ECG Interpretation

Question 1

Describe the normal conduction system

Answer 1

-SAN= pacemaker
-AVN= from atria, delay to facilitate emptying of blood
-Bundle of His
-Bundle branches
-Purkinje fibres= contraction of ventricles

Question 2

What are the different waveforms produced by an ECG and what by?

Answer 2

-p= atrial depolarisation/contraction (+ve deflection)
-PR interval= flat line, time taken for electrical stimulus to reach ventricles from atria, includes delay at AV node (upslope P wave to beginning of QRS complex, 120-200 milliseconds/ 3-5 small squares)
-QRS complex= ventricular depolarisation, duration 120 milliseconds/3 small squares abnormal
-T= ventricular repolarisation (upright except AVR and V1)
-QT interval= start Q wave end T wave, ventricular depolarisation and repolarization, varies with HR (shorter=faster HR)
-QTc interval= estimate at 60bpm HR (>440 men abnormal, >460 milliseconds women)

Question 3

What factors need to be considered by analysing an ECG?

Answer 3

-Rate
-Rhythm
-Axis
-Go though cardiac cycle- p, QRS duration and morphology, T, QT interval

Question 4

Rate determination

Answer 4

-25millimetres per second
-1 Small square= 1 millimetre= 0.04 seconds
-1 Large square= 5 small squares= 5millitres= 0.2 seconds
=300 large squares= 1 minute

Rate= 1500/number of small squares OR 300/number of large squares (one beat= R to R waves)

-Irregular HR= number of RR intervals in 10 seconds x6

Question 5

Rhythm determination

Answer 5

-Sinus/ not sinus (lead 2)
-Look at p waves and their relationship to QRS complexes
-Lead II commonly used

-Regular or irregular?
-If in doubt, use a paper 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

Question 6

Axis determination

Answer 6

1. Quadrant method (lead 1 and aVF)
2. Three lead analysis (Lead 1, 2, and aVF)

Question 7

Quadrant method

Answer 7

-Lead 1 and AVF
=Both positive= R wave greater than S wave in both leads= 0 and 90 degrees so normal

-Positive lead 1, negative aVF= Possible LAD (0 to -90)
-Negative lead 1, positive aVF= RAD
-Both negative= extreme axis

Question 8

Three lead analysis

Answer 8

-Combined evaluation of lead 1 2 and AVF/ lead 3
-More accurate

-All positive leads= normal (0 to 90)
-LAD 0 to -30= 1 +, 2 equiphasic, 3 -ve
-LAD -30 to -90= 1 +, 2 -ve, 3 -ve
-RAD= 1-ve, 2 and 3 +ve
-Extreme axis= all negative
-Indeterminate= all equiphasic

Question 9

What is the cardiac axis?

Answer 9

-Major direction of the overall electrical activity of the heart

=Normal (-30 and 90 degrees)
=L ward= major QRS vector falls between -30 and -90 degrees
=R axis deviation= 90 and 180 degrees
=Indeterminate= 180 and -90 degrees

-Limb leads- QRS

Question 10

Iso-electric method

Answer 10

-If QRS is positive in any given lead, the axis points roughly in the same direction as this lead
-Negative=opposite

-If positive deflection=negative deflection= 90 degrees to lead (R wave height= Q/S wave depth)