06 capnography and pulseoximetry


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06 capnography and pulseoximetry

  2. 2. CAPNOGRAPHIC DEVICES <ul><li>Infrared Absorption Photometry </li></ul><ul><li>Colorimetric Devices </li></ul><ul><li>Mass Spectrometry </li></ul><ul><li>Raman Scattering </li></ul>
  3. 3. INFRARED <ul><li>First developed in 1859. </li></ul><ul><li>Based on Beer-Lambert law: Pa = 1 - e -  DC </li></ul><ul><ul><li>Pa is fraction of light absorbed </li></ul></ul><ul><ul><li> is absorption coefficient </li></ul></ul><ul><ul><li>D is distance light travels though the gas </li></ul></ul><ul><ul><li>C is molar gas concentration </li></ul></ul><ul><li>The higher the CO 2 concentration, the higher the absorption. </li></ul><ul><li>CO 2 absorption takes place at 4.25 µm </li></ul><ul><li>N 2 O, H 2 O, and CO can also absorb at this wavelength </li></ul><ul><li>Two types: side port and mainstream </li></ul>
  5. 5. SIDE PORT <ul><li>Gas is sampled through a small tube </li></ul><ul><li>Analysis is performed in a separate chamber </li></ul><ul><li>Very reliable </li></ul><ul><li>Time delay of 1-60 seconds </li></ul><ul><li>Less accurate at higher respiratory rates </li></ul><ul><li>Prone to plugging by water and secretions </li></ul><ul><li>Ambient air leaks </li></ul>
  6. 6. MAINSTREAM <ul><li>Sensor is located in the airway </li></ul><ul><li>Response time as little as 40msec </li></ul><ul><li>Very accurate </li></ul><ul><li>Difficult to calibrate without disconnecting (makes it hard to detect rebreathing) </li></ul><ul><li>More prone to the reading being affected by moisture </li></ul><ul><li>Larger, can kink the tube. </li></ul><ul><li>Adds dead space to the airway </li></ul><ul><li>Bigger chance of being damaged by mishandling </li></ul>
  7. 7. COLORIMETRIC <ul><li>Contains a pH sensitive dye which undergoes a color change in the presence of CO 2 </li></ul><ul><li>The dye is usually metacresol purple and it changes to yellow in the presence of CO 2 </li></ul><ul><li>Portable and lightweight. </li></ul><ul><li>Low false positive rate </li></ul><ul><li>Higher false negative rate </li></ul><ul><li>Acidic solutions, e.g., epi, atropine, lidocaine, will permanently change the color </li></ul><ul><li>Dead space relatively high for neonates, so don’t use for long periods of time on those patients. </li></ul>
  9. 9. NORMAL CAPNOGRAM <ul><li>Phase I is the beginning of exhalation </li></ul><ul><li>Phase I represents most of the anatomical dead space </li></ul><ul><li>Phase II is where the alveolar gas begins to mix with the dead space gas and the CO 2 begins to rapidly rise </li></ul><ul><li>The anatomic dead space can be calculated using Phase I and II </li></ul><ul><li>Alveolar dead space can be calculated on the basis of : V D = V Danat + V Dalv </li></ul><ul><li>Significant increase in the alveolar dead space signifies V/Q mismatch </li></ul>
  10. 10. NORMAL CAPNOGRAM <ul><li>Phase III corresponds to the elimination of CO 2 from the alveoli </li></ul><ul><li>Phase III usually has a slight increase in the slope as “slow” alveoli empty </li></ul><ul><li>The “slow” alveoli have a lower V/Q ratio and therefore have higher CO 2 concentrations </li></ul><ul><li>In addition, diffusion of CO 2 into the alveoli is greater during expiration. More pronounced in infants </li></ul><ul><li>ET CO 2 is measured at the maximal point of Phase III. </li></ul><ul><li>Phase IV is the inspirational phase </li></ul>
  11. 11. ABNORMALITIES <ul><li>Increased Phase III slope </li></ul><ul><ul><li>Obstructive lung disease </li></ul></ul><ul><li>Phase III dip </li></ul><ul><ul><li>Spontaneous resp </li></ul></ul><ul><li>Horizontal Phase III with large ET-art CO 2 change </li></ul><ul><ul><li>Pulmonary embolism </li></ul></ul><ul><ul><li> cardiac output </li></ul></ul><ul><ul><li>Hypovolemia </li></ul></ul><ul><li>Sudden  in ETCO 2 to 0 </li></ul><ul><ul><li>Dislodged tube </li></ul></ul><ul><ul><li>Vent malfunction </li></ul></ul><ul><ul><li>ET obstruction </li></ul></ul><ul><li>Sudden  in ETCO 2 </li></ul><ul><ul><li>Partial obstruction </li></ul></ul><ul><ul><li>Air leak </li></ul></ul><ul><li>Exponential  </li></ul><ul><ul><li>Severe hyperventilation </li></ul></ul><ul><ul><li>Cardiopulmonary event </li></ul></ul>
  12. 12. ABNORMALITIES <ul><li>Gradual  </li></ul><ul><ul><li>Hyperventilation </li></ul></ul><ul><ul><li>Decreasing temp </li></ul></ul><ul><ul><li>Gradual  in volume </li></ul></ul><ul><li>Sudden increase in ETCO 2 </li></ul><ul><ul><li>Sodium bicarb administration </li></ul></ul><ul><ul><li>Release of limb tourniquet </li></ul></ul><ul><li>Gradual increase </li></ul><ul><ul><li>Fever </li></ul></ul><ul><ul><li>Hypoventilation </li></ul></ul><ul><li>Increased baseline </li></ul><ul><ul><li>Rebreathing </li></ul></ul><ul><ul><li>Exhausted CO 2 absorber </li></ul></ul>
