Normal sinus rhythm

# Overview
An impulse (action potential) that originates from the SA node at a rate of 60 - 100 beats/minute (bpm) is known as normal sinus rhythm.
The sinus node (SA) is located in the upper part of the wall of the right atrium. When the sinus node generates an electrical impulse, first the cells of the right atrium depolarise, then the cells of the left atrium, the AV node (atrioventricular) follows and at last the ventricles are stimulated via the His bundle.
- A P wave (atrial contraction) precedes every QRS complex
- The rhythm is regular, but varies slightly while breathing
- The frequency ranges between 60 and 100 beats per minute
- The P waves maximum height is 2.5 mm in II and/or III
- The P wave is positive in I and II, and biphasic in V1

# Sinus Tachycardia
If SA nodal impulse occur at a rate exceeding 100 bpm, the consequent rapid heart rate is sinus tachycardia.

# Sinus Bradycardia
If SA nodal impulses occur at a rate less than 60 bpm, the heart rhythm is known as sinus bradycardia.  Sinus bradycardia may not always be indicative of disease. Trained athletes, for example, often have heart rates < 60 bpm when not exercising.

# Non Sinus Rhythms
If the SA node fails to initialize the AV junction can take over as the main pacemaker of the heart.  The AV Junction "surrounds" the AV node (the AV node is not able to initialize its own impulses) and has a regular rate of 40 to 60 bpm. These "Junctional" rhythms are characterized by a missing or inverted P Wave.  If both the SA node and the AV Junction fail to initialize the electrical impulse, the ventricles can fire the electrical impulses themselves at a rate of 20 to 40 bpm and will have a QRS complex of greater than 0.12 seconds.

# Depolarization and the ECG

# SA node: P wave
Under normal conditions, electrical activity is spontaneously generated by the SA node, the physiological pacemaker.  This electrical impulse is propagated throughout the right and left atria, stimulating the myocardium of the atria to contract. The conduction of the electrical impulse throughout the atria is seen on the ECG as the P wave.
As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the AV node.

# AV node/Bundles: PR interval
The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles. The delay in the AV node forms much of the PR segment on the ECG. And part of atrial repolarization can be represented by PR segment.
The distal portion of the AV node is known as the Bundle of His. The Bundle of His splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch. The left bundle branch activates the left ventricle, while the right bundle branch activates the right ventricle. The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage.

# Purkinje fibers/ventricular myocardium: QRS complex
The two bundle branches taper out to produce numerous Purkinje fibers, which stimulate individual groups of myocardial cells to contract.
The spread of electrical activity through the ventricular myocardium produces the QRS complex on the ECG.

# Ventricular repolarization: T wave
The last event of the cycle is the repolarization of the ventricles.

# Sinus arrhythmias
Some variants of sinusrhythm exist:
- Asystole
- Sinus tachycardia (>100 beats per minute)
- Sinus bradycardia (<50 beats per minute)
- Sinus arrest or pause
- Sino-atrial exit block
- Sick Sinus Syndrome
If the heart rate exceeds 100 bpm, the tachycardia flow chart should be followed.

# Additional resources
- ECGpedia: Course for interpretation of ECG
- The whole ECG - A basic ECG primer
- 12-lead ECG library
- Simulation tool to demonstrate and study the relation between the electric activity of the heart and the ECG
- ECG information from Children's Hospital Heart Center, Seattle
- ECG Challenge from the ACC D2B Initiative
- National Heart, Lung, and Blood Institute, Diseases and Conditions Index
- A history of electrocardiography
- EKG Interpretations in infants and children