Base principles and physics Flashcards
Thermodynamics definition
Branch of physics dealing with interaction between heat (thermal energy) and other forms of energy
First law of thermodynamics
the change in internal enegry of a system is equal to the heat added minus the work done by the system
i.e. in a vaccuum the total energy in a chamber cannot change without input or output from the system or external sources (conservation of energy)
First law of thermodynamics equation
Second law of thermodynamics
Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature without work beign performed
Third law of thermodynamics
The temperature absolute zero is unattainable
When a gas expands is energy done?
Yes
ADiabatic expansion refers to an insulated process where no external heat transfer is done so system total energy si stable
Joule thomson efffect
A gas changes temperature when it moves from higher pressure to lower pressure, and for most gasses they cool e.g. bike tire pumped = hot
What happens to medical gasses as they escape a compressed cylinder? Why are they kept as dry gasses?
Temperature rises when a gas is compressed, and as it expands it cools
Therefore released gas from compressed gas cylinders cool and if there was too much humidity the gas cylinder would obstruct with ice
Viscocity
Fluids resistance to flow
Newtonian fluid
Constant viscocity regardless of flow rate
Non newtonian fluid
Viscocity changes with flow rate
Give an example of a non newtonian fluid and its mechanism
Blood
Red cells thicken blood as they have a tendancy to agregate
The cells change shape the faster they travel becoming more elongated and fall in to a line having lower resistane and a drop in viscocity as flow rate increases
What is the SI unit for viscocity
Pascal second also known as poiseuille
Poise (dyne . sec. cm^2)
Surface tension
The result of attraction between moleciles across the surface of a liquid - as the molecules on the surface have reduced molecules to interact with compared to those deeper they form stronger bonds leaving the surface with the smallest possibel surface area for a given volume
Wall tension
Vessel wall that is an elasticated solid and the attraction between molecules across the surface of the solid (similar ot surface tension)
Laplace’s law
The larger the radius of the vessel the greater the wall tension required to withstand a given internal fluid pressure
What is Laplace’s equation for a spherical bubble
What is Laplace’s law for a cylinder
Explain why a dilated cardiomyopathy causes issues using Laplace’s law
The distended radius of the LV but the same pressure required during ejection is needed, using Laplace’s equation for a sphere the wall tension must therefore be greater for a heart dilated than for a smaller heart and as a consequence it strains to generate this
Explain aortic aneurysms using Laplace’s law
if a weak spot gives way and bulges the effective radius fo the artery increases and if BP remains constant then the wall tension of the artery increases becoming a vicious cycle - the only way for wall tension to decrease is for cylindrical bulge to move towards a sphere which has half the wall tension for the same radius i..e why aneurysms form spherical bulges
What is a surfactant?
A compound that loweres surface tension
Explain alveoli and surfactant effect using Laplace’s law
A high pressure is needed to oppose a given surface tension if the radius is small because alveoli when small have significant surface tension from water
By surfactant exisitng the air in smaller alveoli does nt simply flow into neighbouring larger alveoli with a lower pressure gradient
Work equation
Force applied x distance moved
Amount of energy applied to a system
ie. holding a shopping bag is not work because there is no distance moved
Energy definition
Capacity to do work
Measured in joules - the energy required to exert a force of one newton through a distance of one metre
Power equation
Work done/ time taken
Units watt (1 J/sec)
The rate at which work is done or the rate of transfer of energy
What is pressure by definiition?
Force divided by area
If you were to try and calculate how much work is required to drive a ventilator where it delivers a constant pressure how would you figure it out?
Gas flow is by pressure difference so a pressure must first be created –> pressure = force / area
OR force = pressure x area
As work done = force x distance
Work done = pressure x area x distance
Volume is equal to area x length
Therefore Work done = pressure x volume
Pressure equalisation takes time due to resistance
Draw a diagram representing the work done by the lung through inspiration and expiration - including energy inflating/deflating the lung and energy lost to friction
What is elastic hysteresis
The lungs like many elastic materials do not return to their original size in an identical fashion - retaining instead a memory of being stretched. So whilst at identical pressures during inhalation and exhalation the volumes are different - more during exhalation,
Define compliance
The change of volume with respect to pressure and a measure of the ease of expansion
Units metres/newton
Compliance equation
Change in volume / change in pressure
How would one represent compliance on a volume pressure graph
Gradient of the curve
What is elastance
The opposite, or reciprocal of compliance
Chnage in pressure/change in volume
What effewct does increased lung compliance have on breathing and what pathologies does this occur in?
Increases with age, emphysema
Elasticity decreases
Extra work required to breathe in exhalation as elastic recoil diminished
Low compliance conditions in the lung include
Atelectasis
Fibrosis
Deficit in surfactant
oedema
Pneumonia
How would you calculate the power of breathing? How does this compare to the power output generally in the body?
Work of one inspiration for 450mLs at a fixed pressure of 0.8kPa is 0.36 joules (once SI units adopted)
Therefore 0.36 x 15/60 gives you 0.09J/sec = 90mW
Because the respiratory muscles are only approximately 10% efficient, the power required for resting breathing is actually closer to 900 mW. The total metabolic rate of a subject at rest is 80– 90 W, so the respiratory power represents approximately 1% of the energy consumed by the body. The power of breathing during exercise is greatly increased due to a much greater tidal volume in addition to increased airway resistance (see Section 3.6 below) and respiratory rate. During strenuous exercise
How to calculate the energy required to move a volume througha tube?
