ECG Analysis - developinganaesthesia



ECG ANALYSIS

“Absence Makes the Heart Grow Fonder”, oil on canvas, John William Godward, 1912.

The Great Nineteenth century Neo-Classicist painters such John William Godward were able to beautifully illustrate the romantic inner “affairs of the heart”. In the medical field we capture the inner “affairs of the heart” by the somewhat less romantic medium of the electrocardiograph.

ECG ANALYSIS

INDEX

1. Normal parameters.

2. P Wave Abnormalities

3. P-R Interval Abnormalities

4. P-R Segment Abnormalities

5. Q Wave Abnormalities

6. QRS Abnormalities

7. Q-T Interval Abnormalities

8. T Wave Abnormalities

9. U Waves Abnormalities

10. Left Bundle Branch Block

11. Right Bundle Branch Block

12. Fascicular Block

13. Left Axis Deviation

14. Right Axis Deviation

15. ST Segment Abnormalities

16. Abnormalities of AVR

17. Ventricular Hypertrophy

For paediatric considerations see separate guidelines

For ECG changes associated with specific conditions, see separate individual guidelines for these conditions.

Normal Parameters

Standard tracing speed and ECG grid spacing:

ECG grid makings 1

1. The standard ECG trace velocity is 25 mm per second.

● One small square is 1 mm.

● One large square is 5 mm.

2. The horizontal axis is a measure of time.

● Each small square is 0.04 seconds

● Each large square is 0.20 seconds.

3. The vertical axis is a measure of voltage and polarity.

● Each small square is 0.1 mV.

● Each large square is 0.5 mV.

● 1 millivolt is therefore 10 mm or 2 large squares.

Morphology of the Electrical Tracing:

P Wave

The P wave represents depolarization through both the atria.

Characteristics of the normal P wave include:

1. Width:

● Duration is < 0.12 seconds, (< 3 small squares)

2. Amplitude:

● Amplitude is ≤ 3 mm (3 small squares)

3. Polarity:

● Upright (ie positive) in I, II, AVF and V4-V6

● Inverted in AVR

● May be positive, negative or biphasic in III, AVL, V1-V3

4. Morphology:

● Shape, is normally smoothly curved (as opposed to an abnormal notched or peaked)

PR Interval

● This is measured from the beginning of the P wave to the beginning of the Q wave (or the first ventricular deflection).

● This represents the length of time taken for the electrical impulse to travel from the atria to the ventricles.

● The normal PR interval is 0.12 to 0.20 seconds, (or 3-5 small squares)

PR Segment

● This is measured from the end of the P wave to the beginning of the QRS complex. It is part of the PR interval.

● The PR segment is normally isoelectric.

Q Wave

● A normal q (usually expressed as a small letter) wave is often normally found in leads in leads I, III, aVL, V5, and V6 2

1. Width

● It is less than 0.04 seconds (1 small square) in duration.

2. Amplitude:

● It is less than 25% of the amplitude of the R wave.

3. Polarity

● By definition a q wave has negative polarity.

QRS Interval

Nomenclature in QRS complexes:

● Q wave: Any initial negative deflection

● R wave: Any positive deflection

● S wave: Any negative deflection after an R wave

1 Width:

● The normal QRS width is ≤ 3 mm (or 3 small squares.

● Note that often there will not be a definite q wave at the beginning or a definite S wave at the end of the complex. The “QRS” in these cases is measured from the moment it leaves the iso-electric line to the moment it returns.

● The QRS represents the intra-ventricular conduction time.

2. Amplitude:

The QRS amplitude (voltage) can vary under normal circumstances.

It may be taller in:

● The young

● The thin

It may be smaller in:

● The obese

● COPD, (hyper-expanded lung) may cover the heart.

Its amplitude will also be affected in a number of pathological conditions, (see below)

Normal Measurements

● The upper limit of normal in a pre-cordial lead is 30 mm.

● The upper limit of normal in a limb lead is 20 mm.

In the pre-cordial leads the voltage of the QRS complex should not be less than the following: 1

● V1 and V6 6 mm

● V2 and V5 8 mm

● V3 and V4 10 mm

3. Polarity:

● Positive to equiphasic: I, II, V3-V6

● Positive, negative or equiphasic: AVL and AVF

● Negative: AVR

4. Transitional zone:

● The R wave in the precordial leads steadily increases in amplitude from lead V1 to V6, with a corresponding decrease in S wave depth, culminating in a predominantly positive complex in V6. Thus, the QRS complex gradually changes from being predominantly negative in lead V1 to being predominantly positive in lead V6

● Around V3-V4 the R and S waves are roughly equiphasic and this defines the transitional zone.

ST segment

● This is the interval between the end of the QRS complex and the beginning of the T wave.

● It represents part of the re-polarization phase of the heart and is normally iso-electric but may slant slightly upward into the T wave.

● Elevation or depression of this segment is highly significant, (see below) A non-pathological elevation of the ST segment may be seen with benign early repolarisation which is particularly common in young males, athletes, and Africans. 2

QT Interval

● This is an important measurement. It is the interval from the beginning of the Q wave (or ventricular complex) to the end of the T wave.

