Dräger Cato Operating Manual
I) Anesthetic Workstation Cato:
It is a universally applicable anesthetic workstation for:
- Inhalation anesthetic in semi-closed systems
- Inhalation anesthetic in virtually closed systems with “low flow” and “minimal
flow” techniques for minimum gas and anesthetic consumption with:
1. Automatic ventilation (IPPV)
2. Synchronized intermittent mandatory ventilation (SIMV)
3. Manual ventilation (MAN)
4. Spontaneous breathing (SPONT)
II) Measurement and Monitoring Functions:
The available functions are:
- Measurement of ventilation parameters (pressure, flow, O
inspiratory and expiratory).
- Continuous measurement of CO
(halothane, enflurane, isoflurane, sevoflurane, desflurane). The flow rate for
sampling the measuring gas can be varied and is returned to the circulation.
- Automatic adjustment of the alarm limits for automatic ventilation (IPPV).
- Anesthetic vaporizer with automatic vapor recognition.
- Optional:
• Continuous non-invasive measurement of functional O
• Measurement of inspiratory breathing gas temperature.
III) Indicated Values:
The indicated values are:
- Continuous curve for airway pressure, peak and plateau pressure, mean pressure
and PEEP1.
- Patient compliance.
- Expiratory minute volume, tidal volume and respiration rate.
- Expiratory flow curve.
- Inspiratory and expiratory O
2
- Inspiratory and expiratory concentration of N
enflurane, isoflurane, sevoflurane, and desflurane.
- Inspiratory and end-expiratory CO
- Continuous CO
curve.
2
- List entries and trend displays.
1
PEEP: Positive End-Expiratory Pressure
concentration
2
concentration and N2O/anesthetic concentration
2
saturation.
2
concentration.
O and anesthetic halothane,
2
concentration.
2
- Optional:
• Functional O
• Inspiratory breathing gas temperature.
IV) Monitored Parameters:
The monitored parameters include:
- Airway pressure.
- Expiratory minute volume.
- Inspiratory O
concentration.
2
- Inspiratory and expiratory CO
- Inspiratory anesthetic concentration.
- Optional:
• Functional O
• Inspiratory breathing gas temperature with invariable upper alarm limit.
V) Keys:
A) Setting operating modes:
saturation, pulse rate, and plethysmogram.
2
concentration.
2
saturation and pulse.
2
B) Setting ventilation parameters:
Located below the display window:
VI) Structure of Display Screen:
Figure 1 - Structure of Dräger Cato Display Screen
Figure 8 illustrates the diagram of the display screen structure on a Dräger Cato
ventilation machine. The description of each section is given as follows:
- Status field: contains information on current alarm mode of monitor.
- Alarm field: indicates alarms and their priority.
- Graphic field: curves and bar graphs.
- Measured value field: for most important numerical values.
- Operator prompts: prompts to guide the operator.
- Soft keys: for rapid selection of functions displayed on screen.
VII) Screens:
The available screens are:
a. Standard screen: with CO
measured values are indicated to the right).
b. Data screen: contains all measured values with their units of measurement;
simplifies completion of anesthesia record.
c. Trend screen: for displaying the changes in the measured values since the
measurement started (current measured values are shown on the right).
VIII) O
Flush Capability:
2
The O
flush capabilities are possible with O
2
IX) Oxygen Ratio Control (ORC):
ORC regulates the delivery of O
0.8 L/min proportionally with O2 flow for ORC low-flow, and N2O flow decreases to
“0” proportionally with O2 flow for SORC. When the mode is switched to “air”, the
N2O decreases to “0”.
X) Automatic Calibration of O
/Flow Sensors:
2
The side-stream calibration of O2 measurement (O2 sampling) and flow measurement
are performed during the first breaths after starting ventilation. There’s therefore no
cause for panic if measured values for minute ventilation still appear in grey
immediately after self-test. CO2 measurement must function correctly before
automatic flow calibration can be performed, otherwise the flow sensor must be
calibrated manually.
XI) Manual Calibration of O
Sensor:
2
This manual calibration is only performed when the O2 measurement has been set in
inspiratory line.
Calibrating the O
sensor with 21% O2 by volume – air:
2
The O
sensor can be calibrated while the flow calibration is being conducted:
2
1. Remove the sensor from inspiratory valve and expose it to ambient air.
2. Place it on table and wait for at least 2 minutes.
3. Use rotary control to select “Calibrating” in “Standby/configuration” menu
and press to confirm.
