ECG Flashcards
What is the isoelectric line?
Straight line where there is no positive or negative charges of electricity to create deflections
Size of small and large sqaures
-small squares: 1mm x 1mm -large sqaures: 5mm x 5mm
What determines direction of waveforms?
-towards the lead= positive deflection -away from the lead=negative deflection
Biphasic waves
Wave forms that are above and below the isoelectric line
QRS complex
- ventricular depolarisation
- 3 waveforms: Q wave downwards, R wave upwards, S wave downwards
Length of normal QRS complex
0.04-0.12 seconds
P wave
- represents atrial depolarisation from SA node towards AV node
- seen as a small positive deflection
Duration and height of normal P wave
Duration: 0.1 seconds
Height: 2.5mm
Which lead can you see P wave best in?
Lead II
T wave
- represents ventricular repolarisation following ventricular depolarisation
- rounded, taller and wider than P wave
Height of normal T wave
5-10mm
What is the U wave?
- comes after T wave
- similar in shape to P wave
- not usually seen on an ECG
- represents late repolarisation of Purkinje Fibres
Interval vs segment
- Interval: length of wave plus isoelectric line that follows it. It ends before the next wave begins. They are named using letters of both waves on either side. It cotains waves.
- Segments: baseline between the end of one wave and the beginning of the next wave. Lines between waves
PR interval
- length along baseline from beginning of P wave to beginning of QRS complex
- Normal duration: 0.12-0.20s (3-5 small squares)
QT interval
- from the BEGINNING of Q wave to the END of the T wave
- if U wave present measure till END of U Wave
ST segment
Length between end of S wave of QRS complex and beginnning of the T wave.
Electrically neutral
PR segment
Represents the delay in conduction from atrial depolarisation to the beginning of ventricular depolarisation.
Electrically neutral.
Estimating time or rate: useful standards
Small square: 1mm (0.04s)
Large square: 5mm (0.2s)
5 large squares=1s
Vertical scale on ECG
- vertical lines measure amplitude- measured in mV
- one small square= 0.1 mV
- one large squae=0.5 mV
Length of a normal 12 lead ECG
Just over 10 seconds (25cm).
i.e. 50 large blocks
How to calculate HR from ECG?
- count the number of QRS complexes in 10 seconds
- multiply by 6 to find number in 60s (1 minute)
Quick count
Count the large blocks that fall between 2 R waves.
Start by finding an R wave that falls on or close to a dark line.
Quick guide to estimating heart rate (by counting large squares between R waves)
- find out how many seconds are represneted by the large squares (=n)
- 60/n=approx HR
How to determine rhythm of an ECG?
- measure distance between 2 P waves or between 2 R waves
- see if the PP or PR intervals are consistent
How do you define a BBB?
-if the length of QRS complex exceeds 0.12 seconds
*NB: BBB can occur with ANY rhythm. The RHYTHM determines what the P wave looks like as well as atrial and ventricular RATE so don’t rely on the P wave or RATE.
**BUNNY EAR APPEARANCE OF QRS COMPLEXES (as conduction is delayed along one ventricle)
Causes of BBB
- normal
- pericarditis
- myocarditis
- congested heart failure
- congenital heart disease
Right BBB
MoRRoW
V1: QRS complex looks like M
V6: QRS complex looks like W
Left BBB
WiLLiaM
V1: QRS complex looks lie W
V6: QRS complex looks like M
What is 1st Degree AV block?
A rhythm in which the electrical impulse from the SA node through the atria, AV node, Bundle of His to PK fibres is slower than normal.
Defined by PR interval greater than 0.20 seconds.
