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
How is gas flow simply measured?
A rotameter
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.
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 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
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
How would you calculate the partial pressure of oxygen change between dry air at standard H20 pressures, and alveolar gas pressures?
Describe the relationship between pressure and volume in gasses and draw a diagram to represent the same
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
Ficks law of diffusion
Add the diffusion constant to Fick’s law of diffusion
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?
What two factors does the diffusion constant involve
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
Raoults law
the fall in vapour pressure of a solvent is proportional to the molar concentration of the solute
Mathematics of the Beer-Lambert law
Draw a simple circuit with a battery, direction of currrent flow and a resistory
Write the relationship between current, potential difference and resistance
Draw the Ohm;s law pyramid
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
When voltage over time is considered for AC circuits what does it look like?
What factors determine the severity of an electrical shock?
Draw a simplified cross section of an ultrasound transducer head
Define absorption
Define scattering
Define reflection
Define dispersion
What methods are there for measuring pressureX
Methods for measuring flow
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
Hagen Poiseuille law
Turbulent flow is proportional to? and inversely proportional to?
Reynolds number
What affects flow