4. Move the cursor frame to “O
and press it to confirm starting the calibration.
curve and another selectable curve (most important
2
+
button.
2
, N2O, and air where N2O flow decreases below
2
Sensor 21 Vol %” by means of a rotary control
2
5. Plug the O
sensor onto the inspiratory valve again.
2
XII) Manual Calibration of Flow Sensor:
A) Without Removal: On anesthetic unit:
1. Set the AIR/N
O selector to “AIR”.
2
2. Close the delivery valves for O
thoroughly flush the breathing system with air.
3. Close the delivery valve for air.
4. Press the rotary control on the monitor to start the calibration (timer icon
appears and then tick appears when calibration has been completed correctly).
5. Tick off in the checklist, then the cursor frame automatically jumps to the
“Ventilator Start-up Test”.
B) With Removal:
1. Set the AIR/N
O selector to “AIR”.
2
2. Close the delivery valves for O
thoroughly flush the breathing system with air.
3. Close the delivery valve for air.
4. Press the rotary control on the monitor to start the calibration (timer icon
appears and then tick appears when calibration has been completed correctly).
5. Remove the flow sensor as follows:
- Unscrew the expiration nozzle.
- Remove the flow sensor.
- Briefly swing it to flush with ambient air.
- Hold it horizontally with the cable connection pointing toward downwards
(calibration in installation position improves the measuring accuracy).
- Seal off one or both sides as shown on right preferably with thumb or
palm.
- Press the rotary control on the monitor to start the calibration.
6. Replace the flow sensor.
XIII) Ventilation Modes:
A) Manual Ventilation:
1. Deliver fresh gas.
2. Switch the pressure limiting valve APL to “MAN”; turn the lever to set
the pressure limitation on the scale.
On ventilator:
3. Press “MAN/SPONT” for at least 1 second.
4. Ventilate the patient with a breathing bag.
and N2O, open the delivery valve for air and
2
and N2O, open the delivery valve for air and
2
B) Mechanical Ventilation in IPPV Mode:
1. Deliver fresh gas.
On ventilator:
2. “IPPV Mode?” message appears.
3. Press the rotary control to confirm.
4. Activate the ventilation parameters by pressing the corresponding keys.
5. Select and confirm with the rotary control.
C) Mechanical Ventilation in SIMV Mode:
1. Deliver fresh gas.
On ventilator:
2. “SIMV Mode?” message appears.
3. Press the rotary control to confirm.
4. Activate the ventilation parameters by pressing the corresponding keys.
5. Select and confirm with the rotary control.
XIV) Parameters in Standby:
The parameters in standby are:
- SpO
measurement (on/off).
2
- Side-stream measurement O
- Sample rate: 60 or 200 mL/min.
- CO
units: mmHg, kPa, or % by volume.
2
XV) Calibration Options:
The calibration options include:
- Calibrate the O
sensor with 21 vol. % O2.
2
- Calibrate the flow sensor.
- Perform the ventilation start up test.
- More:
¾ O
sensor calibration with 100% O2 by volume.
2
¾ Linearity check of O
¾ Calibration of CO
(on/off).
2
sensor.
2
sensor.
2
XVI) Curve Selection:
The curve selection is only possible in the standard screen. A second curve for bottom
half of the screen can be selected from the menu presented to complement the CO
concentration curve which is always displayed. The following can be selected:
- PAW: Airway pressure
- Flow: Expiratory flow
- Volumeter: Display showing minute ventilation plus PAW and V
- Pleth.: Plethysmogram (optional)
- O
: Oxygen concentration of breathing bag (optional)
2
XVII) Simplified Pneumatic Schematic:
Figure 9 illustrates the simplified pneumatic schematic of the Dräger Cato ventilation
machine. It shows the valves and subsystems.
as bar graph.
T
2
Figure 2 - Simplified Pneumatic Diagram of Dräger Cato Ventilation System
XVIII) Care Schematic:
Figure 10 shows the care schematic of the Dräger Cato ventilator.
Figure 3 - Care Schematic of Dräger Cato Ventilation System
XIX) Choice of SpO
Sensor:
2
The choice of SpO2 sensor (Nellcor) is based on the following criteria:
- Patient’s weight
- Mobility of patient
- Possible application site
- Perfusion of patient
- Period of use (short or long term monitoring)
XX) Checking Operational Readiness:
The operational readiness of the machine must be restored and checked whenever it
has been cleaned, disinfected, sterilized, repaired or serviced. Supplementary
equipment must be checked in accordance with the respective “Instructions for Use”.
