ST Segment Changes and ECG Interpretation

• It is important to recognize ECG presentations that could show ST segment changes

ECG presentations may show ST segment changes including:

• Left Bundle Branch Block

• Pericarditis

• Pre-excitation

• Left Ventricular Hypertrophy

• Early Repolarisation

• Hyperkalaemia

• It's important to explain the clinical significance of ST segment changes in certain conditions

• The following ECGs can show ST segment changes: Left Bundle Branch Block, Pericarditis, Pre-excitation, Left Ventricular Hypertrophy, Early Repolarisation, and Hyperkalemia

• It's important to use an ECG along with the clinical findings, and not in isolation

STE-MICS

• STE-mics refers to ECG rhythms that show ST-elevation but are not associated with an acute MI
• STE-mics are ST-elevation "mimics"
• LBBB, pericarditis, pre-excitation, left ventricular hypertrophy, early-repolarisation changes, paced rhythms and Brugada syndrome, can all exhibit ST-elevation
• STE mimics aren't associated with blockage or narrowing of the coronary arteries

Terminology

• In the context of ECG interpretation, the same P-QRS-T complex both occur positive or both with negative deflections
• Concordance is when QRS complex and T waves are in the same direction
• Discordance is when QRS complex and T waves are in opposite directions, being positive and the other being negative
• In LBBB, the QRS complex and T waves should be discordant. The QRS complex is negative and the T wave is positive, or vice versa

LBBB (Left Bundle Branch Block)

• LBBB is often caused by myocardial ischaemia and associated ischaemia of the left bundle branch
• Patients with new onset LBBB and symptoms of acute coronary syndrome should be treated as having a STEMI
• Blockage in the left bundle branch prevents the electrical impulse from reaching the left ventricle's myocardium via its normal pathway
• This results in a delayed left ventricular contraction as the impulse takes an alternative, slower route
• The ECG shows a prolonged QRS complex over 0.12 seconds (3 small squares), which indicates asynchronous contraction between the left and right ventricles
• A left bundle branch block (LBBB) can be a pre-existing condition, but it's always pathological
• Causes include either a new or old MI
• LBBB also causes ST / T wave changes, with T wave inversion in the left ventricular leads
• A new LBBB caused by an Acute Coronary Syndrome identifies a very high-risk patient associated with > 40% mortality without treatment
• Diagnosing AMI in the presence of LBBB on an ECG is challenging, with possible marked ST-segment abnormalities
• Algorithms exist to aid ECG diagnosis of STEMI with LBBB, but the ESC notes they lack diagnostic certainty
• Concordant ST-segment elevation in leads with positive QRS deflections may suggest an ongoing MI
• Patients with clinical suspicion of ACS and LBBB should be treated similarly to those with STEMI, regardless of LBBB's known history
• A new LBBB does not automatically indicate a MI
• If the QRS is broad (>3 small squares)
• If the rhythm is supraventricular (there is a P wave before each QRS) then look at V1 for signs of left or right bundle branch block
• No initial Q wave in lead V6
• No Secondary R wave in V

STEMICS - Pericarditis

• Pericarditis involves inflammation or infection of the pericardium, the sac surrounding the heart
• It leads to widespread ST-elevation, typically upwardly concave ("smile-like")
• ST-elevation is present in almost every lead without reciprocal changes
• P-R depression is a key characteristic
• This effect can exaggerate the appearance of ST-elevation
• For accurate assessment, ST-elevation should be compared to the T-P segment, not the P-R segment
• Pericarditis Symptoms can be described as dull, sharp, burning or pressing
• Pericarditis is either barely perceptible or up to a severe level
• Pericarditis is felt in the sub-sternal or precordial region
• Pericarditis radiates to the neck, trapezius ridge (especially the left) or shoulders
• Pericarditis is aggravated by inspiration, swallowing, coughing and lying flat
• Pericarditis is relieved by sitting up and leaning forward

