06 capnography and pulseoximetry

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CAPNOGRAPHY andCAPNOGRAPHY andPULSE OXIMETRYPULSE OXIMETRY

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UCAPNOGRAPHIC CAPNOGRAPHIC

DEVICESDEVICES• Infrared Absorption PhotometryInfrared Absorption Photometry• Colorimetric DevicesColorimetric Devices• Mass SpectrometryMass Spectrometry• Raman ScatteringRaman Scattering

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UINFRAREDINFRARED

• First developed in 1859.First developed in 1859.• Based on Beer-Lambert law: Pa = 1 - eBased on Beer-Lambert law: Pa = 1 - e- - DC DC

– Pa is fraction of light absorbedPa is fraction of light absorbed is absorption coefficientis absorption coefficient– D is distance light travelsD is distance light travels though the gasthough the gas– C is molar gas concentrationC is molar gas concentration

• The higher the COThe higher the CO22 concentration, the higher the concentration, the higher the absorption.absorption.

• COCO22 absorption takes place at 4.25 µm absorption takes place at 4.25 µm

• NN22O, HO, H22O, and CO can also absorb at this wavelengthO, and CO can also absorb at this wavelength

• Two types: side port and mainstreamTwo types: side port and mainstream

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UABSORPTION BANDSABSORPTION BANDS

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USIDE PORTSIDE PORT

• Gas is sampled through a small tubeGas is sampled through a small tube• Analysis is performed in a separate chamberAnalysis is performed in a separate chamber• Very reliableVery reliable• Time delay of 1-60 secondsTime delay of 1-60 seconds• Less accurate at higher respiratory ratesLess accurate at higher respiratory rates• Prone to plugging by water and secretionsProne to plugging by water and secretions• Ambient air leaksAmbient air leaks

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UMAINSTREAMMAINSTREAM

• Sensor is located in the airwaySensor is located in the airway• Response time as little as 40msecResponse time as little as 40msec• Very accurateVery accurate• Difficult to calibrate without disconnecting Difficult to calibrate without disconnecting

(makes it hard to detect rebreathing)(makes it hard to detect rebreathing)• More prone to the reading being affected by More prone to the reading being affected by

moisturemoisture• Larger, can kink the tube.Larger, can kink the tube.• Adds dead space to the airwayAdds dead space to the airway• Bigger chance of being damaged by Bigger chance of being damaged by

mishandlingmishandling

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UCOLORIMETRICCOLORIMETRIC

• Contains a pH sensitive dye which undergoes a Contains a pH sensitive dye which undergoes a color change in the presence of COcolor change in the presence of CO22

• The dye is usually metacresol purple and it The dye is usually metacresol purple and it changes to yellow in the presence of COchanges to yellow in the presence of CO22

• Portable and lightweight.Portable and lightweight.• Low false positive rateLow false positive rate• Higher false negative rateHigher false negative rate• Acidic solutions, e.g., epi, atropine, lidocaine, Acidic solutions, e.g., epi, atropine, lidocaine,

will permanently change the colorwill permanently change the color• Dead space relatively high for neonates, so don’t Dead space relatively high for neonates, so don’t

use for long periods of time on those patients.use for long periods of time on those patients.

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UNORMAL CAPNOGRAMNORMAL CAPNOGRAM

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• Phase I is the beginning of exhalationPhase I is the beginning of exhalation• Phase I represents most of the anatomical dead Phase I represents most of the anatomical dead

spacespace• Phase II is where the alveolar gas begins to mix Phase II is where the alveolar gas begins to mix

with the dead space gas and the COwith the dead space gas and the CO22 begins to begins to rapidly riserapidly rise

• The anatomic dead space can be calculated using The anatomic dead space can be calculated using Phase I and IIPhase I and II

• Alveolar dead space can be calculated on the basis Alveolar dead space can be calculated on the basis of : Vof : VDD = V = VDanat Danat + V+ VDalvDalv

• Significant increase in the alveolar dead space Significant increase in the alveolar dead space signifies V/Q mismatchsignifies V/Q mismatch

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• Phase III corresponds to the elimination of COPhase III corresponds to the elimination of CO22 from the alveolifrom the alveoli

• Phase III usually has a slight increase in the slope Phase III usually has a slight increase in the slope as “slow” alveoli emptyas “slow” alveoli empty

• The “slow” alveoli have a lower V/Q ratio and The “slow” alveoli have a lower V/Q ratio and therefore have higher COtherefore have higher CO22 concentrations concentrations

• In addition, diffusion of COIn addition, diffusion of CO22 into the alveoli is into the alveoli is greater during expiration. More pronounced in greater during expiration. More pronounced in infantsinfants

• ET COET CO22 is measured at the maximal point of Phase is measured at the maximal point of Phase III.III.