Note the applicable time limits for filters, maintenance and calibration, e.g. in
conjunction with the equipment for measuring blood pressure or body temperature.
Checking the operational readiness includes the check based on the checklist, the selftest and the following:
- O
Shortage Signal:
2
1. Open O
2. Interrupt O
3. O
2
delivery valve.
2
supply.
2
shortage signal must be given after approximately 3 seconds:
continuous tone for at least 7 seconds.
- Supply of Anesthetic Agent:
1. Check filling level.
2. Replenish anesthetic agent if necessary – as specified in separate
“Instructions for Use” of anesthetic vaporizers. If the last inspection took
place more than 6 months previously.
3. Check vapor.
4. Lock hand-wheel in zero position.
- Temperature Measurement (Optional):
1. Remove the temperature sensor from the Y-piece and place in water bath
(21º → 49º C).
2. Compare the measured value on the screen with that of the second
thermometer with known accuracy.
- SpO
Measurement (Optional):
2
1. Apply Dura-sensor DS 100A to own finger.
2. Indication must be plausible.
- Power Failure Alarm:
1. Interrupt power supply, e.g. disconnect plug from the socket.
2. Switch on machine – press power switch, turn to I. Power failure alarm is
given. Continuous tone – volume must remain constant for 30 seconds. If
not, reconnect to mains and leave machine on for 24 hours so that the
battery can recharge. Repeat test.
3. Switch off machine, press power switch again and turn to “0”. Alarm goes
out.
4. Reconnect to power supply.
- Self-test:
The self-test must be completed successfully in order to establish readiness for
operation. Supplementary equipment must not be switched on until after self-test.
Start self-test: Switch on machine by pressing the power switch then conduct
ventilator start-up test.
- Calibrate Flow Sensor:
Check the manual calibration of the flow sensor.
- Calibrate Inspiratory O
Sensor with 100% O2:
2
This is necessary if the side-stream O
measurement is not used.
2
1. Remove O
2. Allow an O
sensor and place test adapter 6801349 on sensor.
2
flow of approximately 1 L/min to flow over the O2 sensor for
2
approximately 2 minutes.
3. Select “More” with rotary control.
4. Select “O
5. Display: When the O
Sensor 100 Vol. %” with rotary control.
2
sensor has been flushed with O2 for approximately
2
2 minutes:
a) Press rotary control to confirm.
b) Calibration is started and continuous automatically.
c) Calibration is then complete when “9” tick shows instead of the
clock item.
6. Replace the O
sensor in its mounts.
2
- Calibrate O
Sensor for Side-stream:
2
It is necessary for monthly linearity check. Prepare substitute sampling line:
1. Cut sampling hose through the middle.
2. Unscrew the original sampling hose from water trap and screw on slit
sampling hose.
On Cato:
1. Disconnect fresh gas hose from breathing system.
2. Set an O
flow of 1 L/min O2 at O2 flow tube and slide sampling hose into
2
fresh gas hose.
3. Use rotary control to select “Calibrating” and then “More”.
4. Select “O
Sensor 100 Vol. %” with rotary control.
2
5. Display.
6. Allow O
to flow for approximately 2 minutes.
2
7. Press the rotary control to confirm; calibration is then started and
continues automatically. The clock icon “¥” is then replaced by a tick
“9” icon.
8. Screw the original sampling hose back onto the water trap.
9. Plug the fresh gas hose back into the breathing system.
- Check Linearity:
The linearity must be checked every month.
1. Calibrate the O
measurement used (inspiratory or side-stream) with 100%
2
O2.
2. Expose inspiratory O
sensor to ambient air for approximately 2 minutes
2
(inspiratory).
OR
Unscrew sampling hose from water trap and allow it to take in the air for
approximately 2 minutes (side-stream).
3. Display on screen should show 18 – 24 vol. % O
. If the value displayed is
2
outside 18 – 24 vol. % O2, then the sensor capsule is faulty.
4. Put the inspiratory O
sensor back and calibrate (inspiratory).
2
OR
Put the O2 sensor for side-stream O2 measurement back and calibrate
(side-stream).
5. Put the O
sensor back.
2
6. Screw the sampling hose back onto the water tap.
- Manual Ventilation Function:
This function can be selected when the machine is in “Manual/Spontaneous”
mode.
