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Pathophysiology
Clinical meaning
Cardiac rhythm interpretation is a foundational competency for registered nurses caring for patients on telemetry, in critical care, and in emergency departments. Understanding dysrhythmias requires knowledge of the electrical properties of cardiac cells, the mechanisms by which abnormal rhythms develop, and the clinical significance of each rhythm in guiding treatment decisions.
Mechanisms of Dysrhythmia Formation at the Cellular Level:
All dysrhythmias arise from one of three fundamental mechanisms: disorders of impulse formation, disorders of impulse conduction, or a combination of both. Understanding these mechanisms is essential because they determine both the clinical behavior of the rhythm and the appropriate treatment.
Disorders of Impulse Formation (Automaticity):
Normal automaticity resides in the pacemaker cells of the SA node, AV node, and His-Purkinje system. These cells possess the unique ability to spontaneously depolarize during phase 4 of the action potential due to the funny current (If), a mixed sodium-potassium inward current that gradually moves the membrane potential toward threshold. The SA node has the fastest intrinsic rate (60-100 bpm) and therefore normally suppresses all lower pacemakers through a mechanism called overdrive suppression: because the SA node depolarizes first, it resets the phase 4 depolarization of lower pacemakers before they can reach threshold.
Abnormal automaticity occurs when cells that normally do not have pacemaker activity develop the ability to spontaneously depolarize. This can occur in ischemic or damaged myocardium where cellular injury alters ion channel expression, partially depolarizing the resting membrane potential. At a resting potential of approximately minus 60 mV (compared to normal minus 90 mV), the fast sodium channels are inactivated, and depolarization occurs through slow calcium channels, producing cells with pacemaker-like behavior. These abnormal automatic foci can generate ectopic impulses that override the SA node, producing premature beats or tachyarrhythmias. Clinical examples include accelerated idioventricular rhythm (AIVR) frequently seen after reperfusion in acute MI, and multifocal atrial tachycardia (MAT) seen in severe pulmonary disease with hypoxia.
Triggered activity is a related mechanism where abnormal depolarizations (afterdepolarizations) occur during or after repolarization, triggered by the preceding action potential. Early afterdepolarizations (EADs) occur during phase 2 or 3 (before repolarization is complete) and are associated with conditions that prolong the action potential duration, particularly prolonged QT interval. The prolonged plateau phase allows L-type calcium channels to reactivate before the cell has fully repolarized, generating an additional depolarization. If this afterdepolarization reaches threshold, it triggers a premature action potential that can initiate torsades de pointes. This mechanism explains why hypokalemia, hypomagnesemia, and QT-prolonging medications are dangerous: they all prolong repolarization and increase EAD susceptibility. Delayed afterdepolarizations (DADs) occur after repolarization is complete (during phase 4) and are caused by intracellular calcium overload. Excess calcium is released from the sarcoplasmic reticulum and exchanged for sodium via the sodium-calcium exchanger, generating a transient inward current. If this current is sufficient to depolarize the cell to threshold, a triggered beat occurs. DADs are the mechanism underlying digitalis toxicity (digoxin inhibits the sodium-potassium ATPase, increasing intracellular sodium and secondarily increasing intracellular calcium via the sodium-calcium exchanger) and catecholamine-induced arrhythmias.
Disorders of Impulse Conduction (Reentry):
Reentry is the most common mechanism for sustained tachyarrhythmias and requires three conditions: two functionally distinct pathways connected proximally and distally forming a circuit, unidirectional block in one pathway (one pathway has a longer refractory period and is blocked when the impulse arrives), and slow conduction through the alternative pathway that allows the blocked pathway sufficient time to recover excitability. When these conditions are met, the impulse conducts slowly down the unblocked pathway, arrives at the distal connection point, and then travels retrograde through the previously blocked pathway (which has now recovered). Upon reaching the proximal connection, it re-enters the slow pathway, establishing a self-sustaining circuit.
Atrial flutter is a classic example of macroreentrant reentry, with the reentrant circuit traveling around the tricuspid valve annulus in the right atrium. The circuit fires at approximately 300 bpm, producing characteristic sawtooth flutter waves on ECG. The AV node acts as a gatekeeper, typically conducting every second or third flutter wave to the ventricles (2:1 or 3:1 block), producing ventricular rates of 150 or 100 bpm. A ventricular rate of exactly 150 bpm should always raise suspicion for atrial flutter with 2:1 block.