Causes of 1AVBT
- Coronary Heart Disease
- Inferior wall MIs
- Hyperkalaemia
- Congenital abnormalities
- Medications: quinidide, digitalis, beta blockers, CCBs
Characteritsics of 1st Degree AV block
Rate: atrial and ventricular rate can vary. 1:1 ratio between P waves and QRS complexes
Rhythm: Usually regular but can be irregular
P wave: usually normally shaped. 1:1 ratio with QRS complexes. Prolonged PR interval (>0.2s)
QRS complex: Within normal limits or may have a bundle branch block
ST segment: within normal limits for the intrinsic rhythm
T wave: within normal size and configuration
Second degree AV Block Type I (Mobitz): Definition
- progressive delay of conduction at AV node until conduction is completely blocked
- happens because impulse arrives at the absolute refractory period- absence of conduction–>loss of QRS complex
- next P wave occurs and cycle begins again
Causes of Second degree AV block Mobitz Type 1
- Acute inferior wall MI
- Digitalis
- Beta blockers
- CCBs
- rheumatic fever
- myocarditis
- excessive vagal tone
Features of AV block Mobitz Type 1
Rate: 60-100 bpm
Atrial rhythm: regular
Ventricular rhythm: irregular
P wave: normal configuration
PR interval: gets longer with each beat until QRS complex disappears
QRS complex: normal but eventually dropped (then cycle starts again)
ST segment: normal
T wave: normal
Second degree AV block type 2 Mobtiz
- 2-4 P waves before each QRS complex-Ventricular rate: depends on no. of waves conducted through AV node. LESS THAN ATRIAL RATE
- Rhythm: BOTH atrial and ventricular are irregular
- P wave: two/three/four to 1 ratio to QRS complex
- PR interval: CONSTANT for each P wave prior to QRS
- QRS: may be within normal limits for the intrinsic rhythm
- ST segment: normal in size and configuration
**CAN PROGRESS TO TYPE 3**
Causes of second degree AV block Mobitz Type II
- Acute anterior or anteroseptal myocardial infarction
- cardiomyopathy
- coronary artery disease
- rheumatic heart disease
- digitalis
- beta blockers
- CCBs
Third degree AV heart block
- complete loss of electrical conduction from atria at the AV junction i.e. COMPLETE heart block
- atria and ventricles thus beat independently of each other
- significantly decreases cardiac output and could lead to aysystole (complete cardiac arrest)
Causes of third degree heart block
- anterior or inferior myocardic infarction
- coronary heart disease
- excessive vagal tone
- myocarditis
- endocarditis
- age
- oedema from heart sugrery
- drugs: digitalis, CCBs, beta blockers
Characteristics of third degree heart block
Atrial rate: faster than ventricular rate
Rhythm: regular but normal configuration
PR interval: no relationship between P waves and QRS complexes
QRS complex: variates depending on intrinsic rhythm
ST segment and T wave: normal configuration
Causes of ST segment elevation
- impending MI
- pericarditis
- vasospastic(variant) angina
- can be normal in some healthy adults- EARLY REPOLARISATION
How do you measure the height of the ST segment?
At a point 2 boxes from the end of the QRS complex, you measure the vertical distance from the isoelectric line
When is ST segment elevation considered abnormal?
Limb lead: >1mm
Prcordial lead: >2mm
What is a transmural infarction?
An MI that covers the entire thickness of the myocardium
ST segment elevation in MI
- one of the first manifestations in a transmural MI
- will be seen in those leads involved in impending infarction
- ST segment elevation decreases as T wave inversion begins
- ST segment elevation may remain when ventricular aneurysm develops
ST segment elevation and ventricular aneurysm
- if ST segment elevation persists 3 months beyond following MI–>suggests venrticular aneurysm
- 1/3 of venrtiuclar aneurysms present with ST segment elevation
- in ventricular aneurysm patients presenting with acute chest pain, a baseline ECG is useful to rule out impending MI thus avoiding unnecessary thrombolytic drugs
What is vasospastic angina?
- severe type of ischaemia that can present with ST segment elevation (aka Prinzmetal’s angina)
- exercise angina involves the subendocardium while vasospastic angina causes loss of blood flow transmurally
- ST segment elevation simply indicates injury- this may be due to coronary thrombosis OR coronary spasm (Prinzmetal’s angina). At this point injury is reversible
ST segment elevation in pericarditis
- pericarditis: inflammation of the space between pericardial sack and outer surface of the heart
- causes widespread ST segment elevation- in ALL leads, not just distribution of the coronary arteries (“current of injury”). Classic indication.
