P wave alternans
Synonyms and keywords: p alternans
Related chapter: electrical alternans

# Overview
Electrical alternans is an electrocardiographic phenomenon which describes teh beat to beat alternation in the height (ie.e 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

## Normal physiology
- Normal P wave resembles the atrial depolarization on the ECG.
- The atrial depolarization reflects the travel of the depolarization signal from the sinoatrial node to both atria then to the atrioventricular node.
- Sinus P wave is upright in lead II which means it is positive and downward in lead aVR which means it is negative. So, when there is abnormal ectopic pacemaker other than the sinoatrial node, this can reverse the P wave into negative in lead II and positive in lead aVR.
- P wave is less than 120 milliseconds in normal duration references. It is under 0.15 - 0.20 mV in the lead II.

## Pathogenesis
- Hyponatremia is a major cause of electrical changes in the heart generally and P wave alternans.
- The changes in the QRS complex is due to swinging of the heart with every contraction. As the heart swings, the electrical signals detected by the electrodes travel over a greater distance through the fluid resulting in reduced voltage at the electrodes.

## 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.
It is underappreciated that the movement of the heart in hypertrophic cardiomyopathy can cause electrical altrical alternans as well.

## Electrical Alternans Due to Changes in Electrical Conduction of the Heart
Electrical laternans 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
- Hypertrophic cardiomyopathy
- Left ventricular dysfunction
- Myocardial contusion
- Myocardial ischemia
- Pericardial effusion
- Pulmonary embolism
- Rheumatic heart disease
- Wolff-Parkinson-White syndrome

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

## Due to Changes in Impulse Conduction
- Atrial fibrillation
- Atrioventricular reentrant tachycaria
- 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.

# Epidemiology and Demographics
- The prevalence of  is approximately  per 100,000 individuals worldwide.
- In , the incidence of  was estimated to be  cases per 100,000 individuals in .

## Age
- Patients of all age groups may develop .
-  is more commonly observed among patients aged  years old.
-  is more commonly observed among .

## Gender
-  affects men and women equally.
-  are more commonly affected with  than .
- The  to  ratio is approximately  to 1.

## Race
- There is no racial predilection for .
-  usually affects individuals of the  race.
-  individuals are less likely to develop .

# Electrocardiographic Examples
Shown below is an ECG showing electrical alternans in a person with pericardial effusion. Note the beat to beat change in the height and axis of the QRS complexes.
Shown below is an ECG showing electrical alternans in a person with pericardial effusion. Note the beat to beat change in the height and axis of the QRS complexes.
Shown below is an ECG showing electrical alternans in a person with pericardial effusion. Note the beat to beat change in the height and axis of the QRS complexes.
Shown below is an ECG showing electrical alternans in a person with pericardial effusion. Note the beat to beat change in the orientation of the QRS, and axis of the QRS complexes.[[

# Treatment

## Medical Therapy
- There is no treatment for ; the mainstay of therapy is supportive care.
- The mainstay of therapy for  is  and .
-  acts by .
- Response to  can be monitored with  every .

## Surgery
- Surgery is the mainstay of therapy for .
-  in conjunction with  is the most common approach to the treatment of .
-  can only be performed for patients with  .

## Prevention
- There are no primary preventive measures available for .
- Effective measures for the primary prevention of  include , , and .
- Once diagnosed and successfully treated, patients with  are followed-up every . Follow-up testing includes , , and .