STE-mics – Early Repolarisation Changes

• Early ST-elevation is referred to as 'high take off'
• Early ST-elevation is common and normal among young individuals and athletes
• Early ST-elevation is primarily observed in right precordial leads (V1-V3)
• Early ST-elevation is typically a benign feature and isn't linked to ischemia or infarction
• Distinguishing benign from pathological ST elevation requires patient history and prior ECG comparisons
• Importance of checking for reciprocal changes to aid differentiation
• Benign early repolarisation can be misleading and it is not always benign
• Studies link early repolarisation in inferior ECG leads to increased cardiac-related death risk in middle-aged men
• Early repolarisation is most prevalent in younger men. Up to 90% show 1-3 mm ST segment elevation in at least one precordial lead
• he likelihood of early repolarisation decreases with age in men, while cardiac risk factors increase
• Women have a lower prevalence of early repolarisation (~20%), which does not increase with age
• In clinical practice, a ‘high take off' diagnosis can be a dangerous conclusion to make when assessing an ECG
• If the patient's clinical presentation were enough to prompt an ECG to be obtained in the first place, then any J point deviation should be considered a serious finding
• If the patient is presenting with ACS symptoms, then a diagnosis of early repolarisation isn't appropriate

Early Repolarisation (BER)

• Benign early repolarisation (BER) is an ECG pattern most seen in young, healthy patients < 50 years of age
• BER is less common in the over 50s, in whom ST elevation is more likely to represent myocardial ischaemia
• It is rare in the over 70s.
• In STEMI vs. Acute Pericarditis/BER, AMI can either be Diffuse or localized STE, frequent reciprocal STD or Usually diffuse STE, no reciprocal STD (except V1 or aVR)
• Straight, horizontal, AMI ST segment morphology is convex or concave or concave upwards
• If STE III > II (usually) it is STEMS or If STE II > III (nearly 100%) it is Pericarditis and BER
• For evolving Evolving the STE, ST segments and Ts is STEMI or Unlikely in ED for Pericarditis/BER

STE-mics - Pre-Excitation

• Pre-excitation causes widening of the QRS and consequently ST and T wave changes
• It commonly causes ST-depression and T-wave inversion
• These can appear similar to the ECG signs of Ischaemia (such as in angina) or non-STEMI
• Not all the QRS complexes in the different leads may show pre-excitation so it is important to look for signs of STEMI in the normal complexes
• The following ECG abnormalities are considered red flags and in these cases the patient must be conveyed to the Emergency Department for assessment
• New conduction abnormalities (complete right or left bundle branch block or any degree of heart block)
• Evidence or prolonged (>440 ms for males or >460 ms for females) or shortened QTc intervals (<350 ms for both)
• Any ST segment or T-wave abnormalities (e.g. brugada syndrome, abnormal T-wave inversion)
• Pathological Q waves.Paced rhythm
• Inappropriate persistent bradycardia
• Ventricular arrhythmia
• Ventricular pre-excitation (e.g. Wolff-Parkinson-White syndrome)
• Sustained atrial arrhythmias
• The PR interval is short (<3 small squares)
• The QRS is greater than 2.5 small squares
• There is an initial delta wave or slur at the beginning of the QRS
• This is due to an abnormal conduction pathway between the atria and the ventricles, bypassing the normal (and slow) conduction through the AV node
• An accessory pathway predisposes the patient to dangerous arrhythmias such as AVRT (a type of supra-ventricular tachycardia (SVT)
• Because it bypasses the AV node the ventricles begin to contract while the atria may still be contracting. This can prevent proper cardiac filling and compromise cardiac
• https://www.nhs.uk/conditions/wolff-parkinson-white-syndrome/

Hyperkalaemia

• Potassium is vital for regulating the normal electrical activity of the heart
• Increased extracellular potassium reduces myocardial excitability, with depression of both pacemaking and conducting tissues
• Progressively worsening hyperkalaemia leads to suppression of impulse generation by the SA node and reduced conduction by the AV node and His-Purkinje system, resulting in bradycardia and conduction blocks and ultimately cardiac arrest
• Tall 'tented' T waves
• Worsening hyperkalemia is associated with progressive flattening of P waves, prolongation of the PR interval (PR interval > 200 ms) and eventually disappearance of P waves. Bradycardia is common and AV block may complicate hyperkalemia
• As serum K+ levels rise the qrs complex becomes wider eventually passing the upper limit of normal. At least think of hyperkalemia if you see this combination of wide qrs complexes and tall T waves
• As K+ levels rise further, the situation is becoming critical. The combination of broadening QRS complexes and tall T waves produces a sine wave pattern on the ECG readout. Cardiovascular collapse and death are imminent
• The end game for untreated hyperkalemia is chaotic depolarisation of ventricular myocardium: ventricular fibrillation. No cardiac output is present. This situation is not compatible with life
• https://litfl.com/hyperkalaemia-ecg-library/