Biological measurement Flashcards
What changes would you expect to see in the electrocardiogram if the patient began to take exercise?
T would get shorter as the flat part of trace would shrink
State two ways of ensuring good electrical contact between the electrodes and the person.
electrodes must be made from a material which does not become polarised
electrodes must be coated with conducting gel
hair and dead skin must be removed from the patient
State two properties of the amplifier needed to amplify the signal from the electrodes.
The amplifier Must have:
high gain
high input impedance
low noise
frequency response should be even across the frequency range
Describe the processes involved in the production of such an action potential when a nerve is stimulated
=Na+ ions move into the cell .
=The pd rises (from –70 to 0) , this is called depolarisation
=K+ ions move out of the nerve . The pd returns/falls to –70/resting potential, this is called repolarisation
=Na+ moving from 0 to +30 called reverse polarisation
=To restore starting equilibrium of ions, the Na/K pump operates
Describe the response of the heart to the action potential originating at the sino-atrial node
The pulse causes atria muscles to contract
Blood is forced into the ventricles
The pulse is delayed before firing ventricular node
ventricles contract
this forces blood out of heart to lungs and body
Describe the ion movement which produces depolarisation.
Na+ move from from outside to inside
Describe the ion movement which produces repolarisation
K+ from inside to outside
State what electrical event happens at points A and B and the physical change that results.
position A:
event - sino atrial node fires
result - atria contracts
position B:
event - ventricular node fires
result - ventricles contract
State, giving a reason, one precaution you would take when attaching the electrodes to the surface of the skin to ensure a good signal is obtained.
precaution: remove dead skin [or use conducting/electrode gel]
reason: to give best possible contact between person and electrode
The amplifier used must have a high gain. State two other properties of the amplifier.
low noise
high input impedance
When obtaining such a trace, electrodes are attached to the patient. State and explain two precautions which should be taken when attaching the electrodes to ensure reception of the best signal.
attach firmly = stop noise
remove dead skin/hair = reduce contact resistance
use conducting gel = remove air for better electrical contact
positioning of electrodes = to get largest pd
State what is meant by depolarisation and repolarisation and, in terms of ion movement, explain how each effect is caused.
Depolarization (change of voltage from a negative to positive value) is due to the opening of Na+ channels causing a rapid increase in the membrane conductance to Na+ and thus a rapid influx of Na+ ions into the cell; a Na+ current.
Repolarization (change of voltage from a positive back to a negative value) - the sodium channels close, while the K+ channels are open. This ensures a net outward current, corresponding to a negative change in membrane potential.
State how the action of the atria and ventricles correspond to the waveform PQRST shown in the diagram
Atrial contractions shows up as the P wave.
Ventricular contractions show as a series known as the QRS complex.
The T wave is the electrical activity produced when the ventricles are recharging for the next contraction (repolarizing).
An ECG trace is to be obtained for a patient. State and explain the procedure and some design features of the equipment needed to ensure a good trace is obtained.
To reduce contact resistance
• sandpaper skin/shave to remove hairs and some dead skin
• apply conducing gel
• securely attach more than one electrode
To remove unwanted signals
• electrodes should be non-reactive
• patient to remain relaxed and still
• shielded leads/reducing interference from ac sources
Properties of amplifier
• amplifier has large input impedance/high gain/low noise
State the electrical events which give rise to the points P, R amd T
P depolarisation of atria
R depolarisation of ventricles (and repolarisation of atria)
T repolarisation of ventricles
