capnography erika a. gibson, dvm michigan state university small animal clinical sciences
TRANSCRIPT
Capnography
Erika A. Gibson, DVMMichigan State University
Small Animal Clinical Sciences
What is capnography?
Capnography is the practice of recording the amount of carbon dioxide present
A capnogram is a graphic representation of the partial pressure of carbon dioxide over time
Gravenstein, Paulus, Hayes Capnograpy in Clinical Practice © 1989
PhysiologyJust 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 capillariesAs exhalation begins, CO2 rich air is expelled from first the upper and then the lower airwaysThe 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
Types of CapnographsTrend capnographVolume capnographTime capnograph
displays the partial pressure of CO2 as a function of time
• Main-stream
• Side-stream
Main-streamCO2 monitor is located between the ET tube and the breathing circuit Changes in vapor pressure do not affect the CO2 concentration
Immediate recording of patient’s CO2 concentration
Sensor is expensive to clean, heavy, and can become clogged with secretions
Side-streamCO2 travels through a sampling tube to a sensor in the main unitDelayed recording due to time it takes gas to travel through tubeExhaled water vapor pressure affects the recorded CO2 pressure
Can be used in awake patients
Clinical Applications
Check placement of endotracheal tubeMonitor patient ventilationIdentify potentially life-threatening situations (apnea, malignant hyperthermia)Monitor intracranial pressure and ventilation of comatose patients
Phases of the CapnogramPhase I
Expiration
Represents anatomical dead space
Phase II
Expiration
Mixture of anatomical and alveolar dead space
Phase III
Expiration
Plateau of alveolar expiration
Phase 0
Inspiration
Rapid fall
in CO2 concentration
Phase IV
Exhalation
Compromised thoracic compliance
Waveform EvaluationHeight
Evaluate the partial pressure of CO2
Assess ventilatory capability
BaselineIs soda lime adequately removing CO2?
ShapeAre slopes too steep? To gradual?
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 plateauIncreased frequency of waveforms
Cardiogenic OscillationsRipples during Phase II and Phase III
Due to changes in pulmonary blood volume and ultimately CO2 pressure as a result of cardiac
contractions
Curare CleftShallow dips in phase II 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 SystemSmaller wave form represents rebreathing of CO2
Slow ventilationIncompetent inspiratory
valveProlongation of Phase 0
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