Introduction to Acute Coronary Syndrome

Acute Coronary Syndrome (ACS) is a coronary artery disease causing many changes in the ECG such as ST elevation & angina.

  1. Ischemia: Interruption of coronary heart supply temporarily, usually 70% blocked or more where cardiac oxygen demand is higher than supply. This results in cardiac chest pain symptoms as the heart cannot get electrolytes nor remove waste which impairs repolarisation or relaxation
    1. Hence T-wave changes
    2. ST depression can occur with prolonged ischemia
  2. Injury: Complete occlusion (blockage) of the coronary vessel resulting in spasms or rupture which injure the surrounding muscles. This is an acute phase for acute myocardial infarction (AMI). This phase only injures muscles, it has not died or necrotic yet – The vessel can be reopened and halt AMI to prevent further damage.
  3. Infarction: Prolonged interruption resulting in cardiac muscle death / necrosis. ST elevation can form acutely as it cannot lie normally on the isoelectric line normally.
    1. Q-wave develop (wider > 0.04s) in multiple leads and more than ¼ R-wave’s height.
    2. ST elevation is significant if > 1mm high in limb lead & > 2mm in chest leads in same group



Ischemia can be transmural (entire wall thickness) &/or subendocardial (only subendocardium impacted).

ST Elevation Myocardial Infarction (STEMI)

Reflection of ischemic area if it is shown in two (2) or more leads of the same anatomical lead group such as V1 & V2 / V5 & V6 / aVF & Lead III etc. Changes in one (1) lead only is not significant and is termed ‘involvement.’

For example - ST elevation in V1-V5 would be called “anterior AMI with lateral involvement”

Reciprocal ECG on a STEMI

Refer to ST depression present in other regions of the heart whilst there is ST elevation – This reflects on the view of the AMI from opposite sides and not clinically significant

  • ST depression with ST elevation present = Reciprocal
  • ST depression by itself = Prolonged ischemia / injury
LeadLead IaVRV1V4
RegionHigh Lateral
Reciprocal changesII, III, aVF
ArteryCircumflex Left Anterior Descending
Left Anterior Descending
LeadLead IIaVLV2V5
RegionInferiorHigh Lateral
Reciprocal changesI, aVL
ArteryRight Coronary
CircumflexLeft Anterior DescendingCircumflex
LeadLead IIIaVFV3V6
Reciprocal changesI, aVLI, aVL II, III, aVF
ArteryRight CoronaryRight CoronaryLeft Anterior DescendingCircumflex
Non-ST Elevation Myocardial Infarction (NSTEMI)

Beware of the 'normal ECG' on arrival; run multiple sequential ECGs, which may show progressive changes as the disease state continues.

For ACS symptoms that are not STEMI where an artery is partially blocked therefore there is still circulation therefore oxygen can reach the endocardium (but not the sub-endocardium).

ECG features include ST depression with T-wave changes (flat waves or inverted) – These ST-depression may not reflect ischemic area. Pathological Q-waves are not developed (hence infarction is usually small unless subendocardium infarction).

T-wave inversions with ST depressions indicates acute/ongoing ischemia where isolated T-wave inversion indicates post-ischemia after the ischemia episode. 

Ischemic changes to the ECG
Q-Wave Changes

A fragmented QRS complex can be seen during or after infarction; in bundle branch blocks, >2 notches can be seen in in R-wave or S-wave.

  • QRS complex with > 1 R-wave and/or
  • Notch in descending limb of R-wave and/or
  • Notch in descending limb of S-wave

QTc interval may also be prolonged, shortened or unchanged in ischemia

Q-wave & R-wave Changes

Q-wave behaviours includes being wider and deeper usually in transmural or inferior STEMI with diminishing waves overtime.


Inferior Q waves (II, III, aVF) with ST elevation due to acute MI

The R-wave has a reduced amplitude with an abnormal R-wave progression overtime – more specifically being lower than the pathological Q-waves – However, acute transmural ischemia may show increase in R-wave amplitude due to delayed depolarisation in the ischemic area.

ST-T Changes

Works with T-wave as they correlate to Phase 2 & 3 of electrical activity which is repolarisation. Changes depend on localisation, extension and time of ischemia (ie. It differs in later phases)

  • ST segment may be elevated or depressed (in relation to PR segment)
  • T-wave may lose (flat) increase amplitude (hyperacute), become negative (inversion)
T-Wave behaviours include;
  • 1. Isolated T-wave inversions occur after ischemic episode (same as flat T-wave)
  • 2. Hyperacute T-wave indicate isolated sign for SMI – Seen as broad and high
U-wave Changes

New U-waves (without bradycardia) may indicate ischemia – If U-waves are present in previous ECG recordings, then the amplitude must increase to suggest ischemia. Changes to U-wave are accompanied by ischemic ST-T changes in NSTEMI & STEMI - Inverted U-waves are typical signs of ischemia.

