Cardiology & Internal Medicine & Family Medicine
ECG (electrocardiogram)
The electrocardiogram (ECG) is a non-invasive, painless and rapid test that records the heart's electrical activity using electrodes placed on the surface of the body. It is the most widely prescribed first-line cardiac examination in medicine - available in less than 5 minutes, without risk and at low cost - and provides essential diagnostic information on heart rhythm, atrioventricular and intraventricular conduction, the morphology of the heart chambers and the state of the myocardium (ischemia, necrosis, pericarditis). The physical principle is based on the detection of electrical potential variations generated by the successive depolarization and repolarization of myocardial cells: the P wave reflects atrial depolarization; the QRS complex, ventricular depolarization; the T (and U) wave, ventricular repolarization. A standard 12-lead ECG explores the heart in the frontal plane (limb leads: DI, DII, DIII, aVR, aVL, aVF) and in the horizontal plane (precordial leads: V1 to V6). The ECG is produced at a standard paper speed of 25 mm/s (1 small square = 0.04 sec; 1 large square = 0.20 sec) and a calibration of 10 mm/mV (1 mm = 0.1 mV). The systematic and rigorous reading of an ECG follows a standardized sequence: heart rate → rhythm (sinus or non-sinus) → electrical axis → intervals (PR, QRS, QT/QTc) → wave morphology (P, QRS, ST, T, U) → conclusion. A normal ECG does not exclude significant heart disease - up to 50 % of ECGs are normal in stable angina, and an ECG may be normal in the early hours of a myocardial infarction (MI); clinical correlation is therefore always essential.
Clinical indications for ECG
- Cardiac symptoms : chest pain (suspected acute coronary syndrome - ACS - or pulmonary embolism); palpitations (arrhythmia - tachycardia, atrial fibrillation, flutter, ESV, SVT); syncope or lipothymia (atrioventricular block, Brugada syndrome, long QT, aortic stenosis); unexplained dyspnea (heart failure, pericarditis, tamponade); lower-limb edema (decompensated heart disease).
- Preoperative and pre-anaesthetic workup : recommended for patients ≥40-50 years of age or with cardiovascular risk factors (hypertension, diabetes, smoking, dyslipidemia, history of coronary heart disease) before any intermediate- or high-risk surgery; perioperative cardiac risk assessment (Lee score - Revised Cardiac Risk Index)
- Chronic disease and drug monitoring: hypertension (left ventricular hypertrophy); heart failure (ejection fraction monitoring - indirect correlation); chronic renal failure (hyperkalemia - sharp T waves, QRS widening); QTc-prolonging drugs (class IA/III antiarrhythmics, antipsychotics, antiemetics - domperidone, azithromycin, hydroxychloroquine, methadone); tricyclic antidepressants (QRS + PR + QTc prolongation)
- Preventive health check and sport : screening for cardiopathies at risk of sudden death in athletes (Brugada syndrome, hypertrophic cardiomyopathy - HCM, Wolff-Parkinson-White syndrome - WPW, congenital long QT); pre-employment check-up (certain high-risk jobs - drivers, pilots, divers); annual check-up for patients with high cardiovascular risk factors
Systematic reading and frequent anomalies
| Parameter / Anomaly | Normal values, mechanism and ECG presentation | Clinical significance and treatment |
|---|---|---|
| Heart rate and sinus rhythm 1st reading stage |
Normal frequency: 60-100 bpm (rapid method: 300 ÷ number of large squares between 2 QRS complexes); bradycardia: 100 bpm; sinus rhythm: positive P wave in DII, negative in aVR, identical morphology before each QRS, with constant PR interval 120-200 ms; normal electrical axis: between -30° and +90° (positive QRS complex in DI and aVF); left axial deviation (-30° to -90°): left anterior hemibloc, LVH, inferior MI; right axial deviation (+90° to +180°): DAH, pulmonary embolism, VHD, BPH | Sinus bradycardia: often benign (sportsman, vagotonia, sleep, medications - beta-blockers, digoxin, amiodarone); symptomatic bradycardia (<40 bpm, syncope) → cardiological evaluation + Holter ECG; sinus tachycardia: always secondary (fever, pain, anemia, hypovolemia, hyperthyroidism, PE, heart failure, anxiety, medication) → treat underlying cause; non-sinus rhythm → analysis of P waves and P/QRS ratio to identify mechanism |
