Olle Pahlm and Galen S. Wagner

Chapter Outline

Limitation in Use of Conventional Displays of the 12 Standard ECG Leads for Diagnosis of Acute Coronary Syndromes

STEMI diagnosis requires at least 0.1-mV ST elevation in at least two contiguous leads.¹ Contiguous leads, of course, refers to the spatial locations of the leads in the body, rather than their positions on the ECG display, whether on paper or on screen. However, the conventional way of displaying the 12-lead ECG, based on the historical development of leads, presents a challenge for identifying spatially contiguous limb leads. The standard ECG display format provides five potential pairs of contiguous chest leads (V1/V2, V2/V3, V3/V4, V4/V5, and V5/V6) but only three potential pairs of contiguous limb leads (I/aVL, II/aVF, and III/aVF). This difference occurs because only the six chest leads are displayed in their orderly sequence, while the six limb leads are displayed as two groups, each consisting of three leads. Leads I, II, and III are displayed as one group, and aVR, aVL, and aVF are displayed as a second group. The six limb leads can, however, be integrated into one sequence, creating a similarly logical display as that used routinely for the chest leads. This Cabrera sequence has been used routinely in Sweden for many years.¹² Spatial contiguity is more readily appreciated in the Cabrera sequence format than in the classical display format. In the classical format, nonspatially contiguous limb leads are displayed adjacently; therefore it is possible for “pattern-oriented” ECG readers to erroneously consider lead pairs such as I and II as contiguous.

Challenges in Considering ECG Limb Lead Relationships

Limb leads are strictly related mathematically; all six leads can be derived from any pair of two leads (e.g., leads I and II) using

Einthoven’s law (i.e., lead II = lead I + lead III at any point in time) and the way that electrodes are connected to produce the augmented leads aVR, aVL, and aVF. Specifically, the remaining four limb leads can be expressed in terms of leads I and II in the following way:

Reciprocal ECG Leads

It becomes necessary to introduce the term reciprocal for understanding the relationships among the appearances of ECG waveforms in different leads. Truly reciprocal views are those presenting inverted or “mirror-lake” images. No pair of standard ECG leads is separated by 180 degrees; however, “almost reciprocal” views are available only in leads such as aVL and III, with their positive poles oriented 150 degrees apart. When ST elevation indicative of acute ischemia of the anterior left ventricular (LV) wall is present in lead aVL, “almost reciprocal” ST depression is typically seen in lead III, and when ST elevation indicative of acute ischemia of the inferior LV wall is present in lead III, “almost reciprocal” ST depression is typically noted in lead aVL.

As indicated in the above formulas, lead aVR is the average of standard leads I and II with a negative sign. The inverted or “reciprocal” lead −aVR is the average of leads I and II and therefore fits appropriately in the orderly clockwise Cabrera sequence of aVL, I, −aVR, II, aVF, III, as has been mentioned.

Challenges in Considering ECG Chest Lead Relationships

The chest leads in their typical orderly display sequence (V1 to V6) present a different challenge for their optimal use for diagnosis of acute coronary syndromes. When anterior-posterior views of cardiac electrical activity are added to the left-right and superior-inferior views provided by the limb leads, the chest leads provide key diagnostic information about LV regions at risk from left anterior descending and left circumflex coronary artery occlusions.

ECG recording of the six chest leads requires use of the Wilson central terminal¹⁴ to provide the negative electrode for each lead from the averaged limb lead acquisitions. The positive electrode is provided by the additional placement of a positive electrode over each of the six bony thoracic landmarks. However, unfortunately, Wilson et al. considered this single electrode to provide “anterior unipolar leads,” leading to the common current mistaken use of the term “anterior leads,” which causes the mistaken conception that these leads provide information only from the “anterior” myocardium. Also, because placement of the positive electrodes is determined by reference to bony thoracic landmarks closer to the cardiac source, they lack the “equal” inter-lead spacing provided by the Einthoven triangle already described for the limb leads. However, it has been demonstrated by Brody et al.¹⁵ that the frontal plane triangle is not really equi-angular; therefore it becomes feasible to consider that approximately 30-degree angles are present between each of the six adjacent positive and their six truly reciprocal negative limb and chest leads.

ECG Imaging to Overcome These Challenges

Optimal use of the ECG in the clinically challenging emergency evaluation of patients with acute coronary syndromes requires a change in the way that ECGs are typically interpreted by even skilled cardiologists. This can be accomplished by expanding the definition of STEMI to include ST depression as “STEMI equivalent”¹⁶ and providing an ECG image of a change in the limb lead display format to the orderly sequence of aVL, I, −aVR, II, aVF, III. This would provide the full five potential pairs of spatially contiguous limb leads (aVL/I, I/−aVR, −aVR/II, II/aVF, and aVF/III), just as there are already five potential pairs of spatially contiguous chest leads, as has been noted.

A study by Perron et al.¹⁷ of patients with complete acute coronary occlusion caused by angioplasty balloon inflation has documented that further extension of the positive and negative limb and chest lead displays around full 360-degree “clockfaces” further increases the accuracy of diagnosis. This ECG image is presented in Figure 65-1 for a patient with acute left circumflex occlusion. Note that ST elevation is seen only in nonstandard leads −V1, −V2, −V3, and −V4. The ST depression in standard leads V1, V2, V3, and V4 fulfills STEMI equivalent criteria in at least two adjacent leads. For patients with STEMI criteria in only a single lead (e.g., aVL or III), threshold ST depression in adjacent leads (e.g., −III or −aVL) would indicate positive STEMI criteria. In the study by Perron et al.¹⁷

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