Physics 2003

Question 1

A fixed performance oxygen delivery system can be obtained with:

• an O2 tent
• a venturi system
• a t-piece suppled by a FGF of 3 times the tidal volume
• by use of a demand valve
• with a Mapleson A circuit with a FGF equal to minute volume

Answer 1

TTFTT

Question 2

Concerning the transfer of heat:

• boiling involves transfer of heat without a change in temperature
• an adiabatic change retains the heat of reaction within the system
• a body with a high heat capacity will transfer heat to one with a low heat capacity at the same temperature
• radiation is proportional to the fourth power of the absolute temperature
• the response time of a thermometer increases with it’s heat capacity

Answer 2

TTFTT

• adiabatic change to the volume of a gas involves no transfer of heat to or from the system
• transfer of heat requires a temperature gradient
• radiation is proportional to AT⁴

Question 3

When gas flow is laminar, the flow rate depends on the:

• square of the radius
• pressure drop along the tube
• viscosity of the gas
• length of tube
• density of gas

Answer 3

FTTTF

Question 4

The following are true of nerve stimulators:

• applied electrical potential is as high as 150V
• tetanic stimulus is a set frequency of 50Hz
• TOF involves a pulse duration of 200ms and a frequency of 2Hz
• higher electrical potentials are necessary if subcut electrodes are used
• the apparatus uses a square wave electrical signal

Answer 4

TTTFT

Question 5

Concerning the measurement of temperature:

• one Kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water
• the triple point of water is at -0.05C
• alcohol thermometers are unsuitable for high temperature as alcohol boils at 55C
• mercury solidifies at -39C
• more than 2 mins are necessary for thermal equilibration to be achieved with a mercury thermometer

Answer 5

TFFTT

BP of alcohol is 78C

Question 6

When considering heat loss from the body during anaesthesia:

• conduction is the most important mechanism
• convection is decreased when the air next to the body is warm
• radiation is decreased by aluminium foil blankets
• respiration accounts for 30% of the total heat loss
• the cooling effect of sweating is decreased when the relative humidity increases

Answer 6

FTTFT

Question 7

With laminar flow, when the diameter of the tube is doubled the flow rate will increase in direct proportion to:

• the density of the gas
• the pressure difference across the tube
• barometric pressure
• ambient pressure
• viscosity of the gas

Answer 7

FTFFF

Question 8

A thermistor:

• responds more quickly to temperature change than a mercury in glass thermometer
• can be used in the measurement of blood flow
• has a linear response of resistance change with temperature
• is well suited to the measurement of small temperature changes
• has a very large thermal capacity

Answer 8

TTFTF

• thermistors consist of a small bead of metal oxide, with low thermal capacity, high sensitivity and fast response time
• resistance falls exponentially as temperature rises
• thermistor responds more quickly to changes in temperature than a thermocouple
• thermistors response is non-linear but they are suitable for detecting small temperature changes and have a small thermal capacity

Question 9

In a rotameter:

• the bobbin must not rotate
• the rotameter is read from the bottom of the bobbin
• the tube must be vertical
• static electricity is a cause of inaccuracy
• the flowmeters are gas specific

Answer 9

FFTTT

Question 10

The following are true of biological potentials:

• the normal signal amplitude of the ECG is of the order of 5-10mV
• recordings are normally a display of the difference in potential between 2 electrodes
• the frequency distribution of an ECG signal is 0.5 - 80Hz
• the EMG can cause interference on the ECG
• the EEG does not interfere with the ECG

Answer 10

FTTTT

Question 11

pH = pK + log (HCO3-/ alpha PCO2). In this equation:

• the p in the pH and pK indicate a partial pressure
• the term HCO3- refers to the standard bicarbonate
• pK varies with temperature
• alpha varies with temperature
• substitution of another acid and it’s radical (eg phosphate) would not alter the value of pK

