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American Journal of Critical Care. 2007;16: 63-71
Copyright © 2007 by the American Association of Critical-Care Nurses.
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Discriminating Between Right Coronary Artery and Circumflex Artery Occlusion by Using a Noninvasive 18-Lead Electrocardiogram

By Shu-Fen Wung, RN, PhD, ACNP, BC. From the College of Nursing, University of Arizona, Tucson, Ariz.

Corresponding author: Shu-Fen Wung, RN, PhD, ACNP, College of Nursing, University of Arizona, 1305 N Martin Ave, Tucson, AZ 85721-0203 (e-mail: shufen{at}nursing.arizona.edu).


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 Limitations of the Study
 Conclusions
 References
 
• Background Differentiating occlusion of the circumflex branch of the left coronary artery (also called the circumflex artery) from occlusion of the right coronary artery is often difficult because either may be associated with a pattern of acute inferior myocardial infarction on the electrocardiogram.

• Objectives To determine if an inexpensive 18-lead electrocardiogram can provide useful information in differentiating sites of coronary occlusion.

• Methods Continuous 18-lead electrocardiograms, including standard 12-lead, right ventricular, and posterior leads, were recorded in 38 and 50 subjects undergoing percutaneous coronary interventions in the right coronary artery and the circumflex artery, respectively.

• Results ST-segment elevation in the posterior leads was twice as frequent during occlusion of the circumflex artery as during right coronary occlusion (P < .001). ST-segment elevation in the right ventricular leads and inferior leads occurred more often during occlusion of the right coronary artery than during occlusion of the circumflex artery. ST-segment depression in lead aVL is highly suggestive of right coronary occlusion, whereas ST-segment elevation in posterior leads without depression of the ST segment in lead aVL is highly sensitive and specific for occlusion of the circumflex artery.

• Conclusions ST-segment changes in the 18-lead electrocardiogram can be used to differentiate between occlusions of the circumflex artery and occlusions of the right coronary artery. Knowing which vessel is occluded before percutaneous coronary intervention can help in planning the procedure and recognizing when patients are at high risk for disturbances in conduction at the atrioventricular node.

Prompt and accurate diagnosis of acute myocardial infarction (AMI) is clinically essential because various reperfusion procedures show promise for reducing the size of infarction. Despite major advances in diagnostic tools, electrocardiography (ECG) remains the cornerstone in the diagnosis of myocardial infarction. Occlusion of the left anterior descending artery can be identified accurately on a standard 12-lead ECG, but acute occlusion of the circumflex branch of the left coronary artery (also called the circumflex artery) does not always produce the ECG changes typical of AMI.1 Furthermore, differentiation of occlusion of the circumflex artery from occlusion of the right coronary artery (RCA) is often difficult because either may be associated with an ECG pattern of inferior AMI.1–4 Patients with right ventricular infarction due to RCA occlusion have a poor prognosis and are at high risk of disturbances in conduction at the atrioventricular node,5,6 whereas patients with occlusion of the circumflex artery are thought to have a good prognosis.7 With the advent of percutaneous coronary intervention (PCI) for AMI, identification of the occluded vessel is key. Knowing which vessel is occluded before PCI can help in stratifying risk and planning the procedure.

Several investigators8–10 have used ST-segment elevation and depression criteria on the standard 12-lead ECG to assist in distinguishing occlusion of the circum-flex artery from RCA occlusion in patients with inferior myocardial infarction. For example, ST-segment elevation that is greater in lead III than in lead II suggests RCA involvement,8,9 and isolated ST-segment depression in leads V2 through V4 suggests involvement of the circumflex artery.10


ECG differentiation of left circumflex and right coronary occlusion is difficult since both may present as inferior infarctions.

 

Some criteria are rather complicated and impractical for clinical use. For instance, a ratio of ST-segment depression in lead V3 to ST-segment elevation in lead III that is less than 0.5 indicates occlusion of the proximal part of the RCA, a ratio greater than 0.5 but less than or equal to 1.2 indicates occlusion of the distal part of the RCA, and a ratio greater than 1.2 indicates occlusion of the circumflex artery.11 Fuchs et al1 suggest that no criteria on the 12-lead ECG allow distinction of RCA occlusion from disease of the circumflex artery. As the use of posterior and right ventricular leads to detect involvement of the posterior wall and right ventricle increases, these additional leads may be helpful in determining which artery has the infarct.