1. Press “MAN/SPONT”.
2. Pressure the limiting valve (APL). Set to “MAN”.
3. Connect lung simulator, and exercise thorax or breathing bag to the
Y-piece.
4. Set the fresh gas flow.
5. Set maximum ventilation pressure between 5 and 70 mbar on APL valve.
Turn the valve head for this purpose.
6. Compress the breathing bag.
7. Compare the pressure indication on the monitor with setting on the
pressure limiting valve. If excess pressure has to be relieved quickly, then
press vertically down on the lever of the pressure limiting valve.
- Automatic Ventilation Function:
1. Connect the lung simulator, and exercise thorax or breathing bag to the
Y-piece.
2. Set fresh gas.
3. Press “IPPV”.
4. Press rotary control.
5. Piston movement is indicated on bar graph. The machine starts with
ventilation parameters set upon delivery.
6. Lung simulator inflates regularly.
7. Pressure profile is displayed on the monitor.
8. Volume measurement yields plausible values.
- Machine and machine parts must be cleaned and disinfected before starting any
maintenance work and before returning to the manufacturer for repair.
- Replace cooling air filter for monitor.
- Replace water separator.
XXI) Operating Elements and Displays on Ventilator:
Figure 11 illustrates the operating elements and displays on the Dräger Cato
ventilator.
Figure 4 - Operating Elements and Displays on Dräger Cato Ventilator
1. Pressure limiting valve (APL) with “Manual/Spontaneous” change over
2. MAN/SPONT key for manual ventilation / spontaneous breathing
3. Bar graph of relative piston movement [%]
4. Indication of set maximum pressure (P
5. Indication of set tidal volume (V
)
T
6. Indication of ventilation frequency (f
max
IPPV
)
)
7. Window for interactive settings and selection
8. Rotary control for settings and selection (confirmation)
9. Key for leakage test and compliance measurement
10. Key for mechanically-aided spontaneous breathing (SIMV)
11. Key for invoking “Standby” mode
12. Key for automatic ventilation (IPPV)
13. Key for setting maximum pressure (P
14. Key for setting tidal volume (V
15. Key for setting ventilation frequency (f
16. Key for setting ratio of inspiration time to expiration time (T
17. Key for setting ratio of inspiration pause time to inspiration time (T
18. Key for setting positive end-expiratory pressure (PEEP). Only possible with
IPPV.
19. Key for setting frequency for mechanically-aided spontaneous breathing (f
XXII) Descriptions:
A) Ventilation with Automatic Adjustment of Breathing to Match Fresh Gas Flow
(review schematic diagram given in section XVII):
Most of the breathing systems used for anesthesia today are based on the rebreathing principle. Part of the expired gas is redelivered to the patient after
absorbing CO
excess anesthetic is scavenged; the amount of scavenged anesthetic gas essentially
depends on the set fresh gas flow. The administration of anesthetic gas with
reduced fresh gas flow (low flow technique) yields a number of major advantages:
lower consumption of anesthetic gases and agents, more effective humidification
and heating of the inspiratory gas, lower environmental burden and good manual
ventilation properties. The design of the breathing system is an aspect of essential
importance for low flow anesthesia. The high degree of fresh gas utilization is a
major prerequisite. Systems suitable for low flow techniques should be designed
so that it is impossible, firstly, for too much expiration gas to disappear in the
anesthetic scavenging line without building up a constant pressure and, secondly,
for fresh gas to escape without first having been administered to the patient. In
closed anesthetic systems, anesthetic gas cannot escape from the breathing system
and no more fresh gas is delivered than is actually required by the patient.
However, closed systems must also meet additional requirements: the breathing
system must be absolutely tight and must feature additional monitoring and
control elements. The breathing system implemented in the Cato automatically
matches its degree of openness to the fresh gas flow. During inspiration, breathing
gas streams from the piston pump to the patient. Valve V2 of the excess gas outlet
and the fresh gas shut-off valve V1 are closed. Expiration is initiated when the
fresh gas shut-off valve V1 is opened. Expired gas from the patient’s lungs
streams into the breathing bag which serves as a reservoir and also into the
retracting piston pump. The excess gas outlet valve V2 is closed. Unlike the case
with conventional semi-closed breathing system, the valve opening time for
)
max
)
T
)
IPPV
)
I:TE
)
IP:TI
IMV
and enriching with anesthetic gases and anesthetic agent. The
2
)
discharging excess gas is controlled as required. The system remains open longer
for anesthesia with high fresh gas flow. If the fresh gas flow is inadequate in
closed anesthesia systems, the pressure measuring function will detect that the
patient’s expiratory pressure has dropped below approximately -3 mbar. This
shortage of fresh gas is signaled by the Cato and the piston pump stops in order to
avoid a negative pressure in the patient.
