What is Normal PR Interval on ECG?
June 7th, 2021
PR Interval : Definition
The heart repeats contraction and relaxation with regular electrical stimulus initiated by Sinus Node. In condition without cardiac issues, the signals pass through the heart along the electrical circuit within limited time.
12-lead ECG(EKG) is gold standard to diagnose cardiac disease. The ECG machine was invented by Dutch Scientist, Willem Einthoven and widely used in medical field even in today. Appearance and size of the machine has been downsized and ring size single-lead ECG device is available.
Even though size of the devices is different, there is a certain protocol reading the electrical signals collected from a patient or those who wear the device. With ECG rhythm strip, a beat of the heart consists of three different parts, P wave, QRS complex and T wave. As mentioned above, each wave should pass through the heart within limited time. Normally, ECG records are printed in certain speed on paper with a certain scale. With girds on the paper, physicians can estimate the speed and width of the electrical signals.
The PR Interval is a segment from starting point of P wave to starting point of QRS complex. The interval indicates that the electrical signal arrived its first destination which is AV node from Sinus Node. Average between 120 ~ 200ms (0.12-0.20s) is observed in healthy adults.
1. First Degree AV Block
Electrical signals pass through the atrioventricular node, His bundle, left and right bundle branches, and are transmitted to the Purkinje’s fiber, causing the left and right ventricles to contract. When the normal path of electrical conduction is delayed or closed, it is called conduction interruption. Atrioventricular block refers to any blockage between the SA node and the Purkinje’s fiber.
First-degree atrioventricular blockage refers to cases where atrioventricular nodules and conduction delays appear in the heath. First-degree atrioventricular blockade is correct to see as a delay rather than blockage because electrical stimulation is eventually transmitted to the ventricle even though there is a conduction delay in the atrioventricular node or His bundle. First-degree atrioventricular block can be diagnosed when the PR Interval is more than 0.2 seconds and more than 5 small scales on the ECG.
2. Second Degree AV Block
In general, there are no symptoms, but some may feel irregular heartbeat, presyncope or fainting. There are times when the conduction between the atrium and the ventricle is incomplete and sometimes not. Mobitz type 1 is common, and the PR interval gradually increases, and conduction is blocked. This can be found at bedtime in healthy people and do not require special treatment. However, the Mobitz type 2, where conduction is blocked after the PR interval is constant, may require an artificial pacemaker to be inserted, similar to the third-degree atrioventricular block.
3. Complete AV Block
A phenomenon in which no electrical signal is transmitted to the ventricle. Although the atrium and the ventricle are physically connected, they are electrically insulated so that an electrical signal cannot flow to the ventricle. It can be caused by acquired or congenital causes and shows a phenomenon in which all P waves and QRS complexes are dissociated on the electrocardiogram.
4. Short PR Interval
The PR Interval is also referred to as the PQ interval, and refers to the interval from the origin of the P wave to the origin of the QRS group. This PR interval represents the time it took for the electrical stimulation of the atrium to pass through the atrioventricular node to the His bundle. The normal PR interval is within 200ms. It is generally harmless but may be associated with Wolff-Parkinson-White syndrome (a symptom that occurs when the heart contracts faster than normal).
5. Pre-excitation syndromes (Wolf-Parkinson-White)
WPW syndrome is the most common cause of paroxysmal supraventricular tachycardia (PSVT) in children and infants and can be inherited from parents. 70% of all patients are male, and about 70% do not have other heart disease.
In the case of a normal heart, the SA node sends an electrical signal to a specific path to regulate the cycle of the heart rate, but in WPW syndrome patients, the electrical impulse is transmitted early from the heart through a subconductor, causing paroxysmal supraventricular tachycardia. In severe cases, increased heart rate can lead to sudden death due to fainting or ventricular tachycardia.
Written by Gerrard Kim
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