ECG Interpretation/ How to read an ECG or EKG/ Characteristics of the normal ECG

 ECG INTERPRETATION BASIC 

An ECG complex represents the electrical events occurring in one cardiac cycle. A complex consists of five waveforms labelled with the letters P, Q, R, S, and T. The middle three letters —Q, R, and S—are referred to as a unit, the QRS complex. ECG tracings represent the conduction of electrical impulses from the atria to the ventricles.

 

 P WAVE 

• Small, Positive and smooth wave.

• Always +ve in lead II during sinus rhythm.

• P wave is virtually always +ve in leads aVL, aVF, I, V4, V5, and V6 and -ve in aVR lead.

• Frequently biphasic in V1 ( occasionally in V2)  the -ve deflection is normally <1mm.

• P wave duration should be <0.12 sec.

• P wave amplitude should be <2.5 mm.

Pathological P wave 

• Tall Tented  :-  Right Atrial Enlargement

• Bifid (looks like an M) :- Left Atrial Enlargement

 PR INTERVAL

• Normal PR Interval :-  3-5 small squares 

                                                  or

                                         120 - 200 milliseconds

                                         ( 0.12 to 0.22 sec )

• A prolonged PR Interval (<0.22 sec) is consistent with lead Ist degree AV-Block.

• A shortened PR interval (<0.12 sec) indicate pre-excitation (presence of an accessory pathway this is associated with delta wave).

                                                     

                                                 

 QRS COMPLEX

Normal QRS Complex 

 

Amplitude :-  <3 small squares

Duration    :-    0.08 to 0.10 sec 

Q WAVE 

• Negative deflection preceding the R wave.
• < 1 small square wide
• < 2 small square deep 

Pathological (large) Q wave  

• Past MI

• MI with last 6 hours

  

  

R WAVE                 

• The only Positive deflection

• Look at leads V1, V2, V3

• If R wave height is equal to or more than S wave

Pathological is either :-

• Right Ventricular Hypertrophy (T wave inversion also present)

• Posterior infarct (normal upright T wave)

S WAVE

• Look at V1, V2, V3

• If the deepest S wave > 4 big squares.....

• Then add it to the large R wave of lead V5 or V6.

• If total >7 :- Left Ventricular Hypertrophy 

NOTE If S wave height in V1, V2. V3, + R wave height in V5 or V6 = LVH 

                                              

QRS COMPLEX WAVEFORM VARIETY

the various configurations of QRS complexes. 

When documenting the QRS complex, use 

uppercase letters to indicate a wave with a normal or high amplitude (greater than 5 mm) and lowercase letters to indicate a wave with a low amplitude (less than 5 mm). In some instances, a second R wave may appear in a QRS complex. 

This is called R'.

ST SEGMENT 

• The ST Segment is the part of the ECG between the end of the S wave and the start of the T wave.

 ST SEGMENT CHANGES

ST Elevation 

• Acute MI ( STEMI)

• Acute Pericarditis 

• Aneurysm

ST Depression 

• Acute Ischemia 

T WAVE 

• Duration :-  0.10 to 0.25

• Amplitude :-  < 5 mm 

• Normal T Wave  :-  same direction as corresponding QRS Complex 

Pathological T wave 

• Inversion  :-  Can be a sign of  :-  Ischemia 

                                                                 Old Infarction 

• Tall T wave  :-  Hyperkalemia 

                                    Hyper-acute Stage of MI

U WAVE 

• The U wave is a wave on an ECG that is not always seen.

• Typically small and by definition, follow the T wave.

• Represent Repolarization of Purkinje fiber.

ELECTROCARDIOGRAPHY BASICS

10-STEP METHOD ( ECG INTERPRETATION)

STEP 1.  Determine the rhythm 

• Regular or Irregular 

• P wave before every QRS Complex (Sinus Rhythm)

• QRS Complex Interval - Constant 

STEP 2.  Determine the rate 

• Normal  :-  60 - 100 bpm 

• Tachycardia :-  > 100 bpm 

• Bradycardia :-  < 60 bpm 

Calculation Method of Heart Rate 

• If a patient has a regular heart rhythm their heart rate can be calculated by using the following method:-

Method 1.

