class III (potassium channel blockers)
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Introduction
Potassium channel blockers are class III drugs. They work by blocking the potassium channels that facilitate potassium transport and mediate repolarization of the cardiac cells. They extend the action potential and slow repolarization. All antidysrhythmic drugs that block potassium channels are proarrhythmic. They lead to a prolonged QT interval, which increases the risk for torsade de pointes. Because of this, all drugs within this class require ECG monitoring for QT interval assessment. The nurse must also be vigilant to consider concomitant drugs that increase the QT interval because concomitant use can have additive effects. Although all drugs within this class block potassium channels, many have additional antidysrhythmic mechanisms.
Illustrated mechanism of action
Potassium channel blockers are considered a class III antiarrhythmic drug. They work by preventing the movement of potassium ions into the heart and other tissues.
Potassium channels are present in all cells of the body. They let potassium ions pass through cell membranes and are involved in many body functions, such as hormone secretion and cardiac activity control.
Medications that block these potassium channels can have a range of effects on the body, depending on the types of cells involved.
In the heart, potassium channel blockers work to slow down the heart’s electrical activity, preventing dangerous arrhythmias.
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Examples of Medication Brand Names
Amiodarone
Sotalol
Dofetilide Capsules
Bretylium Tosylate
Ibutilide Fumarate
Indication
Potassium channel blockers are class III drugs. They work by blocking the potassium channels that facilitate potassium transport and mediate repolarization of the cardiac cells. They extend the action potential and slow repolarization. All antidysrhythmic drugs that block potassium channels are proarrhythmic. They lead to a prolonged QT interval, which increases the risk for torsade de pointes. Because of this, all drugs within this class require ECG monitoring for QT interval assessment. The nurse must also be vigilant to consider concomitant drugs that increase the QT interval because concomitant use can have additive effects. Although all drugs within this class block potassium channels, many have additional antidysrhythmic mechanisms.
Side Effects
Class III antiarrhythmic drugs are a group of medications that can prolong action potentials in the heart. They achieve this by blocking potassium channels or enhancing inward currents from sodium channels. However, these drugs have a unique property of "reverse use-dependence," which is most pronounced at slower heart rates and can lead to torsades de pointes—a specific type of arrhythmia. However, it is essential to note that excessive QT interval prolongation—a measure of the heart's electrical activity—is not always the best predictor of drug-induced torsades de pointes. Other factors, such as action potential stability, triangulation, reverse use dependence, and repolarization time dispersion, can also influence the occurrence of this condition.
Amiodarone is a well-known class III drug structurally similar to thyroxine and is used for severe ventricular and supraventricular arrhythmias. It blocks potassium channels (IKr and IKs), prolonging the heart rate. Amiodarone has side effects such as bradycardia, pulmonary toxicity, liver damage, skin discoloration, corneal microdeposits, visual disturbances, and thyroid dysfunction.
Precautions & Contraindications
All class III drugs are proarrhythmic and can cause QT prolongation, leading to torsade de pointes. Aside from that, the adverse effect profile depends on the agent being used.
Amiodarone has many potential toxicities and adverse effects, which include:
Optic neuropathy and/or optic neuritis, which can lead to permanent blindness; corneal microdeposits that may be reversible
Photosensitivity resulting in a bluish-gray discoloration of the skin
New arrhythmias, including torsade de pointes
Hyperthyroidism or hypothyroidism
Hepatotoxicity
Acute and chronic pulmonary toxicity, including acute respiratory distress syndrome and pulmonary fibrosis
As with all beta-adrenergic blockers and calcium channel blockers, amiodarone can cause bradycardia, hypotension, and fatigue. Amiodarone carries multiple boxed warnings from the FDA related to arrhythmias, pulmonary toxicity, and hepatotoxicity. It is on the Beers Criteria® list of high-risk medications for older adults because of its potential for toxicity (American Geriatrics Society, 2019). Amiodarone should not be used in clients with bradycardia at baseline unless they have a pacemaker. Amiodarone has iodine within its chemical structure; thus, it is contraindicated in clients with iodine hypersensitivity.
Dronedarone has many of the same adverse effects as amiodarone, including heart failure, hepatotoxicity, QT prolongation/torsade de pointes, and pulmonary toxicity. Because of a particularly high risk for adverse reactions, including death, it is contraindicated in clients with heart failure and those in atrial fibrillation who cannot be converted to sinus rhythm; these contraindications are listed as boxed warnings from the FDA. Dronedarone is contraindicated in pregnant clients due to the potential for teratogenicity.
Dofetilide and sotalol are contraindicated in clients with severe kidney disease.
Sotalol, like all beta-adrenergic blockers, can cause bradycardia, hypotension, and fatigue and should be used cautiously in clients with asthma and heart failure.
Monographs
Mohamed Ibrahim .pdfBretylium Drug
Shahd Pharma (1) (1).pdfAmiodarone Drug
Drug monograph (2)... (7).pdfDofetilide Drug
Drug monograph elatarrr (1).pdfVernakalant Drug
Drug Dronedarone.pdfDronedarone Drug
Introduction
Illustrated mechanism of action
Examples of Medication Brand Names (with Images)
Indication
Side Effects
Precautions & Contraindications
Monographs
Reference
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