Electrocardiography Revisited Flashcards
What is the clinical relevance of the ECG?
Conduction abnormalities
Structural abnormalities
Perfusion abnormalities
Are electrodes, cables / wires, and leads all the same thing?
No
Electrodes - sticky parts that stick onto the skin e.g. chest
Cables / wires - connect to the electrodes
Leads - views of the heart (i.e. the graph on the ECG)
What is the job of the leads?
What is a vector?
Pick up cardiac vectors
A quantity that has both, magnitude and direction
What are the advantages of ECGs?
Relatively cheap and easy to undertake
Reproducible between people & centres
Quick turnaround on results/report
What happens if the wave of excitation travels toward the negative electrode?
Downward deflections are towards the anode (-)
What happens if the wave of excitation travels toward the positive electrode?
Upward deflections are towards the cathode (+)
What does the isoelectric line represent?
Represents no net change in voltage. i.e. vectors are perpendicular to the lead
What does the steepness of the line relate to?
What does the width of the line relate to?
Steepness of line denotes the ‘velocity’ of action potential
Width of the deflection denotes the ‘duration’ of the event
What is the P wave?
What is the QRS complex?
What is the T wave?
The electrical signal that stimulates contraction of the atria (atrial systole)
The electrical signal that stimulates contraction of the ventricles (ventricular systole)
The electrical signal that signifies relaxation of the ventricles
What comprises the SAN?
Autorhythmic myocytes
Small amount of muscle that points more positive than negative
What does the action of the SAN show on an ECG?
Deflection is wide (slow)
Not very high (thin muscle)
Positive
What does the action of the AVN show on an ECG?
AVN depolarisation
Isoelectric ECG
Slow signal transduction
Protective
What does the Bundle of His divide into?
What does the action of the bundle branches show on an ECG?
2 bundle branches
Sharp but small downward spike - due to thin wall of muscle
What does the action of the purkinje fibres show on an ECG?
Ventricular depolarisation, followed by late ventricular depolarisation
Upward stroke, QRS peak
What does the action of the ventricular repolarisation show on an ECG?
Period of isoelectric status
Slow, domed T-wave
Where do you place the leads?
Lead I (one L) Right Arm to Left Arm
Lead II (two L’s) Right Arm to Left Leg
Lead III (three L’s) Left Arm to Left Leg
What is the rule of reading for the leads to figure out which are the anodes / cathodes?
English is read left to right and top to bottom.
Polarity does that too.
Drawn as a triangle and reading left to right and top to bottom the first electrode of each pair you reach is the anode (-ve)
Right arm anode
Left Leg cathode
Left arm both
Where are the V1, V2, V3 and V4 electrodes placed?
V1 - Right sternal border
In the 4th intercostal space
V2 - Left sternal border
In the 4th intercostal space
V3 - Halfway between V2 and V4
V4 - Mid-clavicular line
In the 5th intercostal space
Where are the V5 and V6 electrodes placed?
V5 - Anterior axillary line
at the level of V4
V6 - Mid-axillary line
at the level of V4
An ECG graph paper has big squares, that are further divided up into smaller squares?
What do each of these squares represent?
Big square - 0.2s, 0.5mV
Little square - 0.04s, 0.1mV
What else is seen on an ECG graph?
Top left = details of the patient e.g. name, sex, age
Bottom = date and printing speed
Left bottom = where the ECG was taken e.g. ICL occupational health service
Top = shows voltage
Names of the 12 leads labelled
Bottom = rhythm strip
Top = HR
Machine can also tell you whether it is a normal sinus rhythm / what the issues may be
What view of the heart does V1 and V2 look at?
What view does V5 and V6 look at?
V1 and V2 = septum
V5 and V6 = lateral side of the heart
What are the three main coronary arteris?
Left circumflex (LCx)
Left anterior descending (LAD)
Right coronary artery (RCA)
Which leads correspond to which coronary arteries?
LCx = I, aVL, V5, V6 LAD = V1, V2 RCA = II, III, aVF, V3, V4
Which single lead is most commonly used to identify sinus rhythm / arrhythmias?
Lead II
What is the normal duration for:
R-R interval P wave P-R interval QRS QT interval T wave
R-R interval = 0.6-1.2 s P wave = 80 ms P-R interval = 120-200 ms QRS = <120 ms QT interval = 420 ms T wave = 160 ms
What are the bipolar leads?
Where are they found?
Leads I, II and III - as they all have an electrode that is positive, and an electrode that is negative
I = between right arm (-ve) and left arm (+ve) II = between right arm (-ve) and left leg (+ve) III = between left arm (-ve) and left (+ve)
What are the augmented leads?
aVF, aVL, aVR - they use fixed cathodes but virtual anodes
Where are the fixed cathodes and virtual anodes for the leads:
aVF
aVL
aVR
aVF = left leg cathode, lead I anode (between right and left arms)
aVL = left arm cathode, lead II anode (between right arm and left leg)
aVR = right arm cathode, lead III anode (between left arm and left leg)
Where are the right angles formed between the different leads?
