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).

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”

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. 

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.

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)

• 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

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 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 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

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.

• V1-V3 has ST depression & positive T-waves
• V1-V3 has larger R-waves

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.

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.

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).

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).