Detailed Explanation of This 12-Lead ECG

Technical Details (from the tracing)

• Heart rate: 90 bpm (shown top-right)
• Paper speed: 25 mm/sec (x-axis)
• Amplitude calibration: 10 mm/mV (y-axis)
• Leads shown: Standard 12-lead layout — I, II, III (limb leads) + aVR, aVL, aVF (augmented limb leads) + V1–V6 (precordial leads) + a rhythm strip (Lead II at bottom)

How to Read This ECG — Step by Step

1. Rate

"Rate = 300 / (number of large boxes between R waves)" — Medical Physiology (Boron & Boulpaep)

2. Rhythm

• Look at the rhythm strip (Lead II, bottom row): The R-R intervals appear regular and consistent throughout.
• Each QRS complex is preceded by a P wave → confirms sinus rhythm (impulse originates from the SA node, travels through the AV node → ventricles normally).

3. P Waves

• Normally <120 ms (3 small boxes) wide, upright in Leads I, II, and aVF; inverted in aVR.
• In this tracing, P waves are visible and upright in Lead II, confirming sinus origin.
• P waves appear normal in morphology (not broad or bifid — no P mitrale; not tall/peaked — no P pulmonale).

4. PR Interval

• Measured from onset of P wave to onset of QRS.
• Normal: 0.09–0.20 sec (90–200 ms).
• In this ECG the PR interval appears within normal limits (roughly 1 large box or slightly less = ~160–180 ms).
• A PR >200 ms = 1st-degree AV block; a short PR may suggest Wolff-Parkinson-White (pre-excitation).

5. QRS Complex

• Normal duration: 75–110 ms (just under 3 small boxes).
• In this ECG, QRS complexes appear narrow (< 3 small boxes wide) — indicating normal, rapid ventricular depolarization through the His-Purkinje system (no bundle branch block).
• Capital letters (R, S, Q) = deflections ≥ 5 mm; lowercase (r, s, q) = deflections < 5 mm.

• Normally the R wave is small in V1 and progressively increases to become dominant by V5–V6 ("R-wave progression").
• A loss of this progression (persistent small r or rS pattern) can suggest anterior MI.
• In this ECG, there appears to be normal R-wave progression across the precordial leads.

• Small septal q waves (< 1 small box wide and < 2 mm deep) are normal in Leads I, aVL, V5–V6.
• Pathological Q waves (≥ 1 small box wide OR ≥ 2 mm deep) suggest prior myocardial infarction. None appear clearly pathological here.

6. ST Segments

• The ST segment connects the end of the QRS to the beginning of the T wave.
• Normally it sits at the isoelectric line (baseline).
• ST elevation → acute transmural ischemia / STEMI (needs urgent intervention).
• ST depression → subendocardial ischemia, posterior STEMI pattern, or digoxin effect.
• In this ECG, ST segments appear isoelectric — no obvious elevation or depression.

7. T Waves

• Normally upright in I, II, V3–V6; inverted in aVR (normal); can be inverted in aVL, III, V1.
• T wave inversions in V1–V4 can suggest anterior ischemia or right ventricular strain.
• Looking at this ECG: T waves appear upright in most leads, though some flattening may be present — overall appearance is within normal/borderline range.

8. QT Interval

• Measured from onset of QRS to end of T wave; must be corrected for rate (QTc).
• Using Bazett's formula: QTc = QT / √(RR in seconds).
• Normal: males 390–450 ms; females 390–460 ms.
• A prolonged QTc increases risk of torsades de pointes (a dangerous arrhythmia).

9. Electrical Axis

• Normal axis: −30° to +90° in the frontal plane.
• Quick method: If QRS is positive in both Lead I and Lead II → normal axis.
• Left axis deviation (LAD): positive in I, negative in II/aVF → seen in left anterior hemiblock, inferior MI, LVH.
• Right axis deviation (RAD): negative in I, positive in aVF → right ventricular hypertrophy, RBBB, lateral MI.
• In this ECG, the QRS is upright in Lead I and Lead II → suggesting a normal axis.

Summary Interpretation of This ECG

Lead Groupings and What They "Look At"