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					UTHSCSA
Pediatric Resident Curriculum for the PICU




              PULSE OXIMETRY
              CAPNOGRAPHY and
          Pediatric Resident Curriculum for the PICU
                                                           CAPNOGRAPHIC DEVICES
                                                       •   Infrared Absorption Photometry
                                                       •   Colorimetric Devices
                                                       •   Mass Spectrometry
                                                       •   Raman Scattering
UTHSCSA
          Pediatric Resident Curriculum for the PICU
                                                                     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 travels though the gas
                                                          – C is molar gas concentration
                                                       • The higher the CO2 concentration, the higher the
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                                                         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
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                   ABSORPTION BANDS
          Pediatric Resident Curriculum for the PICU
                                                                        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
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                                                       •   Ambient air leaks
          Pediatric Resident Curriculum for the PICU
                                                                      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
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                                                       •   Larger, can kink the tube.
                                                       •   Adds dead space to the airway
                                                       •   Bigger chance of being damaged by mishandling
          Pediatric Resident Curriculum for the PICU
                                                                   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
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                                                       • 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.
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                   NORMAL CAPNOGRAM
          Pediatric Resident Curriculum for the PICU
                                                           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
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                                                         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
          Pediatric Resident Curriculum for the PICU
                                                           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
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                                                         greater during expiration. More pronounced in
                                                         infants
                                                       • ET CO2 is measured at the maximal point of Phase
                                                         III.
                                                       • Phase IV is the inspirational phase
          Pediatric Resident Curriculum for the PICU
                                                               ABNORMALITIES
                                                       • Increased Phase III   • Sudden  in ETCO2 to 0
                                                         slope                    – Dislodged tube
                                                          – Obstructive lung      – Vent malfunction
                                                            disease
                                                                                  – ET obstruction
                                                       • Phase III dip
                                                                               • Sudden  in ETCO2
                                                          – Spontaneous resp
                                                                                  – Partial obstruction
                                                       • Horizontal Phase III
                                                         with large ET-art CO2    – Air leak
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                                                         change                • Exponential 
                                                          – Pulmonary embolism    – Severe
                                                          –  cardiac output        hyperventilation
                                                          – Hypovolemia           – Cardiopulmonary
                                                                                    event
          Pediatric Resident Curriculum for the PICU
                                                                ABNORMALITIES
                                                       • Gradual                 • Gradual increase
                                                          – Hyperventilation         – Fever
                                                          – Decreasing temp          – Hypoventilation
                                                          – Gradual  in volume   • Increased baseline
                                                       • Sudden increase in          – Rebreathing
                                                         ETCO2                       – Exhausted CO2
                                                          – Sodium bicarb              absorber
                                                            administration
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                                                          – Release of limb
                                                            tourniquet
          Pediatric Resident Curriculum for the PICU
                                                            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
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                                                       • 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
          Pediatric Resident Curriculum for the PICU
                                                                     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
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                                                       • Low cardiac output will decrease the reading
          Pediatric Resident Curriculum for the PICU
                                                                           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
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                                                          – Measure of effectiveness in CPR
                                                          – Diagnosis of pulmonary embolism: measure
                                                            gradient
          Pediatric Resident Curriculum for the PICU
                                                             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
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                                                            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
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #1




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #2




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #3




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #4




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #5




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #6




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #7




J Int Care Med, 12(1): 18-32, 1997
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                     CAPNOGRAM #8




J Int Care Med, 12(1): 18-32, 1997
          Pediatric Resident Curriculum for the PICU
                                                                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
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                                                         in light transmission
                                                       • Measures 4 types of hemoglobin: deoxy, oxy,
                                                         carboxy, and met
                                                       • Estimates functional hemoglobin saturation:
                                                         oxyhemoglobin/deoxy + oxy
UTHSCSA
Pediatric Resident Curriculum for the PICU
                                   ABSORPTION SPECTRA
          Pediatric Resident Curriculum for the PICU
                                                             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
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                                                         calibration is certain only down to 80%
          Pediatric Resident Curriculum for the PICU
                                                             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
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                                                       • Low perfusion state
                                                       • Ambient Light
                                                       • Delay in reading of about 12 seconds
          Pediatric Resident Curriculum for the PICU
                                                             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
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                                                            absorbing too much light at both 660nm and
                                                            940nm. Saturations are overestimated, but drop
                                                            no further than 85%, which occurs when methgb
                                                            reaches 35%.
          Pediatric Resident Curriculum for the PICU
                                                             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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posted:9/19/2012
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