ECG (Electrocardiogram)

ECG Definition

A graphic recording electrical activity of heart, used to evaluate cardiac function and to diagnose arrhythmias and other disorders.

ECG leads

electrodes placed on the surface of the body to the electrocardiograph are called leads.

3- Lead ECG → A 3-wire lead set can monitor one of three ECG vectors (I, II, or III).

5- Lead ECG → A 5-wire lead set can monitor seven ECG vectors (I, II, III, aVR, aVL, aVF, and V)

12 Lead ECG → A 10-wire ECG lead set can monitor 12 ECG vectors (I, II, III, aVR, aVL, aVF, V1,

V2, V3, V4, V5, and V6)

12 leads usually comprise a diagnostic ECG recording. Six limb leads (3 bi-polar and 3 uni-polar) and six uni-polar precordial leads.

(a) Bipolar limb leads

The bipolar limb leads record the voltage between electrodes placed on the wrists and legs.

I lead : → left arm (+) - right arm (-)

II lead : → left leg (+) - right arm (-)

III lead : → left arm (-) - left leg (+)

(b) unipolar limb leads are aVR, aVL & VF

Colour code for limb leads / Placement of electrodes

Limb electrodes

RA -Red On the right arm , avoiding thick muscle

LA- yellow On the left arm

RL - Black On the right leg, lateral calf muscle

LL- Green On the left leg

c) unipolar chest leads are V1,V2,V3,V4,V5, and V6

ECG chest electrodes placement

V1 - Fourth intercostal space, Right sternum border.

V2 - Fourth intercostal space, left sternum border.

V3 - Midway between V2 and V4.

V4 - Fifth intercostal space left midclavicular line.

V5 - Level with V4 (5th ICS ), left anterior axillary line.

V6 - Level with V4 (5th ICS ), left mid axillary line.

ECG graph paper

ECG recorder is a graph with time represented on the X-axis and voltage represented on the Yaxis.

ECG paper speed - 25 mm/sec
Voltage calibration → 1 mV = 1 cm
Each small square → 1 mm
Each large square → 5 mm

Timings

1 small square → 0.04 sec
1 large square → 0.2 sec
25 small squares → 1 sec
5 large squares → 1 sec

NORMAL ECG

Polarization

P wave →Atrial depolarization (Atrial contraction)
QRS complex → Ventricular depolarization (Ventricular contraction)
T wave → Ventricular Repolarization (Ventricular relaxation and passive filling)
Duration P wave → <0.12 sec
Duration PR interval → <0.20 sec
Duration QRS interval → 0.08 to 0.10 sec
Duration QT interval → 0.36 to 0.40 sec

Simplified normal position of leads on ECG graph

Lead I → Upward PQRST
Lead II → Upward PQRST
Lead III → Upward PQRST
Lead aVR → Downward PQRST
Lead aVL → Upward PQRST
Lead aVF → Upward PQRST
Chest lead V1 → Downward PQRST
R wave slowly increase in height from V1 to V6

normal ECG only aVR and V1 have negative or downward deflections.

best way to learn ecg interpretation (step by step)

1. Rate

2. Rhythm

3. Cardiac Axis

4. P wave

5. PR interval

6. Q wave

7. QRS complex

8. ST segment

9. T wave

10. QT interval

11. U wave

12. Other ECG signs

1 .ECG Rate ( how to calculate heart rate from ECG ?

(A) calculate 300 rule for ECG

Rate → 300 ➗ No. of large squares between two R waves

(B) calculate 1500 rule for ECG

Rate = 1500 ➗ No. of small squares between two R waves

Rate = (Number of R waves in 6-second strips) x 10

Note: Normal R-R interval is 0.6 to 1.2 sec

If R-R interval is more than normal, it is suggestive of bradycardia
If R-R interval is less than normal, it indicates tachycardia

2. RHYTHM ( Regular or irregular? )

Look at P waves and their relationship to QRS complexes.
Lead II is commonly used
If in doubt, use a papar strip to map out consecutive beats and see whether the rate is the same further along the ECG.
Measure ventricular rhythm by measuring the R-R interval and atrial Rhythm by measuring P-P interval.

1) Locate the 'P wave

- If absent and rhythm is irregular, think of atrial fibrillation.
- If present, check rate (< 60 - bradycardia, If > 100 - tachycardia)
- In general, if narrow complex, tachycardia is present.

- If Heart Rate is

100 - 150 think of sinus tachycardia
150 - 250 think of supraventricular tachycardia
250 - 350 think of atrial flutter
>350 think of atrial fibrillation

2) Establish the relationship between P wave and QRS complex

. If 1:1, it is normal
If more P waves than QRS complexes, think of AV block.
If more QRS complexes than P waves think of ventricular arrhythmias. (hypokalemia can casuse cardiac arrhythmias)

4. P WAVE

Normal values -

→ Up in all leads except aVR

→ Duration : <0.12 sec or <2.5 mm

→ P waves are best seen in lead II and V1 ( hyperkalemia can casues flat p waves)

Abnormalities

1. Inverted P wave / may be absent → Junctional rhythm

2. Wide P wave (P mitrale) → Left Atrial Enlargement

3. Peaked P wave (P Pulmonale) → Right Atrial Enlargement

4. Absent normaly P wave → Atrial fibrillation

5. Saw tooth appearance→ Atrial flutter

Atrial fibrillation

A-fib is the most common cardiac arrhythmia involving atria. Rate= -150bpm, irregularly irregular, baseline irregularity, no visible p waves, QRS occur irregularly with its length usually <0.12s

Atrial flutter

Atrial Rates--300bpm,
similar to A-fib, but have flutter waves,
ECG baseline adapts 'saw-toothed' appearance'

5. P-R INTERVAL

Definition : The time interval between beginning of P wave to beginning of QRS complex.

