Fundamentals of Electrocardiography (EKG): Anatomy & Physiology Study Notes

The Basics of Electrocardiography (EKG)

Introduction to EKG

The electrocardiogram (EKG or ECG) is a diagnostic tool that records the electrical activity of the heart. It is essential for evaluating cardiac rhythm, conduction, and identifying various cardiac pathologies. The EKG does not measure mechanical function or blood flow, but rather the electrical potentials generated during cardiac depolarization and repolarization.

• EKG Function: Detects and records electrical signals that initiate heartbeats.

• Key Ions in Cardiac Activity: Sodium (Na+) influx initiates depolarization; Potassium (K+) efflux maintains resting membrane potential; Calcium (Ca2+) is crucial for conduction and contraction.

• Graphic Representation: The EKG tracing is a graphic representation of voltage (millivolts) over time (seconds).

Cardiac Electrical Events and EKG Waves

The EKG tracing consists of several waves and intervals, each corresponding to specific electrical events in the heart.

• P wave: Represents atrial depolarization.

• QRS complex: Represents ventricular depolarization.

• T wave: Represents ventricular repolarization.

• PR interval: Time from the start of the P wave to the start of the QRS complex; reflects conduction through the AV node.

• PR segment: Delay in conduction through the AV node.

Cardiac Cells and Their Properties

Cardiac cells are specialized for generating and conducting electrical impulses. Their properties are essential for coordinated heart function.

• Automaticity: Ability to generate impulses without external stimulation (e.g., SA node).

• Conductivity: Ability to transmit impulses to neighboring cells.

• Excitability: Ability to respond to stimuli.

• Resting Membrane Potential: Approximately -90 mV, maintained by the sodium-potassium pump.

EKG Paper and Measurement

EKG tracings are recorded on standardized paper, allowing for accurate measurement of intervals and amplitudes.

• Small square: 0.04 seconds horizontally.

• Large square: 0.20 seconds horizontally (5 small squares).

• Standard speed: 25 mm/sec.

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

• Standard calibration: 1 millivolt = 10 mm deflection.

EKG Lead Placement and Function

Electrodes placed on the body surface detect electrical potentials, allowing for the recording of the heart's electrical activity from different angles.

• Limb leads: Provide views in the frontal plane.

• Precordial leads: Provide views in the horizontal plane.

• Lead II: Commonly used for rhythm interpretation due to alignment with the heart's general depolarization direction.

Key EKG Intervals and Segments

Intervals and segments on the EKG provide information about conduction times and cardiac function.

• Normal QRS duration: Less than 0.12 seconds.

• PR interval: Normal range is 0.12–0.20 seconds.

• QT interval: Varies with heart rate; corrected QT (QTc) is calculated using formulas such as Bazett's formula: $QTc = \frac{QT}{\sqrt{RR}}$

Clinical Relevance of EKG Findings

EKG findings are crucial for diagnosing arrhythmias, conduction blocks, electrolyte imbalances, and myocardial infarction.

• Arrhythmias: Abnormal rhythms can be identified by changes in waveforms and intervals.

• Electrolyte Imbalances: Hyperkalemia and hypokalemia affect T wave and QRS morphology.

• Myocardial Infarction: Specific leads show changes corresponding to infarcted regions.

Summary Table: EKG Waves and Their Cardiac Events

EKG Component	Cardiac Event
P wave	Atrial depolarization
QRS complex	Ventricular depolarization
T wave	Ventricular repolarization
PR interval	Conduction from atria to ventricles (AV node delay)
PR segment	Delay in AV node conduction

Example: Calculating Heart Rate from EKG

• Count the number of large squares between two consecutive R waves.

• Heart rate formula: $\text{Heart Rate} = \frac{300}{\text{Number of large squares between R waves}}$

• Example: If there are 4 large squares between R waves, heart rate = $\frac{300}{4} = 75$ bpm.

Additional info:

• EKG interpretation requires understanding of both anatomy and physiology of the heart, as well as the principles of electrophysiology.

• Systematic analysis of EKGs improves diagnostic accuracy and clinical decision-making.