Shockable Rhythms: VF and Pulseless VT, Defibrillation, and Antiarrhythmics Practice Questions

25 multiple-choice questions on Shockable rhythms: VF and pulseless VT, defibrillation, and antiarrhythmics for paramedic and EMT students, from Chapter 40 (Responding to the Field Code). Read each question, pick your answer, then tap Show answer to check it and read the explanation.

25 practice questions

1. For an adult in VF using a biphasic defibrillator, the initial energy should be:

   1. A. 50 J, then 100 J for any later shocks
   2. B. 2 J/kg, then 4 J/kg for any later shocks
   3. C. 360 J only after 2 minutes of CPR
   4. D. Manufacturer's dose, usually 120-200 J
   Show answer

   Answer: D. Manufacturer's dose, usually 120-200 J

   Biphasic devices should be used at the manufacturer's recommended energy, typically 120 to 200 J, and if that is unknown, the maximum available energy is reasonable. 2 J/kg then 4 J/kg is the pediatric dosing sequence.

2. After a shock for VF, the shock is considered successful when:

   1. A. A pulse returns within 2 minutes
   2. B. VF stops for at least 5 seconds
   3. C. The patient begins to move or wake
   4. D. The rhythm becomes sinus at once
   Show answer

   Answer: B. VF stops for at least 5 seconds

   Shock success is defined as termination of VF for at least 5 seconds after the shock; the heart may then show asystole, PEA, or an organized rhythm, which is why CPR resumes immediately. ROSC within 2 minutes is a separate outcome that depends on perfusion and the underlying cause, not the definition of a successful shock. Movement or waking may come much later, if at all. A sinus rhythm right away is not required; a brief pause or slow rhythm after the shock is common. (ERC 2021 ALS; AHA 2025 Part 9.)

3. A 66-year-old man on the monitor with pads applied goes into VF in the back of your ambulance. You should:

   1. A. Do 2 minutes of CPR before the first shock
   2. B. Give 1 mg of epinephrine before any shock
   3. C. Shock as soon as the defibrillator is charged
   4. D. Give amiodarone 300 mg before the first shock
   Show answer

   Answer: C. Shock as soon as the defibrillator is charged

   For a monitored, witnessed VF arrest, the chance of success is highest in the first moments, so the shock should be delivered immediately. A period of CPR before the first shock is not recommended when a defibrillator is ready.

4. For an unwitnessed adult cardiac arrest, current AHA guidance for the first shock is to:

   1. A. Give 5 cycles of CPR before analyzing
   2. B. Analyze after the airway has been placed
   3. C. Shock after epinephrine has been given
   4. D. Do CPR while pads go on, then shock promptly
   Show answer

   Answer: D. Do CPR while pads go on, then shock promptly

   Trials found no benefit from a fixed 1.5 to 3 minutes of CPR before the first rhythm analysis, so CPR is performed while the defibrillator is readied and the patient is shocked as soon as possible. An airway or drug should never delay a shock.

5. A 200-J biphasic shock fails to end VF, and your device can deliver higher energy. For the next shock, 2025 AHA guidance says:

   1. A. Higher energy may be considered
   2. B. Energy should drop to 120 J
   3. C. Energy must stay at 200 J
   4. D. Shocks must wait for amiodarone
   Show answer

   Answer: A. Higher energy may be considered

   The 2025 AHA adult ALS guidelines state that higher energy for the second and later shocks may be considered if the first shock fails (Class 2b), following the manufacturer's escalation if the device allows it. Lowering the energy makes conversion less likely. Staying at 200 J is acceptable on fixed-energy devices but is not required. Shocks should never be delayed for an antiarrhythmic; amiodarone or lidocaine is given during CPR after the third shock. (2025 AHA Part 9.)

