Physics & Equipment Flashcards
(171 cards)
1
Q
Define heat.
A
Thermal energy that flows from one body in contact with another when they are at different temperatures
2
Q
Define specific heat capacity.
A
Heat required to raise unit mass of the substance by one degree of temperature
3
Q
State the equation for specific heat capacity.
A
Q = mcΔT
Q: amount of heat needed (in joules or calories (1 calorie = 4.18 J))
m: mass of the body
c: specific heat capacity
Expressed in J/g/K
ΔT: change in temperature
4
Q
Define temperature.
A
Mean kinetic energy of molecules within a body
5
Q
Define conduction with regards to heat transfer.
A
Occurs via direct collisions between atoms and molecules of warmer and cooler regions and the resultant transfer of kinetic energy
Unit of conductivity is W/K/m
6
Q
Define convection with regards to heat transfer.
A
Transfer of heat from a body by the liquid or gas that surrounds it
Passive: still air
Active: moving air (e.g. bear hugger)
7
Q
Define radiation with regards to heat transfer.
A
Hot bodies emit thermal energy in the form of electromagnetic radiation resulting in heat transfer
8
Q
State Stefan-Boltzmann Law.
A
Radiation energy per unit time from a black body is proportional to the fourth power of the absolute temperature
9
Q
Define emissivity.
A
Is a dimensionless quantity which quantifies the ability of a body to radiate heat
This is expressed as a comparison to a perfect black body surface at the same temperature (a black body has a value of 1)
10
Q
Define a black body.
A
A body that absorbs all radiation that falls upon it
11
Q
State the routes of heat loss from the body and their relative proportions.
A
Radiation: 40%
Convection: 30%
Sweating: 20%
Respiration: 10%
NOTE: average emission is 50 watts per square metre
12
Q
State the first law of thermodynamics.
A
Energy cannot be created or destroyed, only transformed from one form to another.
ΔU = Q − W
Where:
ΔU = change in internal energy of a system
Q = heat added to the system
W = work done by the system
13
Q
State the second law of thermodynamics.
A
In any natural thermodynamic process, the total entropy of a system and its surroundings always increases.
ΔS = ΔQ/T
ΔS: change in entropy
ΔQ: heat entering the system
T: temperature
14
Q
State the third law of thermodynamics.
A
As the temperature of a system approaches absolute zero (0 K), the entropy of a perfect crystalline substance approaches zero.
The temperature absolute zero is unattainable.
15
Q
Define adiabatic expansion.
A
Thermodynamic process in which a gas expands without exchanging heat with its surroundings.
Any change in internal energy comes solely from work done by the gas.
It usually leads to a drop in temperature, since the gas does work but receives no heat to compensate.
16
Q
Describe the Joule-Thompson effect.
A
A gas changes temperature when it moves from a higher pressure to a lower pressure and for most gases, they cool
17
Q
Define a wave.
A
Series of repeating disturbances that propagates in space and time
Longitudinal Waves: oscillations occurring in the direction of travel
E.g. sound waves
Transverse Waves: oscillations perpendicular to the direction of travel
E.g. Mexican wave
18
Q
State the equation that gives the velocity of a wave.
A
Velocity = Frequency x Wavelength
19
Q
Which sound wavelengths and frequencies can be heard by the human ear?
A
20-20,000 Hz
170 mm to 0.17 mm
20
Q
How does the speed of light differ from the speed of sound in terms of the densities of the media it travels through?
A
Speed of light travels faster through less dense materials
Sound travels faster through more dense material
21
Q
Define simple harmonic motion.
A
Characterised by periodic oscillation about the equilibrium position and each oscillation is one cycle. The acceleration of an object is proportional and in opposition to its displacement from the equilibrium position.
Example: child on a swing
22
Q
Define resonance.
A
Natural tendency to oscillate
When maximal amplitude is gained for the effort put in
23
Q
Define natural frequency.
A
Frequency at which a system oscillates when displaced from its equilibrium position and allowed to vibrate freely without any external forces acting on it
24
Q
Define damping.
A
Tendency to resist oscillation
25
What is a harmonic series?
Most oscillators have a tendency to resonate at several frequencies
The lowest frequency is the natural frequency
Integer multiples of the fundamental frequency are called harmonics
26
What is Fourier analysis?
Mathematical model for analysing a periodic waveform to find its component frequencies
It will plot a spectrum of frequencies (amplitude against frequency)
27
State the Doppler shift equation.
28
What is the radiant flux of a light source?
Number of joules of energy emitted each second - expressed in watts
29
What is the intensity or irradiance of a light source?
