Reading a rhythm strip and knowing what to do about it is a core NCLEX skill, and the single most tested piece is shockable versus non-shockable. Work through these 10 practice questions on ECG basics, sinus rhythms, a-fib, SVT, VT and VF, heart blocks, and cardioversion versus defibrillation, each with a full rationale.
Dysrhythmias & ECG: Rhythms, Shockable vs Not & Drugs
The ECG tells a predictable story: the P wave is atrial depolarization, the QRS is ventricular depolarization, and the T wave is ventricular repolarization. Normal sinus rhythm is regular at 60-100 beats/min with a P wave before every QRS. Read rate, then regularity, then look for P waves and their relationship to the QRS. Sinus bradycardia (under 60) is treated only when the client is symptomatic (dizziness, hypotension, altered mentation), with atropine first, then transcutaneous pacing; for symptomatic Mobitz II or third-degree block, transcutaneous pacing is preferred because atropine is often ineffective. Sinus tachycardia (over 100) is a response to something else, so you treat the underlying cause: fever, pain, hypovolemia, hypoxia, or anxiety.
Atrial fibrillation is irregularly irregular with no discrete P waves; the great danger is a clot forming in the fibrillating atria and traveling to the brain, so the priority is anticoagulation to prevent embolic stroke, plus rate and rhythm control. Atrial flutter shows sawtooth flutter waves. Supraventricular tachycardia (SVT) is a fast, narrow-complex rhythm: try vagal maneuvers first, then adenosine by rapid IV push followed by a fast saline flush. On the ventricular side, know pulses. Ventricular tachycardia with a pulse but unstable gets synchronized cardioversion; pulseless VT and ventricular fibrillation are lethal, shockable rhythms treated with immediate defibrillation and high-quality CPR. Torsades de pointes gets IV magnesium.
The single most tested concept: shockable versus non-shockable. Shockable = ventricular fibrillation and pulseless VT (defibrillate). Non-shockable = asystole and PEA (never shock; give high-quality CPR and epinephrine and correct the reversible H's and T's). Synchronized cardioversion (timed to the R wave) is for unstable tachydysrhythmias that still have a pulse, such as unstable atrial fibrillation with rapid ventricular response, SVT, or monomorphic VT with a pulse; defibrillation is unsynchronized, and unstable polymorphic VT (torsades) with a pulse is also treated with an unsynchronized shock. Heart blocks: first-degree is a benign prolonged PR; Wenckebach (type I) lengthens the PR until a beat drops; type II drops beats with a constant PR and is dangerous; third-degree (complete) shows independent P waves and QRS and needs a pacemaker.
Cardiac Dysrhythmias & ECG NCLEX Practice Questions
Question 1 · ECG Basics
A nursing student asks the nurse to explain what the P wave on a client's ECG rhythm strip represents. Which response by the nurse is correct?
- A The P wave represents atrial depolarization
- B The P wave represents ventricular depolarization
- C The P wave represents ventricular repolarization
- D The P wave represents the resting state of the ventricles
Show answer & rationale
Correct answer: A
The P wave reflects atrial depolarization, the electrical event that triggers the atria to contract. On a normal strip a P wave precedes every QRS complex, confirming that the impulse originated in the SA node and traveled through the atria before reaching the ventricles. The choice naming ventricular depolarization is wrong because ventricular depolarization produces the QRS complex, the large, sharp deflection that follows the P wave and drives the pulse you palpate. The choice naming ventricular repolarization is wrong because repolarization of the ventricles produces the T wave, the rounded deflection after the QRS; disturbances here (peaked or inverted T waves) suggest electrolyte problems or ischemia. The choice describing a resting ventricular state is wrong because the electrically silent, flat baseline periods are isoelectric segments, not a wave. Clinical pearl: walk the waveform in order, P then QRS then T equals atrial depolarization, ventricular depolarization, ventricular repolarization. A quick memory aid is 'P before every QRS keeps the rhythm sinus.' In normal sinus rhythm the rate is 60-100, the rhythm is regular, and one upright P wave sits in front of each QRS. When P waves disappear or lose their consistent relationship to the QRS, suspect atrial fibrillation or a heart block, which is why identifying the P wave is a foundational NCLEX skill.
Question 2 · Sinus Bradycardia
A client's monitor shows sinus bradycardia at 42 beats/min. The client is dizzy, diaphoretic, and has a blood pressure of 84/50 mm Hg. Which medication should the nurse anticipate administering first?
