The rhythm is either sinus rhythm or not sinus rhythm. Sinus rhythm refers to the origination of the electrical activity coming from the sinus node — also known as the sinoatrial node, or SA node.
This results in an upright P wave in lead II on the ECG.
If there is a P wave before every QRS complex, and it has a sinus morphology, then normal sinus rhythm, or NSR, is said to be present. A sinus morphology is an upright P wave in lead II and biphasic (up and down) P wave in lead V1.
The first ECG strip below shows a P wave with sinus morphology, thus normal sinus rhythm. If the P wave has a morphology different from the typical sinus morphology, it is termed ectopic, meaning coming from somewhere other than the sinus node. The second ECG strip below shows an ectopic atrial rhythm. Note that the P wave is down in lead II and only up (not biphasic) in lead V1.
If there is sinus rhythm, and the heart rate is less than 60 beats per minute, then sinus bradycardia is present. If there is sinus rhythm, and the heart rate is greater than 100 bpm, then sinus tachycardia is present. The links below take you to examples of each of these.
If there are no P waves present, or the P wave morphology is not normal, then the exact rhythm must be determined. Various arrhythmias — including atrial fibrillation, atrial flutter, and ventricular rhythms such as ventricular tachycardia or ventricular fibrillation — are discussed in detail in their respective sections in ECG Reviews and Criteria.
Below are three more examples of rhythms other than sinus rhythm: atrial fibrillation, atrial flutter and multifocal atrial tachycardia.
Note that when atrioventricular, or AV, dissociation is present (complete heart block or VT), as previously described, there may not be a P wave before every QRS complex. However, as long as the P wave is upright in lead II, sinus rhythm is still said to be present.