Cardiovascular Physiology - ECG Flashcards
What are the necessary components of an electrocardiogram?
Systematically from the patient end:
Electrodes - Adherent electrical conductors with a silver/silver chloride layer that connects to the patient’s skin, and detects tiny electrical potentials conducted from the myocardium.
Skin needs to be dry, clean and free from hair
Cables - To safely conduct the signals to the processing unit, the cables must be insulated electrical conductors with sufficient shielding to prevent significant interference from other instruments
Processor - Filters and amplifies the incoming signal to enhance the desired signal and reduce interference
Monitor - To display the electrocardiogram
What is Einthoven’s law?
IMAGE
In a standard 12-lead ECG, the total electrical potential of the limb leads (individually between 0.1 and 2mV), is 0.
I + II + III = 0
The positive (R) deflections and negative (Q) deflections in each lead are summed to give a net value for each lead.
These limb leads form the sides of a triangle (Einthoven’s triangle) around the heart, with the electrodes at the points.
Einthoven’s law can be used to verify correct lead placement, as well as to calculate a bipolar lead when the other two leads are known (eg. II = I + III)
What are ECG leads?
When two electrodes are placed on the skin either side of the heart, then the potential difference between these two electrodes can be measured
This generates a ‘lead’. The change in amplitude reflects the direction of action potential propagation within the heart, relative to the positions of the two electrodes
There are limb, precordial, and augmented leads.
Limb leads
I - From R to L arm
II - From R arm to L leg
III - From L arm to L leg
Precordial leads
V1-V6, six electrodes across the precordium. Each is compared against a virtual indifferent electrode to produce a lead.
Augmented leads
Generated by the ECG processor, using the indifferent electrode
aVF - From indifferent to L leg
aVL - From indifferent to L arm
aVR - From indifferent to R arm
The indifferent electrode is the virtual electrode formed by averaging the values generated by the three limb leads.
What happens to the electrical signal of an ECG as it passes from patient to monitor?
Potential differences detected by skin electrodes are very small, requiring processing in order to be usefully displayed on a monitor or printout.
First, background interference needs to be removed:
Common mode rejection
Identical electrical activity in multiple leads is removed from the signal, as it is likely to represent interference rather than genuine cardiac activity. This is enabled by having a ground electrode, usually on the right leg
Sensitivity and frequency filters
Diagnostic ECG is more sensitive, using a higher range of frequencies, but subject to more interference
Monitoring ECG (3 lead ECG) has a smaller frequency range, and therefore less resolution, but less prone to interference.
This filtered signal is then amplified to produce a waveform
Explain the cardiac axis on an ECG
IMAGE
A graph can be drawn using lead I (drawn from Left to Right) on the X axis, and aVF (drawn Cranio-caudally) on the Y axis, as these leads are perpendicular to each other
Normal axis is between -30° and 90°, as a result of the action potential passing antero-infero-laterally from RA to LV.
If both leads have largely positive deflections, then the axis will fall within the normal quadrant.
If Lead I is +ve, but aVF is -ve, then that reflects left axis deviation.
If Lead I is -ve, but aVF is +ve, then that reflects right axis deviation
If both leads are negative, then that suggests an extreme or indeterminate axis.
What causes left axis deviation on an ECG?
Left bundle branch block (LBBB)
Left anterior fascicular block
Inferior MI
Left ventricular hypertrophy
Ventricular pacing and arrhythmias
Abdominal distension
Short body shape
Hyperkalaemia
What causes right axis deviation on an ECG?
Normal in children and teenagers
Right bundle branch block (RBBB) Left posterior fascicular block
Anterolateral MI
Right ventricular hypertrophy
Pulmonary embolism
Dextrocardia
Tall thin body shape
What can cause errors with ECG monitoring?
Poor signal
Electrode not stuck properly
Sweaty, dirty, or hairy skin
External interference
Diathermy
Movement
Shivering
What ECG changes are seen in hypokalaemia and hyperkalaemia?
IMAGES
Hypokalaemia
Large P waves
Widening PR interval
ST depression
Flattened T waves
U waves
May progress to Torsades des Pointes
Usually only seen below 2.6mmol/L
Hyperkalaemia
Flattened P waves
Prolonged PR interval
Broad QRS
Tall tented T waves (most sensitive marker)
Usually only seen above 5.6mmol/L
What ECG changes are seen with hypocalcaemia?
Prolonged QT interval
Usually only seen if adjusted calcium below 1.8mmol/L
May progress to Torsades des Pointes although less commonly so than in hypokalaemia
Explain Torsades des Pointes
Polymorphic ventricular tachycardia
Often follows extreme prolongation of QT interval
Treatment is magnesium replacement or overdrive pacing
Will usually progress to VF if untreated
What ECG changes are seen in hypothermia?
Firstly bradycardia and shivering
PR prolongation as hypothermia worsens
Osborn (J) waves, which may be bigger than the QRS itself if temperature below 29°C.
