Understanding Tachycardia on an ECG: A Comprehensive Guide
When the heart rate exceeds 100 beats per minute (bpm), it is generally classified as tachycardia. The electrocardiogram (ECG) records and represents the heart's electrical activity as a series of waveforms and intervals such as P-QRS-T. Sinus tachycardia is identified when the heart rate increases to 100 bpm because it starts with a P wave. Other forms of tachycardia can be indicated by the absence of a P wave or by an abnormal P wave and QRS complex at a rate of 100 bpm.
Understanding a 12-Lead ECG
A standard 12-lead ECG includes six leads on the left side, representing different views of the electrical changes in the coronal plane, and six on the right side, representing different views in the axial plane. Each small square along the bottom of the ECG paper represents 0.2 seconds.
To determine the heart rate, identify the QRS complex, which represents the ventricular contraction, and count the number of small squares between successive R-waves. Divide this number into 300 to get the heart rate. If the rhythm is irregular, count several complexes and multiply the result by the number of complexes counted to get an average heart rate.
Normal Resting Heart Rate
The typical normal resting heart rate for adults is between 60 and 100 bpm. Therefore, any heart rate over 100 bpm is considered a tachycardia.
Mechanisms of Tachycardia
There are several mechanisms that can cause tachycardia, including supraventricular tachycardia, ventricular tachycardia, and atrial fibrillation. Detailed analysis of the ECG can help diagnose these conditions.
Useful Resources for Learning ECG Interpretation
When I was a junior doctor, the Oxford Textbook of Medicine provided excellent explanations for ECG interpretation. Although it is more of a reference book than a textbook, it is an invaluable resource for medical students and practitioners. Its detailed nature means that it may be too comprehensive for a general practitioner, but it is very useful for those training to become a specialist.
For a more concise and portable guide, the Oxford Handbook of Medicine is highly recommended. It contains essential information that is not always covered in medical school. For example, when facing a task like passing a nasogastric tube in a clinical setting, it often suggests trying to have a nurse attempt the task first, as they may successfully do it, and if not, you can use their feedback to assist in the process.