| Atrial fibrillation (AF) and flutter Most frequent sustained arrhythmia |
AF: absence of distinct P waves → irregular isoelectric line (coarse or fine fibrillation) + totally irregular RR intervals + fine QRS complexes (unless associated bundle branch block or WPW); variable ventricular rate (often 100-160 bpm if uncontrolled); atrial flutter: sawtooth F waves at 300 bpm (best seen in DII, DIII, aVF and V1) - regular - variable ventricular conduction (most often 2:1 → FC ~150 bpm or 3:1 → FC ~100 bpm); slow ventricular response AF: regularly spaced QRS complexes + AF → think of associated complete AV block (especially under digoxin) | AF: most frequent arrhythmia - prevalence 1-2 % adults, 10 % after age 80; major embolic risk (ischemic stroke ×5 - CHA₂DS₂-VASc score for anticoagulation); frequency control (target <110 bpm at rest): beta-blockers or non-HDP calcium channel blockers (verapamil, diltiazem) - or digoxin if LVEF <40 %; cardioversion (electrical or pharmacological - amiodarone, flecainide) if AF 95 %) |
| Atrioventricular block (AVB) Nodal conduction disorders |
1st degree BAV: PR >200 ms - constant - each P leads a QRS → benign; 2nd degree BAV Mobitz I type (Wenckebach): progressive PR lengthening → blocked P wave (not followed by a QRS) → cycle starts again → often nodal - benign (vagotonia, athletes, inferoinferior MDI); 2nd degree BAV Mobitz II type: fixed PR + intermittently blocked P wave without prior PR lengthening → often infranodal (His bundle/branches) → risk of progression to full BAV; 3rd degree BAV (full BAV): complete atrioventricular dissociation - independent P waves and QRS - junctional escape rhythm (fine QRS, 40-60 bpm) or ventricular escape rhythm (wide QRS, 20-40 bpm) | 1st degree BAV and asymptomatic Mobitz I: monitoring, no urgent treatment; Mobitz II and complete BAV: indication for definitive cardiac pacing (pace-maker) - urgently if complete BAV with syncope or unstable hemodynamics (atropine IV 0.5-1 mg bridging + transcutaneous pace-maker); common causes: inferior MI (transient nodal BAV - spontaneous recovery), Lenegre-Lev disease (idiopathic degeneration of conductive tissue), drugs (digoxin, beta-blockers, verapamil, amiodarone), myocarditis, sarcoidosis, Lyme disease |
| Branch blocks (BBD / BBG) Intraventricular conduction disorders |
Complete right bundle branch block (BBD): QRS ≥120 ms + rSR' or RSR' in V1 (rabbit ears) + wide S wave in DI and V6 + negative T wave in V1-V3 (secondary repolarization); incomplete BBD: same criteria but QRS 100-119 ms; complete left bundle branch block (BBG): QRS ≥120 ms + wide, monophasic R wave in DI, aVL, V5, V6 (absence of q septal) + QS or rS appearance in V1 + discordant T wave (negative where QRS positive); new complete LBBB: STEMI equivalent until proven otherwise (Sgarbossa criteria); left anterior hemiblock (LAH): left axial deviation (-30° to -90°) + qR in DI/aVL + rS in DII/DIII/aVF + QRS <120 ms | Isolated BBD : often benign (idiopathic, elderly) - no treatment if asymptomatic; BBD + hemiblock = bifascicular block → risk of progression to complete BAV → monitoring + Holter; new complete BBG in a patient with chest pain → treat as STEMI (urgent coronary angiography); pre-existing complete LBBB + chest pain → Sgarbossa criteria to detect superadded ischemia (ST elevation ≥1 mm concordant with QRS - most specific criterion) |
| Ischemia and myocardial infarction (ACS) Diagnostic and therapeutic urgency |
STEMI (ST elevation infarction): ST elevation ≥1 mm in ≥2 contiguous limb leads or ≥2 mm in V1-V4 → coronary territory: anterior (V1-V4 → IVA); inferior (DII, DIII, aVF → CD or Cx); lateral (DI, aVL, V5-V6 → Cx); posterior (ST sub-shift in V1-V3 + dominant R → mirror - leads V7-V9); hyperacute T wave (sharp, symmetrical T - very early stage); necrosis Q wave (Q ≥0.04 sec and/or ≥25 % of the following R wave); NSTEMI / Unstable angina: horizontal or descending ST sub-shift ≥0.5-1 mm; symmetrical T-wave inversion; normal ECG possible in 50 % of NSTEMIs | STEMI: emergency reperfusion - primary PCI (angioplasty) within 90 min (balloon holder delay) or thrombolysis if PCI delay >120 min - call 911 immediately - aspirin 325 mg + ticagrelor or clopidogrel as soon as diagnosed; NSTEMI : IV heparin + double antiaggregation + coronary angiography within 24-72 h depending on risk score (GRACE, TIMI); continuous ECG monitoring; ECG repeated at 15-30 min if initial ECG non-diagnostic with persistent chest pain; troponins hs at H0 and H1 or H0 and H3 (ESC 2023 algorithm) |
