I85PREP
⌘ K
CRAS study materials
CRAS study resources

CRAS practice questions

One question at a time. Build your CRAS understanding.

First 50 of 199 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Basic Cardiac Anatomy and Physiology

Which chamber of the heart normally generates the highest systolic pressure during contraction?

  1. Right ventricle
  2. Right atrium
  3. Left atrium
  4. Left ventricle Correct answer.

Explanation

The left ventricle must eject blood into the high-resistance systemic circulation, so it develops the greatest systolic pressure. The right ventricle pumps into the low-pressure pulmonary circuit, and the atria generate only modest pressures.

Question 2Basic Cardiac Anatomy and Physiology

Blood returning from the systemic circulation enters the heart through which vessel?

  1. Inferior vena cava Correct answer.
  2. Pulmonary vein
  3. Pulmonary artery
  4. Superior vena cava

Explanation

The inferior vena cava carries deoxygenated systemic blood from the lower body into the right atrium. The superior vena cava drains the upper body, the pulmonary veins carry oxygenated blood from the lungs, and the pulmonary artery carries blood away from the heart.

Question 3Basic Cardiac Anatomy and Physiology

Which valve prevents backflow of blood from the left ventricle into the left atrium during systole?

  1. Tricuspid valve
  2. Mitral valve Correct answer.
  3. Pulmonary valve
  4. Aortic valve

Explanation

The mitral (bicuspid) valve sits between the left atrium and left ventricle and closes during ventricular systole to prevent regurgitation. The aortic and pulmonary valves are semilunar outflow valves, and the tricuspid valve guards the right side of the heart.

Question 4Basic Cardiac Anatomy and Physiology

What is the normal resting heart rate range for a healthy adult?

  1. 20 to 50 beats per minute
  2. 60 to 100 beats per minute Correct answer.
  3. 100 to 140 beats per minute
  4. 40 to 60 beats per minute

Explanation

A normal adult resting sinus rhythm is conventionally defined as 60 to 100 beats per minute. Rates below 60 are termed bradycardia and rates above 100 tachycardia.

Question 5Basic Cardiac Anatomy and Physiology

Which structure is the normal primary pacemaker of the heart?

  1. Atrioventricular node
  2. Sinoatrial node Correct answer.
  3. Purkinje fibers
  4. Bundle of His

Explanation

The sinoatrial node in the upper right atrium has the fastest intrinsic firing rate and therefore sets the normal sinus rhythm. The atrioventricular node and Purkinje system serve as slower backup pacemakers.

Question 6Basic Cardiac Anatomy and Physiology

Which blood vessel type is the primary site of resistance to blood flow in the circulation?

  1. Arterioles Correct answer.
  2. Large veins
  3. Capillaries
  4. Elastic arteries

Explanation

Arterioles have thick smooth muscle and small lumens, producing the greatest resistance and allowing regulation of tissue blood flow. Capillaries are exchange vessels, and large arteries and veins contribute little to resistance.

Question 7Basic Cardiac Anatomy and Physiology

During which phase of the cardiac cycle do the coronary arteries receive most of their blood flow?

  1. Ventricular ejection
  2. Ventricular diastole Correct answer.
  3. Isovolumetric relaxation
  4. Isovolumetric contraction

Explanation

Coronary perfusion occurs mainly during ventricular diastole, when the myocardium is relaxed and intramyocardial pressure is low. During systole, contracting muscle compresses the coronary vessels and limits flow.

Question 8Basic Cardiac Anatomy and Physiology

A patient's stroke volume is 70 mL and heart rate is 80 beats per minute. What is the cardiac output?

  1. 560 mL/min
  2. 5.6 L/min Correct answer.
  3. 56 L/min
  4. 0.56 L/min

Explanation

Cardiac output equals stroke volume multiplied by heart rate: 70 mL × 80 beats/min = 5600 mL/min, or 5.6 L/min. This falls within the typical adult resting range.

Question 9Basic Cardiac Anatomy and Physiology

According to the Frank-Starling relationship, what happens to stroke volume when ventricular end-diastolic volume increases within physiologic limits?

  1. It falls because of reduced contractility
  2. It decreases because afterload rises
  3. It increases because of greater myocardial stretch Correct answer.
  4. It remains unchanged

Explanation

Greater end-diastolic filling stretches myocardial fibers, increasing actin-myosin overlap and force of contraction, which raises stroke volume. This length-tension relationship is the basis of the Frank-Starling law.

