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CAPNOGRAPHY and PULSE OXIMETRY

CAPNOGRAPHY and PULSE OXIMETRY. CAPNOGRAPHIC DEVICES. Infrared Absorption Photometry Colorimetric Devices Mass Spectrometry Raman Scattering. INFRARED . First developed in 1859. Based on Beer-Lambert law: Pa = 1 - e -  DC Pa is fraction of light absorbed  is absorption coefficient

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CAPNOGRAPHY and PULSE OXIMETRY

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  1. CAPNOGRAPHY andPULSE OXIMETRY

  2. CAPNOGRAPHIC DEVICES • Infrared Absorption Photometry • Colorimetric Devices • Mass Spectrometry • Raman Scattering

  3. INFRARED • First developed in 1859. • Based on Beer-Lambert law: Pa = 1 - e-  DC • Pa is fraction of light absorbed •  is absorption coefficient • D is distance light travelsthough the gas • C is molar gas concentration • The higher the CO2 concentration, the higher the absorption. • CO2 absorption takes place at 4.25 µm • N2O, H2O, and CO can also absorb at this wavelength • Two types: side port and mainstream

  4. ABSORPTION BANDS

  5. SIDE PORT • Gas is sampled through a small tube • Analysis is performed in a separate chamber • Very reliable • Time delay of 1-60 seconds • Less accurate at higher respiratory rates • Prone to plugging by water and secretions • Ambient air leaks

  6. MAINSTREAM • Sensor is located in the airway • Response time as little as 40msec • Very accurate • Difficult to calibrate without disconnecting (makes it hard to detect rebreathing) • More prone to the reading being affected by moisture • Larger, can kink the tube. • Adds dead space to the airway • Bigger chance of being damaged by mishandling

  7. COLORIMETRIC • Contains a pH sensitive dye which undergoes a color change in the presence of CO2 • The dye is usually metacresol purple and it changes to yellow in the presence of CO2 • Portable and lightweight. • Low false positive rate • Higher false negative rate • Acidic solutions, e.g., epi, atropine, lidocaine, will permanently change the color • Dead space relatively high for neonates, so don’t use for long periods of time on those patients.

  8. NORMAL CAPNOGRAM

  9. NORMAL CAPNOGRAM • Phase I is the beginning of exhalation • Phase I represents most of the anatomical dead space • Phase II is where the alveolar gas begins to mix with the dead space gas and the CO2 begins to rapidly rise • The anatomic dead space can be calculated using Phase I and II • Alveolar dead space can be calculated on the basis of : VD = VDanat + VDalv • Significant increase in the alveolar dead space signifies V/Q mismatch

  10. NORMAL CAPNOGRAM • Phase III corresponds to the elimination of CO2 from the alveoli • Phase III usually has a slight increase in the slope as “slow” alveoli empty • The “slow” alveoli have a lower V/Q ratio and therefore have higher CO2 concentrations • In addition, diffusion of CO2 into the alveoli is greater during expiration. More pronounced in infants • ET CO2 is measured at the maximal point of Phase III. • Phase IV is the inspirational phase

  11. Increased Phase III slope Obstructive lung disease Phase III dip Spontaneous resp Horizontal Phase III with large ET-art CO2 change Pulmonary embolism  cardiac output Hypovolemia Sudden  in ETCO2 to 0 Dislodged tube Vent malfunction ET obstruction Sudden  in ETCO2 Partial obstruction Air leak Exponential  Severe hyperventilation Cardiopulmonary event ABNORMALITIES

  12. Gradual  Hyperventilation Decreasing temp Gradual  in volume Sudden increase in ETCO2 Sodium bicarb administration Release of limb tourniquet Gradual increase Fever Hypoventilation Increased baseline Rebreathing Exhausted CO2 absorber ABNORMALITIES