E = pressure x volume
How are power and flow related? (laminar)
If the pressure difference remains constant when E = P x V then
power = pressure x the rate of change of volume (or flow rate)
therefore since pressure is directly related to flow in laminar conditions
Power directly related to flow squared for laminar flow
In turbulent flow how is this related to pressure?
Power is directly proportional to flow ^ 3
How would you graphically represent the work fo the heart?
How would you calculate the power requirements of the heart?
Explain pressure as a concept then define it
Gas in a box contains millions of molecules zipping around in all directions bouncing off one antoher and off the walls, the combined effect of these collisions with the walls of the box create pressure
Pressure = force per unit of area
What is absolute pressure in comparison to gauge pressure?
Absolute pressure - true pressure where zero is the pressure of a vaccuum
Gauge pressure is a relative pressure taking the zero point of the scale as a convenient reference
e.g. 101.3 kPa = 760mmHg = 1033 cmH20 = 1atm
What device compares the pressures of two gasses?
Manometer
Define manometer
An instrment to measure the pressure in a gas by the vertical displacement of a liquid in a tube - comparing the pressure of two gasses one being at reference pressure i.e. atmospheric rpessure
Draw a diagram of a simple manometer
How is the height and pressure understood in the following set up?
Weight or gravitational force exerted by the colum of liquid - the pressure exerted as a result of a liquids weight is a hydrostatic pressure
pressure = force/area
Force of gravity = mass x accelration
The mass of the column = density x volume (area x height)
therefore measured gauge pressure = (density (p) x area x height) x gravity / area
Simplieifed out remove the A –> pressure = density x gravity x height
How would you set up a manometer for CVP
What is a bourdon tube
Bourdon tube measures gauge pressure and is a C shaped hollow spring like tube extending outwards at the seal end when the pressure within it rises - compares pressure to outside in accordance with Boyles law as pressure rises so does volume
What is a barometer
A special type of manometer for measurement of atmospheric pressure
Unlike a normal manometer instead of measuring pressure relative to a reference pressure the barometer measures absolute atmospheric pressure
The reference pressure is a vaccuum formed by a closed tube filled with liquid. Gravit pulls the liquid downward and a gap forms at teh top of the closed tube while a resevoir of liquid is exposed directly to atospheric pressure balancing the weight of the actual fluid
mercury is the ideal fluid for this as it has a high density - as pressure = density x mass x height
A 2mL and a 50mL syringe have the same force applied ot depress their plungers - which wil deliver a greater pressure?
Pressure = force / area
A 50ml syringe has the same force over a bigger area therefore lower pressure
Explain an adjustable pressure limit relief valve e.g. that on the end of the BVM to adjust PEEP
Draw and explain a pressure regulator valve
Draw a runaway exponential curve and give an example
Depolarisation of a nerve cell emmbrane once threshold is reached i.e. development of a nerve action potnetial. Or the initial growth of a bacterial colony from a single cell
Describe a washout curve and give an example
Washout of nitrogen from the lung when breathing 100% oxygen from a non rebreathing circuit
Give an example of a washin curve
What is a bourdon gauge
Rugged, robust and reliable pressure gauge used in long term measurement of pressures where absolute accuracy is not as important
It consists of a coiled tube connected to a high pressur esource - the coil is somewhat flattened and as a pressure increased occurs within the tube it causes it to uncoil slightly. This movement is connected to a pointer which moves across a calibrated scale so the pressure cna be read
What is an aneroid gauge
Gauges for pressure which do not contain liquid
e.g. Bourdon gauge or bellows gauge
How does a bellows gauge work? Where is it used?
Bellows - used in some symphomanometers
Principle is pressure applied to the inside of the bellows causes outward movement of the walls and this is sense by a mechanism that moves a pointer over a calibrated scale
The pressure measured is gauge pressure as it measures the pressure difference between isnide and and outside the bellows i.e. referenced against atmospheric pressure
How does the pressure relate to contents in oxygen cylinders?
Linearly
How is a nitrous oxide pressure gauge different to oxygen?
Oxygen content and pressure have a linear relationship
Nitrous oxide contains liquid nitrous oxide and nitrous oxide vapour. As long as liquid nitrous oxide remains the rpessure will remain the same. This pressure will be the saturated vapour pressure at this temperature
If the cylinder is emptied rapidly liquid N20 evaporates to replace gaseous N2O drawn off until there is no more liquid left the saturated vapour pressure remains the same; only then does it start to fall
Once all the nitrous oxide is used the final pressure is 1 atmosphere
If it is drawn off very quickly the cylinder may not cool apreciably due to the latent heat of vapourisation and SBP will be a lower at a lower temperature so the cylinder pressure will fall progressively until all the liquid content has evaporated
If it is drawn off slowly the cylinder obtains heat from its surroundings and the temperature remains stable and therefore the saturated vapour pressure also remains stable
WHat is a transducer
Device converting energy from one form into another
Define ultrasound
sound waves of wavelengths 2-20 MHz are applied through tissue and their reflections are used to produce images used in diagnosis and management.
Sound energy can be attenuated as it passes through tissue because of?
- Reflection - sound waves are reflected at interfaces between soft tissues of differing acoustic impedance, increased percentages of sound are reflected the larger the difference in acoustic impedance mismatch and additionally depends on angle of approach. The angle of incidence = angle of reflection
- Scattering - occurs at interfaces within the sound beam path and refers to the separation of the sound wave in multiple directions where the interface of contact is equal to the wavelength (scatter assymetrical with pattern dependent on angle of approach and frequency) or much smalller than wavelength where Rayleigh scattering equal in all directions occurs.
