Antidysrhythmic Medications and Action Potential

• Antidysrhythmic medications are used to treat and prevent cardiac rhythm disturbances.
• A dysrhythmia is any deviation from the heart's normal rhythm.

Action Potential

• Cardiac muscle contraction results from the movement of ions across the cell membrane resulting in electrical impulses.
• It creates contraction in the atrium or ventricle.
• A change in ion distribution causes cardiac cells to become excited.
• The resting cardiac cell has a net negative charge relative to the outside.
• Uneven ion distribution (sodium, potassium, calcium) results in electronegative charge across the cell membrane
• Sodium-Potassium ATPase pump maintains this difference.

Electrical Activity of the Heart

• Sinoatrial (SA) node is the natural pacemaker.
• Sinoatrial node rate is 60-100 bpm
• AV node rate is 40-60 bpm
• Bundle of His rate is 40-60 bpm
• Left and Right Bundle Branches rate is 20-40 bpm
• Purkinje fibers rate is 20-40 bpm

Electrocardiography

• Key components include the P wave, PR internal, QRS complex, ST segment, and T wave

Electrical Depolarization/Repolarization

• Atrial depolarization is represented by the P wave.
• Ventricular depolarization occurs at the QRS complex.
• Ventricles are contracting during the QRS complex.
• Ventricular repolarization happens at the T wave.

Common Dysrhythmias

• Include Supraventricular Tachycardia, Atrial Fibrillation and Atrial Flutter

Ventricular Dysrhythmias

• Include Ventricular Tachycardia and Ventricular Fibrillation.

Antidysrhythmic Drugs

• Mechanisms of Action: prolong the AV node, increase/reduce conduction speed, alter ectopic pacemakers/SA node, reduce myocardial excitability, and simulate autonomic nervous system.
• Multiple medication classes exists.
• These include Sodium Channel Blockers, Beta-Adrenergics, Potassium Channel Blockers, Calcium Channel Blockers, and additional classes.
• Inotropic affects force of contraction; can be positive or negative
• Chronotropic relates to the rate of contractions.
• Dromotropic relates to electrical current.
• Increasing inotropic causes more force and is positive

Vaughan Williams Classification

• Commonly used to classify antidysrhythmic drugs based on electrophysiologic effects on action potential.
• Class I drugs are Sodium Channel Blockers (Ia, Ib, Ic)
• Class II drugs are Beta Blockers
• Class III drugs are Potassium Channel Blockers
• Class IV drugs are Calcium Channel Blockers
• Other drugs include Digoxin and Adenosine

Drugs Affecting the Cardiac Action Potential

• Class 1: Sodium Channel Blockers alter sodium influx
• 1a (moderate): Quinidine, Procainamide
• 1b (weak): Lidocaine, Phenytoin
• 1c (strong): Flecainide, Propafenone
• Class 2: Beta Blockers
• Propranolol
• Metoprolol
• Class 3: Potassium Channel Blockers
• Amiodarone
• Sotalol
• Class 4: Calcium Channel Blockers
• Verapamil
• Diltiazem

Vaughan Williams Class 1a: Sodium Channel Blocker

• Class Ia drugs include procainamide (Pronestyl), quinidine gluconate, and disopyramide (Norpace).
• They block sodium (fast) channels and delay repolarization.
• Used for Atrial fibrillation/flutter, Ventricular/Supraventricular Tachycardia, and Wolff-Parkinson-White Syndrome.
• Adverse effects of procainamide include Systemic Lupus Syndrome, neutropenia/thrombocytopenia, cardiotoxicity, and hypotension.
• Contraindicated in pregnancy (Category C) and liver/renal disorders.
• Interacts with antidysrhythmics and antihypertensives.
• Do not crush or chew sustained-release medications.

Vaughan Williams Class 1b: Sodium Channel Blockers

• Class Ib drugs (most common) include lidocaine (Xylocaine), mexiletine (Mexitil), and tocainide (Tonocard).
• These decrease electrical conduction and increase repolarization rate.
• Primarily used for ventricular dysrhythmias.
• Can cause CNS effects such as twitching, convulsions, and respiratory depression.
• Contraindicated in liver and renal dysfunction.
• Interacts with beta blockers and phenytoin, increasing lidocaine serum levels.
• IV administration usually begins with a weight-based dose and transitions to a maintenance dose; use for no more than 24 hours.

Vaughan Williams Class 1c: Sodium Channel Blockers

• Class Ic drugs include propafenone (Rhythmol) and flecainide (Tambocor).
• They decrease electrical conduction and excitability, while increasing the repolarization rate.
• Used for Supraventricular Tachycardia.
• Adverse effects include bradycardia, heart failure, dizziness, and weakness.
• Contraindicated in severe heart failure and hypotension.
• Interacts with digoxin, oral anticoagulants, and beta blockers.
• Closely monitor ECG, bradycardia, and hypotension.
• Take these medications with food.
• Do not crush.

