An electrocardiogram (ECG) is a medical test that detects heart problems by measuring the electrical activity that occurs when the heart contracts. ECGs are simple, inexpensive, noninvasive, and readily available. Adhesive electrodes attach to the surface of the skin, allowing you to measure your heartbeat from multiple angles. An electrocardiogram provides 3-dimensional information about the heart, myocardium, and other systems. In this article, we’ll discuss what you need to know about reading an ECG.
Examine the heart rate and rhythm
- The patient’s heart rhythm can be regular or irregular. An irregular rhythm is usually a recurring irregular pattern or irregularity in which the heartbeat is completely disorganized.
If the patient has an arrhythmia, the heart rate calculation method for normal heart rate does not work (because the R-R interval on the ECG varies greatly). So you need to take a different approach:
– Count the number of complexes on the rhythm bar (each rhythm is usually 10 seconds long).
– Multiply the number of complexes by 6 (average number of complexes in 1 minute).
- Normal heart rate should be around 60-100 bpm (beats per minute)
If the patient has a regular heart rhythm, their heart rate can be calculated using the following formula:
– Count the number of large squares that appear in the R-R interval.
– Divide this number by 300 to calculate heart rate.
Understanding the cardiac axis, intervals, and segments
Types of axis
The mandrel describes the general direction of electrical propagation within the heart.
- Normal cardiac axis
-Lead II has the most significant positive deflection compared to leads I and III.
- Right axis deviation
– Lead III has the most significant positive deflection, lead I should be negative.
– Right axis deviation is associated with right ventricular hypertrophy.
- Left axis deviation
-Lead I have the most significant positive deflection.
– Leads II and III are negative.
– Left axis deviation is associated with cardiac conduction abnormalities.
Types of intervals
- PR interval
The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. The normal PR interval is 0.12-0.20 seconds (120-200 milliseconds).
- Prolonged PR interval
PR interval prolongation suggests atrioventricular delay (AV block).
- Shortened PR interval
If the PR interval is shortened, it could mean one of two things:
-P waves originate closer to the AV node and therefore take less time to conduct (the SA node is not in a fixed location, and some people have smaller atria than others).
-Atrial impulses take a quicker shortcut to the ventricles, rather than slowly traversing the atrial wall. This is an auxiliary pathway, possibly related to delta waves.
Segments
- ST-segment
The ST segment is the part of the ECG between the end of the S wave and the beginning of the T wave. In a healthy individual, it should be an isoelectric line (neither rising nor falling). ST-segment abnormalities should be examined to rule out pathology.
- J point segment
The J point segment is where the S wave joins the ST segment. This point can be raised, causing the ST segment behind it to also raise (this is called a “high takeoff”). Elevated takeoff (or benign early repolarization for short) is a normal variant that causes a lot of anxiety and confusion because it looks like ST elevation.
