Makindo Medical Notes

Related Subjects: |ECG Basics |ECG Axis |ECG Analysis |ECG LAD |ECG RAD |ECG Low voltage |ECG Pathological Q waves |ECG ST/T wave changes |ECG LBBB |ECG RBBB |ECG short PR |ECG Heart Block |ECG Asystole and P wave asystole |ECG QRS complex |ECG ST segment |ECG: QT interval |ECG: LVH |ECG RVH |ECG: Bundle branch blocks |ECG Dominant R wave in V1 |ECG Acute Coronary Syndrome |ECG Narrow complex tachycardia |ECG Ventricular fibrillation |ECG Regular Broad complex tachycardia |ECG Crib sheets

Introduction

• The electrocardiogram (ECG) is a critical diagnostic tool in emergency medicine. It is a simple, cost-effective test frequently used in the assessment of patients presenting with chest pain.
• The ECG is essential for diagnosing cardiac ischaemia and determining eligibility for thrombolytic therapy.
• A systematic approach to ECG interpretation is crucial to avoid misinterpretation. This module guides the learner through a step-by-step approach to ECG analysis.
• Specific examples of various abnormal ECG patterns are included, along with a quiz at the end for practice.

Objectives

• By the end of this module, you should have a systematic approach to ECG interpretation and be able to identify common ECG abnormalities.

The 12-Lead ECG

• The 12-lead ECG includes three standard limb leads (I, II, III), three augmented limb leads (aVR, aVL, aVF), and six precordial leads (V1 through V6).

Waves and Complexes

Intervals and Segments

Normal Values

Difficulties and Errors

Systematic Approach to ECG Interpretation

Step 1: Determine Heart Rate

Locate a QRS complex that falls on a major grid line. Count large squares to the next QRS complex, then divide 300 by this number to calculate heart rate. There are several ways to do this

Step 2: Measure Key Intervals and axis

Measure the PR, QRS, and QT intervals. On a 25 mm/sec ECG, each small square represents 0.04 seconds, and each large square represents 0.2 seconds.

Axis Measurement

Step 3: Evaluate Rhythm

Check if the rhythm is regular by confirming a consistent RR interval. Ensure there is a P wave before each QRS complex.

Step 4: Inspect P Waves for Atrial Enlargement

• P wave amplitude over 3 mm in lead II suggests right atrial enlargement.
• In lead V1, a terminal negative P wave deflection exceeding 1 mm indicates left atrial enlargement.

Step 5: Inspect QRS Complexes for Hypertrophy or Low Voltage

In Left Ventricular Hypertrophy (LVH), tall R waves are seen in V5, V6, and aVL, while deep S waves are seen in V1 and V2.
V1 or V2		V5, V6 or aVL		The voltage criteria for LVH are met if the sum of the S wave (V1 or V2) and R wave (V5 or V6) is ≥ 35 mm.

Step 6: Assess Q Waves for Myocardial Infarction

Pathological Q waves (≥ 0.04 seconds and > 1/3 R wave amplitude) suggest MI, especially if seen in multiple contiguous leads.

Step 7: Evaluate ST Segments and T Waves

ST elevation suggests myocardial injury, while depression can indicate ischaemia. T wave abnormalities may also signal pathology.

Step 8: Measure QT Interval

Corrected QT (QTC) is calculated to account for heart rate:
QTC = QT observed / √(RR interval)

This ECG is from a male patient with familial prolonged QT syndrome. Normal QTC: ≤ 0.40 seconds for males; ≤ 0.44 seconds for females.

Credits

This module was developed by Adam Szulewski, edited by Dr. Bob McGraw, Dr. Jason and has been heavily edited and added to.