Answer 11

FFTTF

• the p indicates a negative logarithmic function
• the P in PCO2 indicates partial pressure
• the bicarbonate in the equation is actual bicarbonate
• both pK and alpha depend on the dissociation of a weak acid, this is a temperature dependent process
• since pK is the pH at which the weak acid is half dissociated, it will depend on the acid involved

Question 12

The conventional oxygen fail-safe mechanism of anaesthetic machines:

• is pressure sensitive
• eliminates the possibility of anoxic gas mix
• cuts off all gas flows when activated
• is flow sensitive
• depends on an intact electrical supply to the machine

Answer 12

TFFFF

Question 13

Laplace’s Law:

• relates tension to the square root of the radius
• is applicable to structures with high compliance
• implies that interconnecting bubbles stabilise to equalise radii
• needs the length of tube to be known when applied to a tubular structure
• can be used to calculate MAP

Answer 13

FTFFF

P = 2T/R

Laplace’s Law

(for a tube P = T/R, therefore length is not required)

Question 14

The critical temperature of a gas is the temperature:

• at which it’s pressure is the critical pressure
• at which freezing takes place
• at which the attraction between the molecules is negligible
• above which it cannot be liquefied by pressure alone
• at which Boyle’s law is perfectly obeyed

Answer 14

FFFTF

• as gases cool the attraction between molecules is increased and their behaviour deviates from the gas laws
• the critical pressure is the vapour pressure of a substance at it’s critical temperature

Question 15

Gas chromatography:

• uses the principle of selective retardation of the passage of gases through a tube
• can be used to detect enflurane
• measures individual components in a mix of gases
• uses CO2 as a carrier gas
• is unsuitable for detecting N2O

Answer 15

TTTFF

• very accurate method of analysing a discrete sample of gas
• a small sample of unknown gas mix is injected into a stream of inert carrier gas (helium/nitrogen) flowing through a column of liquid coated particles (stationary phase)
• as the gas flows through the column the component gases are slowed down according to their solubility in the stationary phase liquid and are separated out by the end of the column where they can be analyzed by a non-specific gas analyzer
• the gases are identified by the comparison of the time lags of the different peaks with those of known gas samples; their concentrations are given by the height of the peaks

Question 16

Helium:

• has a viscosity similar to oxygen
• is stored as a liquid in cylinders
• when inhaled causes voice changes
• decreases the work of breathing in bronchospasm
• supports combustion

Answer 16

TFTFF

• helium is stored as a gas in brown cylinders at 137 bar
• helium is less dense than nitrogen and therefore useful in reducing WOB where obstruction of the airway produces turbulent flow, eg upper airway obstruction, by reducing Reynolds number and promoting laminar flow
• not useful in bronchospasm because flow in the alveolar bronchioles is already mainly laminar
• inert gas so doesn’t support combustion

Question 17

In the measurement of temperature:

• the fixed point of the Kelvin scale of temperature is the freezing point of water at one atmosphere pressure
• one degree Kelvin and one degree centigrade are exactly equal divisions of temperature
• Bourdon Gauge can be used to measure temperature
• response time of an electrically measuring thermometer is shortened by increasing the size of the measuring probe
• mercury thermometers are too inaccurate for use in intensive care

Answer 17

FTTFF

Question 18

The following are true:

• critical temperature is the temperature above which a substance cannot be liquefied no matter how much pressure is applied
• critical temp of O2 = -119C
• nitrous oxide cylinders always contain some liquid nitrous oxide
• the critical temperature of nitrous oxide is 48.5C
• critical pressure is the vapour pressure of a substance at it’s critical temperature

Answer 18

TTFFT

Question 19

The Severinghaus CO2 electrode:

• has sodium bicarbonate in the electrolyte solution
• is affected by nitrous oxide in the sample
• has CO2 sensitive glass
• is more accurate for blood than gas samples
• is affected by changes in temperature

Answer 19

TFFFT

• meausures PCO2 indirectly via H+ concentration
• H+ sensitive glass is surrounded by a mesh impregnated by bicarbonate
• glass electrode is covered by a membrane permeable only to CO2 and is unaffected by N2O presence
• no difference between blood/gas samples due to speed of diffusion of CO2, unlike an O2 electrode which is more accurate for gas samples