Describes how much energy falls on a surface and is measured in watts per square metre
30
Which equation describes the intensity of light at a distance of R from a source?
I = P/4πr2
31
Describe how a simple manometer works.
Measures pressure in a gas by vertical displacement of a liquid in a tube
If the density of the liquid is known, the difference in column height can be used to calculate the hydrostatic pressure of the displaced column which is equal to the gas pressure
Derivation
P = F/A
P = mg/A
P = ((Density x Area x Height) x Gravity)/Area
Gauge Pressure = Density x Gravity x Height
32
What is a Bourdon gauge?
Consists of a C-shaped hollow spring-like tube that extends outwards at the sealed end when pressure rises in the tube
The expansion and contraction is relayed to a pointer that indicates the pressure on a calibrated dial
33
What is a barometer and how does it work?
Barometer – Measures absolute atmospheric pressure.
A sealed, liquid-filled tube is inverted into a reservoir.
Atmospheric pressure pushes on the reservoir, balancing the column of liquid.
The height of the liquid column is proportional to atmospheric pressure.
NOTE: Mercury is used due to its high density, keeping the column height manageable.
34
What is a pressure relief valve?
Relieves pressure beyond a specified limit and re-closes upon return to the normal pressure range
35
Describe how an adjustable pressure limiting valve works.
Turning a screw raises or lowers the spring and diaphragm
When lowered, more pressure is required to raise the diaphragm enough for gas to escape through the apertures
The diaphragm is pushed upwards and reaches a point where the gas can escape
The adjustable pressure limit valve allows the blow-off pressure to be adjusted by turning a screw that raises or lowers the spring and diaphragm
When the spring is lowered, the spring must be compressed further so more pressure is needed to raise the diaphragm enough for the gas to escape
36
Describe how a pressure regulator valve works.
Used to supply a flow of gas to an outlet (e.g. facemask) at a much lower pressure than the supply (e.g. cylinder)
Gas flows into a chamber via an aperture around a piston
If the chamber pressure gets too high, the diaphragm and piston rise to narrow the aperture
This restricts gas flow into the chamber, thereby reducing outflow pressure
37
What is the siphon effect?
Liquid from one vessel is drawn off via a tube to another vessel at a lower level
38
Why is a vent necessary for infusions running from glass vials?
The subatmospheric pressure created in the bottle as liquid flows from the bottle to the vein would mean that the pressure will equilibrate with the pressure in the vein, thereby stopping drug delivery
A vent in the bottle enables air to replace the liquid so the pressure in the bottle remains atmospheric
39
Why might an elevated syringe driver empty faster?
If a syringe driver is placed at a significant height above the patient, the weight of the column of fluid (hydrostatic pressure) could overcome the friction between a plunger and the barrel, thereby making the syringe empty faster
40
What is an oscillometer?
Instrument for measuring the changes in pulsations in the arteries, especially of the extremities
The cuff is inflated and slowly deflated through a valve whilst a sensor monitors the cuff pressure and the small oscillations which begin as soon as cuff pressure drop below the systolic
Max amplitude occurs when the cuff pressure is equal to the mean pressure
41
Describe the Penaz volume clamp technique.
Monitors arterial blood volume via light absorption (photoplethysmography).
Increased b lood volume → more light absorbed → less detected.
A cuff inflates until blood volume is constant—this occurs at mean arterial pressure.
A feedback system adjusts cuff pressure in real time to maintain constant arterial volume, approximating MAP.
NOTE: only works in vasodilated patients
42
How does an invasive blood pressure monitor work?
Uses a pressure transducer with strain gauges in a Wheatstone bridge to detect small pressure changes.
A fluid-filled catheter (saline-heparin) transmits arterial pressure to the transducer.
Sensor is placed at cannula level and zeroed to atmospheric pressure by exposing it briefly to air.
Continuous saline flush (3–5 mL/hr) prevents clotting and maintains patency.
43
How does underdamping affect the blood pressure result in invasive blood pressure monitoring?
Over-estimates both systolic and diastolic blood pressure
44
How does overdamping affect the blood pressure result in invasive blood pressure monitoring?
Under-estimates systolic
Over-estimates diastolic
45
List some causes of over-damping of arterial line trace.
Long catheter
Narrow catheter
Air bubble (compressible meaning that a lot of the pressure change is absorbed by the bubble and not transmitted to the transducer)
46
Define saturated vapour pressure.
Maximum pressure exerted by the evaporated molecules above the liquid at equilibrium
47
What is the dew point?
The temperature at which air becomes saturated with water vapor, causing condensation to exceed evaporation.
Below this temperature, water vapor condenses into liquid (dew), as the air can no longer hold all the moisture.
