ECG Interpretation Study Notes

ECG Interpretation Simplified

Objectives

  • Describe the purpose of electrocardiography (ECG)

  • Recognize normal and abnormal ECG recordings

  • Recognize arrhythmias

  • Steps to perform a 12 lead ECG

  • Recognize artifact

ECG / EKG Overview

  • Definition: ECG (Electrocardiography) is a non-invasive procedure that provides a graphic representation of the heart’s electrical activity.

  • Equipment: The machine used is called an electrocardiograph.

  • Limitations: Only measures electrical activity; mechanical activity cannot be obtained.

  • Output: The printed record is termed as a rhythm strip or EKG strip.

Types of Lead Systems

  • 3 Lead System: Common in telemetry, consisting of three leads.

  • 5 Lead System: More advanced for monitoring.

  • 12 Lead EKG: Provides views from 12 different angles (using 10 leads) and is a useful diagnostic tool. It is inexpensive and easy to obtain.

Indications for ECG

Chief Complaints

  • Chest pain

  • Dyspnea on exertion

  • Orthopnea

  • Paroxysmal nocturnal dyspnea

  • Pedal edema

  • Fainting spells (syncope)

  • Palpitations

Past Medical History

  • History of heart disease

  • History of cardiac surgery

Physical Examination Findings

  • Unexplained tachycardia at rest

  • Hypotension

  • Decreased capillary refill

  • Abnormal heart sounds and murmurs

  • Cool, edematous, cyanotic extremities

  • Diaphoresis (profuse sweating)

  • Positive jugular venous distension (JVD)

  • Hepatojugular reflex

  • Bilateral inspiratory crackles in dependent lung zones

  • Abnormal electrolytes.

Limitations of ECG

  • Does not measure the pumping ability of the heart.

  • Cannot show abnormalities in cardiac structure.

  • Lacks predictive value.

  • Artifacts, operator technique, lead placement limitations, and technical issues might affect results.

Intrinsic Conduction System

  • Components:

    • Sinoatrial (SA) Node: Located in the right atrium; primary pacemaker of the heart.

    • Atrioventricular (AV) Node: Electrical relay station that slows the impulse before it enters the ventricles.

    • Bundle of His: Pathway for electrical signals from AV node to the ventricles.

    • Purkinje Fibers: Spread impulses through the ventricles.

Pacemaker Sites

  • Dominant Pacemaker (SA node): 60-100 beats/min

  • Escape Pacemaker (AV junction): 40-60 beats/6Y

  • Bundle Branches / Purkinje network: 20-40 beats/min

Basic Electrophysiology

  • Impulse Conduction: The impulse-conducting system stimulates heart muscle cells.

  • Depolarization: Sodium ions (Na+) influx inside the cell leads to contraction.

  • Repolarization: The return to the polarized state after contraction, where electrical imbalance is reestablished.

  • Ectopic Beat: A heartbeat originating outside of the SA node.

Autonomic Nervous System and Heart Rate

Parasympathetic Nervous System

  • Neurotransmitter: Acetylcholine

  • Effects: Reduces electrical impulses, decreases heart rate, affects atrioventricular nodes, induces pupil constriction.

Sympathetic Nervous System

  • Neurotransmitter: Norepinephrine

  • Effects: Increases electrical impulses and heart rate, affects all areas of the heart, converts glycogen to glucose, causes pupil dilation.

Characteristics of Cardiac Cells

  1. Automaticity: The ability to initiate an impulse without external stimuli.

  2. Excitability: The response of cardiac cells to stimuli.

  3. Conductivity: The ability to conduct impulses throughout the cardiac tissue.

  4. Contractility: The ability of heart muscles to contract.

  5. Refractoriness: The state in which cardiac tissue cannot respond to a new impulse.

Terminology in ECG

  • Systole: Contraction phase of the heart cycle.

  • Diastole: Relaxation phase of the heart cycle.

  • Stroke Volume: Volume of blood pumped from the left ventricle per beat.

  • Cardiac Output (CO): The amount of blood the heart pumps in one minute, calculated as CO = SV imes HR (Stroke Volume x Heart Rate).

  • Preload: Initial stretching of the cardiac myocytes prior to contraction; influenced by venous return.

  • Afterload: The load against which the heart must contract to eject blood.

