Abstract
An atypical "tails-up" capnograph pattern was noticed in a patient during the use of an accidentally crushed sampling tube with a slit-like hole. We investigated the mechanics involved in the observed capnograph pattern. Forty consenting ASA I patients of both sexes presenting for tonsillectomy were included in this study. After intravenous induction of anaesthesia, intermittent positive pressure ventilation (IPPV) using a mechanical ventilator was maintained for 20 min and the capnograph trace, ETCO2, and inspiratory/expiratory sevoflurane were compared using a breached sampling tube and then an intact sampling tube. Similar comparisons were made during spontaneous breathing. During IPPV, an atypical "tails-up" capnograph was noted using the breached sampling tubing. At similar inspiratory sevoflurane (2.0 +/- 0.03) levels, expiratory levels (0.9 +/- 0.03) were significantly lower when using the breached sampling tube than the intact 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 shape with both sampling tubes, but ETCO2 and both insp./exp. sevoflurane levels were lower with the breached sampling tube. During IPPV, pressure in the breathing circuit is lower during exhalation, thus allowing air to enter through the slit-like hole in the sampling tube causing erroneously low ETCO2 and expiratory sevoflurane. With inspiration, positive pressure in the breathing circuit, transmitted to the sampling tube, prevents air admixture and the upsurge in CO2 is displayed giving the capnograph an atypical "tails-up" appearance. During spontaneous breathing, since pressure in the breathing circuit barely becomes positive during exhalation and is negative during inspiration, air mixes with the sampled gas during both phases and so the capnograph shape was normal but with lower values for ETCO2, insp./exp. sevoflurane, and nitrous oxide levels. If undiagnosed, this defect in the sampling tube can lead to significant errors in the measurement of inspired and expired gas concentrations.
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