- Refraction - deviation in the path of the sound wave beam where it passes through a soft tissue interface with differing acoustic impedance and the angle of incidence is not perpendicular.
- Absorption - Transfer of some of the energy to the maternal through which the sound is travelling. This increases with increasing sound frequency. Frequency therefore limits depth penetration due to energy loss
Define reflection and its effect and base principles in the context of ultrasound
- Reflection - sound waves are reflected at interfaces between soft tissues of differing acoustic impedance, increased percentages of sound are reflected the larger the difference in acoustic impedance mismatch and additionally depends on angle of approach. The angle of incidence = angle of reflection
Define scatter and its effect on sound energy and base principles
- Scattering - occurs at interfaces within the sound beam path and refers to the separation of the sound wave in multiple directions where the interface of contact is equal to the wavelength (scatter assymetrical with pattern dependent on angle of approach and frequency) or much smalller than wavelength where Rayleigh scattering equal in all directions occurs.
*
Define refraction and its base principles in the context of ultrasound
- Refraction - deviation in the path of the sound wave beam where it passes through a soft tissue interface with differing acoustic impedance and the angle of incidence is not perpendicular.
Define absorption and its principles in the context of ultrasound
- Absorption - Transfer of some of the energy to the maternal through which the sound is travelling. This increases with increasing sound frequency. Frequency therefore limits depth penetration due to energy loss
Explain the base principle to probe function in ultrasound
The ultrasound probe both propagates the sound and receives returning echos
* Returning echos are transduced to signals by piezoelectric transducers - crystals expand or contract depending on polarity of voltage applied, when they resonate with returning sound waves it converts this back to electricity.
* Returning echos occur at different strengths over a different timeframe - amplitude of the reflected echo is a function of acoustic mismatch of the tissues and angle of incidence
Explain ultrasound beam profiles
- Beam profile consists of
◦ Near field - transition distance and the zone useful for imaging purposes. A narrow beam is required to produce high resolution
◦ Far field - a divergent beam
◦ Transition point - the point at which the near field becomes the far field - Diffraction pattern
◦ A beam profile plotting maximum pressure amplitude has primary beam displaying characteristics as described above
◦ Additional side lobes of energy; “off axis energy” and grating lobes from array transducers are also produced which contribute to artefacts interfering with the primary beam image
How do different ultrasound probes function differently
Probes
* Linear probes - straight line crystals to produce linear beams 90 degrees to the transducer face
* Curved array - face of the transducer is convex and the beam paths are 90 degrees to the transducer face resulting in a wider field of view at the bottom of the image
What are the different ultrasound modes of display
Modes of display
* Returning energy can be displayed in 2D, M mode, Doppler
◦ B mode or brightness mode is the standard 2D mode
◦ M mode -captures returning echos over time in one line fo the B mode image and displays them over a time axis
◦ Colour Doppler utilises the Doppler effect to show blood flow or tissue motion ina. Selected 2D area - direction and velocity are shown superimposed on the 2D image
Outline briefly the concepts of artefact and key examples in the context of ultrasound
Artefacts
* Sound beam to soft tissue interface interactions are assumed to follow a series of rules in ultrasound scanning which produces artefacts when these assumptions are incorrect e.g. constant speed of sound, all echos detected come from the central axis of the beam and ultrasound beams travelling in straight lines
* This results in images not accurately reflecting the anatomy e.g. acoustic shadowing where little sound is transmitted deeper, or enhancement where sound is attenuated less through fluid filled structures; and mirror image artefacts can occur
Artefacts
* Reverberation
* Mirror images
* Reflection
* Enhancement
* Attenuation
Define the doppler effect and explain it
Doppler effect - Change in frequency of a sound wave reflected by a moving target;reflected frequency increases if moving towards and decreases if moving away. The best Doppler images are obtained with lower frequencies and colour Doppler
* The change in detected frequency of the ultrasound waves is a reflection of velocity of encountered object - in analysis of vessels this is the flow of blood cells
What limits depth penetration in ultrasound
Limits of depth penetration - longer wavelengths and varying properties of human tissue regarding refraction and attenuation - little refraction in human tissue; air attenuates
Define flow
the movement of gas through a tube or system
Volume / time
What is the conservation of flow?
Flow remains constant although if cross sectional area changes the velocity will also change to account for flow being conserved therefore
Q = A1 x V1 = A2 x V2
What is laminar flow
Orderly movement of a fluid that complies with a model in which parallel layers have different velocities relative to one another
What is the velocity profile within a blood vessel
Parabolic - fastest at the middle, decreasingly fast either side
Flow occurs when…
There is a difference in pressure between two points
What effect does resistance have on flow?
If resistance is increased a greater driving pressure is needed to maintain a fixed flow rate, BUT it will not prevent flow
How does resistance to flow account for hypertension
Higher SVR therefore a nigher driving presssure needed to maintain sufficent flow
Describe the relationship between flow and resistance
Flow = change in pressure or driving pressure / flow resistance of the tube
What is Ohms law
Current = potential difference or voltage/ resistance
Flow has what relationship to pressure
Directly proportional
What is resistance defined by
Hagen Poiseuelle law
What are the assumptions of the hagen Poiseuille equation
liquid is incompressible
Viscosity is stable
Flow is laminar
Hagen poiseuelle equation
What equation is this
Hagen Poiseulle equation
What is the equation for low solved for the Hagen POisuelle equation
Define turbulent flow
Movement of a fluid in which small scale currrents in the fluid move in irregular patterns while the overall flow is one directoin
What is Reynolds number?
