Capnography is the practice of recording the amount of carbon dioxide present over time using a capnogram, which is a graphic representation of carbon dioxide levels. The capnogram shows carbon dioxide levels during inhalation and exhalation, and any deviations from a normal waveform can indicate physiological or equipment issues. There are different types of capnographs that can be main-stream, measuring carbon dioxide directly from the breathing circuit, or side-stream, measuring it from a sampling tube. Capnography has various clinical applications like checking endotracheal tube placement and monitoring patient ventilation.

1. Capnography Erika A. Gibson, DVM Michigan State University Small Animal Clinical Sciences

2. 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

3. Physiology Just after inhalation, the lungs are rapidly filled with oxygen and other gases (negligable CO 2 ) CO 2 diffuses across capillaries and into the alveoli as O 2 moves from alveoli to capillaries As exhalation begins, CO 2 rich air is expelled from first the upper and then the lower airways The capnogram represents the movement of CO 2 over time All normal, healthy patients should ideally produce identical capnograms

4. 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 CO 2 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

5. Types of Capnographs Trend capnograph Volume capnograph Time capnograph displays the partial pressure of CO 2 as a function of time Main-stream Side-stream

6. Main-stream CO 2 monitor is located between the ET tube and the breathing circuit Changes in vapor pressure do not affect the CO 2 concentration Immediate recording of patient’s CO 2 concentration Sensor is expensive to clean, heavy, and can become clogged with secretions

7. Side-stream CO 2 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 CO 2 pressure Can be used in awake patients

8. 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

9. 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 CO 2 concentration Phase IV Exhalation Compromised thoracic compliance

10. Waveform Evaluation Height Evaluate the partial pressure of CO 2 Assess ventilatory capability Baseline Is soda lime adequately removing CO 2 ? Shape Are slopes too steep? To gradual?

11. Hyperventilation Progressively lower plateau (phase II) segment Baseline remains at zero Decreasing CO 2 levels

12. Hypoventilation Steady increase in height of Phase II Baseline remains constant

13. Spontaneous Ventilation Short Alveolar plateau Increased frequency of waveforms

14. Cardiogenic Oscillations Ripples during Phase II and Phase III Due to changes in pulmonary blood volume and ultimately CO 2 pressure as a result of cardiac contractions

15. 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

16. Inspiratory Valve Malfunction Increasing elevation of baseline Increasing elevation of Phase II Smaller waveform represents rebreathing of CO 2

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

18. Rebreathing of Soda Lime Contamination with CO 2 Elevation of Phase II segment and baseline Elevation of baseline and Phase II, smaller inspiratory efforts Progressive elevation of Phase II and baseline

19. Bain System Smaller wave form represents rebreathing of CO 2

20. Slow ventilation Incompetent inspiratory valve Prolongation of Phase 0

21. Images reprinted from Capnography.com homepage Designed by Bhavani Shankar Kodali