Ecg interpretation

ECG interpretation

Dr Sudhir DevHouse OfficerDept. Of GP and Emergency Medicine

Objectives

• Justify the reasons for performing an ECG

• Develop a structured approach to interpreting an ECG

• Practice interpreting ECGs

The ECG

“The ECG (electrocardiogram) is a transthoracic interpretation of the electrical activity of the

heart.”

The ECG

SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

Lead Placement

EKG Distributions

• Anteroseptal: V1, V2, V3, V4• Anterior: V1–V4• Anterolateral: V4–V6, I, aVL• Lateral: I and aVL• Inferior: II, III, and aVF• Inferolateral: II, III, aVF, and V5 and V6

Why perform an ECG?

• It’s part of the admission bundle• Indicated by the patient’s symptoms

- symptoms of IHD/MI- symptoms associated with dysrhythmias

• Indicated by the patient’s examination findings- cardiac murmur

ECG Interpretation• Quality of ECG?

• Rate • Rhythm• Axis

• P wave• PR interval• QRS duration• QRS morphology• Abnormal Q waves• ST segment• T wave• QT interval

Quality of the ECG

• Patient name • Date of the ECG• Is there any interference?• Is there electrical activity from all 12 leads?

Calibration• Standard calibration of the ECG is 10mm/mV and normal speed 25mm/sec. At this calibration, 1 miliVolt calibration signal is expected to produce a rectangle of 10 mm height and 5 mm width.

• When ECG waves are tall, the R or S waves may extend into the QRS complexes above or below them. To prevent this superimposition, the whole ECG may be calibrated at 5mm/mV .

LOW VOLTAGELow voltage of the limb leads is present when the amplitude of the QRS complex in each of the three standard limb leads (I, II, and III) is <5 mm . Low voltage of all leads is diagnosed when the average voltage in the limb leads is <5 mm and the average voltage in the chest leads is <10 mm.

An example of correct calibration at 10 mm/mV and speed of 25 mm/sec : The calibration signal is a rectangle making 90 degrees of angles.

ECG interpretation• Quality of ECG?

• Rate • Rhythm• Axis

• P wave• PR interval• QRS duration• QRS morphology• Abnormal Q waves• ST segment• T wave• QT interval

Rate

• The 6 Second Rule: All but very slow heart rates can be determined by counting the number of cycles that occur within 6 seconds, and then multiplying that number by 10.

• The rule of 300 : 300/number of big squares between R waves

• Rate is either: - Normal– Bradycardia – HR < 40bpm– Tachycardia HR > 120bpm

Rate

Rhythm

• Look for:• Are there P waves?• Are they regular?• Does one precede every QRS complex?

Rythm could either be of sinus or non sinus in origin . Having P wave is sinus origin. Without P wave or buried with preceeding T wave is either Venticular or Supraventricular

origin.

AxisDefinition:the mean direction of electrical forces in the frontal plane ( limb leads) as measured from the zero reference point (lead 1)Normal values

P wave: 0 to 75 degreesQRS complex: -30 to 90 degressT wave: QRS-T angle <45 degrees frontal or <60 degrees precordial

The Quadrant Approach

• QRS up in I and up in aVF = Normal

Axis

Positive in I and II = NORMAL

Positive in I and negative in II = LAD

Negative in I and positive in II = RAD

D/D of Right Axis Deviation (RAD)

• Differential diagnosis Right Ventricular Hypertrophy (RVH) — most common• Left Posterior Fascicular Block (LPFB) — diagnosis of exclusion• Lateral and apical MI• Acute Right Heart Strain, e.g. acute lung disease such as pulmonary embolus• Chronic lung disease, e.g. COPD• Dextrocardia• Ventricular pre-excitation (WPW) — LV free wall accessory pathway• Ventricular ectopy• Hyperkalemia• Sodium-channel blockade, e.g. tricyclic toxicity• Normal in infants and children• Normal young or slender adults with a horizontally positioned heart can also

demonstrate a rightward QRS axis on the ECG.

D/D ofLeft Axis Deviation (LAD)

• left ventricular hypertrophy (LVH)• Left Anterior Fascicular Block (LAFB) — diagnosis

of exclusion• LBBB• inferior MI• ventricular ectopy• paced beats• Ventricular pre-excitation (WPW)

ECG interpretation• Quality of ECG?

• Rate • Rhythm• Axis

• P wave• PR interval• QRS duration• QRS morphology• Abnormal Q waves• ST segment• T wave• QT interval

P wave• The P wave is the first positive deflection on the ECG• It represents atrial depolarisationCharacteristics of the Normal Sinus P Wave• Morphology• Smooth contour• Monophasic in lead II• Biphasic in V1• Axis• Normal P wave axis is between 0° and +75°• P waves should be upright in leads I and II, inverted in aVR• Duration• < 120 ms• Amplitude• < 2.5 mm in the limb leads,• < 1.5 mm in the precordial leads

P wave abnormalities

Common P wave abnormalities include:• P mitrale (bifid P waves), seen with left atrial

enlargement.• P pulmonale (peaked P waves), seen with right atrial

enlargement.• P wave inversion, seen with ectopic atrial and

junctional rhythms.• Variable P wave morphology, seen in multifocal atrial

rhythms.

