The Electrocardiogram Flashcards
What does the P wave represent?
Why is it a small upward depolarisation?
Atrial depolarisation
Due to: smaller muscle mass and depolarisation moving towards the electrode
What does the QRS complex represent?
Ventricular contraction
What does Q represent?
Septal depolarisation spreading to ventricles
What does R represent?
Main ventricular depolarisation
What does S represent?
End of ventricular depolarisation
When does atrial repolarisation take place?
During the same time as ventricular depolarisation
– during QRS complex
What does the T wave represent?
Ventricular repolarisation
Describe the T wave shape
Upward signal as repolarisation is away from electrode
Medium sized as timing in the different cells is dispersed
What does the PR interval measure?
Time taken for impulse to reach ventricles from the SA node
What is a normal time for a PR interval?
0.12 to 0.2 seconds
3-5 small squares
What does a short PR interval indicate?
Atria have been depolarised close to the AV node or the is an abnormality of conduction from the atria to the ventricles.
What does the duration of the QRS complex represent?
How long excitation takes to spread through the ventricles
What is the normal duration of a QRS complex?
0.12 seconds or less
What does a widened QRS complex represent?
An abnormality of conduction through the ventricles
E.g. Bundle branch block
What are the V leads?
Six chest leads
Made from 6 positions overlying the 4th and 5th rib spaces
– look in horizontal plane from the front and from the left
What do V1 and V2 show?
Right ventricle
What do V3 and V4 show?
Septum and anterior wall of left ventricle
What do V5 and V6 show?
Anterior and lateral walls of left ventricle
What is the cardiac axis?
The average direction of spread of the depolarisation wave from the front
Where can you deduct the cardiac axis?
What is the normal shape?
From leads I, II and III
Defects signal upwards as depolarisation is spreading towards the three leads
When does right axis deviation occur?
What is it associated with?
When the right ventricle becomes hypertrophied.
Usually associated with pulmonary condition putting a strain on the right side of the heart as well as congenital heart defects.
What do you see form the leads in right axis deviation?
Deflection in I becomes negative
Deflection in III becomes positive
– axis swings towards the right
When does left axis deviation occur?
What is it usually due to?
When the left ventricle becomes hypertrophied
Usually due to a conduction defect
– unlikely to be due to increased bulk of left ventricle
What do you usually see from the leads in left axis deviation?
QRS complex become negative in III
Only is significant when QRS deflection is predominantly negative in lead II
What is first degree heart block?
Prolonged PR interval
What is first degree heart block a sign of?
Coronary artery disease
Acute rheumatic carditis
Digitalis toxicity
Electrolyte disturbances
What is second degree heart block?
Erratic PR interval
Excitations fails to pass through the AV node or the bundle of His
What are the causes of second degree heart block?
Same as first degree heart block: Coronary artery disease Acute rheumatic carditis Digitalis toxicity Electrolyte disturbances
What is third degree heart block?
Complete atrioventricular block
- atrial contraction is normal but no beats are conducted to the ventricles
- the two are disconnected electrically
How are the ventricles excited in third degree heart block?
By a slow escape mechanism
How can you recognise third degree heart block?
P wave rate is normal (90)
QRS rate is low (36)
No relationship between P and QRS
Abnormal shaped QRS complexes due to abnormal spread of depolarisation from ventricular focus
What can cause third degree heart block?
Acutely – transient heart attack
Chronically – fibrosis around bundle of His
When can a right bundle brach block be normal?
When the duration of the QRS complex is normal
What do bundle branch blocks indicate?
RBBB indicates problems with the R side of the heart
LBBB always indicates heart disease, usually on the L side
What changes occur in the ECG during right bundle brach block?
V1 lead – normal R wave, second R wave due to failure of conduction pathway
V6 lead – smaller Q wave, normal R wave, wide deep S wave
What changes occur in the ECG during left bundle brach block?
V1 lead – small Q, R wave (inspire of smaller mule mass), S wave (late depolarisation of LV)
V6 lead – R wave, S wave (appears as a notch), second R wave (due to late depolarisation of LV)
Where is right bundle brach block most easily seen?
In V1, where there is an RSR pattern
Where is left bundle brach block best seen?
In V6 where there is a “rabbit ears” pattern
Describe the branching of the bundle of His
From the AV node, two branches arise. The right bundle branch and the left bundle branch.
The right bundle branch has no main divisions.
The left bundle branch has two main divisions – anterior and posterior fascicles
Which pacemaker cells have priority?
SAN pacemaker cells. They pre-empt other pacemakers due to a faster firing rate.
What can you use in order to determine the cardiac axis?
The lead with the smallest possible R wave which will be the lead at 90 degrees to the cardiac axis.
When analysing an ECG what do you look for?
Rate Rhythm Axis P wave P-R segment QRS complex Q-T interval T wave
When is the P wave absent?
In atrial fibrillation
What are abnormalities that can be seen in the P-R interval?
First degree heart block = prolonged P-R interval
Second degree heart block = erratic P-R interval
Third degree heart block = no relationship between P and QRS complex
What is sinus rhythm?
Pacemaker cells in the SAN are controlling the heart rate
– normal depolarisation
What are the features of myocardial infarction?
ST elevation
Pathological Q waves – greater than one small square across
Inverted T waves
What changes from the MI are permanent?
Pathological Q waves are permament
– present in full thickness MI
– due to scar tissue forming which effectively blocks the circuit
Inverted T waves
What do broader QRS complexes represent?
Escape rhythm
– ventricle is taking over as the pacemaker
Where can you see an inferior MI?
Which coronary artery is likely to be responsible?
Leads II, III and aVf
Right coronary artery
Where can you see an anteroseptal MI?
Which coronary artery is likely to be responsible?
V1 and V2
Left anterior descending coronary artery
Where can you see an anteroapical MI?
Which coronary artery is likely to be responsible?
V3 and V4
Distal left anterior descending coronary artery
Where can you see an anterolateral MI?
Which coronary artery is likely to be responsible?
V5, V6, I, aVL
Circumflex coronary artery
Where can you see an extensive anterior MI?
Which coronary artery is likely to be responsible?
V1 - V6, I, aVL
Proximal left coronary artery
Where can you see a posterior MI?
Which coronary artery is likely to be responsible?
Wat are you likely to see in this case?
V1 and V2
Right coronary artery
ST depression in anterior leads or as tall R waves
Where is the MI if there are changes in leads II, III and aVf?
Inferior
Where is the MI if there are changes in leads V1 and V2?
Anteroseptal
Where is the MI if there are changes in leads V3 and V4?
Anteroapical
Where is the MI if there are changes in leads V5, V6, I and aVL?
Anterolateral
Where is the MI if there are changes in leads V1 - V6, I and aVL?
Extensive anterior
Where is the MI if there are tall R waves in V1 and V2?
Posterior aspect