ST Segment Changes
  • ST Depressions
    • Either horizontal or down sloping segment with potential abrupt transition from ST to T wave – This can occur in STEMI or NSTEMI.
  • ST Elevation
    • Can present in many forms, however concaved ST elevation are less likely from ischemia; can have differential diagnosis besides STEMI.
Transmural Ischemia (Full thickness)Effects seen


Vector directed from endocardium to epicardium

  • Hyperacute T-wave
  • ST elevation
  • Reciprocal ST depression


Subendocardial Ischemia (Partial thickness)Effects seen


Vector directed from endocardium to epicardium

  • Hyperacute T-wave
  • ST elevation
  • Reciprocal ST depression



STEMI without ST-Elevation

Transmural ischemia in posterolateral portion of left ventricles AND/OR right ventricles usually don’t show up on the ECG – These regions may have simultaneous ischemia at one time

Posterolateral STEMI (Posterior & Inferobasal)

Masked by the ECG – however leads V1-V3 (maybe V4) can detect injury currents as ST depressions (mirroring ST elevations) with positive T-wave in V1-V3 (mirrors T-wave inversion) as well as larger R-waves as reciprocal to posterolateral Q-waves.
It is recommended V4-V6 becomes V7-V9 by placing electrodes on the back.


This picture illustrates the reciprocal relationship between the ECG changes seen in STEMI and those seen with posterior infarction. The previous image (depicting posterior infarction in V2) has been inverted. See how the ECG now resembles a typical STEMI!

  • V1-V3 has ST depression & positive T-waves
  • V1-V3 has larger R-waves
Above: Both ECG's are of the same patient, but the latter ECG has recorded vectors V7, V8, V9 (posterior leads) instead of V4, V5, V6. You'll notice that V4/V5/V6 have been crossed out and now indicate V7, V8, V9; this obviously cannot be achieved pre-hospitally, so it's vitally important that when patching a call through to a hospital with regards to a transmitted ECG, you convey clearly the lead changes that have occurred.
Right Ventricle STEMI

Rare blockage, however, V1-V2 can assist in determining STEMI in the right ventricles – the elevation are short duration and rarely persist after six hours, the ST elevations can be accurately recorded using right-sided ECG placement in V3R & V4R.

  • ST elevation in V1 & V2, but elevation is higher in V1 than V2
  • Most cases, ST elevation in Lead I, II, & aVF as it also sees the RCA.
ECG diagnosis of ischemia

Multiple ECGs must be taken to identify any ST-T changes as it can be missed due to the limited time the ECG captures the activity – This is bound to happen if ischemia occurs; furthermore, an ECG must be compared where ST-T changes suggest ongoing AMI.

A patient complaining of cardiac-related symptoms, chest region, or breathing difficulties should warrant further history collection and examination, including an ECG to rule out STEMI / NSTEMI.

  • Normal / Inconclusive ECG
  • Isolated T-wave inversion
  • ST depressionST depression
  • ST elevation
  • ST elevation & ST depression
  • LBBB
Normalisation of the ECG after ischemia

Occur days or weeks after event (longer if infarction occur) – ST-T changes normalise with the permanent changes usually relate to the QRS complex (mostly Q-waves).


Localising the infarction area
Right Coronary Artery (RCA)
Infarcted ArteryECG Features
Inferior WallSTE in Lead II, III, & aVF (STE highest in Lead III) with reciprocal ST depression aVL & Lead I
Inferior & Posterior (Inferobasal)STE in Lead II, III, aVF & V7-V9; with reciprocal ST depression V1-V3, aVL, & Lead I – V1 to V3 can show high R-waves and positive T-waves
Inferior & right ventricle STE in V3R to V6R with potential for STE in V1 & V2 (STE highest in V1)
Left Anterior Descending Coronary Artery (LAD)
Infarcted ArteryECG Features
Proximal LADSTE in V1-V4, aVL & Lead I with reciprocal ST depression in Lead II, III, aVF, -aVR, & V5-V6.
Distal LADSTE in V2-V6
Left Circumflex Coronary Artery (Left Cx)
Infarcted ArteryECG Features
Posterior LCxSTE in V7-V9 with reciprocal ST depression in V1-V3 with high R-waves & positive T-wave in same leads.
Inferoposterior LCxSTE in Lead II, III, aVF (occasion in aVL, Lead I); just like inferior blockage
Main LCASTE in most ECG leads due to large wall.
Determining infarction age

Identified by Q-waves & ST elevation (STE) as a reference

  1. ST Elevation in a region but NO Q-waves = Acute infarction Figure C & D
    1. Due to muscle not yet died – important as this indicate the vessel can be reopened and re-perfused.
  2. ST Elevation in a region WITH Q-waves = Infarction is > 2hrs ago Figure E
    1. Q-wave build from 2 to 4 hours after blockage, STE with Q-waves can indicate damage has been done but is still acute – Some cardiac muscle can be saved by doctors.
  3. No ST Elevation WITH pathological Q-waves = Old infarction Figure F
    1. ST segments return to baseline and T-wave can be inverted after an AMI has evolved. Furthermore, the T-wave can be upright along with no STE. Pathological Q-waves occur if the AMI is large (usually wide & deep wave).



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Page contributors:

60825Thanh Bui, AP60825
Event Medic, Emergency Medical Technician &
Volunteer Development Officer



Andrew Moffat, AP16790
Volunteer Training Manager & Volunteer Development Officer

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