Question 10Basic Cardiac Anatomy and Physiology

Which component of the ECG represents ventricular depolarization?

  1. P wave
  2. T wave
  3. PR segment
  4. QRS complex Correct answer.

Explanation

The QRS complex reflects depolarization of the ventricular myocardium. The P wave represents atrial depolarization and the T wave represents ventricular repolarization.

Question 11Basic Cardiac Anatomy and Physiology

What is the primary function of the atrioventricular valves during ventricular systole?

  1. To prevent backflow into the atria Correct answer.
  2. To close the coronary ostia
  3. To open and allow atrial filling
  4. To regulate flow into the aorta

Explanation

During ventricular systole, rising ventricular pressure closes the atrioventricular valves, preventing blood from regurgitating into the atria. The semilunar valves open to permit ejection.

Question 12Basic Cardiac Anatomy and Physiology

Which layer of the heart wall is responsible for the force of myocardial contraction?

  1. Epicardium
  2. Endocardium
  3. Pericardium
  4. Myocardium Correct answer.

Explanation

The myocardium is the thick muscular middle layer composed of cardiac muscle cells and generates contractile force. The endocardium lines the chambers, the epicardium is the outer serous layer, and the pericardium is the surrounding sac.

Question 13Basic Cardiac Anatomy and Physiology

Which factor most directly determines myocardial oxygen consumption?

  1. Heart rate, contractility, and wall stress Correct answer.
  2. Respiratory rate
  3. Heart rate alone
  4. Blood pH

Explanation

Myocardial oxygen demand is determined chiefly by heart rate, contractility, and ventricular wall stress, which reflects preload and afterload. These factors together set the workload of the myocardium.

Question 14Basic Cardiac Anatomy and Physiology

During isovolumetric ventricular contraction, which statement is true?

  1. The aortic valve is open
  2. Blood flows into the atria from the ventricles
  3. The mitral valve is open
  4. All four valves are closed Correct answer.

Explanation

In isovolumetric contraction, ventricular pressure rises while both the atrioventricular and semilunar valves remain closed, so ventricular volume is constant. Once ventricular pressure exceeds aortic pressure, the aortic valve opens.

Question 15Basic Cardiac Anatomy and Physiology

Which structure conducts the electrical impulse from the atria to the ventricles?

  1. Sinoatrial node
  2. Purkinje fibers only
  3. Atrioventricular node and bundle of His Correct answer.
  4. Chordae tendineae

Explanation

The atrioventricular node and bundle of His provide the only normal electrical pathway from atria to ventricles. This conduction path also introduces a delay that allows the atria to empty before ventricular contraction.

Question 16Basic Cardiac Anatomy and Physiology

A patient has a mean arterial pressure of 90 mm Hg and a right atrial pressure of 10 mm Hg. What is the approximate driving pressure for systemic flow?

  1. 80 mm Hg Correct answer.
  2. 100 mm Hg
  3. 10 mm Hg
  4. 90 mm Hg

Explanation

The perfusion driving pressure equals mean arterial pressure minus right atrial pressure: 90 − 10 = 80 mm Hg. This gradient drives blood through the systemic circulation.

Question 17Basic Cardiac Anatomy and Physiology

Which of the following best explains why the left ventricular wall is thicker than the right ventricular wall?

  1. The right ventricle has more Purkinje fibers
  2. The right ventricle receives more blood
  3. The left ventricle has a larger chamber volume
  4. The left ventricle pumps against higher resistance Correct answer.

Explanation

The left ventricle must overcome the high resistance of the systemic circulation, so its myocardium hypertrophies to generate greater pressure. The right ventricle pumps into the low-resistance pulmonary circuit and requires less muscle mass.

Question 18Basic Cardiac Anatomy and Physiology

Which change would most increase cardiac output in a healthy person during exercise?

  1. Increased sympathetic tone to the heart Correct answer.
  2. Decreased venous return
  3. Increased afterload alone
  4. Increased parasympathetic tone

Explanation

Sympathetic stimulation raises heart rate and contractility, increasing cardiac output during exercise. It also improves venous return through venoconstriction, further supporting stroke volume.

Question 19Basic Cardiac Anatomy and Physiology

Which statement best describes the relationship between preload and afterload in determining stroke volume?