| QT prolongation and ion disorders Risk of torsade de pointes |
QT interval measured from beginning of QRS to end of T wave in DII or V5; QTc (corrected for HR) by Bazett's formula: QTc = QT / √RR (seconds); normal QTc: male <440 ms, female 450 ms (male) or >470 ms (female) - risk of torsade de pointes if QTc >500 ms; causes: drugs (IA/III antiarrhythmics, antipsychotics, domperidone, azithromycin, methadone, hydroxychloroquine), hypokalemia (flattened T-wave + prominent U-wave), hypomagnesemia, hypocalcemia (ST prolongation), congenital long QT (Romano-Ward, Jervell-Lange-Nielsen); hyperkalemia: sharp, symmetrical T waves (K⁺ 6-7 mmol/L) → PR widening + QRS → sine wave + standstill (K⁺ >8-9 mmol/L) | QTc >500 ms: discontinue or substitute responsible drug + correct electrolytes (target K⁺ >4.0 mmol/L, Mg²⁺ >0.8 mmol/L); torsade de pointes: magnesium sulfate IV 2 g in 5-10 min + temporary pacemaker (isuprel or pacemaker to accelerate HR → QT shortening); threatening hyperkalemia (widened QRS + sine wave): calcium gluconate IV 1 g in 2-3 min (membrane stabilization) + bicarbonate + glucose-insulin + kayexalate or patiromer + dialysis if ARF; monitor QTc before and after introduction of any QT-prolonging drug (riskQT.com - complete list) |
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Visit Wolff-Parkinson-White (WPW) syndrome is a congenital conduction anomaly characterized by the presence of an atrioventricular accessory pathway (Kent's bundle) which short-circuits the AV node → early ventricular pre-excitation. On ECG: short PR interval (<120 ms) + onde delta (montée lente et élargie du début qrs) qrs élargi ≥120 ms anomalies secondaires de repolarisation (onde t discordante). le wpw prédispose aux tachycardies par réentrée (avrt — 80 % fins si conduction orthodromique, larges antidromique) et, en cas fa associée, à une rapide antérograde la voie accessoire (fa pré-excitée) → risque ventricular fibrillation and sudden death. Pre-excited AF on WPW (irregular short RR + polymorphic wide QRS) is a life-threatening emergency - formal contraindication to AV node-braking drugs (adenosine, digoxin, verapamil, diltiazem) → immediate electrical cardioversion. Definitive treatment: catheter ablation of the accessory pathway (successful >95 %).
Emergency - ECG abnormalities requiring immediate intervention
Dial 911 immediately if an ECG shows any of the following abnormalities in a compatible clinical context: ST elevation in ≥2 contiguous leads (STEMI - reperfusion within 90 min); ; Complete BAV with severe bradycardia (<30-40 bpm) or hemodynamic instability (hypotension, syncope); ; sustained ventricular tachycardia (wide QRS ≥120 ms, regular, >100 bpm, >30 sec) or ventricular fibrillation (chaotic pattern with no identifiable complexes) → immediate defibrillation ; FA pre-excited on WPW (irregular short RR + polymorphic wide QRS) → urgent electrical cardioversion ; sinusoidal wave (severe hyperkalemia - risk of asystole).
A normal ECG in the context of chest pain does not rule out SCA - repeat ECG at 15-30 min and measure troponin hs.
Consult at Clinique Omicron
Clinique Omicron offers the production and interpretation of 12-lead ECGs for preventive assessment, pre-operative assessment, chronic disease monitoring and cardiovascular symptom evaluation. Results are interpreted by physicians and transmitted rapidly. In the event of abnormalities requiring specialized follow-up, a cardiology referral is organized. Consultations are available at our points of service in Quebec and via telemedicine for follow-up. To book an appointment, visit cliniqueomicron.ca.
The contents of this page are provided for information purposes only and do not replace the advice of a qualified healthcare professional. Any acute cardiac symptoms (chest pain, palpitations, syncope) require immediate medical evaluation.
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