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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
THANK YOU