- A Adenosine
- B Atropine
- C Amiodarone
- D Magnesium sulfate
Show answer & rationale
Correct answer: B
This client has symptomatic bradycardia: a slow rate plus signs of poor perfusion (dizziness, diaphoresis, hypotension). The first-line drug is atropine, an anticholinergic that blocks vagal tone at the SA node and increases the heart rate. If atropine is ineffective, the next step is transcutaneous pacing (and possibly dopamine or epinephrine infusions). Adenosine is wrong because it slows conduction through the AV node to break a fast narrow-complex tachycardia such as SVT; giving it to a bradycardic client could worsen the slow rate or cause a pause. Amiodarone is wrong because it is an antidysrhythmic used for ventricular tachycardia, ventricular fibrillation, and some atrial dysrhythmias; it can further slow conduction and is not indicated for symptomatic bradycardia. Magnesium sulfate is wrong because it is the treatment for torsades de pointes, not bradycardia. Remember the key qualifier: asymptomatic bradycardia (common in athletes and during sleep) needs no drug at all, only monitoring. Treat the client, not the number. Clinical pearl: for symptomatic bradycardia think 'Atropine, then Pace, then Pressors.' Atropine is dosed at 1 mg IV every 3-5 minutes up to a maximum of 3 mg. Before medicating, ensure the airway is patent and oxygen is applied, because hypoxia is a reversible cause of bradycardia.
Question 3 · Atrial Fibrillation
The nurse is reviewing the plan of care for a client newly diagnosed with atrial fibrillation. Which statements about atrial fibrillation are accurate? Select all that apply.
- A Clients are at high risk for embolic stroke from atrial clot formation
- B Sawtooth-shaped waves are the hallmark finding
- C It appears as an irregularly irregular rhythm with no discrete P waves
- D Rate control is a treatment goal
- E The rhythm always requires immediate defibrillation
- F Anticoagulation is a central part of management
Show answer & rationale
Correct answer: A, C, D, F
Atrial fibrillation is the classic irregularly irregular rhythm; the atria quiver chaotically instead of contracting, so no organized P waves are visible, only a wavy baseline before the QRS complexes. Because blood pools and stagnates in the non-contracting atria, clots can form and embolize, and the most feared complication is an embolic stroke. That is why anticoagulation (warfarin or a DOAC such as apixaban, guided by CHA2DS2-VASc scoring) is a cornerstone of management, alongside rate control (beta blockers, diltiazem) and sometimes rhythm control or ablation. Rate control is therefore a legitimate treatment goal, keeping the ventricular response from becoming dangerously fast. The statement about sawtooth waves is incorrect: sawtooth flutter waves are the hallmark of atrial flutter, not atrial fibrillation. The statement that a-fib always requires immediate defibrillation is incorrect; defibrillation is for pulseless shockable rhythms (VF and pulseless VT). An unstable client in a-fib with a pulse would receive synchronized cardioversion, not defibrillation, and stable clients are managed with medications. Clinical pearl: think 'a-fib = irregularly irregular, no P waves, prevent the stroke.' Before elective cardioversion of a-fib lasting more than 48 hours, the client is typically anticoagulated for about 3 weeks or screened with a transesophageal echocardiogram to rule out an atrial clot that cardioversion could dislodge.
Question 4 · SVT
A stable client presents with a regular, narrow-complex tachycardia at a rate of 190 beats/min consistent with supraventricular tachycardia (SVT). Which intervention should the nurse expect to try first?
- A Immediate unsynchronized defibrillation
- B Vagal maneuvers such as bearing down
- C Synchronized cardioversion
- D Intravenous magnesium sulfate
Show answer & rationale
Correct answer: B
For a stable client in SVT, the first-line intervention is vagal maneuvers, such as having the client bear down (Valsalva) or applying carotid sinus massage per provider order. Stimulating the vagus nerve slows AV node conduction and can convert the rhythm without drugs. If vagal maneuvers fail, the next step is adenosine given by rapid IV push into a proximal vein, immediately followed by a fast 20 mL saline flush, because adenosine has an extremely short half-life (seconds) and must reach the heart quickly; clients should be warned they may briefly feel flushing, chest pressure, or a sense that the heart pauses. Immediate unsynchronized defibrillation is wrong because defibrillation is reserved for pulseless shockable rhythms (VF and pulseless VT); this client has a pulse and is stable. Synchronized cardioversion is wrong for a stable client; it is used when SVT causes instability (hypotension, chest pain, altered mentation, signs of shock). Magnesium sulfate is wrong because it treats torsades de pointes, a polymorphic ventricular tachycardia, not a narrow-complex supraventricular rhythm. Clinical pearl: for stable SVT go 'Vagal, then Adenosine'; for unstable SVT go straight to synchronized cardioversion. Always assess stability first, because the client's hemodynamic status, not just the rate, decides how aggressively you treat.
