Chapter 1: The Basics of EKG

Introduction to EKG

The electrocardiogram (EKG or ECG) is a diagnostic tool that records the electrical activity of the heart over time. It is essential for evaluating cardiac rhythm, conduction, and detecting abnormalities.

• EKG Function: Detects and records electrical signals that initiate heartbeats, not mechanical function or blood flow.

• Graphic Representation: The EKG tracing is a graphical representation of voltage changes over time.

• Standard EKG Paper: Each small square equals 0.04 seconds horizontally; five small squares (one large box) equal 0.20 seconds.

• Amplitude: One large box vertically equals 0.5 mV; five boxes equal 2.5 mV.

• Standard Calibration: 1 mV = 10 mm deflection.

Key Terms and Concepts

• Depolarization: Occurs when the inside of the cell becomes more positive due to sodium influx.

• Repolarization: Potassium efflux restores the negative resting potential.

• Resting Membrane Potential: Maintained by the sodium-potassium pump, typically around -90 mV.

• Baseline (Isoelectric Line): Indicates no detectable electrical activity.

Properties of Cardiac Cells

• Automaticity: Ability to generate impulses without external stimulation.

• Conductivity: Ability to transmit impulses.

• Excitability: Ability to respond to stimuli.

EKG Components and Interpretation

• P Wave: Represents atrial depolarization.

• QRS Complex: Represents ventricular depolarization.

• T Wave: Represents ventricular repolarization.

• PR Interval: Reflects conduction time from the start of the P wave to the start of the QRS complex (normal: 0.12–0.20 seconds).

• QT Interval: Covers total ventricular activity (depolarization and repolarization).

• ST Segment: Represents the period between ventricular depolarization and repolarization.

Electrolyte Effects on EKG

• Potassium: Abnormalities can flatten or invert T waves.

• Calcium: Plays a key role in conduction and contraction.

• Magnesium: Maintains resting membrane potential.

Lead Placement and Function

• Electrodes: Placed on the body surface to detect electrical potentials.

• Ground Electrode: Reduces electrical interference.

• Reference Lead: Lead II is often used for rhythm interpretation.

Table: EKG Waves and Their Cardiac Events

Types of Cardiac Cells

The heart is composed of three major types of cells: pacemaker cells, electrical conducting cells, and myocardial cells. These cells work together to generate, conduct, and respond to electrical impulses.

• Pacemaker Cells: Located primarily in the sinoatrial (SA) node, initiate the heartbeat through spontaneous depolarization.

• Electrical Conducting Cells: Transmit impulses rapidly through the heart's conduction system (e.g., His bundle, Purkinje fibers).

• Myocardial Cells: Contract in response to impulses but do not initiate them.

Pacemaker Cells and Automaticity

• SA Node: The primary pacemaker of the heart, sets the rhythm by spontaneous depolarization.

• Automaticity: Pacemaker cells possess automaticity and generate impulses independently.

• Conduction System: Includes the AV node, His bundle, bundle branches, and Purkinje fibers.

Resting Membrane Potential and Ion Exchange

• Resting Potential: Maintained by potassium efflux and sodium-potassium pump ().

• Depolarization: Sodium influx initiates depolarization; calcium is essential for conduction and contraction.

• Repolarization: Potassium efflux restores the negative potential.

Table: Cardiac Cell Types and Functions

Key Equations

• Resting Membrane Potential:

• EKG Paper Timing:

• EKG Paper Calibration:

Additional info:

• EKG interpretation is foundational for understanding cardiac physiology and pathology in Anatomy & Physiology.

• Electrolyte imbalances (e.g., potassium, calcium) can significantly alter EKG waveforms and cardiac function.

• Knowledge of cardiac cell types and their properties is essential for interpreting arrhythmias and conduction abnormalities.