OHD1: Electronic tonometers week 7

schiotz

a digital scale readout

subject to the same errors

mackay - marg

hand held

1.5mm diameter probe in tonometer head 

movements create electrical current 

know how much force required to applanate 1.5mm²

analyse current to measure applanation 

how does mackray marg work?

middle of baseplate that sticks out

when taking pressure, touches the central cornea

plunger pushes in and movement is sensed by magnets which creates an electrical impulse

by the time the cornea has hit the rest of the plate it stops moving 

at that point it has 1.5 mm applanation

tonopen

hand held mackray-marg 

battery operated 

read IOP directly - digital display 

record time taken 

grolmann (NCT)

uses air to applanate the cornea

constant area applanation

no need for anaesthetic

little risk of corneal damage

principles of grolmann

3 components:

alignment system

pneumatic system

applanation detector

alignment system of NCT 

ensures ocrrect positioning 

image forms when object is at the centre of the curvature of surface 

approx 11mm from the eye 

cant operate unless aligned 

fires when right distance away

pneumatic system

this creates the puff of air

narrow collimated pulse9 air going in same direction)

pressure increases linearly gradually

peaks at 12ms

indents/ flattens cornea

applanation reached before indentation

applanation detector 

need to know at what point the cornea flattens

collimated IR beam and has IR detector 

uses reflection: cornea reflects IR light bounces of the flat cornea and is detected by the detector 

time measured to applanation is proportional to IOP 

measure puff of air at applanation 

procedure of NCT

demonstrate and explain

align pateint then instrument

take number of readings recommended by manufacturers guidance: usually 4

pulsair nct

hand held nct

automatic activation

measures system pressure at applanation

softer pulse under 25mmHg

take 4 readings

readings of the pulsair 

the machine will indicate when 2 readings within 1mmHg of eachother have been achieved 

do 4 readings 

displays average so press R to get the individual readings 

what to look for in pulsair

when bowtie is seen hold it there and it should fire

reichert xpert (NCT)

Manual or automatic modes

measures system pressure

pulse cut off at applanation

TV monitor

results printed

errors with nct 

ocular pulse: pressure fluctuates every sections 

distorted corneal surface -applanation and allignment requires cornea to be shiny for the reflection

eye movement 

lids/lashes 

high IOP- reliability decreases

advantages of nct

speed and ease of use

repeated readings are possible

non contact with the cornea

clinically accurate

disadvantages of nct

patients dislike

expensive

not good on scarred cornea

can produce spurious readings

complicated to calibrate

rebound tonometry

gently touching the cornea with a probe

induction coil propels a magnetised steel wire (with a plastic tip) towards the cornea 

probe hits cornea and rebounds

 returning movement induces current in coil 

bouncier cornea= higher iop  

6 measurements 

advantages of rebound tonometry

patients prefer over NCT

no drugs required

self assessment versions available

less affected by corneal thickness

disadvantages

probes are needed which are costly

less reliable at high IOPs

affected by corneal rigidity : elasticity of cornea, how bouncy it is

calibrating goldman tonometer 

calibration bar - a weight at side

marked at 0.2 and 6mgs : 20 mmHg and 60mmHg 

if set at 6: hae 6g of weight pushing against tonometer bar 

if dialled up to 6 aswell it equalises so probe will be in the middle 

can compensate 

calibrating perkins

lie on plastic disc

puts weight on cone- 3g and 5g

20mmHg 50mmHg

can compensate f calibration out

must be linear : if its out my 1mmHg at 2, and out by 1mmHg at 6

perkin calibration

add the 2g and twist the wheel until it balances

compensating measurements of goldmann

if use 2g and rocks forward by 1.8g then it reads under by 2mmHg 

so need to add 2mmHg to the result