The ECG Flashcards
What is an ECG measuring?
It is a measure of the currents generated in the EXTRACELLULAR FLUID by the changes occurring
simultaneously in many cardiac cells
What is Ohm’s Law?
V = IR
V: Voltage
I: Current
R: Resistance
What are the Phases of myocyte action
potential?
Phase 0 - rapid depolarisation. Peak Summits to Sodium in
Phase 1 - partial depolarisation. Peak Plummets so potassium out
Phase 2 - plateau curve Continues so Calcium influx leading to Contraction
Phase 3 - repolarisation. peak Plummets so K+ open and influx of potassium as calcium channels closed
Phase 4 - Resting membrane potential reached. Heart is in diastole so no action potential needed to lead to contraction
What is the P wave on the ECG representing?
The atrial depolarisation
What is the QRS complex on the ECG representing?
Ventricular depolarisation
What is the T wave on the ECG representing?
Ventricular repolarisation
What is the normal PR interval values?
120-200 ms (3-5 small squares)
What is the normal QRS complex width?
Less than 120 ms (3 small squares)
0.06-0.1s
What is the normal value for the QT interval?
- Measure of time of ventricular repolarisation
Men: 350-440 ms
Women: 350-460 ms
What is Tachycardia?
increased heart rate (>100 bpm)
What is Dextrocardia?
heart on right side of chest instead of left
What is Bradycardia?
decreased heart rate (<60 bpm)
What are the 2 types of ECG can we have?
A 12 lead and a 3 lead
How do electrical impulses in the heart move?
- Electrical impulses in the heart move in 3 dimensions
- ECG only measure voltage in 1 dimension
Why does an impulse look big or small?
- If an impulse is towards the electrode it looks big
- If an impulse is away from the electrode it looks small or even negative
What is the difference between an ECG electrode and lead?
Electrode:
- Physical connection to patient in order to measure potential at that point
- 10 electrodes to record a 12 lead ECG
Lead:
- Graphical representation of electrical activity
- Calculated by the machine from electrode signals
- 12 leads for a 12 lead ECG
What is a bipolar vs unipolar lead?
Bipolar:
- Measures potential difference (voltage) between 2 electrodes
- One electrode designated positive and the other negative
Unipolar:
- Measures potential difference between an electrode (+ve) and a combined reference electrode (-ve)
- Sometimes known as augmented leads
Where do the standard limb leads go?
- form a triangle between electrodes on the wrists and left leg (right leg is a ground electrode)
- the negative poles are REFERENCE electrodes and the positive poles are RECORDING electrodes
Where do augmented leads go?
- Augmented leads (aVR, aVL & aVF) bisect the angles of the triangle by combining two electrodes as reference e.g. for lead aVL, the right wrist & foot are combined as the negative pole, thus creating a reference point along the line between them, pointing toward the recording electrode on the left wrist
Where do the precordial leads go?
recording electrodes placed on the chest. 6 leads
When reading an ECG what does each square represent?
When reading an ECG, the graph shows changes in voltage over time, each small square across represents 40ms & each big square across represents 0.2s
In a normal ECG the P and T waves should be?
- In a normal ECG the p waves are POSITIVE in EVERY LEAD (apart from the aVR)
- T waves are POSITIVE in EVERY LEAD (apart from the aVR & sometimes the V1 and V2 depending on trace)
How many of each leads go where in an ECG summary?
3 unipolar - avR, avL, avF
3 bipolar - I, II, III (4 sticky pad)
6 chest (6 sticky pad)
