How to Read an ECG: Waves, Intervals, Axis Deviation

A 55-year-old man with diabetes arrives at the emergency room complaining of sweating and chest pain. His ECG reveals reciprocal changes in I and aVL along with ST elevation in leads II, III, and aVF. The cath lab is turned on right away by the treating physician. Twelve leads and a few seconds of recording on this one piece of paper just saved a life. The ability to read an ECG systematically distinguishes passing from topping, and it continues to be the most tested clinical skill in NEET PG.

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An electrocardiogram, or ECG, is a graphical representation of the electrical activity of the heart over time. Atrial depolarization is represented by the P wave, ventricular depolarization by the QRS complex, and ventricular repolarization by the T wave. PR (0.12-0.20s), QRS (<0.12s), and QTc (<0.44s in men, <0.46s in women) are examples of normal intervals. The predominant direction of ventricular depolarization is indicated by axis deviation.

NEET PG RELEVANCE

ECG interpretation appears in 8-12 questions across NEET PG papers annually. Focus areas include: interval calculations, axis deviation determination, lead localization of MI, and arrhythmia recognition. Recent papers emphasize QTc calculation, Sgarbossa criteria, and Wellens syndrome identification.

An ECG – What is it?

The electrical signals produced by cardiac myocytes during their depolarization and repolarization are captured on an ECG. Consider it as a voltage versus time graph with the x-axis representing duration (in seconds) and the y-axis representing amplitude (in millivolts).

The typical ECG paper moves at a speed of 25 mm per second. Each tiny square stands for 0.1 mV vertically and 0.04 seconds (40 ms) horizontally. Each big square is equivalent to 0.5 mV and 0.20 seconds. Because of this standardization, intervals and amplitudes can be precisely measured on any machine in the world.

Electrical views from various perspectives are provided by the 12-lead ECG. The heart is seen in the frontal plane by the limb leads (I, II, III, aVR, aVL, and aVF). It is seen in the horizontal plane by the Precordial leads (V1-V6). When combined, they produce an electrical map of cardiac activity in three dimensions.

What Leads to Abnormalities in the ECG?

Changes in the ECG are indicative of underlying cardiac structural, electrical, or metabolic problems.

Repolarization patterns are altered by ischemia and infarction, resulting in pathological Q waves and ST-T abnormalities. Which coronary territory is impacted is indicated by the location of the changes. Whether caused by fibrosis, ischemia, or infiltration, conduction system disease lengthens intervals and produces bundle branch blocks.

Myocyte membrane potentials are directly impacted by electrolyte imbalances. Peaked T waves, widened QRS, and sine wave patterns are the results of hyperkalemia. By lengthening the ST segment, hypocalcemia increases the QT interval. Chamber enlargement modifies axis and raises the voltage amplitude.

Digoxin (scooped ST segments, shortened QT), amiodarone (prolonged QT), and tricyclic antidepressants (widened QRS, rightward axis shift) are among the drug effects that are frequently tested.

Comprehending ECG Waves

P-Wave

Atrial depolarization is represented by the P wave, which starts at the SA node and travels through both atria. A typical P wave has an amplitude of less than 2.5 mm and a duration of less than 0.12 seconds (three tiny squares).

P wave morphology changes with atrial pathology. P mitrale (bifid P wave in lead II, >0.12s) indicates left atrial enlargement—classic in mitral stenosis. P pulmonale (peaked P wave >2.5mm in lead II) suggests right atrial enlargement from pulmonary hypertension or tricuspid disease.

Atrial fibrillation is characterized by irregular rhythm and the absence of P waves. Atrial flutter is indicated by 300/min flutter waves with a distinctive sawtooth pattern in the inferior leads.

Complex QRS

Ventricular depolarization is recorded by the QRS complex. The typical duration is between 0.06 and 0.10 seconds. Any QRS that is longer than 0.12 seconds (three small squares) is considered abnormal ventricular conduction.

Septic depolarization is represented by the Q wave (left to right). In lateral leads (I, aVL, V5-V6), small septal Q waves are typical. A previous myocardial infarction is indicated by pathological Q waves, which are wider than 0.04s or deeper than 25% of R wave height.

