Conduction Pathway
What is the primary function of the SA node?
It is the natural pacemaker of the heart, firing at 60–100 bpm
What does the P wave represent?
Atrial depolarization
How many seconds is one small box on ECG paper?
0.04 seconds
What is the normal adult heart rate range?
60–100 bpm
What defines sinus bradycardia on an ECG?
HR <60 bpm, regular rhythm, P before each QRS
Which part of the conduction system delays the impulse so the atria can finish contracting?
The AV node
What does the PR interval represent on an ECG?
The time from the start of atrial depolarization to the start of ventricular depolarization (the delay at the AV node)
How many seconds is one large box?
0.20 seconds
What makes a rhythm “sinus”?
P wave before every QRS, upright and consistent.
What defines sinus tachycardia?
HR >100 bpm, regular rhythm, normal P waves
Which structure carries the electrical impulse from the AV node into the ventricles?
The Bundle of His
What does the QRS complex show?
Ventricular depolarization
How do you calculate heart rate using the 6-second method?
Count QRS complexes and multiply by 10.
What is considered a normal QRS duration?
Less than 0.10 seconds.
What is the hallmark ECG finding of atrial fibrillation?
No P waves; irregularly irregular rhythm
What do the right and left bundle branches do?
Conduct impulses through the ventricles
What should the ST segment normally look like?
Flat or isoelectric
What is the 300 rule for heart rate calculation?
300 divided by the number of large boxes between R waves.
What describes a normal rhythm?
Regular, evenly spaced R–R intervals
What is first-line treatment for symptomatic bradycardia?
IV atropine
Which conduction fibers cause the ventricles to contract and fire at 20–40 bpm?
Purkinje fibers
What does the T wave represent?
Ventricular repolarization
What is the 1500 method?
1500 divided by the number of small boxes between R waves.
What is the normal QT interval?
0.34–0.43 seconds (less than half the R–R interval)
Name two common treatments for atrial fibrillation
Rate control (e.g., beta blockers), anticoagulation, amiodarone, cardioversion, or ablation