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 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

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 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

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

6. 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

7. 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

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 CO2 concentration
Phase IV
Exhalation
Compromised thoracic compliance

10. 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?

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

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

18. 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

19. Bain System
Smaller wave form represents rebreathing of CO2

20. Slow ventilation Incompetent inspiratory valve
Prolongation of Phase 0

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