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 of ECG
- Heart 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-anesthetic Evaluation: 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)
- Monitoring of chronic diseases and medications: 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-up and sports: 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
| Setting / Anomaly | Normal values, mechanism, and ECG presentation | Clinical significance and management |
|---|---|---|
| Heart rate and sinus rhythm First reading step |
Normal frequency: 60-100 bpm (quick method: 300 ÷ number of large squares between 2 QRS complexes); bradycardia: 100 bpm; sinus rhythm: P wave positive in DII, negative in aVR, identical morphology before each QRS, with constant PR interval 120-200 ms; normal electrical axis: between −30° and +90° (qrs complex positive in DI and aVF); left axis deviation (−30° to −90°): left anterior hemiblock, LVH, inferior MI; right axis deviation (+90° to +180°): RA, pulmonary embolism, RVH, left posterior hemiblock | Sinus bradycardia: often benign (athletes, vagotonia, sleep, medications—beta-blockers, digoxin, amiodarone); symptomatic bradycardia (<40 bpm, syncope) → cardiological evaluation + ECG Holter; sinus tachycardia: always secondary (fever, pain, anemia, hypovolemia, hyperthyroidism, PE, heart failure, anxiety, medications) → treat underlying cause; non-sinus rhythm → analyze P waves and P/QRS ratio to identify mechanism |
| Atrial fibrillation (AF) and flutter Most frequent sustained arrhythmia |
Atrial fibrillation (AFib): absence of distinct P waves → irregular isoelectric line (coarse or fine fibrillation) + completely irregular RR intervals + narrow QRS complexes (unless there's an associated bundle branch block or Wolff-Parkinson-White syndrome); variable ventricular rate (often 100–160 bpm if uncontrolled); atrial flutter: sawtooth F waves at 300 bpm (best seen in leads II, III, aVF, and V1) — regular — variable ventricular conduction (most often 2:1 → HR ~150 bpm or 3:1 → HR ~100 bpm); AFib with slow ventricular response: regularly spaced QRS complexes + AFib → consider associated complete AV block (especially with digoxin). | FA : arythmie la plus fréquente — prévalence 1–2 % adultes, 10 % après 80 ans ; risque embolique majeur (AVC ischémique ×5 — score CHA₂DS₂-VASc pour l'anticoagulation) ; contrôle de la fréquence (cible <110 bpm au repos) : bêtabloquants ou inhibiteurs calciques non DHP (vérapamil, diltiazem) — ou digoxine si FEVG <40 % ; cardioversion (électrique ou pharmacologique — amiodarone, flécaïnide) si FA 95 %) |
| Atrioventricular block (AVB) Nodal conduction disorders |
First-degree AV block: PR >200 ms — constant — each P wave conducts a QRS complex → benign; Second-degree AV block, Mobitz I (Wenckebach) type: progressive PR prolongation → blocked P wave (not followed by a QRS complex) → cycle repeats → often nodal — benign (vagotonia, athletes, inferior MI); Second-degree AV block, Mobitz II type: fixed PR + intermittently blocked P wave without prior PR prolongation → often infranodal (His bundle/branches) → risk of progression to complete AV block; Third-degree AV block (complete AV block): complete atrioventricular dissociation — independent P waves and QRS complexes — junctional escape rhythm (narrow QRS, 40–60 bpm) or ventricular (wide QRS, 20–40 bpm) | First-degree AV block and asymptomatic Mobitz I: monitoring, no urgent treatment; Mobitz II and complete AV block: indication for permanent cardiac pacing (pacemaker) — urgent if complete AV block with syncope or hemodynamic instability (0.5–1 mg IV atropine drip + transcutaneous pacing); common causes: inferior MI (transient nodal AV block — spontaneous recovery), Lenegre-Lev disease (idiopathic conduction tissue degeneration), medications (digoxin, beta-blockers, verapamil, amiodarone), myocarditis, sarcoidosis, Lyme disease |
| Branch blocks (BBD / BBG) Intraventricular conduction disorders |
Complete Right Bundle Branch Block (RBBB): QRS ≥120 ms + rSR' or RSR' in V1 (rabbit ears) + wide S wave in I and V6 + negative T wave in V1–V3 (secondary repolarization); Incomplete RBBB: same criteria but QRS 100–119 ms; Complete Left Bundle Branch Block (LBBB): QRS ≥120 ms + wide monophasic R wave in I, aVL, V5, V6 (absence of septal q) + QS or rS pattern in V1 + discordant T wave (negative where QRS is positive); New Complete LBBB: equivalent to STEMI until proven otherwise (Sgarbossa criteria); Left Anterior Fascicular Block (LAFB): left axis deviation (−30° to −90°) + qR in I/aVL + rS in II/III/aVF + QRS <120 ms | Isolated bundle branch block: often benign (idiopathic, elderly) - no treatment if asymptomatic; bundle branch block + hemiblock = bifascicular block → risk of progression to complete heart block → monitoring + Holter; new complete bundle branch block in a patient with chest pain → treat as STEMI (urgent coronary angiography); pre-existing complete bundle branch block + chest pain → Sgarbossa criteria to detect superimposed ischemia (ST elevation ≥1 mm concordant with the QRS - most specific criterion) |