The purpose of this study was to determine whether the noninvasive 18-lead ECG, including the standard 12 leads, posterior leads V7 through V9, and right ventricular leads V3R through V5R (Figure 1Go) can be useful for distinguishing between acute occlusions of the circumflex artery and RCA occlusions.


Figure 1
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  		Figure 1 Locations of electrodes for the posterior and right ventricular leads. (A) Locations for right ventricular leads V3R through V5R: V3R is located halfway between V2R (or V1) and V4R, V4R is located at the right midclavicular line in the fifth intercostal space, V5R is located at the right anterior axillary line at the same horizontal level as V4R. (Modified from Drew and Ide. 12) (B) Location for posterior leads: V7 is located at the posterior axillary line at the same level as V4 through V6, V8 is located halfway between V7 and V9, and V9 is at the left paraspinal line at the same level as V4 through V6.

 

   Methods
Top
 Abstract
 Methods
Results
 Discussion
 Limitations of the Study
 Conclusions
 References
 
Sample and Setting
This study was a secondary analysis of a larger study that included 320 subjects with suspected coronary artery disease who underwent a nonemergent cardiac catheterization at either a community hospital or 1 of 3 academic medical centers. Nineteen subjects having an AMI at the time of PCI were excluded from this analysis because of the difficulty in distinguishing ST-segment changes induced by balloon inflation from ST-segment changes due to an evolving AMI. Also excluded were 117 subjects who didn’t undergo PCI, 71 subjects with occlusion of the left anterior descending artery, 16 subjects with prior inferior or posterior wall myocardial infarction, 6 subjects with visible collateral flow or in whom perfusion-type balloon catheters were used, and 3 subjects who had very brief inflation of the balloon (<30 seconds).

Patients who were taking digitalis (n=10) or patients with a right (n = 6) or left (n = 3) bundle branch block were not excluded from this analysis because the baseline ST-segment abnormalities could be distinguished from acute ischemic changes in the ST segment when continuous trends in the ST segment were analyzed. Thus, there remained 38 subjects who underwent PCI in the RCA and 50 subjects who underwent PCI in the circumflex artery. Of the 38 subjects with RCA occlusion, 10 had coronary occlusion in the proximal part of the RCA, proximal to the right ventricular branch. Informed consent was obtained in a manner approved by each institution’s committee on human research.

Instruments and Procedure
A continuous 18-lead ECG was recorded beginning when the subjects entered the catheterization laboratory and continuing throughout the entire procedure (Figure 1Go). The 18-lead ECGs were recorded by using 2 Mortara ELI 100 ST monitors (Milwaukee, Wis). The Mortara monitor is a portable, programmable microprocessor-based device that acquires, analyzes, and stores 12-lead ECGs at a programmed interval. This Mortara monitor was designated to record the standard 12-lead ECG; a second Mortara monitor was used to record posterior leads V7 through V9 and right ventricular leads V3R through V5R. For the purpose of this study, the monitors were programmed to analyze and store the ECGs every 20 seconds during the PCI. The monitors were time synchronized and programmed identically with filter settings of 0.05 to 100 Hz, as recommended by the American Heart Association for ST-segment analysis.13 In accordance with standards used for clinical practice, a calibration of 10 mm/mV and a paper speed of 25 mm/s were used. Radiolucent ECG wires and electrodes were used to minimize disruption of coronary artery visualization.

At the end of the monitoring session, all stored ECGs were downloaded to a personal computer with additional software for ST-segment analysis (Mortara ST Review Station). The ST Review Station provided quantitative measurements of the ST segment in microvolts for each of the 18 leads. ST-segment values measured with this computer-assisted technique are more accurate, reliable, and less biased than manual measurements made by experts.14,15 One reason for this difference was that computer-assisted measurements offer better resolution. When ischemia was defined as a ST-segment deviation of 1 mm (or 100 µV) or less in the posterior leads, the detection of minimal ST-segment change was important. Computerized monitoring systems were capable of measuring ST-segment deviation to a resolution of 0.01 mm, whereas humans were capable of measuring to a resolution of 0.5 mm. "Noisy" ECGs were eliminated according to published procedures.16

