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.
Understanding Cardiac Conduction & EKG Basics
Heart muscle cells create electrical signals because charged particles move across their cell membranes. Sodium and calcium moving into a cell help start activation. Potassium moving out helps the cell return to its resting state.
This change in charge is called depolarization, followed by repolarization. The electrical event happens just before the muscle squeezes. That timing matters because the chambers must fill with blood before they contract.
An EKG records these changing electrical charges from the skin. It does not directly measure blood flow, pressure, or the strength of each squeeze.
Some heart cells can slowly reach a firing point without a message from the brain. This property is called automaticity. Nerves and hormones can change how quickly these cells fire.
For example, exercise, fear, fever, and thyroid hormone can speed the rhythm. Sleep, athletic training, or some medicines can slow it.
A short pause at the AV node is useful because it gives the lower chambers time to fill after the upper chambers contract. If this delay becomes too long, too short, or inconsistent, the pattern can provide clues about a conduction problem.
Each EKG lead views the heart from a different direction. A wave moving toward a lead's positive electrode usually creates an upward deflection. A wave moving away usually creates a downward deflection.
This is why the same heartbeat can have different shapes in different leads. The tracing is a map of electrical direction and timing, not a photograph of the heart.
A large wave does not always mean a stronger contraction. Body shape, electrode placement, lung disease, and heart position can all change wave size.
EKG paper gives time and voltage a fixed scale when the machine uses standard settings. Small boxes help measure short events, while large boxes make longer timing easier to estimate. The large box heart rate shortcut works best when the spaces between R waves stay nearly equal.
With an irregular rhythm, a longer strip is more useful. Count the beats in six seconds and multiply by ten to estimate beats per minute.
Students should first check that the tracing is clean. Loose electrodes, movement, muscle tension, and nearby electrical equipment can create artifacts that look like abnormal waves.
A careful reading follows the same order every time. Start with rhythm regularity, then examine whether atrial activity appears before each ventricular beat. Next, measure timing and width, then compare the waves across leads.
This method prevents a reader from focusing on one unusual feature while missing the whole pattern. In real clinical settings, an EKG is interpreted alongside symptoms, pulse, blood pressure, medicines, and medical history.
Chest pain, fainting, shortness of breath, or a very fast or slow pulse need prompt professional assessment. Learning the normal pattern first makes later rhythm disorders easier to recognize without assuming that every unusual tracing has one simple cause.