48
Explain how clouds form.
Sun heats water → evaporation increases water vapor
Water vapor displaces heavier gases → air becomes less dense and rises
Rising air expands and cools (adiabatic expansion) → condensation occurs
Condensation forms clouds and leads to rise in air density
49
What is absolute humidity?
Mass of water molecules per unit volume (grams per metre cubed)
50
What is relative humidity?
Relative Humidity = (Actual Vapour Density/Max Vapour Density) x 100
51
Explain how a wet and dry bulb hygrometer works.
Uses two thermometers: a dry bulb to measure ambient temperature, and a wet bulb wrapped in a water-saturated wick.
The rate of evaporation from wick depends on relative humidity of surrounding air
Evaporation from wick causes a drop in temperature on the wet bulb
Temperature difference correlates to relative humidity
52
Explain how a hair tension hygrometer works.
Hair contains cytokeratin molecules linked by disulphide bridges and hydrogen bonds.
H bonds absorb moisture from the air, causing the hair to lengthen.
The degree of lengthening depends on relative humidity and can be calibrated to give a humidity reading.
53
Explain how a Regnault's hygrometer works.
Air is passed through ether in a silver tube, causing it to evaporate and cool the tube via latent heat loss.
Condensation on the tube’s surface marks the dew point temperature.
Modern versions use a chilled mirror and electronic sensors to detect condensation.
54
What is the absolute humidity of alveolar air?
44 mg/L
Gives a partial pressure of around 6.3 kPa
55
What is a heat and moisture exchanger made of?
Calcium chloride or silica
56
Explain how an HME works.
Exhaled warm, moist air condenses on the material, releasing latent heat and warming it.
On the next inhalation, dry gas passes over the warm, moist material, becoming warmed and humidified.
Approx. 80% efficient in conserving heat and moisture.
A 0.2 µm filter can be added to block bacteria/viruses, preventing circuit contamination.
57
How much dead space and resistance is added to the breathing circuit by an HME?
100 mL in adults
Up to 2 cm H2O resistance
NOTE: it can get blocked with secretions which increases the resistance
58
What are the two types of water bath humidifiers?
Active: Use heating elements and thermostats to reach up to 100% efficiency, but are bulky—used mainly for long-term ventilation.
Passive: Bubble inspired gas through water to saturate it with humidity.
59
Why is temperature control important for active water bath humidifiers?
Water temperature limits humidity; evaporation cools the bath (latent heat loss).
Active systems maintain temperature (typically 40–60°C) with a thermostat.
40°C: Less scald risk, more microbial growth.
60°C: More scald risk, lower infection risk.
60
Describe how the size of the droplets generated by a nebuliser relates to the pharmacological effect.
< 3 µm: enter the alveoli and THERAPEUTICALLY BENEFICIAL
1 µm = ideal
5-10 µm: deposit in the upper airways
61
How does a jet-driven nebuliser work?
High-pressure gas flows over a capillary tube immersed in fluid.
Gas passes through a small orifice, creating negative pressure via the Venturi effect.
This draws fluid up and converts it into an aerosol mist.
62
How does an ultrasound-driven nebuliser work?
A ceramic piezoelectric transducer converts electrical energy into mechanical vibrations (~1.5 MHz).
Vibrations pass through water to a flexible diaphragm.
The diaphragm vibrates the solution, breaking it into fine aerosol particles. The particle size is more consistent.
63
Define flow.
The movement of a gas or liquid through a tube or other system (volume passing a point per unit time)
64
Define laminar flow.
Smooth, orderly movement of fluid in parallel layers.
65
How does Ohm's law apply to laminar flow?
In laminar flow, flow is directly proportional to driving pressure
Q = ΔP/R
66
State the equation for calculating resistant in laminar flow.
67
State Poiseuille's equation.
68
Define turbulent flow.
Turbulent flow is a chaotic, irregular fluid motion characterised by swirling eddies and vortices, where layers of fluid mix unpredictably.
69
State the formula for Reynolds number.
Interpretation
< 2000 = Laminar
2000-4000 = Transitional
> 4000 = Turbulent
70
Describe the relationship between driving pressure and flow rate in turbulent flow.
Q ∝ √ΔP
Q² ∝ ΔP
The gradient is resistance which increases with flow rate
71
Define the Bernoulli principle.
Fluid moving rapidly (e.g. through a constriction) exerts less pressure than a static fluid
72
Explain the theory behind the Bernoulli principle.
As velocity increases at a constriction, the kinetic energy increases and, for the total energy within the system to remain constant, the potential energy must decrease
The potential energy of a gas is proportional to the pressure it exerts, so the pressure exerted on the walls of the tube will decrease with increased velocity
73
What is the Venturi effect?
