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Pharmacology · chapter 17 · Antidysrhythmic Drugs

Class III: Potassium Channel Blockers

The 14 things this section of the textbook says that you are most likely to be asked about — quoted word for word, not retold. Then 6 practice questions with the reasoning.

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Key points

  1. What potassium channel blockers do

    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.

  2. All are proarrhythmic: watch the QT

    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.

  3. Other QT-prolonging drugs add up

    The nurse must also be vigilant to consider concomitant drugs that increase the QT interval because concomitant use can have additive effects.

  4. Amiodarone lasts for months

    The nurse should be aware that even after amiodarone administration is discontinued, its effects will be apparent for several weeks to months.

  5. Amiodarone raises digoxin levels

    Therefore, it is recommended that digoxin doses be decreased (with careful monitoring) or discontinued altogether if amiodarone is initiated.

  6. Amiodarone and warfarin

    Warfarin: Concomitant administration of warfarin and amiodarone increases the anticoagulant effect of warfarin.

  7. Dronedarone is a hazardous drug

    Because of the potential for teratogenicity, dronedarone is also classified as a hazardous drug; nurses should wear gloves when handling or administering it and follow institutional policies for hazardous drugs (Institute for Safe Medication Practices, 2018).

  8. Starting dofetilide

    Therefore, dofetilide must be initiated in a health care setting capable of providing cardiac resuscitation services and continuous ECG monitoring for a minimum of 3 days.

  9. Electrolytes before dofetilide

    Before starting dofetilide, clients should have electrolyte levels checked (potassium, magnesium), and electrolyte supplements should be administered to maintain adequate levels. Renal function must also be monitored because dofetilide is excreted by the kidneys.

  10. Watching the client after ibutilide

    It requires continuous ECG monitoring and observation for at least 4 hours after administration.

  11. Sotalol and baseline QTc

    If the client’s baseline QTc interval is longer than 450 msec, sotalol should not be initiated.

  12. Amiodarone contraindications

    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.

  13. Dronedarone in pregnancy

    Dronedarone is contraindicated in pregnant clients due to the potential for teratogenicity.

  14. Torsade symptoms need emergency care

    Seek emergency medical attention if they experience symptoms of torsade de pointes (dizziness, palpitations, lightheadedness, syncope).

Terms to know

QTc
This correction is referred to as the QTc and is usually used in drug titration and safety algorithms.
cytochrome P450
A key component of this system is an enzyme complex known as cytochrome P450 or CYP450.
Dofetilide and ibutilide
Dofetilide and ibutilide are both pure class III antidysrhythmic medications with no additional mechanisms of action.

Practice questions

Stuck on select-all-that-apply? How to take them one option at a time.

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Every question here, with the answer

  1. A client on warfarin is started on amiodarone. Which laboratory value should the nurse expect to be monitored closely?

    1. Prothrombin time
    2. Serum calcium
    3. Blood glucose
    4. Serum sodium

    Answer: Prothrombin time

    Amiodarone increases the anticoagulant effect of warfarin, so the warfarin dose is lowered and prothrombin time is monitored. The other values are not linked to this interaction in the section.

  2. A nurse is teaching a client who is going home on amiodarone. Which instructions should the nurse include? Select all that apply.

    1. Avoid grapefruit and grapefruit juice.
    2. Avoid direct sunlight and wear sunscreen and protective clothing.
    3. The drug leaves the body within a day if you stop it.
    4. Report heat sensitivity, weight loss, fatigue or cold sensitivity.
    5. Report any vision changes or breathing difficulties.

    Answer: Avoid grapefruit and grapefruit juice., Avoid direct sunlight and wear sunscreen and protective clothing., Report heat sensitivity, weight loss, fatigue or cold sensitivity., Report any vision changes or breathing difficulties.

    Amiodarone causes photosensitivity, eye and lung toxicity, and thyroid problems, and grapefruit should be avoided. Its half-life is longer than a month, so its effects last weeks to months after stopping.

  3. Before a client's first dose of dofetilide, which action by the nurse is most important?

    1. Give the dose with grapefruit juice.
    2. Check the serum cholesterol.
    3. Check potassium and magnesium levels.
    4. Arrange discharge the same day.

    Answer: Check potassium and magnesium levels.

    Electrolytes are checked and corrected before starting dofetilide, and it must be started in a monitored setting for at least 3 days. Cholesterol is not relevant, and same-day discharge contradicts the required monitoring.

  4. A client is scheduled to start sotalol. The baseline ECG shows a QTc of 470 msec. What should the nurse do?

    1. Give the dose with food.
    2. Hold the drug and notify the health care provider.
    3. Give the first dose and recheck the ECG tomorrow.
    4. Give half the dose.

    Answer: Hold the drug and notify the health care provider.

    Sotalol should not be started when the baseline QTc is longer than 450 msec because it prolongs the QTc and can cause torsade de pointes. Any dose, full or reduced, carries that risk.

  5. A nurse is preparing a dose of dronedarone. Which precaution should the nurse take?

    1. Give it only with digoxin.
    2. No special handling is needed.
    3. Crush the tablet to speed absorption.
    4. Wear gloves when handling the drug.

    Answer: Wear gloves when handling the drug.

    Dronedarone is classified as a hazardous drug because of potential teratogenicity, so nurses wear gloves and follow hazardous-drug policies. The section does not support crushing it, and digoxin is an interacting drug, not a required partner.

  6. Which nursing actions apply to a client starting a class III antidysrhythmic drug? Select all that apply.

    1. Monitor the ECG including the QTc interval.
    2. Take a complete medication history for other QT-prolonging drugs.
    3. Be prepared to perform cardiac resuscitation.
    4. Stop ECG monitoring after the first dose.
    5. Monitor electrolyte levels.

    Answer: Monitor the ECG including the QTc interval., Take a complete medication history for other QT-prolonging drugs., Be prepared to perform cardiac resuscitation., Monitor electrolyte levels.

    Class III drugs prolong the QT interval and can cause torsade de pointes, so the nurse checks for interacting drugs, monitors ECG and electrolytes, and is ready to resuscitate. Stopping monitoring after one dose ignores the ongoing risk.

Where every quote comes from

Section 17.4 Class III: Potassium Channel Blockers of Pharmacology for Nurses by Tina Barbour-Taylor, Leah Mueller (Sabato), Donna Paris, Dorie Weaver, OpenStax, 2024. Read the whole section free at openstax.org.

The textbook is licensed under CC BY-NC-SA 4.0. This page quotes it word for word and adds the headings, the order and the practice questions; references to the book's figures and stray spaces left by its formatting are removed. The page is shared under the same licence. Nothing here is sold.