Transcript Gas Analyzers 205b - Respiratory Therapy Files

GAS ANALYZERS
205B
BLOOD GAS ANALYZERS KEY
COMPONENTS
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Blood Gas Analyzers consist of a 3 electrode system
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pH Electrode
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PCO2 Electrode
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PO2 Electrode
Calibrating gas tanks
 Reagent containers containing buffers used for
calibration and rinse solutions
 Waste containers
 Results display, storage and transmittal systems
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THE PH ELECTRODE AND THE
POTENTIOMETRIC METHOD
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Consists of 2 half cell
electrodes.
Measuring electrode
 Reference electrode
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Measuring half cell contains a silver-silver
chloride rod surrounded by a solution of
fluid with a constant ph of 6.840 and is
capped by a pH sensitive glass membrane
The reference electrode contains a
mercury/mercurous chloride rod
Surrounded by a solution of potassium
Chloride which creates a small electric voltage
PH ELECTRODE
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The Reference electrode creates a known voltage
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The pH sensitive glass in the measuring electrode
comes into contact with the blood.
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H+ ions in the blood diffuse into the measuring
electrode thru the glass
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+
The difference in H ions on either side of the glass
changes the potential charge within the measuring
electrode
This change in voltage is compared with the reference
electrode and converted into a pH reading.
The potential difference in current between the 2
electrodes creates the pH reading thus the name
“Potentiometric Method”
THE PO2 ELECTRODE AND THE
POLAROGRAPHIC METHOD
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The most common oxygen electrode used in blood
gas analysis is the Clark Electrode.
THE PO2 ELECTRODE AND THE
POLAROGRAPHIC METHOD
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Blood is separated from the electrode terminals by the use
of an O2 permeable membrane
Oxygen diffuses easily thru this membrane into the
electrolyte solution
The Cathode attracts oxygen molecules where they react
with the H2O in the electrolyte solution
The chemical reaction at the cathode consumes 4 O2
electrons which are rapidly replaced as the silver and
chloride react at the Anode. The more electrons consumed,
the greater the electron flow between the poles.
The current generated will be in direct proportion to the
amount of dissolved oxygen present at the cathode
A Polarogram graph shows the direct relationship between
the PO2 and the voltage at the cathode
Polarogram
A
m
p
s
PO2 (mmHg)
THE SEVERINGHAUS PCO2 ELECTRODE
•Differences:
1. Blood does not come into contact
with the pH sensitive glass.
2. Blood comes into contact with a CO2
permeable membrane
3. On the other side of the membrane is
bicarbonate solution that is in direct
contact with the pH-sensitive glass
4. A hydrolysis reaction occurs within the
bicarbonate solution as CO2 diffuses in.
5. This reaction results in the
+
production of H Ions and a pH change
of the bicarbonate solution.
6. The pH change is in direct proportion
to the PCO2, thus the corresponding
voltage change can be converted into
PCO2 units
Modified version of the pH electrode
OXYGEN ANALYZERS
Used to analyze the FiO2 of inspired
gas
There are 2 common types:
Clark Electrode (Polarographic)
Galvanic Fuel Cell
Galvanic fuel cell analyzer
Clark Electrodes: Function is similar to ABG machines
Galvanic fuel cells use a gold anode and a lead cathode. Current is
generated by the chemical reaction of potassium hydroxide and oxygen.
The greater the oxygen, the more reaction with the potassium, the more
current generated which is converted to %O2.
Once the potassium is consumed, the fuel cell must be replaced. The
fuel cell is covered when not in use, and placed proximal to any
humidification device.
Oxygen analyzers must be calibrated using R/A and
100% O2
OXIMETRY
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Oximetry is the measurement of hemoglobin saturation
using spectrophotometry.
Oximetry works because every substance emits its own
unique pattern of light (absorption/emission).
Each form of hemoglobin (e.g., HbO2, HbCO) has its own
pattern of light absorption.
For example, HbO2 absorbs less red light and more
infrared light.
An oximeter is an instrument that measures the amount of
light transmitted through, or reflected from a sample of
blood at two or more specific wavelengths.
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Oximetry first described in 1932
PULSE OXIMETRY
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The pulse oximeter uses light absorption patterns to indicate saturation
levels of the “pulsed” blood which is arterial blood.
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Is a Trending/Monitoring device, Not a Diagnostic Tool.
