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Spirometers:
are devices that measure volume of gas; if the Spirometer is
equipped with timing devices that mark off the seconds, then
exhaled flow rates can be measured.
1.
Water-sealed Spirometer: this first PFT spirometer used a
circuit attached to a huge cylinder [called the bell] that
was isolated from the rest of the atmosphere by a water
seal.
·
As the patient exhales into the circuit, the bell rises
·
The top of the bell is attached to a pulley and a chain with
a weight that counter balanced the bell to remove gravity
from the equation.
·
This in turn was attached to a pen that records the changing
volumes onto a rolling cylinder called a kymograph.
·
The kymograph rolls at a set speed so that the seconds can
be known s our measurement of volume can now include flow
rate volume/time]
·
The graph created by this device was the spirogram
·
This Spirometer was equipped with soda lime to absorb the
C02 to prevent rebreathing [during VT determinations during
the testing]
·
As the bell went up the pen went down
2.
Stead-wells Spirometer: replaced the metal bell with a
lighter plastic one so that the counter weight was no longer
needed. Because the pulleys were no longer used as the bell
went up the pen also went up. The actual spirogram changed.
3.
Bellows Spirometer: the bell was replaced by a flexible
rubber or plastic bellows that opened as gas entered it and
closed as gas left
4.
Dry-rolling Spirometer: a piston is attached to cylinder.
As the patient breaths the piston is displaced. This was an
improvement over the bellows which tend to have more
resistance to movement. A potentiometer attached to the
cylinder records the cylinder movement, and then sends the
information to the pen.
5.
Wright’s type Spirometer: hand-held spirometers used to
measure exhaled VT and VE on patients at the bedside. These
devices used a rotating vane that rotated as gas entered it.
While these will react to inhaled gases, the resulting VI
are not dependable.
Flow meters:
are devices that measure flow. In the case of these types of
flow meters, we are not measuring the flow of gas coming from
the lines in the wall, rather we are measuring the patient’s
exhaled and sometimes inhaled flow rates
1.
Dedicated flow meters: called ‘peak exhaled flow meters ‘to
differentiate them from the 02 flow meters in the wall,
these devices can only measure PEFR.
·
Most of the cheaper disposable units use a piston and a
spring to move a needle to the displayed PEFR.
·
Exhaled water vapor on these devices makes them less
accurate so they need to be dried by placing next to 02 flow
meter
2.
Thermal flow meters [also called anemometer]: these devices
uses a heated wire or a thermistor bead. Both of these
devices cool as faster gas passes over them.
·
Changes in temperature change the electrical resistance
through the two types of devices.
·
These devices are unidirectional and cannot measure gas in
both directions
·
These are pretty accurate until they get wet in which the
heat loss from evaporation will make them read inaccurately
3.
Turbine flow meters:
·
Uses a rotating vane that speeds up as faster gas moves
through it
·
The vane rotations are counted by [1] a light beam that is
interrupted , or mechanically with a needle attached to a
calibrated display
·
Some dedicated PEFR use turbine technology
Pneumotachograph [differential pressure pneumotachometers]
1.
Fleisch Pneumotachograph: uses copper wires to create a
known resistance. It measures the pressure before and after
the resistance to determine the flow. This is called
pressure differential.
·
This can measure bi-directional flow.
·
A heater will keep the temperature of the copper consistent
and will prevent water from building up on the copper tubes
which would change the resistance
·
More accurate with laminar flow rather than turbulent flows
·
It is quite sensitive to the different viscosities of gas
mixtures used so needs to be calibrated to the gas mixture
needed for each test
2.
Screen Pneumotachograph: in this device the resistance is
created by the screen. Again, both particulate matter and
water vapor will change the resistance and the readings
3.
Variable orifice Pneumotachograph: use a set and a variable
orifice to create the differentials. These are bidirectional
General word on all differentiate flow devices: Because the
flow resistance through these devices determine the accuracy
of the differential flow meters, it is important to select
only those patient interfaces [mouthpieces and circuits]
that conform to the manufacturers’ specs for resistance to
flow otherwise-- the numbers we collect are worthless.
