PULSE OXIMETRY : PULSE OXIMETRY PRESENTED BY
P.G.STUDENT [OMFS] History : History 1935 Carl Matthes – 1st to build the device
Development of LED’s, photodetectors and microprocessors refined the technique of pulse oximetry.
1980 William New – Modern pulse oximeter.
1987- standard care for GA included pulse oximeter.
2008- HRPO is used for screening and testing for sleep apnea at home. INTRODUCTION : INTRODUCTION Pulse oximetry is an optical measurement of oxygen saturation in tissues
Continuos monitoring of O2 saturation
Measures cardiorespiratory function
Quickly detects hypoxaemia Slide 4: Modern Pulse oximetry was born with realization that pulsatile changes in light transmission through living tissue are due to alteration of the arterial blood volume in the tissue.
Measurement of the pulsatile component would eliminate the variable absorption of light by bone, tissue,skin, pigment,etc from analysis. PULSE OXIMETRY : PULSE OXIMETRY Uses spectrophotometry based on the Beer-Lambert law
Differentiates oxy from deoxy Hb by the differences in absorption of light at 660 nm and 940 nm. At 660nm deoxyhb absorbs 10times the light compared to oxyhb. At 940nm oxyhb absorbs more light than deoxyhb.
Minimizes tissue interference by separating out the pulsatile signal
Estimates HR by measuring cyclic changes in light transmission
Estimates functional Hb by comparing amounts of oxy and deoxy Hb PRINCIPLES : PRINCIPLES Spectrophotometry [light absorbance of oxyhb is different from deoxyhb]
Plethysmography [measurement of the fluctuations in the volume of arterial blood between source and the detector]
Beer’s law: states that intensity of transmitted light decreases exponentially as the concentration of the substance increases.
Lambert’s law: states that intensity of transmitted light decreases exponentially as the distance travelled through the substance increases. USES OF PULSE OXIMETRY : USES OF PULSE OXIMETRY Monitoring oxygenation: O.T,ICU, PACU,Pre hospital rapid sequence intubation,during transport and to diagnose obstructive sleep apnoea.
Avoidance of hyperoxaemia:retrolental fibroplasia
Accurate usage of O2 and avoid wastage: maintaining hypoxic ventilatory drive in resp failure
Can replace ABG analysis
Monitoring vascular volume SOURCES OF ERROR : SOURCES OF ERROR Sensitive to motion
Sats below 85% have increased error
Calibration is performed by company on normal patients breathing various gas mixtures, so cal is accurate only down to 80%
Low perfusion state increases error
Ambient light interferes with reading
Delay in reading of about 12 seconds SOURCES OF ERROR : SOURCES OF ERROR Skin pigmentation
Darker color may make the reading more variable due to optical shunting
Dark nail polish has the same effect, especially black, blue, and green…red is OK
Hyperbilirubinemia has no effect
Methylene blue and indigo carmine (dyes) cause underestimation of the saturation SOURCES OF ERROR : SOURCES OF ERROR Dysfunctional hemoglobin
Carboxyhemoglobin leads to overestimation of sats because it absorbs at 660 nm like oxyHb does
MetHb can mask the true saturation because it absorbs at both wavelengths used…sats are overestimated
Anaemichypoxia is not detected
Pulsatile veins Slide 12: Fails to detect hypoventilation, hypercarbia and hyperventilation.
Fails to detect disconections or oesophageal intubation or assess the adequacy of ventilation.
A capnograph is necessary to detect these complication NORMAL CAPNOGRAM : NORMAL CAPNOGRAM Phase I: the beginning of exhalation…CO2 level is zero
Phase II: alveolar gas begins to mix with the deadspace gas and the CO2 rises rapidly
Phase III: elimination of CO2 from the alveoli…usually has a slight upward slope
Phase IV: end exhalation
Phase 0: inspiration THE NORMAL CAPNOGRAM : THE NORMAL CAPNOGRAM ABNORMALITIES : ABNORMALITIES Increased Phase 3 slope:
Obstructive lung dx
Phase 3 dip:
Horizontal Phase 3 with large ET-art gradient:
hypovolemia Sudden decrease to 0
CP event ABNORMALITIES : ABNORMALITIES Gradual decrease
Gradual decrease in volume
Release of limb tourniquet Gradual increase
Exhaused CO2 absorber The Arterial Blood Gas analysis : The Arterial Blood Gas analysis Information Obtained from an ABG: : Information Obtained from an ABG: Acid base status
Dissolved O2 (pO2)
Saturation of hemoglobin
Levels of carboxyhemoglobin and methemoglobin Indications: : Indications: Assess the ventilatory status, oxygenation and acid base status
Assess the response to an intervention Contraindications: : Contraindications: Bleeding diathesis
Severe peripheral vascular disease, absence of an arterial pulse
Infection over site Why an ABG instead of Pulse oximetry? : Why an ABG instead of Pulse oximetry? Pulse oximetry does not assess ventilation (pCO2) or acid base status.
Pulse oximetry becomes unreliable when saturations fall below 70-80%.
Technical sources of error (ambient or fluorescent light, hypoperfusion, nail polish, skin pigmentation)
Pulse oximetry cannot interpret methemoglobin or carboxyhemoglobin. Which Artery to Choose? : Which Artery to Choose? The radial artery is superficial, has collaterals and is easily compressed. It should almost always be the first choice.
Other arteries (femoral, dorsalis pedis, brachial) can be used in emergencies. Preparing to perform the Procedure: : Preparing to perform the Procedure: Make sure you and the patient are comfortable.
Assess the patency of the radial and ulnar arteries. Collection Problems: : Collection Problems: Type of syringe
Plastic vs. glass
Use of heparin
Specimen handling and transport Type of Syringe : Type of Syringe Glass-
Impermeable to gases
Expensive and impractical
Somewhat permeable to gases
Disposable and inexpensive Heparin : Heparin Liquid
Dilutional effect if <2-3 ml of blood collected
Preloaded dry heparin powder
Eliminates dilution problem
Mixing becomes more important
May alter sodium or potassium levels The Kit : The Kit Air bubbles : Air bubbles Gas equilibration between ambient air (pO2 ~ 150, pCO2~0) and arterial blood.
pO2 will begin to rise, pCO2 will fall
Effect is a function of duration of exposure and surface area of air bubble.
Effect is amplified by pneumatic tube transport. Transport : Transport After specimen collected and air bubble removed, gently mix and invert syringe.
Because the wbcs are metabolically active, they will consume oxygen.
Plastic syringes are gas permeable.
Key: Minimize time from sample acquisition to analysis. Transport : Transport Placing the AGB on ice may help minimize changes, depending on the type of syringe, pO2 and white blood cell count.
Its probably not as important if the specimen is delivered immediately. Performing the Procedure: : Performing the Procedure: Follow asepsis
Prepare the site
Drape the bed
Cleanse the radial area with a alcohol
Position the wrist (hyper-extended, using a rolled up towel if necessary)
Palpate the arterial pulse and visualize the course of the artery. Performing the Procedure: : Performing the Procedure: If you are going to use local anesthetic, infiltrate the skin with 2% xylocaine.
Open the ABG kit
Line the needle up with the artery, bevel side up.
Enter the artery and allow the syringe to fill spontaneously. Performing the Procedure: : Performing the Procedure: Withdraw the needle and hold pressure on the site.
Remove any air bubbles
Gently mix the specimen by rolling it between your palms
Place the specimen on ice and transport to lab immediately. Slide 38: . Slide 39: THANK YOU