3.3 Cardiac Arrhythmias, Conduction Blocks & Device Rhythms (Pacemakers/ICDs)

3.3 Cardiac Arrhythmias, Conduction Blocks & Device Rhythms (Pacemakers/ICDs)

Continuous telemetry monitoring in cardiac rehabilitation requires swift rhythm discrimination. Clinicians must differentiate benign ectopy from life-threatening ventricular dysrhythmias, recognize progressive conduction system failure, and safely supervise patients with pacemakers and implantable cardioverter-defibrillators (ICDs).

---

Supraventricular Arrhythmias & Hemodynamic Compromise

Supraventricular arrhythmias arise above the bifurcation of the bundle of His, typically producing narrow QRS complexes ($< 0.12\text{ s}$).

1. Sinus Node Rhythms & Premature Atrial Contractions (PACs)

• Sinus Bradycardia ($HR < 60\text{ bpm}$): Common in endurance conditioning or beta-blockade; pathologic if symptomatic or failing to rise with exertion (chronotropic incompetence).
• Sinus Tachycardia ($HR > 100\text{ bpm}$): Exertional tachycardia is physiologic; resting tachycardia signals deconditioning, hypovolemia, anemia, or heart failure exacerbation.
• Premature Atrial Contractions (PACs): Ectopic atrial beats with abnormal P-wave morphology followed by a narrow QRS. Generally benign, but frequent PACs may trigger paroxysmal atrial flutter or fibrillation.

2. Atrial Flutter

Driven by a right atrial reentrant circuit (atrial rate 250 to 350 bpm), atrial flutter produces classic "sawtooth" flutter ($F$) waves in leads II, III, aVF, and $V_1$. Conduction ratios include:

• 2:1 AV Conduction: Ventricular rate fixed at 150 bpm (any regular narrow-complex rhythm at 150 bpm warrants evaluation for atrial flutter).
• 4:1 AV Conduction: Ventricular rate regular at 75 bpm.

3. Atrial Fibrillation (AF)

Chaotic atrial activation (electrical rate 350 to 600 bpm) with absent organized mechanical contraction. The ECG displays absent P waves, fibrillatory ($f$) baseline waves, and an irregularly irregular ventricular response.

Hemodynamic Consequences

In sinus rhythm, coordinated atrial systole ("atrial kick") contributes $15% \text{ to } 25%$ of left ventricular end-diastolic volume (up to $40%$ in non-compliant ventricles). AF eliminates the atrial kick, substantially diminishing stroke volume and cardiac output. A rapid ventricular response ($HR > 100\text{ bpm}$) curtails diastolic filling time, elevating atrial pressures and inducing exertional dyspnea or angina.

Thromboembolic Risk ($CHA_2DS_2\text{-VASc}$)

Stroke risk is evaluated via $CHA_2DS_2\text{-VASc}$: Congestive Heart Failure (1), Hypertension (1), Age $\ge 75$ (2), Diabetes (1), Prior Stroke/TIA (2), Vascular Disease (1), Age 65–74 (1), Female Sex (1). Anticoagulation (DOACs or warfarin) is indicated for scores $\ge 2$ in men or $\ge 3$ in women.

---

Ventricular Arrhythmias & Clinical Management Protocols

Ventricular dysrhythmias originate below the bundle of His, producing wide ($QRS \ge 0.12\text{ s}$), bizarre complexes with discordant T waves.

graph TD
    VE["Ventricular Ectopy on Telemetry"] --> ISO["Isolated Unifocal PVCs"]
    VE --> COMPLEX["Complex PVCs (Multifocal, Couplets, Bigeminy)"]
    VE --> NSVT["Non-Sustained VT (>= 3 beats, < 30 s)"]
    VE --> SVT["Sustained VT (> 30 s or Collapse)"]
    
    ISO --> ACT1["Assess symptoms; continue exercise if suppressed"]
    COMPLEX --> ACT2["Pause exercise; check BP; notify Medical Director"]
    NSVT --> ACT3["Immediate test cessation; 12-lead ECG; physician evaluation"]
    SVT --> ACT4["Activate Code Blue; immediate CPR; Defibrillation"]

1. Premature Ventricular Contractions (PVCs)

• Patterns: Unifocal (uniform) vs Multifocal (multiple foci, higher ischemic risk); Couplets (two consecutive); Bigeminy (every other beat); Trigeminy (every third beat).
• R-on-T Phenomenon: A PVC falling on the vulnerable downslope of the preceding T wave, risking degeneration into polymorphic VT or ventricular fibrillation.
• Exercise Response: Isolated PVCs that suppress during warm-up are typically benign. PVCs appearing or multiplying with exertion indicate active ischemia, ventricular dysfunction, or electrolyte derangements.

2. Ventricular Tachycardia & Fibrillation Protocols

• Non-Sustained Ventricular Tachycardia (NSVT): Defined as $\ge 3$ consecutive ventricular beats at $> 100\text{ bpm}$ lasting $< 30\text{ seconds}$ without hemodynamic collapse. Immediate action: stop exercise, assess vitals, obtain a 12-lead ECG, and notify the medical director.
• Sustained Ventricular Tachycardia (VT): Persists for $> 30\text{ seconds}$ or causes hemodynamic collapse. Action: stop exercise, activate emergency response, assess pulse. If pulseless, initiate CPR and attach an AED/defibrillator.
• Ventricular Fibrillation (VF): Chaotic, lethal undulating rhythm without identifiable QRS complexes or pulse. Immediate action: call Code Blue, initiate CPR, and deliver an immediate high-energy unsynchronized defibrillator shock (120–200 J biphasic).

---

Atrioventricular (AV) Conduction Blocks

---

Pacemakers and Implantable Cardioverter-Defibrillators (ICDs)

NBG Pacemaker Code

Pacemaker modes are classified by a five-letter standard: Position I = Chamber Paced (A, V, D, O); Position II = Chamber Sensed (A, V, D, O); Position III = Response to Sensing (T, I, D, O); Position IV = Rate Modulation (R, O); Position V = Multisite Pacing (A, V, D, O).

• DDDR Mode: Senses and paces both chambers, inhibits or triggers pacing as needed, and modulates heart rate using activity sensors (accelerometers).
• Telemetry Appearance: Sharp vertical pacer spikes preceding P waves (atrial pacing) or wide QRS complexes (ventricular pacing). Clinicians watch for failure to capture, undersensing (spikes firing into intrinsic beats; R-on-T risk), and oversensing (inappropriate inhibition from muscle artifact).

ICD Exercise Safety Thresholds

ICDs detect ventricular tachyarrhythmias and deliver antitachycardia pacing (ATP) or defibrillator shocks.

[!IMPORTANT] The 10-to-20 Beats/Min Safety Margin: The prescribed peak exercise target heart rate must be set at least 10 to 20 beats/min below the lowest programmed tachycardia detection threshold.

$\text{Peak Exercise HR} \le \text{Lowest Programmed ICD Therapy Threshold} - 10\text{ to } 20\text{ bpm}$

Example: If an ICD detection threshold is programmed at $165\text{ bpm}$, the patient's exercise heart rate ceiling must not exceed $145\text{ to } 150\text{ bpm}$ to avoid inappropriate shocks triggered by exertional sinus tachycardia.