- Clinical science
Electrocardiography (ECG) is an important diagnostic tool in cardiology. ECG uses external electrodes to measure the electrical conduction signals of the heart and record them as characteristic lines. These lines allow the axis, rate, and rhythm, as well as the amplitudes of specific parts of the heart (e.g., the P wave, PR interval, QRS complex, ST segment) to be examined–all important interpretive criteria. This learning card provides an overview of the most essential components of the ECG.
Leads: A 12-lead ECG with six limb leads (I, II, III, aVL, aVF, aVR ) and six precordial leads (V1–V6) is standard.
- Interpretation of the limb leads
- Interpretation of the precordial leads
- For ECGs, a paper speed of 25 mm/s is usually used in the United States: 1 mm = 0.04 s
- Alternatively, in other countries a paper speed of 50 mm/s is used: 1 mm = 0.02 s
- Unfortunately, the grids on the graph paper are sometimes faded or not used at all!
- Amplitude: 1 mm (vertical) = 0.1 mV
- Definition: : A continuous, ambulatory battery operated ECG worn by patients for 24-48 hours
- Daily or near-daily symptoms of:
- Patients who are unable to use other ambulatory ECG monitoring devices
- Effect of new atrial fibrillation rate control medication (e.g., metoprolol)
- Screening for ventricular ectopy in high-risk patients (e.g., cardiomyopathy, acute coronary syndrome)
- Daily or near-daily symptoms of:
- Common metrics
- When interpreting an ECG, it is important to keep the individual patient in mind and, if possible, to compare it with previous ECGs.
- A personalized algorithmic approach to ECG interpretation that assesses every aspect of the ECG ensures pathologies are not overlooked.
- The heart rate (i.e., the pulse felt on physical exam) can be calculated by assessing the QRS complexes on ECG (correlating with ventricular systole).
- The atrial rate is sometimes calculated (e.g., in assessing some supraventricular arrhythmias). Calculated by applying the same methods to the P waves (correlating with atrial systole) that are used when assessing QRS complexes
If the QRS rhythm is regular; (see below), then the heart rate can be estimated by dividing 300 by the number of “large” (e.g., 5 mm) boxes between successive QRS complexes, or by counting the number of QRS complexes in 6 seconds and multiplying by 10.
- Careful! This method is only a rough estimate.
- Only applies if paper speed is 25 mm/s
Alternatively, the heart rate may be estimated by multiplying 5 with the number of QRS complexes on the rhythm strip of a standard ECG.
- Careful! This method of measuring the heart rate is not very precise and only for initial orientation.
- Only applies to a paper speed of 25 mm/s. Multiply with 10 for a paper speed of 50 mm/s!
- A more exact method to calculate the heart rate (HR)
- If paper speed is 25 mm/s: HR = 150/RR interval in cm
- If paper speed is 50 mm/s: HR = 300/RR interval in cm
- The heart rate is often measured with an ECG ruler in clinical settings.
- If the QRS rhythm is regular; (see below), then the heart rate can be estimated by dividing 300 by the number of “large” (e.g., 5 mm) boxes between successive QRS complexes, or by counting the number of QRS complexes in 6 seconds and multiplying by 10.
- Normal heart rate: 60–100/min
- Tachycardia: > 100/min (see also )
- Bradycardia: < 50-60/min (see also )
- The heart rhythm is assessed by evaluating the appearance and relationship of the various waves (e.g., P, QRS, and T) on ECG.
P wave assessment
- ls it visible in any lead?
- Determine the atrial heart rate (i.e., PP interval).
- Determine the morphology of the P waves.
Relationship of P to QRS
- A 1:1 relationship of P with QRS is normal. If not present:
A P wave before every QRS, and a QRS after every P are normal.
- If no QRS after a P → ectopic atrial beat or AV block
- If QRS without preceding P → ectopic junctional or ventricular beat
- Normal duration: 0.07–0.10 seconds
- Wide QRS: > 0.12 seconds or 3 “small” blocks
- If normal in all leads → supraventricular rhythm
- If wide (> 0.12 sec, or 3 “small” blocks) → aberrant conduction, pre-excitation, ventricular origin, or ventricular pacing
- Some arrhythmias have characteristic features which can help in diagnosis (see ).
- Associate any findings with your patient (e.g., history of heart disease, drug ingestion, etc.)
