The Swedish ECG display is logical and easy to understand

The EKG recording is more than 100 years old. Dutchman Willem Einthoven received the Nobel Prize in 1924 for the first clinically useful ECG recordings in humans [1]. Einthoven provided the first, second, and third designations of the three ECG leads that result when the electrical potential difference is derived between the right and left arm (I), left foot, right arm (II), left foot, and left arm (III), respectively. The so-called third-party derivatives (aVR, aVL, and aVF) were not added until several decades after Einthoven’s first recordings. For these historical reasons, and given that the ECG printing technology in the middle of the last century for space reasons did not allow the simultaneous recording of more than a few channels, the limb ends were presented in two groups: first I, II, III, then aVR, aVL, aVF. This type of ECG display is often referred to as the “classic” or “historical” view.

Cabrera Show for Sweden

Electrophysiologists and cardiologists at this time were aware that this presentation of the ECG was impractical and illogical. An alternative view was available, however, in which the extremities of the limbs were presented in the order they “look at” to the heart, from left cranial through left lateral to right caudal: aVL, I, -aVR, II, aVF, III, where -aVR It is the inverted lead aVR.

This method of presentation is usually called the Cabrera Show (after the Cuban cardiologist, Ernesto Cabrera). However, Cabrera’s show did not have a widespread impact as commercial ECG machines did not allow prints in that order. It wasn’t until the 1970s that the Swedish company Elema-Schönander AB (later Siemens-Elema AB) began offering many display systems. On this ECG machine, the view method can easily be selected manually.

In Sweden, hospital ECGs have opened their eyes on the Cabrera Show [2]. Several studies have been conducted in central hospitals and major university hospitals regarding physicians’ perceptions of the advantages and disadvantages of classic presentation compared to that of Cabrera. In most cases, these studies were in favor of Cabrera’s offer. Moreover, the basic education of the physicians was a need for a suitable ECG textbook and thus Cabrera’s presentation was interesting in action.

After a two-year investigation of the national standardization of ECG indicators, led by the Secretary of the Technical Committee of the Swedish Society of Clinical Physiology, Lennart Gurfeldt, a move was recommended (among others) to Cabrera’s ECG presentation rather than the classic presentation (Figure 1) [3]. The recommendation received support from the Swedish Society of Cardiologists, the Swedish Society for Internal Medicine, the Swedish Society of Pediatric Cardiology and the Swedish Association for Medical Physics and Technology, after the proposal was submitted for an opinion. [4].

This later led to the Swedish Medicines Agency issuing a regulation in 1979 requiring Cabrera to be used in Sweden [5].

The classic symptom, which is by far the most international used, has several drawbacks and may be considered inconsequential (Fig.1).

Differences in width

The ECG criteria for ST elevation infarction are considered fulfilled if ST elevation is present in two anatomically adjacent directions [6]. In the classic view, there are 3 pairs of strands at the terminal ends: aVL / I, II / aVF, and aVF / III, which, however, are not rendered in relation to one another but must instead be preserved.

In the Cabrera model, these leads are displayed next to each other. In addition, two transformation pairs were added: I / –aVR and –aVR / II. Cabra’s logic rendering simplifies understanding of the evolution of waveforms among leads. An example of this was shown in a study involving Swedish and American medical students in which Cabrera’s presentation was shown to result in a faster and more accurate identification of the frontal plane axis. [7]. A better understanding of the relationship between potential clients will likely lead to a better ability to understand and identify reciprocal changes, which is important when evaluating an EKG if acute myocardial infarction is suspected. [8].

Moreover, as mentioned above, in the classic view aVR is used instead of -aVR. With its positive pole in the upper right quadrant of the chest, the amplitudes will be positive at negative amplitudes in the first and second. The Q waves in aVR coincide with the R waves in I and II, and the ST decrease in aVR coincides with the ST-increase of I, II, etc. This is perhaps an explanation that aVR is often neglected when interpreting ECGs with classic presentation [9].

At an international ECG conference, Pahlm and others had 35 ECG experts who evaluated five EKGs, all of which contained multiple potential differential diagnoses, given according to the classic presentation with little change. Instead of aVR, without the study participants’ knowledge, an inverted equivalent – aVR – was introduced. After obtaining the ECG interpretations, a second round of questionnaires were conducted in which the study participants were asked whether they had used all twelve strands in the interpretation, whether they had included the default repetition rate in their assessment and if it was observed that the aVR was presented in the form. Upside down. Only 2 of 35 ECG experts had actually noticed in Round 1 that the aVR was inverted. Of the 26 study participants who indicated that they used all 12 leads, 20 admitted they did not notice the lead was upside down. Pahlm and colleagues concluded that ECG evaluators often ignore the heart rate frequency and that the traditional method of presenting a 12-pin ECG may be considered an “11-line ECG” instead. However, the importance of including aVR (or -aVR) in ECG evaluation has been emphasized in several scientific studies. [10-15].

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Waiting for the rest of the world

Although the benefits of Cabrera’s show have been highlighted in numerous articles in international magazines [8, 16-20] And that the method has already been recommended by the American Heart Association as an alternative [21] It failed to apply internationally, and is today used in addition to Sweden only in parts of Finland, Italy and Japan. More scientific studies comparing the two presentation methods would be welcome.

Recently, an ECG book was launched in Swedish (Hampton J, Hampton J. ECG on easy way. Studentlitteratur AB; 2020), in which an EKG is produced with a classic presentation. The introduction questions the advantages of the Swedish presentation, and instead emphasizes the importance of being able to also interpret an EKG through classic presentation. For starters in interpreting ECG, there might be a point to know that international journals and textbooks present ECG differently than Cabrera. However, we believe that ECG education should be done according to Swedish ECG standards, and there are many modern Swedish textbooks.

Presenting the Cabrera Presentation as part of the national standardization of ECG presentation in Sweden was successful based on careful study, preparation and close cooperation between specialized societies with a common goal: to improve and facilitate understanding of cardiac electrical activity and thus ECG diagnosis, to ultimately provide the best possible care to our patients. This deserves respect, and we must – while waiting for the rest of the world to finally come to terms one day – protect the visual and rational way of delivering the ECG that we use in Sweden today.

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Potential Relationships or Conflicts: None mentioned.

Läkartidningen. 2021; 118: 20162

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