interpreting an ECG Flashcards
first thing you do Before taking an ECG
Confirm the name and date of birth of the patient matches the details on the ECG.
Confirm the date and time the ECG was performed.
12 step ECG algorithm
- rate
- rythm
- cardiac axis
- P waves
- PR interval
- QRS complex
- delta waves
- RS progression
- j point
- ST segment
- T wave
- U waves
13.
1
normal heart rate
Normal = 60 – 100 bpm
Tachycardia > 100 bpm
Bradycardia < 60 bpm
Hint: If there are obviously P-waves present, check the ventricular rate and the atrial rate. The rates will be the same if there is 1:1 AV conduction.
1.1
how to measure HR if REGULAR
Count the number of large squares present within one R-R interval
Divide 300 by this number to calculate the heart rate
e.g. 4 large squares in an R-R interval: 300/4 = 75 beats per minute
how to calc HR if rythm is IRREGULAR
Count the number of QRS complexes on the rhythm strip (each rhythm strip is 10 seconds long)
Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute)
e.g. 10 complexes on a rhythm strip X 6 = 60 beats per minute
3
what are the two types of IRREGULAR heart rythms
how to measure irreg rythms
top for measurement of AV block
- Regularly irregular (i.e. a recurrent pattern of irregularity) 2nd degree AV block
- Irregularly irregular (i.e. completely disorganised) e.g. atrial fibrillation
measurement
Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are the same.
- Hint – if you are suspicious that there is some atrioventricular block,*
- map out the atrial rate and the ventricular rhythm separately (i.e. mark the P waves and R waves). As you move along the rhythm strip, you can then see if the PR interval changes, if QRS complexes are missing or if there is complete dissociation between the two.
4
what is the cardiac axis
which leads determine it
Cardiac axis describes the overall direction of electrical spread within the heart.
In a healthy individual the axis should spread from 11 o’clock to 5 o’clock.
To determine the cardiac axis you need to look at leads I,II and III.
normal cardiac axis
which leads are positive which is negative
n normal cardiac axis:
Lead II has the most positive deflection compared to Leads I
Lead III has negative deflection usually
Right axis deviation ( opposite of normal)
common assoc patholigy
Lead III has the more positive deflection than Lead II
Lead I should be negative
This is commonly seen in individuals with right ventricular hypertrophy
Left axis deviation
assoc pathology
Lead I has the most positive deflection
Leads II and III are negative
Left axis deviation is seen in individuals with heart conduction defects
4
what to look for in the P waves
- Are P-waves present? = Sinus rythm
- If so, is each P-wave followed by a QRS complex? = if not AV block
- Do the P-waves look normal? (check duration, direction and shape)
-
If not present, is there any atrial activity e.g.
- sawtooth baseline → flutter waves
- chaotic baseline → fibrillation waves
- flat line → no atrial activity at all?
Hint – If P-waves are absent and there is an irregular rhythm it may suggest atrial fibrillation
normal duration of P wave
Normal duration: 0.12-2.0 seconds (3-5 horizontal boxes).
This is measured from the onset of the P wave to the onset of the QRS complex regardless if the initial wave is a Q or R wave.
5
P-R interval
normal duration
when is it pathological
The P-R interval should be between 120-200 ms/ 0.12-0.2 (3-5 small squares)
pathologies
- prolonged P-R (over 0.2 secs)= AV block
- shortened (below 0.12) = small atrium or accessory pw
types of Causes of prolonged P-R
how to remember them according to anatomical location
- First degree AV block: Occurs between the SA node and the AV node (i.e. within the atrium)
-
Second degree AV block:
- Mobitz I (Wenckebach) – occurs IN the AV node. This is the only piece of conductive tissue in the heart which exhibits the ability to conduct at different speeds
- Mobitz II – occurs AFTER the AV node in the bundle of His or Purkinje fibres
- Third degree AV block: Occurs anywhere from the AV node down causing complete blockage of conduction
First degree heart block
fixed prolonged PR interval (>200 ms)
Second degree heart block (Mobitz type 1)
PR interval slowly increases/widens then there is a dropped QRS complex (beat), this is MOBITZ TYPE I SECOND DEGREE AV BLOCK (Wenckebach)
Second degree heart block (Mobitz type 2)
PR interval is fixed but there are dropped beats,
MOBITZ TYPE 2 SECOND DEGREE HEART BLOCK
(clarify that by the frequency of dropped beats e.g 2:1, 3:1, 4:1)
Third degree heart block (complete heart block)
If the P waves and QRS complexes are completely unrelated,
this is THIRD DEGREE AV BLOCK (complete heart block)
causes of shortened PR interval
- P-wave is originating from somewhere closer to the AV node so the conduction takes less time (the SA node is not in a fixed place and some people’s atria are smaller than others!)