If the Bernoulli principle is used to create an an area at subatmospheric pressure, it will entrain air
74
What determines the percentage of oxygen delivered by a Venturi device?
Rate of entrainment (determined by the size of the holes)
Rate of flow of oxygen (to a lesser extent)
Entrainment Ratio = Entrained Flow/Driving Flow
75
Define the Coanda effect.
A fluid or gas stream will hug a convex contour when directed at a tangent to that surface
76
Why does the Coanda effect happen?
A fast-moving fluid jet generates a region of lower pressure around it, entraining surrounding fluid into the stream. When the jet encounters a nearby surface, entrainment is reduced on the side facing the surface, leading to a local drop in pressure. This pressure differential causes the jet to be drawn toward and follow the contour of the adjacent surface.
77
Describe how a variable orifice flowmeter works.
It is a tapered vertical tube with a bobbin that rises as gas flow increases
The bobbin equilibrates when the upward force of gas balances its weight
The conical shape allows more air around the bobbin at higher gas flows
It is calibrated for specific gases due to viscosity/density
78
How can lung volume be deduced from respiratory flow rates?
It is the time-integral of the flow rate
I.e. flow is volume passing a certain point per unit time, the area under the curve will be the lung volume
79
How can a volume-time curve be converted into a flow rate-time curve?
It is the differential of the volume-time curve
80
Explain how a rotating vane flowmeter works.
Measures gas flow using a small turbine that spins with the flow. Interruptions of a light beam by the turbine generate a voltage proportional to flow rate. Most accurate at low respiratory flows due to turbine inertia.
81
Explain how a pneumotachometer works.
It measures airflow by detecting the pressure difference across a fine mesh in a tube (fixed resistance).
This pressure difference, sensed by pressure sensors, is proportional to flow rate (per Ohm’s law) under laminar conditions.
A widened tube ensures laminar flow by reducing velocity and the Reynolds number.
82
How does a pitot tube flowmeter work?
Measure flow by comparing pressure in two sealed tubes—one facing into the airflow and one facing away.
The flow-facing tube compresses air, creating a pressure increase. The pressure difference reflects the square of the flow velocity (kinetic energy = 1/2(mv^2)), which is assumed proportional to flow rate.
Unlike pneumotachometers, they use no flow resistor.
83
Explain how a Wright peak flow meter works.
It measures peak expiratory flow by using exhaled air to push a diaphragm against a spring. As the diaphragm moves, it opens a narrow slot for air to escape, reducing pressure. A sliding marker is pushed by the diaphragm and stops when the spring force balances the airflow pressure, indicating the peak flow rate.
84
Explain how a Benedict-Roth spirometer works.
It measures lung volume by collecting exhaled gas in a sealed, movable bell chamber. As air enters, the bell rises or falls, with its displacement proportional to the volume of air exhaled. A water seal ensures airtight movement, and the motion is recorded—traditionally on a rotating drum. It may underestimate volume due to air cooling and condensation inside the chamber.
85
Describe the features of an ideal gas.
Molecules are so far apart that there is no attraction between them
Volume of the molecules themselves is negligible
Molecules are in random motion, obeying Newton's laws of motion
86
What is standard temperature and pressure when discussing gases?
Temperature of 273.15 K (0 degrees)
Atmospheric pressure of 101.3 kPa = 760 mm Hg
87
State Avogadro's law.
Equal volumes of gases, at the same temperature and pressure, contain the same number of molecules
88
Define one mole.
A unit of measurement that is the amount of a pure substance containing the same number of molecules as there are atoms in exactly 12 grams of carbon-12 (i.e., 6.022 x 10^23).
89
What is Avogadro's constant?
Quantifies how many molecules make up one mole
6.02 x 10^23 per mole)
90
What volume does 1 mole of an ideal gas occupy at STP?
22.4 litres
91
State Dalton's law.
For a gas, the total pressure is simply all the partial pressures added together
92
What is the rough partial pressure of water in ambient room air?
1.3 kPa
93
State Boyle's law (First Ideal Gas Law).
The volume of a gas is inversely proportional to its pressure at a fixed temperature
P α 1/V
94
State Charles' law (Second Ideal Gas Law).
At a given pressure, the absolute temperature (in kelvin) is directly proportional to the volume of the gas
95
State Gay-Lussac law (Third Ideal Gas Law).
The pressure of a gas is directly proportional to its temperature within a fixed volume
96
State the combined gas law.
The product of pressure and volume is proportional to the absolute temperature
97
Demonstrate the working going from the combined gas law to the universal gas law.
