearts were reperfused by snare release after 1, 2 and 6 hours of occlusion. After 15 minutes of reperfusion, 1.0 mg of tritiated digoxin (3H-digoxin) was given intravenously, and 2 hours later the hearts were excised and endocardial and epicardial samples from each zone were analyzed for 3H-digoxin concentration. In another group of eight dogs regional myocardial blood flow was assessed utilizing 15 mu of radio-labeled microspheres administered during occlusion and reperfusion. In five dogs with 1 hour of coronary occlusion and subsequent reperfusion, 3H-digoxin uptake was comparable in endocardial and epicardial layers of all three zones. In six dogs undergoing reperfusion after 2 hours of occlusion, mean 3H-digoxin concentration was significantly (P less than 0.001) reduced from the mean nonischemic concentration, by 54 percent in endocardial and 35 percent in epicardial layers of the ischemic zone. Border zone endocardial and epicardial 3H-digoxin uptake was reduced by 21 percent and 16 percent, respectively (P less than 0.05). In six dogs undergoing reperfusion after 6 hours of occlusion, 3H-digoxin uptake in the ischemic zone was significantly (P less than 0.001) reduced by 85 percent in endocardial and 60 percent in epicardial layers from the concentration in the nonischemic zone. Border zone uptake was decreased by 54 percent in endocardial and 36 percent in epicardial regions (P less than 0.01). These alterations of in vivo digoxin binding could not be explained by impaired reflow of blood to ischemic myocardium. We conclude that coronary reperfusion after 2 to 6 hours of occlusion is associated with a marked reduction in myocardial digoxin uptake, which is more pronounced in subendocardial than in subepicardial regions of ischemic tissue.

Digitalis pharmacokinetics and metabolism

The pharmacokinetics of the cardiac glycofides have been elucidated as a result of the development of assays of sufficient sensitivity to measure the concentration of digitalis compounds in biological fluids. Digoxin can accumulate in the body without the administration of a loading dose, and a steady state blood concentration will be reached in 5 to 7 days. Digitoxin requires 35 days to accumulate to a plateau. If a loading dose of digoxin is used, it should be approximately three times the estimated daily maintenance dose. Factors that determine the selection of the appropriate maintenance dose of digoxin include renal function and lean body mass. Digitoxin is less dependent on renal function for its elimination than is digoxin. Knowledge of the pharmacokinetics of digitalis preparations is useful in determining how to change from one cardiac glycoside to another, each with different half-lives. One should wait 3 days before starting digoxin therapy when changing from maintenance digitoxin to digoxin (assuming normal renal function). The pharmacokinetics of changing from ouabain to digoxin without loss of digitalis effect are described. The metabolism of the commonly used digitalis preparations are summarized.