  1. Increased preload lowers stroke volume while increased afterload raises it
  2. Neither preload nor afterload affects stroke volume
  3. Both increased preload and increased afterload raise stroke volume
  4. Increased preload raises stroke volume while increased afterload lowers it Correct answer.

Explanation

Preload increases myocardial stretch and enhances contraction, raising stroke volume. Afterload is the resistance the ventricle must overcome to eject blood, so higher afterload reduces stroke volume.

Question 20Basic Cardiac Anatomy and Physiology

Which of the following sequences correctly traces blood flow through the right side of the heart?

  1. Right atrium, pulmonary vein, right ventricle, pulmonary valve, aorta
  2. Right atrium, mitral valve, right ventricle, aortic valve, aorta
  3. Right ventricle, tricuspid valve, right atrium, pulmonary vein, lungs
  4. Right atrium, tricuspid valve, right ventricle, pulmonary valve, pulmonary artery Correct answer.

Explanation

Deoxygenated blood enters the right atrium, passes through the tricuspid valve into the right ventricle, and is ejected through the pulmonary valve into the pulmonary artery. The mitral and aortic valves belong to the left side of the heart.

Question 21Electrophysiology and Arrhythmia Mechanisms

Which ion carries the rapid depolarizing upstroke (phase 0) of a ventricular myocyte action potential?

  1. Chloride influx through ligand-gated channels
  2. Calcium influx through L-type channels
  3. Sodium influx through fast voltage-gated channels Correct answer.
  4. Potassium efflux through delayed rectifier channels

Explanation

Phase 0 in atrial and ventricular myocytes is produced by a rapid inward sodium current through fast voltage-gated channels, giving the fast upstroke. Calcium current drives the phase 0 of nodal tissue, not the fast-response ventricular upstroke.

Question 22Electrophysiology and Arrhythmia Mechanisms

The plateau phase (phase 2) of a ventricular action potential is maintained chiefly by the balance of which currents?

  1. Inward funny current versus outward sodium current
  2. Inward L-type calcium versus outward potassium currents Correct answer.
  3. Inward sodium versus outward chloride currents
  4. Outward calcium versus inward potassium currents

Explanation

During phase 2, slowly inactivating L-type calcium influx is balanced against outward potassium currents, producing the sustained plateau. This plateau prolongs refractoriness and couples excitation to contraction.

Question 23Electrophysiology and Arrhythmia Mechanisms

In the normal heart, which structure normally serves as the dominant pacemaker because it has the fastest spontaneous firing rate?

  1. Atrioventricular node
  2. Purkinje fibers
  3. Bachmann bundle
  4. Sinoatrial node Correct answer.

Explanation

The sinoatrial node has the steepest phase 4 depolarization and thus the highest intrinsic rate, so it overdrives slower latent pacemakers. Suppression of subsidiary pacemakers by this overdrive is a key feature of normal impulse generation.

Question 24Electrophysiology and Arrhythmia Mechanisms

Which ECG interval best reflects the total duration of ventricular depolarization and repolarization?

  1. QT interval Correct answer.
  2. QRS duration
  3. PR interval
  4. RR interval

Explanation

The QT interval spans from the start of the QRS to the end of the T wave, representing ventricular depolarization plus repolarization. It varies inversely with heart rate, which is why rate-corrected values are used.

Question 25Electrophysiology and Arrhythmia Mechanisms

Enhanced automaticity of an ectopic focus is best characterized by which mechanism?

  1. Reentry around a fixed anatomic obstacle
  2. A steepened phase 4 spontaneous depolarization in a non-pacemaker cell Correct answer.
  3. Reflection of impulses at a conduction block
  4. Triggered activity from early afterdepolarizations

Explanation

Abnormal automaticity occurs when cells that normally do not pacemake develop a steeper phase 4 depolarization and reach threshold spontaneously. This differs mechanistically from reentry and from triggered activity, which depend on afterdepolarizations.

Question 26Electrophysiology and Arrhythmia Mechanisms

Which set of conditions is required for reentry to be sustained around an anatomic circuit?

  1. Unidirectional block plus conduction time exceeding refractoriness Correct answer.
  2. Delayed afterdepolarizations reaching threshold
  3. Two independent ectopic foci firing asynchronously
  4. A short refractory period only

Explanation

Sustained reentry requires a unidirectional block that starts the circuit and a conduction time around the loop longer than the refractory period so the tissue has recovered when the impulse returns. If the circuit time is shorter than refractoriness, the wavefront extinguishes.

