Questions on biological measurement

Question 1

State what is meant by the resting potential of a muscle cell, and state what happens to it during an action potential.

Accepted Answer

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).

Question 2

State what the P wave and the QRS complex of a normal ECG each represent.

Accepted Answer

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).

Question 3

State what the T wave represents, and state why it is broader and lower than the QRS complex.

Accepted Answer

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).

Question 4

On an ECG trace, successive R spikes are 0.60 s apart. Calculate the heart rate in beats per minute.

Accepted Answer

One R to R interval is one beat, so the rate is 60/T (1) = 60/0.60 = 100 beats per minute (1).

Question 5

State two requirements of the amplifier in an ECG machine, and give a reason for each.

Accepted Answer

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 noise added by the amplifier itself does not swamp the millivolt signal (2).

Question 6

State the approximate size of the potential difference an ECG machine detects between two skin electrodes.

Accepted Answer

About 1 mV, which is 10⁻³ V (1).

Question 7

Explain how the electrical activity of the heart produces a potential difference that can be detected at the skin.

Accepted Answer

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).

Question 8

Explain why the amplifier of an ECG machine must have a very high input impedance.

Accepted Answer

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).

Question 9

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.

Accepted Answer

Gain = output/input = 2.4/(1.2 × 10⁻³) (1) = 2000 (1). The largest input is 5.0/2000 = 2.5 × 10⁻³ V = 2.5 mV (1). Beyond that the output would be clipped flat and the shape of the trace lost.

Question 10

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.

Accepted Answer

The P wave is the atria depolarising and the QRS complex is the ventricles depolarising (1). The ventricles carry far more muscle than the atria, so far more charge moves at once and the p.d. produced at the skin is several times larger (1). The atria repolarise while the ventricles are depolarising, so their much smaller signal is buried underneath the QRS complex (1).

Question 11

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.

Accepted Answer

Time = distance/speed = 20/25 (1) = 0.80 s (1). Rate = 60/0.80 = 75 beats per minute (1).

Question 12

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.

Accepted Answer

The signal that has just depolarised the atria is delayed on its way to the ventricles (1). During the delay the atria are contracting and emptying into the ventricles, so the ventricles fill before they are told to contract (1). Very little muscle is depolarising during that time, so there is almost no changing separation of charge and the trace stays close to the baseline (1).

Question 13

State why a conducting gel is used between an ECG electrode and the skin, and why several electrodes are attached rather than two.

Accepted Answer

Dry skin makes a poor contact of high resistance, and the gel lowers that resistance so that more of the signal reaches the amplifier (1). It also keeps the contact steady, so movement of the patient produces less drift on the trace (1). Several electrodes allow the p.d. to be measured between different pairs, and so the heart's dipole to be viewed from several directions rather than only one (1).

Question 14

Describe fully how an ECG trace is obtained, from the electrical event in the heart muscle to the trace on the display.

Accepted Answer

Cardiac muscle cells hold a resting potential across their membranes, and a wave of depolarisation sweeps across the heart, reversing that p.d. cell by cell (1). At any instant part of the heart is depolarised and part is not, so the heart acts as an electric dipole whose size and direction change through the beat (1). The conducting body fluids carry potential differences of about 1 mV to the skin, where electrodes with conducting gel pick them up (1). A differential amplifier of high gain raises the signal to a usable size, while its high input impedance stops the skin contact resistance from swallowing it (1). The amplifier responds to the difference between the electrodes and rejects what they both receive, chiefly 50 Hz mains hum, and filtering removes the rest (1). The amplified signal is plotted against time on a screen or on moving paper, giving the P wave, QRS complex and T wave of one beat (1).

Question 15

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.

Accepted Answer

Intervals: 0.78 s, 0.70 s and 0.75 s (1). Mean interval = (2.95 − 0.72)/3 = 0.743 s (1). Mean rate = 60/0.743 = 81 beats per minute (2). The three intervals differ by at most 0.08 s, a spread of about 10 per cent, so the rhythm is regular within the accuracy of reading the trace (1).

Question 16

Explain why 50 Hz interference appears on an unscreened ECG, and explain how the machine is designed to reject it.

Accepted Answer

Mains cables around the room produce alternating electric and magnetic fields at 50 Hz, and the patient and the leads act as an aerial that picks them up (1). The interference is comparable with or larger than the millivolt signal being measured, so it would otherwise dominate the trace (1). It arrives at every electrode almost equally, while the wanted signal is a difference between two electrodes, so an amplifier that amplifies only the difference between its inputs and ignores what is common to both rejects the hum (1). Screened leads and a filter tuned to remove what is left complete the job (1).

Question 17

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.

Accepted Answer

An ECG plots potential difference at the skin against time, and that p.d. comes from the wave of depolarisation crossing the muscle, not from any movement (1). Each chamber contracts shortly after it has depolarised, so the mechanical action follows the trace rather than appearing on it (1). A normal trace has the P wave, QRS complex and T wave present and in that order, with the QRS much the largest and the T wave broader and lower (1), the intervals within their usual ranges and the beats evenly spaced (1).

Question 18

A resting cardiac muscle cell maintains a constant potential difference across its membrane even though ions leak across the membrane the whole time. Explain how the resting potential is held steady, and state what happens to the membrane p.d. when the cell is triggered.

Accepted Answer

Ion pumps carry ions back across the membrane at the same rate as the leak carries them across, so the two currents balance and the membrane potential does not change (1); the cell goes on holding its separated charge like a very small charged capacitor (1). When the cell is triggered, ion channels open and ions rush through, so the p.d. briefly reverses: depolarisation (1).