AnaesthesiaVolume 58, Issue 2 p. 189-190 Free Access Another use for capnography First published: 23 September 2008 https://doi.org/10.1046/j.1365-2044.2003.03005_12.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat A 55-year-old, 78 kg male patient was scheduled to undergo a laparoscopic Nissen's fundoplication. Pre-operative assessment was unremarkable. Mouth opening was Mallampati class II, neck extension was reduced and dentition was normal. After placement of standard monitoring equipment, a 20G cannula was inserted into the dorsum of the left hand. After pre-oxygenation, general anaesthesia was induced with fentanyl 100 µg and propofol 150 mg. Bag and mask ventilation was straightforward, and vecuronium 10 mg were administered. Direct laryngoscopy revealed a Cormack and Lehane grade 3 view with only the epiglottis visible, despite readjustment of the patient's head position and use of a long-bladed Mackintosh laryngoscope. Nevertheless, intubation was achieved without difficulty by passing a size 8 cuffed tracheal tube over a disposable gum-elastic bougie. Correct placement of the tube was confirmed by auscultation and capnography. The patient's lungs were ventilated using a Penlon Nuffield 200 ventilator attached to a coaxial Mapleson D (Bain) breathing system (inspiratory/expiratory ratio = 1 : 3). Anaesthesia was maintained using isoflurane 1% administered in a mixture of 4 l.min−1 oxygen and 4 l.min−1 nitrous oxide. At these flow rates, the partial pressure of carbon dioxide was measured by capnography as 5.8 kPa at the end of expiration, and 1.3 kPa at the start of inspiration. Subsequent passage of a nasogastric tube proved not to be possible under direct vision. Consequently, passage was attempted ‘blind’, meeting little tissue resistance. In order to confirm the position of the tube, suction was applied to the proximal end of the tube. At this point, it was noticed that the capnograph trace had altered; the partial pressure of carbon dioxide had decreased to 0 kPa at the start of inspiration, and the measured partial pressure of carbon dioxide decreased throughout expiration (Fig. 4). When suction was discontinued, the capnograph trace returned to normal. Auscultation of air injected down the nasogastric tube between tidal volumes confirmed tracheal placement of the nasogastric tube. Figure 4 Open in figure viewerPowerPoint Schematic diagram of changes observed after application and discontinuation of suction. When a Bain circuit is used in intermittent positive pressure ventilation with a Penlon Nuffield 200 ventilator, there is a non-zero baseline because the fresh gas flow is usually less than the patient's minute volume [1]. To prevent significant rebreathing, the fresh gas flow for the system should be at least 2.5 times the minute volume (approximately 20 l.min−1). Anaesthetic suction apparatus should be capable of displacing free air at a flow rate of at least 35 l.min−1[2]. Suction applied to the intratracheal gas mixture (as occurred in this case) is therefore likely to aspirate gas at a rate of 35 l.min−1, and the capnograph will only be capable of recording expired carbon dioxide when the flow rate of carbon dioxide past the sampling port exceeds 35 l.min−1. The expiratory phase of positive pressure ventilation is passive; the expiratory flow rate throughout the expiratory phase declines exponentially, but is initially in excess of 35 l.min−1. Furthermore, the physiological decline in flow rate may explain the abnormal decline in plateau phase CO2 partial pressure observed. It has previously been suggested that carbon dioxide may be measured to detect inadvertent endobronchial placement of nasogastric tubes [3,4]. The effect on the capnograph trace of applying suction to a nasogastric tube, as reported above, is another simple method of detecting nasogastric tube misplacement into the trachea. S. M. White St. Thomas' Hospital, London, SE1 7EH, UK E-mail:igasbest@hotmail.com References 1 Normal and abnormal waveforms. In: O'Flaherty D, ed. Capnography. London: BMJ Publishing Group 1994, 55 – 66. 2 Medical suction apparatus. In: Ward CS, ed. Anaesthetic Equipment . Physical principles and maintenance. London: Ballière-Tindall 1985, 288 – 95. 3 White NA. Confirmation of placement of fine-bore nasogastric tubes. Anaesthesia 2001; 56: 1123. 4 Thomas BW, Falcone RE. Confirmation of nasogastric tube placement by colorimetric indicator detection of carbon dioxide: a preliminary report. Journal of the American College of Nutrition 1998; 17: 195 – 7. Volume58, Issue2February 2003Pages 189-190 FiguresReferencesRelatedInformation
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