GAS ANALYZERS 205B

BLOOD GAS ANALYZERS KEY COMPONENTS Blood Gas Analyzers consist of a 3 electrode

system pH Electrode PCO2 Electrode PO2 Electrode

Calibrating gas tanks Reagent containers containing buffers used for

calibration and rinse solutions Waste containers Results display, storage and transmittal systems

THE PH ELECTRODE AND THE POTENTIOMETRIC METHOD

Consists of 2 half cell electrodes. Measuring electrode Reference electrode

Measuring half cell contains a silver-silverchloride 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 The Reference electrode creates a known voltage The pH sensitive glass in the measuring electrode comes

into contact with the blood. H ions in the blood diffuse into the measuring electrode

thru the glass 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”

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THE PO2 ELECTRODE AND THE POLAROGRAPHIC METHOD The most common oxygen electrode used in

blood gas analysis is the Clark Electrode.

THE PO2 ELECTRODE AND THE POLAROGRAPHIC METHOD

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

PO2 (mmHg)

Amps

Polarogram

THE SEVERINGHAUS PCO2 ELECTRODE

Modified version of the pH 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

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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

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OXIMETRY Oximetry first described in 1932 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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PULSE OXIMETRY A convenient, portable, continuous and non-invasive method of

determining SpO2

The pulse oximeter uses light absorption patterns to indicate saturation levels of the “pulsed” blood which is arterial blood.

Is a Trending/Monitoring device, Not a Diagnostic Tool.

Is adversely affected by: High ambient light Painted/false/long fingernails Movement Decreased local or systemic perfusion Can be adversely affected by Hb variants: HbCO, Methemoglobin Can be affected by vascular dyes (Methylene Blue, Indocyainie Green, Indigo

Carmine) Needs to be correlated with the HR or HR plethysmography and patient

clinical appearance Does not measure CaO2 or PCO2; patients suspected of having O2 transport

issues or hypoventilation should have an ABG

CO-OXIMETRY Measures:

SaO2%, MetHb, HbCO% SaO2 is measured as a percentage of the

Oxyhemoglobin compared with all measured forms of Hb including dyshemoglobin species (functional Hb)

Potential measurement errors occur in neonates with substantial quantities of fetal hemoglobin - May show increased levels of HbCO, decreased SaO2

Usually run in tandem with arterial ABGs

TRANSCUTANEOUS OXYGEN AND CO2 MONITORING 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 Indications:

Continuous monitoring of adequacy of oxygenation/ventilation

Need for real time assessment of therapeutic interventions

Contraindications: Patients with poor skin integrity and adhesive

allergies Precautions:

False-negative or false-positive results may lead to inappropriate treatment

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 Factors affecting accuracy:

Patient age: agreement between sensed gas and actual PaO2 or PaCO2 decreases with age. The best correlation occurs only in neonates.

Poor perfusion either localized or systemic.

Calibration must be done prior to application.

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

2 TECHNIQUES FOR MONITORING PETCO2

Two methods in obtaining a gas sample for analysisMainstreamSidestream

Mainstream (Flow-through or In-line)Adapter placed in the breathing circuitNo gas is removed from the airwayAdds bulk to the breathing circuitElectronics 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 gasesCan be used in non-intubated patientsPotential for disconnect or leaks giving false

readingsWithdraws 50 to 500ml/min of gas from

breathing circuit (most common is 150-200ml/min)

Water vapor from circuit condenses on its way to monitor A water trap is usually interposed between the

sample line and analyzer to protect optical equipment

LOCATION 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

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SidestreamMainstream

HOW ETCO2 WORKS Photo detector measures the amount

of infrared light absorbed by airway gas during inspiration and expirationCO2 molecules absorb specific

wavelengths of infrared light energyLight absorption increases directly with

CO2 concentration A monitor converts this data to a CO2

value and a corresponding waveform (capnogram)

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CAPNOMETRY (CONT.) The normal capnogram shows an PCO2 of zero at the

start of the expiratory breath.

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 B-C Mixed airway/alveolar gasC-D Alveolar gas D-E Inspiration

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COLORIMETRIC CO2 ANALYZERS Used to detect CO2 in exhaled gases Simple, inexpensive, inline detector especially useful for detection

of successful intubations 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