A number used to predict whether turbulent or laminar flow would occur in a given system. It has no units
Reynaulds number equation
Reynaulds number equation
What is the unit for density
kg/m cubed
What is the unit for viscocity
newton x seconds/ metres cubed
What are the cutoffs for the Reynolds number
<2000 predominantly laminar
>4000 turbulen flow predominant
2000-4000 transitional with eddies and vortices
What effect does viscocity have on laminar flow?
Increasing viscocity reduces the reynolds number proportionally and therefore makes flow more laminar
What effect does density have on laminar flow
the more dense something is the hgiher the reynolds number and the more turbulent the flow will be
Why are sounds heard when a BP cuff is used or in the context of ejection murmurs or caortid bruits?
Reducing radius, preservation of flow and increased velocity all increases the reynolds number –> increasing turbulent flow –> generating sound waves
Where is turbulent flow in the lungs?
Large airways
What relationship does pressure and flow have in the context of turbulence?
Flow is directly proportional to the square root of driving pressure (however in laminar flow it is directionally proportional)
What is Bernoulli’s principle?
Fluid moving rapidly through a constriction exerts less pressure than a static fluid - the nergy of a flowing fluid exists in two forms : kinetic (velocity) and potential (pressure). The total energy remains the same, so potential enery falls (pressre) so the pressure exerted on the wall of the tube decreases with increasing velocity
Draw a tube and describe the effect of having a constriction in the middle on velocity, pressure, kinetic energy, potential energy. What principle is involved?
Describe this
Explain how driving past a truck results in being pulled towards it?
What is jet entrainment
An application fo the Bernoulli principle - whereby introducing a constriction to the passage of flow results in a reduction of pressure at the constriction and if at this time an opening existed entrainment would occur
What is the venturi effect
An application fo the Bernoulli principle - whereby introducing a constriction to the passage of flow results in a reduction of pressure at the constriction and if at this time an opening existed entrainment would occur
What applications are there of the venturi principle
Venturi mask
Nebuiliser - liquid is entrained and broken down
Car engine
What is the entrainment ratio
Entrained flow /driving flow
Draw a diagram of a venturi mask
What is the COanda effect
fluid or gas stream will hug the convex contour when directed at a tangent to the surface
Explain the coanda effect
As has already been seen with the Venturi effect, when the water leaves the tap at speed, the flowing fluid entrains fluid (in this case air) into the stream of flow. When there is an obstruction, such as the spoon’s surface, this entrainment is dramatically reduced on the spoon side. There is a drop in pressure on the spoon side of the jet and this causes a deflection in the flow towards the spoon.
How is gas flow simply measured?
A rotameter
Describe how a rotameter works
This type of flowmeter is a simple design with the important benefit of not requiring a power supply. A control knob allows precise adjustment of gas flow through a needle-valve. Above this, a vertical glass tube, tapered so it is wider at the top than the bottom, contains a bobbin which is supported by the upward flow of gas through the tube. When flow commences, the bobbin rises until the downward pull of gravity is balanced by the upward force of gas. This upward force decreases the further the bobbin rises, because the gas escapes around the bobbin more easily as the tube gets wider. The position of the bobbin is thus dependent on the rate of gas flow. Common designs feature a flat-topped bobbin so the flow rate is read off from the top of the bobbin onto a scale. Flutes in the bobbin cause it to rotate which reduces the risk of the bobbin sticking to the sides of the tube. Alternatively a spherical bobbin may be used and the flow rate is read from the bobbin’s centre.
What is another name for a variable orifice flowmeter
Rotameter
What important calibration is involved in a flowmeter/rotameter?
Gas density and viscocity as this will affect the bobbins position
How is volume measured in respiratory devices
Lung volume is just th etime integral of the flow rate
How is volume calculated from flow?
According to Equation 8.9, the differential of the volume–time curve (gradient of the curve) produces the flow at a given time. This can be plotted as a flow–time curve as shown. Similarly, the area under the flow–time curve gives the inspired and expired volumes.
What are two commonloy used respiratory volume flowmeters?
Rotating vane
Differential pressure flowmeters
How does a rotating vane flowmeter work
Sensor has a small turbine in the flow patha dn the rotation of the turbine can e related to the volume flow of the gas and interruption of a light beam by the tubrine can be sensed, converted into a voltage which is proprtional to flow and displayed continuously - volume is found by integrating flow with respect to time
Most accurate at low flow rates because inertia of the vane. Found in mouthpeices of lung function equipment
Explain a differnetila pressure flowmeter or a pneumotachometer?
Circular tube containing fine metal mesh providing resisatnce ot flow - during breathing flow causes small pressure difference on either side of the mesh and this is measured by a pair of pressure sensors connected by small tubes place in the wall of the tube at either side of the mesh. No moving parts, reliable, calculated flow rate is proportional to pressure difference according to Ohms law
What is another name for a pneumotachometer
Differential pressure flow meter
Descirbe the construction of a pneumotachometer
neumotachometers are designed with a large diameter at the point of measurement, which keeps the flow velocity down, resulting in laminar flow, as predicted by the Reynolds number equation
What assumption does a pneumotachometer make and how does it try to ensure this assumption is correct
ASsume laminar flow as flow calculated from pressure change using Ohms law
Pneumotachometers are designed with a large diameter at the point of measurement, which keeps the flow velocity down, resulting in laminar flow, as predicted by the Reynolds number equation
Problems with a pneumotachometer
preventing water vapour condensation as the tubes can be blocke by them - a resistance element is heated to get around this
What alternate arrangement of a differential pressure flowmeter can be constructed to allow for no flow resistor?
