By Laurence Sharifi

ISCHAEMIC CHANGES ON THE ECG

Some theory if you’re interestedECGs and acute coronary syndromesECGs and stable angina

WHAT WE WILL BE COVERING

WHY THE HEART PRODUCES AN ECG SIGNAL

The heart contracts because of the cardiac conduction system

A wave of depolarisation spreads across the myocardium in a co-ordinated way, making the heart contract in the right way through every heart beat

HOW THE HEART BEATS

67 year old man, obese, HTN, DMRetrosternal pain, tight, radiating to arm and jaw lasting past 30 minutesNot relieved with GTN spray, now sweating and distressed

Time to take an ECG!!

CASE 1: ACS

WHY THE ECG LOOKS THE WAY THAT IT DOES

Don’t think of an ECG lead as the bit of wire stuck on the chest – this isn’t what it isAn ECG lead is actually a calculated voltage difference between two electrodes– not REALLY vital to know about the detailsThink of a lead as a “photo” of the heart taken at a particular orientation

ECG LEADS

=

ECG LEADS ARE “VIEWS”

Think of the ECG leads as photographs of the heart taken at different angles. This slide illustrates this concept using the limb leads

=V5

V1

V6

This slide illustrates the same concept but on the chest leads. Cheryl remains the same, we just look at her from different directions

The ECG leads all look at the heart from different angles:

ECG LEADS ARE VIEWS

Each lead is looking at a different part of the heart’s surface

Depolarisation travels from the endocardium through to the epicardium (in to out)Repolarisation goes from the epicardium to the endocardium (out to in)

HOW THE HEART BEATS

• Causes a positive deflection in ECG if travelling towards lead

• Causes a negative deflection if travelling away from lead

Depolarisation

• Causes a negative deflection in ECG if travelling towards lead

• Causes a positive deflection if travelling away from lead

Repolarisation

RULES ABOUT ECG LEADS

THE CORONARY CIRCULATION – WHEN IT

ALL GOES WRONG

• The myocardium is adequately perfused

• The conduction system works

The heart pumps properly when:

• Left Main coronary artery. Branches:• Left anterior descending

artery• Circumflex artery

• Right coronary artery. Branches:• Acute marginal branch• AV node branch• Posterior descending artery

Key vessels are:

THE CORONARY ARTERIES

Ischaemic tissue• Myocytes are still

working – still get an ECG signal from the tissue

• Signal is different though as anaerobic cells behave differently

Infarcted tissue• Tissue is dying/dead– no

ECG signal from the tissue

• Heart is beating abnormally due to presence of dead tissue

• ECG signal looks different

ISCHAEMIC TISSUE BEHAVES DIFFERENTLY TO INFARCTED TISSUE

The blood supply can become compromised in:• Stable angina• Acute coronary syndrome (ACS)

Coronary artery lumen

becomes narrowed

Partly(NSTEMI

)

Tissue becomes ischaemic

Tissue dies slowly

Wholly(STEMI) Tissue becomes

extremely ischaemic

(transmural) Tissue dies quickly

ACUTE CORONARY SYNDROME

K+ channels open earlier in ischaemic tissue

Repolarisation normally causes a positive deflection if moving away from lead (think T wave)Ischaemic tissue repolarises early so you get a positive deflection earlier, otherwise known as ST ELEVATIONST Elevation often means SEVERE TRANSMURAL ISCHAEMIA

ISCHAEMIC CELLS REPOLARISE FASTER

+20

-100

100msm

emb

potl

(mV)

Red = normal cardiac action potentialBlue = ischaemic tissue action potential

ECG in suspected ACS is used to• Confirm or refute the diagnosis• Guide therapy• Estimate prognosis

Consider ECG alongside:• Major cardiovascular risk factors (fat, old, DM, HTN)• The description of the pain (central, crushing >20 min,

at rest)• History of cocaine indulgence• How the patient looks. Are they sweaty, grey and

breathless? Beware the “silent MI” however – a patient may be having a heart attack but look entirely normal. Diabetic patients are particularly susceptible to this

THE ECG IN ACS

ECG and ACS Therapy• ST elevation MI will benefit from

thrombolysis / PCI• Other ACSes need aggressive anti-

platelet therapyECG and ACS prognosis• Assess ACS using risk scoring tools• Resting ECG changes are important

variables

THE ECG IN ACS

Our patient has had a series of ECGsWhat could they show?

CASE 1: ACS

ECG CHANGES IN STEMI

0

• No ECG changes

1

• Hyper-acute T waves

2: Infarction• ST

elevation

3: Fibrosis

• ST normal• T wave

inversion• Q wave

development

4

• Re-inversion of inverted T waves

5

• Late loss of Q waves

OnsetLasts a

few minute

s

Minutes to

hoursHours to days

Days to

weeks

Years to

decades

STEMI PATTERNS

•I•II•aVL•V1-5/6Leads

showing ST

elevation

•AnterolateralInfarct

description

•Proximal LAD (left anterior desciending)

Artery occluded

STEMI PATTERNS

•II•V1-3/4

Leads showing

ST elevation

•AnteroseptalInfarct

description

•Left anterior descending

Artery occluded

STEMI PATTERNS

•II•III•aVFLeads

showing ST

elevation•InferiorInfarct

description

•Right coronary if ST II>III

•Circumflex if ST III>II

Artery occluded

STEMI PATTERNS

•I•II•V5/6Leads

showing ST

elevation•LateralInfarct

description

•Diagonal branch LAD

•Obtuse marginal branch circumflex

Artery occluded

STEMI PATTERNS

•V1 - 3

Leads showing

ST depressi

on

•PosteriorInfarct descriptio

n

•CircumflexArtery occluded

This happens because the ST elevation on the posterior wall causes a RECIPROCAL change on the opposite side. Confirm by doing a posterior ECG

Patient has had a posterior STEMI

ST depression in V1-3We would see ST elevation in posterior chest leads

CASE 1: ACS

CASE 1: ACS

74 year old lady, Hx of HTN, PC: chest pain at restNormal ECG but raised Trop T 12 hours later – ischaemic damage to myocardium highly likely

Sounds like ACS, so why normal ECG?

CASE 2: ACS

Normal ECG is possible in NSTEMI• Only small coronary artery affected• Posterior infarct• Diagnosis is made clinically and with TropT

NSTEMI ECG Changes• T wave flattening• T wave inversion – follows the coronary artery

distribution• ST depression – ominous sign, high predictor of

mortality

NSTEMI ECG PATTERNS

NSTEMI ECG PATTERNS

Inverted T waves

ST depression

45 year old man, smoker, sedentary lifestyleChest pain on mild inclines, relieved by sitting still againECG to confirm stable angina

CASE 3: ANGINA

ECG changes in stable angina:-• Normal when resting (unless evidence of a

previous MI like Q waves)

Exercise ECG:-• Treadmill (Bruce protocol) gradually getting

harder• ST depression and developing symptoms• Exercise test isn’t 100% though

STABLE ANGINA ECG

STABLE ANGINA EXERCISE ECG

ST depression

ECG leads look at the heart from lots of different directions and give information about the conduction system and myocardial contractionECGs are key investigative tools helping delineate the acute coronary syndromes and their correct interpretation influences management dramaticallyStable angina is more of a clinical diagnosis but exercise ECG can assist where there is uncertainty

SUMMARY