thumb|300px|right|Schematic representation of normal sinus rhythm showing standard wave, segments, and intervals
A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is necessary, but not sufficient, for normal electrical activity within the heart.
thumb|right|Example of a sinus rhythm with [[bifascicular block]]
Other types of sinus rhythm that can be normal include sinus tachycardia, sinus bradycardia, and sinus arrhythmia. Sinus rhythms may be present together with various other cardiac arrhythmias on the same ECG.
Characteristics
In humans, for an ECG to be described as showing a sinus rhythm, the shape of the P wave in each of the 12 standard ECG leads should be consistent with a "typical P vector" of +50° to +80°. This means that the P wave should be:
- always positive in lead I, lead II, and aVF
- always negative in lead aVR
- any of biphasic (–/+), positive or negative in lead aVL
- positive in all chest leads, except for V1 which may be biphasic (+/–) Criteria therefore include:
- Normal heart rate (classically 60 to 100 beats per minute for an adult).
- Regular rhythm, with less than 0.16-second variation in the shortest and longest durations between successive P waves
- The sinus node should pace the heart – therefore, P waves must be round, all the same shape, and present before every QRS complex in a ratio of 1:1.
- Normal P wave axis (0 to +75 degrees)
- Normal PR interval, QRS complex and QT interval.
- QRS complex positive in leads I, II, aVF and V3–V6, and negative in lead aVR.