6. A patient in VF still has a nonrebreather mask running at 15 L/min on his face. Before the shock, you should:

   1. A. Turn the flow down to 2 L/min and shock
   2. B. Move it to his abdomen just below the pads
   3. C. Move the mask at least 1 m from his chest
   4. D. Delay the shock until the cylinder is off
   Show answer

   Answer: C. Move the mask at least 1 m from his chest

   Free-flowing oxygen near the pads can feed a spark into a fire, so a mask or nasal cannula should be removed and placed at least 1 m from the chest before shocking; a bag connected to an ET tube or supraglottic airway can stay attached. Turning the flow down still leaves oxygen-enriched air around the pads, and resting the mask on the abdomen keeps it well within 1 m of them. Turning off the cylinder is unnecessary and delays the shock; moving the mask away takes a second. (ERC 2021 ALS; AHA 2025 Part 9.)

7. A monitored patient goes into VF while the defibrillator is ready at the bedside. A precordial thump:

   1. A. Should be given before the shock
   2. B. Should not delay the shock
   3. C. Replaces the first shock in VF
   4. D. Is used in unwitnessed arrests
   Show answer

   Answer: B. Should not delay the shock

   When a defibrillator is ready, the shock should be delivered at once; a precordial thump rarely converts VF and must not delay CPR or defibrillation. AHA allows it only as a possible option for witnessed, monitored unstable VT when a defibrillator is not immediately available, so it is not a routine step before the shock. It does not replace a shock. It should not be used in unwitnessed arrests, when the heart has been ischemic for an unknown time. (AHA 2015 and 2020 ALS; 2025 Part 9.)

8. In the AHA adult cardiac arrest algorithm, amiodarone or lidocaine is first given for VF that persists after the:

   1. A. Third shock
   2. B. First shock
   3. C. Fifth shock
   4. D. Second dose of epinephrine
   Show answer

   Answer: A. Third shock

   The algorithm places epinephrine after the second shock and an antiarrhythmic after the third shock. Waiting for two doses of epinephrine would delay the antiarrhythmic beyond the point where it is most likely to help.

9. You press SHOCK for VF, but the defibrillator does not discharge. The screen shows that sync mode is on. You should:

   1. A. Hold the button until it fires
   2. B. Lower the energy, then retry
   3. C. Resume CPR for 2 full minutes first
   4. D. Turn sync off and shock at once
   Show answer

   Answer: D. Turn sync off and shock at once

   In sync mode the defibrillator waits for an R wave to time the shock, and VF has no R waves, so it will not fire; turn sync off and deliver an unsynchronized shock immediately. Holding the button longer will not help, because the device is still waiting for an R wave. Lowering the energy does not fix the sync problem and makes conversion less likely. Resuming CPR for 2 minutes delays a shock that should have been given already; keep compressions going only while you correct the setting. (AHA 2025 Part 9; ch40 shockable rhythms section.)

10. Why must compressions stop while an AED analyzes the rhythm?

    1. A. Compressions drain the AED's battery
    2. B. The pads may shock the compressor
    3. C. The AED cannot charge during CPR
    4. D. Motion artifact can mimic or hide VF
    Show answer

    Answer: D. Motion artifact can mimic or hide VF

    Chest compressions create electrical artifact that can look like VF or hide it, so AEDs need a few seconds without movement to analyze accurately; the pause should be as short as possible, and compressions resume while the device charges if the AED allows it. CPR does not drain the battery. The rescuer is at risk only during the shock itself, which is why everyone clears before it is delivered, not during analysis. Many AEDs and all manual defibrillators can charge during compressions. (AHA 2025 adult BLS and Part 9.)

11. Standard anterolateral defibrillation pad placement is:

    1. A. Left infraclavicular and right apex
    2. B. Right infraclavicular and left apex
    3. C. Left sternal border and left apex
    4. D. Right infraclavicular and left epigastrium
    Show answer

    Answer: B. Right infraclavicular and left apex

    One pad goes to the right of the upper sternum below the clavicle and the other in the left midaxillary line at the cardiac apex, so current crosses the ventricles. Mirroring the pads to the left clavicle and right side is not the standard position and moves the lateral pad away from the cardiac apex. A pad at the left sternal border sits too close to the apex pad, and a pad in the epigastrium lies below the heart, so in both cases much of the current bypasses the myocardium.