Question 5 · Shockable vs Non-Shockable
During a code, the nurse must quickly identify which rhythms are shockable (require defibrillation) per current resuscitation guidelines. Which rhythms are shockable? Select all that apply.
- A Asystole
- B Ventricular fibrillation
- C Sinus bradycardia
- D Pulseless electrical activity (PEA)
- E Pulseless ventricular tachycardia
Show answer & rationale
Correct answer: B, E
This is the single most tested dysrhythmia concept: only two rhythms are shockable in cardiac arrest, ventricular fibrillation and pulseless ventricular tachycardia. Both are shockable ventricular rhythms in which the heart cannot perfuse: ventricular fibrillation is chaotic and disorganized, while pulseless VT is organized but far too fast to produce a pulse. Delivering an unsynchronized shock (defibrillation) depolarizes the entire myocardium at once so the heart's normal pacemaker can, ideally, resume an organized rhythm. Every defibrillation is sandwiched between cycles of high-quality CPR. Asystole is not shockable; it is the absence of electrical activity (a flat line), and there is nothing for a shock to reorganize, so treatment is high-quality CPR and epinephrine while correcting reversible causes. Pulseless electrical activity is not shockable either; the monitor shows organized electrical activity but the heart is not generating a pulse, so, like asystole, it is treated with CPR, epinephrine, and correction of the H's and T's (hypoxia, hypovolemia, hydrogen ion/acidosis, hypo/hyperkalemia, hypothermia, tension pneumothorax, tamponade, toxins, thrombosis). Sinus bradycardia is a perfusing rhythm with a pulse and is never defibrillated; if symptomatic it is treated with atropine and pacing. Clinical pearl: 'Shock the Vs' equals VF and pulseless VT; asystole and PEA get compressions and epi, never a shock. Defibrillating asystole or PEA is a classic wrong answer and can cause harm by delaying compressions.
Question 6 · Ventricular Fibrillation
A hospitalized client suddenly collapses. The monitor shows ventricular fibrillation, and the nurse confirms the client is pulseless and unresponsive. After calling for help, which action is the priority?
- A Administer atropine 1 mg by intravenous push
- B Perform synchronized cardioversion immediately
- C Defibrillate, then resume high-quality CPR
- D Apply pads and begin transcutaneous pacing
Show answer & rationale
Correct answer: C
Ventricular fibrillation is a lethal, shockable rhythm: the ventricles quiver chaotically and produce no cardiac output, so the client is pulseless. The priority is early defibrillation, delivered as soon as a defibrillator is available, immediately followed by resuming high-quality CPR (push hard and fast, minimize interruptions). Early defibrillation combined with quality compressions is the intervention most associated with survival in VF arrest. Atropine is wrong; it treats symptomatic bradycardia and has no role in VF. Synchronized cardioversion is wrong because synchronization requires the machine to detect an R wave to time the shock, and VF has no organized QRS, so the device cannot synchronize; VF must be treated with unsynchronized defibrillation. Attempting to synchronize would dangerously delay the shock. Transcutaneous pacing is wrong because pacing addresses rhythms that are too slow (symptomatic bradycardia, high-grade heart block), not a chaotic pulseless rhythm. While CPR continues, the team establishes IV/IO access and gives epinephrine every 3-5 minutes and considers amiodarone for refractory VF. Clinical pearl: for a pulseless shockable rhythm, 'CPR and shock' win; do not waste time hunting for a pulse in obvious VF, and do not try to synchronize a rhythm with no discernible R wave. Verify the leads are connected to rule out artifact, but never delay defibrillation of true VF.
Question 7 · Cardioversion vs Defibrillation
The nurse is preparing to assist with synchronized cardioversion. For which client is synchronized cardioversion the appropriate intervention?
