SS Pulse Ox - Fall23.pptx
Transcript
Pulse Oximetry Clay Freeman, DNP, CRNA Science in Anesthesia 1 Objectives Readings: Nagelhout: Chap. 15, p.242243 Nagelhout: Chap. 18, p.320321 Davis: Chap 18 • Describe the calculation of oxygen carrying capacity • Describe the components of the Beer-Lambert Law and applications within anesthe...
Pulse Oximetry Clay Freeman, DNP, CRNA Science in Anesthesia 1 Objectives Readings: Nagelhout: Chap. 15, p.242243 Nagelhout: Chap. 18, p.320321 Davis: Chap 18 • Describe the calculation of oxygen carrying capacity • Describe the components of the Beer-Lambert Law and applications within anesthesia practice • Detail the mechanics of pulse oximetry • Analyze advantages, disadvantages and causes of inaccuracies with pulse oximetry • Detail the principles of cerebral oximetry 2 Oxygen Carrying Physiology CaO2: Arterial oxygen content CaO2 = SaO2 · (Hgb · 1.34) + (0.003 · PaO2) Bound oxygen (SaO2) PaO2: Arterial oxygen tension 3 Dissolved & Bound O2 Logarithmic relationship of available O2 and the saturation of Hgb Oxyhemoglobin Dissociation Curve SaO2 90% = 60 PaO2 SaO2 of 70% = 40 PaO2 (Cyanosis apparent) 4 Pulse Ox enters the chat SaO2 is inferred from interpretation of SpO2 Pulse Oximetry calculates SpO2 based on: Spectrophotometry (measuring optical properties of materials over various wavelengths) and Plethysmography (measuring changes in volume) 5 Spectroscopy Light consists of photons which can exist at various wavelengths • Photons are carriers of electromagnetic radiation Matter interacts with complementary photons Spectroscopy is the science of analyzing the reaction between matter and specific wavelengths Wavelength range of 6 Beer-Lambert Law Calculation of SpO2 is done using an algorithm derived from the Beer-Lambert Law Ratio is Calculated: Itrans = Iin · e-DC ltrans lin Itrans = Intensity of transmitted light Iin = Intensity of incident light D = The distance through the medium C = Concentration of the solute = The extinction coefficient of the solute D e 7 Beer-Lambert Law Lambert's laws Beer's law 1) The luminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source. Absorptivity is established by when the amount of light absorbed is proportional to the solution concentration 2) If the rays meet the surface at an angle, then the luminance is proportional to the cosine of the angle with the normal. 3) The luminous intensity of light decreases exponentially with distance as it travels through an absorbing medium. 8 Spectrophotometry 1) Pulse oximeter probe contains two lightemitting diodes (LED) which are activated in alternating sequence • One in the red band • One in the near infrared band 2) A photosensor detects the amount of Incident Light for each band 3) Absorption ratios of the red/infrared bands is compared and calculated Oxyhemoglobin absorbs infrared light 940nm Deoxyhemoglobin absorbs visible red light 660nm oxyhemoglobin and reduced hemoglobin (deoxyhemoglobin) demonstrate different extinction coefficients for chosen wavelengths 9 Plethysmography - Dawn of the Pulse Ox Plethysmographic analysis is used to differentiate the pulsatile ‘arterial’ signal from the nonpulsatile signals and tissue. Pulsations cause systolic volume expansion of arteriole beds which increases optical path length Detection of absorption during pulsatile flow occurs several hundred times/second 10 But wait…There’s more Light absorbance at both wavelengths is then divided by the absorbance of light from extraneous tissue D C AC D C AC 11 Risk : Benefit Pros: Non-invasive Easy to apply Continuous monitoring Earlier detection of desaturation: SpO2 70% = PaO2 40mmHg cyanosis • Inexpensive • • • • Cons: • Prone to artifact • Delayed measurements 30-60 seconds • Inaccurate at SpO2 values below 70% • Rare risk of burns in poor perfusion states 12 Reduced False Alarms Delay in readings related to probe location (circulation time) and Calculation of readings; Ear Finger Toe 7-20 sec 20-35 sec 40-60 sec Lower accuracies due to low flow or low pulsatile flow states (cardiac bypass, hypotension, NIBP cuff inflation, tourniquets, vasoconstriction) 13 Reduced Hemoglobins Pulse Oximetry is calibrated to focus on 2 components that affect light absorption: oxyhemoglobin and deoxyhemoglobin - However, the light frequencies lack specificity and are subject to error from alternate species of hemoglobin Errors of various hemoglobin species: Carboxyhemoglobin (COHb): false high SpO2 Methemoglobin (MetHb-ferric iron): false high SpO2 14 Artifacts & Inaccuracies Dyes cause an underestimation of SpO2: • Methylene Blue • Indigo Carmine • Indocyanine Green Other sources of error: • Ambient light • Deep skin pigmentation/Scar tissue • Electrosurgery decreases SpO2 readings • Motion Artifact • Fingernail polish underestimates SpO2 15 Physiologic Derangements Left Shift (Increased affinity) Right Shift (Decreased affinity) 16 Improving Standards 17 Improving Standards Plethysmography Variability Index 18 AANA Standards of Care Standard 6: Equipment Adhere to manufacturer’s Standard 5: Documentation operating instructions and other safety precautions Communicate anesthesia care data to complete a daily anesthesia equipment check. and activities through legible, timely, Verify function of anesthesia equipment prior to accurate, and complete each anesthetic. Operate equipment to minimize documentation in the patient’s the risk of fire, explosion, electrical shock, and healthcare record. equipment malfunction. Standard 9: Monitoring, Alarms Monitor, evaluate, and document the patient’s physiologic condition as appropriate for the procedure and anesthetic technique. When a physiological monitoring device is used, variable pitch and threshold alarms are turned on and audible. Document blood pressure, heart rate, and respiration at least every five minutes for all anesthetics. a. Oxygenation Continuously monitor oxygenation by clinical observation and pulse oximetry. The surgical or procedure team communicates and collaborates to mitigate the risk of fire. b. Ventilation Continuously monitor ventilation by clinical observation and confirmation of continuous expired carbon dioxide during moderate sedation, deep sedation or general anesthesia. Verify intubation of the trachea or placement of other artificial airway device by auscultation, chest excursion, and confirmation of expired carbon dioxide. Use ventilatory monitors as indicated. c. Cardiovascular Monitor and evaluate circulation to maintain patient’s hemodynamic status. Continuously monitor heart rate and cardiovascular status. Use invasive monitoring as appropriate. d. Thermoregulation When clinically significant changes in body temperature are intended, anticipated, or suspected, monitor 19 Cerebral Oximetry Near-infrared Spectroscopy is utilized to monitor regional tissue oxygen saturation Light-emitting diodes provide various wavelengths in the range 600-1000 nm Scattering and Reflection of electromagnetic radiation occurs Skull and scalp are relatively transparent. Sampled tissue is assumed to be 75% venous and 25% arterial The path of EMR is unpredictable: • Beer-Lambert law must be modified • Difficult to predict factors of both absorption and scattering 20 Additional Resources • Anesthesia Equipment: Principles and Applications. 3rd Ed. Ehrenwerth, Eisenkraft, Berry. Chap 11. • https://issuu.com/aanapublishing/docs/standards_for_nu rse_anesthesia_practice_2.23?fr=sOGNhNjU2NDAxMjU 21
