Capnography Erika A. Gibson, DVM Michigan 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

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 time All 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 malfunction Any factor that affects the production, diffusion, elimination or partial pressure of CO2 will affect the capnogram The anesthetist must be able to distinguish between abnormal waveforms due to equipment failure/malfunction from those due to physiological conditions

Types of Capnographs Trend capnograph Volume capnograph Time capnograph displays the partial pressure of CO2 as a function of time Main-stream Side-stream

Main-stream CO2 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-stream CO2 travels through a sampling tube to a sensor in the main unit Delayed recording due to time it takes gas to travel through tube Exhaled water vapor pressure affects the recorded CO2 pressure Can be used in awake patients

Clinical Applications Check placement of endotracheal tube Monitor patient ventilation Identify potentially life-threatening situations (apnea, malignant hyperthermia) Monitor intracranial pressure and ventilation of comatose patients

Phases of the Capnogram Phase 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 Evaluation Height Baseline Shape Evaluate the partial pressure of CO2 Assess ventilatory capability Baseline Is soda lime adequately removing CO2? Shape Are slopes too steep? To gradual?

Hyperventilation Progressively lower plateau (phase II) segment Baseline remains at zero Decreasing CO2 levels

Hypoventilation Steady increase in height of Phase II Baseline remains constant

Spontaneous Ventilation Short Alveolar plateau Increased frequency of waveforms

Cardiogenic Oscillations Ripples during Phase II and Phase III Due to changes in pulmonary blood volume and ultimately CO2 pressure as a result of cardiac contractions

Curare Cleft Shallow dips in phase II plateau Can 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

Bronchospasm Airway Obstruction COPD Sloping of inspiratory and expiratory segments Prolonged Phase II and Phase III

Rebreathing of Soda Lime Contamination with CO2 Elevation of Phase II segment and baseline Progressive elevation of Phase II and baseline Elevation of baseline and Phase II, smaller inspiratory efforts

Bain System Smaller wave form represents rebreathing of CO2

Slow ventilation Incompetent inspiratory valve Prolongation of Phase 0

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