B) Automatic Leakage Test IPPV:
This leakage test identifies any leaks of relevance for automatic ventilation in
subsystems 1 and 2 of the Cato ventilation system. It also encompasses the
breathing hoses up to the Y-piece, as well as the measured gas sampling and
return lines if installed. The overall system compliance is determined at the same
time. The IPPV leakage test and the leakage test started in “Standby” mode are
carried out by building up a constant pressure of 30 mbar. The piston movement
necessary to compensate the gas escaping through the leaks is measured,
calculated and indicated as a volume per unit time. The effective leakage over the
complete ventilation cycle is lower than the value indicated, since the effective
mean pressure P
in IPPV ventilation mode is considerably lower than the test
mean
pressure. The relationship depends on the rate of the pressure increase, the plateau
time and the ratio TI:TE. The effective leakage value varies with the value
measured in the leakage test as P
mean:Ptest
. The effective leakage is defined as:
Effective Leakage = Test Leakage x P
mean/Ptest
C) Automatic MAN Leakage Test:
This leakage test is also part of the self-test and locates any leaks of relevance for
normal ventilation in subsystem 3 (review Figure 9). The breathing bag, fresh gas
hose, vapor and internal connections up to the bank of measuring tubes are tested
for leaks. The test is normally carried out at a pressure of 30 mbar. If the leakage
value remains below 300 mL/min, this is not indicated and the self-test continues.
This subsystem contributes only marginally to the overall leakage, since the mean
pressure is normally below 5 mbar.
D) Automatic Compliance Test:
The stroke volume applied by a ventilator not only ventilates the patient’s lungs,
but also the hose system connecting the patient to the ventilator. This means that
only part of the stroke volume is effectively used to ventilate the lungs, the rest
remaining in the compressible hose volume. This compressible volume must be
known for ventilation to be effective (P = VT/C). The compliance of the breathing
system (breathing gas block, soda lime container, hoses, etc…) is determined
during the leakage test and saved by the Cato system. This calculated compliance
value is used to calculate the volume stored in the breathing system and hoses for
each ventilation pressure. In order to correct it, the Cato starts with the set tidal
volume and reaches the correct volume after 3 – 6 breaths. The corrected volume
is constantly verified automatically. The measured value must be limited to
plausible ranges for safety reasons. The limit is set at 3.9 mL/mbar when using
adult hoses (V
(V
< 200 mL). The maximum length of the breathing hoses should therefore not
T
> 200 mL) and at 0.8 mL/mbar when using infant hoses
T
be exceeded.
E) Fresh Gas Flow:
- The patient’s uptake of gas depends on the anesthesia and primarily comprises
the consumption of O2 and N2O. The consumption of O2 can be calculated
approximately using Brody’s equation:
BW = Body Weight in kg
O2 flow = 10 x BW
0.75
in mL/min
This corresponds to an O2 uptake of roughly 3.5 mL/min per kg body weight.
Higher O2 consumption results in lower inspiratory O2 concentration due to
re-breathing when low fresh gas flow is set.
- The uptake of N
O varies with time and can be approximated by following the
2
rule of thumb: 1/√t (approximately steady-state value N2O per kg body
weight: 1.5 mL/min)
- The leakage from ventilation system depends on airway pressure (mean value)
and can be determined with the aid of automatic leakage test.
- The measuring gas sampled to measure the CO
, N2O, and anesthetic agent
2
can be set on the system monitor (60 or 200 mL/min).
- The gas is returned to the breathing circulation via the measured gas return
line.
F) Relationship Between Fresh Gas Flow and Gas Concentration in the Breathing
System:
The inspiratory concentration differs from the set fresh gas concentration due to
re-breathing and the O
, N2O, and anesthetic agent uptake by the patient. The
2
lower the fresh gas, the larger the concentration gradient between the fresh gas,
inspiratory, and expiratory gas concentration becomes. Since the concentration of
the fresh gas flow in this flow range bears a little resemblance to the concentration
at the patient, it is important to measure the anesthetic agent concentration as
close to the inspiration tube as possible in this mode. The measuring system is
integrated into the system monitor.