Calculate Big Square Boxes between one R-R interval,

Then divide 300 by this number to calculate heart rate. 

Method 2.

Calculate small square boxes between one R-R interval,

Then divide 1500 by this number to calculate heart rate.

• If a patient has a irregular heart rhythm their heart rate can be calculated by using the following method:- 

Method 1.

6 Second strip method 

Simply count the no. Of complete QRS complex in a 6 second strip. 

Then multiple by 10 in this number to calculate heart rate.

STEP 3.  Cardiac Axis 

Cardiac axis describes the overall direction of electrical spread within the heart. 

Normal Axis :-  Positive QRS Complex in Lead I and Lead II / Lead aVF

Right Axis Deviation :-

• Negative QRS Complex in Lead I

• Positive QRS Complex in Lead II /Lead aVF

Left Axis Deviation :-

• Positive QRS Complex in Lead I 

• Negative QRS Complex in Lead II /Lead aVF

STEP 4.  Evaluate the P wave

• Are P wave present ?

• If so, is each P wave followed by a QRS complex ?

• Do the P wave look normal ?
• Check :- Duration, Direction and Shape 

• If  P waves are absent, is there any atrial activity ?

1. Sawtooth baseline :- Flutter Waves 
2. Chaotic baseline :-  Fibrillation Waves 
3. Flat line :-  No atrial activity at all 

STEP 5.  Determine the duration of the PR Interval 

• Is the PR Duration a normal 0.12 to 0.20 second ?

• Is the PR Interval constant ?

• ( To measure the PR interval, count the small squares between the start of the P wave and the start of the QRS complex, then multiply the number of squares by 0.04 second )

STEP 6.  Determine the duration of the QRS Complex 

• Is the QRS Complex Duration a normal 0.08 to 0.10 second ?

• Are all QRS Complexes the same size and shape ?
• If not, measure each one and describe it individually. 

• ( To calculate QRS Complex Duration, count the number of small squares between the beginning and end of the QRS complex and multiply this number by 0.04 second. )

• Does a QRS Complex appear after every P wave ?

STEP 7.  Evaluate the ST Segment 

ST - Elevation 

ST - Elevation is significant when it is greater than 1mm ( 1 small square ) in two or more contiguous limb leads or  >2 mm in two or more chest leads indicate myocardial infarction. 

ST - Depression 

ST - Depression >  0.5 mm in more than and equal to two contiguous leads indicate myocardial ischemia.  

STEP 8.  Evaluate the T Wave 

• Are T wave present ?

• Do they all have a normal shape, amplitude and duration?

• Do they all have the same amplitude ?

• Do the T waves have the same deflection as the QRS complexes ?

STEP 9.  Evaluate the U Wave

• U wave are not a common finding. 

• The U wave is a > 0.5 mm deflection after the T wave best seen in V2 or V3.

• These become larger the slower the bradycardia.
• U waves are seen in various electrolyte imbalance. 

STEP 10.  Evaluate any other components 

• Check for the ectopic beats and other abnormalities.

• Origin of the rhythm ( for example, sinus node, atrial, AV node or ventricles).

• Rate Characteristics ( for example, bradycardia or tachycardia ).

• Rhythm Abnormalities ( for example, Flutter, Fibrillation, Heart Block, Escape rhythm or other arrhythmias ).

               

NORMAL SINUS RHYTHM

• Regular rhythm

• Normal rate

• A P wave for every QRS complex; all P waves similar in size and shape

• All QRS complexes similar in size and shape

• Normal PR and QT intervals

• Normal (upright and round) T waves

THE EFFECT OF REVERSING THE ELECTRODE ATTACHED IN LEFT AND RIGHT ARMS ( LA and RA)

• Compare with Normal Sinus Rhythm diagram.

• Inverted P wave in Lead I.

• Abnormal QRS complex and T wave in Lead I.

• Upright T waves in Laed aVR are most unusual.

Human Circulatory System (Flow of blood, Electrical activity of the heart etc. )