12. A patient in VF has an implanted pacemaker visible under the skin of the right upper chest. When placing pads, you should:

    1. A. Place a pad directly on top of the device
    2. B. Shock after placing a magnet on the device
    3. C. Keep the pad off the device itself
    4. D. Lower the energy to 100 J to protect it
    Show answer

    Answer: C. Keep the pad off the device itself

    Pads placed over a device can block current and damage the device, so they should be placed away from it, often in an AP position. A magnet is not needed and only delays the shock, and lowering the energy makes conversion of VF less likely; standard energy is used.

13. Firefighters have a bystander's AED attached, and CPR is under way when you arrive. The best time to switch to your manual monitor is:

    1. A. At the next planned rhythm check
    2. B. Right now, by stopping CPR to swap pads
    3. C. Only after ROSC has been achieved
    4. D. Never; keep the AED for the whole call
    Show answer

    Answer: A. At the next planned rhythm check

    Changing devices at the next scheduled pause avoids an extra interruption in compressions, and many systems let the manual monitor's cables connect to the same pads. Stopping CPR now to swap equipment adds an unnecessary pause. Waiting until ROSC means the team cannot use manual rhythm interpretation, faster shocks, or pacing during the arrest. Keeping the AED for the whole call gives up the manual monitor's capabilities, such as charging during compressions and 12-lead monitoring after ROSC. (AHA 2025 Part 9; ch40 shockable rhythms section.)

14. For anterior-posterior defibrillation in an adult, the posterior pad is placed:

    1. A. Over the right shoulder blade, near the neck line
    2. B. On the lower back at the level of the waist
    3. C. Under the right flank over the kidney
    4. D. Below the left shoulder blade, near the spine
    Show answer

    Answer: D. Below the left shoulder blade, near the spine

    In the anterior-posterior position, the front pad goes over the left precordium and the back pad goes below the left scapula, beside the spine, so current passes through the heart from front to back; this is the position used for vector change. Over the right shoulder blade, the current path misses much of the left ventricle. The lower back and right flank are too far below the heart to direct current through the ventricles. (AHA 2025 Part 9; DOSE-VF trial methods.)

15. Refractory VF is usually defined as VF that:

    1. A. Persists after 3 or more shocks
    2. B. Recurs after a period of ROSC
    3. C. Lasts more than 30 minutes
    4. D. Begins after an epinephrine dose
    Show answer

    Answer: A. Persists after 3 or more shocks

    VF that continues despite at least three standard shocks is called shock-refractory. VF that returns after successful conversion is recurrent VF, a different problem managed by resuming CPR and shocking at the previous energy.

16. In the context of refractory VF, 'vector change' defibrillation means:

    1. A. Raising the energy to 360 J biphasic
    2. B. Switching to a synchronized shock
    3. C. Moving pads from anterolateral to AP
    4. D. Shocking twice from one defibrillator
    Show answer

    Answer: C. Moving pads from anterolateral to AP

    Vector change moves the pads to an anterior-posterior position so current takes a different path through the heart. Firing two defibrillators in rapid sequence is double sequential external defibrillation, a separate technique.

17. For refractory VF, the 2023 and 2025 AHA guidelines describe double sequential external defibrillation and vector change as:

    1. A. Required after the third failed shock
    2. B. Not established as beneficial
    3. C. Harmful and not to be used in the field
    4. D. Proven superior in every adult arrest
    Show answer

    Answer: B. Not established as beneficial

    The DOSE-VF trial suggested better survival with both techniques, but AHA judged the evidence insufficient and states their usefulness has not been established. Some EMS systems use them by protocol, so follow local medical direction.

18. A 70-year-old man has a wide-complex tachycardia at 190 per minute on the monitor and no palpable pulse. You should:

    1. A. Perform synchronized cardioversion at 100 J
    2. B. Give adenosine 6 mg by rapid IV push
    3. C. Defibrillate with an unsynchronized shock
    4. D. Give amiodarone 150 mg over 10 minutes
    Show answer

    Answer: C. Defibrillate with an unsynchronized shock

    Pulseless VT is treated exactly like VF, with unsynchronized defibrillation and CPR. Synchronized cardioversion is for VT with a pulse, and the monitor may fail to synchronize and delay the shock.