G) Relationship Between Fresh Gas Flow and Time Constants in the Breathing
System:
The response time following a change of concentration of O
, N2O or anesthetic
2
agent in the fresh gas depends on the set fresh gas flow. The inspiratory
concentration in the breathing system corresponds more and more accurately with
the fresh gas concentration as the fresh gas flow increases. At a low fresh gas
flow, a change of concentration takes effect in the breathing system very slowly.
This process can be speeded up by increasing the fresh gas flow abruptly.
The rule of thumb for estimating the system response overtime:
T = VC/ FG
where T: time constant of system (min)
VC: system volume (L): i.e. breathing system ventilation hoses, residual
volume of lungs
FG: fresh gas flow (L/min)
H) O
Measurement:
2
Measuring Principle of Galvanic Cell:
The O2 sensor is based on the principle of galvanic cell. The oxygen molecules
from the gas mixture to be measured diffuse through a plastic membrane into the
electromechanical cell and are reduced on precious metal electrodes. Figure 12
illustrates a diagram of the galvanic cell construction.
Figure 5 - Galvanic Cell Module of O
Sensor in Dräger Cato Ventilator
2
A base electrode is oxidized at the same time. It is depleted by the oxidation
process and essentially determines the service life of the sensor. The current
flowing through the cell is proportional to the partial oxygen pressure in the gas
mixture to be measured. At constant pressure and constant temperature of the gas
mixture to be measured, then measured value is directly proportional to the partial
oxygen pressure.
I) Flow Measurement:
- Measurement Principle and Signal Processing:
The sensor is based on the principle of a constant – temperature hot-wire
aerometer. The breathing gas flows round a very thin, electrically heated
platinum wire in a measuring tube. The wire is heated to a constant
temperature of 180º C which is controlled by a control circuit. Heat is
dissipated when gas flows past this wire. The larger the volume of gas flowing
per unit time, the more heat will be dissipated. The heating current required to
maintain a constant wire temperature is taken as an indicator for the gas
stream.
- Gas Type Compensation:
The effect of the various types of gas contained in the breathing gas is
compensated by a second heated platinum wire. The heat dissipated by the
second wire in the stationary gas column in the measuring tube is determined
during a period in which there is no gas flow (i.e. during inspiration when the
sensor is positioned on the expiration side). The gas compensation is
determined on the basis of the specifically different thermal conductivity of
the types of gases present in the breathing gas. Linearization is performed
with the aid of internal calibration tables for the gas mixtures O2/N2O, air and
100% O
.
2
J) Measurement of CO
and Anesthetic Agent:
2
- Measuring Principle:
CO2 and anesthetic agent absorb infrared light. A pump contains a small
amount of breathing gas through a measuring cuvette irradiated with
infrared light. Different filters make it possible to select a frequency band
in which one of the gases to be identified is absorbed. All the gases can be
measured quasi continuously by changing filters rapidly. The absorption
reflects the gas concentration in the cuvette. The gas concentrations in the
breathing gas can be calculated by simultaneously measuring the
temperature and absolute temperature in the cuvette.
- Cross-sensitivity of the Anesthetic Gas Measurement:
The measurement of the anesthetic agent can be falsified by vapors of
organic substances (such as those contained in cleaning agents or
disinfectants) in the air round about, the sampling hose or the T-piece.
Elevated values for anesthetic agent will be displayed, particularly when
using halothane, if the patient’s breathing air contains alcohol. Mixtures of
different anesthetic vapors may considerably impair the accuracy with
which the concentration is measured.
K) SpO
Measurement:
2
Measuring Principle:
The light absorption properties of oxygenated arterial blood (oxyhemoglobin
HbO2) differ from those of unsaturated venous blood (reduced hemoglobin Hb).
O2 saturation is a logarithmic function of the irradiated light intensity (LambertBeer’s law).
The effect of such dysheomoglobins as carbon monoxide hemoglobin HbCO and
methemoglobin MetHb is normally negligible. The sensor comprises from two
LEDs which alternately emit infrared and red light at typical wavelengths of
920 nm and 660 nm respectively. The radiation intensity is measured by a
photodetector opposite the diodes. The sensor is positioned on a limb in which the
arterial blood vessels can be irradiated, such as a finger, toe or nose (see Figure 13
for further illustration).