Atrial fibrillation involves multiple simultaneous reentrant wavelets circulating chaotically throughout both atria, depolarizing the atrial myocardium in a completely disorganized fashion. This produces the irregularly irregular rhythm with no discernible P waves that is the hallmark of AFib. The loss of organized atrial contraction eliminates the atrial kick, reducing cardiac output by approximately 15 to 25 percent. More importantly, the stagnant blood in the poorly contracting atria, particularly the left atrial appendage, creates conditions for thrombus formation. If a thrombus forms and embolizes, it most commonly travels to the cerebral circulation, causing ischemic stroke. This is why anticoagulation assessment using the CHA2DS2-VASc score is essential for all patients with atrial fibrillation.
AV Nodal Reentrant Tachycardia (AVNRT) is the most common form of paroxysmal supraventricular tachycardia (PSVT). It involves a reentrant circuit within or near the AV node utilizing a fast pathway and a slow pathway. The typical form conducts antegrade down the slow pathway and retrograde up the fast pathway, producing a narrow-complex tachycardia at 150-250 bpm with P waves buried in or just after the QRS complex (often invisible). Treatment involves vagal maneuvers (carotid sinus massage, bearing down, cold water immersion) which increase parasympathetic tone and slow AV nodal conduction, potentially terminating the reentrant circuit. If vagal maneuvers fail, adenosine (6 mg rapid IV push, may repeat 12 mg) is administered. Adenosine transiently blocks AV nodal conduction for several seconds, breaking the reentrant circuit. Because of its ultra-short half-life (less than 10 seconds), adenosine must be given by rapid IV push followed by an immediate 20 mL normal saline flush through a proximal IV site.
AV Reentrant Tachycardia (AVRT) involves a reentrant circuit using an accessory pathway that connects the atria and ventricles outside the normal AV node-His-Purkinje system. Wolff-Parkinson-White (WPW) syndrome is the most common example, where the accessory pathway (Bundle of Kent) can conduct impulses both antegrade and retrograde. In sinus rhythm, pre-excitation produces the characteristic short PR interval and delta wave (slurred QRS upstroke) because the impulse reaches the ventricle through the accessory pathway before the impulse traveling through the normal AV node. During orthodromic AVRT (the most common tachycardia in WPW), the impulse conducts antegrade through the AV node (normal narrow QRS) and retrograde through the accessory pathway. During antidromic AVRT, the impulse conducts antegrade through the accessory pathway (wide, bizarre QRS) and retrograde through the AV node. A critical clinical concern is atrial fibrillation in WPW: if AFib impulses conduct down the accessory pathway (which lacks the rate-limiting properties of the AV node), extremely rapid ventricular rates can occur, potentially degenerating into ventricular fibrillation. Therefore, AV nodal blocking agents (adenosine, beta-blockers, calcium channel blockers, digoxin) are CONTRAINDICATED in wide-complex tachycardia with known or suspected WPW because they block the AV node while leaving the accessory pathway unopposed, potentially accelerating conduction down the accessory pathway. Treatment in this scenario is procainamide or electrical cardioversion.
Ventricular Dysrhythmias and Their Significance:
Premature ventricular contractions (PVCs) arise from ectopic foci in the ventricular myocardium. They produce wide, bizarre QRS complexes (greater than 0.12 seconds) because the impulse originates in the ventricle and spreads slowly through myocardium rather than the rapid His-Purkinje system. Isolated PVCs are common and often benign. However, PVCs in certain patterns are concerning: PVCs occurring on the T wave (R-on-T phenomenon) can trigger ventricular fibrillation because they stimulate the ventricle during the vulnerable period of repolarization when cells have heterogeneous recovery states. Frequent PVCs (more than 10 percent of all beats) can cause tachycardia-mediated cardiomyopathy over time. Couples (two consecutive PVCs) and salvos (three or more, which technically constitute ventricular tachycardia) indicate increased ventricular irritability.
Ventricular tachycardia (VTach) is defined as three or more consecutive ventricular beats at a rate greater than 100 bpm. Monomorphic VTach has uniform QRS morphology, typically arising from a single reentrant circuit, often around scar tissue from prior MI. Polymorphic VTach has varying QRS morphology and axis. Torsades de pointes is a specific form of polymorphic VTach associated with prolonged QT interval, characterized by a twisting-of-the-points appearance on ECG where the QRS axis appears to rotate around the baseline. Treatment of torsades is IV magnesium (2 grams over 10 minutes), overdrive pacing to shorten the QT interval, and correction of the underlying cause (stopping offending medications, correcting hypokalemia).
