Monday, July 27, 2015

ECG Blog #114 (Standard Recording — Cabrera Format — Recording Speed)

How would you interpret the 12-lead ECG in Figure-1? This tracing was obtained from a previously healthy middle-aged woman who presented with back pain over the previous month.
  • Does her ECG provide any clue to the etiology of her symptoms?
HINT: Can you explain why the QRS complex in lead aVR shows global positivity (ie, positive P wave, QRS complex and T wave)?
Figure-1: 12-lead ECG obtained from a middle-aged woman with back pain over the past month. NOTEEnlarge by clicking on FiguresRight-Click to open in a separate window.
ANSWER: The ECG in Figure-1 provides no clue to the etiology of this patient’s symptoms. The reason the P wave, QRS complex and T wave are all positive in aVR, is that instead of the usual 12-lead format (that interpreters in the United States are accustomed to) — the Cabrera format has been used instead.
  • 12-lead ECGs that are recorded in the United States typically display simultaneous recording of 4 sets of 3 leads (leads I,II,III; aVR,aVL,aVF; V1,2,3; and V4,5,6). One or more long lead rhythm strips are typically displayed immediately below the 12-lead. In contrast — Note that a simultaneously-recorded long lead rhythm strip for each of the 12-leads is displayed in Figure-1 — and, that the vertical sequence used for the limb leads is markedly different from the usual format. That is, rather than lead I — the first lead displayed is lead aVL.
  • Note that a minus sign appears before the designation aVR (ie, -aVR). The reason for global positivity is that the polarity of lead aVR has been reversed. As a result — the mirror image picture (ie, global positivity rather than negativity) is displayed for –aVR.
Our Interpretation of Figure-1: Unfortunately, there is some distortion of this 12-lead tracing, arising from the fact that this figure represents a smart phone photograph rather than a flat scan of the ECG. In addition, there is some baseline drift movement; interference artifact (the thick, vertical geometric lines most prominently seen in lead V1, with finer baseline undulations perhaps due to muscle contraction in several other leads); and other motion or improper skin-electrode contact artifact (seen best at various points in the long lead V2 rhythm strip). That said — the overall quality of this tracing is sufficient for appropriate interpretation.
  • The rhythm is sinus at about 80/minute. The PR, QRS and QT intervals are normal. The axis is leftward, but not enough to qualify as LAHB (Left Anterior HemiBlock) — since the QRS complex in the long lead II is not consistently more negative than positive. There is no chamber enlargement.
  • Regarding Q-R-S-T Changes — a small q wave is seen in lead aVL; Transition may be slightly delayed (the R wave only becomes consistently more positive than the S wave is deep between V3-to-V5) — but there are no more than minimal nonspecific ST-T wave abnormalities (slight flattening in several leads) that do not at all appear acute. Thus, we find no explanation for this patient’s symptoms from her ECG that is shown in Figure-1.
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The Cabrera Format:  Used in Other Countries
Electrocardiography has long been recognized as an essential diagnostic tool used across the world. While a similar display format (with no more than minor variation) is used for ECG recording throughout the United States — variations in format are used in many countries. Since many U.S. clinicians only rarely (if ever) see ECG recording formats that differ significantly from their own — these clinicians may fail to recognize other formats. Given the exponentially increasing use of the internet for international medical correspondence (with now literally scores of international Facebook ECG forums) — plus ever expanding daily use of smart phone photographs for rapid remote consultation with experts regarding problematic ECGs  the clinical reality is that virtually all clinicians will from time-to-time encounter ECG formats that differ from the format they are most accustomed to. This clinical reality is my purpose for presenting this ECG Blog post.
  • Illustration of the principal features of the Cabrera format is seen in Figure-2 — in which we highlight the vertical lead orientation (with blue and red rectangles) — and insert the Cabrera axis system in the upper righthand corner of the tracing. Note that vertical limb lead display begins with lead aVL — employs reversed polarity for aVR (within the blue rectangle) — and that each of the 6 limb leads are separated by the equal increment of 30 degrees in the frontal plane. Thus, lead –aVR is situated at a frontal plane angle corresponding to +30 degrees (in between lead I at 0 degrees and lead II at +60 degrees).
Figure-2: The 12-lead ECG from Figure-1 has been labeled, highlighting features of the Cabrera format. Note vertical limb lead sequencing begins with lead aVL — and that reversed polarity is used for aVR, with lead –aVR being situated in between leads I and II at +30 degrees (within the blue rectangle).
Although first description of the sequential limb lead format shown in Figure-2 was made by Fumagalli (in 1949) — development of this format is attributed to Cabrera. The Cabrera sequential lead system for ECG recording has been in general use in Sweden since 1977, with current use in several other countries. 
  • In many ways, the Cabrera format offers a much more logical display of limb lead sequencing. As opposed to the traditional U.S. format (in which limb leads are grouped into standard leads I,II,III and augmented leads aVR,L,F) — there is gradually progressive (equally spaced) sequencing with the Cabrera format from lead aVL (at -30 degrees) to lead III (at +120 degrees). 
  • Using the Cabrera format enhances the clinical utility of aVR by effectively adding lead –aVR as a transition lead between lateral and inferior frontal plane location. This allows greater specificity of the extent of high lateral and inferior ischemia or infarction. It also simplifies both axis and ST-T wave vector calculation in the frontal plane — since no more than a glance at the 6 sequential Cabrera leads is now needed for instant determination of which lead(s) manifests greatest net QRS and/or ST-T wave deflection. 
  • Finally, comparison with serial tracings in a given patient is easier — because sequential limb lead display makes serial variation in Q wave presence, QRS amplitude, and ST-T wave displacements much more evident as to what represents probable “real change” in patient status vs serial alteration in ECG waveforms primarily due to a difference in lead placement.
Despite the above potential advantages of more logical sequential lead placement — the Cabrera format seems unlikely to replace the non-sequential traditional U.S. format, at least for the immediate future. Old established habits are difficult to break ... — even when a newer approach seems technically easy to implement and clinically advantageous. 
  • As a result — it is important for clinicians to be aware that their particular system of lead system recording is not universally used across the globe.
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P.S. Another variation in format to be aware of is the recording speed of the ECG. Whereas most of the world employs a standard recording speed of 25mm/second (which is the recording speed used in Figure-1) — some countries favor ECG recording at twice this speed ( = 50mm/second). Awareness of this variation is essential when interpreting tracings submitted from other parts of the world, especially since notation of the recording speed used may be unintentionally deleted from that part of the ECG within the photograph being sent. At double the recording speed (ie, 50mm/second) — the QRS complex appears twice as wide, and the R-R interval appears twice as long as these parameters appear when the recording speed is 25mm/second. In addition  ST-T wave appearance looks markedly different at twice the recording speed, such that familiar patterns of ST-T wave abnormalities may no longer be recognized ...
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ACKNOWLEDGMENT: My thanks to Alexandru Radulian (of Bucharest, Romania) for allowing me to reproduce this tracing and use this clinical case.
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