A look at the Isolated Posterior MI.

ST-Elevation Myocardial Infarction (STEMI) is an ECG presentation that meets the criteria for reperfusion with Primary Percutaneous Coronary Intervention (PPCI) – But do all types of Myocardial Infarction (MI) present with ST-Elevation on a Standard 12-Lead ECG?

A look at the Isolated Posterior MI         

 

          Time is of the essence to restore coronary blood flow in patients with ST-segment myocardial infarction (STEMI), as the benefits of primary percutaneous coronary intervention (PPCI) or thrombolytic treatments are essentially time dependant. Current guidelines, as set out by the Joint Royal Colleges Liaison Committee (JRCALC), state that the ECG criteria for reperfusion includes ST-segment elevation ≥ 2mm in two standard or two adjacent precordial leads, not including V1 [1]. This is an intriguing criteria to set in stone, as many patients with myocardial infarction do not show these changes on a routine 12-lead ECG, and hence are denied rapid reperfusion therapy [2]. Although the incidence of STEMI has declined over the past 20 years, it varies between regions of the UK, and still averages approximately 750 cases per million people each year [21]. The current population of the UK is approximately 64.5 million [28], therefore if the previous statistic remains true, this equates to more than 48,000 cases of STEMI per annum.

The National Institute for Health and Care Excellence (NICE) describe STEMI as one end of the spectrum of related conditions called Acute Coronary Syndromes. The common pathophysiology involves a sudden rupture or erosion of an atheromatous plaque within the wall of a coronary artery. This stimulates thrombosis within the affected artery, obstructing blood flow, which eventually leads to heart muscle death. Without intervention, infarction progressively worsens over time. Rapidly and effectively restoring coronary blood flow limits the extent of myocardial damage, and reduces the likelihood of death or future heart failure. It is the obstruction of coronary blood flow and transmural ischaemia that gives rise to ST-segment elevation as a presentation on an ECG trace. Typical STEMI causes the onset of acute chest pain, although associated symptoms may include nausea, shortness of breath and diaphoresis (profuse sweating). Symptoms may be atypical, particularly in women, the elderly, and in patients with diabetes [21]. JRCALC suggest having a low threshold for performing a 12-Lead ECG in any patient presenting as ‘unwell’ [1].

Myocardial infarction of the posterior wall of the left ventricle is a result of the occlusion of either the left circumflex (LCx), or the right coronary artery (RCA). These occlusions lead to eventual necrosis of the posterior infra-atrial area of the left ventricle, located beneath the atrioventricular sulcus [27]. More often than not, a posterior wall MI occurs along with acute inferior or lateral MI, due to the occluded artery and the shared blood supply involved [3]. Posterior MI is associated with 15-20% of STEMIs and is statistically less likely to be missed when associated with other anatomical infarctions [5]. But what about an isolated posterior MI?

Isolated Posterior Myocardial Infarction is an indication for emergency reperfusion therapy, yet it is challenging to diagnose due to the absence of ST-segment elevation on a standard 12-Lead ECG; even in the setting of complete coronary artery occlusion and transmural infarction [4] [5]. Several studies indirectly reveal that the percentage of STEMIs that are isolated posterior, is approximately 8% [7]. It is suggested to be one of the most commonly missed types of acute myocardial infarction electrocardiographic patterns [27]. Those with posterior MI are much more likely to present with onset of chest pain but with normal vital signs [7].

Research suggests that a possible 4000 patients per annum with AMI, that are suitable for PPCI, are not being identified in the prehospital setting, resulting in significant treatment delays [17]. JRCALC guidelines advise a time critical transfer for patients with Acute Coronary Syndrome (ACS), yet a STEMI (as per their protocol) must be present to undertake a direct admission to a cardiac facility [1]. Therefore, unidentified STEMI patients can be initially treated as a non ST-segment elevation myocardial infarction (NSTEMI) based on a delayed marker of cardiac necrosis at the receiving hospital. Unfortunately, these patients can receive suboptimal initial therapy and delays to definitive care (PPCI/Thrombolytics) [13]. It is known that treatment delays in MI are associated with increased mortality and morbidity. Nallamothu et al. [19] found that the 6-month mortality increased by 0.18% per 10 minute delay in door-to-balloon time, between 90 and 150 minutes post onset, for patients undergoing PPCI. PPCI has been in place since 2009 and it is estimated by NICE that since 2012, more than 95% of the population in England & Wales are covered by a PPCI care pathway [21]. PPCI is now the most common form of reperfusion treatment, and involves the reopening of the occluded artery by angioplasty and the insertion of a stent [1] [21].