PV/T = Constant (R)
Constant is proportional to the no. of moles of gas:
PV/T = nR
Rearrange the equation to get universal gas equation:
PV = nRT
98
What is the molar gas constant?
8.31 J/K/mol
99
Define diffusion.
Movement of a substance from an area of high concentration to an area of lower concentration
100
What is the difference between diffusion and flow?
Flow is the bulk movement of fluid driven by pressure differences
Diffusion is the passive, random movement of molecules resulting in net transport from high to low concentration.
101
State Fick's Law of Diffusion.
States that the rate of transfer of a gas through the membrane is:
- Proportional to tissue area
- Proportional to difference in gas partial pressure between two sides
- Inversely proportional to tissue thickness
102
State the equation for velocity of diffusion across a membrane.
This is Fick's law multiplied by a diffusion constant
103
State Graham's law of diffusion.
Rates at which gases diffuse are inversely proportional to the square root of their densities
104
State how the diffusion constant is derived.
105
Which two factors affect the diffusion constant of a gas in a liquid?
Solubility
Molecular Mass (Square Root)
106
What is the thickness of the respiratory membrane in a normal healthy lung?
0.5-1.0 µm
107
What is the transit time for a red blood cell to pass through a pulmonary capillary?
0.75 s
NOTE: CO2 diffusion is complete in 0.10 s and O2 diffusion is complete in 0.30-0.40 s
108
State the steps to derive the equation for lung diffusion capacity.
109
Why is carbon monoxide a useful agent for measuring the diffusion capacity of the lung?
CO has similar diffusion properties to oxygen
CO is safe in small amounts
CO partial pressure in plasma is basically zero (i.e. P2 is zero)
Negligible blood levels of CO at baseline
110
How is the diffusion capacity of the lung measured using the carbon monoxide breath hold method?
Measured via a single-breath test using a gas mix containing ~0.3% CO and 10% He. The patient inhales to TLC, holds their breath for 10 seconds, then exhales. The first portion is discarded (dead space), and the next sample is analysed.
VCO = rate of CO absorbed = difference between inspired and expired CO divided by time
PACO = Mean alveolar CO partial pressure during the breath-hold
DLCO = VCO / PACO
Helium (not absorbed) is used to calculate alveolar volume by dilution. The result is corrected for haemoglobin levels.
111
Define osmosis.
Diffusion of a solvent across a membrane whilst the solute remains
Quantified in terms of osmotic pressure which is the pressure required to stop the flow from one side of the membrane to the other
112
Define solubility.
The amount (in moles) of a solute that can be dissolved in a unit volume of solvent under specified conditions
113
Define Henry's law.
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid
114
How is the rate of CO absorption deduced using the single breath method?
115
State the Henry's law equation.
116
Describe how temperature affects the solubility of a gas in a liquid.
Solubility decreases
Molecules gain kinetic energy as temperature rises, making it easier for it to escape from the liquid phase
117
Why does a fizzy drink fizz when it's opened?
Opening the bottle lowers the pressure above the liquid, creating a partial pressure gradient that causes dissolved CO₂ to escape into the gas phase. On a hot day, higher temperatures further reduce gas solubility, making the drink go flat faster.
118
Define partition coefficient.
Measures the tendency of a solute to dissolve in two different immiscible solvents (phases). It is the ratio of concentrations of a substance in two phases of a mixture of two immiscible solvents.
Written as λ(a,b)
119
Why is a low blood-gas partition coefficient favourable for volatile anaesthetic agents?
A low blood-gas partition coefficient means the anaesthetic is less soluble in blood, allowing alveolar partial pressure to rise quickly. This creates a steep gradient for rapid diffusion into the brain, leading to a faster onset of action.
In contrast, high solubility delays this process by slowing the buildup of alveolar partial pressure and reducing the drug's drive to cross into the brain.
120
Define minimum alveolar concentration.
Concentration of vapour in the lungs required to prevent a reflex response to a skin incision in 50% of patients, it is a measure of anaesthetic potency
121
Define colligative properties.
Properties of solutions that depend on the NUMBER of molecules of solute in a given volume of solvent rather than the properties of the solute molecules themselves
122
List some examples of colligative properties.
Vapour pressure
Osmotic pressure
Variations in boiling and freezing point
123
What effect does dissolving salt in water have on the boiling and freezing point and why?
Dissolving salt in water RAISES the boiling point because solute particles reduce the number of solvent molecules at the surface, hindering evaporation. This means a higher temperature is needed for the vapour pressure to match atmospheric pressure.