- P wave assessment
- Criteria for a sinus rhythm
- The axis represents the spread of intraventricular electrical activity projected along the frontal plane (determined from limb leads I, II, III, aVR, aVL, aVF).
The key here is to evaluate the QRS complex, and specifically whether it is positive or negative.
- Positive: : if the area above the isoelectric line (i.e., the amplitude) is larger than the area beneath
- Negative: : if the area below the isoelectric line is larger than the area above
- The main QRS vector (position of the electrical axis of the heart) is close to the lead with the highest positive QRS amplitude.
- The normal axis of the heart is between -30° and +90°.
- A rapid approximation of the axis may be made by assessing the QRS in leads I and aVF:
|Left-axis deviation||+||-||(-30°)–(-90°)||LVH, LBBB, LAFB, inferior MI|
|Right-axis deviation||-||+||(+90°)–(+180°)||RVH, LPFB, lateral MI, RV strain (e.g., PE), chronic lung disease (e.g., COPD)|
|Extreme right-axis deviation||-||-||(-90°)–(-180°)||Severe RVH, lateral MI|
- Exact determination of the axis is done with the Cabrera circle
- Find a limb lead in which the QRS complex is isoelectric.
- Find the angle of that lead on the Cabrera circle, e.g., the angle of lead I is 0°, aVF is 90°, etc.
- The approximate axis is either +90° or -90° from that angle.
- Utilize another limb lead to assess which of the two possible angles is correct.
- Find the two possible axes on the Cabrera circle and find the limbs that are nearest to them.
- The axis whose nearest limb lead QRS complex is positive is the approximate axis of depolarization.
|P wave||Interpretation||Pathophysiology||Possible etiology|
| || |
Effect of right atrial enlargement
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Effect of left atrial enlargement
| || |
Effect of biatrial enlargement
- The QRS complex represents the electrical conduction in the ventricles and corresponds approximately to ventricular systole.
Interpretation of the duration
- ≤ 100 ms = physiological
- 100–110 ms = incomplete bundle branch block (BBB)
≥ 120 ms = complete bundle branch block (BBB)
- Signs of right bundle branch block (RBBB) are primarily seen in leads V1,2
- Signs for left bundle branch block (LBBB) are primarily seen in leads I, V5,6
Interpretation of amplitude
- Amplitude of the QRS complex in the precordial leads is used to assess for ventricular hypertrophy
Various unique grading criteria exist for electrocardiographic determination of ventricular hypertrophy. The Sokolow-Lyon criteria are utilized below:
- Left ventricular hypertrophy (LVH): SV1 or 2 + RV5 or 6 ≥ 3.5 mV
- Right ventricular hypertrophy (RVH): RV1 or 2 + SV5 or 6 ≥ 1.05 mV
R1ght 5ignS (R in V1 and S in V5) → Sign of right ventricular hypertrophy!
Remember that patients with myocardial hypertrophy may not exhibit these signs on their ECG: These may become apparent later in the course of the disease or they may even be absent in some cases (e.g., severe obesity). However, ECG changes associated with clinical signs confirm the diagnosis of hypertrophy!
Intrinsicoid deflection = start of the last downstroke within the QRS complex (final negativity); tool for distinguishing between LBBB and RBBB
- A delayed start of the final negativity indicates delayed ventricular conduction and therefore suggests a bundle branch block
- Special type: bifascicular block
Interpretation of R-wave progression and S-wave persistence
- In healthy patients, the R wave increases from V1/2 to V6, whereas the S wave continuously decreases
- The time between the beginning of the P wave and the beginning of the Q wave
- The PR interval represents atrioventricular transmission.
|PR interval ≤ 0.2 s||Normal|
|PR interval > 0.2 s|
|PR intervals become progressively longer (but PP intervals remain constant) until a dropped QRS complex occurs after a regular atrial depolarization.||,|
|Constant PR intervals (which are usually normal but may be prolonged) followed by one or more non-conducted P waves.||,|
|P waves and QRS complexes occur independently of each other, but in regular intervals → complete dissociation of P waves and QRS complexes.|
- The Q wave represents the beginning of ventricular depolarization, correlating with the left → right depolarization of the interventricular septum.