So what are we looking for on a 12-Lead ECG? The ECG leads that monitor the posterior wall are not present on a standard 12-lead ECG, therefore only reciprocal changes are visible for an isolated PMI in the right precordial leads V1, V2 ± V3 [6]. UK Paramedics are currently taught to recognise AMI by identifying ST-segment elevation in the standard 12-Lead ECG [15], and current JRCALC guidelines only cover use of the standard 12-Lead ECG to assess chest pain [1]. Therefore, it is important to have a good understanding of what presentation may be found on a standard trace.

 

Presentation of an Isolated Posterior MI on a standard 12-Lead ECG

[2] [3] [4] [5] [6] [15] [16] [22]

Screen Shot 2016-03-24 at 16.29.15

(https://meds.queensu.ca/central/assets/modules/ECG/acute_posterior_mi.html)

  • Horizontal ST-segment depression ( V1, V2 ± V3 )

    • ST-depression in V1 – V3 is much more likely to be posterior than subendocardial ischaemia. Subendocardial ischaemia does not tend to localise as does the expected presentation of Posterior MI. Global ST-segment depression would be expected in the case of subendocardial ischaemia, for example in II, III, aVF and V4 through V6 [7].
    • Localisation of the maximum ST-depression in lead V2 or V3 has a reported sensitivity of 96% and specificity of 70% for identifying ischaemia due to left circumflex artery occlusion [24].
  • Tall and/or Broad R waves ( V1, V2 ± V3 )

    • The increase of the R wave in PMI is said to be opposite to the Q wave formation associated with traditional ST-segment elevated myocardial infarction [27]. It can be seen as broad (> 30ms) and with increased amplitude in later stages [16].
    • In some patients, the increase of the R wave in V1 is a result of a conduction failure in the His-Purkinje system which is located in the left fascicle of the septum [27].
  • Tall upright T waves ( V1, V2 ± V3 )

    • The combination of horizontal ST-segment depression with an upright T wave increases the diagnostic accuracy of the two different ECG findings [3].
    • This finding is stated by many authors on it’s own as an important indication, although evidence for this area is lacking [7].
  • R/S wave ratio > 1.0 ( V2 only )

    • Specifically, meaning a taller R wave than the S wave in V2.

 

In a study by Khan et al. in 2012 [13], Doctors in the emergency department and acute medical unit at two teaching hospitals and WMAS Paramedics were asked to interpret a 12-Lead ECG illustrating ST-depression and a dominant R-wave in V1 & V2 in the context of cardiac chest pain. The aim of the study was to see if Posterior MI could be identified as a potential diagnosis. 44 out of 117 Doctors (38%) identified Posterior MI as a potential diagnosis. 5 out of 50 Paramedics (10%) identified Posterior MI as a potential diagnosis; only 7 (14%) would have pre-alerted the Emergency Department based on the ECG. 50% of Doctors who identified potential Posterior MI knew that posterior leads were required to confirm diagnosis.

 

Posterior Chest Leads

[2] [4] [7] [13] [14] [16] [17] [18] [22] [25] [26] [27] [29] [30]

The electrodes that monitor the posterior wall are not on a standard 12-Lead ECG. These are V7, V8 and V9[6]. These leads are not actually extra wires attached to a 12-Lead monitor, they are a relocation of either V1, V2 and V3 or V4, V5 and V6 to a different anatomical position, in order to further investigate the possibility of a Posterior MI. The 12-Lead ECG is then recorded and a trace printed as normal, being sure to cross out the relocated leads and to rename them as V7, V8 and V9.

These positions are illustrated well by Life in the Fast Lane (http://lifeinthefastlane.com/ecg-library/pmi/) in the above right image.

Posterior Myocardial Infarction is diagnosed with ST-segment elevation of  ≥ 0.5mm in any one of the posterior leads (V7, V8, V9). 

This is the most sensitive and specific criteria.

 

 Posterior-MI-V789An example ECG from Life in the Fast Lane (http://lifeinthefastlane.com/posterior-myocardial-infarction/) demonstrating horizontal ST-depression in the right precordial leads V1-V3, and ST-elevation in the posterior leads V7-V9. The R waves are noticeably broad in the right precordials, the R/S ratio in V2 is > 1.0, and the T waves remain upright even in the presence of ST-depression.

 

Errors or delay in diagnosis can be prevented with the use of the posterior leads, following a normal 12-Lead ECG. Risk factors, clinical presentation and differential diagnosis are no different than with other myocardial infarctions [27]. Of course, there are differentials for ST-segment elevation, for example Pericarditis. Although with Pericarditis, you would expect global ST-segment elevation changes and possible PR-segment depression, not a specific localisation to the posterior leads [7]. Taha et al. [25] state that ST-Elevation up to 0.5mm measured at the J point relative to the PR segment in all three posterior leads can be a normal variant.