● It represents the sum of depolarization and re-polarization periods of the heart.

The normal QT interval is depended on the age of the patient and the heart rate. As the heart rate increase the QT interval shortens and as it slows down the QT lengthens.

The normal Q-T interval range: 0.35 – 0.43 seconds.

The normal QT interval can be corrected for variations in heart rate by using Bazzet’s formula. The corrected QT is symbolized by Q-Tc.

Bazett’s formula:

● Q-Tc = Q-T duration (in seconds) divided by the square root of the R-R interval.

● The Q-Tc corresponds to Q-T duration (in seconds) at a heart rate of 60.

As a rough estimate:

● Q-T should be < ½ the R-R interval (at rates of 0.12 seconds (> 3 squares), (in complete blocks)

2. V1 has:

● A QS morphology, (without antero-septal MI)

Or

● An rS morphology, (with antero-septal MI)

There may also be a notch within the complex (W-shaped)

3. V6 has:

● A monophasic R wave (no Q wave and no S wave) (without antero-septal MI)

Or

● A qR morphology, (with antero-septal MI)

There may also be a notch within the complex (M-shaped)

4. T wave discordance:

● The T wave should be deflected opposite the terminal deflection of the QRS complex. This is known as appropriate T wave discordance with bundle branch block. A concordant T wave may suggest ischemia or myocardial infarction.

5. Left axis deviation

Incomplete LBBB:

● Incomplete LBBB looks like “normal” LBBB but the QRS duration is within normal limits, with less ST-T wave changes. This is often a progression of LVH.

Causes of LBBB

1 Myocardial infarction

2. Extensive CAD

3. Hypertension

4. Primary disease of the cardiac electrical conduction system

5. Cardiomyopathy

Right Bundle Branch Block

Morphology of RBBB

1. QRS widened > 0.12 seconds (> 3 squares), (in complete blocks)

2. V1 has

A triphasic complex rSR/, (without antero-septal MI)

Or

A biphasic complex (QR), (with antero-septal MI)

3. V6 has:

qRS (a deep terminal S wave), (without antero-septal MI)

Or

RS (a deep terminal S wave), (with antero-septal MI)

See below.

4. T wave discordance:

● The T wave should be deflected opposite the terminal deflection of the QRS complex. This is known as appropriate T wave discordance with bundle branch block. A concordant T wave may suggest ischemia or myocardial infarction.

5. Right axis deviation.

Incomplete RBBB looks like “normal” RBBB but has a QRS duration within normal limits with the same terminal QRS features. This is often a normal variant.

Causes of RBBB

1. May be a normal variant.

2. Myocardial infarction

3. Hypertension.

4. Myocarditis

5. Primary disease of the cardiac electrical conduction system

6. Cardiomyopathy.

7. Cor pulmonale due to chronic lung disease

8. ASD

Fascicular Block

LAHB

1. Normal QRS duration.

2. Left axis deviation

3. QRS morphology:

● Small q in leads 1 and AVL

● Small r waves in the inferior leads

LPHB

1. Normal QRS duration.

2. Right axis deviation

3. QRS morphology:

● q waves in II, III and AVF

● r waves in leads I and AVL

Uni-fascicular Block

● RBBB

● LAHB

● LPHB

● 1st Degree Heart Block

Bi- fascicular Block

● RBBB and LAHB

● RBBB and LPHB

● LBBB

Tri- fascicular Block

● RBBB and LAHB and 1st Degree Heart Block

● RBBB and LPHB and 1st Degree Heart Block

● LBBB and 1st Degree Heart Block

● Alternating RBBB and LBBB

Left Axis Deviation

Causes

1. LHV

2. LAHB

3. LBBB

Right Axis Deviation

Causes

1. Body habitus:

● Pregnancy

● Obesity

2. Infants and young children

3. RVH

4. LPHB

5. RBBB

ST Segment Abnormalities

Causes of an Elevated ST Segment

Note that ST segment elevation is measured from the iso-electric line to the J point. The iso-electric point (line) is taken as the ECG trace between beats, (ie the end of the T wave to the beginning of the next P wave) 4

1. ST segment elevation myocardial infarction

● Persistent S-T elevation of > 2 small squares (2 mm) in 2 or more contiguous pre-cordial leads.

● Persistent S-T elevation of > 1 small square (1 mm) in 2 or more contiguous limb leads.

● Note the new LBBB is also considered evidence of STEMI

2. Pericarditis

3. Myocarditis

4. Prinzmetal angina

5. Brugada’s syndrome

6. LBBB

7. Ventricular aneurysm

8. Benign Early Repolarization, (BER); (normal variant).

9. Pseudo-ST elevation, such as may be seen in hyperkalemia.

Causes of a Depressed ST Segment

ST segment depression has a wide range of causes. The exact morphology of the depressed segment can give important clues to aetiology.

1. ST segment depression that is upward sloping:

This may be:

● A normal variant if the depression is mild. The normal ST segment curves very slightly upward into the beginning of the T wave. 1

● Seen in rapid tachyarrhythmias

2. ST segment depression with a horizontal base:

● This pattern is seen with myocardial ischemia. In fact an absolutely horizontal depressed ST segment, which forms a sharp angle with the T wave is highly suggestive of ischemia.

