Localizing Infarcts On a 12-Lead EKG



UNDERSTANDING EKGS BY UNDERSTANDING ANATOMY ~ Amy Gutman MD, FACEP

References: Mosby, Bates, AHA, eMedicine Google Images, NAEMSE, NAEMT, AmbulanceTechnicianStudy.co.uk

Three main vessels feed the heart: the Right Coronary Artery (RCA), Left Anterior Descending artery (LAD), and the Left Circumflex Artery (LCX). Their position in each person is slightly different but predictable. Each vessel feeds one heart part or “territory”. These territories are represented on the EKG so that each vessel = one section of the EKG.

The Right Coronary Artery (RCA) perfuses the right side / right ventricle / bottom) of the heart in the Inferior territory. The RCA has a Posterior Descending Artery (PDA) branch feeding the back / posterior heart.

The Left Main Artery (LMA) divides into the Left Anterior Descending (LAD) (anterior left ventricle) & Circumflex (LCX) (lateral left ventricle). The LMA is called the “Widowmaker” as an LMA clot blocks the LAD & LCX taking out the entire left ventricle, or “power pump” of the heart.

As a clot in one vessel blocks blood flow to one heart territory, different vessel occlusions threaten different territories like inferior (RCA), or posterior territories (PDA). It’s important to remember that the vessels overlap each other, therefore territories overlap. More than one vessel feeds more than one area of the heart as a brilliant “back-up system” in the case of a blockage.

The 12 lead EKG is also broken into territories: inferior, anterior, septal, & lateral. The posterior territory is not directly seen on the ECG (but there are tricks to find it, including looking at the ECG “backwards” or doing a “right-sided EKG”). The anterior heart, septum (middle) and anterior left ventricle, are (mostly) perfused by the left anterior descending (LAD) artery. In “ECG” anatomy, the anterior heart = left side, as the heart is rotated in the chest.

The heart lies on its side in the chest, with the right ventricle pointed down & inferiorly. This territory is perfused by the right coronary artery (RCA). The inferior heart is innervated partly by the same nerves as the stomach (vagus nerve) – which is why people with inferior wall ischemia may have nausea, vomiting, or hiccups in place of “chest pain” ~ an “anginal equivalent”. An inferior wall myocardial infarction (MI) is seen in leads II, III & AvF.

Look at the diagram: see how V1 & V2, which are in the middle of the EKG, also are in the “middle” of heart or “septum”. The anterior left ventricle, perfused by the Left Anterior Descending (LAD) is seen by leads V2 (“septal overlap”) V3 & V4. Therefore, LAD blockage causes an anterior wall MI. The septum perfused by both the RCA and LAD. A septal MI is seen in V1 & V2.

The circumflex (CX), which in Latin means “all the way around” is lateral ventricle territory: leads V5, V6, I & AVL. Leads V5 & V6 are placed on the left side of the chest. You can see the lateral territory on both sides of the EKG – or wrapping “all the way around” the EKG!

That’s the basics…and this is how it looks when you put anatomy and territories together on an ECG:

ISCHEMIA VS INFARCT

The heart’s electrical signal travels through a network of conducting cell pathways, which stimulates the upper chambers (atria) & lower chambers (ventricles) to contract.

The electrical signal starts in a group of cells at the top of the heart called the sinoatrial (SA) node or “pacemaker of the heart”. The SA node signal travels down the heart, triggering the two atria to contract, then the atrio-ventricular (AV) node sends an impulse to the two ventricles to contract. As the cells rapidly transmit the electrical charge, the entire heart contracts in one coordinated motion, creating a heartbeat.

One heartbeat = 1 ECG complex. There are four questions to ask for each ECG complex: is the Rhythm Regular (r to r Interval), is the Rate Regular (60–100 beats / min at same intervals), is there a p wave before every QRS complex (the

beat started in the atria), and is the QRS normally shaped?

If all four are normal, you are in a really good place!

ISCHEMIA

Going from ischemia (lack of blood flow) to infarction (tissue death) is a continuum producing different changes in ECG complexes. Ischemia, usually causes ST depression, below the isoelectric line of flipped T waves

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STAGES OF A MYOCARDIAL INFARCTION (MI)

1st Stage: Acute Injury (ST Elevation)

Look at the QRS complex. The r-wave rises from the isoelectric line (flat part after QRS). If clot blocks the vessel, infarction begins. There are 3 stages in progression of the MI. The 1st is “acute injury” lasting 6 hours and represents the period of time between the acute vessel blockage and the start of tissue death in the part of the heart that downstream / “distal” to the clot. This is the time frame that TPA or cardiac catherization must be done within to save the heart muscle. This stage of infarct appears as an ST elevation in all the leads in the heart territory being affected

2nd Stage: Necrosis (Q-waves)

Look at the QRS complex. Is there a q-wave? Big q-waves represent progression of an MI from acute injury to necrosis. They appear as QRS moving downwards first, after the PR interval & called “pathologic” q-waves: greater than a third of the total size of the QRS. The appearance of q-waves means the patient has likely missed the “window” for clot lysis / TPA as it takes about 6 hours for q waves to form. However, if you see ST elevations + q waves at the same time the heart attack is still evolving. ST segments come back down to isoelectric baseline as q waves develop.