  13. PaCO2-PetCO2 gradient • Usually <6mm Hg • PetCO2 is usually less • Difference depends on the number of underperfused alveoli • Tend to mirror each other if the slope of Phase III is horizontal or has a minimal slope • Decreased cardiac output will increase the gradient • The gradient can be negative when healthy lungs are ventilated with high TV and low rate • Decreased FRC also gives a negative gradient by increasing the number of slow alveoli

  14. LIMITATIONS • Critically ill patients often have rapidly changing dead space and V/Q mismatch • Higher rates and smaller TV can increase the amount of dead space ventilation • High mean airway pressures and PEEP restrict alveolar perfusion, leading to falsely decreased readings • Low cardiac output will decrease the reading

  15. USES • Metabolic • Assess energy expenditure • Cardiovascular • Monitor trend in cardiac output • Can use as an indirect Fick method, but actual numbers are hard to quantify • Measure of effectiveness in CPR • Diagnosis of pulmonary embolism: measure gradient

  16. PULMONARY USES • Effectiveness of therapy in bronchospasm • Monitor PaCO2-PetCO2 gradient • Worsening indicated by rising Phase III without plateau • Find optimal PEEP by following the gradient. Should be lowest at optimal PEEP. • Can predict successful extubation. • Dead space ratio to tidal volume ratio of >0.6 predicts failure. Normal is 0.33-0.45 • Limited usefulness in weaning the vent when patient is unstable from cardiovascular or pulmonary standpoint • Confirm ET tube placement

  17. CAPNOGRAM #1 J Int Care Med, 12(1): 18-32, 1997

  18. CAPNOGRAM #2 J Int Care Med, 12(1): 18-32, 1997

  19. CAPNOGRAM #3 J Int Care Med, 12(1): 18-32, 1997

  20. CAPNOGRAM #4 J Int Care Med, 12(1): 18-32, 1997

  21. CAPNOGRAM #5 J Int Care Med, 12(1): 18-32, 1997

  22. CAPNOGRAM #6 J Int Care Med, 12(1): 18-32, 1997

  23. CAPNOGRAM #7 J Int Care Med, 12(1): 18-32, 1997

  24. CAPNOGRAM #8 J Int Care Med, 12(1): 18-32, 1997

  25. PULSE OXIMETRY • Uses spectrophotometry based on the Beer-Lambert law • Differentiates oxy- from deoxyhemoglobin by the differences in absorption at 660nm and 940nm • Minimizes tissue interference by separating out the pulsatile signal • Estimates heart rate by measuring cyclic changes in light transmission • Measures 4 types of hemoglobin: deoxy, oxy, carboxy, and met • Estimates functional hemoglobin saturation: oxyhemoglobin/deoxy + oxy

  26. ABSORPTION SPECTRA

  27. SOURCES OF ERROR • Sensitive to motion • Standard deviation is certified to 4% down to 70% saturation • Sats below 85% increase the importance of error in the reading • Calibration is performed by company on normal patients breathing various gas mixtures, so calibration is certain only down to 80%

  28. SOURCES OF ERROR • Skin Pigmentation • Darker color may make the reading more variable due to optical shunting. • Dark nail polish has same effect: blue, black, and green polishes underestimate saturations, while red and purple have no effect • Hyperbilirubinemia has no effect • Low perfusion state • Ambient Light • Delay in reading of about 12 seconds

  29. SOURCES OF ERROR • Methylene blue and indigo carmine underestimate the saturation • Dysfunctional hemoglobin • Carboxyhgb leads to overestimation of sats because it absorbs at 660nm with an absorption coefficient nearly identical to oxyhgb • Methgb can mask the true saturation by absorbing too much light at both 660nm and 940nm. Saturations are overestimated, but drop no further than 85%, which occurs when methgb reaches 35%.

  30. SOURCES OF ERROR • Affect of anemia is debated • Oxygen-Hemoglobin Dissociation Curve • Shifts in the curve can affect the reading • Oximetry reading of 95% could correspond to a PaO2 of 60mmHg (91% saturation) or 160mmHg (99% saturation)

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