ECG Flashcards
Whats the standard speed of tracing on ECG?
Set at 25mm/s
Every 5 large squares equals 1 second
Whats the standard paper on an ECG?
1 large square = 5mm =0.2 seconds
1 small square = 1mm = 0.04 seconds
What are the limb leads?
These are recorded from the electrodes that attached to the patients limbs.
These leads look at the heart from the vertical plane.
Lead I is between aVR and aVL
Lead 2 is between aVR and aVF
Lead 3 is between aVL and aVF
II, III, aVF - inferior surface of heart
I, aVL - lateral surface
AVR - right atrium
What are the chest leads?
There are six standard chest leads, which are recorded from the electrodes that attach to the patients chest. These leads look at the heart from the horizontal plane.
V1, V2: septal leads. View the right ventricle of the heart and septum between ventricles.
V3, V4: anterior leads. View the anterior wall of the left ventricle
V5, V6: lateral leads. Look at the anterior and lateral wall of the left ventricle.
Why is aVR lead normally negative?
Because it’s in the opposite direction to the heart’s electrical flow
Outline what causes the changes in ECG shape?
The shape appearing on a ECG represents the average direction of the wave of depolarisation.
Depolarisation moving towards a lead causes a predominantly upward/or positive deflection. Depolarisation moving away from a lead causes a predominantly downward/or negative deflection. The opposite is seen with repolarisation.
When we say a lead is ‘positive’, we mean the lead has a predominantly upward deflection in the QRS complex. When we say a lead is ‘negative’, we mean the lead has a predominantly negative deflection in the QRS complex.
What does the p-wave represent?
Atrial depolarisation
What does the QRS complex represent?
Ventricular depolarisation
What does the T-wave represent?
Ventricualr repolarisation
What does the U-wave represent?
-
They arr small deflections immediately following the T wave. Best seen in V2 and V3. Not always observed due to its size
They represent repolarisation of purkinje fibres
What are prominent U waves and what do they represent?
U waves more than 1-2mm or 25% of the height of the T wave
Hypokalaemia - most often
Others - hypercalcaemia, thyrotoxicosis, exposure to digitalis, epinephrine or class 1A and 3 antiarrhytgmics, congenital long QT sundrome and intracranial haemorrhage
What may an inverted U wave represent?
MI or left ventricular volume overload
What does ST segment represent?
the interval between depolarization and repolarization of the ventricles
Whats a normal ST segment length?
120-200ms
Whats a normal QRS length?
70-100ms
What does the QT interval represent?
the duration of ventricular electrical systole, which includes ventricular activation and recovery.
From beginning of QRS complex to the end of the T wave
Whats a normal QT interval?
350-440ms in men
350-460ms in women (slower cardiac repolairsation than men)
Should be half the R-R interval
What is QTc?
Corrects the QT interval for heart rate extremes e.g. at rest longer QT and at exercise shorter QT
What determines the shape of the QRS complex?
Q waves develop because the septum between the ventricles undergoes depolarisation before the walls. The wave of depolarisation within the septum is from left to right. This means towards the septal leads (V1/V2) and away from the lateral leads (V5/V6).
The more muscular left ventricle then exerts more influence on the ECG than the right ventricle leading to a dominant R wave in the lateral chest leads (V5/V6). In the septal leads (V1/V2) we see a small R wave and dominant S wave.
After depolarisation of the whole myocardium, the ECG trace of the QRS complex moves back to the baseline, otherwise known as the isoelectric line.
What is a narrow QRS?
<120ms <3 small squares
How do you calculate HR on ECG?
300 / number of large squares between each R-R interval
Use the method below for an irregular HR:
Count number of complexes on rhythm strip and multiply this number by 6 (as each rhythm strip is typically 10 seconds long)
How can you work out if an ECG heart rhythm is irregular?
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 similar.
What should you do if you are suspicious of an atrioventricular block on an ECG?
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.
What is the cardiac axis?
describes the overall direction of electrical spread within the heart.
In a healthy individual, what is a normal cardiac axis?
the axis should spread from 11 o’clock to 5 o’clock
Lead II should have the most positive deflection compared to leads I and III
What are typical ECG findings for right axis deviation?
QRS positive in lead II, III and aVF (dominant R wave)
QRS negative in lead I (dominant S wave)
QRS axis between -30 and +90 degrees
What causes right axis deviation of the heart?
Normal in children or thin adults with a horizontally positioned heart
Right ventricular hypertrophy
Left posterior fascicular block
Lateral myocardial infarction
Acute lung disease (e.g. Pulmonary Embolus)
Chronic lung disease (e.g. COPD)
Ventricular ectopy
Hyperkalaemia
Sodium-channel blocker toxicity
WPW syndrome
What are typical ECG findings for left axis deviation?
QRS axis < -30 degrees
QRS is positive in lead I (dominant R wave)
QRS is negative in leads II, III and aVF (dominant S wave)
What causes left axis deviation?