Matetzky et al. [17] studied 33 consecutive patients with ischaemic chest pain suggestive of AMI, without ST-elevation in the standard 12-Lead ECG, but with isolated ST-elevation in the posterior chest leads V7 through V9. Acute MI was confirmed in all patients enzymatically, and on discharge ECG pathological Q-waves appeared in leads V7 through V9 in 75% of the patients. Echocardiography showed posterior wall-motion abnormality in 97% of patients and 69% had evidence of Mitral Regurgitation. Agarwal et al. [2] conducted a similar study, looking at posterior chest leads that were recorded in 58 patients with clinically suspected myocardial infarction, but with a non-diagnostic routine ECG. 18 out of 58 patients had ≥ 1mm ST-elevation in ≥ 2 posterior chest leads. All of these patients were confirmed to have myocardial infarction by blood test indicators. 89% of these patients had the LCx as the culprit vessel.

Adjusting the ischaemic criterion from 1mm to 0.5mm in V7 to V9 significantly improved sensitivity from 49% to 94%. However, Wung and Drew [29] found that 12 subjects in their study out of 53 patients undergoing LCx angioplasty, had posterior lead ST-segment elevation of ≥ 0.5mm but did not demonstrate expected ST-depression in V1 – V3.

Posterior leads V7 to V9 contribute significant additional diagnostic information above and beyond the standard 12-Lead ECG, and could potentially reduce the chances of misdiagnosing NSTEMI. The use of posterior leads provides increased sensitivity and odds of detecting ST-segment elevation in AMI with no loss of specificity.

An emphasis needs to be placed on diagnosing Posterior MI during Paramedic training. This is particularly the case where ECG interpretation and diagnosis is increasingly being made in the pre-hospital environment. Since JRCALC guidelines do not currently discuss the use of additional leads, Paramedics working to these guidelines alone, currently make a clinical decision based on the findings of a standard 12-Lead ECG to pre-alert the Emergency Department of a potential Posterior STEMI. This is especially the case where patients have chest pain in conjunction with ST-depression in leads V1 & V2.

In future guidelines and in training, Paramedics should be trained in the use of additional leads, such as V4R, V7, V8 and V9, and encouraged to use them following a standard non-diagnostic 12-Lead ECG. Not only would this be broadening the skill set of the Paramedic, increasing autonomy and allowing for better informed evidence-based decisions, up to 4000 patients per annum will receive definitive treatment rapidly, without delay. A diagnosis of Posterior STEMI in the pre-hospital environment allows patients a potential direct transfer to a cardiac facility, preventing the need for initial Emergency Department screening and reducing the delay for coronary intervention.

 

 

 

References

[1] AACE, Association of Ambulance Chief Executives (2013). UK Ambulance Services Clinical Practice Guidelines 2013. Bristol, Class Publishing.

[2] AGARWAL, J. B., KHAW, K., AURIGNAC, F. and LOCURTO, A. (1999). Importance of posterior chest leads in patients with suspected myocardial infarction, but non-diagnostic, routine 12-lead electrocardiogram. American Journal of Cardiology, 83 (3), 323-326.

[3] BRADY, W. J. (1998). Acute posterior wall myocardial infarction: electrocardiographic manifestations. American Journal of Emergency Medicine, 16 (4), 409-413.

[4] BROWN, L., SIMS, J. and CONFORTO, A. (2003). Posterior myocardial infarction with isolated ST elevations in V8 and V9: Is this an ‘’ST elevation MI’’? CJEM, 5 (2), 115-118.

[5] CELEDON, M. (2015). Posterior MI Recognition. [online]. Accessed 26th February at: http://www.emdocs.net/posterior-mi-recognition/

[6] DIEHL, T. S. et al. (2011). ECG Interpretation made Incredibly Easy. 5th ed. Pennsylvania, Lippincott Williams & Wilkins.

[7] DR. SMITH ECG BLOG (2009). Pure (Isolated) Posterior STEMI — not so rare, but often ignored! [online]. Accessed 26th February at: http://hqmeded-ecg.blogspot.co.uk/2009/04/pure-isolated-posterior-stemi-not-so.html

[8] EISENSTEIN, I., et al. (1985). Electrocardiographic and vectorcardiographic diagnosis of posterior wall myocardial infarction. Significance of the T wave. Chest, 88 (3), 409-416.

[9] NALLAMOTHU, B., et al. (2007). Relationship of treatment delays and mortality in patients undergoing fibrinolysis and primary percutaneous coronary intervention. The Global Registry of Acute Coronary Events. Heart, 93 (12), 1552-1555.

