Anaesthesiologists interpret the patient state by obtaining useful information from a monitor screen. In particular, measurements of carbon dioxide levels related to artificial respiration greatly contribute to the safety of the anaesthetized patient.1,2
We present this report because special attention should be given to the interpretation of carbon dioxide waveforms on the monitor.
Carbon dioxide curves provide much information on the patient state. On the basis of this information, anaesthesiologists can identify the presence of both artificial and spontaneous respiration. However, when an irregular carbon dioxide curve at the early recovery stage of anaesthesia changes into a regular one as anaesthesia recovery progresses as in Fig. 1, care must be taken to accurately assess the patient state. When carbon dioxide concentration is low at the initial anaesthesia recovery, it is estimated that ventilation is minimal. However, while spontaneous respiration resumes after slight anaesthesia recovery, the carbon dioxide curves sometimes appear in regular patterns and the value is high as in Fig. 1. At that time, tidal volume, that is, the flow rate, is not related to carbon dioxide value. For this reason, tidal volume is either extremely low as in Fig. 1(a) or rather high as in Fig. 1(b). The discrepancy between the carbon dioxide curve and the flow rate may be explained by the fact that the carbon dioxide curve does not represent the absolute value, but the ratio of carbon dioxide to the total expiratory gas. The irregular carbon dioxide basically originates from abrupt initiation of inspiration and expiration during early recovery, which also cause abrupt change of the ratio of carbon dioxide to the total expiration gas. Although tidal volume is relatively small as in Fig. 1(a), it can be presented as such regular patterns and high value if the patient breathes regularly and the ratio of carbon dioxide to the total expiratory gas is periodically changed. Likewise, if carbon dioxide concentration does not increase despite the fact that tidal volume increases from the value in Fig. 1(a) to that in Fig. 1(b), the carbon dioxide curve will be unchanged. In Fig. 1, the carbon dioxide curve indicates that expiratory gas contains approximately 4% (=35/760) carbon dioxide, but it gives little information on tidal volume. Therefore, after anaesthesia recovery progresses to some extent, we should evaluate tidal volume by the movement of the bellows or the patient chest as well as the flow-time curve displayed on a spirometre in an anaesthesia machine instead of the carbon dioxide curve.
1 Goldman JM, Gravenstein JS, Paulus DA, Hamburger A. Capnography during anesthesia. In: Gravenstein JS, Jaffe MB, Paulus DA, editors. Capnography
. New York: Cambridge University Press; 2004. pp. 47–58.
2 Ross WT. Monitoring the anesthesia machine and respiratory gases. In: Lake CL, Hines RL, Blitt CD, editors. Clinical monitoring
. Philadelphia: W.B. Saunders; 2001, pp. 293–314.