From V1 to V5, the R wave progression across the precordial leads should gradually increase. Inadequate R-wave progression may indicate lead misplacement, left ventricular hypertrophy, or anterior MI. Wolff-Parkinson-White syndrome, posterior MI, or right ventricular hypertrophy all cause dominant R waves in V1.

T-Wave

Ventricular repolarization is represented by the T wave. With the exception of a VR and occasionally V1, normal T waves are asymmetric—gradual upslope, sharper downslope—and upright in the majority of leads.

The first indication of STEMI is hyperacute T waves, which appear minutes after coronary occlusion and are tall, broad, and symmetrical. Ischemia is suggested by T-wave inversion in adjacent leads, while critical LAD stenosis is indicated by Wellens syndrome (biphasic or deeply inverted T waves in V2-V3).

Hyperkalemia is suggested by narrow, peaked T waves and shortened QT. Hypokalemia causes flattened T waves, though U wave prominence is the traditional indicator.

U Wave

U waves, which are best observed in V2-V3, are tiny deflections that come after T waves. Their origin is still unknown, but they most likely result from the late repolarization of papillary muscles or Purkinje fibers. Ains debated—likely represents late repolarization of Purkinje fibers or papillary muscles.

Hypokalemia is strongly suggested by prominent U waves (>1.5mm or exceeding T wave height). They also manifest as long QT syndrome, digoxin effect, and bradycardia.

Also Read: Thyroid Hormones: Synthesis, Functions, and Clinical Scenario

How to Measure ECG Intervals?

PR Interval

Measure from the start of the P wave to the start of the QRS complex. 0.12-0.20 seconds (3-5 small squares) is the typical range.

First-degree AV block is defined by prolonged PR (>0.20s). Pre-excitation (WPW syndrome) is indicated by a shortened PR (<0.12s) with a delta wave. Ectopic atrial rhythm or accelerated AV nodal conduction is suggested by shortened PR without a delta wave.

The length of QRS

Determine the separation between the initial deflection and termination of QRS. Typical: under 0.12 seconds.

QRS 0.10-0.12s represents an incomplete bundle branch block. QRS >0.12s indicates a complete bundle branch block or ventricular rhythm. QRS >0.16s strongly suggests ventricular tachycardia over SVT with aberrancy.

QT Duration

Measure from the beginning of the QRS to the end of the T wave. This interval needs to be adjusted because it changes with heart rate.

QTc = QT / √RR (RR interval in seconds) is Bazett's formula.

Men's normal QTc is less than 440 ms, while women's is less than 460 ms. The risk of torsades de pointes is greatly increased by QTc >500 ms. Congenital long QT syndrome, hypokalemia, hypomagnesemia, hypocalcemia, and medications (antiarrhythmics, antipsychotics, antibiotics like fluoroquinolones and macrolides) are among the causes of prolonged QT.

How the Cardiac Axis Is Determined?

The predominant direction of ventricular depolarization in the frontal plane is revealed by axis determination. Learn this; almost all NEET PG papers use it.

Fast Two-Lead Technique

Examine only leads I and aVF:

• Normal axis (0° to +90°): Lead I positive, aVF positive
• Lead I positive, aVF negative: Deviation from the left axis (-30° to -90°)
• Right axis deviation (+90° to +180°): Lead I negative, aVF positive
• Extreme axis deviation: Lead I negative, aVF negative (-90° to ±180°)

Also Read: Nerve Muscle Physiology—Important Questions and Answers

Axis Deviation Causes

Left anterior fascicular block (the most frequent cause in exams), inferior MI, left ventricular hypertrophy, WPW with a right-sided pathway, and ostium primum ASD are examples of left axis deviation (LAD).

Right ventricular hypertrophy, pulmonary embolism, lateral MI, left posterior fascicular block, chronic lung disease, and ostium secundum ASD are examples of right axis deviation (RAD).

Mnemonic for LAD causes: "LAD leaves through the LEFT" — Left anterior fascicular block, Left ventricular hypertrophy.

Methodical Approach to ECG Reading

Always follow the same order. Create a habit that will come naturally when you're under pressure to do well on an exam.