Question 27Electrophysiology and Arrhythmia Mechanisms

A patient has a narrow-complex tachycardia with sudden onset and termination and evidence of an accessory pathway on baseline ECG. Which mechanism most likely underlies the tachycardia?

  1. Reentry involving the AV node and accessory pathway Correct answer.
  2. Early afterdepolarizations from prolonged repolarization
  3. Enhanced normal automaticity of the sinus node
  4. Triggered activity from delayed afterdepolarizations

Explanation

Sudden-onset, sudden-offset narrow-complex tachycardia in a patient with an accessory pathway is characteristic of AV reentrant tachycardia, a macroreentrant circuit using the AV node and the bypass tract. The abrupt behavior reflects a reentrant rather than automatic mechanism.

Question 28Electrophysiology and Arrhythmia Mechanisms

Early afterdepolarizations are most likely to arise under which condition?

  1. Rapid pacing at physiologic rates
  2. Action potential shortening with hyperkalemia
  3. Increased phase 0 sodium current
  4. Prolonged repolarization with bradycardia Correct answer.

Explanation

Early afterdepolarizations occur when repolarization is prolonged, allowing reactivation of inward calcium (and late sodium) currents before the membrane fully repolarizes. Bradycardia and QT-prolonging conditions favor them, which is why torsades de pointes is bradycardia-dependent.

Question 29Electrophysiology and Arrhythmia Mechanisms

Delayed afterdepolarizations are classically caused by which cellular disturbance?

  1. Reduced intracellular sodium with decreased calcium entry
  2. Intracellular calcium overload with spontaneous sarcoplasmic reticulum release Correct answer.
  3. Prolonged action potential duration from potassium channel block
  4. Loss of gap junction coupling between cells

Explanation

Delayed afterdepolarizations arise when intracellular calcium overload causes the sarcoplasmic reticulum to release calcium spontaneously, activating the electrogenic sodium-calcium exchanger and producing a transient inward current. Digoxin toxicity and catecholamine excess are classic clinical triggers.

Question 30Electrophysiology and Arrhythmia Mechanisms

Which property of the AV node explains the physiologic delay between atrial and ventricular activation?

  1. Slow conduction from calcium-dependent upstrokes and low gap junction coupling Correct answer.
  2. High density of fast sodium channels
  3. Rapid conduction through large Purkinje fibers
  4. Absence of any resting membrane potential

Explanation

AV nodal cells are slow-response tissue: their upstroke depends on calcium current and they have sparse gap junctions, so conduction is slow. This delay allows the atria to empty before ventricular contraction.

Question 31Electrophysiology and Arrhythmia Mechanisms

Which feature best distinguishes the effective refractory period of a cardiac cell?

  1. The interval after full repolarization during which normal excitability returns
  2. The interval during which a stronger-than-normal stimulus can elicit a propagated response
  3. The interval during which no stimulus, however strong, can elicit a propagated response Correct answer.
  4. The interval during which conduction velocity is maximal

Explanation

During the effective refractory period no stimulus of any strength can produce a propagated action potential because fast sodium channels remain inactivated. Excitability then recovers progressively through the relative and supranormal periods.

Question 32Electrophysiology and Arrhythmia Mechanisms

Why does the ventricular myocardium normally not sustain tetanic contraction?

  1. The long plateau phase keeps cells refractory for most of the contraction Correct answer.
  2. Calcium is not required for ventricular contraction
  3. Gap junctions prevent mechanical summation
  4. Ventricular cells lack contractile proteins

Explanation

The prolonged plateau extends the effective refractory period to nearly the whole contraction, so a second excitation cannot occur before relaxation. This protects the heart from tetany that would impair filling.

Question 33Electrophysiology and Arrhythmia Mechanisms

A 12-lead ECG shows a PR interval of 0.28 seconds with every P wave followed by a QRS complex. Which conduction abnormality is present?

  1. Third-degree AV block
  2. First-degree AV block Correct answer.
  3. Complete intraventricular block
  4. Second-degree AV block type I

Explanation

A PR interval longer than 0.20 seconds with consistent 1:1 AV conduction defines first-degree AV block, which reflects slowed AV nodal conduction. Every P wave conducts, distinguishing it from higher-degree blocks.