[10] HABERL, R. (2000).  ECG Pocket. 2nd ed. California, Borm Bruckmeier Publishing LLC

[11] HAMPTON, J. R. (1986). The ECG Made Easy. 3rd ed. Edinburgh, Churchill Livingstone.

[12] HUANG. C. M., et al. (2010). Acute left ventricular rupture following posterior wall myocardial infarction. International Medicine, 49 (14), 1387-1390.

[13] KHAN, J. N., CHAUHAN, A., MOZDIAK, E., KHAN, J. M. and VARMA, C. (2012). Posterior myocardial infarction: are we failing to diagnose this? Emergency Medicine Journals, 29 (1), 15-18.

[14] KLIGFIELD, P., et al. (2007). Recommendations for the standardisation and interpretation of the electrocardiogram: Part I: The electrocardiogram and its technology: A scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee. Circulation, 115 (10), 1306-1324.

[15] LINDRIDGE, J. (2009). True posterior myocardial infarction: the importance of leads V7-V9. Emergency Medicine Journal, 26, 456-457.

[16] LITFL, Life In The Fast Lane (2007). Posterior Myocardial Infarction. [online]. http://lifeinthefastlane.com/ecg-library/pmi/

[17] MATETZKY, S., et al. (1999). Acute myocardial infarction with isolated ST-segment elevation in posterior chest leads V7-9: ‘’hidden’’ ST-segment elevations revealing acute posterior infarction. Journal of the American College of Cardiology, 34 (3), 748-753.

[18] MELENDEZ, L. J., JONES, D. T., and SALCEDO, J. R. (1978). Usefulness of three additional electrocardiographic chest leads (V7, V8 and V9) in the diagnosis of acute myocardial infarction. CMAJ, 119 (7).

[19] NALLAMOTHU, B., et al. (2007). Relationship of treatment delays and mortality in patients undergoing fibrinolysis and primary percutaneous coronary intervention. The Global Registry of Acute Coronary Events. Heart, 93 (12), 1552-1555.

[20] NICE, National Institute for Health and Care Excellence (2010). Chest pain of recent onset: assessment and diagnosis. [online]. Accessed 26th February at: https://www.nice.org.uk/guidance/cg95/chapter/Introduction

[21] NICE, National Institute for Health and Care Excellence (2013). The acute management of myocardial infarction with ST-segment-elevation. [online]. Accessed 6th March 2016 at: http://www.nice.org.uk/guidance/cg167/documents/myocardial-infarction-with-stsegmentelevation-stemi-appendices2

[22] ORAII, S., et al. (1999). Prevalence and outcome of ST-segment elevation in posterior electrocardiographic leads during acute myocardial infarction. Journal of Electrocardiology, 32 (3), 275-278.

[23] PILBERRY, R. (2013). Emergency care in the streets. 7th ed., Burlington, Jones and Bartlett Learning.

[24] SHAH, A., et al. (1997). Electrocardiographic differentiation of the ST-segment depression of acute myocardial injury due to the left circumflex artery occlusion from that of myocardial ischaemia of non occlusive aetiologies. American Journal of Cardiology, 80, 512-513.

[25] TAHA, B., REDDY, S., AGARWAL, J. and KHAW, K. (1998). Normal limits of ST segment measurements in posterior ECG leads. Journal of Electrocardiology, 31 (suppl), 178-179.

[26] TORTORA, G. J. and GRABOWSKI, S. R. (2000). Principles of Anatomy and Physiology. 9th ed., New York, John Wiley & Sons Inc.

[27] VAN GORSELEN, E. O. F., VERHEUGT, F. W. A., MEURSING, B. T. J. and OUDE OPHUIS, A. J. M. (2007). Posterior myocardial infarction: the dark side of the moon. Neth Heart J., 15 (1), 16-21.

[28] WORLD BANK (2016). Population, Total. [online]. Available at: http://data.worldbank.org/indicator/SP.POP.TOTL

[29] WUNG, S. F. and DREW, B. J. (2001). New electrocardiographic criteria for posterior wall acute myocardial ischaemia validated by a percutaneous transluminal coronary angioplasty model of acute myocardial infarction. American Journal of Cardiology, 87 (8), 970-974.

[30] ZALENSKI, R. J., COOKE, D., RYDMAN, R., SLOAN, E. P. and MURPHY, D. G. (1993). Assessing the diagnostic value of an ECG containing leads V4R, V8 and V9: the 15-lead ECG. Annals of Emergency Medicine, 22 (5), 786-793.

Advertisements

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s