Normally, the SA node acts as the pacemaker with a rate of 60 to 100 bpm. Sinus bradycardia is characterized by a heart rate below 60 bpm while sinus tachycardia presents with a heart rate exceeding 100 bpm. Dysrhythmias are caused by disorders of impulse formation or conduction. Assuming the heart rhythm is regular, the heart rate is calculated by either counting the number of QRS complexes over a 6 second interval and multiplying by 10, or by counting the number of large squares between one R-R interval and dividing the number into 300. For fast rhythms, you can count the number of small squares in between one R-R interval and divide the number into 1500.
A regular rhythm is characterized by the same duration between each R-R interval. Normal sinus rhythm is defined as a rhythm starting at the SA node at a rate between 60 to 100 beats per minute and follows the normal conduction pathway. Dysrhythmias are caused by abnormal cardiac conduction. Although many dysrhythmias may be serious, some are benign and require no treatment.
The presence of P waves is a component of a normal ECG. P waves precede the QRS complex and represent atrial depolarization. The absence of P waves may indicate atrial fibrillation or sinoatrial arrest.
A normal ECG has one upright and rounded P wave before every QRS.
The PR interval indicates AV conduction time. To measure the PR interval on the ECG strip, start from the beginning of the P wave and mark until the beginning of the QRS complex. Normally, this interval should be between 0.12 to 0.20 seconds, or less than 0.20 for simplicity. This interval shortens with increased heart rate.
The QRS complex represents ventricular depolarization. Measure the QRS complex from the end of the PR interval to the end of the S wave. Normally, this interval should be between 0.06 to 0.12 seconds, or less than 0.12 for simplicity.
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