How Cardiac Cells Generate the Heart’s Rhythm: Interpreting EKG Waves

Building on our understanding of cardiac cells, let’s explore how their electrical activity translates into the heart’s mechanical performance and how this is reflected on an electrocardiogram (EKG).

The Sinoatrial (SA) Node and the P Wave

The Sinoatrial (SA) node is the heart’s primary pacemaker, located in the right atrium.

• Initiating the Impulse: The SA node generates an electrical impulse that spreads across the atria. Note that the impulse generated by the SA node is not seen on an EKG!
• Atrial Contraction: This impulse causes the atrial myocardial cells to depolarize and contract, pushing blood into the ventricles.
• EKG Representation: The atrial depolarization is represented by the P wave on an EKG. The P wave reflects the electrical activity from the start to the finish of atrial depolarization. The P-wave is best interpreted in leads II and V1.
  • In II, the initial part of the P-wave represents the conduction in the right atrium, the terminal part represents conduction in the left atrium, and the middle part represents conduction in both atria.
  • In V1, the initial positive deflection reflects right atrial depolarization while the terminal negative deflection reflects the left atrial depolarization. More on this as we go on.

The Atrioventricular (AV) Node: The Gatekeeper

After the atria contract, the electrical impulse reaches the Atrioventricular (AV) node.

• Impulse Delay: The AV node slows down the impulse, ensuring that the ventricles have enough time to fill with blood before they contract. This gives the atria enough time to complete contracting and filling the ventricles.
• Regulation: The AV node is influenced by the autonomic nervous system. Sympathetic stimulation (e.g., during exercise or stress) can speed up conduction, while parasympathetic stimulation (e.g., during rest) can slow it down.

The Bundle of His, Bundle Branches, and Purkinje Fibers

From the AV node, the impulse travels to the ventricles via specialized pathways.

• Bundle of His: This pathway divides quickly into the right and left bundle branches.
  • Right Bundle Branch: The right branch carries the impulse through the right side of the inter-ventricular septum and the right ventricle.
  • Left Bundle Branch: The left bundle further divides into the septal fascicle, depolarizing the inter-ventricular septum from left to right, the anterior fascicle sweeping the anterior surface of the left ventricle, and the posterior fascicle covering the posterior surface of the left ventricle.
  • These branches branch out into countless Purkinje fibers that deliver the electrical cues to the ventricular myocardium.
• Purkinje Fibers: These fibers spread throughout the ventricles, allowing rapid transmission of the impulse.
• Coordination: This electrical network ensures that both ventricles contract simultaneously and efficiently.

The QRS Complex: Ventricular Depolarization

The depolarization of the ventricles is captured on the EKG as the QRS complex– As the name suggests, it may have several waves.

• Components:
  • Q Wave: The first negative deflection, if any. In a normal EKG, the initial small negative deflection of the QRS complex reflects the interventricular septum’s depolarization by the septal fascicle of the left bundle branch. This usually moves from left to right across the inter-ventricular septum.
  • R Wave: The first positive deflection, if any, indicating the main phase of ventricular depolarization. If there’s a second upward deflection, that’s termed the R′ (R-prime).
  • S Wave: A negative deflection following the R wave, representing the final phase of ventricular depolarization.
  • QS Complex: If the only deflection is negative, we call it a QS wave.
• Because the left ventricle has significantly more muscle than the right ventricle, most of the electric activity we see in the QRS complex of a normal heart is from the left ventricle’s electrical activity.
• Significance: The shape and size of the QRS complex provide vital information about ventricular health and conduction pathways.
• Small waves may be represented in a lower-case letter and large waves in an upper-case letter. So qR implies a small Q-wave and a prominent R-wave.

Refractory Period

After depolarization, the myocardial cells take a short break, a refractory period where they’re immune to further stimulation.

• Absolute refractory period: From the end of QRS to the early to mid T wave, when cardiac cells have not fully recovered and a premature beat cannot cause another beat.
• Relative refractory period: The mid to end of the T wave, when cardiac cells are “hyper-polarized” and a premature beat can initiate depolarization. This causes an R-wave to fall on a T-wave (the R on T phenomenon) which could trigger a life-threatening ventricular arrhythmia such as ventricular tachycardia or ventricular fibrillation.

Repolarization and the T Wave

After depolarization and following the refractory period, the ventricles repolarize, returning to their resting state.

• Repolarization Process: Ventricular myocardial cells restore their negative internal charge.
• EKG Representation: This phase is shown as the T wave on the EKG.
• Importance: The T wave indicates that the heart muscle is ready for the next heartbeat.

The Complete EKG Cycle

An EKG cycle reflects the entire process of a heartbeat.

1. P Wave: Atrial depolarization and contraction.
2. PR Interval: Delay at the AV node.
3. QRS Complex: Ventricular depolarization and contraction.
4. ST Segment and T Wave: Ventricular repolarization.