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Is adversely affected by:
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High ambient light
Painted/false/long fingernails
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Movement
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Decreased local or systemic perfusion
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Can be adversely affected by Hb variants: HbCO, Methemoglobin
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Can be affected by vascular dyes (Methylene Blue, Indocyainie Green, Indigo
Carmine)
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Needs to be correlated with the HR or HR plethysmography and patient clinical
appearance
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Does not measure CaO2 or PCO2; patients suspected of having O2 transport
issues or hypoventilation should have an ABG
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A convenient, portable, continuous and non-invasive method of
determining SpO2
CO-OXIMETRY
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Measures:
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SaO2%, MetHb, HbCO%
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SaO2 is measured as a percentage of the Oxyhemoglobin
compared with all measured forms of Hb including
dyshemoglobin species (functional Hb)
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Potential measurement errors occur in neonates with
substantial quantities of fetal hemoglobin - May show
increased levels of HbCO, decreased SaO2
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Usually run in tandem with arterial ABGs
TRANSCUTANEOUS
OXYGEN AND
CO2 MONITORING
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Provides continuous and non invasive estimates of
arterial PO2 and PCO2 through a surface skin
sensor.
Expressed as PtcO2 and PtcCO2
Devices heat the skin to help vascularize the tissue
increasing the permeability of O2 and CO2 from the
capillary bed
TRANSCUTANEOUS
OXYGEN AND
CO2 MONITORING
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Indications:
Continuous monitoring of adequacy of
oxygenation/ventilation
 Need for real time assessment of therapeutic
interventions
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Contraindications:
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Patients with poor skin integrity and adhesive allergies
Precautions:
False-negative or false-positive results may lead to
inappropriate treatment
O
 Tissue injury (burns/tearing) may occur at the sensor site
because sensor heats to 43.5 C
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TRANSCUTANEOUS
OXYGEN AND
CO2 MONITORING
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Factors affecting accuracy:
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Patient age: agreement between sensed gas and
actual PaO2 or PaCO2 decreases with age. The best
correlation occurs only in neonates.
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Poor perfusion either localized or systemic.
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Calibration must be done prior to application.
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Response time: response time of the electrode varies
due to skin thickness, temperature and patient age
CAPNOMETRY
Term capnometry comes from the Greek word
KAPNOS, meaning smoke.
 Measures end tidal CO2: The maximum partial
pressure of CO2 exhaled during a tidal breath
just before the beginning of inspiration;
expressed as PetCO2
 Respiratory context: inspired and expired gases
sampled at the Y connector, mask or nasal
cannula.
 Gives insight into alterations in ventilation,
cardiac output, distribution of pulmonary blood
flow and metabolic activity.
 Capnography is the technique of displaying CO2
measurements as waveforms (capnograms)
throughout the respiratory cycle
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2 TECHNIQUES FOR MONITORING
PETCO2
 Two
methods in obtaining a gas sample
for analysis
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Mainstream
Sidestream
 Mainstream
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(Flow-through or In-line)
Adapter placed in the breathing circuit
No gas is removed from the airway
Adds bulk to the breathing circuit
Electronics are vulnerable to mechanical
damage
2 TECHNIQUES FOR MONITORING
PETCO2
 Sidestream (aspiration)
Gas is aspirated from an airway sampling site
and transported through a tube to a remote
CO2 analyzer
 Provides ability to analyze multiple gases
 Can be used in non-intubated patients
 Potential for disconnect or leaks giving false
readings
 Withdraws 50 to 500ml/min of gas from
breathing circuit (most common is 150200ml/min)
 Water vapor from circuit condenses on its way
to monitor
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A water trap is usually interposed between the
sample line and analyzer to protect optical equipment
LOCATION
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OF
SENSOR
The location of the CO2 sensor greatly affects the
measurement
Measurement made further from the alveolus can
become mixed with fresh gas causing a dilution of
CO2 values and rounding of the capnogram
Mainstream
Sidestream
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HOW ETCO2 WORKS
 Photo
detector measures the amount of
infrared light absorbed by airway gas
during inspiration and expiration
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CO2 molecules absorb specific wavelengths of
infrared light energy
Light absorption increases directly with CO2
concentration
monitor converts this data to a CO2
value and a corresponding waveform
(capnogram)
CAPNOMETRY (CONT.)
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Soon afterward, the PCO2 level rises sharply and plateaus
as alveolar gas is exhaled.
The end-tidal PCO2 (PETCO2) can be used to estimate
deadspace ventilation and normally averages 1 to 5 mm Hg
less than PaCO2
A-B Deadspace
C-D Alveolar gas
B-C Mixed airway/alveolar gas
D-E Inspiration
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The normal capnogram shows an PCO2 of zero at the start
of the expiratory breath.
COLORIMETRIC CO2 ANALYZERS
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Simple, inexpensive, inline detector especially useful for
detection of successful intubations
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Color is purple when CO2 is less than 0.5%
Color is tan when CO2 is Up to 2%
Color is yellow when CO2 exceeds 2%
If patient has no perfusion, ET could be in airway and color will still
be purple
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Used to detect CO2 in exhaled gases