08 capnometry and pulse oximetry

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08 capnometry and pulse oximetry

  1. 1. CAPNOMETRY AND PULSE OXIMETRY
  2. 2. CAPNOGRAPHIC DEVICES <ul><li>Infrared Absorption Photometry </li></ul><ul><li>Molecular Correlation Spectrography </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 the Beer-Lambert law, which describes the absorption of infrared light by CO2 </li></ul><ul><li>The higher the CO2 concentration, the higher the absorption </li></ul><ul><li>N2O, H2O, and CO can also absorb infrared light at the wavelength used </li></ul><ul><li>Two types: mainstream and side stream </li></ul><ul><li>More compact and less expensive than the other types of capnometers </li></ul><ul><li>Requires sampling gas flow of ~150ml/min thru the unit </li></ul>
  4. 4. SIDE STREAM <ul><li>Gas is sampled through a small tube that pulls it out of the main gas stream </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 high rates </li></ul><ul><li>Sampling tube is prone to plugging by water/secretions </li></ul><ul><li>Ambient air leaks affect reading </li></ul><ul><li>Connector is lightweight and doesn’t pull on airway </li></ul><ul><li>Easy to use when patient is in an unusual position, such as prone </li></ul>
  5. 5. MAINSTREAM <ul><li>Sensor is located in the airway </li></ul><ul><li>Response time as quick as 40 msec </li></ul><ul><li>Very accurate </li></ul><ul><li>Difficult to calibrate without disconnecting </li></ul><ul><li>Reading more prone to being affected by moisture </li></ul><ul><li>Larger and heavier than sidestream…can kink the ETT </li></ul><ul><li>Adds deadspace to the airway </li></ul><ul><li>Bigger chance of being damaged by mishandling </li></ul><ul><li>Sensor window can be clogged with secretions </li></ul><ul><li>Difficult to use in unusual positions, such as prone </li></ul>
  6. 6. Molecular Correlation Spectrography <ul><li>Uses an infrared emission that precisely matches the absorption spectrum of CO2 </li></ul><ul><li>Allows for the use of very small samples at very low flow rates </li></ul><ul><li>Samples are measured every 25 msecs and uses a flowrate of 50 ml/min </li></ul>
  7. 7. COLORIMETRIC <ul><li>Contains a pH sensitive dye which undergoes a color change in the presence of CO2 </li></ul><ul><li>The dye is usually metacresol purple and it changes to yellow in the presence of CO2 </li></ul><ul><li>Portable and lightweight </li></ul><ul><li>Low false positive rate…higher false negative rate </li></ul><ul><li>Acidic solutions (eg-lidocaine, epi, atropine) will permanently change the color </li></ul><ul><li>Deadspace high for a neonate – can’t use for long </li></ul>
  8. 9. MASS SPECTROMETRY <ul><li>Separates and counts ionized molecules to determine the concentration of gas </li></ul><ul><li>A gas sample is aspirated into a vacuum chamber when an electron beam ionizes and fragments the components of the sample </li></ul><ul><li>The ions are accelerated into a final chamber which has a magnetic field that allows for determination of the components of the gas and the concentration of each component </li></ul><ul><li>Very expensive and bulky, but have the advantage of being able to monitor multiple patients at a time (eg-OR) </li></ul>
  9. 11. RAMAN SCATTERING <ul><li>Raman scattering occurs when light hits a molecule and it scatters the light…most of the scattered light is the same wavelength as the laser source, but a small amount of light scattered is at a different wavelength </li></ul><ul><li>The different wavelength produced gives information about the molecule </li></ul><ul><li>An argon laser is shone through a gas sample and the CO2 in the sample will scatter it…the amount of scattering is related to the CO2 level </li></ul>
