2.3 Cardiac Vector Analysis, 12-Lead ECG Fundamentals & Axis Determination
Key Takeaways
- A deflection is positive when depolarization travels toward the positive electrode; the amplitude reflects the projection of the mean vector onto that lead's axis.
- Standard calibration is 25 mm/s and 10 mm/mV, so one small box is 0.04 s wide and 0.1 mV tall and one large box is 0.20 s.
- Normal QRS axis is -30 to +90 degrees; left axis deviation is -30 to -90 and right axis deviation is +90 to +180.
- In pre-excitation a positive delta wave in V1 indicates a left-sided accessory pathway and a negative delta wave in V1 indicates a right-sided pathway.
- Lead misplacement is the most common cause of an artifactual axis shift; limb-lead reversal of right and left arm produces a negative P, QRS, and T in lead I.
2.3 Cardiac Vector Analysis, 12-Lead ECG Fundamentals & Axis Determination
The surface ECG is the only recording in the EP lab that requires no catheter, and it remains the reference channel against which every intracardiac timing is measured. CCI lists 12-lead ECG interpretation, cardiac vector analysis, and ECG/EGM rhythm analysis/interpretation as distinct knowledge areas.
1. The Vector Model
Depolarization spreading through myocardium creates a moving wavefront of charge separation — a dipole with magnitude and direction, i.e. a vector. Each ECG lead is a line of sight through the heart with a positive and a negative pole.
The governing rule: a wavefront travelling toward the positive electrode inscribes an upward (positive) deflection; travelling away, a downward one; travelling perpendicular to the lead axis, a biphasic or isoelectric complex. Amplitude equals the projection of the vector onto that lead axis, so the largest complex appears in the lead most parallel to the mean vector and the smallest in the lead most perpendicular to it.
The two lead planes
| Plane | Leads | Axes |
|---|---|---|
| Frontal (limb leads) | I, II, III, aVR, aVL, aVF | Hexaxial reference system |
| Horizontal (precordial) | V1-V6 | Transverse, anterior to lateral |
Hexaxial reference system (frontal plane degrees):
| Lead | Axis |
|---|---|
| I | 0° |
| II | +60° |
| aVF | +90° |
| III | +120° |
| aVR | −150° |
| aVL | −30° |
Leads I, II, and III are bipolar (Einthoven's triangle; lead II = lead I + lead III). Leads aVR, aVL, and aVF are augmented unipolar, each comparing one limb against the average of the other two.
Precordial electrode positions must be memorized exactly, because misplacement invalidates every anterior-lead interpretation:
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Midway between V2 and V4 |
| V4 | 5th intercostal space, midclavicular line |
| V5 | Anterior axillary line, level with V4 |
| V6 | Midaxillary line, level with V4 |
Note that V3 is placed after V4, because it is defined relative to it.
2. Calibration and Measurement
Standard paper speed is 25 mm/s and standard gain is 10 mm/mV.
| Grid element | Time | Voltage |
|---|---|---|
| Small box (1 mm) | 0.04 s (40 ms) | 0.1 mV |
| Large box (5 mm) | 0.20 s (200 ms) | 0.5 mV |
| Five large boxes | 1.00 s | — |
Rate estimation. For a regular rhythm, divide 300 by the number of large boxes between consecutive R waves (300, 150, 100, 75, 60, 50). For an irregular rhythm, count QRS complexes in a 6-second strip and multiply by 10.
Normal adult intervals:
| Interval | Normal | Meaning |
|---|---|---|
| P wave | < 120 ms, < 2.5 mm | Atrial depolarization |
| PR | 120-200 ms | Atrial depolarization + AV nodal delay + His-Purkinje |
| QRS | < 120 ms (normal ≤ 110 ms) | Ventricular depolarization |
| QT | Rate-dependent; QTc < 450 ms men, < 460 ms women | Depolarization + repolarization |
| ST segment | Isoelectric | Plateau (phase 2) |
Bazett's correction: QTc = QT ÷ √RR (RR in seconds). Bazett over-corrects at fast rates and under-corrects at slow rates, which is why it is unreliable above about 100 beats per minute.
3. Axis Determination
The mean QRS axis is the average direction of ventricular depolarization in the frontal plane.
| Axis range | Name |
|---|---|
| −30° to +90° | Normal |
| −30° to −90° | Left axis deviation |
| +90° to +180° | Right axis deviation |
| −90° to −180° (+180 to +270) | Extreme/northwest axis |
The two-lead quadrant method
Inspect the net polarity of the QRS in lead I and lead aVF:
| Lead I | Lead aVF | Quadrant |
|---|---|---|
| Positive | Positive | Normal (0 to +90°) |
| Positive | Negative | Left axis deviation (check lead II: if lead II is also negative, true LAD) |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis |
Refinement: the axis is approximately perpendicular to whichever lead is most isoelectric (equiphasic). If lead aVL is isoelectric (−30°), the axis is near +60° or −120°; the polarity in lead I resolves which.
