3.1 Electrocardiogram (ECG) Interpretation & Dysrhythmia Management

3.1 Electrocardiogram (ECG) Interpretation & Dysrhythmia Management

Introduction and Systematic Approach

Mastering Electrocardiogram (ECG) interpretation is a foundational skill for the Adult-Gerontology Acute Care Nurse Practitioner (AGACNP). In the acute and critical care settings, recognizing and accurately diagnosing rhythm disturbances promptly dictates the urgency of intervention and directly influences patient survival. A systematic approach to ECG interpretation ensures that subtle but clinically significant findings are not overlooked. The standard approach involves assessing rate, rhythm, cardiac axis, hypertrophy, intervals (PR, QRS, QT), and identifying signs of ischemia or infarction.

When analyzing an ECG, always begin with the heart rate. Is the patient bradycardic (less than 60 beats per minute), normocardic (60 to 100 beats per minute), or tachycardic (greater than 100 beats per minute)? Next, evaluate the rhythm. Is it regular or irregular? A crucial step is identifying the P wave—is there a P wave before every QRS complex, and is there a QRS complex following every P wave? Normal sinus rhythm is confirmed when P waves are upright in leads I, II, and aVF, indicating that the electrical impulse originated in the sinoatrial (SA) node. Understanding the normal conduction pathway, from the SA node through the atrioventricular (AV) node, bundle of His, and down to the Purkinje fibers, is essential for pinpointing the origin of conduction blocks or ectopic beats.

Tachyarrhythmias: Narrow and Wide Complex

Tachyarrhythmias are broadly classified by the width of the QRS complex into narrow complex (QRS less than 0.12 seconds) and wide complex (QRS 0.12 seconds or greater).

Narrow Complex Tachycardias: These rhythms typically originate above the ventricles (supraventricular). Common examples include Sinus Tachycardia, Atrial Fibrillation (AFib), Atrial Flutter, and Supraventricular Tachycardia (SVT).

• Atrial Fibrillation (AFib) is the most common sustained arrhythmia encountered in clinical practice. It is characterized by an irregularly irregular rhythm with no discernible, organized P waves, instead showing fibrillatory waves. Management of acute AFib with rapid ventricular response (RVR) focuses on rate control using intravenous beta-blockers (e.g., metoprolol) or non-dihydropyridine calcium channel blockers (e.g., diltiazem). In unstable patients, synchronized cardioversion is required. Long-term management involves assessing stroke risk using the CHA2DS2-VASc score and initiating appropriate anticoagulation.
• Supraventricular Tachycardia (SVT) often presents as a regular, rapid rhythm (frequently 150-250 bpm) with hidden or retrograde P waves. The initial intervention for a hemodynamically stable patient is vagal maneuvers (e.g., Valsalva). If unsuccessful, rapid intravenous push of adenosine (initially 6 mg, then 12 mg if needed) is the drug of choice to temporarily block AV nodal conduction and terminate the reentrant circuit.

Wide Complex Tachycardias: These rhythms usually originate within the ventricles, primarily Ventricular Tachycardia (VT), or they represent a supraventricular rhythm with aberrant conduction, such as a pre-existing bundle branch block.

• Monomorphic VT exhibits uniform, wide QRS complexes. In a stable patient, antiarrhythmic infusions such as amiodarone (150 mg over 10 minutes) or procainamide are indicated.
• Polymorphic VT, notably Torsades de Pointes, is characterized by a shifting electrical axis, giving the appearance of the QRS complexes twisting around the baseline. It is frequently associated with a prolonged QT interval and electrolyte derangements like hypomagnesemia. The hallmark treatment for Torsades is intravenous magnesium sulfate, regardless of the serum magnesium level.
• Pulseless VT and Ventricular Fibrillation (VFib) are lethal arrhythmias requiring immediate initiation of high-quality CPR and early defibrillation, adhering strictly to the ACLS Cardiac Arrest Algorithm.

Bradyarrhythmias and Conduction Blocks

Bradyarrhythmias can range from benign physiological findings in athletes to life-threatening emergencies causing profound hemodynamic collapse.

