Capnogram

Dr.C.N.Chandra Sekhar M.D

Definitions

Capnometry: Measurement and numerical display of CO2 level during resp.cycleCapnometer: Device that performs the measurement and displays the readings.Capnography: Graphic record with display on a screen or paper of carbondioxide concentration.Capnograph: the machine that generates the waveformCapnogram: the actual waveform.

CAPNOGRAPHIC DEVICES

Infrared Absorption Photometry

Colorimetric Devices

Mass Spectrometry

Raman Scattering

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

Pa is fraction of light absorbed is absorption coefficientD is distance light travels though the gasC is molar gas concentration

The higher the CO2 concentration, the higher the absorption.

CO2 absorption takes place at 4.28 µm

N2O, H2O, and CO can also absorb at this wavelengthTwo types: side port and mainstream

ABSORPTION BANDS

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

Ambient air leaks

MAINSTREAMSensor is located in the airwayResponse time as little as 40msecVery accurateDifficult to calibrate without disconnecting (makes it hard to detect rebreathing)More prone to the reading being affected by moistureLarger, can kink the tube.Adds dead space to the airwayBigger chance of being damaged by mishandling

COLORIMETRICContains 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 rateHigher false negative rateAcidic solutions, e.g., epi, atropine, lidocaine, will permanently change the colorDead space relatively high for neonates, so don’t use for long periods of time on those patients.

Physiology

Just after inhalation, the lungs are rapidly filled with oxygen and other gases (negligable CO2)

CO2 diffuses across capillaries and into the alveoli as O2 moves from alveoli to capillaries

As exhalation begins, CO2 rich air is expelled from first the upper and then the lower airways

The capnogram represents the movement of CO2 over timeAll normal, healthy patients should ideally produce identical capnograms

Physiology (continued)

Any deviation from a normal wave is due to an altered physiological state, a pathological state, or equipment malfunctionAny factor that affects the production, diffusion, elimination or partial pressure of CO2 will affect the capnogramThe anesthetist must be able to distinguish between abnormal waveforms due to equipment failure/malfunction from those due to physiological conditions

Normal Capnogram

Normal Capnogram

Phase I is the beginning of exhalationPhase I represents most of the anatomical dead spacePhase II is where the alveolar gas begins to mix with the dead space gas and the CO2 begins to rapidly riseThe anatomic dead space can be calculated using Phase I and IIAlveolar dead space can be calculated on the basis of : VD = VDanat + VDalv

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

Normal Capnogram

Phase III corresponds to the elimination of CO2 from the alveoliPhase III usually has a slight increase in the slope as “slow” alveoli emptyThe “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

PaCO2-PetCO2 gradientUsually <6mm Hg

PetCO2 is usually lessDifference depends on the number of underperfused alveoliTend to mirror each other if the slope of Phase III is horizontal or has a minimal slopeDecreased cardiac output will increase the gradientThe gradient can be negative when healthy lungs are ventilated with high TV and low rateDecreased FRC also gives a negative gradient by increasing the number of slow alveoli

Uses

MetabolicAssess energy expenditure

CardiovascularMonitor trend in cardiac outputCan use as an indirect Fick method, but

actual numbers are hard to quantifyMeasure of effectiveness in CPRDiagnosis of pulmonary embolism:

measure gradient

PULMONARY USESEffectiveness 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.

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

Confirm ET tube placement

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

ABNORMALITIES

Increased Phase III slope Obstructive lung

diseasePhase III dip Spontaneous resp

Horizontal Phase III with large ET-art CO2 change Pulmonary embolism cardiac output Hypovolemia

Sudden in ETCO2 to 0 Dislodged tube Vent malfunction ET obstruction

Sudden in ETCO2 Partial obstruction Air leak

Exponential Severe hyperventilation Cardiopulmonary event

ABNORMALITIESGradual Hyperventilation Decreasing temp Gradual in

volumeSudden increase in ETCO2 Malignant Hyperthermia Increased Metabolic

states Sodium bicarb

administration Release of limb

tourniquet

Gradual increase Fever Hypoventilation

Increased baseline Rebreathing Exhausted CO2

absorber

HyperventilationProgressively lower plateau (phase II) segment

Baseline remains at zero

Decreasing CO2 levels

HypoventilationSteady increase in height of Phase IIBaseline remains constant

Spontaneous Ventilation

Short Alveolar plateau

Increased frequency of waveforms

Cardiogenic OscillationsRipples during Phase III and Phase IV

Due to changes in pulmonary blood volume and ultimately CO2 pressure as a result of cardiac contractions

Curare CleftShallow dips in phase III plateauCan occur when patient is in a light plane of anesthesia

Represent patient attempts to breathe independent of mechanical ventilation

Inspiratory Valve Malfunction

Increasing elevation of baseline

Increasing elevation of Phase II

Smaller waveform represents rebreathing of CO2

BronchospasmAirway Obstruction

COPDSloping of inspiratory and expiratory segments

Prolonged Phase II and Phase III

Rebreathing of Soda LimeContamination with CO2

Elevation of Phase II segment and baseline

Elevation of baseline and Phase II, smaller inspiratory efforts

Progressive elevation of Phase II and baseline

Bain System

Smaller wave form represents rebreathing of CO2

Slow ventilationIncompetent inspiratory

valveProlongation of Phase IV

Images reprinted from Capnography.com homepage

Designed by Bhavani Shankar Kodali

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