Question 34Electrophysiology and Arrhythmia Mechanisms

Progressive lengthening of the PR interval followed by a dropped QRS complex is characteristic of which rhythm?

  1. Wenckebach (Mobitz type I) second-degree block Correct answer.
  2. First-degree AV block
  3. Mobitz type II second-degree block
  4. Ventricular preexcitation

Explanation

Wenckebach periodicity reflects fatigue-like conduction delay in the AV node, producing progressive PR prolongation until a beat fails to conduct, then resetting. The QRS complexes are typically narrow because conduction is still nodal.

Question 35Electrophysiology and Arrhythmia Mechanisms

Which finding suggests the mechanism of a wide-complex tachycardia is ventricular rather than supraventricular with aberrancy?

  1. QRS duration of 0.14 seconds
  2. History of atrial fibrillation
  3. Regular rhythm at 160 beats per minute
  4. AV dissociation with independent P waves Correct answer.

Explanation

AV dissociation means atria and ventricles are driven independently, which is strong evidence for a ventricular origin because the tachycardia is not conducted through the AV node. A wide QRS alone is nonspecific since aberrancy also widens the complex.

Question 36Electrophysiology and Arrhythmia Mechanisms

Which ECG change is most consistent with hypokalemia?

  1. Prominent U waves and flattened T waves Correct answer.
  2. Delta waves with a short PR interval
  3. Peaked T waves and widened QRS
  4. Shortened QT interval with tall R waves

Explanation

Hypokalemia hyperpolarizes the resting membrane potential and prolongs repolarization, producing flattened T waves, ST depression, and prominent U waves. Peaked T waves and QRS widening instead suggest hyperkalemia.

Question 37Electrophysiology and Arrhythmia Mechanisms

Hyperkalemia most directly affects the cardiac action potential by which early mechanism?

  1. Increasing the resting membrane potential negativity
  2. Reducing phase 0 sodium current by shortening the QRS
  3. Prolonging the plateau by increasing calcium current
  4. Depolarizing the resting membrane potential toward threshold Correct answer.

Explanation

Elevated extracellular potassium reduces the potassium gradient, making the resting membrane potential less negative (depolarized). This inactivates fast sodium channels, slowing conduction and eventually widening the QRS.

Question 38Electrophysiology and Arrhythmia Mechanisms

Which maneuver is expected to terminate a reentrant tachycardia that depends on the AV node?

  1. Rapid intravenous potassium
  2. Isometric handgrip sustained for one minute
  3. Carotid sinus massage Correct answer.
  4. Administration of a pure vasodilator

Explanation

Carotid sinus massage increases vagal tone, slowing AV nodal conduction and breaking the reentrant circuit, so it can terminate AV node-dependent tachycardias. Vagal maneuvers act by transiently blocking conduction through the node.

Question 39Electrophysiology and Arrhythmia Mechanisms

Which antiarrhythmic class primarily blocks sodium channels and slows phase 0 depolarization?

  1. Class II
  2. Class IV
  3. Class III
  4. Class I Correct answer.

Explanation

Class I agents block fast sodium channels, reducing the phase 0 upstroke and slowing conduction in fast-response tissue. The subclasses differ in their effects on repolarization and binding kinetics.

Question 40Electrophysiology and Arrhythmia Mechanisms

Which antiarrhythmic mechanism is shared by class III agents such as amiodarone and sotalol?

  1. Blocking potassium channels to prolong repolarization Correct answer.
  2. Increasing sodium current to speed conduction
  3. Blocking beta receptors to slow the sinus node
  4. Opening calcium channels to strengthen contraction

Explanation

Class III agents block potassium channels, delaying repolarization and prolonging action potential duration and refractoriness. Prolonged refractoriness suppresses reentrant circuits but also creates risk of torsades de pointes.

Question 41Electrophysiology and Arrhythmia Mechanisms

Digitalis toxicity commonly produces which arrhythmia mechanism?

  1. Reentry around a congenital accessory pathway
  2. Delayed afterdepolarizations from intracellular calcium overload Correct answer.
  3. Early afterdepolarizations from QT prolongation
  4. Enhanced sinus node automaticity

Explanation

Digitalis inhibits the sodium-potassium pump, raising intracellular sodium and secondarily intracellular calcium, which promotes delayed afterdepolarizations and triggered activity. This underlies the classic toxic arrhythmias such as atrial tachycardia with block and ventricular ectopy.