- The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall. This is an accessory pathway and can be associated with a delta wave (see below which demonstrates an ECG of a patient with Wolff Parkinson White syndrome)
Width of the QRS complex
normal width/ duration
normal QRS interval = below 0.12 seconds
NARROW (< 0.12 seconds)
- occurs when the impulse is conducted down the bundle of His and the Purkinje fibre to the ventricles. This results in well organised synchronised ventricular depolarisation
BROAD (> 0.12 seconds)
- occurs if there is an abnormal depolarisation sequence – e.g.
- a ventricular ectopic where the impulse spreads slowly across the myocardium from the focus in the ventricle. In contrast, an atrial ectopic would result in a narrow QRS complex because it would conduct down the normal conduction system of the heart.
- bundle branch block results in a broad QRS because the impulse gets to one ventricle rapidly down the intrinsic conduction system then has to spread slowly across the myocardium to the other ventricle.
when is a q wave pathologically signifiant
what previous condition can be found by looking at Q waves
Isolated Q waves can be normal.
A pathological Q wave is
- > 25% the size of the R wave that follows it or
-
> 2mm in height and > 40ms in width.
- A single Q wave is not a cause for concern – look for Q waves in an entire territory (anterior / inferior) for evidence of previous MI.
7
Delta waves
what causes it’s characteristic slurred upstroke
conditions for WPW
sign that the ventricles are being activated earlier than normal from a point distant to the AV node.
The early activation then spreads slowly across the myocardium causing the slurred upstroke of the QRS complex.
NB The presence of a delta wave does NOT diagnose Wolff-Parkinson-White syndrome.
WPW = This requires evidence of tachyarrhythmias AND a delta wave.
8
R WAVE progression
which lead shows normal progession
what leads does pathological progression indicate previous condition
Look for R wave progression across the chest leads
- from small in V1 to L A R G E in V6
- The transition from S > R wave to R > S wave should occur in V3 or V4.
- Poor progression (if S is still > R at through to leads V5 and V6) can be a sign of
- previous MI
- but can also occur in very large people due to lead position.
Conditions for ST elevation
what specific pathology does it suggest
ST elevation is significant when it is
- greater than 1 mm (1 small square) in 2 or more contiguous limb leads or
- >2mm in 2 or more chest leads.
It is most commonly caused by acute FULL thickness myocardial infarction.
Conditions for ST
which pathology does it suggest
ST depression ≥ 0.5 mm in ≥ 2 contiguous leads
indicates myocardial ischaemia.
step 11
T waves
normal t wave
pathological conditions
T waves represent repolarisation of the ventricles
normal t waves :
upright in all leads, except aVR, aVL, III and V1 leads.
Highest amplitude of T wave is found at V2 and V3 leads.
The shape of the T wave is usually asymmetrical with a rounded peak
pathological T waves
- Tall
- inverted
- biphasic
- flat
Tall T waves criteria ( same as what?)
pathological conditions
criteria
- > 5mm in the limb leads AND
- > 10mm in the chest leads (the same criteria as ‘small’ QRS complexes)
Pathological conditions
- Hyperkalaemia (“Tall tented T waves”)
- Hyperacute STEMI
When are T waves Inverted normally and pathologically
what is important to comment about the T wave Inversion
normally
- normally inverted in V1
- normal variant inversion in lead III
Pathological
- Ischaemia
- Bundle branch blocks
- V1 – V3 in RBBB
- V4 – 6 in LBBB
- Pulmonary embolism
- Left ventricular hypertrophy (in the lateral leads)
- Hypertrophic cardiomyopathy (widespread)
- General illness ( 50% of ICU pts)
- Comment on the distribution of the T wave inversion
- e.g. anterior / lateral / posterior leads
- Comment on the distribution of the T wave inversion
what are Biphasic T waves
what 2 conditions does it indicate
Biphasic T waves have two peaks and can be indicative of
- ischaemia
- hypokalaemia
what is a flat T wave and what does it suggest
non-specific sign, this may represent
ischaemia or
electrolyte imbalance.
12
what is a u wave
which leads
what axacerbates it
3mconditions it suggests
which 3 drugs can cause it i DAP- U wave
The U wave is a > 0.5mm deflection after the T wave best seen in V2 or V3.
- These become larger the slower the bradycardia
- classically U waves are seen in
- various electrolyte imbalances
- hypothermia,
- antiarrhythmic therapy
- Digoxin, Amiodarone, Procainamide
