3. 12-Lead ECG Interpretation – NORMAL ECG & Sinus Rhythm

Course: EXSS3075 – Exercise & Clinical Cases 1
Topic: 12-Lead ECG Interpretation – “The Normal ECG”
Presenter: Dr Yorgi Mavros, Senior Lecturer, Faculty of Health Science, The University of Sydney

Default rhythm of the human heart; originates from the sino-atrial (SA) node.
Electrical pathway
- Impulse generated at SA node ➔ travels through atrial myocardium ➔ reaches the atrioventricular (AV) node ➔ conducted via the His-Purkinje system to the ventricles.
- Ensures coordinated atrial then ventricular depolarisation.
Resulting surface ECG
- Regular, narrow-complex rhythm
- Heart rate (HR) in adults: 60\text{–}100\;\text{bpm} (age-appropriate values for children).
- QRS complexes appear < 100\;\text{ms} wide (equivalent to < 0.10\;\text{s}).
- PR interval constant; P waves exhibit normal morphology & axis.

P Wave
- Present before every QRS.
- Axis: Upright (positive) in Lead I & Lead II, inverted (negative) in aVR.
PR Interval
- Constant from beat-to-beat.
- Adult normal range: 0.12\text{–}0.20\;\text{s} (3–5 small squares).
QRS Complex
- Duration < 0.10\;\text{s} (unless pre-existing bundle-branch or intraventricular conduction delay).
- Morphology narrow and consistent.
Rate Calculation
- One quick method: \text{HR}=\dfrac{1500}{\text{(# small boxes between successive R waves)}}.
- Example shown: 17 small boxes ➔ \dfrac{1500}{17}\approx88\;\text{bpm}.

Sinus Tachycardia
- Same ECG morphology as NSR but HR > 100\;\text{bpm} (above age-specific threshold in children).
- Example slide: HR 125 bpm satisfying tachycardia criteria yet retaining normal P–QRS–T relationships.
Sinus Bradycardia
- HR < 60\;\text{bpm} in adults (below age-specific limit in kids).
- Example slide: HR 55 bpm, PR 0.18\;\text{s}, narrow QRS, overall normal despite low rate.
Sinus Arrhythmia
- Benign beat-to-beat variation in P-P interval → slight irregularity in ventricular rhythm.
- Often respiratory-modulated; not pathologic in healthy people.

• Case A – HR 82 bpm

Rhythm regular; normal frontal axis.
P upright in II; PR 0.14\;\text{s} constant.
QRS < 0.12\;\text{s}; ST & T waves normal.
• Case B – HR 84 bpm
Same fundamental findings; minor ST elevations in II, III & chest leads recognised as normal early repolarisation variants.
• Case C (Bradycardic) – HR 55 bpm
P upright in II; PR 0.18\;\text{s}.
ST isoelectric; notable T-wave inversion V1–V2 and flattening in III, aVL, V4 (accepted normal variants in some individuals).
• Case D (Tachycardic) – HR 125 bpm
Maintains classic sinus pattern; elevation in II, III & chest leads again judged benign.

Physiologic “normal variant” often causing concave ST elevation.
Epidemiology
- Most common in adults < 50\;\text{yrs}.
- In patients > 50\;\text{yrs} or with cardiovascular disease (CVD/coronary artery disease, CAD), be cautious—do not automatically label as BER.
Clinical challenge: Can mimic acute myocardial infarction (AMI) ➔ risk of misdiagnosis.
Diagnostic clues
- Concave upward ST segments.
- Characteristic J-point notching/slurring (illustrated in II & aVF).
- Absence of reciprocal ST depression or evolving enzyme rise.
Prevalence in ED: Up to 10\text{–}15\% of chest-pain presentations display BER pattern.
Key teaching note: You will not be examined specifically on differentiating BER, but must integrate ECG with entire clinical context to avoid confusing it with an AMI.

Familiarise with on-screen parameters:
- ST Level (e.g., 2.5\;\text{mm} at J+60 ms) & ST Slope (e.g., 2.3\;\text{mV/s}) shown for lead-specific monitoring (V3 displayed).
- Real-time vitals: HR, BP, METs, Double Product (DP).
- Protocol examples: “GRADEDTEST3.5”, treadmill speed/grade (km/h, %).
- Filters applied: 0.05\text{–}150\;\text{Hz} bandwidth, 50\;\text{Hz} notch filter.
Clinical implication: Understanding software outputs helps contextualise exercise-induced ST changes and differentiate artefact from pathology.

ST Elevation in inferior leads (II, III) and some precordial leads may appear in healthy adults.
T-wave patterns can vary with age, sex, and chest lead position; e.g., early puberty T-wave inversions.
Axis: Normal QRS axis lies roughly between -30^{\circ} and +90^{\circ}; confirmed visually by upright QRS in Leads I & II.
Standard recording parameters
- Paper speed: 25\;\text{mm/s}.
- Amplitude: 10\;\text{mm/mV}.

Always relate ECG findings to patient’s presentation (symptoms, age, risk factors).
Avoid sole reliance on pattern recognition; integrate history, exam, and serial/previous tracings.
Document and communicate uncertainty (e.g., potential BER vs AMI) to expedite appropriate follow-up testing (enzymes, imaging).

✔ Identify P-wave morphology and axis.
✔ Measure PR, QRS, QT intervals (know normal limits).
✔ Calculate HR via small-box method or 6-second rule.
✔ Distinguish between NSR, sinus tachycardia, bradycardia, arrhythmia.
✔ Recognise benign early repolarisation features (concave ST, J-notch) without conflating with MI.
✔ Understand software outputs—ST level, slope, filter settings—during exercise studies.