| Ischemia and myocardial infarction (ACS) Diagnostic and therapeutic emergency |
STEMI (ST-elevation myocardial infarction): ST elevation ≥1 mm in ≥2 contiguous limb leads or ≥2 mm in V1–V4 → coronary territory: anterior (V1–V4 → LAD); inferior (II, III, aVF → RCA or Cx); lateral (I, aVL, V5–V6 → Cx); posterior (ST depression in V1–V3 + dominant R → mirror image — V7–V9 leads); hyperacute T wave (peaked and symmetrical T wave — very early stage); Q wave of necrosis (Q ≥0.04 sec and/or ≥25 % of the subsequent R wave); NSTEMI / Unstable angina: horizontal or downward sloping ST depression ≥0.5–1 mm; symmetrical T wave inversion; normal ECG possible in 50 % of NSTEMI | STEMI: Emergency reperfusion — Primary PCI (angioplasty) within 90 min (door-to-balloon time) or thrombolysis if PCI time >120 min — immediate 911 call — aspirin 325 mg + ticagrelor or clopidogrel upon diagnosis; NSTEMI: IV heparin + dual antiplatelet therapy + coronary angiography within 24–72h according to risk score (GRACE, TIMI); continuous ECG monitoring; repeat ECG at 15–30 min if initial ECG is non-diagnostic with persistent chest pain; hs troponins 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 the beginning of QRS to the end of the T wave in lead II or V5; QTc (heart rate corrected) using Bazett's formula: QTc = QT / √RR (seconds); normal QTc: male <440 ms, female 450 ms (male) or >470 ms (female) — risk of torsades de pointes if QTc >500 ms; causes: medications (Class IA/III antiarrhythmics, antipsychotics, domperidone, azithromycin, methadone, hydroxychloroquine), hypokalemia (flattened T wave + prominent U wave), hypomagnesemia, hypocalcemia (prolonged ST segment), congenital long QT syndrome (Romano-Ward, Jervell-Lange-Nielsen); hyperkalemia: peaked, symmetric T waves (K⁺ 6–7 mmol/L) → prolonged PR + QRS → sine wave + arrest (K⁺ >8–9 mmol/L) | QTc >500 ms: stop or substitute the responsible medication + correct electrolytes (target K⁺ >4.0 mmol/L, Mg²⁺ >0.8 mmol/L); torsades de pointes: IV magnesium sulfate 2 g in 5–10 min + temporary pacemaker (isoproterenol or pacemaker to increase HR → shorten QT); concerning hyperkalemia (widened QRS + sine wave): IV calcium gluconate 1 g in 2–3 min (membrane stabilization) + bicarbonate + glucose-insulin + Kayexalate or patiromer + dialysis if ARF; monitor QTc before and after introducing any QT-prolonging medication (riskQT.com — complete list) |
ℹ️
Visit Wolff-Parkinson-White (WPW) syndrome is a congenital conduction anomaly characterized by the presence of an atrioventricular accessory pathway (bundle of Kent) that bypasses the AV node → early ventricular pre-excitation. On ECG: short PR interval (<120 ms) + delta wave (slow and slurred upstroke at the beginning of the QRS) + widened QRS ≥120 ms + secondary repolarization abnormalities (discordant T wave). WPW predisposes to reentrant tachycardias (AVRT — 80 % — narrow QRS if orthodromic conduction, wide QRS if antidromic) and, in case of associated AFib, to rapid antegrade conduction through the accessory pathway (pre-excited AFib) → risk of ventricular fibrillation and sudden death. Pre-excited AFib on WPW (short irregular R-Rs + polymorphic wide QRSs) is a life-threatening emergency—an absolute contraindication for AV node blocking drugs (adenosine, digoxin, verapamil, diltiazem) → immediate electrical cardioversion. Definitive treatment: catheter ablation of the accessory pathway (success >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-segment depression in ≥2 contiguous leads (STEMI - reperfusion within 90 min); ; Complete AV block with severe bradycardia (<30–40 bpm) or hemodynamic instability (hypotension, syncope); ; sustained ventricular tachycardia (QRS wide ≥120 ms, regular, >100 bpm, >30 sec) or ventricular fibrillation (chaotic pattern with no identifiable complexes) → immediate defibrillation ; Atrial Flutter with pre-excitation 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 exclude a SCA - repeat ECG at 15-30 min and measure troponin hs.
Consult at Clinique Omicron
Clinique Omicron offers the performance and interpretation of 12-lead ECGs as part of preventive assessments, pre-operative assessments, chronic disease monitoring, and cardiovascular symptom evaluation. Results are interpreted by physicians and transmitted promptly. In case of an abnormality requiring specialized follow-up, a referral to cardiology is organized. Consultations are available at our service points in Quebec and via telemedicine for follow-up. To book an appointment, visit cliniqueomicron.ca.
The content of this page is provided for informational purposes only and does not substitute for the advice of a qualified healthcare professional. Any acute cardiac symptoms (chest pain, palpitations, syncope) require immediate medical evaluation.
Omicron Clinic
Need to consult a doctor?
Treatment within 24-48 hours. In-clinic or telemedicine, anywhere in Quebec.
Insurance receipts. 7j/7. No family doctor required.