Figure 6 - Diagram of Operation for SpO2 Sensor
These two wavelengths – 920 and 660 nm – are used because meaningful
absorption values are still obtained for oxygenated and reduced blood, even in the
presence of slight perfusion, and because they differ significantly. The light
alternately emitted by the diodes is completely absorbed by the pulsating arterial
blood, skin, finger, nails, muscular tissue, bones and venous blood. Except for the
pulsating arterial blood, the amount of light absorbed by the other components
remains constant as regards the quantity and optical density over a defined unit of
time. The arterial blood pulsating with every beat of the heart, however, produces
a change of volume synchronous with the pulse in the irradiated tissue. In other
words, absorption of the irradiated light also changes in time with the pulse.
Figure 14 shows a graphic scheme of how the absorption levels differ for different
body part with time.
Figure 7 - Absorption Levels of Light for Various Body Parts
during the Pulsed Oxidation Sessions
The light absorbed when there is no pulsating blood (during the diastole) is
determined first. This yields the amount of light absorbed by tissue and nonpulsating blood. The absorption value does not normally change during the pulse
part and provides a reference value for the pulsating part of absorption. Following
the next beat of the heart, the absorption of light is measured again when the
pulsating blood enters the tissue. The absorption of light changes for both
wavelengths due to the pulsating arterial blood. At 660 nm, the absorption and
corresponding pulse amplitude decrease with increasing the O
saturation, but rise
2
at 920 nm. Since the absorption coefficients of HbO2 and Hb are known for both
wavelengths, the system can calculate how much of these two hemoglobin levels
are present. The quotient obtained by dividing the oxygenated hemoglobin
(HbO
) by the reduced and oxygenated hemoglobin (Hb + HbO2) is known as the
2
functional saturation:
% SpO2 (func) = 100 x HbO2/(HbO2 + Hb)
and refers to the hemoglobin capable of transporting oxygen. Dyshemoglobins,
HbCO2 and MetHb are normally negligible, but may affect the accuracy of the
measurement.
L) Temperature Measurement:
Measuring Principle:
The measurement principle is based on temperature – dependent change in the
resistance of an NTC2 thermistor with linearization circuit.
M) Pressure Measurement:
- Pressure Measurement:
The pressure measurement is dependent on the piezoresistive change of
resistance in a membrane.
- Determination of PEEP and Plateau Circuit:
PEEP is the airway pressure at the end of expiration. The plateau pressure is
the airway pressure measured 16 ms before expiration begins.
N) Definitions for “Low Flow” and “Minimal Flow” Anesthesia:
Low flow anesthesia is performed with a fresh gas flow considerably below the
minute ventilation. When starting such low fresh gas volumes, the anesthetic
gases must be returned to the patient via a semi-closed or closed re-breathing
system. The re-breathing volume increases when the fresh gas flow is reduced and
the excess gas volume decreases correspondingly. Although the fresh gas flow
can be infinitely reduced to the gas volume taken up by the patient at a given
moment of anesthesia in a completely hermetic system, a distinction is
nevertheless made between the following methods:
The fresh gas flow is reduced to 1 L/min for low gas flow anesthesia and to
0.5 L/min for minimal flow anesthesia. In case of non-quantitative anesthesia in a
closed system, the gas delivery settings are corrected frequently to adjust the fresh
gas volume in line with the volume of gas taken up by the patient so that the
internal pressure and charge of the breathing system do not decrease and the
ventilation pattern remains unchanged. In case of quantitative anesthesia in a
closed system, the composition of the fresh gas corresponds exactly to the
volumes of oxygen, nitrous oxide and inhalation anesthetic taken up by the patient
at a given moment in anesthetic gas also remains constant, in addition to the gas
charge in the system and the ventilation pattern.
2
NTC: Negative Temperature Coefficient
O) Synchronous Intermittent Mandatory Ventilation or SIMV:
The mixture of mechanical ventilation and spontaneous breathing is known as
synchronous intermittent mandatory ventilation (SIMV). In SIMV mode, the
patient can breathe spontaneously at specified regular intervals. Between these
intervals, mandatory (i.e. automatically delivered) ventilation strokes ensure a
minimum degree of ventilation. The mandatory ventilation strokes are the same as
those for IPPV ventilation. They are defined by the parameters VT, IPPV
frequency fIPPV, T
and TIP. Figure 15 shows a graph of the airway pressure
I:TE
over time.
