Cardiac Antidysrhythmic Drugs and Action Potential

• Antidysrhythmic drugs are used for the treatment and prevention of disturbances in cardiac rhythm

Action Potential

• Variation in distribution of ions leads to cardiac cell excitation
• There is a spread of electrical impulse across cardiac cells due to ion movement across the cell membrane
• The electrical impulse leads to muscle contraction

Cardiac Cell

• Resting cardiac cells have a net negative charge relative to the outside
• Resting Membrane Potential results from an uneven distribution of sodium, potassium and calcium ions
• The Sodium-Potassium ATPase pump maintains this difference

Electrical Activity of the Heart

• Sinoatrial (SA) node is the natural pacemaker
• The SA node inherent rate is 60-100 BPM
• AV node inherent rate is 40-60 BPM
• The bundle of His inherent rate is 40-60 BPM
• Left and right bundle branches inherent rate is 20-40 BPM
• Purkinje fibers inherent rate is 20-40 BPM

Electrocardiography (ECG or EKG)

• The P wave reflects atrial electricity
• The PR interval reflects atrial electricity
• The QRS complex reflects ventricular contraction and electricity
• There is also ST segment and a T wave on an EKG

Common Atrial Dysrhythmias

• Supraventricular Tachycardia present with a PR intervals that are too small
• Atrial Fibrillation presents with the absence of p-waves
• Atrial Flutter presents with a predictable pattern

Ventricular Dysrhythmias

• Include Ventricular Tachycardia
• Include Ventricular Fibrillation

Antidysrhythmic Drug Mechanisms of Action

• Prolong AV node
• Increase or decrease conduction speed
• Alter ectopic pacemakers and SA node
• Reduce myocardial excitability
• Simulate autonomic nervous system
• There are multiple different classes of medications

Inotropic, Chronotropic & Dromotropic

• Inotropic relates to force of contraction and it can be increased or decreased
• Chronotropic affects the rate of contractions
• Dromotropic affects electrical current

Vaughan Williams Classification

• It is a system for classifying antidysrhythmic drugs
• It is based on the electrophysiologic effect of particular drugs on action potential
• Class I drugs are: Sodium Channel Blockers, including Ia, Ib, and 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

Class la: Sodium Channel Blocker

• Drugs include: procainamide (Proneystl), Quinidine gluconate, disopyramide (Norpace)
• Mechanism of Action: Blocks sodium (fast) channels, and delays repolarization
• Used for: Atrial fibrillation, Atrial flutter, Ventricular Tachycardia, Supraventricular Tachycardia, and Wolff-Parkinson-White Syndrome
• Adverse Effects: Systemic Lupus Syndrome, Neutropenia/Thrombocytopenia, Cardiotoxicity, and Hypotension
• Contraindications: Pregnancy Risk Category C, Liver or renal disorders
• Medication/Food Interactions: Interacts with other Antidysrhythmics & Antihypertensives
• Do not crush or chew sustained-released medications during Nursing Administration

Class Ib: Sodium Channel Blocker

• Drugs include: lidocaine(Xylocaine), mexiletine (Mexitil), tocainide (Tonocard)
• Mechanism of Action: Decreases electrical conduction, and increases rate of repolarization
• Used for: Ventricular Dysrhythmias
• Adverse Effects: CNS effects like twitching, convulsions or respiratory depression
• Contraindications: Liver and renal dysfunction
• Medication/Food Interactions: Beta Blockers and Phenytoin increase serum levels of lidocaine
• Nursing Administration: IV administration begins with a weight based dose that is maintained
• Should be used for no more than 24 hours

Class Ic: Sodium Channel Blocker

• Drugs included propafenone (Rhythmol), flecainide (Tambocor)
• Mechanism of Action: deceases electrical conduction & excitability, and increases the rate of repolarization
• Used for: Supraventricular Tachycardia
• Adverse Effects: Bradycardia, Heart failure, Dizziness, and Weakness
• Contraindications: Severe heart failure, Severe hypotension
• Medication/Food Interactions: Digoxin, oral anticoagulants, and beta blockers
• Nursing Administration: Monitor ECG during treatment and administer with food - do not crush tablets, and watch for bradycardia and hypotension