Normal P-R interval

→ 3 to 5 mm or 3 to 5 small squares on ECG graph.

Duration → 0.12 -0.2 sec

Abnormalities

a) Short PR interval

Wolff Parkinson White Syndrome (WPW) - Abnormal, accessory electrical pathway between the atria andVentricles.

ECG variation of WPW syndrome

1) P-R interval is less than 3 small squares.

2) The beginning of Rwave slopes gradually up and is slightly widened called delta wave.

3) There may be ST changes also, like ST

depression and T wave inversion.

b) Long PR interval

First degree heart block PR interval more than 5 small squares or 0.2 sec.
hyperkalemia, hypermagnesemia can cause prolonged PR INTERVAL

6. Q waves

Duration : <0.04 sec
That is less than one small square duration.
Height : <25% or < 14 of Rwave height.

Abnormal Q waves

Duration or width of Q waves becomes more than 1 small square on ECG graph
The depth of Q wave becomes more than 25% of R wave. This pathological 'Q' wave is observed in myocardial infarction.

7. QRS COMPLEX

Interval: 0.08 to 0.10 seconds
That is less than almost two and half small squares. So duration < 2.5 mm
Morphology
'R' wave slowly increase in height from V1 to V6 wave
in the precordial leads must grow from V1 to at least V4

Abnormalities

Wide QRS complex

1) Bundle branch block

2) Ventricular arrhythmia

3) Hyperkalemia & hypermagnesemia

Tall 'R' wave in V1

1) RVH

2) Posterior wall MI

RBBB (Right Bundle Branch Block)

ECG changes of RBBB

- 'M' shaped in QRS complex of V1
- W' shaped in QRS complex of V6

LBBB (Left Bundle Branch Block)

ECG changes

-'W' shaped in QRS complex of V1
-"M' shaped in QRS complex of V6

8. ST SEGMENT

Starts from 'S' wave to the beginning of T wave
hypocalcemia → prolonged ST segment
hypercalcemia → shortened ST segment
hypokalemia → STdepression
J Point→ It is the junction between the termination of QRS complex and beginning of ST segment.

9.T- WAVE

T wave represents the repolarization of the ventricles.
Normal T-wave Physiology
Normal T waves are upright in leads I, II and V3-V6, inverted in AVR.
Less than five mm in limb leads, less than ten mm in precordial leads
hypercalcemia → widened T waves

hypomagnesemia → tall T waves
hyperkalemia → tall peaked T waves

myocardial ischemia → Inverted T waves
T-waves (also called hyper-acute T waves) can be an early sign of ST-elevation myocardial infarction.

Classification AV heart blocks

1st degree block → Uniformly prolonged PR interval
2nd degree, Mobitz Type I → Progressive PR interval prolongation
2nd degree, Mobitz Type II → Sudden conduction failure
3 degree block → No AV conduction

1st degree AV block

Prolonged AV conduction time
PR interval > 0.20 sec
Rhythm - Regular

2nd degree AV block

There is intermittent failure of the supraventricular impulse to be conducted to the ventricles.
Some of the P waves are not followed by a QRS Complex.

types

Mobitz type 1 or Wenckebach phenomenon

Irregular rhythm.
Cycles of gradually increasing PR - intervals followed by dropped QRS complex.

Runs in cycle, first P-R interval is often normal. With successive beat, P-R interval lengthens until there will be a P wave with no following QRS complex. The block is at AV node, often transient, maybe asymptomatic

Mobitz type II - Fixed or classical

PR interval prolonged, but remain constant.
Periodically, wave shows dropped QRS complex.

P-R interval is constant, duration is normal/prolonged. Periodically, no conduction between atria and ventricles- producing a p wave with no associated QRS complex. (blocked p wave). The block is most often below AV node, at bundle of His or BB, May progress to third degree heart block

Complete AV block (3 heart block)

Atrioventricular dissociation
Regular P-P and R-R, but without association between the two(No association between waves)
Atrial rate > ventricular rate
QRS > 0.12 sec

Premature ventricular contractions (PVC)

PVC is a relatively common event where the heart beat is initiated by Purkinje fibres in the ventricles rather than by SA node, the normal heart beat initiator.

PVC's sometimes described by lay people as skipped heart beats.

Ectopic beat has no 'P' wave, as it starts from ventricles.
Wide QRS complex
'T' wave for PVC's are usually large and opposite in polarity to terminal 'QRS'.
3 or more PVC's in a row (goes to ventricular tachycardia)
Any PVC observed in the ECG of a patient who is diagnosed as having acute MI.
These dangerous phenomenon may preclude the occurrence of deadly arrhythmias like ventricular tachy - cardia and ventricular fibrillation.

PVC - Calassification