- A An unstable client in rapid atrial fibrillation who still has a pulse
- B A pulseless, unresponsive client in coarse ventricular fibrillation
- C An unresponsive, pulseless client in asystole confirmed in two leads
- D A pulseless client in PEA showing a narrow-complex rhythm on the monitor
Show answer & rationale
Correct answer: A
Synchronized cardioversion delivers a shock timed to the R wave of the QRS complex, and it is used for unstable tachydysrhythmias in a client who still has a pulse, such as unstable atrial fibrillation with rapid ventricular response, unstable SVT, or unstable VT with a pulse. Timing the shock to the R wave avoids delivering energy during the vulnerable T-wave period, which could otherwise induce ventricular fibrillation. The client in coarse ventricular fibrillation is wrong for synchronized cardioversion because VF has no organized R wave to synchronize to and is pulseless; VF requires unsynchronized defibrillation. Asystole is wrong because it is a non-shockable rhythm (no electrical activity); it is treated with high-quality CPR and epinephrine, never any shock. PEA is wrong for the same reason: despite organized-looking electrical activity, such as a narrow-complex rhythm, there is no pulse, and it is treated with CPR and epinephrine, not shocks. Clinical pearl: 'Sync the pulse, shock the pulseless.' Synchronized cardioversion equals a pulse present but unstable; defibrillation (unsynchronized) equals pulseless VF or pulseless VT. Before elective cardioversion the nurse ensures consent, sedation, and, for a-fib longer than 48 hours, adequate anticoagulation or a TEE to exclude an atrial clot. Confirm the SYNC button is engaged; on many machines it must be re-selected after each synchronized shock because the device defaults back to the unsynchronized (defibrillation) mode.
Question 8 · Heart Blocks
The nurse is teaching a group of new graduate nurses about atrioventricular (AV) heart blocks. Which statements are accurate? Select all that apply.
- A First-degree AV block is a lethal rhythm requiring immediate defibrillation
- B First-degree AV block shows a consistently prolonged PR interval and is usually benign
- C Second-degree type I (Wenckebach) shows progressive PR lengthening until a QRS is dropped
- D Second-degree type II shows a constant PR interval with intermittently dropped QRS complexes and often needs pacing
- E Third-degree (complete) block shows P waves and QRS complexes that occur independently of each other
- F Third-degree (complete) block is typically treated with a pacemaker
Show answer & rationale
Correct answer: B, C, D, E, F
First-degree AV block is simply a consistently prolonged PR interval (greater than 0.20 seconds) with every impulse still conducting to the ventricles; it is usually benign and needs monitoring, not emergency treatment. Second-degree type I, also called Wenckebach or Mobitz I, shows the PR interval progressively lengthening beat to beat until one QRS is dropped, then the cycle repeats; it is often transient and less dangerous. Second-degree type II (Mobitz II) shows a constant PR interval with QRS complexes that drop intermittently without warning; this is a dangerous, unstable block that frequently progresses and often requires pacing. Third-degree (complete) block shows complete AV dissociation: the atria and ventricles beat independently, so P waves and QRS complexes march to their own separate rhythms with no consistent relationship; this is treated with a pacemaker (transcutaneous initially, then permanent). The statement that first-degree block is lethal and needs defibrillation is false: it is benign, and defibrillation is only for VF and pulseless VT, never for a perfusing conduction delay. Clinical pearl: 'Longer, longer, longer, drop equals Wenckebach; if some Ps just don't get through, then you have Mobitz II; if Ps and Qs don't agree, then you have third degree.' The higher the degree of block, the greater the need for pacing.
Question 9 · Torsades de Pointes
A client with a prolonged QT interval develops torsades de pointes, a polymorphic ventricular tachycardia in which the QRS complexes appear to twist around the baseline. The client still has a pulse, is alert, and has a blood pressure of 110/68 mm Hg. Which treatment should the nurse anticipate?
- A Intravenous adenosine by rapid push
- B Intravenous atropine
- C Immediate unsynchronized defibrillation
- D Intravenous magnesium sulfate
Show answer & rationale
Correct answer: D
Torsades de pointes is a distinctive polymorphic ventricular tachycardia associated with a prolonged QT interval, and the specific treatment is intravenous magnesium sulfate, which stabilizes the myocardium and suppresses the triggering activity. The nurse should also identify and remove QT-prolonging causes (many antidysrhythmics, certain antibiotics and antipsychotics) and correct electrolyte deficits, especially low magnesium and potassium. Adenosine is wrong because it is used to break a narrow-complex SVT by slowing AV conduction; it has no role in polymorphic VT. Atropine is wrong because it treats symptomatic bradycardia, not a fast ventricular rhythm. Immediate unsynchronized defibrillation is wrong in this scenario because the client still has a pulse and no signs of instability are present; unsynchronized shocks are indicated for pulseless torsades, pulseless VT, ventricular fibrillation, and polymorphic VT (such as torsades) with a pulse that becomes hemodynamically unstable. Importantly, if torsades becomes pulseless, the client is then defibrillated as a shockable rhythm. Note also that synchronized cardioversion is difficult in torsades because the polymorphic, twisting QRS morphology makes reliable R-wave synchronization hard. Clinical pearl: 'Torsades twists, give magnesium.' Anytime you see a client on QT-prolonging drugs with a low magnesium level, keep torsades on your radar and expect magnesium at the bedside. Reassess the pulse frequently, because the treatment pathway changes the instant the client loses a pulse.
