Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAPHY and
PULSE OXIMETRY
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAPHIC DEVICES
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Infrared Absorption Photometry
Colorimetric Devices
Mass Spectrometry
Raman Scattering
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
ABSORPTION BANDS
Pediatric Resident Curriculum for the PICU
UTHSCSA
SIDE PORT
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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
Pediatric Resident Curriculum for the PICU
UTHSCSA
MAINSTREAM
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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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
NORMAL CAPNOGRAM
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
– Air leak
with large ET-art CO2
• Exponential 
change
– Severe
– Pulmonary embolism
hyperventilation
–  cardiac output
– Cardiopulmonary
– Hypovolemia
event
Pediatric Resident Curriculum for the PICU
UTHSCSA
ABNORMALITIES
• 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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #1
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #2
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #3
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #4
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #5
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #6
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #7
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
CAPNOGRAM #8
J Int Care Med, 12(1): 18-32, 1997
Pediatric Resident Curriculum for the PICU
UTHSCSA
PULSE OXIMETRY
• Uses spectrophotometry based on the BeerLambert 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
Pediatric Resident Curriculum for the PICU
UTHSCSA
ABSORPTION SPECTRA
Pediatric Resident Curriculum for the PICU
UTHSCSA
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%
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
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%.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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)