Atypical "tails-up" capnograph due to breach in the sampling tube of side-stream capnometer

Objective. An atypical "tails-up" capnograph pattern was noticed in a patie nt during the use of an accidentally crushed sampling tube with a slit-like hole. We investigated the mechanics involved in the observed capnograph pa ttern. Methods. Forty consenting ASA I patients of both sexes presenting fo r tonsillectomy were included in this study. After intravenous induction of anaesthesia, intermittent positive pressure ventilation (IPPV) using a mec hanical ventilator was maintained for 20 min and the capnograph trace, ETCO 2, and inspiratory/expiratory sevoflurane were compared using a breached sa mpling tube and then an intact sampling tube. Similar comparisons were made during spontaneous breathing. Results.During IPPV, an atypical "tails-up" capnograph was noted using the breached sampling tubing. At similar inspira tory sevoflurane (2.0 +/- 0.03) levels, expiratory levels (0.9 +/- 0.03) we re significantly lower when using the breached sampling tube than the intac t tube (1.7 +/- 0.03). ETCO2 with the breached sampling tube (26.8 +/- 0.30 mmHg) showed significantly lower values than with the intact sampling tube (37 +/- 0.3). During spontaneous breathing, the capnograph was normal in s hape with both sampling tubes, but ETCO2 and both insp./exp. sevoflurane le vels were lower with the breached sampling tube. Conclusion.During IPPV, pr essure in the breathing circuit is lower during exhalation, thus allowing a ir to enter through the slit-like hole in the sampling tube causing erroneo usly low ETCO2 and expiratory sevoflurane. With inspiration, positive press ure in the breathing circuit, transmitted to the sampling tube, prevents ai r admixture and the upsurge in CO2 is displayed giving the capnograph an at ypical "tails-up" appearance. During spontaneous breathing, since pressure in the breathing circuit barely becomes positive during exhalation and is n egative during inspiration, air mixes with the sampled gas during both phas es and so the capnograph shape was normal but with lower values for ETCO2, insp./exp. sevoflurane, and nitrous oxide levels. If undiagnosed, this defe ct in the sampling tube can lead to significant errors in the measurement o f inspired and expired gas concentrations.