Sinus Bradycardia is often asymptomatic but can cause hemodynamic instability, manifesting as hypotension, altered mental status, ischemic chest pain, or signs of shock. Symptomatic bradycardia is treated primarily with intravenous atropine (1 mg IV every 3-5 minutes, up to a maximum of 3 mg). If atropine is ineffective, the provider must quickly transition to transcutaneous pacing or initiate chronotropic infusions such as dopamine or epinephrine.

Atrioventricular (AV) Blocks involve a delay or failure of electrical conduction through the AV node and are classified into three degrees:

1. First-Degree AV Block: Characterized by a prolonged PR interval (greater than 0.20 seconds). Every P wave is followed by a QRS complex. This is usually benign, asymptomatic, and requires no specific treatment other than monitoring.
2. Second-Degree AV Block, Mobitz Type I (Wenckebach): Shows progressive lengthening of the PR interval until a QRS complex is completely dropped. The cycle then resets. It is often transient, benign, and rarely progresses to complete heart block.
3. Second-Degree AV Block, Mobitz Type II: Manifests as intermittent dropped QRS complexes without the progressive PR lengthening. This is a highly unstable rhythm with a significant risk of sudden progression to complete heart block. It mandates urgent intervention, often requiring temporary pacing and eventually a permanent pacemaker.
4. Third-Degree (Complete) AV Block: Represents complete electrical dissociation between the atria and the ventricles. P waves and QRS complexes march out independently of each other. The ventricular rate is typically slow (escape rhythm). This is a medical emergency requiring immediate transcutaneous pacing followed by permanent transvenous pacemaker placement.

ACLS Algorithms Summary Table

Electrolyte Imbalances and the ECG

Electrolyte derangements dramatically alter myocardial action potentials, and the ECG can serve as an early warning system.

• Hyperkalemia classically presents with tall, peaked T waves and a shortened QT interval initially. As potassium levels rise, the PR interval lengthens, P waves disappear, and the QRS complex widens, ultimately merging with the T wave to form a sine wave pattern, which precedes asystole. Urgent treatment involves intravenous calcium gluconate or calcium chloride (to stabilize the myocardial membrane), followed by insulin with dextrose, and albuterol to shift potassium intracellularly.
• Hypokalemia is characterized by flattened or inverted T waves, prominent U waves (an extra deflection after the T wave), and ST segment depression.
• Hypocalcemia significantly prolongs the QT interval, thereby increasing the vulnerability to Torsades de Pointes.
• Hypercalcemia shortens the QT interval.

Acute Ischemia and Infarction Patterns

Recognizing the electrocardiographic evolution of acute myocardial ischemia and infarction is a critical competency.

• Ischemia typically presents as ST-segment depression or deep, symmetrical T-wave inversion.
• Injury (acute infarction) is marked by ST-segment elevation (STEMI), indicating an acute, complete occlusion of a coronary artery.
• Infarction (old or evolving) may manifest with pathological Q waves (defined as a width greater than 0.04 seconds or depth greater than one-third of the subsequent R wave).

Correlating leads to specific coronary artery territories aids in rapid diagnosis:

• Inferior Wall (Leads II, III, aVF): Supplied by the Right Coronary Artery (RCA). Providers must be highly suspicious of concomitant right ventricular infarction. In RV infarction, preload reduction (e.g., administering nitroglycerin or morphine) can lead to profound hypotension and should be strictly avoided. Volume expansion is often required.
• Anterior Wall (Leads V1-V4): Supplied by the Left Anterior Descending (LAD) artery. Anterior MIs carry a high risk of left ventricular dysfunction, heart failure, and lethal ventricular arrhythmias.
• Lateral Wall (Leads I, aVL, V5-V6): Supplied by the Left Circumflex (LCx) artery.

Rapid identification of STEMI within 10 minutes of patient arrival is the standard of care, immediately directing the patient to reperfusion therapy (primary percutaneous coronary intervention or fibrinolytics) to salvage endangered myocardium.