(2) Adequacy of spontaneous respiration  

Capnography can be used to monitor the adequacy of spontaneous ventilation, not only during general anesthesia and recovery but also in the awake unintubated patient either in the intensive care unit or during regional anesthesia. In addition, C0₂ monitoring can serve as an apnea monitor. The samples can be drawn from the nasal cavity using nasal adaptors or cannulae.^1-4 Gases can also be sampled from the nasal cavity during the administration of oxygen using a simple modification of the standard nasal cannulae. End-tidal C0₂ tension, thus measured, is a good predictor of PaC0₂ even when oxygen is being administered simultaneously.^2,4   This may be of particular benefit in monitoring the ventilatory status of patients with chronic respiratory failure where excessive oxygen therapy can produce C0₂ narcosis. However, the major limiting factor is the admixture of end-tidal gas with air or insufflated oxygen resulting in a falsely low PETCO₂ particularly in mouth breathing patients, or in those who may require more than 4 1. min-l of nasal oxygen, or in patients who are hypoventilating.^2,4

(3) Integrity of anesthetic apparatus

Anesthetic mishaps due to airway problems, leaks and disconnections in the anesthesia system often develop and may become apparent only when a crisis occurs. Circuit leaks which decrease the minute volume may not be indicated by airway pressure monitoring but may be detected by C0₂ monitoring because the PETCO₂ gradually increases. Airway pressure monitors used to detect breathing system leaks occasionally fail to detect some disconnections. Under these circumstances a C0₂ monitor would detect disconnection instantaneously in paralyzed patients.⁵ Carbon dioxide monitoring gives an early warning of C0₂ retention by the patient due to a faulty Bain anesthetic system, an exhausted C0₂ absorbent in a semi-closed anesthetic system, leaks in the anesthetic system, disconnections within the anesthetic machine or malfunction of valves in circle anesthetic systems.^5-12

Further, a total occlusion or accidental extubation of the endotracheal tube results in an abrupt decrease in PETCO₂, whereas a partially kinked or obstructed tube can result in either increased or decreased PETCO₂, or in no change in PETCO₂ depending on the severity of the obstruction.^6,13 Capnography is considered more valuable than capnometry in detecting partially kinked endotracheal tubes, as distortions in C0₂ waveforms (prolonged phase II, steeper phase III, irregular height of the CO₂ waveforms) occur earlier than changes in PETCO₂.^11,13,14   However, it should be noted that endotracheal tube obstruction must be severe (at least 50% occlusion) to produce changes in PETCO₂ or in the C0₂ waveforms.^11,13

(4) Adjustments of fresh gas flow rates (FGF) in rebreathing systems   

The FGF’s required with various rebreathing systems during anesthesia can be adjusted precisely by continuous monitoring of PETCO2 and  doing so prevents  hypercarbia due to inadequate flow rates.  

References:

1. Lenz G, Heipertz W, Epple E. Capnometry for continuous postoperative monitoring of nonintubated, spontaneously breathing patients. J Clin Monit 1991;7:245-8.

2. Roy J, McNulty SE, Torjman MC. An improved nasal prong apparatus for end-tidal carbon dioxide monitoring in awake, sedated patients. J Clin Monit 1991;7:249-52.

3. Brampton WJ, Watson RJ. Arterial to end-tidal carbon dioxide tension difference during laparoscopy. Anaesthesia 1990;45:210-4.

4. Bowe EA, Boyson PG, Broome JA, Klein Jr EF. Accurate determination of end-tidal carbon dioxide administration of oxygen by nasal cannlae. J Clin Monit 1989;5:105-10.

5. Carbon dioxide monitors. Health Devices 1986;15:255-85.

6. Cote CJ, Liu LMP, Szyfelbein SK et al. Intraoperative events diagnosed by expired carbon dioxide monitoring in children. Can Anaesth Soc J 1986;33:315-20.

7.  Lillie PE, Roberts JG.  Carbon dioxide monitoring. Anaesth Intensive Care 1988;16:41-4.

8. Martin DG. Leak detection with a capnograph. Anaesthesia 1987;42:1025.

9. Pyles, ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle  anesthesia circuit-verification by analysis of carbon dioxide waveform. Anesthesia Analgesia 1984;63:536-7.

10.Berman LS, Pyles ST. Capnographic detection of anaesthesia circle valve malfunction.  Can J Anaesth 1988;35:473.5

11. Van Genderingen HR, Granvenstein N, Van der Aa JJ, Gravenstein JS, Computer-assisted capnogram analysis. J Clin Monit 1987;3:194-200.

12. Weingarten M. Respiratory monitoring of carbon dioxide and oxygen: a ten-year perspective. J Clin Monit 1990;6:217-25.

13. Murray IP, Modell JH. Early detection of endotracheal tube accidents by monitoring carbon dioxide concentration in respiratory gas. Anesthesiology 1983;59:344-6.

14. Smallhout B, Kalenda Z. An Atlas of Capnography. 2nd ed, Utrecht:Kerckebosch-Zeist, 1981.