The ST segment was measured at J plus 60 ms, with the PR interval used as the isoelectric reference point. Baseline ECGs were obtained before the controlled balloon inflations for comparison. ST-segment amplitudes at the preinflation baseline were subtracted from maximal ST amplitudes during balloon inflation to create a positive or negative change score ({Delta}ST) for each of the 18 leads. The term "{Delta}ST elevation" was used to describe a change in the ST-segment level in the positive direction from the baseline, whether or not actual ST-segment elevation from the isoelectric line was present. This {Delta}ST value ensured that only "new-onset" ST-segment deviation was considered. Ischemic changes were defined as a {Delta}ST of 1 mm or greater in any of the standard 12 leads or right ventricular leads,17 or a {Delta}ST of 0.5 mm (50 µV) or greater in any of the posterior leads.18 Subjects were considered to have ischemia when the ischemic changes in the ST segment occurred with balloon inflation and disappeared after a brief period of balloon deflation.


ST segment elevation greater in lead III than in lead II suggests right coronary artery involvement, whereas isolated ST depression in V2 through V4 suggests left circumflex involvement.

 

Statistical Analysis
Means and SDs were calculated for continuous variables, whereas other measures of central tendency and frequency were calculated for categorical variables. Data were tabulated to compare the prevalence of ST-segment deviation (elevation or depression) in each of the 18 ECG leads between the patients with circumflex artery occlusion and the patients with RCA occlusion.

In patients with multiple balloon inflations, the ECG that showed the maximal ST-segment elevation during balloon occlusion was selected for analysis. Sensitivity was calculated as the percentage of patients with RCA or circumflex artery occlusion who were correctly identified by each ECG criterion. Specificity was the percentage of patients without coronary artery occlusion at the specific site (RCA or circumflex artery) who were correctly eliminated by each ECG criterion. To avoid type I error when multiple ECG criteria were being examined, a level of significance (P value) of less than .01 was considered statistically significant.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 Limitations of the Study
 Conclusions
 References
 
Sample Characteristics
The mean age of the subjects was 68 years (SD 12 years), and 67% were male. The ethnic breakdown of the sample was 58% white, 18% Asian, 18% Hispanic, and 6% African American. The mean duration of balloon inflations selected for analysis was 62 seconds. None of the subjects in any of the vessel groups had ventricular pacing rhythm, right ventricular hypertrophy, or Wolff-Parkinson-White syndrome.

ST-Segment Changes on the 18-Lead ECG
The sensitivities and specificities of various ECG criteria in the posterior, right ventricular, and standard 12 leads for differentiating occlusions of the circumflex artery from RCA occlusions are summarized in the TableGo. Examples of the ECGs are presented in Figures 2Go and 3Go.


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  		Sensitivity and specificity of ST-segment changes during occlusion of the circumflex artery and the right coronary artery

 

Figure 2
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  		Figure 2 ST-segment changes in the posterior and right ventricular leads during occlusions of the right coronary artery and the circumflex artery. (A) Proximal occlusion of the right coronary artery produces ST-segment elevation in right ventricular leads V3R through V5R; in contrast, occlusion of the circumflex artery produces ST-segment depression in lead V4R. (B) ST-segment elevation in V3R through V5R without significant ST-segment depression in posterior leads V7 through V9 is observed in a patient during an occlusion in the proximal part of the right coronary artery. (C) ST-segment elevation in both the right ventricular and posterior leads is recorded in another patient with occlusion of the right coronary artery. (D) ST-segment elevation in leads V7 through V9 without significant ST-segment depression in right ventricular leads is seen during occlusion of the circumflex artery.

 

Figure 3
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  		Figure 3 ST-segment changes in the precordial leads during occlusions of the right coronary artery and the circumflex artery. Occlusions in either artery produce similar patterns of ST-segment depression in precordial leads V1 through V4. Therefore, ST-segment depression in leads V1 through V4 cannot be used to differentiate between occlusions of the right coronary artery and the circumflex artery. During occlusion of the circumflex artery, slight elevation of the ST segment is observed in leads V5 and V6; however, ST-segment depression in leads V5 and V6 is produced during occlusion of the right coronary artery.