Question 10 · Asystole and PEA
A client's monitor shows asystole, and the nurse confirms the client is unresponsive with no pulse. Which action is appropriate?
- A Defibrillate immediately
- B Perform synchronized cardioversion
- C Begin high-quality CPR and administer epinephrine
- D Deliver a precordial thump followed by a shock
Show answer & rationale
Correct answer: C
Asystole is the absence of electrical activity, a flat line, and it is a non-shockable rhythm. The correct response is to begin high-quality CPR immediately and administer epinephrine every 3-5 minutes while the team searches for and corrects reversible causes, the H's and T's (hypoxia, hypovolemia, hydrogen ion/acidosis, hypo/hyperkalemia, hypothermia, tension pneumothorax, cardiac tamponade, toxins, and thrombosis, coronary or pulmonary). Defibrillating immediately is wrong: there is no organized or fibrillatory electrical activity for a shock to convert, so a shock provides no benefit and only interrupts compressions. Synchronized cardioversion is wrong because it requires an R wave to synchronize to and is used for unstable tachydysrhythmias with a pulse, not for a pulseless flat line. A precordial thump followed by a shock is wrong; the precordial thump has a very limited, specific role and is not indicated for asystole, and shocking asystole is never appropriate. The same principle applies to pulseless electrical activity (PEA), the other non-shockable arrest rhythm: CPR plus epinephrine plus fixing the underlying cause. Clinical pearl: 'You cannot shock a flat line.' Before accepting asystole, confirm it in more than one lead and check that the leads and gain settings are correct to rule out fine VF or a disconnected lead masquerading as asystole.
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Cardiac Dysrhythmias & ECG FAQ
What is the difference between defibrillation and synchronized cardioversion?
Defibrillation is an unsynchronized shock used for pulseless shockable rhythms, ventricular fibrillation and pulseless ventricular tachycardia. Synchronized cardioversion delivers a shock timed to the R wave and is used for unstable tachydysrhythmias in a client who still has a pulse, such as unstable atrial fibrillation with rapid ventricular response, unstable SVT, or unstable monomorphic VT with a pulse (unstable polymorphic VT gets an unsynchronized shock). Memory aid: sync the pulse, shock the pulseless.
Which cardiac rhythms are shockable and which are not?
Only two rhythms are shockable and get defibrillation: ventricular fibrillation and pulseless ventricular tachycardia. The non-shockable arrest rhythms are asystole (a flat line) and pulseless electrical activity (PEA); these are treated with high-quality CPR and epinephrine while you correct reversible causes. Never defibrillate asystole or PEA. This shockable-versus-non-shockable distinction is one of the highest-yield, most frequently tested dysrhythmia concepts on the NCLEX.
How do you treat symptomatic sinus bradycardia?
Treat bradycardia only when the client is symptomatic (dizziness, hypotension, chest pain, altered mentation). The first-line drug is atropine, 1 mg IV every 3-5 minutes up to 3 mg. If atropine is ineffective, move to transcutaneous pacing and consider dopamine or epinephrine infusions. Asymptomatic bradycardia, common in athletes and during sleep, needs monitoring only. Always treat the client, not just the number.
Why are clients with atrial fibrillation placed on anticoagulants?
In atrial fibrillation the atria quiver instead of contracting, so blood stagnates and can form a clot. If that clot embolizes to the brain it causes an ischemic stroke, the most feared complication of a-fib. Anticoagulation (warfarin or a DOAC such as apixaban, guided by CHA2DS2-VASc risk scoring) is central to management, along with rate and rhythm control. Before elective cardioversion of a-fib lasting over 48 hours, clients are anticoagulated or screened with a TEE to rule out an atrial clot.