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 capillaries

As 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

Images reprinted from Capnography.com homepageDesigned by Bhavani Shankar Kodali