Pitot tube flowmeter
Draw a pitot tube flowmeter
In the following diagram is there flow in the sensing tubes?
The tubes are connected to the pressure sensors, so there is no flow in the tubes. However, the air in the flow-facing tube is compressed, causing a pressure increase. The compression arises because the kinetic energy of the moving air (proportional to the square of the velocity) is converted into potential energy (proportional to pressure) in the tube. The pressure difference is therefore equal to the square of the velocity. Note also that flow is not measured directly, and the velocity varies across the tube. It therefore has to be assumed that at a fixed point in the tube, the velocity is proportional to the flow rate.
What is a Wright peak flow meter and how does it work
Simple low cost design measuring maximal expired flow
Air from patients pushes against a diaphragm moving in the idrection of air flow, this is opposed by a spring which causes a narrow slot to open through which expired air escapes reducing the pressure on the diaphragm. A sliding marker indicates peak flow rate until the spring force overcomes air pressure
How could volume be measured directly over time instead of flow?
Benedict Roth spirometer - collecting gas passing through an airway opening. It is an expandable compartment consistent of a moveable statically counterbalanced rigid chamber or bell, a stationary base and a dynamic seal between them. The bell can move up and down freely so pressure inside it is close to atmospheric, the seal is often water but dry seals have been used. Changes in internal volume are proportional to displacement
Describe the way a Benedict Roth spirometer works and draw a diagram
Benedict Roth spirometer - collecting gas passing through an airway opening. It is an expandable compartment consistent of a moveable statically counterbalanced rigid chamber or bell, a stationary base and a dynamic seal between them. The bell can move up and down freely so pressure inside it is close to atmospheric, the seal is often water but dry seals have been used. Changes in internal volume are proportional to displacement
Problems with a Beneditct Roth Spiremter
Underestimates expired volumes due to contraciton of exhaled air and oncdensation fo water vapour as the air cools inside the spirometer
Define an ideal gas
An ideal gas has 3 conditions
- the molecules are assumed to be so far apart there is no attraction between them
- volume of the molecules themselves is negligible
- moleculears in random motion obeying newtons laws of motion
What factors within nature make gasses behave less like an idela gas
Very low temperatures
High pressures
What is standard temperature and pressure?
273.15K or 0 degrees
Atmospheric pressure is 101.3 kPa or 760mmHg
Avogadros law
equal volumes of gasses at the same temperature and pressure contain the same number of molecules
What is a mole
one mole is 6.02 x 10 ^ 23 atoms/molecules such that it represents a standard amount - it is derived from 12g of carbon
What is molar mass
the mass of 6.02 x 10 ^23 partiicles of the substance measured in g / mol
Why is humid air less dense than dry air?
Humid air contains water vapour, examining equal volumes of dry air and humid air at the same temperature and pressure then avogadros law states they contain equal numbers of molecules
Daltons law
for a gas the total pressure is simply all the partial pressures added up
Pt = P1 + P2 + P3
What is the partial pressure of water vapour as a standard
- In air
- In alveoli
1.3 kPa unless otherwise specified
the partial pressur eof water vapour in the alveoli does not alter as the airways always maximally humidify –> 6.3kPa
How would you calculate the partial pressure of oxygen change between dry air at standard H20 pressures, and alveolar gas pressures?
Boyles law
the volume of gas is inversely proportional to its pressure at a fixed temperature
Describe the relationship between pressure and volume in gasses and draw a diagram to represent the same
What is Charle’s law
at a given pressure the temperature is directly proportional to the volume of the gas - linera relationship
Guy-Lussac’s law
the pressure of a gas is directly proportional to its temperature within a fixed volume
Draw a curve representing Guy Lussacs law
Benedict Roth spirometer - collecting gas passing through an airway opening. It is an expandable compartment consistent of a moveable statically counterbalanced rigid chamber or bell, a stationary base and a dynamic seal between them. The bell can move up and down freely so pressure inside it is close to atmospheric, the seal is often water but dry seals have been used. Changes in internal volume are proportional to displacement
What is the combined gas law
What is the univertsal gas equation
Does a new substance diffuse faster through water or air?
Air
Less particle collisions due to reduced density leads to increased motion of the molecules over each period of time. Factors which increase energy will increase rate of diffusion - temperature
Define diffusion
Passive movemen tof a substance from an area of high concentration to that of lower concentration
Ficks law of diffusion
Add the diffusion constant to Fick’s law of diffusion
What factors lead to a faster rate of diffusion
Large surface area
Large concentration gradient
Small thickness ot diffuse through
High solubility in medium diffusing thorugh
Low molecular weight or density
Grahams law of diffusion
rate at which gasses diffuse are inversely proportional to the square root of their densities
Why is Grahams law of diffusion based on density? How can it be rearranged for molecular mass?
When does solubility of a gas matter to diffusion rate?
When diffusion is moving from a gas through a membrane into a liquid the rate of diffusion is proportional to soliubility of the gas
What two factors does the diffusion constant involve
What is osmotic pressure
the pressure required to stop the flow from one side of a semi permeable membrane to another
Isotonic solution
Two or more solutiosn that have the same concentration of a solute
Hypertonic solution
Has a higher concentration fo solute in comparison to another solution
Henry’s law
at a cosntant temperature the amount of gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium wiht that liquid
Define partition coefficient
the ratio of conentrations of a substance in two phases of a mixture of two immiscible solvents
Blood gas coefficient- what does this mean and reflect?
