Lecture 1 Flashcards
what can elevated iop result in
elevated iop can result in glaucoma
which can lead to visual field constriction and ultimnaltey blindness
chronic glaucoma may not be noticed until the optic nerve is permeantly damaged- therefore iop monitoring can be helpful in identifying asympotomatic patients ar risk
what is 14mmhg in hpa
19hpa
what is glaucoma
raised iop
impaired drainage of aqueous fluid from the anterior chamber
permeant damage to optic nerve leading to loss of ganglion cells can be considered a form of optic neuropathy
leads to a biararcuate field defect
what are some example causes of glaucoma
obstruction of aqueous fluid drainage from the anterior compartment of the eye
in order to make clinical inferences from iop it is important to establish a normal range in addition measurements should be non traumatic and non invasive
what is the definition of normal iop
pressure that dosnt lead to glaucomatous damage of the optic nerve - normal pressure is about 15mmhg with a standard deviation of 3mmhg
their is a overlap between non glaucomatous and glaucomatous which demonstrates that diagnosis cannot be purely reliant on iop - most studies would suggest a normal range should be between 10-20mmhg however individual differences cloud this + there is subtypes like low tension glaucoma
thus high iop isn’t necessarily glaucoma and low iop dosnt necaessairly rule it out -
iop= influenced by a number of factors e.g. blood prsessure / drugs etc
what is the measurement of iop termed
the measurement of iop= termed tonomettry and is performed with tonometers - pressure within it can be measured by relating the deformation of the globe (unknown ) to an exrerannly applied force (known)
two types of tonometer- applanantizn and indentation
what relationship do application tonometers rely on
applanation tonometer rely on the relationship between wall tension and pressure in a elastic sphere - the surface tension y (gamma) is related to the pressure difference triangle across the curved wall by the following equation
pressure difference = 2 x surface tension over radius
how are pressure, force and area related
pressure= force / area (opthamologists call this the imbert fick law)
when do the forces balance out when a piston is placed on a cornea
cornea has a central thickness of 0.55mm
in the literature forces balance out at about 3.06mm
how does the Goldmann tonometer work
Goldman type application tonomter applies a force to the surface of the cornea and uses a optical system to determine when the applanated area is 3.06mm in diameter ]
the cornea is anthessied with a topical anaesthetic and the tear film is stained with sodium fluroscein
fluorescence allows visualisation of the meniscus surrounding the application area
which is split into two semicircles by biprisms
when the inner edges of the semicircles touch is when applantion is correct
the iop can then be measured directly from a scale on a tonometer
what can errors be due to when using a tonomter
variations of thickness and curvature of cornea - e.g. keratoconus/ laser surgery
variations in tear film , age , epithelial oedema , bvlephorasm / poor coop , posture , drugs and blood pressure/ tight collar
how do tonometers that use a puff of air work
tonometers are now available that use a puff of air to deform the surface of the cornea
by using a know velocity and volume of air and measuring the resulting deformation of the corneal surface it is possible to provide a reasonable accurate measurement of iop
how much is 1mmhg in hpa
1mmhg= 1.33hpa
1hpa= 0.75mmhg
what do we regard an electrode as
an electrode is a transducer - has to convert the ionic flow of current in the body to an electric flow along a wire to a recorder
what are the different types of electrode
microelectrodes- electrodes which are used to measure the potential either inside or very close to a single cell
needle electrodes- electrodes used to pass through the skin and record potentials from a small area such as a motor unit within a muscle
surface electrodes - electrodes applied to the surface of the body and used to record signals such as the ecg and erg
what are microelectrodes used for
microelectrodes are not routinely used in medical departments
they are electrodes with a tip small enough to penetrate single cell and can only be applied to samples of neural tissue - a very fine wire can be used
smallest electrodes are tube of glass stretched to give a tip size of 0.5um the tube is filled with electroplate kcl
special recording amplifiers used in order to allow for the very high impedence of tiny electrodes
needle electrodes uses
used for routine needle electromyography as it allows for potentials from only a small group of motor units to be recorded
needle electrodes must be sterilised before use and they also must be kept clean in order for them to work satisfactorily
electrodes are suitable for sterilisation by autoclaving but others must be steralised in ethelynee oxide gas
this form of sterilisation requires needles to be pe placed in ethylene oxide gas at 20 psi 140kpa for 1.5hrs at a temperature of 55-55 degrees the needles must not be used for 48hrs following sterilisation this allows for spore tests to be completed
what are the problems with surface electrodes
they are subject to movement artefacts
movement of the electrode - movement of the electrode disturbs the electrochemical equilibrium between the electrode and the skin and results in contact potential changes - that’s why electrolyte (gel) is placed between electrode and skin
a pool of electrolyte is placed between the silver chloride disc and the skin - movment of the electrolyte does not disturb the junction between metal and electrolyte and so does not change the electrode or contact potential
what are other electrodes that can be used
e/..g conductive polymer electrodes can be used to make contact to the cornea and record electrical signals from the eye
what is the largest electrical signal produced by the body
the largest signal produced by the body is the ecg
which has an amplitude of 1mv
this is too small to drive any recorder directly or too input into a computer hence any signal must be amplified before recording
amplification can be either of voltage or current- voltage amplification is required to supply sufficent to current to drive a recording device
what is amplifier gain
if the signal to amplifiers is small then the amplifier gain will be large , an eeg amplifier will have a higher gain than an ecg amplifier
the gain of an amplifier is the ratio of the output and input voltages
in a ideal amplifier the gain is independent of frequency but in a real amplifier this sent the car
frequency response of amplifier must be matched to the frequency content of the signal
what is frequency analysis
Fourier developed the concept of frequency analysis he demonstrated that any receptive signal can be considered a summation of sine waves
what is the ecg
ecg is a perodic signal of which the lowest frequency component is the heart rate
if the heart rate = 60min the lowest frequency component is 1hz
Fourier analysis reveals that the complete ecg waveform can be produced by adding together the sine waves of 1hz 2hz 3hz and so on the. amplitude of components will determine the shape of the ecg
frequency analysis dictates that the largest component of the ecg has a frequency of 17hz and that the components above 100hz are of negligible amplitude an ecg amplifier must be able to handle frequency components between 1 hz and 100hz - it must amplify the components equally and preserve the relative phase of all the components
what is bandwidth
bandwidth of an amplifier = the frequency over which the gain remains constant
in practice = -3db bandwidth - this = frequency range over which the gain is not less than 3db below the maximum gain - the bandwidth of an amplifier must be sufficent to handle all frequency components in the signal of interest
what is sample rate
interfacing signals must be sampled and recorded at regular intervals this= a sample rate - the rate of sampling must be high enough to handle all the frequency components of the desired signal
noise and electrical interference
electrophysiological signals such as the ecg , eeg and emg are susceptible to interference due to their small amplitude
lab instruments , nuclear medicine and computing equipment can be subject to interference from nearby electrical machinery such as lifts and air conditioners