3. ST segment depression that is downward sloping:

● Strain patterns in hypertrophy.

● Digitalis effect, (the “reverse tick”).

4. ST segment depression that is sagging ie has a U shape with the nadir in the middle of the segment.

● Digitalis effect.

5. Non-specific ST segment depression:

● Hypokalemia

● Aortic dissection

● Pulmonary embolism

● Subarachnoid haemorrhage

Localization of STEMI infarction

(Source: Life in the fast lane Website)

Abnormalities of AVR

aVR has been described as the “forgotten lead” in ECG recordings.

ST segment elevation in aVR is often ignored as just being a “reciprocal change”; however it does have two important diagnostic utilities:

● In toxicology, where certain changes in the aVR lead may indicate fast sodium channel blockade.

Diagnostic features of TCA poisoning (and sodium channel blockade in general) in lead AVR include:

● A large terminal R wave in aVR

● An increased R/S ratio, (>0.7)

● In cardiology where ST elevation in the aVR lead may indicate a left main coronary artery critical stenosis.

Ventricular Hypertrophy

Ventricular hypertrophy can essentially be due to a pressure load (eg aortic stenosis or hypertension) or a volume load, (eg mitral incompetence or dilated cardiomyopathies).

A pressure load results when myocardial fibers generate increased systolic force or tension. This results in a concentric hypertrophy in which the ventricular wall thickness is increased in relation to the ventricular cavity.

A volume load results when there is an increased end-diastolic wall stress.

This leads to eccentric hypertrophy, where the left ventricular wall thickness remains normal relative to the increase in the radius of the left ventricle (chamber dilation). In this situation the systolic pressure remains unchanged.

Diagram showing the patterns of hypertrophy, normal on the left, pressure loading in the middle and volume loading on the right.

The gold standard of ventricular hypertrophy is echocardiography. However the ECG remains an invaluable aid to diagnosis of ventricular hypertrophy. Its specificity is good but sensitivity somewhat less.

Cardiac hypertrophy is an important finding as it has an association with a number of cardiovascular complications including:

● Myocardial infarction.

● Stroke

● Heart failure

● Arrhythmias and sudden death.

Left Ventricular Hypertrophy

A number of different criteria and scoring systems exist for the determination of LVH on the ECG.

A commonly used one is the Estes (or more correctly the Romhilt-Estes) system. It has reasonable specificity (85-95%), but sensitivity is low.

Estes Scoring System for Left Ventricular Hypertrophy 3

This is scored o 6 criteria:

1. Voltage criteria:

Any of: 3 points

● R or S wave in limb leads = 20 mm

● S wave in V1 or V2 = 30 mm

● R wave in V5 or V6 = 30 mm

2. ST-T wave abnormalities (a strain pattern)

● Without digitalis 3 points

● With digitalis 1 point

3. Left atrial enlargement in V1 3 points

This refers to a biphasic P wave with a deep and

broad terminal (inverted) “trough” seen in V1.

4. Left axis deviation. 2 points

5. QRS duration> 0.9 seconds 1 point

6. Instrinsicoid deflection in V5 and V6 ≥ 0.05 seconds 1 point

Instrinsicoid deflection is the distance from the onset of the QRS to the peak of the R wave.

A total of 5 points is diagnostic of left ventricular hypertrophy.

A total of 4 points represents probable left ventricular hypertrophy.

See appendix 1 below.

Right Ventricular Hypertrophy

Right ventricular hypertrophy is associated with conditions that cause right ventricular mass to begin competing with the left ventricle for the dominant overall effect on the ECG

Causes include:

1. Pulmonary hypertension from any cause.

● The most common will be COPD

2. Pulmonary stenosis

3. Some congenital heart conditions such as Tetralogy of Fallot.

ECG Changes of RVH:

ECG changes suggestive of RVH include:

1. Right axis deviation, (>90 degrees)

2. Tall or dominant R-waves in the RV leads, (V1-3)

3. Deep or dominant S-waves in LV leads, (V4-6)

4. Slight increase in QRS duration

5. ST-T changes directed opposite to QRS direction in pre-cordial and inferior leads.

6. Right atrial enlargement, (P pulmonale)

See appendix 2 below.

Appendix 1

Left ventricular hypertrophy:

Classic LVH with strain and left atrial enlargement.

Appendix 2

Right ventricular hypertrophy:

Classic RVH with strain and right atrial enlargement.

Appendix 3

Axis determination:

References

1. Conover M.B Understanding Electrocardiography 7th ed 1996.

2. Meek S. ABC of clinical electrocardiography. Introduction II - Basic terminology. BMJ. 2002 February 23; 324(7335): 470–473.

3. Romhilt DW, Estes EH Jr. A point-score system for the ECG diagnosis of left ventricular hypertrophy. Am Heart J. 1968; 75(6):752-758.

4. Hampton J.R, The ECG Made Easy, 4th ed. p. 48.

Dr J. Hayes

Reviewed 8 March 2012

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