Physics MCQ's 1

Question 1

The concentration of iso can be measured by which of these in a breathing system?

Infra-red analyser, mass spectrometer, paramagnetic analyser, ultrasonic analyser, refractometer

Answer 1

All of those except paramagnetic analyser.

Infrared- most common

Mass Spec- gold standard for any gas/vapour in theatre

Ultrasonic- historical only

Refractometer - bulky but does the job, better for industrial settings very accurate

Question 2

Capnograph can be obtained by:

Infrared spec, Raman spec, mass spec, gas chromotography, photo acoustic spectrometry?

Answer 2

Everything except gas chromotography- too slow response time.

Infrared- sufficient response for a trace

Raman- scattered light emitted at different but characteristic wavelengths from incident lights and depends on the gas present. Rapidly responding.

Photoacoustic -uses ultrasound and is rapid response.

Question 3

Flow volume loop:
1. Peak inspiratory flow rate is less than peak expiratory

2. Flow increases then reduces as inspiration progresses?
3. Area within the loop is increased in emphysema than normal?

Answer 3

1. True -
2. true - increases in first half, reaches a rounded plateau then falls. Smooth U shape.
3. False - airway resistance is increased and tidal volume is reduced, so the area within the loop is smaller than normal.

Question 4

Pulse oximeters:
1. Inaccurate in foetal Hb?

2. Have slower response times than transcutaneous oxygen electrodes?

Answer 4

1. False- HbF has identical absorption spectrum as HbA

2. False - transcutaneous oxygen electrodes have a slower response time.

Question 5

Why does methaemoglobin effect accuracy of pulse oximeter?

Answer 5

It absorbs light at an equal wavelengths of 660nm and 940nm, independent of PO2.

Question 6

What is methaemoglobin?

Answer 6

Is a haemoglobin in the form of a metalloprotein, in which the iron in the same group is Fe3+ state not the Fe2+ state.

This means it cannot carry oxygen to the tissue.

Question 7

When testing a transducer-cutaneous catheter used for the measurement of pressures:

1. When is optimal damping?
2. Frequency response should be?
3. What does application of a square wave pressure signal do?

Answer 7

1. OPTIMAL DAMPING: When the damping coefficient (D) is 0.64.
   If D is 1, implies critical damping.
2. FREQUENCY RESPONSE: Ideally above the first 8-10 harmonics of the fundamental frequency (heart rate), ie 10 times the heart rate.
3. SQUARE WAVE PRESSURE SIGNAL: It’s a way of testing a clinical catheter-transduced system connected to an amplifier.

Question 8

Oesophageal Doppler cardiac output measurement:

1. What does it measure?
2. How far is the probe inserted?
3. Is it influenced by body temperature?
4. Does it require US beam to be 90 degrees to direction of blood flow?

Answer 8

1. Measures blood flow (red cell velocity) in the descending aorta.
2. Probe inserted to the 40cm mark.
3. Not affected by temp- would affect pulmonary artery catheter CO measurement.
4. No, no change in wavelength would be seen at 90 degrees; no Doppler shift.

Question 9

Bispectral Index: BIS

1. What is it?
2. Decreases during normal sleep?
3. Predicts response to surgical stimuli?
4. Shows similar change with most anaesthetic agents?
5. Predicts brain damage during cardiopulmonary bypass?

Answer 9

1. A dimensionless number ranging for 0 - 100, using using information from EEG, to monitor the depth of anaesthesia.
2. True.
3. No
4. The BIS shows similar changes with most except ketamine.
5. The BIS falls with both deepening sedation and ischaemia but cannot differentiate the two.

Question 10

Diathermy:
1. How does it work?

2. What frequency flows through the return plate?
3. What wave pattern is used for coagulation and cutting?
4. Can diathermy pass a current the plate becomes detached?

Answer 10

1. Uses high frequency AC current greater than 0.5 - 1 MHz, and the degree of burning or coagulation depends on the current density at the tip of the probe.
2. The same frequency of current flows through the diathermy and return plate in monopolar diathermy.

But, in bipolar diathermy the return plate is effectively one of the tips of the probe.

3. Cut - sine wave.
   Coagulation - damped/ pulse sine pattern.
4. Current can flow if the plate is detached and any part of the patient is touching an unearthed metal object.

Question 11

Are mean pressures affected by damping with invasive BP monitoring?

Answer 11

No

Question 12

What does Grahams law state about the rate of diffusion? Is it affected by ambient pressure?

Answer 12

Rate of diffusion is inversely proportional to the square root of the molecular weight of the gas.

Rate of diffusion is unaffected by ambient pressure.

Question 13

Is a sphygomomanometer affected by hypo or hyperthermia?

Answer 13

No effect on measurement of blood pressure.