(aVF, aVL, aVR, lead I, lead II, lead III)
aVF and lead I
aVL and Lead II
aVR and lead III
How can you calculate the net vector / deflection of the QRS?
Use the boxes on the graph, find the difference between the positive and negative deflections of the QRS complex
Use lead II and aVL to:
Calculate theta in the triangle.
Then calculate the yellow angle as you need to find the angle from 0 degrees.
The yellow angle shows the ventricular axis
tan (theta) = 0.444
theta = 24 degrees
Yellow angle =
60 degrees - 24 degrees = 36 degrees
This is the ventricular axis
So how do you find the ventricular angle axis?
Work out the net vector of 2 leads e.g. of lead II and aVL
Use those 2 net vectors to construct a triangle
Figure out theta in the triangle
Then figure out the angle from 0 degrees to the net vector
What is the ECG reporting procedure?
Is it the correct recording?
Review the signal quality and leads? - e.g. if they have muscular conditions, may give too much background noise
Verify the voltage and paper speed?
Review the patient background if available
What are the 7 steps when looking at an ECG?
Step 1 = rate and rhythm - regular?
Step 2 = P wave and PR interval - how long does it last, and does every P wave result in an R wave?
Step 3 = QRS duration
Step 4 = QRS axis - should be between -30 and +90 degrees
Step 5 = ST segment - should be flat and at the isoelectric point
Step 6 = QT interval - duration
Step 7 = T wave - shape
Can you defibrillate someone in asystole (i.e. a flatline)?
Not a shockable rhythm
Instead good to administer drugs and start CPR (chest compressions)
What is a sinus rhythm?
Sinus = signal is being generated by the SAN
What is a normal sinus rhythm on an ECG?
Each P-wave is followed by a QRS wave (1:1)
Rate is regular (even R-R intervals
What is sinus bradycardia on an ECG?
Each P-wave is followed by a QRS wave (1:1)
Rate is regular (even R-R intervals) and slow (56 bpm)
Can be healthy, caused by medication or vagal stimulation
What is sinus tachycardia on an ECG?
Each P-wave is followed by a QRS wave (1:1)
Rate is regular (even R-R intervals) and fast (107 bpm)
Often a physiological response (i.e. secondary)
What is sinus arrhythmia on an ECG?
Each P-wave is followed by a QRS wave
Rate is irregular (variable R-R intervals) and normal-ish (65-100 bpm)
R-R interval varies with breathing cycle
What is atrial fibrilation?
Oscillating baseline – atria contracting asynchronously
Rhythm can be irregular and rate may be slow
Turbulent flow pattern increases clot risk
Atria not essential for cardiac cycle
What is atrial flutter?
Regular saw-tooth pattern in baseline (II, III, aVF)
Atrial to ventricular beats at a 2:1 ratio, 3:1 ratio or higher
Saw-tooth not always visible in all leads
What are the features of first degree heart block?
Prolonged ST segment/interval caused by slower AV conduction
Regular rhythm: 1:1 ratio of P-waves to QRS complexes
Most benign heart block, but a progressive disease of ageing
What are the two types of second degree heart block?
Mobitz I
Mobitz II
What are the features of Mobitz I?
Gradual prolongation of the PR interval until beat skipped
Most P-waves followed by QRS; but some P-waves are not
Regularly irregular: caused by a diseased AV node
Also called Wenckebach
What are the features of Mobtiz II?
P-waves are regular, but only some are followed by QRS
No P-R prolongation
Regularly irregular: successes to failures (e.g. 2:1) or random
Can rapidly deteriorate into third degree heart block
What are the features of third degree heart block?
P-waves are regular, QRS are regular, but no relationship
P waves can be hidden within bigger vectors
A truly non-sinus rhythm – back-up pacemaker in action
What are the features of ventricular tachycardia?
P-waves hidden – dissociated atrial rhythm
Rate is regular and fast (100-200 bpm)
At high risk of deteriorating into fibrillation (cardiac arrest)
Shockable rhythm – defibrillators widely available
What are the features of ventricular fibrillation?
Heart rate irregular and 250 bpm and above
Heart unable to generate an output
Shockable rhythm – defibrillators widely available
What are the features of ST elevation?
P waves visible and always followed by QRS
Rhythm is regular and rate is normal (85 bpm)
ST-segment is elevated >2mm above the isoelectric line
Caused by infarction (tissue death caused by hypoperfusion)
What are the features of ST depression?
P waves visible and always followed by QRS
Rhythm is regular and rate is normal (95 bpm)
ST-segment is depressed >2mm below the isoelectric line
Caused by myocardial ischaemia (coronary insufficiency)
