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Cardiac cells have automaticity. They
are able to contract on their own or
in relation to nearby cells. They are
excited by the difference in charge in
and out of the cell caused by movement
of electrolytes like Na+, K+ and Ca++
across the cell membrane.
The difference in charge is called the resting membrane potential. The spread of electrical impulse is the action potential. Many antidysrhythmic drugs affect the RMP and/or action potential.
The top two pacers: the SA and AV nodes depend mostly on Ca++ channels to get excited. These are slower channels. The Purkinje fibers depend on Na+ channels to get excited. They are faster.
Review T 23-2 for common dysrhythmias. Vaughn Williams Classification Review T 23-3
We will look at individual classes based on where they work in the heart. Class Ia: membrane stabilizers: although works on fast Na+ channels, does help with atrial and ventricular dysrhythmias, plus WPW...drugs are procainamide (se!), quinidine
Class Ic: part of the membrane stabilizing class that affects atrial and ventricular dysrhthmias, and WPW...drugs are flecainide, propafenone
Class II: Beta Blockers, negative chronotropes, negative inotropes, negative dromotropes---delay signal through AV node too! The "ol" drugs.
Class III: increase the action potential duration. Given for VTach or VFib or for AFlutter or AFib that hasn't responded to other drugs. Common drugs: amiodarone (se!), and ibutilide (given just for Afib). Class Ib: also is membrane-stabilizing, the most common drug is lidocaine. Lidocaine only affects the ventricles so shouldn't be given for anything other. Class IV: calcium channel blockers, only given for dysrhthymias above the ventricles: PSVT, AFlutter or AFib. Common drugs are diltiazem (Cardizem) and verapamil (Calan). Others: digoxin, adenosine, and magnesium don't clearly fall in any of the Vaughn Williams' classes. Digoxin is a positive inotrope, negative chronotrope. Adenosine is a negative chronotrope and dromotrope. It is used to convert patients in PSVT to sinus rhythm. It is given as a true IV bolus. Magnesium is given for Torsades de Pointes (a worsening and less-organized form of ventricular tachycardia) Contraindications, Adverse Effects & Interractions As a rule, these drugs should slow things down so they will be contraindicated in those with slow heart rates and/or heart blocks. Common adverse reactions are h/a, dizziness and blurred vision. And know that any antidysrhythmic can CAUSE A NEW DYSRHYTHMIA! We particularly worry about drugs that extend the QT interval as this raises risk of Torsades. For this reason, many patients will be admitted for hospital observation when starting new drugs. One of the big interactions is with warfarin (Coumadin). Patients should follow up with PT/INRs and report bleeding or bruising. Patients should avoid grapefruit juice, not take antiacids within two hours, limit or avoid caffeine.
Nursing Process Assessing VS is important, especially HR and BP. Hold and check with LIP if HR < 60 and/or SBP < 100, periodic EKGs may be done as well, especially when meds are new. HR sounds are important checking for apical-radial deficit. The big nursing diagnosis for dysrhythmias is decreased cardiac output so check for JVD, edema, prolonged cap refill, decreased urine output, chest pain, and intolerance to activity. Teach patients to journal at home so that symptoms can be related to dysrhythmias or treatment. Patients should wear medic alert bracelets. Beware of drugs with multiple available strengths like lidocaine! Also double check the strength, and never use the topical solution (mixed with epinephrine for wound suturing) for IV use.