What are shockable rhythms in an adult patient with a history of cardiac issues?

Shockable Rhythms in Adult Cardiac Arrest

Shockable rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT), which are the only two cardiac arrest rhythms that require immediate defibrillation. 1, 2

Definition and Core Characteristics

Shockable rhythms consist exclusively of:

• Ventricular fibrillation (VF): Chaotic, disorganized electrical activity with no coordinated ventricular contraction 3, 1
• Pulseless ventricular tachycardia (pVT): Rapid ventricular rhythm without a detectable pulse 3, 1

These rhythms are distinguished from non-shockable rhythms (asystole and pulseless electrical activity), which do not respond to defibrillation and require a different treatment pathway. 1, 2

Clinical Significance and Prognosis

Patients with shockable rhythms have dramatically better survival outcomes compared to non-shockable rhythms. 1 VF is the most common primary rhythm in adult sudden cardiac arrest, particularly in patients with underlying ischemic heart disease. 3, 2

Critical timing factors:

• Over 80% of successful defibrillations occur within the first three shocks 3, 1
• Survival probability decreases progressively with each minute of delay between arrest onset and defibrillation 2
• At least 20% of patients with VF/pVT will remain in a shockable rhythm after 3 shocks, with survival decreasing as the number of required defibrillation attempts increases 3

Recognition and Diagnosis

On cardiac monitoring, you must identify:

• VF: Irregular, chaotic waveform with no discernible QRS complexes, varying amplitude and frequency 3
• Pulseless VT: Wide-complex tachycardia (typically >150-180 beats/min) without a palpable pulse 3, 4

Important caveat: Not all ventricular tachycardias are shockable—the critical distinction is whether the patient has a pulse. VT with a pulse requires synchronized cardioversion, while pulseless VT is treated identically to VF with immediate unsynchronized defibrillation. 1, 5

Automated External Defibrillator (AED) Performance

AEDs demonstrate excellent accuracy in rhythm recognition:

• Specificity for non-shockable rhythms: 99-100% (rarely misidentifies non-shockable rhythms as shockable) 1
• Sensitivity for VF: 94-96% 1
• Sensitivity for rapid VT: 60-71% (lower detection rate, particularly for slower VT) 1, 4

This means AEDs may occasionally miss rapid VT but virtually never inappropriately shock a non-shockable rhythm. 4

Treatment Algorithm

Immediate management priorities:

1. Confirm cardiac arrest: Unresponsiveness, no pulse, no normal breathing 2

2. Attach monitor/defibrillator immediately to assess rhythm 2

3. If VF or pulseless VT is identified:

   • Deliver one shock immediately without any delay 2
   • Do not delay for advanced airway or IV access 2
   • Initial energy: 200 J for first shock (monophasic), or manufacturer-recommended dose for biphasic 3, 2

4. Immediately after shock:

   • Resume chest compressions without checking pulse or rhythm 2
   • Continue CPR for 2 minutes 2
   • Do not check rhythm between shocks if VF/pVT persists on monitor 2

5. After 2 minutes of CPR:

   • Check rhythm 2
   • If VF/pVT persists, deliver another single shock 2
   • Subsequent shocks at 360 J (monophasic) or same/escalating energy (biphasic) 3, 2

Common Pitfalls to Avoid

Critical errors that worsen outcomes:

• Delaying defibrillation to establish airway or IV access when VF/pVT is present—the first shock must be given without delay 2
• Checking pulse after every shock—only check if the waveform changes to one compatible with cardiac output 3, 2
• Misinterpreting post-shock asystole—the monitor often shows a flat line for several seconds after shock due to electrical/myocardial "stunning," which does not mean the rhythm has converted to asystole 3
• Confusing VT with pulse for pulseless VT—always confirm pulse status, as treatment differs dramatically 1, 5

Why These Rhythms Are Shockable

VF and pulseless VT are amenable to defibrillation because the electrical shock can depolarize a critical mass of myocardium simultaneously, allowing the heart's intrinsic pacemaker to potentially re-establish organized electrical activity. 3 In contrast, asystole (no electrical activity) and PEA (organized electrical activity without mechanical contraction) cannot be "reset" by electrical shock and have much worse prognosis. 2