 
  Posterior Leads.   ST-segment elevation in leads V7, V8, and V9 occurred in 88%, 88%, and 80%, respectively, of subjects with circumflex artery occlusion and in 32%, 40%, and 37%, respectively, of subjects with RCA occlusion. During RCA occlusion, only 1 subject (3%) had ST-segment depression in lead V7, 4 subjects (11%) in lead V8, and none in lead V9.

  Right Ventricular Leads.   ST-segment elevation in leads V3R, V4R, and V5R occurred in 29%, 34%, and 50%, respectively, of subjects with RCA occlusion. Of the 10 subjects with RCA occlusion at the proximal site, approximately 80% exhibited ST-segment elevation in leads V3R through V5R. During RCA occlusion, one subject (3%) had ST-segment depression in leads V3R and V4R and none had ST-segment depression in lead V5R. During circumflex artery occlusion, 6 subjects (12%) had ST-segment depression in lead V3R, 8 subjects (16%) had ST-segment depression in lead V4R, and none had ST-segment depression in lead V5R.

  Standard 12 Leads.   ST-segment elevation in inferior leads II, III, and aVF occurred in 76%, 92%, and 84%, respectively, of subjects with RCA occlusions and in 26%, 30%, and 30%, respectively, of subjects with circumflex artery occlusion. ST-segment depression in leads V1, V2, and V3 occurred in 38%, 60%, and 40%, respectively, of subjects with circumflex artery occlusion and in 24%, 63%, and 34%, respectively, of subjects with RCA occlusion. ST-segment elevation in leads V1, V2, and V3 was infrequent, occurring in only 13%, 8%, and 5%, respectively, of subjects with RCA occlusion.


   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
Limitations of the Study
 Conclusions
 References
 
Data from this analysis showed that the 18-lead ECG is useful for discriminating between occlusions of the circumflex artery and RCA occlusions. Specifically, ST-segment depression in lead aVL is highly suggestive of RCA occlusion. Comparatively, ST-segment elevation in posterior leads without ST-segment depression in lead aVL is highly sensitive and specific for occlusion of the circumflex artery. Results from this study are similar to those of Herz et al,9 who studied patients with inferior myocardial infarction (RCA, n = 66; circumflex artery, n = 17) and found that ST-segment depression in lead aVL was significantly more common in the RCA group, with a sensitivity of 94% and a specificity of 71%. These ECG changes may help clinicians identify the occluded vessel before PCI, which can help in stratifying risk and planning the procedure, and in identifying reocclusion after coronary interventions.

In addition, ST-segment depression that is greater in lead aVL than in lead I is highly specific (92%) for RCA occlusions. That ECG pattern is almost 6 times as common in RCA occlusion (47%) as in circumflex artery occlusion (8%). Lead aVL faces the superolateral wall of the left ventricle and is therefore the most sensitive lead reciprocal to the inferior wall. Similar to data in this study, Birnbaum et al19 reported that ST-segment depression in lead aVL is found in most patients with evolving myocardial infarction in the inferior wall. In addition, ST-segment depression in lead aVL is not influenced by extension of the infarction to the right ventricle or to the posterior wall, thus ST-segment depression in lead aVL can be useful in the identification of RCA occlusion.

Subjects with occlusions of the RCA and the circumflex artery manifest inferoposterior ischemic patterns with ST-segment elevation in leads II, III, aVF, and V7 through V9, and reciprocal ST-segment depression in precordial leads V1 through V3. Frequencies of ST-segment elevation and depression, however, differ significantly between RCA occlusions and occlusions of the circumflex artery.


ST depression in lead aVL is highly suggestive of right coronary artery occlusion.

 

ST-segment elevation in the posterior leads and ST-segment depression in the right ventricular leads are more likely to be associated with a lesion of the circumflex artery, whereas ST-segment elevation in the right ventricular leads is related to RCA occlusions exclusively. ST-segment elevation in posterior leads V7 through V9 is twice as common in subjects with circum-flex artery occlusion (98%) than in subjects with RCA occlusion (45%). ST-segment elevation in V3R through V5R is observed more frequently in subjects with RCA occlusion than in subjects with circumflex artery occlusion. ST-segment depression in posterior leads is seen infrequently during RCA or circumflex artery occlusions. ST-segment depression in the right ventricular leads, particularly in lead V4R, is observed in approximately one fifth of subjects with circumflex artery occlusion but is observed infrequently in subjects with RCA occlusion.