19. Current AHA guidance on antiarrhythmic drugs for shock-refractory adult VF states that:

    1. A. Amiodarone or lidocaine may be considered
    2. B. Procainamide is the preferred first agent
    3. C. Magnesium is first-line for all VF
    4. D. Antiarrhythmics should be avoided
    Show answer

    Answer: A. Amiodarone or lidocaine may be considered

    In ROC-ALPS, amiodarone and lidocaine improved survival to admission but not overall survival, with a possible benefit in bystander-witnessed arrest, so either may be considered. Magnesium is reserved for torsades de pointes.

20. Before delivering a shock, the operator should:

    1. A. Pause compressions for 30 seconds
    2. B. Clearly announce 'clear' and look
    3. C. Disconnect the bag from the airway
    4. D. Turn off the cardiac monitor
    Show answer

    Answer: B. Clearly announce 'clear' and look

    The operator must announce the shock and visually confirm no one is touching the patient, including the person managing the airway. Long pauses before shocks lower the chance of success, and the bag can usually stay attached to an advanced airway.

21. The goal of defibrillation in VF is to:

    1. A. Restart a heart that is in asystole
    2. B. Pace the ventricles until a rhythm returns
    3. C. Block the reentry circuit in the AV node
    4. D. Depolarize the heart so it can reset
    Show answer

    Answer: D. Depolarize the heart so it can reset

    A shock depolarizes a critical mass of myocardium at once, ending chaotic activity so the heart's own pacemaker can restore an organized rhythm. Defibrillation does not 'restart' asystole, which is why asystole is not shocked. It is a single depolarizing pulse rather than repeated pacing stimuli, and it does not target an AV nodal reentry circuit the way adenosine does in SVT.

22. During a VF arrest, two IV attempts have failed. The next step for drug access is to:

    1. A. Give drugs down the ET tube instead
    2. B. Give epinephrine IM in the thigh
    3. C. Place an IO line
    4. D. Use the intranasal route with an atomizer
    Show answer

    Answer: C. Place an IO line

    AHA recommends trying peripheral IV access first and using IO access when IV attempts fail or are not feasible, so an IO line is next. The endotracheal route gives low, unreliable drug levels and should not be relied on. IM epinephrine is an anaphylaxis route and is not absorbed reliably without circulation, and intranasal atomizers are used for drugs such as naloxone or midazolam, not for arrest drugs. (2025 AHA Part 9; PARAMEDIC-3 and IVIO 2025.)

23. VF returns on the monitor during transport after an earlier successful shock at 200 J. You should:

    1. A. Resume CPR and shock again at 200 J
    2. B. Start at 50 J to protect the myocardium
    3. C. Give amiodarone before any shock
    4. D. Cardiovert at 100 J synchronized
    Show answer

    Answer: A. Resume CPR and shock again at 200 J

    Recurrent VF is treated with CPR and defibrillation, often at the energy that worked before or higher per the device. Delaying the shock to give a drug first lowers the chance of conversion.

24. Which statement about using an AED on a patient with an implanted cardioverter-defibrillator (ICD) that is firing is correct?

    1. A. CPR must stop while the ICD keeps firing
    2. B. An AED should not be used with an ICD
    3. C. The ICD shock will injure the rescuer
    4. D. Rescuers may feel a tingle but are safe
    Show answer

    Answer: D. Rescuers may feel a tingle but are safe

    ICD shocks deliver little energy to the body surface, so rescuers may feel a tingle but are not harmed, and CPR should continue. If the patient remains in VF, an AED is used with pads placed away from the device.

25. In adult VF, compared with monophasic waveforms, biphasic waveforms:

    1. A. Need higher energy for success
    2. B. Succeed at lower energy levels
    3. C. Cannot be used in pediatric arrest
    4. D. Are used only for cardioversion
    Show answer

    Answer: B. Succeed at lower energy levels

    Biphasic waveforms reverse current direction partway through the shock and terminate VF at lower energy with less myocardial injury. That is why biphasic devices shock at 120 to 200 J while monophasic devices need 360 J.