Question 20

The following statements are correct:

• mainstream capnographs may significantly increase deadspace
• side stream capnography may be used with nasal cannulae
• side stream capnographs measure the dry gas partial pressure of the sample
• the response time of sidestream capnographs is more rapid than main stream capnographs
• end tidal CO2 concentrations may reflect changes in CO

Answer 20

FTTFT

• mainstream capnographs have dead spaces of 1-3mls
• they are rapid response
• inserted into the nostril side stream capnographs would measure CO2 exhaled

Question 21

PEEP increases:

• residual volume
• expiratory reserve volume
• TLC
• FRC
• closing volume

Answer 21

TTFTF

• PEEP increases FRC - FRC is made of expiratory reserve volume + residual volume

Question 22

Gas volumes may be accurately measured by a:

• Wright’s respirometer
• Benedict-Roth wet spirometer
• vitalograph
• pneumotachograph
• dry gas meter

Answer 22

FTTTT

Question 23

Concerning the cryoprobe:

• cooling is an adiabatic process
• rapid gas expansion from a capillary tube produces a fall in temperature
• cooling is due to internal energy change resulting from gas expansion
• CO2 is a suitable gas for routine use
• tip temperatures as low as -90C are required for efficient use

Answer 23

TTTTF

Question 24

The following are true of variable orifice flow meters:

• the pressure across the bobbin is constant irrespective of flow rate
• both the density and viscosity of the gas are important for calibration
• the flow meter must be slightly inclined to the vertical to prevent sticking of the bobbin
• the bobbin has slots to reduce resistance to flow
• electrostatic charges may build up during use

Answer 24

TTFFT

Question 25

Central venous pressure measurement: * must be carried out with the patient supine * must be zeroed at the second intercostal space in the mid-axillary line * should normally show a pressure of more than +10cms water * requires a catheter placed in the right atrium * can be accurately measured with an arterial pressure transducer

Answer 25

FFFFT * CVP is usually measured with the patient supine, reference point for zeroing depends on the patient's position * normal value = 0-8 cm H2O * the transducers for arterial/CVC pressure measurement are identical but calibrated differently

Question 26

The following are true of the measurement of O2 concentration: * oxygen is a paramagnetic gas because it's molecule has unpaired electrons in the outer shell * in a paramagnetic analyser the 2 glass spheres are filled with O2 * in a paramagnetic analyser, rotation of the dumbbell is balanced by tension in a suspending filament * water vapour does not affect paramagnetic O2 analysis

Answer 26

TFTTF * they're filled with nitrogen, which is diamagnetic

Question 27

The operation of a standard cylinder pressure gauge is based on: * Hagen Poiseuille formula * fixed orifice principle * Bernoulli principle * Bourdon tube principle * critical flow rate principle

Answer 27

FFFTF

Question 28

Concerning electrical safety: * with wet hands, contact impedance is increased * electrical shock is not a risk with potentials below 24v AC or 50V DC * the modern diathermy earthing plate is not connected directly to earth * microshock may lead to ventricular fibrillation via an intracardiac catheter with currents in excess of 150 microamps * leakage currents are caused by induced voltages in other circuits resulting from an alternating mains current

Answer 28

FFTTT

Question 29

Radiation from a source may imply the emission of: * beta particles * any rays * neutrons * alpha particles * any wave motion

Answer 29

TTTTT

Question 30

Plasma proteins: * exert an osmotic pressure of approximately 5.3 kPa * provide one half of the buffering capacity of the blood * are mostly in the anionic form * are the main source of carbamino groups * include plasminogen

Answer 30

FFTFT * plasma proteins (albumin, globulin, fibrinogen, plasminogen, protein C etc) can have carboxyl groups free for dissociation * account for 15% of buffering capacity of the blood * they exert an osmotic pressure of about 25mmHg * Hb is a major protein buffer and carbon dioxide in the RBCs reacts with amino groups to form carbamino compounds

