Diagnostics: Alternative EKG Leads

The standard 12-lead ECG is our essential diagnostic tool in the evaluation of patients with suspected cardiac pathology, including myocardial infarction and dysrhythmias. Myocardial infarction due to acute coronary occlusion, however, may occur without ST-segment elevation on 12-lead ECG [1].  20 - 27% of patients diagnosed with NSTEMI who subsequently undergo angiography are found to have a completely occluded culprit vessel [2-4].  In some situations, alternate acquisition strategies, such as modified lead placement or recording speed, can increase its diagnostic yield, a few of which we will discuss through this post.

Figure 1. Placement of posterior leads V7-V9  V7 is placed at the posterior axillary line, in the same horizontal plane as V6. V8 is placed at the tip of the left scapula, in the same horizontal plane. V9 is placed in the left paraspinal region, in the same horizontal plane.

Figure 1. Placement of posterior leads V7-V9

V7 is placed at the posterior axillary line, in the same horizontal plane as V6. V8 is placed at the tip of the left scapula, in the same horizontal plane. V9 is placed in the left paraspinal region, in the same horizontal plane.

Posterior Leads (V7 – V9)

ST-segment elevation is particularly insensitive as a marker for occlusion of the posterior coronary circulation, and specifically the left circumflex artery [5,6].  The standard 12-lead ECG does not directly assess the posterior wall of the left ventricle, and while posterior MI most commonly occurs with concomitant inferior or lateral infarct, isolated posterior MI may occur in up to 8% of acute circumflex occlusions [1].  The predominant 12-lead finding in isolated posterior MI is ST-segment depression ≥0.5 mm in leads V1-V3 [7].  The addition of posterior leads V7-V9 (Figure 1) increases ECG sensitivity for posterior MI [1,8], and current guidelines consider ST-segment elevation ≥0.5 mm in V7-V9 (≥1 mm for men under age 40) to be diagnostic (Table 1) [9,10].  Per both the AHA/ACC and ESC Guidelines, however, isolated anterior ST-segment depression alone is an indication for emergent cardiac catheterization [10,11].  Leads V7-V9 may therefore have the greatest utility in patients with clinically-suspected MI but non-diagnostic 12-lead ECGs by providing an objective finding to expedite emergent intervention.  Notably, concern for ongoing ischemia – as in a patient with refractory angina despite maximal medical therapy – should also prompt immediate catheterization, even with a normal ECG [10,11].  Thus, while the presence of ST-segment elevation in V7-V9 warrants emergent intervention, its absence should not necessarily preclude it.

Table 1. ECG criteria for posterior MI [9,10]

Table 1. ECG criteria for posterior MI [9,10]

Figure 2. Placement of right-sided leads V1R-V6R  Right-sided leads are placed as a mirror image of the standard precordial leads. V1R is placed at the left sternal border, in the 4th intercostal space. V2R is placed at the right sternal border, in the 4th intercostal space. V3R is placed between V2R and V4R. V4R is placed at the mid-clavicular line, in the 5th intercostal space. V5R is placed at the anterior axillary line, in the 5th intercostal space. V6R is placed in the mid-axillary line, in the 5th intercostal space. Alternatively, V4R can be obtained as the sole right-sided lead, with the remaining precordial leads placed in standard fashion.

Figure 2. Placement of right-sided leads V1R-V6R

Right-sided leads are placed as a mirror image of the standard precordial leads. V1R is placed at the left sternal border, in the 4th intercostal space. V2R is placed at the right sternal border, in the 4th intercostal space. V3R is placed between V2R and V4R. V4R is placed at the mid-clavicular line, in the 5th intercostal space. V5R is placed at the anterior axillary line, in the 5th intercostal space. V6R is placed in the mid-axillary line, in the 5th intercostal space. Alternatively, V4R can be obtained as the sole right-sided lead, with the remaining precordial leads placed in standard fashion.

