key: cord-0949033-t7icsuub authors: Diehl, J.-L.; Chamoun, C.; Mercat, A.; Richard, J.C.; Guérot, E. title: An unusual sidestream capnogram date: 2017-12-13 journal: Br J Anaesth DOI: 10.1093/bja/aex113 sha: d1ac080c71a85afd74d09378f42fd986a5751b18 doc_id: 949033 cord_uid: t7icsuub nan further by the authors. Of the six videolaryngoscopes, there are five with highly angulated blades, namely C-MAC D-Blade, GlideScope, KingVision, Airtraq, and A.P. Advance with difficult airway blade. In contrast, the McGrath as the sixth videolaryngoscope was chosen with a MAC blade. Even though the authors stated in their methodology paper 2 that the McGrath MAC is a further development of the original McGrath Series 5, the blade design of the two series differs significantly. The McGrath MAC used in the present study has a Macintosh-type blade that is very similar to a conventional laryngoscope blade that anaesthesiologists use in their daily practice. In contrast, the McGrath Series 5 has a highly angulated blade that would have compared much better with the other five videolaryngoscopes in the present study. Thus, one Macintosh-type blade almost every anaesthesiologist is familiar with was compared with five curved blades. Proper handling of curved blades requires a significant amount of training. The important difference between Macintosh-type and highly angulated blades with regard to handling and intubation success has been addressed by several studies. 3 4 However, this aspect was not addressed further in the present study. Readers might become puzzled because of the incomplete description of the videolaryngoscopes used in this study. In the flowchart, use of the C-MAC is clearly stated as D-Blade, both GlideScope and Airtraq are self-explainable curved blades, and the McGrath blade type has been discussed above. However, A.P. Advance and KingVision have no further description of their blade types, at least in the flowchart, so that it is not clear whether the A.P. Advance is used with the Mac-blade (as usually done) or the difficult airway blade, and the KingVision could also be used with a non-channelled blade. Exact classification of the studied material is of paramount importance, especially for A.P. Advance and KingVision, for which few data are available. Finally, the number of missing data for insertion of the device into the oropharynx, quality of view, and ease of tube insertion are not proportional between the different devices tested, and ranges between 9 (A.P. Advance) and 0 (McGrath). None declared. Editor-We report an unusual record of exhaled CO 2 observed during end-expiratory occlusion in a chronic obstructive pulmonary disease patient receiving invasive mechanical ventilation because of a severe acute exacerbation attributable to a bacterial pneumonia. As the patient fulfilled moderate acute respiratory distress syndrome criteria, he was deeply sedated and under neuromuscular block. The 70-yr-old male patient was ventilated (assist-controlled) with a CARESCAPE R860 ventilator (GE Healthcare, Madison, WI, USA). An end-expiratory occlusion manoeuvre was performed to determine the total PEEP level. The duration of the expiratory pause was extended to 15 s, because of the clinical observation of a very unusual record (Fig. 1a) . The airway CO 2 monitoring system (sidestream sampling, aspiration flow 120 ml min À1 as indicated by the manufacturer) displayed at first a so-called 'curare cleft' 1 and further a short 'plateau-like' line, rapidly followed by a decrease in CO 2 fraction, associated with an increase in measured airway O 2 fraction. Indentations were observed during the decreasing phase at a rate very similar to the cardiac frequency. Such a pattern was repeatedly observed over time, even after switching to another CARESCAPE R860 ventilator. Moreover, prolonged expiration was additionally obtained by disconnection of the respiratory line at the Y-piece level, immediately after the port of the CO 2 monitoring device. We then observed the expected pattern, with a slow CO 2 fraction ascending rate, but without reaching a true plateau (Fig. 1b) . The decrease in CO 2 fraction during the extended expiratory pause could be in relation to contamination of the gas sampling by fresh inspiratory gas. A leak between the port of the CO 2 monitoring device and the patient was ruled out by the pattern (increase in CO 2 fraction) observed after disconnection at the Ypiece (Fig. 1b) . A leak between the port of the CO 2 monitoring device and the ventilator was also ruled out because it would have been associated with a decrease in O 2 fraction, attributable to exposure to a gas mixture with 21% fractional inspired O 2 , contrasting with the observed increase in O 2 fraction (Fig. 1a) . Therefore, we believe that the pattern was attributable to contamination by fresh gas on the ventilator side. We assume that the contamination was in relation with the specific configuration of the CARESCAPE R860 ventilator, with a minimal flow-by circuit set at 2 litres min À1 . Accordingly, the aspiration flow of the sidestream CO 2 monitoring system was nearly twice the flow measured between the CO 2 sampling port and the patient. The oscillations observed in the waveforms were probably related to the cardiac beats in a patient under neuromuscular block with low pulmonary compliance, with adiabatic compressions and decompressions, or both. To the best of our knowledge, this is the first report of such a capnographic pattern during a prolonged end-expiratory occlusion manoeuvre. The pattern was probably explained by the sidestream (rather than mainstream) CO 2 monitoring system and by a minimal flow-by circuit set at 2 litres min À1 leading to frank contamination of the sampled gas by fresh gas issued from the ventilator. Clinicians must be aware of this pattern, because it could considerably underestimate end-tidal CO 2 fraction. This is of particular importance in circumstances in which capnographic monitoring has been considered of value, such as pulmonary embolism diagnosis, 2 cardiopulmonary resuscitation, 3 or acute respiratory distress syndrome. 4 Such patterns could also explain, in part, the disappointing results observed for endexpiratory occlusion CO 2 -based prediction of fluid responsiveness in the operating theatre. Principles and Practice of Intensive Care Monitoring Use of capnography in diagnosis of pulmonary embolism during acute respiratory failure of chronic obstructive pulmonary disease Adult advanced cardiovascular life support. AHA guidelines for CPR and emergency cardiovascular care Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome End-expiratory occlusion manoeuvre does not accurately predict fluid responsiveness in the operating theatre Conception, design, and acquisition of data: J.-L.D., C.C., E.G.