Unit 6 - Cardiovascular - ECG Flashcards
What is the path taken by the electrical activity of the heart?
- Action potential (AP) generated in the sino-atrial node (SAN)
- Atrial depolarisation
- AP delayed at the atrioventricular node (AVN)
- AP travels down the septum to the apex of the heart and around the ventricles
- Wave of AP causes wave of contraction
- Wave of depolarisation follows wave of depolarisation
What does depolarisation of cardiac muscle cause?
Contraction of cardiac muscle
- negative to positive
What does repolarisation of cardiac muscle cause?
Relaxation
- positive to negative
What is depolarisation of cardiac muscle followed by?
Repolarisation
What are the different forms of cardiac action potential?
Pacemaker/nodal/slow response - If - ICa Non-nodal/fast response - INa
Why is direction considered on an ECG?
Cells may repolarise in the same order in which they were depolarised
The order may be reversed
What does the P wave on an ECG show?
Atrial depolarisation
- delay between P and Q
What does the QRS wave on an ECG show?
Ventricular depolarisation
What does the T wave on an ECG show?
Ventricular repolarisation
- delay between S and T
What does the ECG measure?
The electrical activity of the heart
What is used to measure an ECG?
An array of electrodes placed on the body surface
What initiates the cardiac cycle?
Action potentials propagate from the sinoatrial node (SAN) through the myocardium
How is the ECG measured?
The spread of action potentials results in an electromagnetic force vector being generated
How quickly does the trace on an ECG move?
Trace moves at 25 mm/s
- 300 big squares/minute
How is heart rate calculated from an ECG?
Intervals between waves
- normally R-R
What does the PR interval on an ECG show?
- 12 - 0.20 seconds
- represents the time between onset of atrial and ventricular depolarisation
- if > 0.20 seconds, possible AV conduction block
What does the QRS wave on an ECG show?
- 06 - 0.1 seconds
- represents ventricular depolarisation
- if prolonged (> 0.1s) conduction is impaired within the ventricles
- bundle branch block
- movement of pacemaker site
What does the QT interval on an ECG show?
0.2 - 0.4 seconds depending on heart rate
- gets shorter as heart rate increases
- can be correct for heart rate (QTc)
- represents the time for both ventricular depolarisation and repolarisation to occur
- rough estimate of ventricular AP duration
Prolonged QT intervals (> 0.44s) are diagnostic of certain types of tachyarrhythmia
What does the ST interval on an ECG show?
Indicator of myocardial infarction - depression - elevation AP conduction is affected by ischaemia - level will be elevated is transmural Also used to diagnose other conduction defects
When is there no movement on an ECG?
Resting polarised state
- no current
- no measurement
Which direction does the action potential travel on an ECG?
Action potential is stimulated and conducted left to right
- movement of positive charge towards positive electrode
What causes an upward deflection on an ECG?
A positive voltage
What happens on an ECG once depolarisation has finished?
No more movement of charge
- no potential difference again
What does lead 2 measure on an ECG?
On the cardiac axis to measure the P wave
- depolarisation from SA to AV node
- positive deflection on ECG