Right-Sided Leads (V1R – V6R)

Right ventricular infarction (RVI) typically occurs with concomitant inferior wall MI, most often due to occlusion of the right coronary artery proximal to the acute marginal branch [12].  Autopsy data suggests isolated RVI is both rare and non-fatal [13].  On a standard 12-lead ECG, RVI may present with ST-segment elevation in the right precordial leads, most prominent in V1, typically with concurrent inferior ST-segment elevation; in the setting of associated posterior MI, however, elevation in V1 may be attenuated or absent due to superimposed ST-segment depression [14].  Right-sided chest leads V1R-V6R (Figure 2) can be used to improve diagnostic accuracy for RVI, and ST-segment elevation in V4R ≥0.5 mm is roughly 80% sensitive and specific [15].  Current ESC/ACC/AHA/WHF guidelines define this as the appropriate diagnostic cutoff, except in men under age 30, for whom 1 mm should be used (Table 2) [9].  RVI may result in right ventricular dysfunction, compromising left ventricular filling and cardiac output.  Adequate right ventricular preload is essential to maintaining LV function and systemic perfusion, and medications that decrease venous return and RV filling, such as nitrates, should be avoided in RVI [10,16].  That said, there is no long-term mortality benefit to nitrate administration in STEMI [17], and current ESC guidelines recommend against the routine use of nitrates, even without RV involvement, except in the setting of hypertension or heart failure [10].

Table 2. ECG criteria for right ventricular infarction [9]

Table 2. ECG criteria for right ventricular infarction [9]

15-lead ECG (V4R, V8, V9)

So why aren’t these leads in our default ECG acquisition? The overwhelming majority of studies regarding both the diagnostic and prognostic utility of adding posterior and right-sided leads date from the late 1970s to early 2000s; that is, a time when thrombolysis was the mainstay of reperfusion therapy, with diagnostic ST-elevation as a nearly-universal prerequisite for intervention.  In the context of current interventional guidelines, however, the additive benefit of extended-lead ECG is unlikely to affect recommended patient management in the majority of cases.  Extended leads are easy to obtain and may provide additional evidence to justify emergent catheterization, however their acquisition should not delay or deter intervention that is otherwise indicated.  Current ESC guidelines state right-sided and posterior leads “should be considered” in cases of inferior and suspected posterior MI, respectively (Class IIa recommendation, level of evidence B) [10].   AHA/ACC guidelines consider it “reasonable” to obtain leads V7-V9 in patients with a non-diagnostic ECG and high risk of ACS (Class IIa recommendation, level of evidence B) [11].

Figure 3. Lewis lead configuration  RA is placed overlying the manubrium. LA is placed at the right sternal border, in the 5th intercostal space. LL is placed at the right lower costal margin. RL is placed on the right leg as usual. With this configuration, lead I should be monitored.

Figure 3. Lewis lead configuration

RA is placed overlying the manubrium. LA is placed at the right sternal border, in the 5th intercostal space. LL is placed at the right lower costal margin. RL is placed on the right leg as usual. With this configuration, lead I should be monitored.

Lewis Lead (S5)

Evidence of atrioventricular dissociation on ECG in the setting of a wide-complex tachycardia is extremely specific for a ventricular origin, but is relatively insensitive [18].  The Lewis lead (Figure 3) is a modified limb lead configuration that accentuates atrial activity, originally developed to assess atrial fibrillation [19].  In the context of an undifferentiated wide-complex tachycardia, the Lewis lead can aid in recognition of AV dissociation that is not apparent on a standard 12-lead ECG, confirming a diagnosis of ventricular tachycardia [20-25].  Though not well-described in the literature, the Lewis lead has anecdotal utility in other situations where the identification of atrial activity has diagnostic and therapeutic implications (e.g. differentiating complete heart block from junctional bradycardia, atrial fibrillation from atrial flutter from multifocal atrial tachycardia, or SVT from sinus tachycardia).

Double-Speed ECG

In tachycardic patients, particularly those with very high heart rates, narrow intervals on the ECG tracing may make subtle findings difficult to appreciate.  Increasing the paper speed from the standard 25 mm/s – where 1 small box is 0.04 s and 1 large box is 0.2 s – to 50 mm/s artificially elongates the intervals, making findings such as flutter waves or slight irregularity easier to recognize [26].

Summary

The value of alternate ECGs ultimately depends on the extent to which they influence patient management.  Posterior and right ventricular leads may improve the characterization of infarct territories, but likely have minimal impact on clinical decision-making outside select cases (e.g. a patient with isolated posterior MI and no ischemic changes on 12-lead), and intervention that is otherwise indicated should not be delayed to facilitate their acquisition. The Lewis lead and double-speed ECG have the potential to alter management decisions, again in select cases by aiding in the diagnosis of arrhythmias.

Abbreviations:  AHA – American Heart Association, ACC – American College of Cardiology, ESC – European Society of Cardiology, WHF – World Heart Federation


Post by Kate Connelly, MD

Dr. Connelly @kmconnel78 is a PGY-1 Emergency Medicine Resident at the University of Cincinnati

Peer Review by Robert whitford, MD and Ryan LaFollette, MD


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