Question 42Electrophysiology and Arrhythmia Mechanisms

Which conduction property best explains why the Purkinje system activates the ventricles rapidly?

  1. High density of fast sodium channels and large fiber diameter Correct answer.
  2. Low gap junction coupling between cells
  3. Slow calcium-dependent upstrokes
  4. Extensive connective tissue insulation

Explanation

Purkinje fibers are fast-response tissue with abundant fast sodium channels and large diameter, both of which increase conduction velocity. Rapid conduction ensures coordinated ventricular activation.

Question 43Electrophysiology and Arrhythmia Mechanisms

The normal sequence of ventricular activation begins at which location?

  1. The atrioventricular valve annulus
  2. The epicardial apex
  3. The bundle of His and Purkinje network at the endocardium Correct answer.
  4. The left atrial appendage

Explanation

The impulse travels from the AV node through the bundle of His and Purkinje fibers, activating the endocardium first and spreading toward the epicardium. This endocardial-to-epicardial sequence shapes the normal QRS morphology.

Question 44Electrophysiology and Arrhythmia Mechanisms

A prolonged QT interval predisposes to which ventricular arrhythmia?

  1. Ventricular preexcitation
  2. Atrial flutter with 2:1 block
  3. Atrioventricular nodal reentrant tachycardia
  4. Torsades de pointes Correct answer.

Explanation

Prolonged repolarization permits early afterdepolarizations that can trigger polymorphic ventricular tachycardia known as torsades de pointes. The arrhythmia is classically bradycardia-dependent and associated with QT-prolonging drugs or electrolyte deficits.

Question 45ECG Interpretation and Rhythm Recognition

The standard paper speed for a resting 12-lead ECG is 25 mm/s. At this speed, how many seconds does one large box (5 mm) represent?

  1. 0.20 seconds Correct answer.
  2. 0.40 seconds
  3. 0.04 seconds
  4. 0.12 seconds

Explanation

At 25 mm/s, each small 1 mm box is 0.04 seconds, so a large 5 mm box is 5 x 0.04 = 0.20 seconds. This relationship is used to calculate rate and intervals.

Question 46ECG Interpretation and Rhythm Recognition

On a standard ECG, which lead is most commonly used to monitor the rhythm strip at the bottom of the tracing?

  1. Lead II Correct answer.
  2. Lead I
  3. Lead aVR
  4. Lead V1

Explanation

Lead II is typically chosen for the rhythm strip because its axis is roughly parallel to the P wave and QRS vectors, producing clear atrial and ventricular deflections. Many monitors default to lead II for continuous rhythm monitoring.

Question 47ECG Interpretation and Rhythm Recognition

Which rhythm is characterized by an irregularly irregular ventricular response with no discernible P waves?

  1. Multifocal atrial tachycardia
  2. Atrial flutter
  3. Atrial fibrillation Correct answer.
  4. Sinus arrhythmia

Explanation

Atrial fibrillation is defined by disorganized atrial activity with absent P waves and an irregularly irregular R-R interval. Atrial flutter usually has regular flutter waves, and sinus arrhythmia has visible P waves.

Question 48ECG Interpretation and Rhythm Recognition

A regular rhythm has an R-R interval of 4 large boxes. What is the approximate heart rate?

  1. 150 bpm
  2. 100 bpm
  3. 60 bpm
  4. 75 bpm Correct answer.

Explanation

The quick method divides 300 by the number of large boxes between R waves. With 4 large boxes, 300 / 4 = 75 bpm.

Question 49ECG Interpretation and Rhythm Recognition

Which interval is measured from the beginning of the P wave to the beginning of the QRS complex?

  1. ST segment
  2. QT interval
  3. PR interval Correct answer.
  4. QRS duration

Explanation

The PR interval begins at the onset of atrial depolarization and ends at the onset of ventricular depolarization. It normally measures 0.12 to 0.20 seconds.

Question 50ECG Interpretation and Rhythm Recognition

A rhythm strip shows a regular rate of 40 bpm with narrow QRS complexes and upright P waves preceding each QRS in lead II. What is the most likely rhythm?

  1. Ventricular escape rhythm
  2. Complete heart block
  3. Junctional rhythm
  4. Sinus bradycardia Correct answer.

Explanation

Sinus bradycardia has a sinus P wave before every QRS, a normal PR interval, and a rate below 60 bpm. The presence of upright P waves in lead II confirms a sinus origin.