Question 31

Flow: * can be measured from the pressure drop across a fixed orifice * may be measured accurately by the reflection of high frequency sound waves * may be measured using the Fick Principle * become turbulent when Reynolds number exceeds 2000 * in SI units is measured in ml/min

Answer 31

TFTTF - SI unit for flow is L/s - US measures velocity of flow, FLOWRATE requires a measure of cross sectional area

Question 32

Solubility of a gas in a liquid depends on: * the temperature * nature of gas * nature of liquid * Dalton's law * the partial pressure of gas in equalibrium with the liquid

Answer 32

TFFFT

Question 33

An ideal intravascular pressure measurement system includes: * resonant frequency of \<40Hz * compliant manometer tubing * a rigid diaphragm in the transducer * a damping coefficient 0.6 * small diameter manometer tubing

Answer 33

TFTTT * manometer tubing should be rigid and transducer diaphragm should have small displacement and a narrow lumen * optimal damping is 0.64

Question 34

Entonox: * is a 50:50 mix by weight of O2 and N2O * exists only in gaseous form in a cylinder * is supplied in cylinders filled to a pressure of about 140kPa x 100 (140 bar) * has a pseudocritical temperature of 5.5C * cylinders should be stored upright

Answer 34

FTTFF * it's a 50:50 mix by VOLUME * pseudocritical temperature is -5.5C * cylinders contain gas only at 137 bar and can therefore be stored horizontally

Question 35

In the measurement of anaesthetic gases and vapours * a paramagnetic analyser is sensitive to expired nitrogen * infrared analyzers are agent-specific * infrared analysers respond to isopropyl alcohol * mass spectrometry as not necessarily agent specific * acoustics can be used to monitor agent concentration

Answer 35

FFTTT * nitrogen is not paramagnetic (it's diamagnetic) * infrared radiation is absorbed by gases with 2 or more different atoms in their molecular structure, including isopropyl alcohol * the wavelength of peak absorption is specific to each gas and so you can use filters on an infrared analyzer to measure several gases * mass spectrometry measures agents according to mass, so agents of identical weight can be difficult to differentiate

Question 36

Surface tension: * is measured in Newtons per square metre * at the junction of fluid and the wall of a tube, is double that in a tubular structure * generates twice as much pressure in a sphere than in a tube * generates a transmural pressure directly proportional to the radius of a sphere * is directly due to molecular attraction

Answer 36

FTTFT * SI unit of surface tension is the Newton/metre * Newton's per square metre is a Pascal, a unit of pressure, not surface tension * the sphere has two planes of curvature - so the pressure generated is twice that of a cylinder * pressure gradient across the sphere wall is inversely proportional to the radius * surface tension is due to inter-molecular attraction - molecules on the surface are pulled into the body of the fluid whereas those in the centre are attracted from all directions, the surface will therefore tend to shrink

Question 37

In the measurement of arterial blood pressure using a transducer: * the catheter should be stiff-walled * " " should be long and narrow * an 18G cannula is most appropriate * systolic pressure in the aortic arch normally exceeds left ventricular end systolic pressure * systolic pressure in the dorsalis pedis artery normally exceeds radial artery pressure

Answer 37

TFFTT * system should be short, stiff-walled and wide * at the end of systole aortic pressure must be greater than the left ventricle to shut the aortic valve * as the pressure wave travels down the arterial tree, the shape changes and systolic pressure increases * this is due to elasticity of vessel walls and reflection of the waveform from the walls * this effect increases with distance from the aorta

Question 38

Mechanical displacement is converted into electrical current by: * amplifiers * transducers * oscilloscopes * photoelectric cells * transistors

Answer 38

FTFFF * photoelectric cells is a type of transducer but converts light energy to electrical energy

Question 39

Blood gas analyzers directly measure: * O2 content * partial pressure of O2 in arterial blood * Base excess * bicarbonate concentration * atmospheric pressure

Answer 39

FTFFF