A few studies have investigated the use of right ventricular or posterior leads to differentiate between RCA and circumflex artery lesions.8,20 Gupta et al8 reported that an upright T-wave polarity in lead V4R is common (89%) when the RCA is occluded and is not seen with occlusion of the circumflex artery (P < .001); in contrast, an inverted T wave in lead V4R is common (79%) when the circumflex artery is occluded and is not seen with RCA occlusion (P < .001). However, applying these T-wave changes to clinical practice can be difficult because the T wave in lead V4R is usually inverted in healthy persons.

Prieto-Solis et al20 studied 66 patients with an inferior myocardial infarction who subsequently underwent coronary arteriography (RCA, n = 46; circumflex artery, n=20) and found that an ST-segment elevation of 1 mm or greater in leads V3R and V4R is specific for obstructive lesions in the proximal part of the RCA (sensitivity, 74%) and ST-segment depression in leads V3R and V4R is specific for lesions of the circumflex artery. They further report that an ST-segment elevation of 1 mm or greater in leads V3R and V4R is observed in 85% of patients with a lesion in the proximal part of the RCA, in 21% of patients with lesions in the distal part of the RCA, and in 15% of patients with a lesion of the circumflex artery.

Data from the present study show a similar frequency of ST-segment elevation in right ventricular leads in patients with occlusion of the proximal part of the RCA (80%); however, ST-segment elevation in the right ventricular leads is more common with occlusion of the distal part of the RCA (40%). In addition, none of the subjects with occlusion of the circumflex artery showed ST-segment elevation in any of the right ventricular leads, suggesting that ST-segment elevation in the right ventricular leads is highly specific to occlusion of the RCA.

In this study, ST-segment elevation in inferior leads II, III, and aVF occurred almost 3 times more often in subjects with RCA occlusion (92%) than in subjects with occlusion of the circumflex artery (32%). This criterion, however, is not highly specific (68%) for identifying RCA occlusion. In contrast to results reported by Gupta et al8 and Herz et al,9 in this study the fact that the ST-segment elevation is greater in lead III than in lead II, though not a sensitive criterion, is nonetheless a specific criterion for identifying occlusion of the RCA. Although ST-segment elevation is greater in lead III than in lead II in twice as many subjects with RCA occlusion (18%) as subjects with occlusion of the circumflex artery (8%), the difference is not statistically significant (P = .32).


ST depression that is greater in lead aVL than in lead I is specific to right coronary artery involvement and occurs 6 times more frequently than in left circumflex involvement.

 

The presence or absence of ST-segment elevation in precordial leads V5 and V6 or of ST-segment depression in leads V1 through V3 does not provide discriminatory value. Approximately one third of our subjects with occlusion of the circumflex artery showed ST-segment elevation in lead V6, which is similar to the finding reported by Blanke et al.21 ST-segment elevation in leads V5 and V6 is slightly more common in occlusions of the circumflex artery (35%) than in occlusion of the RCA (24%); however, that difference is not statistically significant. ST-segment depression in the anterior leads V1 through V3 is thought to be due to the reciprocal changes of the true posterior wall of the left ventricle. However, data from this study show that ST-segment depression in V1 through V3 is frequently associated with occlusion in both the RCA and the circumflex artery. ST-segment depression in lead V2 plus ST-segment elevation in lead V6 cannot be used to differentiate occlusions of the RCA from occlusions of the circumflex artery because of the low sensitivity of that ECG pattern.

The composite ECG pattern of ST-segment elevation in the inferior leads without ST-segment elevation in leads V5 and V6 has a high specificity (80%) and is 4 times more common in RCA occlusion (68%) than in occlusion of the circumflex artery (16%). This finding is supported by results of an earlier thallium scanning study22 in which inferior defects without lateral defects are most common in RCA-related disease, whereas lateral defects without inferior defects are most common in disease related to the circumflex artery.


