ECG Blog #83 — Every-Other-QRST is Different

     Interpret the 3-lead rhythm strip shown in Figure-1 — obtained from a patient in a fairly (but not completely) regular SVT rhythm.

• What is the reason for the slight change in QRS morphology from beat-to-beat?

Figure-1: 3-lead rhythm strip — obtained from a patient in a regular SVT rhythm. What is the reason for the slight change in QRS morphology from beat-to-beat? 

Answer to Figure-1: 

Although one might at first be tempted to interpret the rhythm as a form of bigeminy — a more accurate interpretation would be electrical alternans. While we would accept general description of this rhythm as representing a “regular” SVT (SupraVentricular Tachycardia) — there is in fact slight-but-real phasic variation in the R‑R interval occurring every-other-beat. This is not due to a form of bigeminy — but rather to R‑R alternans. In addition — there is both QRS alternans (red and blue double arrows in Figure-2) — and T wave alternans (red and blue circles in Figure-2). That is — QRS morphology changes every-other-beat. This is subtle in lead V1 — but more noticeable in lead V2 where the initial R wave manifests an obvious difference in height from one beat to the next. Similarly — T wave morphology changes every-other beat, with this clearly more noticeable in lead V2 which manifests extra peaking of every-other-T wave (red and blue circles in lead V2).

• Clinical implications of these forms of electrical alternans in a patient with SVT — are that reentry is almost certain to be involved in the mechanism. There may or may not be a concealed accessory pathway.

Figure-2: We have labeled the 3-lead rhythm strip recorded in Figure-1. There is slight shortening of the R-R interval every-other beat = R‑R alternans. In addition — there is both QRS alternans (red and blue double arrows) — and T wave alternans (red and blue circles in Figure-2). That is — QRS and T wave morphology changes every-other-beat.

What is Electrical Alternans?

The fascinating phenomenon of electrical alternans — is a relatively uncommon clinical entity that is frequently misunderstood. It is often overlooked when it does occur. A look at Figure-1 explains why: This ECG sign can be subtle indeed.

• Electrical alternans is a general term that encompasses a number of different pathophysiologic mechanisms. Its occurrence is not limited to pericardial tamponade — but instead has been associated with an expanding array of clinical conditions.
• Distinction should be made between electrical and mechanical alternans. The term “alternans” itself — merely indicates that there is phasic fluctuation in some cardiac signal from one beat to the next within the cardiac cycle. This may be in the strength of the pulse (or the blood pressure recorded) — or it may be in one or more waveforms in the ECG recording.

NOTE: It may be helpful to first define other alternans phenomena that may sometimes be confused with the various ECG manifestations (especially since these other forms of alternans phenomena may also be seen with cardiac tamponade).

• Pulsus alternans — is a mechanical form of alternans. The rhythm is regular — but cardiac output varies from beat-to-beat. It is seen with severe systolic dysfunction. Pulsus alternans should be distinguished from a bigeminal pulse — in which a weaker beat follows the stronger beat by a shorter time interval (as occurs when the alternating beat is a PVC, which understandably generates less cardiac output).
• Pulsus alternans should also be distinguished from pulsus paradoxus — in which there is a palpable decrease in pulse amplitude (or a measured drop of >10mm in blood pressure) during quiet inspiration. While pulsus alternans and paradoxus may both be seen with pericardial tamponade — they are different phenomena than the various types of electrical alternans.

Electrical Alternans: Definition/Features/Mechanisms

Electrical alternans — is a beat-to-beat variation in any one or more parts of the ECG recording. It may occur with every-other-beat — or with some other recurring ratio (3:1; 4:1; etc.). Amplitude or direction of the P wave, QRS complex, ST segment and/or T wave may all be affected (although P wave alternans is rare). Alternating interval duration (of PR, QRS or QT intervals) may also be seen.

• Electrical alternans — was first observed in the laboratory by Herring in 1909. It was reported clinically by Sir Thomas Lewis a year later, who characterized the phenomena as occurring, “either when the heart muscle is normal but the heart rate is very fast or when there is serious heart disease and the rate is normal”. This 1910 description by Lewis serves well to this day to remind us of the 2 principal clinical situations in which electrical alternans is most often encountered: i) Supraventricular reentry tachycardias; and ii) Pericardial tamponade.

Mechanisms: 

There are 3 basic types of electrical alternans phenomena — each relating to a different pathophysiologic mechanism: i) Repolarization alternans; ii) Conduction and Refractoriness alternans; and iii) Alternans due to abnormal cardiac motion. A common cellular mechanism may underlie each of these processes relating to abnormal calcium release or reuptake within the sarcoplasmic reticulum.

