rebreathing circuits,

The case report on “Failure To Detect CO2-absorbent Exaustion:  Seeing and Believing” by Pond et al is an interesting reading.1  However, the interpretation of the abnormal capnogram as obtained by the authors (figure above) warrants further analysis.

First of all, the differential diagnosis of the abnormal capnogram could be a curare cleft capnogram (if the second peak occurred during expiration), or a rebreathing capnogram (if the second peak occurred during inspiration).2  Understandably, the authors were not certain if the second peak was a part of expiration or inspiration because the capnographs do not have a device yet for marking inspiration and expiration on the time capnogram, thereby delaying the diagnosis of the problem.3,4

Secondly, the mechanism underlying the causation of the abnormal capnogram needs further elucidation. If it was simple dilution of expired gas by the fresh gas, then it should have resulted in a capnogram similar to the computer generated capnogram (fig.3) with a uniform rise in the horizontal baseline.  However, the capnogram obtained by the authors resembles very much  a ‘signature capnogram,’  which is classically obtained during the use of rebreathing circuits such as Bain anesthetic system or Mapelson D circuit.2 In the absence of CO2 absorption, the closed circuit functions as a rebreathing circuit. The only difference between the two circuits, under the circumstances described, is that the fresh gas flow is added to the circuit at the patient end in the Bain or Mepelson D system, whereas it is at the machine end in the circle system. Therefore, before the next inspiration, the inspiratory limb of the closed system consists of predominantly CO2 rich gases at the patient end, and predominantly fresh gases (gases accumulated  during the expiratory pause resulting in a further decrease of  the CO2 concentration) towards the machine end of the inspiratory limb.  During inspiration (shown in     ), the relative differences in the CO2 concentration in the inspired limb results in an inspiratory peak (which is less than the expiratory peak), followed by an inspiratory dip in the CO2 concentration, thereby producing the characteristic shape of the capnogram (fig 1).

Lastly, a faulty expiratory valve has been stated as one of the differential diagnosis of the abnormal capnogram. But a sharp decline in the CO2 concentration at the end of the expiratory plateau (the angle between the plateau and the inspiratory downstroke, also known as beta angle, of nearly 90 degrees) rules out this possibility.3,4,5 The beta angle is more obtuse in the capnograms obtained during valve dysfunction. Moreover, the expiratory plateau is usually prolonged in these circumstances to include a part of inspiration as well. 3,4

(Animated version of the capnogram produced with permission from authors reference 1)

 References:

1.   Pond D, Jaffe RA, Brock-Utne JG. Failure to detect CO2-absorbent exhaustion: Seeing and Believing. Anesthesiology 2000;92:1196-8.

2.   Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y.  Capnometery and Anaesthesia: Review article.   Can J Anaesth 1992;39:6:617-32.

3.    Yasodananda Kumar A, Bhavani-Shankar K, Moseley  HSL, Delph Y.  Inspiratory valve malfunction in a circle system: pitfalls in capnography. Can J Anaesth 1992;39:997-9.

4.    Bhavani-Shankar K, Kumar AY, Moseley HSL, Ahyee-Hallsworth R.  Terminology and the current limitations of time capnography: A brief review. J Clin Monit 1995;11:175-82.

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