  13. 13. PaCO 2 -PetCO 2 gradient <ul><li>Usually <6mm Hg </li></ul><ul><li>PetCO 2 is usually less </li></ul><ul><li>Difference depends on the number of underperfused alveoli </li></ul><ul><li>Tend to mirror each other if the slope of Phase III is horizontal or has a minimal slope </li></ul><ul><li>Decreased cardiac output will increase the gradient </li></ul><ul><li>The gradient can be negative when healthy lungs are ventilated with high TV and low rate </li></ul><ul><li>Decreased FRC also gives a negative gradient by increasing the number of slow alveoli </li></ul>
  14. 14. LIMITATIONS <ul><li>Critically ill patients often have rapidly changing dead space and V/Q mismatch </li></ul><ul><li>Higher rates and smaller TV can increase the amount of dead space ventilation </li></ul><ul><li>High mean airway pressures and PEEP restrict alveolar perfusion, leading to falsely decreased readings </li></ul><ul><li>Low cardiac output will decrease the reading </li></ul>
  15. 15. USES <ul><li>Metabolic </li></ul><ul><ul><li>Assess energy expenditure </li></ul></ul><ul><li>Cardiovascular </li></ul><ul><ul><li>Monitor trend in cardiac output </li></ul></ul><ul><ul><li>Can use as an indirect Fick method, but actual numbers are hard to quantify </li></ul></ul><ul><ul><li>Measure of effectiveness in CPR </li></ul></ul><ul><ul><li>Diagnosis of pulmonary embolism: measure gradient </li></ul></ul>
  16. 16. PULMONARY USES <ul><li>Effectiveness of therapy in bronchospasm </li></ul><ul><ul><li>Monitor PaCO 2 -PetCO 2 gradient </li></ul></ul><ul><ul><li>Worsening indicated by rising Phase III without plateau </li></ul></ul><ul><li>Find optimal PEEP by following the gradient. Should be lowest at optimal PEEP. </li></ul><ul><li>Can predict successful extubation. </li></ul><ul><ul><li>Dead space ratio to tidal volume ratio of >0.6 predicts failure. Normal is 0.33-0.45 </li></ul></ul><ul><li>Limited usefulness in weaning the vent when patient is unstable from cardiovascular or pulmonary standpoint </li></ul><ul><li>Confirm ET tube placement </li></ul>
  17. 17. CAPNOGRAM #1 J Int Care Med, 12(1): 18-32, 1997
  18. 18. CAPNOGRAM #2 J Int Care Med, 12(1): 18-32, 1997
  19. 19. CAPNOGRAM #3 J Int Care Med, 12(1): 18-32, 1997
  20. 20. CAPNOGRAM #4 J Int Care Med, 12(1): 18-32, 1997
  21. 21. CAPNOGRAM #5 J Int Care Med, 12(1): 18-32, 1997
  22. 22. CAPNOGRAM #6 J Int Care Med, 12(1): 18-32, 1997
  23. 23. CAPNOGRAM #7 J Int Care Med, 12(1): 18-32, 1997
  24. 24. CAPNOGRAM #8 J Int Care Med, 12(1): 18-32, 1997
  25. 25. PULSE OXIMETRY <ul><li>Uses spectrophotometry based on the Beer-Lambert law </li></ul><ul><li>Differentiates oxy- from deoxyhemoglobin by the differences in absorption at 660nm and 940nm </li></ul><ul><li>Minimizes tissue interference by separating out the pulsatile signal </li></ul><ul><li>Estimates heart rate by measuring cyclic changes in light transmission </li></ul><ul><li>Measures 4 types of hemoglobin: deoxy, oxy, carboxy, and met </li></ul><ul><li>Estimates functional hemoglobin saturation: oxyhemoglobin/deoxy + oxy </li></ul>
  27. 27. SOURCES OF ERROR <ul><li>Sensitive to motion </li></ul><ul><li>Standard deviation is certified to 4% down to 70% saturation </li></ul><ul><li>Sats below 85% increase the importance of error in the reading </li></ul><ul><li>Calibration is performed by company on normal patients breathing various gas mixtures, so calibration is certain only down to 80% </li></ul>
  28. 28. SOURCES OF ERROR <ul><li>Skin Pigmentation </li></ul><ul><ul><li>Darker color may make the reading more variable due to optical shunting. </li></ul></ul><ul><ul><li>Dark nail polish has same effect: blue, black, and green polishes underestimate saturations, while red and purple have no effect </li></ul></ul><ul><ul><li>Hyperbilirubinemia has no effect </li></ul></ul><ul><li>Low perfusion state </li></ul><ul><li>Ambient Light </li></ul><ul><li>Delay in reading of about 12 seconds </li></ul>
  29. 29. SOURCES OF ERROR <ul><li>Methylene blue and indigo carmine underestimate the saturation </li></ul><ul><li>Dysfunctional hemoglobin </li></ul><ul><ul><li>Carboxyhgb leads to overestimation of sats because it absorbs at 660nm with an absorption coefficient nearly identical to oxyhgb </li></ul></ul><ul><ul><li>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%. </li></ul></ul>
  30. 30. SOURCES OF ERROR <ul><li>Affect of anemia is debated </li></ul><ul><li>Oxygen-Hemoglobin Dissociation Curve </li></ul><ul><ul><li>Shifts in the curve can affect the reading </li></ul></ul><ul><ul><li>Oximetry reading of 95% could correspond to a P a O 2 of 60mmHg (91% saturation) or 160mmHg (99% saturation) </li></ul></ul>