Left anterior fascicular block
Left bundle branch block
Left ventricular hypertrophy
Inferior MI
Ventricular ectopy
Paced rhythm
Wolff-Parkinson White syndrome
What should you look for when checking for P waves?
- Are P waves present?
- If so, is each P wave followed by a QRS complex?
- Do the P waves look normal? – check duration, direction and shape
- If P waves are absent, is there any atrial activity?
What does absent P waves and irregular rhythm represent?
AF
What does a sawtooth baseline on ECG represent?
Flutter waves
What does a chaotic baseline on ECG represent?
Fibrillation waves
Whats a normal PR interval?
120-200mg (3-5 small squares)
Whats considered a prolonged PR interval?
> 200ms
What does a prolonged PR interval suggest?
AV block
How do you recognise first degree heart block?
fixed prolonged PR interval (>200 ms).
How do you recognise second degree heart block type 1?
Aka Mobitz type 1 AV block
progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.
AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.
How do you recognise second degree heart block type 2?
Aka Mobitz type 2 AV block
consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.
The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.
How do you recognise third degree heart block?
This occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.
Whats the anatomical location of block within the conducting system in each AV block?
First-degree AV block: between the SA node and the AV node (i.e. within the atrium).
Second-degree AV block: type 1 occurs IN the AV node, type II AV block occurs AFTER the AV node in the bundle of His or Purkinje fibres.
Third-degree AV block: Occurs at or after the AV node resulting in a complete blockade of distal conduction.
What does a shortened PR interval mean?
Either..
The P wave is originating from somewhere closer to the AV node so the conduction takes less time e.g. someone with a smaller atria
Or…
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
What should you assess when looking at QRS complexes?
Width
Height
Morphology
Outline the aetiology of a narrow QRS and broad QRS complexes?
Narrow - occurs when the impulse is conducted down the bundle of his and purkinje fibres to the ventricles
Broad - abnormal depolarisation sequences e.g. a ventricualr ectopic
What are small QRS complexes?
Small is <5mm in limb leads or <10mm in chest leads
What is a delta wave? Whats it a sign of?
A slurred upstroke of the QRS complex
It’s a sign that the ventricles are being activated earlier than normal from a point distant to the AV mode. The early activation then spreads slowly across the myocardium causing the slurred upstroke of the QRS complex
Are Q waves pathological?
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.
Q waves in V2-V4 with T wave inversion is suggestive of a previous anterior MI
How do you assess R wave progression and what can this tell you?
Assess the R wave progression across the chest leads (from small in V1 to large in V6). The transition from S > R wave to R > S wave should occur in V3 or V4.
Poor progression (i.e. S > R through to leads V5 and V6) can be a sign of previous MI but can also occur in very large people due to poor lead position.
What is the J point?
where the S wave joins the ST segment
What is known as ‘high take-off’?
What does it mean?
When the J point is elevated which results in the ST segment that follows it also being raised
Aka benign early repolairsation
A normal variant that causes a lot of angst and confusion as it LOOKS like ST elevation.
What is a normal ST segment height?
Isoelectric
What is ST elevation?
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.
What is ST depression?
ST depression ≥ 0.5 mm in ≥ 2 contiguous leads indicates myocardial ischaemia.
What do T waves represent?
Repolairsation of the ventricles
When are T waves considered tall?
> 5mm in the limb leads AND
10mm in the chest leads
What can cause tall T waves?
Hyperkalaemia (“tall tented T waves”)
Hyperacute STEMI
When are T waves normally inverted?
In V1 and lead III
What can inverted T waves in leads other than V1 and III indicate?
Ischaemia
Bundle branch blocks (V4-6 in LBBB and V1-V3 in RBBB)
Pulmonary embolism
Left ventricular hypertrophy (in the lateral leads)
Hypertrophic cardiomyopathy (widespread)
General illness
(50% of patients admitted to ITU have some evidence of T wave inversion)
What are biphasic T waves and what do they indicate?
They have 2 peaks
Indicate ischaemia and hypokalaemia
What can flattened T waves indicate?
Ischaemia and electrolyte imbalance
What 2 cells is myocardial contraction dependant on?
Autorhythmic cells 1% - generate action potentials to drive contractile cells
Contractile cells 99% - responsible for myocardial contarction
Where is the SAN located?
At the junction of SVC and right atrium
Whats the rate of authorhythmicity?
70-80bpm
Where is the AVN located?
near the septum at the base of the right atrium.
Whats the rate of autorhuthmicity at the AVN?
40-60bpm
Where is the bundle of His?
Forms a tract from the AVN to the interventricular septum.
Whats the rate of autorhythmicity at the bundle of His?
20-40 bpm.
Does
What ECG changes are seen in left ventricular hypertrophy?
Increased R wave amplitude in I, aVL, V4-6 (left leads)
Increased S wave depth in III, aVR, V1-3 (right leads)
Increased R wave peak time (due to prolonged depolarisation) and ST and T-wave abnormalities (due to delayed repolarisation) in lateral leads - due to thickened LV wall