It also LOWERS the freezing point because solute particles disrupt the formation of the solid lattice, making it harder for water molecules to freeze.
124
State Raoult's law.
The vapour pressure of a solvent above a solution is equal to the vapour pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present
NOTE: the more solute you add, the lower the mole fraction of the solvent
125
How does a freezing point change osmometer work?
Measures osmotic pressure indirectly by detecting changes in the freezing point (colligative property related to solute concentration).
The sample (e.g. urine or plasma) is super-cooled below its expected freezing point while being stirred. As it begins to freeze into a slush-like state (ice-liquid equilibrium), the freezing point is recorded using a temperature probe.
126
How does a side stream gas analyser work?
Uses a thin sampling tube to draw a small constant flow of gas (150-200 mL/min) from the breathing circuit to a detector - this is drawn with a slight negative pressure
A water trap prevents condensation from entering the analyser.
127
What are some issues with a side-stream gas analyser?
Time lag of a few seconds
Gas mixing in the tube reduces precision
Vulnerable to occlusion
128
Describe the difference in the absorption of polyatomic gases vs monoatomic gases.
Polyatomic (e.g. CO2, N2O, H2O) absorbs
Monoatomic (e.g. O2, N2) does not
129
State Beer's law.
Concentration of a gas is directly proportional to the absorbance of electromagnetic waves
130
What is the collision broadening effect?
Interactions between gas molecules in a mixture cause variations in absorption wavelengths.
This leads to a broadened absorption peak and can shift the peak wavelength.
NOTE: nitrous oxide and CO₂ interact and alter each other's absorbance, so gas analysers must correct for this when measuring ETCO₂ to ensure accuracy.
131
How does an infrared gas analyser work?
Generates infrared radiation from a heated wire, which is filtered to a specific wavelength absorbed by the target gas.
The gas passes through a chamber with infrared-permeable windows (e.g. sapphire), and a photodetector measures how much radiation passes through. The amount of absorbed radiation indicates gas concentration (Beer's law).
Some systems include a parallel air-filled control chamber to correct for fluctuations in light source intensity.
132
Why does the plateau of the ETCO2 trace have a slight upstroke?
Due to continued diffusion of CO2 across the alveolar membrane during expiration (i.e. alveoli emptying last will contain more CO2 than the ones emptying first)
133
How does a null deflection paramagnetic oxygen analyser work?
A beam with 2 nitrogen-filled spheres (weakly diamagnetic) is suspended in a magnetic field using a torsion balance
Without oxygen, the beam is aligned over the magnetic poles
When oxygen enters the chamber, it is drawn to the strongest point of the magnetic field pushing the nitrogen spheres and rotating the beam
A mirror on the beam reflects light onto a scale, allowing oxygen levels to be read.
134
What are some issues with a paramagnetic oxygen analyser?
Interference from water vapour, vibrations and variations in ventilation pressure
Slow response time
NOT suitable for real-time expired gas monitoring (used to measure O2 concentration in gas supplies)
135
How does a pulsed-field paramagnetic oxygen analyser work?
Sampled gas and a reference gas are passed via two separate inlets into the analyser
A magnetic field is pulsed (~100 Hz) which attracts the paramagnetic oxygen molecules towards the field
The subsequent change in flow in the sample gas leads to a pressure difference on one side of a double-sided pressure transducer which corresponds to the difference in partial pressure of oxygen between the two gases
The alternating pressure signal allows for fast, real-time oxygen monitoring, overcoming the slow response time of older paramagnetic analysers.
136
Explain how a galvanic fuel cell works.
Detects O2 by producing an electrical current proportional to the partial pressure of oxygen. O2 enters and reacts with the gold cathode:
O₂ + 4e⁻ + 2H₂O → 4OH⁻
The OH- migrate through the KOH solution to the lead anode, where they react:
Pb + 2OH⁻ → PbO + H₂O + 2e⁻
This flow of electrons generates a measurable current.
137
Where are galvanic fuel cells used and what is one major issue that limits their use?
Used in anaesthetic machines to measure oxygen concentration in supply gas
30 s response time
Limited lifespan as the anode decreases in size over time
138
What are the three types of electrodes used in arterial blood gas analysis?
Clark - Oxygen
Severinghaus - Carbon Dioxide
Sanz - pH
139
How does the Clark electrode work?
Measures pO2 using an electrochemical cell consisting of a platinum cathode and a silver/silver chloride anode, immersed in KCl solution. A small constant voltage (~0.6 V) is applied across the electrodes.