The ratio fo the concentration of an anaesthetic agent in blood to that in the same volume of gas in contact with that blood at equilibrium
This reflects the solubility of the gas in blood
Oil/gas coefficeint - what does it reflect and why does it matter
the ratio of the concentration of an anaeshtetic agent in oil (adipose) to that in the same volume of gas in contact with that oil at equilibrium
this reflects the solubility of the gas in adipose/brain tissues in comparison to blood - it will reflect how easily it crosses the BBB. If the oil:blood coefficient is >1 (or blood:oil <1) then the concentration or amount of gas dissolved in adipose tissue will be higher than that in blood
What characteristics are ideal in an inhaled agent?
Low solubility in blood
If highly soluble it transfers quickly from the lungs, but has a lower partial pressure in blood once dissolved therefore staying in solution rather than passing to brian tissue therefore taking longer to work, longer to exit the body.
What is a colligative property
vapour pressure, opsmotic pressure and variations in boiling and freezing point are proprties of solutions that depend simply on the number of molecules of a solute in a given volume of solvent rather than the properties (e.g. size, mass or identities) of the solute molecules)
Raoults law
the fall in vapour pressure of a solvent is proportional to the molar concentration of the solute
How is osmotic pressure measured
Osmotic pressure is related to the concentration of solutes in solution - it can be measured surrogately by analysing the colligative properties of the solution e,g. freezing point where a sample chamber, stirrer and temperature probe along with a cooling chamber analyse the freezing point. The change in freezing point is proportional to osmolarity
Mathematics of the Beer-Lambert law
Define current
the flow of an electric charge - this can be electrons or flow of charged particles e.g. ions
Wire resistance is proportional to?
Increases with length
Decreases with cross sectoinal area increases
Conduct better at lower temperatures
Electrical current is measured in amperes but what does this equate to?
1 ampere is 1 coulomb of charge passing a fixed point in 1 second
When current flows from a battery electrons flow from where to where
negative ot positive
By convention when drawing a battery and direction of current what direction does it travel
positive to negative
Draw a simple circuit with a battery, direction of currrent flow and a resistory
What is voltage
an informal term for electrical potential difference - this is the amount of energy required to move a unit of charge between two points.
1 volt is if 1 coulomb were to move through a potential differenceof 1 volt it would require 1 joule of energy ; or the electrical potential required to move 1 ampere through 1 ohm resistor
Write the relationship between current, potential difference and resistance
Ohms law
the potential difference between two points is the product of the resitance and the current flowing
Draw the Ohm;s law pyramid
What is an insulator electircally
a material whose charge carriers cannot move freely so does not conduct electricity
What is an electrical conductor
A material where charges can move freely and readily conduct electricity
What is a semiconductor
a material allowing a limited conduction of electivity under certain conditions - this can be manipulated
What is a diode
a device only allowing current to pass in one direction
What is a transistor
a tiny switch alllowing a small current to control a larger current
How does resistance vary with temperature
COnductors - electrical reisstance inreases with temperature as vibtraing atoms impede flow
Semiconductors decrease resisatnce with increased temperature as it allows more electrons to jump from valence to the conduction band (freed from bonds)
Kirchoffs first law
current in = current out
Or the sum of all currents going in and out is zero
Kirchoffs second law
closed loop netowrk the total voltage around the loop is equal tot he sum of all voltage drops within the same loop - this is also equal to zero (as the battery produces the voltage gain)
What is power in electircal terms
the rate of electrical energy usage/transferrance per second measured in watts - 1 watt is 1 joule transfered per second
Power in electrical terms =
Voltage x current
voltage squared / resistance
Current squared x resistance
Resistance in series calculation
the total resistance is greater than the largest reisstors
Resistors in parallel equation
the total resisstance is smaller than the smallest resistor
In a wheatstone bridge how is the unknown voltage solved?
Variable resistor is adjusted until current/voltage flow is zero
What is the difference between a magnet and electircity
A north pole cannot exist without a directly related south pole
Whereas a negative and positive charge can exist in totally separate locations without being coupled
What are the 3 types of magnetic materials
ferromagnetic 0 strongly attracted to magnetic fields
Diamagnetic - repelled by magnetic fields - have paired electrons e.g. copper, silver, fold, water, most organic compounds
Paramagnetic - weakly attracted - magnesium, lithium, oxygen, molybdenum, tantalum
What is electromangetic induction
the proicess of generating a current with a magnetic field - when a magnetic field and electrical conductor move relative ot one another
Impedence
the resistance to thef low of an alternating current - instead of resistance using in DC
Calculations for power remain the same
What is a dyanmo
Converts mechanical energy to electrical energy - constructed the same way as a motor
Uses a coil moving through a magnetic field or a magnet moving around a coil
When voltage over time is considered for AC circuits what does it look like?
What is hertz in the context of electricity
the frequency of AC supply - the rate at which it switches back and forth per second
What is a transformer
a device transfering current from one circuit to another via inductance - requires alternating current for the magnetic field to change relative to another coil of wire
What factors determine the severity of an electrical shock?
WHat is a fuse?
a thin wire isnerted into a circuit - its length and cross sectional area (reisstance) are calculated such that when a certain current is reached the heating effect melts the wire breaking the circuit
What is an RCD and how does it work?