Rate: Determine the number of big squares in between R waves. The rate is calculated by dividing 300 by the number of large squares. As an alternative, multiply the number of QRS complexes in the 6-second strip by ten.

Is the rhythm consistent? Do P waves exist? Does QRS come after every P? Does P come before every QRS?

Axis: Apply the previously mentioned two-lead technique.

Intervals: Systematically measure PR, QRS, and QT.

Compare: Dynamic changes are more important than static findings if older ECGs are available.

Also Read: Pressure–Volume Loop of the Cardiac Cycle Explained: Phases, Graph & ECG Correlation

High-Yield Points for NEET PG

• For regular rhythms, the heart rate is calculated as 300/number of large squares between R-R intervals.
• First-degree AV block is defined as a PR interval >0.20s; WPW syndrome is defined as a PR < 0.12s with a delta wave.
• QRS > 0.12s is always abnormal; consider ventricular origin or bundle branch block.
• RBBB pattern: wide S in V6, RSR' in V1 ("M-shaped")
• LBBB pattern: QS or rS in V1, broad notched R in V6 ("M-shaped")
• Sgarbossa criteria: ≥ 1mm concordant ST elevation = highly specific; used to diagnose STEMI in the presence of LBBB
• Bifascicular block = left axis deviation + RBBB = high-yield combination question
• Progression of hyperkalemia: Peaked T → Prolonged PR → Widened QRS → Sine wave → Asystole
• Low voltage QRS (less than 5 mm in the limb leads): Consider hypothyroidism, obesity, COPD, and pericardial effusion.
• Electrical alternans: classic for large pericardial effusion with tamponade; beat-to-beat variation in QRS amplitude

"Rate, Rhythm, Axis, Intervals, Waves" (RRAIW) is a mnemonic for systematic ECG reading.

Frequently Asked Questions

What distinguishes RBBB from LBBB on an ECG?

RBBB shows RSR' pattern (M-shaped) in V1 with a wide slurred S wave in V6. LBBB shows a broad notched R wave in lateral leads (V5-V6) with a QS pattern in V1. LBBB obscures ischemic changes and requires Sgarbossa criteria for STEMI diagnosis; RBBB typically does not.

How is heart rate determined from an ECG?

Divide 300 by the number of large squares that separate successive R waves for regular rhythms. Count QRS complexes in a 6-second strip (30 large squares) for irregular rhythms, then multiply the result by 10. For 1-2-3-4-5-6 large squares, a fast estimate uses the 300-150-100-75-60-50 sequence.

When is QTc deemed prolonged?

QTc is prolonged when it exceeds 440 ms for men or 460 ms for women. A QTc of more than 500 ms greatly raises the risk of torsades de pointes and necessitates an urgent medication review and electrolyte correction. Use Bazett's formula to calculate: QTc is calculated by dividing the square root of the RR interval (in seconds) by the QT interval.

What does an ECG's ST elevation signify?

When present in contiguous leads, ST elevation greater than 1 mm in limb leads or 2 mm in precordial leads (at J-point) indicates acute transmural ischemia (STEMI). Pericarditis (diffuse, concave-up elevation), benign early repolarization, left ventricular aneurysm, and Brugada syndrome are additional causes.

How can you determine which coronary artery is impacted?

RCA territory (85%) or LCx (15%) is indicated by inferior leads (II, III, aVF). LAD territory is indicated by anterior leads (V1-V4). LCx is suggested by lateral leads (I, aVL, V5-V6). Posterior MI (LCx or RCA) may be indicated by ST depression in V1-V3 with dominant R waves.

What do pathological Q waves mean?

A previous transmural myocardial infarction with irreversible myocyte death is indicated by pathological Q waves (>0.04s wide or >25% of R wave amplitude). They usually develop six to twelve hours after the onset of STEMI and last forever. The location's lead distribution is identical to that of the infarct territory.

PEARL CLINICAL

"The ECG doesn't tell the whole truth, but it also doesn't lie." Never forget that the best ECG interpretation occurs at the patient's bedside, not in isolation, even though ST elevation in a symptomatic patient necessitates action. Connect with cardiac biomarkers, previous tracings, and clinical presentation. Systematic reading consistently outperforms pattern recognition in both practice and NEET PG.

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