At the cathode, oxygen is reduced:
O₂ + 4e⁻ + 2H₂O → 4OH⁻
At the anode, silver is oxidised:
4Ag + 4Cl⁻ → 4AgCl + 4e⁻
The hydroxide ions (OH⁻) generated at the cathode carry charge through the electrolyte, and the resulting current is directly proportional to the partial pressure of oxygen (pO₂).
140
How does the Sanz electrode work?
There is a reference electrode made of Ag/AgCl in a KCl solution kept at a fixed voltage.
The measuring electrode is also Ag/AgCl in a pH buffer solution in contact with H+ sensitive glass
H+ from the sample binds to the hydrated gel layer on the outside of the glass causing an imbalance in H+ between the inside and outside of the glass
This creates a potential difference that is detected by the measuring electrode and compared to the reference electrode
141
How does the Severinghaus electrode work?
CO2 from the sample passes through a membrane into a bicarbonate solution
It reacts with water to form carbonic acid
The H+ is detected by a pH-sensitive glass electrode
Ag/AgCl is used as a reference
142
How does gas chromatography work?
Separates components of a gas mixture using a mobile (gas) and stationary (liquid/solid) phase.
Sample is carried through a column by an inert gas (e.g., helium).
Components separate based on how strongly they interact with the stationary phase.
A detector measures retention time (identity) and peak height (concentration).
143
Explain how mass spectrometry work?
Gas molecules are ionised by high-speed electrons.
Positive ions are accelerated and deflected by a magnetic field.
Deflection depends on mass-to-charge ratio (m/z).
Detectors measure ion abundance and m/z.
Heavier ions deflect less; lighter ions more.
144
How does Raman spectroscopy work?
A beam of EM radiation (photons) is directed at the sample.
Most photons scatter elastically (Rayleigh scattering), with no energy change.
In Raman scattering, a photon excites an electron to a higher energy level.
The electron then relaxes to a different energy level, releasing a photon with a different energy (wavelength).
These energy shifts are unique to molecular structures and are recorded as a Raman spectrum.
145
What is the only vaporiser-in-circle system used at present?
Oxford miniatuHre vaporiser
Vapour is added to the gas mixture after it has entered the breathing system
146
How do plenum vaporisers (variable bypass vaporisers) work?
Fresh gas flow splits: some goes through a vaporising chamber, the rest bypasses it.
Positive pressure pushes carrier gas through the chamber, picking up vapour from the volatile agent.
Gases rejoin before reaching the patient.
Splitting ratio controls anaesthetic concentration and can be adjusted.
Vaporiser must maintain the agent's SVP for accurate delivery.
147
How does evaporative cooling affect the vapour concentration achieved by a vaporiser?
When a volatile liquid evaporates, high-energy molecules escape, lowering the average energy of the remaining molecules.
This lowers the temperature of the liquid due to latent heat of vaporisation. This affect SVP.
To prevent excessive cooling, a heat sink (material with high specific heat capacity) is used in the vaporiser.
The heat sink absorbs thermal energy to counteract temperature drops, ensuring stable vapour concentration and minimizing fluctuations in the liquid temperature.
148
How do plenum vaporisers adjust for changes in temperature?
Anaesthetic output rises with temperature due to increased SVP.
To compensate, variable bypass systems adjust the splitting ratio:
-Higher temperature → more gas bypasses the vaporising chamber.
Thermal compensation mechanisms include:
- Bimetallic strips (most common) that bend with temperature.
- Expanding metal rods or liquid-filled bellows to reduce carrier gas flow at higher temps.
The vaporiser's metal body also helps by conducting heat to stabilise temperature.
149
How does a dual-circuit vaporiser for desflurane work?
Desflurane boils at 22.8°C, so it’s stored in the vaporiser at 39°C and 200 kPa to keep it stable.
The vaporiser doesn’t use carrier gas to pick up vapour like others do.
Instead, pure desflurane vapour is injected into the fresh gas stream.
A pressure transducer compares the pressure of fresh gas and vapour.
A pressure correction valve adjusts vapour flow to ensure accurate delivery.
150
Would you need to change the vaporiser settings if delivering anaesthetic at altitude?
Partial pressure, not percentage, determines clinical effect.
SVP is constant (temperature-dependent, not pressure-dependent), so partial pressure of vapour remains the same.
However, since atmospheric pressure drops at altitude, the percentage concentration increases.
This means the same partial pressure delivers a higher concentration of agent (e.g., higher % on Everest), but the anaesthetic effect is determined by the partial pressure of the agent.
151
What is a Bodok seal?
A neoprene washer placed between the gas cylinder and the cylinder yoke to ensure a gas-tight seal
152
What is the pin index system?
