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G AS A NALYZERS 205 B. B LOOD G AS A NALYZERS K EY C OMPONENTS Blood Gas Analyzers consist of a 3 electrode system pH Electrode PCO 2 Electrode PO 2 Electrode.

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Presentation on theme: "G AS A NALYZERS 205 B. B LOOD G AS A NALYZERS K EY C OMPONENTS Blood Gas Analyzers consist of a 3 electrode system pH Electrode PCO 2 Electrode PO 2 Electrode."— Presentation transcript:


2 B LOOD G AS A NALYZERS K EY C OMPONENTS Blood Gas Analyzers consist of a 3 electrode system pH Electrode PCO 2 Electrode PO 2 Electrode Calibrating gas tanks Reagent containers containing buffers used for calibration and rinse solutions Waste containers Results display, storage and transmittal systems

3 T HE PH E LECTRODE AND THE P OTENTIOMETRIC M ETHOD Consists of 2 half cell electrodes. Measuring electrode Reference electrode 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

4 PH E LECTRODE 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” + +

5 T HE PO 2 E LECTRODE AND THE P OLAROGRAPHIC METHOD The most common oxygen electrode used in blood gas analysis is the Clark Electrode.

6 T HE PO 2 E LECTRODE AND THE P OLAROGRAPHIC METHOD Blood is separated from the electrode terminals by the use of an O 2 permeable membrane Oxygen diffuses easily thru this membrane into the electrolyte solution The Cathode attracts oxygen molecules where they react with the H 2 O in the electrolyte solution The chemical reaction at the cathode consumes 4 O 2 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) AmpsAmps Polarogram

7 T HE S EVERINGHAUS PCO 2 E LECTRODE 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 CO 2 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 CO 2 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 PCO 2, thus the corresponding voltage change can be converted into PCO 2 units +

8 O XYGEN A NALYZERS Used to analyze the FiO 2 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 %O 2. 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% O 2

9 9 O XIMETRY 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., HbO 2, HbCO) has its own pattern of light absorption. For example, HbO 2 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.

10 10 P ULSE O XIMETRY A convenient, portable, continuous and non-invasive method of determining SpO 2 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 CaO 2 or PCO 2 ; patients suspected of having O 2 transport issues or hypoventilation should have an ABG

11 CO-O XIMETRY Measures: SaO 2 %, MetHb, HbCO% SaO 2 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 SaO 2 Usually run in tandem with arterial ABGs

12 T RANSCUTANEOUS OXYGEN AND CO 2 MONITORING Provides continuous and non invasive estimates of arterial PO 2 and PCO 2 through a surface skin sensor. Expressed as P tc O 2 and P tc CO 2 Devices heat the skin to help vascularize the tissue increasing the permeability of O 2 and CO 2 from the capillary bed

13 T RANSCUTANEOUS OXYGEN AND CO 2 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 O

14 T RANSCUTANEOUS OXYGEN AND CO 2 MONITORING Factors affecting accuracy: Patient age: agreement between sensed gas and actual PaO 2 or PaCO 2 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

15 C APNOMETRY Term capnometry comes from the Greek word KAPNOS, meaning smoke. Measures end tidal CO 2: The maximum partial pressure of CO2 exhaled during a tidal breath just before the beginning of inspiration; expressed as PetCO 2 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 CO 2 measurements as waveforms (capnograms) throughout the respiratory cycle

16 2 T ECHNIQUES FOR M ONITORING PETCO2 Two methods in obtaining a gas sample for analysis Mainstream Sidestream Mainstream (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

17 2 T ECHNIQUES FOR M ONITORING P ETCO2 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 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

18 L OCATION OF S ENSOR The location of the CO 2 sensor greatly affects the measurement Measurement made further from the alveolus can become mixed with fresh gas causing a dilution of CO 2 values and rounding of the capnogram

19 19 SidestreamMainstream

20 H OW ETCO 2 W ORKS Photo detector measures the amount of infrared light absorbed by airway gas during inspiration and expiration CO2 molecules absorb specific wavelengths of infrared light energy Light absorption increases directly with CO 2 concentration A monitor converts this data to a CO 2 value and a corresponding waveform (capnogram)

21 21 C APNOMETRY ( CONT.) The normal capnogram shows an P CO 2 of zero at the start of the expiratory breath. Soon afterward, the P CO 2 level rises sharply and plateaus as alveolar gas is exhaled. The end-tidal P CO 2 (P ETCO 2 ) can be used to estimate deadspace ventilation and normally averages 1 to 5 mm Hg less than Pa CO 2 A-B Deadspace B-C Mixed airway/alveolar gas C-D Alveolar gas D-E Inspiration

22 22 C OLORIMETRIC C O 2 A NALYZERS 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

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