Heart Blocks and Conduction Abnormalities:
First-degree AV block (PR interval greater than 0.20 seconds) represents delayed conduction through the AV node. It is generally benign and requires only monitoring. Second-degree AV block Type I (Wenckebach) shows progressive PR prolongation until a QRS is dropped, then the cycle repeats. This is typically located at the AV node level and is often benign, associated with inferior MI, digitalis, and increased vagal tone. Second-degree AV block Type II shows a constant PR interval with intermittent dropped QRS complexes. This is located below the AV node (His-Purkinje level) and is dangerous because it frequently progresses to complete heart block without warning. Immediate transcutaneous pacing standby and preparation for permanent pacemaker are indicated. Third-degree (complete) AV block shows complete dissociation between P waves and QRS complexes, with no conduction from atria to ventricles. The ventricular rate depends on the location of the escape pacemaker: junctional escape (40-60 bpm, narrow QRS) or ventricular escape (20-40 bpm, wide QRS). Complete heart block requires pacing.
Systematic Rhythm Interpretation Approach:
Every rhythm strip should be analyzed using a consistent systematic approach. First, assess the rate by counting the number of QRS complexes in a 6-second strip and multiplying by 10, or dividing 300 by the number of large boxes between consecutive R waves. Second, assess regularity by measuring R-R intervals across the strip: regular, regularly irregular (a repeating pattern), or irregularly irregular (completely random). Third, identify P waves: are they present, are they upright in lead II, is there one P wave before each QRS, and do all P waves look the same. Fourth, measure the PR interval (normal 0.12-0.20 seconds) and determine if it is constant. Fifth, measure the QRS duration (normal less than 0.12 seconds). Sixth, examine the ST segment and T wave for ischemic changes. This systematic approach prevents missed findings and ensures accurate rhythm identification.
Advanced rhythm interpretation requires understanding the cellular mechanisms of impulse formation and conduction abnormalities. Automaticity, the ability of certain cardiac cells to spontaneously depolarize, is normally confined to the SA node, AV junction, and His-Purkinje system. Enhanced automaticity occurs when latent pacemaker cells or even working myocytes develop accelerated Phase 4 depolarization due to sympathetic stimulation, hypokalemia, hypomagnesemia, digitalis toxicity, or myocardial ischemia. This produces ectopic rhythms including premature atrial contractions, junctional tachycardia, and accelerated idioventricular rhythm. Triggered activity represents a distinct arrhythmia mechanism where afterdepolarizations during or following repolarization reach threshold and initiate additional action potentials. Early afterdepolarizations (EADs) occur during Phase 2 or 3 when action potential duration is prolonged (as in Long QT syndrome or hypokalemia) and are the cellular mechanism of torsades de pointes. Delayed afterdepolarizations (DADs) occur after complete repolarization and are caused by intracellular calcium overload from conditions such as digoxin toxicity, catecholamine excess, or heart failure, which causes spontaneous calcium release from the sarcoplasmic reticulum through leaky ryanodine receptors, activating the sodium-calcium exchanger and generating a depolarizing current. Re-entry is the most common mechanism of sustained tachyarrhythmias and requires three conditions: two distinct pathways with different conduction velocities and refractory periods, unidirectional block in one pathway, and sufficient conduction delay in the alternate pathway to allow the blocked pathway to recover excitability. Atrial flutter is a classic macro-re-entrant circuit, typically traveling counterclockwise through the right atrium around the tricuspid annulus, with the cavotricuspid isthmus serving as the critical slow conduction zone. Catheter ablation of this isthmus permanently interrupts the circuit. Ventricular tachycardia in the setting of prior myocardial infarction involves re-entry circuits through the border zone of scar tissue, where surviving myocyte bundles within fibrotic tissue create channels of slow conduction that form the substrate for sustained re-entrant arrhythmias.
Exam Focus
Exam relevance
Risk factors:
- Electrolyte imbalances (hypo/hyperkalemia, hypomagnesemia)
- Myocardial ischemia or infarction
- Heart failure
- Drug toxicity (digoxin, beta-blockers, calcium channel blockers)
- Thyroid disorders
- Caffeine and stimulant use
- Structural heart disease
Diagnostics:
- Systematic ECG interpretation (rate, rhythm, P waves, PR interval, QRS width, ST segment)
- Continuous telemetry with alarm management
- Electrolyte panel (K+, Mg2+, Ca2+) correlation with rhythm
- Drug levels when applicable (digoxin, antiarrhythmic levels)
- 12-lead ECG for new-onset arrhythmias
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Clinical scenario
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