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| __NOTOC__
| | #REDIRECT [[Digoxin#Pharmacology]] |
| {{Digoxin}}
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| {{CMG}}; {{AE}} {{AK}}
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| '''''For patient information, click <u>[[Digoxin (patient information)|here]]'''''</u>.
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| ==12 CLINICAL PHARMACOLOGY==
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| ===12.1 Mechanism of Action===
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| All of digoxin’s actions are mediated through its effects on Na-K ATPase. This enzyme, the “sodium pump,” is responsible for maintaining the intracellular milieu throughout the body by moving sodium ions out of and potassium ions into cells. By inhibiting Na-K ATPase, digoxin
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| causes increased availability of intracellular calcium in the myocardium and conduction system, with consequent increased inotropy, increased automaticity, and reduced conduction velocity
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| indirectly causes parasympathetic stimulation of the autonomic nervous system, with consequent effects on the [[sino-atrial node|sino-atrial]] (SA) and [[atrioventricular (AV) nodes]]
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| reduces [[catecholamine ]]reuptake at nerve terminals, rendering blood vessels more sensitive to endogenous or exogenous catecholamines
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| increases [[baroreceptor ]]sensitization, with consequent increased carotid sinus nerve activity and enhanced sympathetic withdrawal for any given increment in mean arterial pressure
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| increases (at higher concentrations) [[sympathetic ]]outflow from the central nervous system (CNS) to both cardiac and peripheral sympathetic nerves
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| allows (at higher concentrations) progressive efflux of intracellular [[potassium]], with consequent increase in serum potassium levels.
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| The cardiologic consequences of these direct and indirect effects are an increase in the force and velocity of myocardial systolic contraction (positive inotropic action), a slowing of the heart rate (negative chronotropic effect), decreased conduction velocity through the AV node, and a decrease in the degree of activation of the sympathetic nervous system and renin-angiotensin system (neurohormonal deactivating effect).
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| ===12.2 Pharmacodynamics===
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| The times to onset of pharmacologic effect and to peak effect of preparations of digoxin are shown in Table 7.
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| [[image:diginj10.png]]
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| '''Hemodynamic Effects:''' Short- and long-term therapy with the drug increases cardiac output and lowers pulmonary artery pressure, [[pulmonary capillary wedge pressure]], and systemic vascular resistance in patients with heart failure. These hemodynamic effects are accompanied by an increase in the left ventricular [[ejection fraction ]]and a decrease in end-systolic and end-diastolic dimensions.
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| '''ECG Changes:''' The use of therapeutic doses of digoxin may cause prolongation of the[[ PR interval]] and depression of the[[ ST segmen]]t on the electrocardiogram. Digoxin may produce false positive ST-T changes on the electrocardiogram during exercise testing. These electrophysiologic effects are not indicative of toxicity. Digoxin does not significantly reduce heart rate during exercise.
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| ===12.3 Pharmacokinetics===
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| Note: the following data are from studies performed in adults, unless otherwise stated.
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| Comparisons of the systemic availability and equivalent doses for oral preparations of digoxin are shown in Table 6 [see [[DIGOXIN injection dosage and administration|Dosage and Administration]] (2.6)].
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| '''Distribution:''' Following drug administration, a 6 to 8 hour tissue distribution phase is observed. This is followed by a much more gradual decline in the serum concentration of the drug, which is dependent on the elimination of digoxin from the body. The peak height and slope of the early portion (absorption/distribution phases) of the serum concentration-time curve are dependent upon the route of administration and the absorption characteristics of the formulation. Clinical evidence indicates that the early high serum concentrations do not reflect the concentration of digoxin at its site of action, but that with chronic use, the steady-state post-distribution serum concentrations are in equilibrium with tissue concentrations and correlate with pharmacologic effects. In individual patients, these post-distribution serum concentrations may be useful in evaluating therapeutic and toxic effects [see[[DIGOXIN injection dosage and administration|Dosage and Administration]] (2.1)].
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| Digoxin is concentrated in tissues and therefore has a large apparent volume of distribution (approximately 475 to 500 L). Digoxin crosses both the blood-brain barrier and the placenta. At delivery, the serum digoxin concentration in the newborn is similar to the serum concentration in the mother. Approximately 25% of digoxin in the plasma is bound to protein. Serum digoxin concentrations are not significantly altered by large changes in fat tissue weight, so that its distribution space correlates best with lean (i.e., ideal) body weight, not total body weight.
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| '''Metabolism''': Only a small percentage (13%) of a dose of digoxin is metabolized in healthy volunteers. The urinary metabolites, which include dihydrodigoxin, digoxigenin bisdigitoxoside, and their glucuronide and sulfate conjugates are polar in nature and are postulated to be formed via hydrolysis, oxidation, and conjugation. The metabolism of digoxin is not dependent upon the cytochrome P-450 system, and digoxin is not known to induce or inhibit the[[ cytochrome P-450]] system.
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| '''Excretion:''' Elimination of digoxin follows first-order kinetics (that is, the quantity of digoxin eliminated at any time is proportional to the total body content). Following intravenous administration to healthy volunteers, 50 to 70% of a digoxin dose is excreted unchanged in the urine. Renal excretion of digoxin is proportional to creatinine clearance and is largely independent of urine flow. In healthy volunteers with normal renal function, digoxin has a half-life of 1.5 to 2 days. The half-life in anuric patients is prolonged to 3.5 to 5 days. Digoxin is not effectively removed from the body by dialysis, exchange transfusion, or during cardiopulmonary bypass because most of the drug is bound to extravascular tissues.
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| '''Special Populations:'''Geriatrics: Because of age-related declines in renal function, elderly patients would be expected to eliminate digoxin more slowly than younger subjects. Elderly patients may also exhibit a lower volume of distribution of digoxin due to age-related loss of lean muscle mass. Thus, the dosage of digoxin should be carefully selected and monitored in elderly patients [see Use in [[DIGOXIN injection specific populations|Specific Populations]] (8.5)].
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| '''Gender:''' In a study of 184 patients, the clearance of digoxin was 12% lower in female than in male patients. This difference is not likely to be clinically important.
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| '''Hepatic Impairment:''' Because only a small percentage (approximately 13%) of a dose of digoxin undergoes metabolism, hepatic impairment would not be expected to significantly alter the pharmacokinetics of digoxin. In a small study, plasma digoxin concentration profiles in patients with acute hepatitis generally fell within the range of profiles in a group of healthy subjects. No dosage adjustments are recommended for patients with hepatic impairment; however, serum digoxin concentrations should be used, as appropriate, to help guide dosing in these patients.
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| '''Renal Impairment:''' Since the clearance of digoxin correlates with creatinine clearance, patients with renal impairment generally demonstrate prolonged digoxin elimination half-lives and greater exposures to digoxin. Therefore, titrate carefully in these patients based on clinical response and based on monitoring of serum digoxin concentrations, as appropriate.
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| '''Race:''' The impact of race differences on digoxin pharmacokinetics has not been formally studied. Because digoxin is primarily eliminated as unchanged drug via the kidney and because there are no important differences in creatinine clearance among races, pharmacokinetic differences due to race are not expected.
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| ====Drug-drug Interactions====
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| Based on literature reports no significant changes in digoxin exposure were reported when IV digoxin was co-administered with the following drugs:
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| [[clarithromycin]], [[carvedilol]], [[isradipine]], [[losartan ]]and [[rifampin]]
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| ==References==
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| {{Reflist|2}}
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| http://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=58f45aba-ff6f-43cc-bb88-be40a9f7beda
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| [[Category:Cardiovascular Drugs]]
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| [[Category:Drugs]]
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