A sinus rhythm is any

cardiac rhythm The cardiac conduction system (CCS) (also called the electrical conduction system of the heart) transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through the ...

in which

depolarisation In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

of the

cardiac muscle Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle ...

begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram (ECG). Sinus rhythm is necessary, but not sufficient, for normal electrical activity within the

heart The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...

. The term normal sinus rhythm (NSR) is sometimes used to denote a specific type of sinus rhythm where all other measurements on the ECG also fall within designated normal limits, giving rise to the characteristic appearance of the ECG when the electrical conduction system of the heart is functioning normally; however, other sinus rhythms can be entirely normal in particular patient groups and clinical contexts, so the term is sometimes considered a misnomer and its use is sometimes discouraged. Other types of sinus rhythm that can be normal include

sinus tachycardia Sinus tachycardia is an elevated sinus rhythm characterized by an increase in the rate of electrical impulses arising from the sinoatrial node. In adults, sinus tachycardia is defined as a heart rate greater than 100 beats per minute (bpm). The ...

, sinus bradycardia, and

sinus arrhythmia Sinus arrhythmia is a commonly encountered variation of ''normal'' sinus rhythm. Sinus arrhythmia characteristically presents with an irregular rate in which the variation in the R-R interval vary by more than 0.12 seconds (120 milliseconds). Addit ...

. Sinus rhythms may be present together with various other

cardiac arrhythmia Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adult ...

s on the same ECG.

Characteristics

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 (+/-) If the P waves do not meet these criteria, they must be originating from an abnormal site elsewhere in the atria and not from the sinus node; the ECG cannot, therefore, be classed as showing a sinus rhythm. In general, each P wave in a sinus rhythm is followed by a QRS complex, and the sinus rhythm therefore gives rise to the whole heart's depolarization. Exceptions to this include complete heart block and certain ventricular artificial pacemaker rhythms, where the P waves may be completely normal in shape, but ventricular depolarization bears no relation to them; in these cases, the speed of the "sinus rhythm of the atria" and the speed of the ventricular rhythm must be calculated separately.

Characteristics of normal sinus rhythm

By convention, the term "normal sinus rhythm" is taken to imply that not only are the P waves (reflecting activity of the sinus node itself) normal in morphology but that all other ECG measurements are also normal. 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 In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization); ...

, QRS complex and

QT interval The QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart. It is calculated as the time from the start of the Q wave to the end of the T wave, and approximates to the time taken ...

. # QRS complex positive in leads I, II, aVF and V3–V6, and negative in lead aVR.

References

External links

* {{cite web, url=http://pediatriccardiology.uchicago.edu/PP/abnl%20rhythm%20for%20parents%20body.htm, title=Normal Sinus Rhythm, archiveurl=https://archive.today/20121215060930/http://pediatriccardiology.uchicago.edu/PP/abnl%20rhythm%20for%20parents%20body.htm, archivedate=15 December 2012, url-status=dead Cardiology