# Unit 3: Normal ECG Waveforms and Variants (CV301) ## Normal ECG Components **[Figure: Normal ECG complex]** One cardiac cycle showing P, Q, R, S, T and U waves with PR, QRS and QT intervals marked. ### Atrial Activation and the Normal P Wave - **P wave** = atrial depolarisation from the **SA node**, right atrium first then left. - **Normal**: upright in I, II, aVF, V3-V6; **inverted in aVR**; duration **< 0.12 s (3 small squares)**; amplitude **< 2.5 mm** in limb leads; biphasic in V1 (initial positive right atrial, terminal negative left atrial component). - Abnormal: tall peaked P (**P pulmonale**, right atrial enlargement); broad notched P (**P mitrale**, left atrial enlargement). ### Atrial Repolarisation and AV Node Conduction - **Atrial repolarisation wave (Ta)**: opposite to the P wave, hidden in the QRS complex; may cause PR depression. - **AV node conduction**: slow conduction (decremental) gives the physiological delay that allows ventricular filling; not recorded directly, seen as the **PR segment**. ### PR Segment and PR Interval - **PR segment**: isoelectric, from end of P to start of QRS (delay in AV node and His bundle). - **PR interval**: start of P to start of QRS; **0.12-0.20 s (3-5 small squares)**. Short in pre-excitation (WPW) or junctional rhythm; prolonged in **first-degree AV block**. ### Ventricular Activation and the QRS Complex - **Q wave**: first negative deflection, septal depolarisation (left to right); physiological small Q in I, aVL, V5-V6 (**< 0.04 s and < 25% of R**). - **R wave**: first positive deflection; **S wave**: negative deflection after R. - **R wave progression**: R increases from V1 to V5 while S decreases; **transition zone** (R = S) at V3-V4. - **Duration** **0.06-0.10 s**; voltage criteria vary with age and body build. - Wide QRS (> 0.12 s): bundle branch blocks, ventricular rhythms, hyperkalaemia, pre-excitation. ### Ventricular Recovery: ST-T Wave and U Wave - **J point**: junction of QRS and ST segment. **ST segment**: from J point to start of T; **isoelectric** (within +/- 1 mm). Elevation (MI, pericarditis) or depression (ischaemia, digoxin) is abnormal. - **T wave**: ventricular repolarisation (epicardium to endocardium so it is concordant with QRS); upright in I, II, V3-V6; inverted in aVR; may be inverted in V1 (and V2-V3 in juvenile pattern); amplitude < 5 mm in limb leads and < 10 mm in chest leads. - **QT interval**: start of QRS to end of T; corrected **QTc = QT / sqrt(RR)** (Bazett); normal about < 440 ms (men) and < 460 ms (women); prolonged QT predisposes to **torsades de pointes**. - **U wave**: small positive wave after T, best seen in V2-V3; likely repolarisation of Purkinje fibres or mid-myocardium; prominent in **hypokalaemia**, bradycardia, drugs. | Parameter | Normal | |---|---| | P wave | < 0.12 s, < 2.5 mm | | PR interval | 0.12-0.20 s | | QRS | 0.06-0.10 s | | QTc | < 0.44-0.46 s | | Heart rate | 60-100 /min | | Axis | -30 to +90 degrees | ## Normal ECG Variations - **Age**: infants (rapid rate, right axis, RV dominance, inverted T V1-V3), children (juvenile T-wave inversion), older adults (left axis tendency, longer PR). - **Sinus arrhythmia**: rate varies with respiration (physiological, common in the young). - **Athlete's heart**: sinus bradycardia, first-degree AV block, voltage criteria for LVH, early repolarisation. - **Early repolarisation**: concave J-point elevation with ST elevation in V2-V5 and tall T waves, common in young males (benign). - **Persistent juvenile T-wave pattern**, **normal incomplete RBBB** (rSR' in V1), posture and respiration-related axis shift, **dextrocardia** (reversed lead polarity - check lead placement), vagotonia. - **Misplaced leads** (e.g. limb lead reversal gives inverted P, QRS and T in lead I) should be excluded before diagnosing abnormality. ## Likely Exam Questions 1. Write the normal values of P wave, PR interval, QRS and QT. (2 marks) 2. Describe the normal P wave and its significance. 3. Explain the origin of the QRS complex and R wave progression. 4. Describe the ST segment, T wave and U wave. 5. What is QTc and why is it important? 6. List common normal variants of the ECG.