Chapter 2: Electrocardiography (ECG), our old friend

Introduction

As per the guidelines of the ESC, an ECG is required to diagnose heart rhythm disorders. In this article, we'll run you through what an ECG is and how it can be used as a diagnostic tool.

Key messages

✔️ A traditional 12-lead ECG is considered the harmonized gold standard in cardiology to assess the electrical conduction system and the cardiac rhythm.

✔️ Physicians with expertise in ECG rhythm interpretation establish a diagnosis based on the rhythm strip, not the machine, nor the ECG algorithm.

✔️ The morphology of the ECG provides inherent information about the conductive behavior within the heart tissue

✔️ The distance between consecutive R-peaks can be used to determine the heart rate and assess the regularity of the heart rhythm

What is an electrocardiogram or ECG?

Today, a traditional 12-lead electrocardiogram (ECG) is still considered the harmonized gold standard in cardiology to assess the conduction morphology and interval analysis of the electrical cardiac activity. The ECG is visualized in a standardized grid and looks like a wavy line with spikes that result from the electrical impulses which we described in chapter 1. These spikes provide healthcare providers with information about the amplitude of the electrical signal on the Y-axis (mV) and the duration between the fiducial points (P-wave, QRS complex, and T-wave, together with the corresponding onset, offset, and peak points) on the X-axis (ms). The standardized and detailed visualization of the traditional ECG-signal allows healthcare providers to investigate and diagnose cardiac conditions.

Next to the traditional 12-lead ECG, there is a widespread of alternative ECG-based devices (handheld devices, implantable devices, Holter devices, etc.) that are able to graphically present the electrical activity of the heart. These technologies can be based on a single-lead analysis, 3-lead analysis, 12-lead analysis, etc. The more leads used, the more accurate the signal is as more vectors are covered for analysis and interpretation.

The ECG offers two unique features for diagnostic purposes

The morphology of the ECG waves and complexes provides inherent information about the conductive behavior within the heart tissue, derived from different leads. Abnormalities in electrical conduction can be clearly identified based on the morphology of the spikes. This is crucial to identify features such as ST-elevation, identification of P-waves, detection of delta-waves, etc.

Next to the morphological features of the waveform, the ECG-signal provides important information regarding the distance between consecutive R-peaks. This information can be used to determine the heart rate, but also the regularity of the heart rhythm, which enables physicians to identify patients with sinus arrhythmia, heart rate variability, atrial fibrillation, etc.

⏩ Now that you know more about the ECG, let’s dive into the world of photoplethysmography.