• Phase IV is the inspirational phasePhase IV is the inspirational phase

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UABNORMALITIES ABNORMALITIES

• Increased Phase III Increased Phase III slopeslope– Obstructive lung Obstructive lung

diseasedisease• Phase III dipPhase III dip

– Spontaneous respSpontaneous resp• Horizontal Phase III Horizontal Phase III

with large ET-art COwith large ET-art CO22 changechange– Pulmonary embolismPulmonary embolism cardiac outputcardiac output– HypovolemiaHypovolemia

• Sudden Sudden in ETCO in ETCO22 to 0 to 0– Dislodged tubeDislodged tube– Vent malfunctionVent malfunction– ET obstructionET obstruction

• Sudden Sudden in ETCO in ETCO22 – Partial obstructionPartial obstruction– Air leakAir leak

• Exponential Exponential – Severe Severe

hyperventilationhyperventilation– Cardiopulmonary Cardiopulmonary

eventevent

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UABNORMALITIESABNORMALITIES

• Gradual Gradual – HyperventilationHyperventilation– Decreasing tempDecreasing temp– Gradual Gradual in in

volumevolume• Sudden increase in Sudden increase in

ETCOETCO22

– Sodium bicarb Sodium bicarb administrationadministration

– Release of limb Release of limb tourniquettourniquet

• Gradual increaseGradual increase– FeverFever– HypoventilationHypoventilation

• Increased baselineIncreased baseline– RebreathingRebreathing– Exhausted COExhausted CO22

absorberabsorber

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UPaCOPaCO22-PetCO-PetCO22 gradient gradient

• Usually <6mm HgUsually <6mm Hg• PetCOPetCO22 is usually less is usually less• Difference depends on the number of Difference depends on the number of

underperfused alveoliunderperfused alveoli• Tend to mirror each other if the slope of Phase III is Tend to mirror each other if the slope of Phase III is

horizontal or has a minimal slopehorizontal or has a minimal slope• Decreased cardiac output will increase the gradientDecreased cardiac output will increase the gradient• The gradient can be negative when healthy lungs The gradient can be negative when healthy lungs

are ventilated with high TV and low rateare ventilated with high TV and low rate• Decreased FRC also gives a negative gradient by Decreased FRC also gives a negative gradient by

increasing the number of slow alveoliincreasing the number of slow alveoli

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ULIMITATIONSLIMITATIONS

• Critically ill patients often have rapidly Critically ill patients often have rapidly changing dead space and V/Q mismatchchanging dead space and V/Q mismatch

• Higher rates and smaller TV can increase the Higher rates and smaller TV can increase the amount of dead space ventilationamount of dead space ventilation

• High mean airway pressures and PEEP restrict High mean airway pressures and PEEP restrict alveolar perfusion, leading to falsely decreased alveolar perfusion, leading to falsely decreased readingsreadings

• Low cardiac output will decrease the readingLow cardiac output will decrease the reading

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UUSESUSES

• MetabolicMetabolic– Assess energy expenditureAssess energy expenditure

• CardiovascularCardiovascular– Monitor trend in cardiac outputMonitor trend in cardiac output– Can use as an indirect Fick method, but Can use as an indirect Fick method, but

actual numbers are hard to quantifyactual numbers are hard to quantify– Measure of effectiveness in CPRMeasure of effectiveness in CPR– Diagnosis of pulmonary embolism: measure Diagnosis of pulmonary embolism: measure

gradientgradient

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UPULMONARY USESPULMONARY USES

• Effectiveness of therapy in bronchospasmEffectiveness of therapy in bronchospasm– Monitor PaCOMonitor PaCO22-PetCO-PetCO2 2 gradientgradient– Worsening indicated by rising Phase III without Worsening indicated by rising Phase III without

plateauplateau• Find optimal PEEP by following the gradient. Should be Find optimal PEEP by following the gradient. Should be

lowest at optimal PEEP.lowest at optimal PEEP.• Can predict successful extubation. Can predict successful extubation.

