Option - Chapter 3 Flashcards
What happens each time your heart beats?
The electric potential of the heart changes by more than 100mV as the muscles of the heart contract and relax in a sequence that forces blood from the two atrial chambers of the heart into the corresponding ventricles then out to your arteries.
What causes your heart to beat?
The Sino-atrial node which generates an electrical impulse, which causes positive ions to move into, then out of, the nerves and muscle cells in the heart. This makes them contract and relax in a sequence.
What is depolarising and repolarising?
ASK DAD LOL
What does ECG stand for?
ECG stands for electrocardiogram
What does an ECG do?
An ECG designed to measure and record the potential difference between two points on the surface of the body. And can display information regarding the condition of the heart.
What is the PD between any two point due to?
The conductivity of the body fluids, the position of the points of the body and the potential of the heart.
Where are the electrodes of the ECG attached to?
To two limbs and a third limb to act as an earth.
On page 2 of chapter 3 use the ECG in order to describe the main features of each point P,Q,R,S and T
P - due to depolarisation and contraction of the atrial heart muscles when blood that has entered the heart is forced into the ventricles.
Between P and Q, the impulse is delayed at the atrioventricular node to allow the ventricles to fill
QRS - is due to depolarisation after Q and contraction after R of the ventricles’ heart muscles when blood is pushed out of the heart.
T - is caused by repolarisation and relaxation of the heart muscles after blood is pushed out of the heart from the ventricles.
Describe the circuit which is used in order for an ECG to be displayed on a screen
Two electrodes are placed on the skin at the point at which you want to see the PD between, and a third electrode is added as an earth (normally the leg). The pulses detected by the electrodes are then passed into an amplifier which is able to amplify the signal from 1mV to 1V this allows it to be displayed on an oscilloscope. This boosted signal is then displayed on a VDU screen allowing it to be read.
What is voltage gain?
Voltage gain = the amplifier output PD / the input PD to the amplifier
What characteristics must an amplifier have?
Its voltage gain must be of the order of 1000 over the frequency range up to about 20Hz. Beyond this frequency, unwanted signals due to muscle activity or from mains supply must be filtered out.
Its frequency response should be even across the frequency range otherwise the output voltage from the amplifier will be distorted.
A high input impedance is important otherwise a large fraction of the input PD will be lost due to body resistance
The amplifier must have a high signal to noise ratio otherwise the output signal is masked by random electrical signals generated in the amplifier its self.
How is the contact resistance lowered on the skin?
Body hair is removed, skin is cleaned and a conduction paste is used between the skin and the electrode.
