Click image to open full size
Cardiac Conduction & EKG Basics Cheat Sheet
A printable reference covering cardiac conduction pathways, EKG waves, intervals, heart rate calculation, and basic rhythm interpretation for grades 11-12.
Related Labs
Cardiac conduction is the electrical system that coordinates each heartbeat. This cheat sheet explains how impulses move through the heart and how those impulses appear on a basic EKG tracing. Students need this reference to connect anatomy, physiology, and clinical measurements in one organized guide. It is especially useful for reviewing normal rhythm patterns before studying more advanced cardiac conditions. The core pathway is SA node, AV node, bundle of His, bundle branches, and Purkinje fibers. On an EKG, the P wave, QRS complex, and T wave show atrial depolarization, ventricular depolarization, and ventricular repolarization. Important measurements include heart rate, PR interval, QRS duration, and QT interval. A normal rhythm is usually evaluated by checking rate, regularity, P waves, PR interval, and QRS width.
Key Facts
- The normal conduction pathway is SA node to AV node to bundle of His to right and left bundle branches to Purkinje fibers.
- The P wave represents atrial depolarization, which is the electrical activation that leads to atrial contraction.
- The QRS complex represents ventricular depolarization, which leads to ventricular contraction.
- The T wave represents ventricular repolarization, which is the recovery phase of the ventricles.
- Normal resting adult heart rate is about 60 to 100 beats per minute.
- Heart rate on a regular rhythm can be estimated by heart rate = 300 divided by the number of large boxes between two R waves.
- A normal PR interval is about 0.12 to 0.20 seconds, which equals 3 to 5 small boxes on standard EKG paper.
- A normal QRS duration is less than 0.12 seconds, which is fewer than 3 small boxes on standard EKG paper.
Vocabulary
- SA node
- The sinoatrial node is the heart's natural pacemaker that normally starts each electrical impulse.
- AV node
- The atrioventricular node briefly delays the electrical signal before it travels to the ventricles.
- Depolarization
- Depolarization is the electrical activation of heart muscle cells that prepares them to contract.
- Repolarization
- Repolarization is the electrical recovery of heart muscle cells after activation.
- PR interval
- The PR interval is the time from the start of atrial depolarization to the start of ventricular depolarization.
- QRS complex
- The QRS complex is the EKG pattern that shows ventricular depolarization.
Common Mistakes to Avoid
- Confusing mechanical contraction with electrical activity is wrong because the EKG records electrical signals, not the actual squeezing of the heart.
- Calling every upward spike an R wave is wrong because an EKG waveform must be identified by its position in the P, QRS, and T sequence.
- Ignoring the paper scale is wrong because each small box usually represents 0.04 seconds and each large box usually represents 0.20 seconds.
- Assuming a normal heart rate means a normal rhythm is wrong because rate, regularity, P waves, PR interval, and QRS width all matter.
- Measuring the PR interval from the peak of the P wave is wrong because the PR interval begins at the start of the P wave and ends at the start of the QRS complex.
Practice Questions
- 1 A regular EKG rhythm has 4 large boxes between two R waves. Estimate the heart rate in beats per minute.
- 2 A PR interval measures 4 small boxes on standard EKG paper. How many seconds is the PR interval, and is it in the normal range?
- 3 A QRS complex measures 2 small boxes wide. How many seconds is the QRS duration, and is it considered narrow or wide?
- 4 Explain why the AV node delay is important for efficient heart pumping.