An Electrocardiogram (ECG), also known as an electrocardiogram (EKG), is a non-invasive test used to record the electrical activity of the heart. By monitoring the electrical impulses that trigger heart contractions, clinicians can identify various pathological states, ranging from mild irregularities to life-threatening emergencies. Understanding these rhythms requires a firm grasp of how electrical signals travel through the heart's conduction system—from the sinoatrial (SA) node through the atria, the AV node, the Bundle of His, and finally to the Purkinje fibers.
Calculating Heart Rate: The Big Box Method
Before diagnosing specific rhythms, one must be able to determine the heart rate. A common clinical tool for a quick assessment is the "Big Box Method". By counting the number of large squares between consecutive R-waves (the R-R interval), you can calculate the beats per minute (BPM) using the following formula:
For example, if there are 3 large boxes between R-waves, the heart rate is BPM.
Clinical Thresholds:
- Tachycardia: A heart rate that is abnormally fast.
- Bradycardia: A heart rate that is abnormally slow.
Sinus Rhythms
Sinus rhythms originate from the SA node, the heart's natural pacemaker.
Normal Sinus Rhythm
This is the baseline standard for a healthy heart, where the electrical impulse follows the expected physiological pathway.
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