13. Electrocardiogram Flashcards
What are the two signals seen by skin electrodes with each systole?
Depolarisation and repolarisation.
Why do cardiac myocytes need to be connected by gap junctions?
For coordinated depolarisation and coordinated contraction.
What is the path of conduction across the heart?
Conduction starts at the SA node. Depolarisation then spread across the intermodal tracts, that allow coordinated contraction of the atria. The signal is then held at the AV node for a short time to allow full contraction of the atria before ventricular contraction. The depolarisation spreads down the bundle of His, the left bundle branch conducts marginally faster than the right bundle branch because the left ventricle is slightly larger than the right. Blood is pushed from the bottom upwards through the large vessels. The Purkinje fibres then feed conduction to the rest of the myocardium.
When does repolarisation take place?
280ms after depolarisation.
What is the route of repolarisation in the heart?
The opposite direction over the ventricles to repolarisation. The epicardial surface repolarises first and then the endocardial surface repolarises last.
Which two planes are inspected in an ECG?
The frontal and horizontal planes.
What are the two types of leads used in electrocardiograms?
Unipolar leads - read from labelled positive electrode and utilise several other electrodes as the negative.
Bipolar leads - Use one positive and one negative electrode form standard limb leads in the configuration of Einthoven’s triangle.
What are the directions of leads in Einthoven’s triangle?
Lead I looks from the positive left arm to negative right arm.
Lead II looks from positive left leg to negative right arm.
Lead III looks from positive left left to negative left arm.
When does depolarisation and repolarisation cause positive and negative deflection?
Positive deflection from depolarisation moving towards an electrode or repolarisation moving away from an electrode.
Negative deflection from depolarisation moving away from an electrode or repolarisation moving towards an electrode.
What does the amplitude of a signal of an ECG depend on?
How much muscle is depolarising and how directly it’s moving towards the electrode.
Summarise what causes the signals of an ECG.
P wave - atrial depolarisation
Q wave - septal depolarisation spreading to the ventricle
R wave - main ventricular depolarisation
S wave - end ventricular depolarisation
T wave - ventricular repolarisation
What can cause confounding of ECG results?
Lead misplace meant, muscle contraction (from talking, moving etc.), interference from other machines etc, poor electrode contact (from sweating, or hair etc.).
What is the electrocardiogram?
The large changing electrical field which can be detected by electrodes on the body surface.
