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Patient/anamnesis: n/a
ECG interpretation/discussion: This ECG is a printout from a telemetry station, derived from a 5 lead patient monitoring. So, what have we here? The underlying rhythm is sinoatrial, but both PR and RR interval varies. Remember, Wenckebach is not the name of a certain type of block, but rather a type of conduction. This is often misunderstood, as Second Degree AV Block Type 1 or Mobitz 1 is also often labelled Wenckebach block. A more precise term would be Wenckebach periodity, phenomenon, conduction. Wenckebach conduction is usually considered benign and can be recognized by the following criteria:
1) PR interval is progressively prolonged until a P wave is blocked
2) The shortest PR interval is the one immediately following the dropped beat. The longest PR interval is the one immediately before the dropped beat. The incremental change in PR interval is in the beginning of the Wenckebach cycle, thus between the first and second PR interval in a sequence.
3) The RR intervals progressively shorten until a QRS is ‘dropped’ due to the non-conducted atrial/sinoatrial impulse.
Now, looking at this ECG, the two first beats are at the end of a Wenckebach cycle. After the second QRS a non-conducted P wave occurs. The following beat is wide and bizarre and is a ventricular escape beat that occurs due to the long preceding pause. After the escape beat, a new Wenckebach cycle starts. The PR interval lengthens until a P wave is blocked. After this pause, a narrow QRS is preceded by a P wave with a very, very short PR interval. This is a junctional escape beat. Then, the Wenckebach cycle restarts.
When counting P waves and QRS complexes in the cycles, we’ll see that for every three QRS complexes there are four P waves, since one of the QRS complexes gets dropped repeatedly. This gives a 4:3 atrioventricular (AV) ratio, which is also called 4:3 conduction.
What a beauty! Thanks to my colleague and fellow ECG-dork for bringing me this rare gem!
September 6, 2009
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Patient: Woman, 82 y/o with permanent atrial fibrillation. Accidental digitalis intoxication. Serum digoxin level when arriving in the ER is 6.6 ng/ml. General fatigue, but no recent history of syncopal episodes.
ECG description:
- Irregular, narrow QRS bradycardia at approx. 35 bpm
- Atrial fibrillation with slow ventricular response
- Normal axis at ca. 60 degrees
- Prominent U waves, best seen in leads V2 -V3
- Cohn effect: ST segment depression and flattened T wave in leads V4-V6
- Poor R wave progression
- Low amplitude in limb leads
- Baseline noise artefact
Discussion: This 12 lead ECG displays atrial fibrillation with slow ventricular response. There is a high degree of AV block, resulting in a bradycardic rate at ca 35 bpm (50 mm/s). The axis is in the normal quadrant and at ca. 60 degrees. Limb leads show T wave flattening, and there is perhaps a slight ST segment depression visible in leads II and AVF. There is a quite prominent U wave. Normally, the U wave is best appreciated in the lateral precordial leads (V5-V6). Here however, it is seen in leads V2 and V3. The classic ST segment morphology induced by digitalis both at therapeutic and toxic serum levels, is the “coved” or “scooped”, or sometimes referred to as “bowl shaped” ST segment depression. It is sometimes described as if the ST segment has been dragged downwards from a point at the middle of the segment. Digitalis intoxications may however, manifest with or without the classic morphology even at high serum levels. The classic digitalis effect on the ST segment is sometimes called the Cohn Effect, named after Alfred E. Cohn, the American cardiologist, for his early 1900-century studies on the effect of digitalis on T wave morphology. It is generally recognized by ST segment depression together with T wave flattening in the same lead. Although this ECG lacks the coving ST segment, the Cohn effect is present in leads V4-V6.
Overall, digitalis has a positiv inotropic effect and a negative chronotropic effect. The negative chronotropy is due to both decreased automaticity of the SA node as well as prolongation of the refractory period of the AV nodal tissue, thus inducing higher degrees of AV block. It also increases AV nodal automaticity which often results in for instance accelerated junctional rhytm and junctional extrasystolia.
This ECG is in the low bradycardic range at around 35 bpm, which is due to the high serum levels of digoxin. Different AV ratios can occur, but 2:1 is rare. With second degree AV blocks, Wenckebach conduction is common. In this ECG it’s impossible to determine the AV ratio, due to atrial fibrillation, and one can only conclude that it is varying. Following is a continous rhythm strip (25 mm/s) of the patient that was obtained 10 minutes later, showing the development of higher degrees of AV block, resulting in long bradycardic cycles.
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Note the long, asystolic pauses. The first and third cycles are so long (>2 seconds) that ventricular escape occurs.
June 18, 2009