- NB: inferolateral transmural infarction with pre-existing junctional ST elevation in the anterior leads could also produce widespread ST elevation- DON’T CONFUSE WITH PERICARDITIS.
- later in the course of pericarditis, ST segment elevation resolves without the development of Q waves
- after days to months ST segment elevation is replaced by T wave inversions
Early repolarisation and ST segment elevation
- typically occurs in young healthy males
- T wave begins early thus contributing to ST segment elevation
- usually shows elevation of J point (junction between end of QRS coplex and beginning of ST segment)
- can also see concave upward curve towards the T wave (called J waves!)
- usually seen in anterior precordial leads but to a lesser extent in the limb
- difficult to differentiate between MI and early repolarisation so with the chest pain patient always assume MI until proven otherwise by revieiwng a previous ECG or by obtaining serial ECGs
What is the normal axis of depolarisation of the heart?
- as the left ventricle is thicker the mean QRS complex is left and down
- origin of the vector is the AV node, with the left ventricle being down and to the left of this
Where does the axis of depolarisation point in hypertrophy and infarction?
Hypertrophy: vector points TOWARDS hypertrophy
Infarction: vector points AWAY from hypertrophy
12 lead ECG viewpoints
Normal axis
-30 to +90 degrees
Left axis deviation
-30 to -90 degrees
Right axis deviation
+90 to +/-180 degrees
Indeterminate (extreme) axis deviation
-90 to +-180 degrees
Lead I
Runs from right to left arm, with left arm being positve
Which two leads can you use to estimate the axis?
Lead I and aVF- as they are perpendicular to each other
What does positive depolarisation in Lead 1 indicate about the average axis of depolarisation?
If QRS complex in Lead I is positive it shows that the direction of depolarisation is in the positive half (right half) of the circle.
Lead aVF
Runs from the top to bottom across a patient’s body, positive at the feet
What does positive depolarisation in lead aVF indicate about the average axis direction?
If the QRS omplex in lead aVF is positive then the direction of depolarisation will be in the positive half (lower half) of the circle.
Lead I and aVF in normal axis (0-+90)
Lead I: positive
Lead aVF: positive
Lead 1 and aVF in left axis deviation (-30 to -90)
Lead I: positive
Lead aVF: negative
Lead I and aVF in right axis deviation (+90 to -180)
Lead I: negative
Lead avF: positive
True left axis deviation (-30 to -90)
Lead I: positive
Lead aVF: negative
MUST CHECK LEAD II: negative
Lead II is the key here.
Lead I and aVF in indeterminate axis
Lead I: negative
Lead aVF: negative
Differentials for left axis deviation
LVH
Left anterior fascicular block
Inferior wall MI
Differentials for right axis deviation
RVH
Left posterior fascicular block
Lateral wall MI
Summary of different positions in circle
Presenting an ECG
- Name and age of patient
- Date of ECG
- Report if patient had any chest pain when ECG was conducted
- Rate:
a) Count no. of QRS complexes in 10s and multiply by 6
b) Count no. of large squares between 2 QRS complexes and divide 300 by that - Rhythm: sinus, regular, irregular, irregularly irregular
- Look for prolongation of PR interval
- Look for extra beats or deflections (eg with a paced rhythm)
- Axis:
- if S wave is greater than R wave in lead I=left axis deviation
- if S wave is greater than R wave in lead II= right axis deviation - Look at configuration of QRS complexes
- Look for ST segment elevation/depression
- Look at T waves. Are they norma, inverted or peaked?
- Gives summary and diagnosis
When can ST segment be depressed?
During ischaemia
Another quick way to calculate heart rate from ECG
Measure the RR interval
Count number of LARGE squares between two R waves (=n)
300/n
Normal: 3-5 large squares
What is R wave progression?
When you go from V1-V6, the ratio of R wave to S wave should increase (i.e. R wave should get progressively taller).
This is NORMAL.