EKG Leads

  • Electrodes: Conductive gel applied to the skin.

  • Leads: Wires that connect electrodes to the EKG machine.

Lead Types

  • Limb Leads:

    • Bipolar leads: I, II, III

    • Unipolar leads: aVR, aVL, aVF

    • Lead II is the most commonly used for monitoring.

  • Chest Leads:

    • Unipolar leads: V1, V2, V3, V4, V5, V6

3 Lead EKG

  • Utilizes leads I, II, and III (bipolar leads).

  • Placement forms an imaginary triangle known as Einthoven’s Triangle.

  • Lead II is preferred due to its ability to visualize P waves effectively.

5 Lead System Placement

  • Leads:

    • RA (Right Arm)

    • LA (Left Arm)

    • RL (Right Leg)

    • LL (Left Leg)

Getting a Good EKG

  • Prepare the patient and skin for electrode application.

  • Clean the site with an alcohol swab, especially if the patient is diaphoretic or has lotion on their skin.

  • Shave excessive hair where electrodes will be placed.

  • Ensure proper lead placement; ask the patient to relax their shoulders and legs; instruct them not to move.

Troubleshooting EKG Artifacts

Common Artifacts

  1. Wandering Baseline: A slow wander of the baseline due to body movement or respiration swings.

  2. AC Interference: Variability in amplitude due to electrical power leakage and proximity to electrical equipment.

  3. Muscle Tremor: Narrow and rapid spikes in the ECG possibly caused by shivering or tremors (e.g., in Parkinson's disease).

  4. Motion Artifact: A large swing in the baseline due to epidermal stretching or movement like coughing or ambulation.

EKG Graph Paper

  • Time Measurement:

    • 1 mm = 0.04 seconds

    • 5 mm = 0.20 seconds

Electrical Events of the Cardiac Cycle

  1. P Wave: Atrial depolarization.

  2. QRS Complex: Ventricular depolarization.

  3. T Wave: Ventricular repolarization.

  4. PR Interval: Conduction time from atria to ventricles (normal duration: 0.12-0.20 seconds).

  5. QT Interval: Time from the beginning of QRS to the end of T wave.

ECG Measurements

  • P Wave: Duration < 0.11 seconds (atrial depolarization).

  • Q Wave: First negative deflection in the QRS complex (represents ventricular septal depolarization).

  • R Wave: First positive deflection in the QRS complex, early ventricular depolarization.

  • S Wave: First negative deflection after an R wave.

  • T Wave: Represents ventricular repolarization.

5-Step Approach for ECG Interpretation

  1. Heart Rate

  2. Heart Rhythm

  3. P Wave Analysis

  4. PR Interval Analysis

  5. QRS Complex Analysis

  • Always observe and treat the patient based on clinical presentation irrespective of the rhythm observed.

Determining Heart Rate

Methods

  • 6-Second Method: In a 6-second EKG strip, count the number of QRS complexes and multiply by 10. This method is used for irregular rhythms.

  • R-R Interval Method: Count the number of boxes between two R waves and divide it into 300. This method is applicable for regular rhythms.

Heart Rhythm Analysis

  • Defined as the sequential beating of the heart due to electrical impulses generated.

  • Ventricular Rhythm: Measure intervals between R to R waves.

  • Atrial Rhythm: Measure intervals between P to P waves.

  • Classifications:

    • Rhythm is “regular” if intervals vary by less than 0.06 seconds (1.5 small squares).

    • Rhythm is “irregular” if intervals vary by greater than 0.06 seconds.

P Wave Characteristics

  • Check for P waves presence, regular occurrence, and the configuration.

  • Suitable criteria:

    • One P wave for every QRS complex.

    • P wave should be smooth, rounded, and upright or inverted, looking consistent across the strip.

PR Interval Characteristics

  • Normal PRI: 0.12 – 0.20 seconds (3 to 5 small squares).

  • Assess for:

    • PR intervals greater than 0.20 seconds (indicating potential block).

    • PR intervals less than 0.12 seconds.

    • Consistency of PR intervals across the strip.

QRS Complex Analysis

  • Normal duration is 0.12 seconds or less (narrow).

  • Assess for wide complexes (greater than 0.12 seconds) or narrow, and consistency across the strip.