A narrow (≤ 40 ms) Q wave is physiological in:
- All limb leads
- V5 and V6
Pathological Q waves are characteristically:
- Abnormally wide (≥ 40 ms)
- Abnormally deep (≥ 2 mV or > 25% of the R wave amplitude) or, detectable in V1–V3
- The ST segment represents the interval between ventricular depolarization and repolarization
- It is physiologically horizontal on the isoelectric line.
- An ST elevation is significant if:
The hallmark ECG finding of myocardial infarction!
- If significant ST elevations are present in ≥ 2 anatomically contiguous leads (corresponding to occlusion of a specific artery)
- The ischemia can be localized by which leads show ST elevation:
- Diffuse ST elevations suggest pericarditis
- LBBB may cause ST elevations due to repolarization abnormalities, therefore ST elevation cannot be used to diagnose MI in the presence of an LBBB.
- Small, concave ST elevations may be a normal finding in young, healthy adults due to early repolarization.
- From descending R: The most important cause is a myocardial infarction.
- From (deep) S:
- Epidemiology: : most common in Asian males
- Often an incidental finding; during a routine check, as patients are mostly asymptomatic
- Sudden cardiac death
Brugada pattern on ECG: Pseudo-RBBB with ST elevation in leads V1-2
- J wave ≥ 2 mm, negative T wave, coved type ST elevation with gradually descending terminal portion or
- J wave ≥ 2 mm, positive or biphasic T wave, saddleback ST elevation with terminal portion elevated ≥ 1 mm or
- J wave ≥ 2 mm, positive T wave, saddleback ST elevation with terminal portion elevated < 1 mm
- Rule out underlying heart disease (e.g., stress test and echocardiography)
- Brugada pattern on ECG: Pseudo-RBBB with ST elevation in leads V1-2
- Implantable cardiac defibrillator (ICD) placement
- Screen all 1st-degree relatives annually with clinical exam and ECG
- Subendocardial myocardial ischemia; (MI) (i.e., NSTEMI)
- Stress-induced MI (sign of coronary artery disease)
- Reciprocal change from MI
- Ventricular hypertrophy
- The shape of the ST segment suggests the etiology of the depression.
- Downsloping ST depression or horizontal ST depression: myocardial ischemia
- Upsloping ST depression: mild manifestations may be normal, but may also occur in cases of tachycardia; sign of coronary heart disease if significantly manifested
- Sagging type ST-segment depression: characteristic of digoxin intake
The stages of myocardial ischemia are associated with characteristic (but variable) ECG findings:
- : very early and transient; usually have disappeared by the time ECG is performed
- ST elevation at the J point: point at which the QRS complex completes and returns to the isoelectric line (i.e., the intersection of the S wave and the ST segment)
- Progressive ST segment elevation, with added convexity
- ST merges with T wave; , forming a QRS-T segment; (i.e., tombstone): usually with associated reciprocal ST depressions (see )
- ST segment returns to isoelectric line, Q wave develops; , and R wave loses amplitude
- T-wave inversion
- Progressive Q wave deepening and R wave shrinkage
- T wave may or may not return to upright position
- The T wave represents the electrical repolarization of the ventricles
- The T wave is physiologically concordant to the QRS complex: positive if the QRS complex is positive or negative if the QRS complex is negative.
- Small T-wave inversions may be normal in the limb leads
- Differential diagnosis
- The shape of the T wave may help to narrow the differential diagnosis.
- Pre-terminal T-wave inversion: If the T wave is bisected, it points to the left. It may occur in:
- Terminal T-wave inversion: If the T wave is bisected, it points either to the right or upwards. It may occur in:
- Tall, narrow, symmetrically-peaked
- High vagal tone
- Broad, asymmetrically-peaked
- Differential diagnosis
- Measured from the beginning of the Q wave to the end of the T wave
- Represents the entire duration of intraventricular depolarization
- Varies with heart rate, so correction for the heart rate is necessary (=QTc )
- QTc normally < 350–440 ms
Possible differential diagnoses include:
- Inflammatory heart diseases (myocarditis, pericarditis)
- Bundle branch block
- High vagal tone
- Rare congenital syndromes (e.g., congenital long QT syndromes such as Romano-Ward syndrome)
- Drug side effect (e.g., antiarrhythmic agents, antidepressants, phenothiazines, 1st-generation antihistamines)
Possible differential diagnoses include:
- Digoxin effect
- Increased sympathetic tone (e.g., hyperthyroidism or fever)