Residual current device is device measuring the currentsin the live and neutral wires - the sum of the currents entering and leaving any node must be zero. If there is a difference te current must be leaking triggering the RCD to disconnect the power
What is a capacitor
a device for storing electrical charge
What is capacitance
a measure of the charge a device can hold measured in Farads
The cpacitance = charge stored in coulombs/ potential difference in volts
Energy stored = 1/2 capacitance x voltage squared
What is a dielectric
two or more parallel conductive platelets electrically separated by air or some form of isnulation which is the part called the dielectric
The charge Q in a capacitor is given by what equation
Q = C x Vc
C is caacitance
Vc is appplied voltage
What factors affect capacitance
Area of platets
Separation between them
Propoerties of the dielectic
What is an ultrasound wave
sound waves with a frequency >20 kHz (upper threshold of human hearing)
What is the velocity of sound waves in tissue
1500m/s in water - tissue is 1540m/s
Pulse echo principle
measures the presence of and distance from an object from which an ultrasound wave has been reflected
How do you calculate distance in the pulse echo principle
2d = v x t
How are ultrasound waves created
piezoelectrical crystals - contracts and expands in response to an alternating current allowing sound waves to be generated
How does an ultrasound probe process or receive signals
Piezoelectric crystals both convert electric to mechanical energy as well as convert mechanical or sound energy back to electrical energy by sound waves compressing and rarefaction of the crystal producing current in opposite directions - leading to timing, amplitude and frequency changes
How do ultrasound probes emit and detect simultaneously
They don’t - Ultrasound waves are emitted in short bursts with pauses allowing the probe to detect - it cannot be an emitter and detector at the same time
Draw a simplified cross section of an ultrasound transducer head
In the context of ultrasound what does phased array mean? What effect does it allow probes to have?
Phased array has many small transducer units (emitter/receivers) ina line each capable of performing independently - simultaneous transmission leads to a parallel wavefront, sequenced emission can lead to angled release - this is because of constructive interference where spherical wavefronts combine to form a single wavefront
What modes can ultrasound operate in
B mode - 2 d mode
A mode - single transducer scans a line through the body with echos plotted on the screen as a function of depth in one dimension. No virtually obseleted
M mode - motion mode, ultrasound pulses in quick succession and a one dimensional A mode image is taken where each image represented as a vertical line with brightness reflected intensity and successive images plotted next to one another. useful for finding boundaries between different regions of a beating heart
Doppler mode - measuring and visualising flow
Continuous wave doppler - doppler information sampled along a line through the body and all velocities detected at each time point plotted against time
A more sophisticated method samples doppler information form a smalll tissue volume and represents it one a timeline - this metho duses pulsed doppler to resolve the depth as well as velocty
Bue towards, red away
Duplex is simultaneous two dimensional and doppler ifnromation
What is attentuation in ultrasound
diminuation of energy associated with sound wave as a result of absorption, spreading, reflection and scattering
What are the forms of attenuation
Absorption
Scattering
Reflection
Dispersion
Define absorption
Define scattering
Define reflection
Define dispersion
Transducer frequencies in ultrasound imaging range from
2-19MHz
What is acoustic impedence
acoustic energy is reflected at interfaces between tissues with differing acoustic impedence - the ratio of impedence between two structures is critical to the way sound behaves. If difference is great reflection is predominant (acoustic impedence mismatch
Acoustic shadowing is
when near total reflection at an interface occurs due to acoustic impedence mismatch the area beyond is obstructed from view
Define resolution in ultrasound terms
smallest size of an object that can be displayed in the image
Axial resolution define
distinguish between two objects lying alone the beam axis
Lateral resolution
the ability to distinguish two neighbouring objects perpendicular to the beam and parallel to the transudcer face - determined by the width fo the ultrasound beam and improved by focus
Temporal resolution
Frame rate or pictures taken per second
How to directly measure the oxygen conten
What is a rotameter?
A brand of rotating bobbin flowmeter wqhich is used ot measure steady gas flow - it is a brand name
What i the principle behind how a rotameter works?
Variable orifice constant pressure flowmeter - principle is that gas flows up a vertical tube and the bobbin rises up the tube. The glass tube is shaped as an inverted cone so as the bobbin rises the space between the outside of the bobbin and the inside increases, this is the variable origice. The pressure drop is equal to thr weight of the bobbin so this is constant. As flow is varied the bobbin rises up or down, and flow is read from the top of the bobbin on the scale on the tube
How accurate are rotameters?
+/- 2.5% provided the correct gas is flowing and that no dust is floating, and the correc tbobbin is in the correct tube
Why is the rotameter shaped as an inverted cone?
Inverted cone is necessary so the distanc ebetween the bobbin and the tube wall increases as the bobbin rises up, the orifice size increases as flow increases and for any particular flwo the bobbin wil come to lie at a unique position, If the walls were parallel and the clearance around the bobbin remained constant there would be no stable bobbin position in the tube which was unique to a particular flow
How are ball flowmters different from bobbin flowmeters?
Flow is read from the middle of the ball; whereas from the top of a bobbin
The flow control is downstream from the ball so if the tube becomes cracked high pressure gas will leak even if the flow control knob is off
the ball flowmeter is less accurate but more robust and portable
What does Henry’s law say
Refers to gas dissolved in liquid - at equilibrium the amount of gas dissolved is proportional to the partial pressure of the gas at the surface of the liquid. The constant of proportionality (k) is a convenient measure of the solubility of a gas in the liquid
Amount of gas dissolved = k x partial pressure (once at equilibrium)
How do we make use of Henry’s law
Used every time we consider gas dissolved in liquid
What is Daltons law
In a mixture of gasses each gas exerts a pressure which it would exert if it alone occupied the entire volume - the partial pressure
the sum of all partial pressures is equal to the total pressure
What is the saturated vapour pressure of water at 37 degrees
47mmHg
What methods are there for measuring pressureX
Methods for measuring flow
What is an anaeroid gauge?