• Repolarization alternans — entails beat-to-beat variation in the ST segment and/or T wave. Alternation in ST segment appearance (or in the amount of ST elevation or depression) — is often linked to ischemia. In contrast — T wave alternation is more often associated with changes in heart rate or in QT duration (especially when the QT is prolonged). In patients with a long QT — T wave alternans may forebode impending Torsades de Pointes. Both ST segment and T wave alternans have been known to precede malignant ventricular arrhythmias. Thus, this type of electrical alternans may convey important adverse prognostic implications when seen in certain situations. That said — a variety of clinical conditions have been associated with repolarization alternans, such that adverse prognostic implications do not always follow. Among these clinical conditions are congenital long QT syndrome — severe electrolyte disturbance (hypocalcemia; hypokalemia; hypomagnesemia) — alcoholic or hypertrophic cardiomyopathy — acute pulmonary embolus — subarachnoid hemorrhage — cardiac arrest and the post-resuscitation period — and various forms of ischemia (spontaneous or induced by treadmill testing or other stimulus).
• Conduction and Refractoriness alternans — entails variance of impulse propagation along some part of the conduction system. This may result from fluctuations in heart rate or in nervous system activity or from pharmacologic treatment. ECG manifestations from this form of alternans may include alternating appearance of the P wave, QRS complex or alternating difference in P-R or R-R interval duration. In particular — QRS alternans during narrow SVT rhythms has been associated with reentry tachycardias. While identification of QRS alternans during a regular SVT often indicates retrograde conduction over an AP (Accessory Pathway) — this phenomenon has also been seen in patients with simple PSVT/AVNRT that exclusively limits its reentry pathway to the AV Node. Therefore — identification of QRS alternans during a regular SVT does not prove the existence of an accessory pathway. Conduction and refractoriness alternans may be seen with WPW-related as well as AV Nodal-dependent reentry tachycardias — atrial fibrillation — acute pulmonary embolus — myocardial contusion — and severe LV dysfunction. NOTE: On occasion — Alternans may be seen with monomorphic VT (Maury and Metzger).
• Cardiac Motion alternans — is the result of cardiac movement rather than electrical alternation. The most important clinical entity associated with motion alternans is large pericardial effusion — though motion alternans has also been observed in some cases of hypertrophic cardiomyopathy. It is important to appreciate that not all pericardial effusions produce electrical alternans. Development of total electrical alternans (of P wave, QRS complex and T wave) — is likely to be a harbinger of impending tamponade. Unfortunately — the sensitivity of total electrical alternans is poor for predicting tamponade (ie, most patients who develop tamponade do not manifest preceding electrical alternans). Therefore — it may be helpful if you see total electrical alternans in a patient with a large pericardial effusion — but failure to see this ECG sign in no way rules out the possibility that tamponade is occurring. Echo studies in patients with documented cardiac tamponade confirm that electrical alternans is synchronous with and a direct result of the pendulous movement of the heart within the enlarged, fluid-filled pericardial sac of a patient with large pericardial effusion.

Electrical Alternans: KEY Clinical Points

In summary, electrical alternans is not common — but it does occur. You will see it. You have probably already seen it a number of times without even realizing it. Electrical alternans is a fascinating but advanced concept.

• In our experience — electrical alternans is most often seen in association with regular SVT rhythms (as seen in Figure-1). Seeing it in this context suggests (but does not prove) the existence of an AP (Accessory Pathway). Regardless of whether the mechanism of the regular SVT is AVNRT or AVRT — it is likely that reentry is involved. This conclusion may prove useful in contemplating potential investigative and therapeutic interventions.
• In a patient with pericarditis — a large heart on chest X-ray — or simply unexplained dyspnea — recognition of electrical alternans should suggest the possibility of a significant pericardial effusion that may be associated with tamponade. That said — electrical alternans is a nonspecific ECG sign that may also indicate myocardial ischemia, LV dysfunction and/or possibility of any of a number of other precipitating factors. BOTTOM Line: If you see electrical alternans — Look for an underlying clinical condition that may be responsible for this ECG sign.
• Development of electrical alternans per se — conveys no adverse prognostic implications beyond those associated with severity of the underlying disorder. Two exceptions to this general rule are: i) In a patient with QT prolongation or severe ischemia — recognition of electrical alternans may portend deterioration to Torsades or VT/VFib; and, ii) In a patient with a large pericardial effusion — development of total electrical alternans (of P wave, QRS complex and T wave) suggests there may now be tamponade.
• P.S. — Keep in mind that not all cases of pericardial effusion will manifest low voltage and electric alternans ... (The pathophysiology behind electrical alternans with a large pericardial effusion — is a "swinging heart" within the pericardial sac. Thus, alternans is unlikely to be seen with smaller effusions — and even with larger effusions, not all cases manifest alternans ...).

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ACKNOWLEDGMENT: My appreciation to Jenda Enis Stros for allowing me to use the ECG in Figure‑1.

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- For more information — GO TO:

• For a pdf of Section 14.0  — on Electrical Alternans (from our ECG-2014-ePub).

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