4.
Ultrasonic Pneumotachograph [vortex type]: measures flow
with ultrasonic sound waves that detect a vortex of flow
created by baffles [struts] in the stream. The sound beam
hits a receiver that notes the differences in the vortex.
Are unidirectional
5.
Ultrasonic Pneumotachograph [non-vortex type] sends sound
waves perpendicular to the flow to measure it.
Body plethysmography
An airtight, rigid-walled container inside which the patient
sits.
·
It is equipped with pressure sensors in the walls that
notice the tiny changes of pressure created by the patient’s
chest wall getting larger and smaller during breathing.
·
It has airway pressure monometers in the circuit
·
The circuit of the body plethysmography contains a shutter
that will close- stopping flow so the alveolar pressure can
be read for RAW calculations,
·
The flow rates needed for the RAW formula come from a
differential pressure pneumotachometer
Gas Analyzers:
1.
Helium analyzers: thermal conductivity/ wheatstone bridge;
due to the GMW of different gases, the ability of a gas to
dissipate heat differs.
·
A sample is brought into the sample chamber where it will
cool a wire which changes the electrical resistance.
·
This change in electrical conductivity is compared between
the reference chamber which holds another gas and the
incoming He sample.
·
Because cooling of the wires by rapidly flowing or damp gas
will confuse the equipment, we must have dry static samples
2.
02 analyzers
·
Paramagnetic: 02 molecules disrupt a magnetic field.
o
Must have dry, static sample, so the gas is pulled into
device. Older-- less used
·
Galvanic analyzers/polargraphic: uses a 02-mediated chemical
reaction to generate an electronic flow.
o
When an electrical current is added to speed up this
reaction, the polargraphic type works faster and requires a
smaller surface area for gas diffusion.
o
These can be operated in line the circuits and have such
fast response that they are probably used the most for
measuring 02 in mechanical ventilators and inside hoods and
02 tents
·
Thermal conductivity: same principle but the 02 analyzer is
calibrated for 02 rather than for Helium. Same problems with
needing a dry, static sample
3.
N2 analyzers: photointensity; this device sucks N2 into a
chamber where it is ionized by electrical current. This
creates blue light which is filtered so that only the blue
light enters the photocell. Excessive levels of C02 and of
He can make this inaccurate
4.
C02 analyzers [capnometers]: C02 absorbs infrared light
waves. The amount of absorption equals the concentration of
the gas in the sample.
·
The gas inters the chamber where it separates the infra red
light from the light detector which notes the decreased
light. A beam splitter [or chopper] rotates to interrupt
the light going to the photodetector.
5.
CO analyzers: measurement of CO is also done by infrared
light waves
6.
Mass spectrometers: spectrum analyzers count the relative
number of ionized molecules of each gas in the sample. First
the gases are ionized [given positive charges. More dense
gases travel to a different place to be attracted by the
polarity
7.
Gas chromatograph: first some gases are separated by
absorption technologies [such as molecular sieves] and other
gases are separated by porous polymers, then each gas is
analyzed
Standards
American Thoracic Society [ATS] established standards for
PFT in 1979, and most equipment conforms to these standards,
so that a patient’s PFT results should be the same --not
only from lab to lab but within the different brands of
equipment.
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Accuracy is the ability of the equipment to relate to the
true value of the quantity measured.
·
Precision: is the ability of the equipment to accurately
reproduce a measurement
Calibration of the equipment prior to testing
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A 3 Liter syringe is used to calibrate the Spirometers
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While flows are calibrated by comparing a number of
standardized waveforms
Tolerance of the equipment
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Spirometers that measure VC and FRC, FEV1 need to be
accurate within+/- 3%
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Pneumotachigraphs that measure PEFR, FEF25-75% need to be
accurate within 5% of the reading
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