Figure 8 - Graph of Airway Pressure Over Time for Dräger Cato Ventilator
Each mandatory breath is followed by a pause in which the patient can breathe
spontaneously. In order to prevent the next mandatory breath being applied during
the expiratory phase of spontaneous breathing, a trigger function ensures that the
mandatory ventilation stroke is synchronized with the inspiratory spontaneous
breathing phase during an expectation period. The time between the end of each
mandatory ventilation stroke and the beginning of the next is subdivided into a
spontaneous breathing time T
and a trigger time T
SPONT
Trigger
.
During the trigger time, the system checks whether the airway pressure drops at
least 1 mbar below the pressure measured at the end of the expiration phase. The
mandatorily applied minute volume may increase if an automatic ventilation
stroke is applied at the beginning of each trigger period. The duration of a
mandatory stroke plus the spontaneous breathing time is calculated as follows:
1/f
IPPV
+ T
SPONT
This corresponds to a frequency of approximately 6 per minute and the applied
minute volume increases to 6 per minute * VT.
P) ORC Low Flow (with N
O Bypass):
2
The delivery of N
reciprocal relationship between N
O is controlled as a function of the O2 flow on a count of the
2
O and O2 flow, thus ensuring that the minimum
2
O2 fresh gas concentration cannot drop below 25% by volume. ORC is not
effective at an O2 flow of less than 0.5 L/min. A separate N2O delivery can be
used for low flow applications at such a low O2 flow. ORC low flow can be
bypassed to permit operation in the low flow range. When ORC low flow blocks
the flow of N2O at an O2 flow of less than 0.5 L/min, between 0.5 and 0.8 L/min
still flow through the bypass to the measuring tube and can be delivered there.
This means that the O2/N2O fresh gas delivery must be individually adjusted by
hand in the low flow range. The proportional action control of the ORC low flow
becomes effective when the O2 flow increases above 0.5 L/min. The O2 and N2O
flow may have to be adjusted several times on account of pressure fluctuations.
Regardless of whether or not ORC low flow is effective, care must be always
taken to ensure that sufficient O2 is delivered when the delivery fresh gas is in the
low flow range. If the O2 supply pressure, for instance, drops below 3 bar
unnoticed, the O2 flow can be reduced to such an extent that the O2 concentration
of the fresh gas declines to less than 21% by volume. This is particularly possible
if a low O
flow – less than 0.5 L/min – was set. The O2 shortage is signaled until
2
the pressure drops to between 2 and 1.8 bar. ORC low flow cannot detect O2 as a
type of gas and does not offer any protection if gases have inadvertently been
confused. For both these reasons, therefore: ensure that the O2 concentration is
always monitored.
Q) Oxygen Ratio Control S-ORC:
Figure 16 illustrates the diagram of a sensitive oxygen ratio control subsystem.
Figure 9 - Sensitive Oxygen Ratio Control (S-ORC) Subsystem on Dräger Cato
Cato is equipped with a sensitive oxygen ratio control (S-ORC) function in order
to prevent the concentration of O2 in fresh gas dropping too low when the O2
supply is inadvertently deactivated or defective or if the O2 flow has been set too
low. The O2 and N2O flows which are set on the delivery valves build up control
pressures for the control diaphragms of the S-ORC in 2 flow restrictors. The N2O
flow is controlled by the pressure ratio on the control diaphragms: the N
limited when O
flow is set. N2O is disabled when the O2 flow drops below
2
O flow is
2
approximately 200 mL/min; it is slowly re-enabled as the value rises. The
proportion of N2O contained in the fresh gas can be set when the O2 flow reaches
approximately 300 mL/min again, the S-ORC ensuring that the O2 concentration
does not fall below 21% by volume. Since the O2 and N2O flow set on the flow
tubes is limited by the S-ORC (but at least 9 L/min for each type of gas), the
minimum O
concentration increases above approximately 3 L/min until both
2
delivery valves are completely open and the mixing ratio of O
approximately 50:50. The principle is illustrated in the diagram of Figure 16. The
curve is displayed towards lower O2 concentrations when anesthetic vapors (e.g.
up to 18% by volume in the case of desflurane) are administered. O2
concentration must be monitored via the airway monitor since the S-ORC is not
specifically for monitoring O2 and does not offer any protection if gases are
inadvertently confused.
R) Abbreviations:
APL: Adjustable Pressure Limitation
ET CO2: End-expiratory CO2 Concentration
IPPV: Intermittent Positive Pressure Ventilation: Automatic Ventilation Mode
Paw: Airway Pressure
: Inspiratory and expiratory flow
E: Minute Ventilation
FG: Fresh gas flow
VT: Tidal or Stroke Volume
2:N2
O is
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