Comprehensive Study Guide on Cardiac Emergencies, Dysrhythmias, and ECG Interpretation

Premature Ventricular Complexes (PVCs) and Ventricular Tachycardia

• Definition & Mechanism:

  • Ectopic Origin: Early complex originating from a site within the ventricles rather than the sinoatrial (SA) node.

  • Rhythm Impact: Occurs earlier than expected, shortening the R-R interval and causing an irregular overall rhythm.

• ECG Characteristics:

  • P Waves & PRI: Absent before the PVC; consequently, there is no PR interval (PRI).

  • QRS Complex: Wide and bizarre, measuring greater than or equal to $0.12\,\text{s}$ ($120\,\text{ms}$).

  • Rate: Depends entirely on the underlying baseline rhythm.

  • Compensatory Pause: Followed by a full pause where the R-R interval containing the PVC equals exactly twice ($2 \times$) the baseline R-R interval.

• Morphological Classifications:

  • Unifocal PVCs: Single ectopic focus; identical ECG waveforms.

  • Multifocal PVCs: Multiple ectopic foci; variable ECG waveforms.

  • Couplet: Two consecutive PVCs without an intervening sinus beat.

  • Run of VT (Salvos/Bursts): Three or more consecutive PVCs in a row.

  • Ventricular Bigeminy: Repetitive pattern where every second beat is a PVC (normal-PVC-normal-PVC).

  • Ventricular Trigeminy: Repetitive pattern where every third beat is a PVC (normal-normal-PVC).

• Etiology & Clinical Severity:

  • Primary Cause: Most commonly originates from myocardial ischemia in ventricular tissue.

  • Relative Severity: PVCs are more serious than PACs or PJCs. Multifocal, couplets, and bigeminal patterns carry higher severity than isolated unifocal PVCs.

  • R-on-T Phenomenon: PVC falls on the T wave during vulnerable ventricular repolarization, frequently triggering Ventricular Fibrillation (VF).

• Management Strategies:

  • PVC Management: Prehospital treatment is deferred unless cardiac output is severely compromised. Treat underlying causes (e.g., ischemic pain) before considering antidysrhythmics.

  • Unstable VT: Requires immediate synchronized cardioversion.

  • Stable VT: Treated pharmacologically, or deferred if transport duration is short.

  • Polymorphic VT: Characterized by continuously changing QRS contours and axis.

Ventricular Fibrillation (VF)

• Pathophysiology:

  • Mechanism: Lethal rhythm where organized ventricular contraction stops; myocardium quivers randomly due to multiple uncoordinated ectopic foci.

• ECG Manifestations:

  • Baseline: Chaotic and disorganized with no P waves, PRIs, or QRS complexes.

  • Coarse VF: Early phase with intact cellular energy reserves; produces high-amplitude chaotic waves.

  • Fine VF: Late phase as myocardial energy is depleted over time without perfusion; low-amplitude waves.

  • CPR Impact: High-quality chest compressions slow cellular energy decay, delaying degradation into fine VF.

• Epidemiology & Resuscitation:

  • Most common initial rhythm in adult cardiac arrest.

  • Early defibrillation within the first few minutes offers the highest resuscitation success.

  • Prolonged arrest requires chest compressions to supply oxygen and restore energy reserves prior to defibrillation.

Asystole and Agonal Rhythms

• Asystole (Flatline):

  • Complete absence of electrical activity and mechanical contraction due to global myocardial hypoxia.

  • ECG Criteria: Flat baseline ($0\,\text{beats/min}$); absent P, QRS, and T waves. Serves as visual confirmation of clinical death.

• Agonal Rhythm:

  • Variation of asystole with occasional slow, broad, sinusoidal complexes ($< 20\,\text{beats/min}$).

  • Represents residual electrical discharge from a dying heart.

  • Key Distinction: Unlike idioventricular rhythm, agonal rhythm generates no mechanical contraction and no palpable pulse.

Artificial Pacemakers and Pacemaker Malfunctions

• ECG Features:

  • Pacemaker Spike: Sharp, narrow vertical line indicating electrical discharge.

  • Ventricular Pacemaker: Single spike followed immediately by a wide QRS complex.

  • Dual-Chamber (AV Sequential): Initial spike followed by a P wave, then a second spike followed by a wide QRS complex.

  • Demand Pacemaker: Paces only when intrinsic heart rate drops below a preset threshold (typically $60\,\text{beats/min}$).

• Pacemaker Malfunctions:

  • Loss of Capture:

  • Etiology: Battery failure or dislodged lead wire.

  • ECG Features: Visible spikes fail to produce a P wave or QRS complex.

  • Clinical Impact: Heart falls back on intrinsic escape rate, reducing cardiac output.

  • Management: Immediate Transcutaneous Pacing (TCP).

  • Runaway Pacemaker:

  • Etiology: Circuitry failure causing rapid impulse discharge.

  • Management: Place a strong magnet over the generator to reset to preset default rate ($60\text{ to }80\,\text{beats/min}$).

Preexcitation Syndromes and Specific ECG Waveform Abnormalities

• Wolff-Parkinson-White (WPW) Syndrome:

  • Anatomy: Accessory pathway (bundle of Kent) bypasses the standard AV node delay.

  • Delta Wave: Slurred, rapid upslope of the R wave immediately following the P wave.

  • ECG Triad: Delta wave, shortened PR interval, and widened QRS complex ($> 0.12\,\text{s}$ / $120\,\text{ms}$).

  • Clinical Risk: Susceptible to paroxysmal SVTs; aberrant conduction can cause SVTs to be misidentified as ventricular in origin.

• Preexcited Atrial Fibrillation:

  • ECG: Irregular wide-complex tachycardia with ventricular rate exceeding $200\,\text{beats/min}$.

  • Contraindications: All AV-nodal blocking agents (adenosine, beta-blockers, calcium channel blockers) are strictly contraindicated as they can force impulses down the accessory tract and trigger VF.

  • Treatment: Immediate synchronized cardioversion.

• Osborn (J) Wave:

  • Clinical Association: Pathognomonic marker for systemic hypothermia.

  • Appearance: Notch at the QRS-ST junction; wave amplitude correlates directly with hypothermia depth.

Electrolyte Imbalances and ECG Manifestations

• Hyperkalemia:

  • Mild to Moderate: Tall, narrow, peaked T waves (height $\ge$ QRS complex).

  • Profound: Severe conduction delay creates a continuous sine wave pattern.

• Hypokalemia:

  • Flattened or absent T waves with the appearance of a U wave (small deflection after T wave and before P wave).

• Calcium Imbalances:

  • Hypercalcemia: Shortens the QT interval.

  • Hypocalcemia: Mildly lengthens the QT interval.

12-Lead Electrocardiography

• Single/3-Lead vs. 12-Lead:

  • Single Lead (e.g., Lead II): Single-view monitoring for rhythm identification and rate evaluation.

  • 12-Lead ECG: Captures 12 simultaneous electrical views to localize myocardial injury, ischemia, or infarction.

• Interpretation & Transmission:

  • Automated software provides supportive hints but requires independent manual clinician interpretation.

  • Prehospital transmission enables destination hospitals to mobilize intervention resources (e.g., cardiac catheterization labs) ahead of patient arrival.