Posterior lead (V7 through V9) ST elevation is twice as frequent in left circumflex than in right coronary artery occlusion.

 


   Limitations of the Study
Top
 Abstract
 Methods
 Results
 Discussion
 Limitations of the Study
Conclusions
 References
 
ST-segment changes in the 18-lead ECG were useful for differentiation of transient total occlusions of the RCA and the circumflex artery during PCI. ECG changes during spontaneous AMI may differ from ECG changes during PCI because controlled coronary occlusion was usually brief. However, to discern whether the ST-segment changes of AMI had a pattern similar to the changes induced by controlled occlusion during PCI, Wagner et al23 conducted a study in patients undergoing elective angioplasty in whom an AMI developed subsequently. They found that the sequential ECG manifestations of AMI mimicked the changes that occurred during balloon occlusion during angioplasty. In half of the study sample, both the magnitude and the time course of ST-segment elevation during angioplasty were the same as occurred during the initial seconds of a spontaneous AMI. Similar findings were reported by Quyyumi et al,24 who found that the ST-segment changes at the onset of balloon occlusion were indistinguishable from—though of greater magnitude than—the ST-segment changes seen with AMI.

Another limitation was that 12 subjects (RCA, n=7; circumflex artery, n = 5) without a prior myocardial infarction did not show ECG changes during balloon inflation. This lack of ECG changes may be caused by the brief duration of balloon occlusions or by collateral circulation that was invisible on the angiogram.


Inferior lead ST elevation occurs 3 times more often in right than in left circumflex coronary artery occlusions.

 


   Conclusions
Top
 Abstract
 Methods
 Results
 Discussion
 Limitations of the Study
 Conclusions
References
 
Differentiation of occlusion of the circumflex artery from occlusion of the RCA is often difficult because either may be associated with an ECG pattern of inferior AMI. Patients with right ventricular infarction due to RCA occlusion have a poor prognosis and a high risk of disturbances in conduction at the atrioventricular node. Identification of the occluded vessel by using ECG signs and their validation can help in planning the PCI procedure and stratifying risk.

Data from this study showed that ST-segment depression in lead aVL is highly suggestive of RCA occlusion. The ECG pattern of ST-segment elevation in posterior leads without ST-segment depression in lead aVL is highly sensitive and specific for occlusion of the circumflex artery. Additionally, ST-segment elevation in posterior leads (V7 through V9) is significantly more common during occlusion of the circumflex artery than during RCA occlusion. Conversely, ST-segment elevation in right ventricular leads (V3R through V5R), inferior leads (II, III, aVF), and precordial leads (V1 through V3) and ST-segment depression in lateral leads (I, aVL) are significantly more frequent during RCA occlusion. Data from this study can be used to augment the value of noninvasive ECG in distinguishing occlusion of the circumflex artery from occlusion of the RCA.

In summary, in this study we were able to overcome the limitations encountered by Hasdai et al,25 who could not differentiate RCA occlusion from occlusion of the distal part of the circumflex artery by means of ECG. Data from this study can be used to augment the value of noninvasive ECG in discriminating occlusion of the circumflex artery from occlusion of the RCA. ST-segment depression in lead aVL is highly suggestive of RCA occlusion, whereas ST-segment elevation in posterior leads without ST-segment depression in lead aVL is highly sensitive and specific for occlusion of the circumflex artery.


   ACKNOWLEDGMENTS
 
The author sincerely appreciates the contribution of the women and men who graciously participated in this study. The author also thanks Barbara Drew, RN, PhD, at the University of California, San Francisco, for her support and mentoring. This work was performed at the Seton Medical Center, University of California San Francisco Medical Center, Southern Arizona Veterans Health Care System, and University Medical Center, Tucson, Ariz.

FINANCIAL DISCLOSURES
This work was supported by grants from the National Institute of Nursing Research/National Institutes of Health (RO1 NR008092), Bethesda, Md, and the American Association of Critical-Care Nurses, Sigma Theta Tau International Honor Society for Nursing, and Emergency Nurses Association Foundation.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.


   REFERENCES
Top
 Abstract
 Methods
 Results
 Discussion
 Limitations of the Study
 Conclusions
 References
 

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