Causes
| Left axis deviation | Right axis deviation |
|---|---|
| Left anterior fascicular block | Left posterior fascicular block |
| Inferior myocardial infarction | Right ventricular hypertrophy |
| Left ventricular hypertrophy | Chronic lung disease, pulmonary embolism |
| Some right-sided accessory pathways | Lateral MI; some left-sided accessory pathways |
| Ostium primum ASD; hyperkalemia | Ostium secundum ASD; normal in children and thin adults |
4. Patterns the EP Specialist Must Recognize
Bundle branch block (QRS ≥ 120 ms)
| Right bundle branch block | Left bundle branch block | |
|---|---|---|
| V1 | rSR′ ("rabbit ears"), terminal R | Broad QS or rS |
| V6 / I | Wide, slurred S wave | Broad, notched, monophasic R |
| Septal q in I, V6 | Preserved | Absent |
| Repolarization | Discordant T in V1-V3 | Discordant T in lateral leads |
The terminal portion of the QRS points toward the delayed ventricle: rightward and anterior in RBBB, leftward and posterior in LBBB. This is a pure vector deduction and the exam expects it to be reasoned rather than memorized.
Fascicular blocks: left anterior fascicular block gives left axis deviation beyond −45° with qR in aVL and rS in II, III, aVF, and a normal or minimally widened QRS. Left posterior fascicular block gives right axis deviation with rS in lead I and qR in III, and requires exclusion of right ventricular hypertrophy.
Chamber enlargement
- Right atrial enlargement (P pulmonale): P wave ≥ 2.5 mm tall in lead II.
- Left atrial enlargement (P mitrale): P wave ≥ 120 ms in lead II, often notched, with a terminal negative deflection in V1 ≥ 1 mm × 40 ms.
- Left ventricular hypertrophy (Sokolow-Lyon): S in V1 + R in V5 or V6 > 35 mm.
- Right ventricular hypertrophy: R > S in V1, right axis deviation, and right atrial enlargement.
Pre-excitation localization
In manifest Wolff-Parkinson-White the delta wave is the initial slurred ventricular activation coming through the accessory pathway, and its vector points away from the pathway insertion.
| Delta wave in V1 | Pathway side |
|---|---|
| Positive (dominant R) | Left-sided (classically "type A") |
| Negative (QS or rS) | Right-sided (classically "type B") |
Inferior-lead delta polarity then separates superior from inferior insertions: negative delta waves in II, III, and aVF indicate an inferior (inferoseptal or posterior) insertion, while positive inferior delta waves indicate a superior/anterior insertion.
Ventricular tachycardia origin
The same vector logic localizes a ventricular focus:
- LBBB morphology in V1 → origin in the right ventricle or the interventricular septum.
- RBBB morphology in V1 → origin in the left ventricle.
- Inferior axis (tall R in II, III, aVF) → outflow tract origin (RVOT or LVOT).
- Superior axis (negative in II, III, aVF) → apical or inferior origin.
- Precordial transition earlier than V3 with an LBBB pattern suggests an LVOT rather than RVOT origin.
5. Artifact and Lead Misplacement
Lead misplacement is the most common cause of a spurious ECG diagnosis, and the EP lab — where electrodes compete with defibrillation pads, ICE cables, and drapes — is where it happens most.
| Error | Signature |
|---|---|
| Right arm / left arm reversal | Negative P, QRS, and T in lead I; aVR looks like a normal aVL |
| Arm / leg reversal | Near-flat line in one limb lead |
| V1-V2 placed too high (2nd-3rd space) | Pseudo-incomplete RBBB, inverted P in V1-V2, spurious anterior Q waves |
| Precordial leads reversed | Loss of orderly R-wave progression, abrupt transition reversal |
| 60 Hz interference | Uniform fine oscillation on all leads; check ground and unplug nearby devices |
| Baseline wander | Slow undulation from respiration, patient motion, or poor electrode contact |
| Muscle tremor | Irregular high-frequency noise, worst in limb leads |
The correct response to noise is always to fix the source — electrode contact, skin preparation, cable routing — before reaching for filters, because aggressive filtering distorts the ST segment and can eliminate the low-amplitude signals the study depends on.
A 12-lead ECG shows a QRS that is predominantly positive in lead I, predominantly negative in lead aVF, and predominantly negative in lead II. What is the axis, and what is the most likely conduction explanation if the QRS duration is 100 ms?
During a pre-excitation workup, the surface ECG shows a short PR interval with a dominant positive delta wave and tall R wave in V1, plus negative delta waves in leads II, III, and aVF. Where is the accessory pathway most likely inserted?
A 12-lead ECG recorded in the EP lab shows an inverted P wave, inverted QRS, and inverted T wave in lead I, while the precordial leads appear entirely normal with orderly R-wave progression. What is the most likely explanation?