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Biological measurement questions
Every heartbeat starts as an electrical event, and the body is salty enough to conduct it to the skin. What arrives there is about a millivolt, buried in mains hum, and turning that into a trace a cardiologist can read is a problem in amplification before it is a problem in medicine.
17 original questions · 51 marks · the biological measurement notes · Medical physics
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State what is meant by the resting potential of a muscle cell, and state what happens to it during an action potential.
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The resting potential is the potential difference held across the cell membrane by the unequal distribution of ions, about 90 mV with the inside negative (1). During an action potential ions cross the membrane, the p.d. briefly reverses (depolarisation) and is then restored (repolarisation) (1).State what the P wave and the QRS complex of a normal ECG each represent.
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The P wave is the depolarisation of the atria, which then contract (1). The QRS complex is the depolarisation of the ventricles, which then contract (1).State what the T wave represents, and state why it is broader and lower than the QRS complex.
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The T wave is the repolarisation of the ventricles as they recover and relax (1). It is broader and lower because repolarisation is slower than depolarisation and less well synchronised across the muscle, so the same recovery is spread over more time (1).On an ECG trace, successive R spikes are 0.60 s apart. Calculate the heart rate in beats per minute.
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One R to R interval is one beat, so the rate is 60/T (1) = 60/0.60 = 100 beats per minute (1).State two requirements of the amplifier in an ECG machine, and give a reason for each.
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Any two of: high gain, because the signal at the skin is only about a millivolt and must reach a volt or so to be displayed; high input impedance, so that the large resistance of the skin contact does not take most of the signal; low noise, so that the 50 Hz mains interference picked up by the patient does not swamp the trace (2).State the approximate size of the potential difference an ECG machine detects between two skin electrodes.
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About 1 mV (1). That is why the amplifier is the central component of the machine.Explain how the electrical activity of the heart produces a potential difference that can be detected at the skin.
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A wave of depolarisation sweeps across the heart muscle, so at any instant part of the heart is depolarised and part is not (1). That leaves a separation of charge with a size and a direction, so the heart behaves as an electric dipole which grows, turns and shrinks through the beat (1). The body fluids conduct, so this dipole sets up small potential differences across the torso, and two electrodes on the skin measure the p.d. between the points they touch (1).Explain why the amplifier of an ECG machine must have a very high input impedance.
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The electrode and the skin beneath it form a contact of considerable resistance, tens of kilohms even with gel (1). The contact and the amplifier input act as a potential divider across the source of the signal (1). If the amplifier input resistance were comparable with the contact resistance it would draw current and most of the signal would be dropped across the contact instead of reaching the amplifier, so the input impedance must be very much the larger of the two (1).An ECG amplifier turns a 1.2 mV input into a 2.4 V output. Calculate its voltage gain. The output cannot exceed 5.0 V: calculate the largest input the amplifier can handle without distorting the trace.
Explain why the QRS complex of a normal ECG is much larger than the P wave, and state why the repolarisation of the atria is not seen as a separate feature.
An ECG is printed on paper moving at 25 mm s−1. Successive R spikes on the printout are 20 mm apart. Calculate the interval between beats and the heart rate in beats per minute.
A normal ECG shows a flat section of about 0.15 s between the end of the P wave and the start of the QRS complex. Explain what is happening during this time.
State why a conducting gel is used between an ECG electrode and the skin, and why several electrodes are attached rather than two.
Describe fully how an ECG trace is obtained, from the electrical event in the heart muscle to the trace on the display.
On an ECG trace, R spikes occur at 0.72 s, 1.50 s, 2.20 s and 2.95 s. Calculate the three intervals between beats, the mean heart rate, and comment on whether the rhythm shown is regular.
Explain why 50 Hz interference appears on an unscreened ECG, and explain how the machine is designed to reject it.
A student says that an ECG shows the heart pumping. Explain why this description is wrong, and state what has to be true of a trace for it to be called normal.
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