The ECG plays a crucial role in diagnosing various cardiac conditions. It is a non-invasive test that records the electrical activity of the heart, providing insights into its health and functioning. If you are a medical professional seeking to refresh your knowledge, this blog offers a variety of topics, from general information to frequently diagnosed conditions and the latest in ECG technology. You are also welcome to download a brochure.
In this blog you will learn:
How the ECG works
The ECG is used to assess and monitor cardiac conditions, making it an essential tool in modern medicine. This chapter takes a look at what the ECG is and when we should perform it. It also offers a short introduction to the conditions an ECG can detect, ranging from arrhythmias to myocardial infarctions (for a comprehensive list, descriptions and illustrations, click here). It also presents some cases when ECG results may not be entirely accurate and the factors to consider while interpreting the findings.
What is an ECG?
‘ECG’ stands for ‘electrocardiogram’. This is a recording of the electrical activity of the heart from different angles. It helps detect and locate various abnormal heart conditions (pathologies). [1]
There are 3 main ECG types:
- a resting ECG: the most common; performed with the patient lying down in a comfortable position.
- a stress or exercise ECG: performed with the patient using a treadmill or an exercise bike.
- an ambulatory ECG (Holter monitor): a portable device worn by the patient for one or more days.
When to perform an ECG?
Whenever a patient experiences chest pain, palpitations (feelings of heart pounding, having irregular or fast heart beats), dyspnea (breathlessness), dizziness, a syncope (a blackout due to a drop in the blood flow to the brain) or an unexplained fall. [1]
What kind of conditions can it detect?
Heart attacks
Blockage of blood supply to the heart due to coronary artery disease or other conditions.
Arrhythmias
Too quick, too slow or irregular heartbeat.
Coronary artery disease (CAD)
Restriction of blood supply due to a build-up of fatty substances in the arteries that supply blood to the heart.
Cardiomyopathy
Thickening or enlargement of heart walls.
For a comprehensive list of the most frequent conditions, their descriptions and ECG illustrations, click here.
What does an ECG record?
The ECG records electrical impulses that a heart produces, their direction and amplitude. We can then estimate where in the heart they start and how they flow through it. These impulses make the heart muscle contract and relax, and their pattern is called ‘heart rhythm’. [2]
One heartbeat (i.e. one cardiac cycle) consists of one heart muscle contraction and one relaxation. The electrical activity of the heart starts in the internal ‘pacemaker’ – the sinoatrial node, located in the right atrium. The electric impulse then travels through different parts of the heart as presented below.
The normal heart rhythm is called ‘sinus rhythm’. The way electrical impulses flow through the heart is called ‘conduction’.
Abnormalities in the electrical activity of the heart can result in abnormal conduction or rhythms; the heart may beat too quickly, too slowly or irregularly. Changes to the normal electricity flow through the heart can be shown on an ECG and may indicate a variety of conditions.
Is the ECG result always accurate?
When conducting an electrocardiogram, certain factors or conditions may interfere with the result, including: [3]
- movement during the test,
- exercise or smoking before the test,
- pregnancy,
- obesity,
- fluid build-up in the abdomen (ascites),
- anatomical factors, e.g. the size of the chest and the location of the heart in the chest,
- certain medications,
- electrolyte imbalances, such as too much or too little potassium, magnesium or calcium in blood.
How to conduct a resting ECG measurement
Electrodes and their placement
Depending on the preferences in individual medical practices, various types of self-adhesive and vacuum electrodes may be used.
Patient preparation in case of self-adhesive electrodes
Good electrical contact between the electrodes and the skin is vital. For this reason, the skin must be clean and dry. Any moisturisers or creams (e.g. with patients with skin conditions) should be removed with alcohol; then, the alcohol must be wiped off. Skin abrasion is essential; in most patients, a rub with a paper towel is enough. [4]
Hair is a poor conductor of the electrical signal and prevents the electrodes from sticking to the skin. Shaving may be required; afterwards, the skin needs to be cleaned with alcohol or a soapy wipe. If the hair can be parted and firm contact made with the electrodes, this is acceptable.
Patient preparation in case of vacuum electrodes
For optimal electrical conductivity, electrode gel or appropriate conductivity spray must be used. [4]
The MESI mTABLET ECG screen provides a suggestion on different electrode placements.*
*DISCLAIMER: Please adapt the suggestions to your clinical practice and always consult the latest placement guidelines.
Filters
Filters play a crucial role in processing and enhancing ECG signals, helping us obtain accurate and reliable data for analysis. Click here for a detailed explanation of various filter types, including:
- Low-pass filter
- High-pass filter (baseline filter)
- Myogram filter (muscle filter)
- Notching filter (mains filter)
We also take a look at signal enhancement, used to further improve the signal quality, and shed light on the typical filter settings and configurations for understanding their application and impact on ECG interpretation.
How to interpret the ECG
In ECG interpretation, a combination of medical expertise and automatic algorithms is combined. Click here to learn more about:
- distances on the ECG graph,
- waves and waveforms,
- intervals,
- duration,
- cardiac axis and
- the role of Glasgow Interpretation Algorithm, its functionality and role in automating ECG analysis.
For a practical example of the role of automatic ECG interpretation, see this case study for STEMI myocardial infarction and this one for NSTEMI myocardial infarction.
Frequently diagnosed conditions
Here are some of the most common heart conditions diagnosed with the ECG. For a detailed description of each of them and how they look on the ECG, click here:
- Acute coronary syndromes:
Myocardial infarction (STEMI and NSTEMI)
Unstable angina pectoris
- Pericarditis and myocarditis
- Atrial fibrillation
- Brugada syndrome
- Left and right ventricular hypertrophy
- Ventricular tachycardia
- Conduction disturbances:
Atrioventricular blocks
- First-degree AV block
- Second-degree AV block Mobitz type 2
- Third degree AV block
Left and right bundle branch blocks
- Left Bundle Branch Block (LBBB)
- Right Bundle Branch Block (RBBB)