P mitrale

P pulmonale

PR interval

• The PR interval is the time from the onset of the P wave to the start of the QRS complex.

• It reflects conduction through the AV node.

• Start of P wave to start of QRS complex

• Normal = 0.12 - 0.2 seconds (3-5 small squares)

• Decreased = can indicate an accessory pathway

• Increased = indicates AV block (1st/2nd/3rd)

ECG interpretation• Quality of ECG?

• Rate • Rhythm• Axis

• P wave• PR interval• QRS duration• QRS morphology• Abnormal Q waves• ST segment• T wave• QT interval

QRS complexMain Features to Consider• Width of the complexes: Narrow versus broad.• Voltage (height) of the complexes.• Spot diagnoses: Specific morphology patterns that are important to

recognise.

• Normal = <0.12 seconds• Narrow complexes (QRS < 100 ms) are supraventricular in origin. • Broad complexes (QRS > 100 ms) may be either ventricular in origin, or

may be due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade)

Low Voltage QRS

• Low Voltage• The QRS is said to be low voltage when:• The amplitudes of all the QRS complexes in

the limb leads are < 5 mm; or• The amplitudes of all the QRS complexes in

the precordial leads are < 10 mm

QRS complex

• Is there LVH?• Probably the most commonly used are the Sokolov-Lyon criteria (S wave

depth in V1 + tallest R wave height in V5-V6 > 35 mm).

>35mm is suggestive of LVH

Q waves

• Q waves are allowed in V1, aVR & III

• Pathological Q waves can indicate previous MI

ECG interpretation• Quality of ECG?

• Rate • Rhythm• Axis

• P wave• PR interval• QRS duration• QRS morphology• Abnormal Q waves• ST segment• T wave• QT interval

ST segment• The ST segment is the flat, isoelectric section of the ECG between the end of the S

wave and the beginning of the T wave.• It represents the interval between ventricular depolarisation and repolarisation.• The most important cause of ST segment abnormality (elevation or depression) is

myocardial ischaemia / infarction.

• ST depression (upsloping, horizontal and downsloping)- downsloping or horizontal = ABNORMAL

• ST elevation- infarction - pericarditis (widespread)

ST segment depression

ST segment elevation

T wave• Small = hypokalaemia

• Tall = hyperkalaemia

• Inverted/biphasic = ischaemia/previous infarct

• Hyperkalemia: Potassium reduces myocardial excitability, with depression of pacemaking and conducting tissues.

• K+ >5.5 meq/l is a/w repolarization abnormalities causing peaked T waves (earliest sign of hyperkalemia)

• K+>6.5 a/w progressive paralysis of artia , ECG shows wide and flat P wave, incerase PR and P wave eventually dissapear.

• K+ more than 7 causes conduction abnormalities and bradycardia and eventually asystole.

T wave

T wave

T wave

QT interval

• Start of QRS to end of T wave

• Needs to be corrected for HR

• Normal QTc = < 400ms

• Long QT can be genetic or iatrogenic

QT interval

Blocks

AV blocks

First degree block PR interval fixed and > 0.2 sec

Second degree block, Mobitz type 1 PR gradually lengthened, then drop QRS

Second degree block, Mobitz type 2 PR fixed, but drop QRS randomly

Type 3 block PR and QRS dissociated

Some important ECG examples

Lateral MI

Reciprocal changes

Inferolateral MI

ST elevation II, III, aVF

ST depression in aVL, V1-V3 are reciprocal changes

Anterolateral / Inferior Ischemia

LVH, AV junctional rhythm, bradycardia

Left Bundle Branch Block

Monophasic R wave in I and V6, QRS > 0.12 secLoss of R wave in precordial leadsQRS T wave discordance I, V1, V6Consider cardiac ischemia if a new finding

Right Bundle Branch Block

V1: RSR prime pattern with inverted T waveV6: Wide deep slurred S wave

First Degree Heart Block, Mobitz Type I (Wenckebach)

PR progressively lengthens until QRS drops

Supraventricular Tachycardia

Narrow complex, regular; retrograde P waves, rate <220

Retrograde P waves

Right Ventricular Myocardial Infarction

Found in 1/3 of patients with inferior MI

Increased morbidity and mortality

ST elevation in V4-V6 of Right-sided EKG

Ventricular Tachycardia

Prolonged QT

QT > 450 ms

Inferior and anterolateral ischemia

Second Degree Heart Block, Mobitz Type II

PR interval fixed, QRS dropped intermittently

Acute Pulmonary Embolism

SIQIIITIII in 10-15%

T-wave inversions, especially occurring in inferior and anteroseptal simultaneously

RAD

Wolff-Parkinson-White Syndrome

Short PR interval <0.12 secProlonged QRS >0.10 secDelta waveCan simulate ventricular hypertrophy, BBB and previous MI

Hypokalemia

U wavesCan also see PVCs, ST depression, small T waves

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