Class II: Beta Blocker

• Drugs included propranolol hydrochloride (Inderal), esmolol hydrochloride (Brevibloc), and metoprolol (Lopressor)
• Mechanism of Action: Prevents sympathetic nervous system stimulation of the heart, decreases heart rate, and slows rate of conduction through the SA node
• Used for: Atrial fibrillation, Atrial flutter, and SVT
• Adverse Effects: Bradycardia, hypotension, and impotence can occur
• Contraindications: Severe heart failure and asthma
• Medication/Food Interactions: Verapamil and Diltiazem, and it can mask the hypoglycemic effect of insulin
• Nursing Administration: Take apical pulse daily and administer IV form slowly

Class III: Potassium Channel Blockers

• Drugs included amiodarone (Cordarone, Pacerone), sotalol (Betapace), dofetilide (Tikosyn)
• Mechanism of Action: Prolongs the action potential and refractory period of the cardiac cycle, decreases rate of repolarization, electrical conduction and contractility
• Used for: Atrial Fibrillation , and recurrent ventricular fibrillation/tachycardia
• Can cause Pulmonary fibrosis/stiffness
• Adverse Effects: Pulmonary toxicity, sinus bradycardia and visual disturbances
• Contraindications: AV block, bradycardia, and liver/thyroid/ respiratory dysfunction
• Medication/Food Interactions: Diltiazem, digoxin, and warfarin may interact
• Amiodarone is highly toxic.
• Nursing Administration: Discontinued medications have a risk for adverse effects and patients are provided information regarding toxicities

Class IV: Calcium Channel Blockers

• Drugs included verapamil (Calan), diltiazem (Cardizem)
• Mechanism of Action: Depresses depolarization and decreases myocardial oxygen demand, decreases force of contraction and heart rate, and slows rate of conduction through SA and AV Node
• Used for: Atrial Fibrillation/Aflutter or SVT
• Adverse Effects: Bradycardia, hypotension, and constipation
• Contraindications: SA/AV dysfunction and heart blocks
• Medication/Food Interactions: Beta-blockers and Digoxin
• Nursing Administration: May cause orthostatic hypotension, so change positions slowly and notify the provider for peripheral edema, or shortness of breath

Classification: Other Antidysrhythmics

• Some medications have properties of several classes and are not placed into one particular class
• Digoxin and Adenosine

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
• Has a long half life and duration of action
• High risk for toxicity that can lead to renal impairment
• Follow Digoxin levels to determine dose
• Adversely affects cardiac arrhythmias and Yellow halos
• The Antidote is Digoxin Immune Fab (Digibind)
• Inotropic effects: ↑ force of contraction
• Chronotropic effects: ↓ rate of contractions
• Dromotropic effects: ↓ electrical current

Unclassified Antidysrhythmic Adenosine (Adenocard)

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

Nursing Implications

• Thorough drug and medical history collection is important
• Measure baseline BP/P, I & O, and cardiac rhythm
• Measure serum potassium levels before initiating therapy
• During therapy, monitor cardiac rhythm, heart rate, BP, general well-being, skin color, temperature, heart and lung sounds
• Assess plasma drug levels as indicated
• Monitor for toxic side effects
• Instruct patients to take medications as scheduled and not to skip doses or double up for missed doses
• Instruct patients to contact their physician for instructions if a dose is missed
• Instruct patients not to crush or 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 full minute, and to notify their physician if the pulse is less than 60 beats/minute before taking the next dose
• Ensure that the patient knows to notify health care provider of any worsening of dysrhythmia or toxic effects
• This is due to changes in cardiac output
• Potential symptoms include: Shortness of breath, Edema, Dizziness, Syncope, Chest pain, Gl distress, Blurred Vision

Expected Outcomes

• Decreased BP in hypertensive patients
• Decreased edema
• Decreased fatigue
• Regular pulse rate
• Pulse rate without major irregularities
• Improved regularity of rhythm
• Improved cardiac output
• Goals should include target parameters for HR and BP

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