Designed to ensure the correct gas cylinder is attached to the correct inlet on the pressure regulator or anaesthetic machine
The positions of the holes on the cylinder valve correspond with pins fitted to the yoke attached to the equipment
The pin positions for each gas are unique
153
What features enable identification of the type of gas within a cylinder?
Pin index system
Engraved chemical formula of the gas
The shoulder of the cylinder should be colour-coded
An expiry date should be present
154
How often does a cylinder need to be internally examined with an endoscope?
Every 5 years
155
What are the main sizes of gas cylinder used in clinical practice?
A: smallest
J: biggest (6800 L)
D: standard size used for anaesthetic machines (340 L)
156
Describe the change in state of the nitrous oxide with decreases in volume.
At 40°C (above critical temperature), nitrous oxide stays a gas regardless of pressure; pressure increases smoothly when compressed.
At 20°C, once pressure reaches 52 bar (its SVP), gas begins to condense into liquid.
Further volume decrease causes more gas to condense without increasing pressure.
After full condensation, any more volume reduction causes a rapid pressure rise, since liquid is incompressible.
157
What is the filling ratio of a cylinder and why is it important?
Filling ratio = mass of gas in a cylinder ÷ mass of water that would fill it.
Cylinders are not filled to 100% to avoid dangerous pressure rises with temperature.
Some gas is left above the liquid to absorb expansion safely.
Rising temperature causes liquid to expand, but excess vapour condenses, limiting pressure increase.
In the UK, the maximum filling ratio is 0.75 for safety.
158
Draw a graph showing how the pressure of a gas cylinder containing liquid will change as it empties.
The pressure will just show SVP until all the liquid has evaporated
When empty, it will contain 1 atmosphere (hence, there is no gradient drawing more gas out)
159
What important storage-related issue affects Entonox cylinders and how can this be mitigated?
Must be kept above pseudocritical temperature of - 5.5°C
Below this, lamination occurs (Poynting effect): nitrous oxide liquefies, and the mixture separates into two layers:
Lower layer: liquid nitrous oxide with some dissolved oxygen (~20%).
Upper layer: oxygen-rich gas.
As gas is used, the patient first receives mostly oxygen, then hypoxic nitrous oxide-rich vapour.
Large cylinders use dip-tubes to delay hypoxia by drawing from the liquid layer.
160
What is the pseudocritical temperature of a gas mixture?
Temperature below which a gas mixture begins to behave like a single component fluid, meaning it can separate into distinct liquid and gas phases, even though it's a mixture.
NOTE: nitrous oxide has a much higher boiling point than oxygen
161
How is oxygen produced?
Fractional distillation of liquefied air and yields oxygen that is 99.6%
Chemical oxygen generator (zeolite molecular sieve) absorbs nitrogen and can produce oxygen and is used for home oxygen concentrators
Electrolysis of water can also be used to generate oxygen (e.g. on submarines)
162
What is the critical temperature of oxygen?
-118°C
163
What is critical pressure?
The minimum pressure required to liquefy a gas at its critical temperature.
164
What is the critical pressure of oxygen?
50 bar
165
What is the boiling point of oxygen at atmospheric pressure?
-182.5°C
166
How do cylinder manifolds work?
Connect multiple gas cylinders to supply hospitals centrally.
Two banks of cylinders are used; when pressure drops in one, the system automatically switches to the other.
Each bank should provide gas for at least 2 days.
Oxygen cylinders are stored at 137 bar, and pressure-reducing valves drop this to 4 bar for safe pipeline delivery.
167
How do vacuum-insulated evaporators work?
Oxygen is stored as a liquid at high pressure (7 bar) and low temperature (-150 to –180 degrees)
The VIE is a thermally insulated vessel
It has an inner shell of stainless steel and an outer shell of carbon steel
1 L of liquid oxygen gives 842 litres of gaseous oxygen at STP
An electrical warmer brings the oxygen to room temperature
168
At what pressure is medical air delivered?
400 kPa for ventilation
700 kPa for pneumatic tools
169
What are the three methods of achieving subatmospheric pressure for vacuum suction?
Mechanical pump
Centralised piped vacuum source
Venturi effect driven by gas from cylinders
170
What negative pressure must be achieved by a suction system?
A suction system needs to generate a negative pressure of at least 400 mm Hg
NOTE: Surgical suction needs to achieve negative 500 mm Hg within 10 seconds and remove 25 L/min
171
What is stoichiometric composition?
It’s the chemically correct air-fuel ratio for complete combustion — no leftover fuel or oxygen.
Represents the most efficient and often most explosive mixture.
All fuel is fully oxidized using exactly enough oxygen.
Example: For ethanol, the stoichiometric air-fuel ratio is 9:1.