– Dead space ratio to tidal volume ratio of >0.6 Dead space ratio to tidal volume ratio of >0.6 predicts failure. Normal is 0.33-0.45predicts failure. Normal is 0.33-0.45

• Limited usefulness in weaning the vent when patient is Limited usefulness in weaning the vent when patient is unstable from cardiovascular or pulmonary standpointunstable from cardiovascular or pulmonary standpoint

• Confirm ET tube placementConfirm ET tube placement

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UCAPNOGRAM #1CAPNOGRAM #1

J Int Care Med, 12(1): 18-32, 1997J Int Care Med, 12(1): 18-32, 1997

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UPULSE OXIMETRYPULSE OXIMETRY

• Uses spectrophotometry based on the Beer-Uses spectrophotometry based on the Beer-Lambert lawLambert law

• Differentiates oxy- from deoxyhemoglobin by the Differentiates oxy- from deoxyhemoglobin by the differences in absorption at 660nm and 940nmdifferences in absorption at 660nm and 940nm

• Minimizes tissue interference by separating out Minimizes tissue interference by separating out the pulsatile signal the pulsatile signal

• Estimates heart rate by measuring cyclic changes Estimates heart rate by measuring cyclic changes in light transmissionin light transmission

• Measures 4 types of hemoglobin: deoxy, oxy, Measures 4 types of hemoglobin: deoxy, oxy, carboxy, and metcarboxy, and met

• Estimates functional hemoglobin saturation: Estimates functional hemoglobin saturation: oxyhemoglobin/deoxy + oxyoxyhemoglobin/deoxy + oxy

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UABSORPTION SPECTRAABSORPTION SPECTRA

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USOURCES OF ERRORSOURCES OF ERROR

• Sensitive to motionSensitive to motion• Standard deviation is certified to 4% down to Standard deviation is certified to 4% down to

70% saturation70% saturation• Sats below 85% increase the importance of Sats below 85% increase the importance of

error in the readingerror in the reading• Calibration is performed by company on normal Calibration is performed by company on normal

patients breathing various gas mixtures, so patients breathing various gas mixtures, so calibration is certain only down to 80%calibration is certain only down to 80%

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• Skin PigmentationSkin Pigmentation– Darker color may make the reading more Darker color may make the reading more

variable due to optical shunting.variable due to optical shunting.– Dark nail polish has same effect: blue, black, Dark nail polish has same effect: blue, black,

and green polishes underestimate and green polishes underestimate saturations, while red and purple have no saturations, while red and purple have no effecteffect

– Hyperbilirubinemia has no effectHyperbilirubinemia has no effect• Low perfusion stateLow perfusion state• Ambient LightAmbient Light• Delay in reading of about 12 secondsDelay in reading of about 12 seconds

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• Methylene blue and indigo carmine Methylene blue and indigo carmine underestimate the saturationunderestimate the saturation

• Dysfunctional hemoglobinDysfunctional hemoglobin– Carboxyhgb leads to overestimation of sats Carboxyhgb leads to overestimation of sats

because it absorbs at 660nm with an because it absorbs at 660nm with an absorption coefficient nearly identical to absorption coefficient nearly identical to oxyhgboxyhgb

– Methgb can mask the true saturation by Methgb can mask the true saturation by absorbing too much light at both 660nm and absorbing too much light at both 660nm and 940nm. Saturations are overestimated, but 940nm. Saturations are overestimated, but drop no further than 85%, which occurs drop no further than 85%, which occurs when methgb reaches 35%.when methgb reaches 35%.

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• Affect of anemia is debatedAffect of anemia is debated• Oxygen-Hemoglobin Dissociation CurveOxygen-Hemoglobin Dissociation Curve

– Shifts in the curve can affect the readingShifts in the curve can affect the reading– Oximetry reading of 95% could correspond Oximetry reading of 95% could correspond

to a Pto a PaaOO22 of 60mmHg (91% saturation) or of 60mmHg (91% saturation) or 160mmHg (99% saturation)160mmHg (99% saturation)

06 capnography and pulseoximetry

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