Vaughan Williams Class II: Beta Blockers

• Beta Blockers will not work properly in asthma patients
• Class II drugs include propranolol hydrochloride (Inderal), esmolol hydrochloride (Brevibloc), and metoprolol (Lopressor).
• They prevent sympathetic nervous system stimulation of the heart, decrease heart rate, and slow conduction through the Sinoatrial (SA) node.
• Used for Atrial fibrillation, Atrial flutter, and Supraventricular Tachycardia (SVT).
• Can cause bradycardia, hypotension, and impotence.
• Contraindicated in cases of severe heart failure and asthma.
• Interacts with verapamil and diltiazem.
• Can mask the hypoglycemic effect of insulin.
• Take apical pulse daily and administer IV form slowly.

Vaughan Williams Class III: Potassium Channel Blockers

• Potassium Channel Blockers can cause Pulmonary Fibrosis
• Class III drugs include amiodarone (Cordarone, Pacerone), sotalol (Betapace), and dofetilide (Tikosyn).
• These prolong the action potential and refractory period, decrease repolarization/electrical conduction/contractility.
• Used for Atrial Fibrillation (oral route), recurrent ventricular fibrillation, and ventricular tachycardia.
• Adverse effects include pulmonary toxicity, sinus bradycardia, and visual disturbances.
• Contraindicated in AV block and bradycardia, liver/thyroid/respiratory dysfunction.
• Interacts with diltiazem, digoxin, and warfarin, as well as grapefruit juice.
• Administer carefully because amiodarone is highly toxic.
• Adverse effects may continue until the drug is discontinued.

Vaughan Williams Class IV: Calcium Channel Blockers

• Class IV drugs include verapamil (Calan) and diltiazem (Cardizem).
• Depress depolarization, decreasing myocardial oxygen demand, force of contraction and heart rate, and slow conduction through SA/AV nodes.
• Used for Atrial Fibrillation/Flutter and SVT.
• Adverse effects consist of bradycardia, hypotension, and constipation.
• Contraindicated in SA/AV dysfunction and heart blocks.
• Interacts with beta-blockers and digoxin.
• May cause orthostatic hypotension, and patients should change positions slowly.
• Notify the provider with peripheral edema or shortness of breath.

Vaughan Williams Classification: Other Antidysrhythmics

• These have properties of multiple classes and are not placed into any one particular class.
• Digoxin
• Adenosine used in emergent situations

Unclassified Antidysrhythmic: Digoxin (Lanoxin)

• Slows conduction between the SA & AV node.
• Used to slow the rate in atrial fibrillation/flutter
• Can be administered PO or IV
• Long half life and duration of action
• Narrow therapeutic index means it has a high risk for toxicity.
• Follow Digoxin levels to determine dose
• Adverse Effects: Cardiac arrhythmias, Yellow halos
• Antidote: Digoxin Immune Fab (Digibind)
• Low heart rate and seeing yellow halos
• Increases inotropic force of contraction
• Decreases chronotropic rate of contractions
• Effects dromotropic electrical current.

Unclassified Antidysrhythmic: Adenosine

• Slows and stops conduction through the AV node
• Used to convert paroxysmal supraventricular tachycardia to sinus rhythm.
• Has a very short half-life of less than 10 seconds.
• Only administered as fast IV push (6mg).
• May cause asystole for a few seconds.
• Other adverse effects are minimal.

Adenosine Administration

• This is a first dose

Nursing Implications

• Obtain a thorough drug and medical history.
• Measure baseline BP/P, I & O, and cardiac rhythm.
• Measure serum potassium levels before initiating therapy.
• Monitor cardiac rhythm, heart rate, BP, general well-being, skin color, temperature, heart and lung sounds during therapy.
• Assess plasma drug levels as indicated and monitor for toxic side effects.
• Instruct patients to take medications as scheduled and not to skip doses/double up for missed doses.
• Instruct patients to contact physician for instructions if a dose is missed.
• Instruct patients not to crush/chew oral sustained-release preparations.
• Administer IV infusions with an IV pump.
• Teach patients taking beta-blockers, digoxin, and other drugs how to take their own radial pulse for 1 minute, and notify their physician if pulse is less than 60 beats/minute before taking the next dose.
• Ensure patients know to notify health care provider of worsening dysrhythmia or toxic effects such as shortness of breath, edema, dizziness, syncope, chest pain, GI distress, and blurred vision.

Expected Outcomes

• Decreased BP in hypertensive patients, decreased edema, and decreased fatigue.
• Improved regular pulse rate/rhythm and cardiac output.
• Pulse rate without major irregularities.
• Goals should include target HR and BP.

Social Determininants of Health (SDOH)

• Older adults with HTN, heart failure, diabetes and thyroid disease are more at risk of developing dysrhythmias.
• Lidocaine - $5 for 20 doses
• Propranolol – 30 day supply $30-$80
• Digoxin - $50 for 100 tablets