  10. 12. NORMAL CAPNOGRAM <ul><li>Phase I: the beginning of exhalation…CO2 level is zero </li></ul><ul><li>Phase II: alveolar gas begins to mix with the deadspace gas and the CO2 rises rapidly </li></ul><ul><li>Phase III: elimination of CO2 from the alveoli…usually has a slight upward slope </li></ul><ul><li>Phase IV: end exhalation </li></ul><ul><li>Phase 0: inspiration </li></ul>
  11. 13. THE NORMAL CAPNOGRAM
  12. 14. ABNORMALITIES <ul><li>Increased Phase 3 slope: </li></ul><ul><ul><li>Obstructive lung dx </li></ul></ul><ul><li>Phase 3 dip: </li></ul><ul><ul><li>Spont resp </li></ul></ul><ul><ul><li>Curare cleft </li></ul></ul><ul><li>Horizontal Phase 3 with large ET-art gradient: </li></ul><ul><ul><li>Pulm. Embolism </li></ul></ul><ul><ul><li>Decreased CO </li></ul></ul><ul><ul><li>hypovolemia </li></ul></ul><ul><li>Sudden decrease to 0 </li></ul><ul><ul><li>Ventilator malfunction </li></ul></ul><ul><ul><li>ETT disconnect </li></ul></ul><ul><ul><li>ET obstruction </li></ul></ul><ul><ul><li>Extubation </li></ul></ul><ul><li>Sudden decrease </li></ul><ul><ul><li>Partial obstruction </li></ul></ul><ul><ul><li>Air leak </li></ul></ul><ul><li>Exponential decrease </li></ul><ul><ul><li>Severe hyperventilation </li></ul></ul><ul><ul><li>CP event </li></ul></ul>
  13. 15. ABNORMALITIES <ul><li>Gradual decrease </li></ul><ul><ul><li>Hyperventilation </li></ul></ul><ul><ul><li>Decreased T </li></ul></ul><ul><ul><li>Gradual decrease in volume </li></ul></ul><ul><li>Sudden increase </li></ul><ul><ul><li>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 CO2 </li></ul></ul><ul><ul><li>Exhaused CO2 absorber </li></ul></ul>
  14. 16. PaCO2-PetCO2 GRADIENT <ul><li>Usually <6 mm Hg </li></ul><ul><li>PetCO2 is usually less than arterial </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 3 is horizontal or minimal </li></ul><ul><li>Decreased CO will increase the gradient </li></ul>
  15. 17. LIMITATIONS <ul><li>Critically ill patients often have rapidly changing deadspace and V/Q mismatch </li></ul><ul><li>Higher rates and small Vt can increase the amount of deadspace ventilation </li></ul><ul><li>High mean airway pressures and PEEP restrict alveolar perfusion leading to falsely decreased readings </li></ul><ul><li>Low CO will decrease the reading </li></ul>
  16. 18. 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 </li></ul></ul><ul><ul><li>Measure of effectiveness in CPR </li></ul></ul><ul><li>Diagnosis of pulmonary embolism: measure the gradient </li></ul>
  17. 19. PULMONARY USES <ul><li>Effectiveness of bronchodilator therapy </li></ul><ul><ul><li>Monitor gradient </li></ul></ul><ul><ul><li>Worsening indicated by rising Phase 3 w/o plateau </li></ul></ul><ul><li>Find optimal PEEP by following the gradient …should be lowest at optimal PEEP level </li></ul><ul><li>Can predict successful extubation…Vd/Vt > 0.6 predicts failure </li></ul><ul><li>Limited pulm usefulness if CV unstable </li></ul>
  18. 20. CAPNOMETRY <ul><li>Measures and displays a numerical value of the CO2 level </li></ul><ul><ul><li>30-43 mm Hg </li></ul></ul><ul><ul><li>4.0-5.6% </li></ul></ul>
  19. 21. ESOPHAGEAL INTUBATION/ DISCONNECTION FROM VENTILATOR/TOTALLY OBSTRUCTED ETT
  20. 22. Air Leak/Loose Connection between sampling tube and capnograph
  21. 23. Increasing Temperature/Metabolism
  22. 24. Hypothermia/Reduced Metabolism/ Hyperventilation/Decreased CO <ul><li>Cause a gradual decrese in end-tidal CO2 </li></ul>
  23. 25. Cardiac Oscillations
  24. 26. Bronchospasm/COPD/obstructed ETT <ul><li>Slanting and prolonged phase 2 and increased slope of phase 3 </li></ul><ul><li>Sometimes there’s a reverse phase 3 slope seen in patients with emphysema </li></ul>
  25. 27. Ventilator IMV breath during spontaneous ventilation
  26. 28. Sticking Inspiratory Valve
  27. 29. Hypoventilation