Do nto contain liquid - convenient method for measuring hgih pressures comapred to liqudi gauges
What is the most common aneroid gauge
Bourdon gauge
What is a Bourdon gauge
A method of measuring pressure in the class aneroid gauges
It consists of a flexible tube that is flattened and coiled inside a emtal case - calibrated dial is found ont he front and the tube is connected to a pointer at the centre of the coil via a set of gears. The other end of the coil is exposed to the gas supply pressure causing ti to uncoil and rotates the pointer
Advantages of a Bourdon gauge
Accurate 0.1-0.2%
Sensitive to small changes in pressure
Linear relationship between uncoiling and pressure
No electrical power supply
Disavantages of a bourdon gauge
Sensitive to shokc and vibration
Demonstrates hysteresis
Sensitvie to temperature change
Does the Bourdon gauge measure low or high pressure
high pressure
What is an example of a low pressure aneroid gauge
bellows gauge
How does a bellows gauge work
Measures pressure, an anaeroid gauge
elastic chamber, through which pressure to be measured is applied. The bellows expand as the pressure increases causing a pointer to move across a calibrated dial via mechanical link. Range of pressure dictated by compliance of the bellows. Used for airway pressure monitoring
Problems with a bellows gauge
large swings in temperature can render reading inaccurate due to Gay Lussacs law
What is a barometre
A device that measures atmospheric pressure
What is a classic barometer design
Atmospheric pressure exerted on the mercury at the open end forcing it around the U
How does an aneroid manometre work?
Measures gauge rpessure as opposed to true pressure; commonest used is NIBP
What devices are used in bobbin flowmetres to reduce error
The cone of the fl owmeter is made of plastic or glass and is coated with a transparent anti-static
material; there is also sometimes a conductive strip to prevent the bobbin sticking due to static.
The shape of the bobbin varies, but there is often a groove to make them spin in the gas fl ow and
a visual indicator to make it easy to see that they are spinning and have not become stuck.
With a flowmetre where on the spherical bobbin is it read
paralell with the equator
If it is a cylindrical bobbin in a flowmetre where is the measurement read off
the top
How does a Bobbin flowmetre work - what physical principles underlie its use
As the needle valve is opened, gas enters the fl owmeter and exerts a force on the bobbin, pushing
it upward. The gas fl ows around the bobbin and on into the back bar, however, the bobbin acts as
an obstruction to fl ow and so there is a pressure drop across it. At equilibrium, the pressure drop is
equal to the bobbin’s weight (mass multiplied by gravity) divided by its cross-sectional area, and
is therefore constant, hence ‘constant pressure’. As the bobbin moves upwards inside in the centre
of the cone, the gap (or ‘orifi ce’) between the bobbin and the side of the fl owmeter increases, hence
‘variable orifi ce’
At any fiven flow the bobbin reaches an equilibrium
Why does the specific gas used matter in a flowmtre?
The characteristics of the flowmeter at high flows are different to those at low flows. When the
bobbin is at the bottom of the meter, the gas flow and velocity are low and the orifice is small.
Therefore, flow around the bobbin at the bottom of the meter is laminar and is partially dependent
on the viscosity of the gas. When the flow is increased, the gas velocity increases and the orifice
becomes larger. The flow around the bobbin at the top of the meter therefore becomes turbulent
and the flow is then partially dependent on the density of the gas. Flowmeters are therefore
calibrated for a particular gas. The tapering of an air flowmeter will be different to that of a helium
flowmeter because although these gases have similar viscosities, their densities are very different.
The density of a gas varies with its temperature and pressure, while its viscosity varies only
with its temperature. These variables therefore theoretically introduce a source of error into the
flowmeter reading.
What effect does temperature have on flow of gasses?
Laminar flow depends on viscocity - viscocity varies with temperature. As temperature increases so does viscoity –> from the Hagen Poiseuelle equation as viscocity increases laminar flow decreases. Therefore a reduced gas flow will occur in the flowmetre if the temperature is high
What happens at high flow rates to bobbin flowmetres based on Hagen Poiseulle and turbulent flow equations
High flow results in turbulence, and the flow then becomes
more dependent on the gas density. As the temperature of
the gas increases, its density decreases. As the density of a gas
decreases, turbulent flow increases. Therefore, an increased
gas flow will occur in the flowmeter if the temperature is high
under turbulent conditions. Again, the scale cannot change and
so although the true rate of flow has increased, the reading is
lower than the true value – this is known as under-reading. Similarly, at low temperatures the
flowmeter will over-read under turbulent conditions.
Hagen Poiseuille law
Turbulent flow is proportional to? and inversely proportional to?
Reynolds number
What affects flow
Define ultrasound
High frequency sound waves
Higher frequency than the upper limit of human hearting (>20kHz)
Frequencies in medicine commonly use 2-20MHz
What are the different modes of USS
B mode
Doppler
M mode
Single beam
3D
How does the display of 2D images work with USS
Linear array of piezoelectric crystals
Thin slice through the body being interrogated
Vertical axis is the time taken for reflection to reach transducer i.e. distance form transducer
Brightness indicates strength of reflection ie. amplitude of reflected echo
What is the Doppler effect
Change in frequency that occurs when a wave is reflected off an object that is moving relative to the observer
Change in frequency is the Doppler shift
What factors affect the doppler shift
Fd = 2 x Ft x V x Cos theta / average velocity fo sound in soft tissue
Fd = doppler shift
Ft = transmitted freqyenct
V = velocity
Cos thea is incident angle
Define tonicity
Effective osmoles
