Heart Conduction and EKG

Overview

This lecture builds on heart structure to explain how the heart contracts, focusing on autorhythmic cells, the intrinsic conduction system, and interpreting EKG readings.

Cardiac Muscle Cell Types

• The heart contains two types of cells: autorhythmic (1%) and non-autorhythmic (99%).
• Autorhythmic cells form the intrinsic conduction system, generating electrical impulses for heart contraction.
• Non-autorhythmic cells are contractile, responsible for the heart's actual pumping action.

Intrinsic Conduction System Pathway

• Autorhythmic cells are concentrated in specific regions: SA node, AV node, AV bundle (bundle of His), bundle branches, and Purkinje fibers.
• The SA node (sinoatrial node) in the right atrium acts as the heart’s pacemaker, generating signals about 75 times per minute.
• The signal quickly spreads across both atria, causing simultaneous atrial contraction.
• The electrical signal slows at the AV node, allowing atria to contract before the ventricles receive the signal.
• From the AV node, the signal travels down the AV bundle, divides into right and left bundle branches, then into Purkinje fibers, causing coordinated ventricular contraction.

Effects of Damage in Conduction Pathway

• If the SA node is damaged, the AV node becomes the pacemaker at a slower rate (about 50 beats/min).
• If the AV node is damaged, the AV bundle takes over as pacemaker, producing even slower ventricular contractions (about 30 beats/min).
• Damage can cause lack of coordination between atrial and ventricular contractions.

Introduction to EKG

• An EKG (electrocardiogram) measures the electrical activity of the heart, not the actual contractions or valve movement.
• The typical EKG waveform consists of the P wave, QRS complex, and T wave.
• The P wave represents atrial depolarization.
• The QRS complex represents ventricular depolarization and overlaps with atrial repolarization.
• The T wave represents ventricular repolarization.

Interpreting EKG Readings

• The duration and shape of EKG waves can indicate heart health or damage (e.g., extended P wave, inverted T wave).
• EKG grid paper allows measurement of time intervals between heartbeats, helping to determine heart rate and rhythm.
• The tip of the R wave is often used for measuring intervals between heartbeats.

Key Terms & Definitions

• Autorhythmic cells — Heart cells that generate electrical impulses automatically.
• Intrinsic conduction system — Network of autorhythmic cells coordinating the heartbeat.
• SA node (Sinoatrial node) — Primary pacemaker of the heart.
• AV node (Atrioventricular node) — Secondary pacemaker; slows signal for proper timing.
• AV bundle (Bundle of His) — Pathway for signals from AV node to ventricles.
• Bundle branches — Split pathways carrying signals to right and left ventricles.
• Purkinje fibers — Fibers that spread the electrical signal through ventricular walls.
• Depolarization — Electrical activation of cardiac muscle cells.
• Repolarization — Recovery phase of cardiac muscle cells after activation.
• EKG (Electrocardiogram) — Recording of heart’s electrical activity.

Action Items / Next Steps

• Complete the EKG activity to practice identifying and interpreting P waves, QRS complexes, and T waves.
• Be prepared to explain how different areas of heart damage affect heart rate and rhythm.