PR interval alternans
Synonyms and keywords: PR alternans
Related chapter: electrical alternans

# Overview
Electrical alternans is an electrocardiographic phenomenon which describes the beat to beat alternation in the height (ie. amplitude), duration (the length of the interval) or the direction (up or down) of any of the EKG complexes or intervals.  While electrical alernans is usually thought of as alternation in the height or direction of the QRS complex, there can also be P wave, PR interval, QRS complex, R-R interval, ST segment, T wave, or U wave forms of alternans (see classification below).

# Historical Perspective
- 1909: First described by Hearing
- 1910: Elaborated upon by Sir Thomas Lewis
- 1948: First description of electrical alternans on the surfce EKG by Kalter and Schwartz 
- 1978: The term pseudoelectrical alternans was first described by Klein, Segni and Kaplinsky

# Pathophysiology

## Electrical Alternans Due to Motion of the Heart
When the word alternans is used, the underlying pathophysiology that is most often thought of is alternans due to motion of the heart and its shifting position in relationship to the surface electrodes.  The pathophysiologic mechanism underlying the alternation in the height or amplitude of the QRS complex is the swinging or shifting or the electrical axis of the heart.  It should be noted that there can also be P wave and T wave alternans attributable to the motion of the heart.
While electrical alternans is frequently thought of in association with pericardial effusion, it should be noted that not all pericardial effusions cause electrical alternans, and that total electrical alternans (involving the P wave, QRS complex and the T wave) is present in just 5-10% of cases of cardiac tamponade.Therefore, these ECG findings are poorly diagnostic of pericardial effusion and tamponade as well.
It is underappreciated that the movement of the heart in hypertrophic cardiomyopathy can cause electrical alternans as well.

## Electrical Alternans Due to Changes in Electrical Conduction of the Heart
Electrical alternans can also be due to variation in the velocity of conduction through any of the anatomic structures of the heart.  Usually this variation in the conductivity is caused by a change in heart rate, autonomic tone, vagal tone, or drugs.  At a more fundamental level, the abnormality is thought to be due to changes in calcium handling in the sarcoplastic reticulum.  The following complexes, either alone or in combination, may exhibit electrical alternans due to changes in electrical conduction: the P wave, the PR interval, the QRS complex, or the RR interval. The appearance of QRS alternans as part of a narrow complex tachycardia suggests the diagnosis of orthodromic atrioventricular (AV) reentrant tachycardia which conducts retrograde over an accessory pathway.

# Causes

## All Causes in Alphabetical Order
- Atrial fibrillation
- Atrioventricular reentrant tachycaria
- Cardiac tamponade
- Drug induced
- Hypertrophic cardiomyopathy
- Left ventricular dysfunction
- Myocardial contusion
- Myocardial ischemia
- Pericardial effusion
- Pulmonary embolism
- Rheumatic heart disease
- Severe asthma exacerbation
- Wolff-Parkinson-White syndrome

## Due to Motion of the Heart
- Cardiac tamponade
- Hypertrophic cardiomyopathy
- Pericardial effusion
- Severe asthma exacerbation

## Due to Changes in Impulse Conduction
- Atrial fibrillation
- Atrioventricular reentrant tachycardia
- Drug induced
- Left ventricular dysfunction
- Myocardial contusion
- Myocardial ischemia
- Pulmonary embolism
- Rheumatic heart disease
- Wolff-Parkinson-White syndrome

# Differentiating Electrical Alternans form other Disorders
The electrical form of alternans should be differentiated from the mechanical form alternans in which case there is alternation of the strength of the pulse as is observed in pulsus alternans.  Electrical and mechanical alternans may coexist.

# Pseudoelectrical alternans
Presence of electrical alternans phenomenon in the absence of pericardial effusion is often termed as Pseudoelectrical alternans.