3rd Stage: Resolution (Persistent Q’s, or flipped T’s)

The resolution stage represents the development of scar tissue in the infarcted part of the heart roughly two weeks after the necrotic stage of the MI. The affected part of the heart will still show EKG changes, possibly forever – “persistent Q-waves”, or flipped T-waves.

WHAT IS ELECTRICAL RECIPROCITY?

Some EKG lead “mirror each other” electrically, so that ST elevations in one group show up as ST depressions in the other. Not ischemia – “mirroring”. There are only two main areas of the heart that do this: inferior with lateral, and septal plus posterior. Inferior ST elevations in II, III and AVF can produce lateral ST depressions in I, AVL, V5 and V6. it’s a reflection, bouncing electrically across the heart from the inferior injury, showing up in reverse. The trick is to remember that when assessing EKG changes, ST elevations always come FIRST. If there are ST elevations anywhere on an EKG, consider that any ST depressions on the same EKG might be reciprocal, instead of ischemic.

Inferior MI, Lateral Reciprocity

See the ST elevations in II, III & AvF? That’s an inferior MI. Now, see the ST depressions in I & AVL lateral leads? That’s not ischemia – it’s the “mirrored reflection” of the inferior MI. See the ST depressions in V1 & V2? This is posterior reciprocity, it makes sense that it would show up in this situation (think back to coronary artery anatomy).

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Lateral MI, Inferior Reciprocity

See the subtle ST elevations in I & AVL? See the ST depressions in II, III & AvF? The primary process is a lateral MI with inferior reciprocity.

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Posterior Reciprocity

The septal leads (front of the heart) reflect a posterior event (back of the heart) in the same way that the inferior/lateral ones do, but it’s difficult to see as there aren’t posterior leads on a normal 12 lead EKG. On an EKG, the way you look at the back of the heart, is by looking at the leads that look at the front of the heart, only backwards and upside down. Here are a couple of sets of septal leads: V1 and V2.

ST Depressions

Now – here are the same groups of leads, flipped and reversed:

ST Elevations

You can “see” posterior leads by flipping over the ECG & holding it up to the light looking “through” it! Sometimes the clue to finding the posterior wall MI is first identifying an inferior MI. Because the RCA perfuses both the inferior and posterior areas, an ST elevation in II, III & AVF plus ST depression in V1 & V2 means the patient is having an “inferior-posterior MI” – the septal leads give a “mirror” of an infarct process, rather than showing ischemia.

Right-Ventricular MI

Posterior events are linked with inferior ones, because the posterior heart is perfused by the posterior descending artery (PDA), which branches off the right coronary artery (RCA) affecting these two areas at once. Both inferior & posterior territories can be affected by an RCA clot, causing an infero-posterior MI.

Infarcting this territory produces a “right ventricular” MI / RVMI. 50% of all inferior MIs affect enough RV territory to “stun” them into “hypokinetic inactivity” – dead tissue doesn’t pump. To see this on an EKG, you must do a 12-lead with the chest leads applied backwards – going around towards the right, instead of towards the left.

Right-sided MIs are managed differently than left-sided MIs. In a right-ventricular / right-sided MI, the ventricle becomes hypokinetic instead of sending pumping effectively. If the blood doesn’t get pumped to the lungs, it doesn’t get pumped to the arteries, and you drop your blood pressure or “pre-load”. These patients get liters of volume, to flood the sluggish right ventricle and keep the blood moving - the opposite of a left-sided MI. Never, ever give nitroglycerine as this coronary artery vasodilator drops the pressure and may cause cardiogenic shock.

Look at the ST elevations in leads II, III and AVF in EKG below. What kind of MI is this? Which territory is affected? Anterior? Inferior? Lateral? Septal? Is it too late for clot lysis (hint: are there Q waves)?

The EXG above shows is an inferior wall MI with ST elevations in II, III & AvF. There is also lateral wall reciprocity with ST depressions in I & AvL. Which coronary artery is being affected? Now look at the ECG below. Very tricky and different story here!

Lead I, AvL, V5, V6 show ST elevation in lateral territory = lateral wall MI caused by either the circumflex or LMA. There are ST depressions in lead III & AvF which indicates reciprocal inferior wall changes. But look closer: there is a q waves in V1, V2, V3 & V4 – big anterior ST elevations – a 2nd infarct area! So there are two areas with ST elevations: lateral and anterior group. This means blockages in the circumflex plus the LAD…in other words - an LMA left main artery lesion. This is the Widowmaker!

If you look at the ECGs systematically, you will never miss ischemia or a myocardial infarction. It’s overwhelming at first, but if you practice, practice, practice the same way every time, you will become a master of interpretation.

RHYTHM ANALYSIS

Normal Sinus Rhythm

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Rhythm – Regular; Rate - 60-100 bpm; QRS Duration – Normal; P Wave - Visible before each QRS complex

P-R Interval - Normal ( ................
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