The electrocardiogram

The SA node's pacemaker potential climbs fastest, so it reaches threshold first and resets every slower pacemaker.

• The AV node:
• Correct: The SA node:
• The Purkinje fibers:

The AV node conducts at only about 0.05 meters per second because of its small cells, few gap junctions and slow calcium-driven upstrokes.

• Correct: AV node cells are small, joined by few gap junctions, and have slow calcium-driven upstrokes:
• The AV node is several centimeters long:
• The vagus nerve holds the signal until the atria are empty:

During the plateau, Ca²⁺ entering through L-type channels balances K⁺ leaving, so the cell stays depolarized near 0 mV for about 0.2 second.

• At its resting potential:
• Correct: Depolarized, near 0 mV:
• Hyperpolarized:

The P wave is the wave of depolarization spreading across both atria after the SA node fires.

• Correct: Depolarization of the atria: Correct. The atria contract just after the P wave.
• Firing of the SA node alone: The SA node is too small for its own activity to show from the skin. The P wave comes from the much larger mass of atrial muscle.
• Depolarization of the ventricles: Ventricular depolarization is the QRS complex.
• Repolarization of the atria: Atrial repolarization happens during the QRS complex and is hidden by it.

The R spikes are 4 large squares apart. 4 × 0.20 second = 0.80 second per beat, and 60 ÷ 0.80 = 75 beats per minute. The shortcut gives the same: 300 ÷ 4 = 75.

• 60 beats per minute: 60 per minute would need 5 large squares (1.0 second) between beats.
• 100 beats per minute: 100 per minute would need 3 large squares (0.60 second) between beats.
• Correct: 75 beats per minute: Correct. The R spikes are 4 large squares apart, and 300 ÷ 4 = 75.
• 375 beats per minute: This uses 0.04 second, the width of a small square, for each large square: 4 × 0.04 = 0.16 second, and 60 ÷ 0.16 = 375.

The PR interval runs from the start of atrial depolarization to the start of ventricular depolarization. Most of that time is the AV nodal delay, so slower AV conduction lengthens it.

• The QRS complex: The QRS complex measures how fast the ventricles depolarize, after the signal has left the AV node.
• The ST segment: The ST segment is the plateau after the ventricles have depolarized. The AV node plays no part.
• The QT interval: The QT interval covers ventricular depolarization and repolarization, which start after the AV node.
• Correct: The PR interval: Correct. The PR interval is the window on the AV node.

A PR interval longer than 0.20 second means the AV node conducts slowly. Because every P wave still reaches the ventricles, it is first-degree heart block.

• Third-degree heart block: In third-degree block no signals cross, so P waves and QRS complexes are unrelated. Here every P wave is followed by its QRS.
• Atrial fibrillation: Atrial fibrillation has no P waves and an irregular rhythm. This rhythm is regular with a P wave before every beat.
• Correct: First-degree heart block: Correct. Every beat conducts, but each is delayed at the AV node.
• Ventricular tachycardia: Ventricular tachycardia is fast, with wide QRS complexes and no linked P waves.

During the ST segment the ventricles are fully depolarized, sitting in the plateau of their action potential. With every cell at about the same voltage, little current flows between regions, so the line is flat.

• The P wave shows atrial depolarization.: This statement is right.
• The PR interval runs from the start of P to the start of QRS and is mostly the AV nodal delay.: This statement is right. The PR interval spans the atria, the AV node and the conducting fibers, and the AV node takes most of the time.
• The QRS complex shows ventricular depolarization.: This statement is right.
• Correct: The ST segment is flat because the ventricles are fully repolarized.: This is the error. The ventricles are fully depolarized, not repolarized, during the ST segment. They repolarize during the T wave.
• The T wave shows ventricular repolarization.: This statement is right.

This is atrial fibrillation. Chaotic atrial wavelets replace the P wave, and the AV node lets signals through at irregular times, giving irregularly irregular narrow QRS complexes.

• Correct: The atria are depolarizing chaotically, and the AV node passes signals at irregular moments: Correct. Narrow QRS complexes show the ventricles are still activated through the normal conducting system.
• The SA node is firing faster than normal but conduction is normal: Sinus tachycardia would be regular, with a P wave before each QRS complex.
• The ventricles are depolarizing chaotically and pump no blood: In ventricular fibrillation there are no QRS complexes at all. Hers are clear and narrow.
• No signals are crossing the AV node at all, so a backup pacemaker in the ventricles has taken over: A ventricular backup pacemaker would give a slow, regular rhythm with wide QRS complexes.

An ECG records only electrical events. Normally contraction follows each one, but electrical activity can continue even when the muscle fails to pump, for example after severe blood loss or when fluid compresses the heart. The patient needs treatment for that cause, not a shock.

• The monitor is showing a delayed recording: Monitors display in real time. The mismatch is real, not a display delay.
• Correct: The ECG records electrical activity, and the muscle is not contracting effectively in response: Correct. Electrical activity and pumping are separate events, and the ECG shows only the first.
• The atria are pumping but the ventricles are not: The QRS complexes show the ventricles are depolarizing; the problem is that the muscle is not pumping effectively after that.
• A normal rate and rhythm prove the heart is pumping, so the pulse check must be mistaken: An ECG cannot show pumping. A normal-looking rhythm with no pulse is a recognized emergency.