  28. 30. Leak/Partial Disconnect in Circuit/ETT too high
  29. 31. Pulmonary Embolism/Pneumonia/ Hypovolemia/ Hyperventilation
  30. 32. Curare Cleft
  31. 33. Spontaneous Breathing
  32. 34. Rebreathing of CO2
  33. 35. CAUSES OF INCREASED PetCO2 <ul><li>Increased CO2 production and delivery to the lungs </li></ul><ul><ul><li>Fever </li></ul></ul><ul><ul><li>Sepsis </li></ul></ul><ul><ul><li>Bicarb administration </li></ul></ul><ul><ul><li>Increased metabolic rate </li></ul></ul><ul><ul><li>Seizures </li></ul></ul><ul><li>Decreased alveolar ventilation </li></ul><ul><ul><li>Respiratory center depression </li></ul></ul><ul><ul><li>Muscular paralysis </li></ul></ul><ul><ul><li>Hypoventilation </li></ul></ul><ul><ul><li>COPD </li></ul></ul><ul><li>Equipment malfunction </li></ul><ul><ul><li>Rebreathing </li></ul></ul><ul><ul><li>Exhausted CO2 absorber </li></ul></ul><ul><ul><li>Leak in ventilator circuit </li></ul></ul>
  34. 36. CAUSES OF DECREASED PetCO2 <ul><li>Decreased CO2 production and delivery to the lungs </li></ul><ul><ul><li>Hypothermia </li></ul></ul><ul><ul><li>Pulmonary hypoperfusion </li></ul></ul><ul><ul><li>Cardiac arrest </li></ul></ul><ul><ul><li>Hemorrhage </li></ul></ul><ul><ul><li>Hypotension </li></ul></ul><ul><li>Increased alveolar deadspace </li></ul><ul><ul><li>Decreased CO </li></ul></ul><ul><ul><li>Pulmonary embolism </li></ul></ul><ul><ul><li>High PEEP levels </li></ul></ul><ul><li>Increased alveolar ventilation </li></ul><ul><ul><li>Hyperventilation </li></ul></ul><ul><li>Equipment malfunction </li></ul><ul><ul><li>Ventilator disconnect </li></ul></ul><ul><ul><li>Esophageal intubation </li></ul></ul><ul><ul><li>Complete airway obstruction </li></ul></ul><ul><ul><li>Poor sampling </li></ul></ul><ul><ul><li>Leak around ETT cuff </li></ul></ul><ul><ul><li>Water in sampling line </li></ul></ul><ul><ul><li>Air entrainment into sampling line </li></ul></ul><ul><ul><li>Inadequate tidal volume </li></ul></ul>
  35. 37. CAUSES OF INCREASED P(a-et)CO2 <ul><li>Pulmonary hypoperfusion </li></ul><ul><li>Pulmonary embolism </li></ul><ul><li>Cardiac arrest </li></ul><ul><li>Positive pressure ventilation, especially with PEEP </li></ul><ul><li>High rate/low tidal volume ventilation </li></ul>
  36. 38. PULSE OXIMETRY <ul><li>Uses spectrophotometry based on the Beer-Lambert law </li></ul><ul><li>Differentiates oxy from deoxy Hb by the differences in absorption of light at 660 nm and 940 nm </li></ul><ul><li>Minimizes tissue interference by separating out the pulsatile signal </li></ul><ul><li>Estimates HR by measuring cyclic changes in light transmission </li></ul><ul><li>Estimates functional Hb by comparing amounts of oxy and deoxy Hb </li></ul>
  37. 40. SOURCES OF ERROR <ul><li>Sensitive to motion </li></ul><ul><li>Sats below 85% have increased error </li></ul><ul><li>Calibration is performed by company on normal patients breathing various gas mixtures, so cal is accurate only down to 80% </li></ul><ul><li>Low perfusion state increases error </li></ul><ul><li>Ambient light interferes with reading </li></ul><ul><li>Delay in reading of about 12 seconds </li></ul>
  38. 41. 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 the same effect, especially black, blue, and green…red is OK </li></ul></ul><ul><ul><li>Hyperbilirubinemia has no effect </li></ul></ul><ul><li>Methylene blue and indigo carmine (dyes) cause underestimation of the saturation </li></ul>
  39. 42. SOURCES OF ERROR <ul><li>Dysfunctional hemoglobin </li></ul><ul><ul><li>Carboxyhemoglobin leads to overestimation of sats because it absorbs at 660 nm like oxyHb does </li></ul></ul><ul><ul><li>MetHb can mask the true saturation because it absorbs at both wavelengths used…sats are overestimated </li></ul></ul>

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