Alternans electrical on ECG

كيف يظهر في التخطيط

In electrical alternans, the amplitude of the R wave (and the entire QRS complex) periodically changes from contraction to contraction. The classical variant is 2:1 alternans, where a tall QRS alternates with a low one every other beat. Due to the difference in amplitude, low complexes can be missed during a quick glance at the tracing and the rate can be erroneously calculated as approximately half the true value; sometimes the phenomenon mimics 2:1 block.

All available leads are evaluated; the phenomenon is often noticeable in precordial leads, but localization and severity depend on the direction of heart movement and electrode position — changes can also be recorded in limb leads. In severe cases (large effusion, tamponade), not only QRS, but also P and T waves alternate — this is called total electrical alternans.

It is important to differentiate true electrical alternans from mechanical pulse alternans (pulsus alternans), which is detected by pulse palpation or on a blood pressure curve and has a different mechanism — it is associated with the alternation of LV contraction strength, rather than the position of the heart.

الآلية

The most studied mechanism — with a large pericardial effusion, the heart moves freely within the pericardial cavity (swinging heart) and can perform periodic pendulum-like movements; if their frequency is synchronized with the cardiac cycle, often at a cycle of about two contractions, the position of the heart's electrical axis relative to the electrodes changes — and the amplitude, axis, and morphology of QRS in the leads change cyclically.

Another mechanism is repolarization alternans: alternation of action potential duration and refractoriness of cardiomyocytes, which underlies T-wave alternans and can precede ventricular tachyarrhythmias; this variant is not associated with heart movement and is more often recorded at the cellular/microvolt level (microvolt T-wave alternans).

الأسباب

• Cardiac tamponade and large pericardial effusion (the most frequent and clinically significant cause)
• Pronounced tachyarrhythmia (supraventricular or ventricular tachycardia with high heart rate)
• Severe left ventricular dysfunction, decompensated heart failure
• Pneumothorax with mediastinal shift (rare)
• Respiratory modulation of QRS amplitude due to changes in heart position and lung volume can mimic alternans; it usually follows the respiratory cycle rather than alternating strictly 2:1
• Electrolyte disturbances and severe myocardial ischemia — more often as T-wave alternans rather than QRS

التشخيص التفريقي

• Respiratory modulation of QRS amplitude: the amplitude changes smoothly, cyclically with respiration (several complexes per respiratory cycle), rather than strictly every other beat.
• Technical artifact (poor electrode contact, tremor): amplitude changes irregularly, often with baseline "noise", and lacks strict 2:1 periodicity.
• Alternans in bundle branch block dependent on heart rate (rate-dependent block phenomenon): QRS shape and width change, and this is usually associated with reaching a critical rate rather than smooth cyclical amplitude alternation.
• Mechanical (pulse) alternans: detected clinically via pulse or blood pressure; ECG QRS amplitude may remain unchanged.
• Sinus arrhythmia with a shifting axis: amplitude changes, but the rhythm is irregular rather than strictly regular.

الأهمية السريرية والتكتيكات

The combination of tachycardia, low QRS voltage, and electrical alternans is a classic, yet nonspecific and low-sensitivity sign of pericardial effusion and tamponade: its absence does not rule out tamponade, and its presence is possible without it. When tamponade is suspected, the physician assesses hemodynamics — hypotension, elevated jugular venous pressure, muffled heart sounds, pulsus paradoxus, and other signs of obstructive shock (Beck's triad — hypotension, jugular venous distension, and muffled heart sounds).

Echocardiography for suspected tamponade should be performed immediately at the bedside if possible without delaying treatment. In a hemodynamically unstable patient with overt obstructive shock due to tamponade, echocardiography must not delay life-saving decompression — pericardiocentesis or surgical drainage should be performed urgently; waiting for a routine echocardiogram in this situation is unacceptable. In a stable condition, management is determined by the cause, volume, and hemodynamic impact of the effusion.

When alternans occurs against the background of tachyarrhythmia, the goal is to identify and abort the underlying rhythm (using algorithms for narrow or wide QRS tachycardia). Microvolt T-wave alternans, detected by special methods during stress tests, is a research/adjunct marker of electrical instability; its routine use for clinical decisions is not recommended, and the results do not replace standard risk assessment for ventricular arrhythmias.

الأخطاء الشائعة أثناء قراءة تخطيط القلب

• Mistaking respiratory amplitude modulation for true alternans — one must check whether the period of changes coincides with respiration or with strict alternation every 1 complex
• Failing to notice concomitant low QRS voltage and tachycardia — this combination increases suspicion of tamponade, although nonspecific
• Confusing it with an artifact due to poor electrode contact — check recording quality and repeat the ECG
• Failing to link the finding to clinical presentation: alternans without hemodynamic evaluation remains a "mute" ECG phenomenon, and echocardiography in an unstable patient must not be awaited at the expense of decompression
• Confusing electrical and mechanical (pulse) alternans — these are phenomena of different origins

الأسئلة الشائعة

المصطلحات ذات الصلة

المزيد في قسم «التامور، الرئتين، الدماغ وأخرى»

القسم بأكمله

المصادر

• Goldberger's Clinical Electrocardiography، الإصدار الأخير
• Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
• أورلوف ف.ن. دليل تخطيط القلب الكهربائي

مادة تعليمية لطلاب كليات الطب والأطباء. لا تحل محل التوصيات السريرية والفحص الطبي؛ يُفسر تخطيط القلب دائماً جنباً إلى جنبกับการحالة السريرية.