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Prolongation of Apnea Time in Obese Patients—Concerns With Rate of Rise of CO2

Makkar, Jeetinder Kaur MD, DNB, MAMS, FRCA; Singh, Narinder Pal MD, DNB; Singh, Preet Mohinder MD, DNB

doi: 10.1213/ANE.0000000000002391
Letters to the Editor: Letter to the Editor

Published ahead of print August 8, 2017.

Department of Anesthesia and Intensive Care, Post Graduate Institute of Medical Education and Research, Chandigarh, India,

Alchemist Hospital, Panchkula, India

Department of Anesthesia and Intensive Care, All India Institute of Medical Sciences, New Delhi, India

Published ahead of print August 8, 2017.

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To the Editor

In their study titled “Apneic Oxygenation During Prolonged Laryngoscopy in Obese Patients: A Random ized, Controlled Trial of Buccal RAE Tube Oxygen Administration,” Heard et al1 administered buccal oxygen via a modified 3.5-mm Ring-Adair-Elwyn tube apposed to the left internal cheek purportedly to provide a safety margin during potentially difficult intubations. We have a few concerns pertaining to the design of the study.

First, there is no clear definition of the apnea time. The authors appear to have measured the time from the start of induction after which the anesthesia team did not manually or mechanically assist ventilation. This is not the same thing as apnea as it would be unlikely that all patients actually became apneic right after the start of target-controlled infusions. Further, as the authors state that “11 of 20 patients in both arms had at least the first twitch (T1) of a train-of-four present 150 seconds after rocuronium administration,” it is intuitive that some of these patients continued breathing on their own during this period. Thus, the apnea time may have been overestimated and perhaps explains in part their observation that “peak end-tidal CO2 levels remained within a safe range.”

Second, continuous insufflation of gas allows both delivery of oxygen and clearance of CO2 from the point of its measurement, but the rate depends on the device used. Rate-of-rise of CO2 has been plotted in a number of studies using different continuous apneic insufflation conditions.2–4 Except for use of high-flow tracheal cannula at 45 L/min, no device has achieved steady state in CO2 levels.4 Use of intratracheal catheters with flow of 0.5 L/min was associated with increase in CO2 of 1.8 mm Hg/min and transnasal humidified rapid-insufflation ventilatory exchange with 1.2 mm Hg/min.4 Heard et al1 in the present study state that benefits of the 10 L/min flow rate used in their study were probably restricted to pure apneic oxygenation, and there was little contribution from continuous positive pressure ventilation. Authors report a peak end-tidal carbon dioxide of 44 (39–50) mm Hg in the study group, but there is lack of clarity regarding the methodology section. It is important to explain the time point at which Etco2 was measured. If it was measured at first breath after intubation, the almost negligible difference between Etco2 in the standard care group at around 400 seconds and the Etco2 at 750 seconds (almost 6 minutes later) seems implausible based on a simple rate of rise calculation. Further, in the event of such high-flow gas insufflation, description of the method used to measure Etco2 will assure greater robustness of study methodology.

To conclude, confirmation of this technique as a safe and effective method of apneic ventilation should include not only a safe period of reasonable saturation of peripheral oxygen but also a safe rise in Etco2. Reporting the safe rise of CO2 in obese patients with this technique could have been supported with better data.

Jeetinder Kaur Makkar, MD, DNB, MAMS, FRCADepartment of Anesthesia and Intensive CarePost Graduate Institute of Medical Education and ResearchChandigarh,

Narinder Pal Singh, MD, DNBAlchemist HospitalPanchkula, India

Preet Mohinder Singh, MD, DNBDepartment of Anesthesia and Intensive CareAll India Institute of Medical SciencesNew Delhi, India

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1. Heard A, Toner AJ, Evans JR, Aranda Palacios AM, Lauer S. Apneic oxygenation during prolonged laryngoscopy in obese patients: a randomized, controlled trial of buccal RAE tube oxygen administration. Anesth Analg. 2017;124:1162–1167.
2. Eger EI, Severinghaus JW. The rate of rise of PaCO2 in the apneic anesthetized patient. Anesthesiology. 1961;22:419–425.
3. Fraioli RL, Sheffer LA, Steffenson JL. Pulmonary and cardiovascular effects of apneic oxygenation in man. Anesthesiology. 1973;39:588–596.
4. Patel A, Nouraei SA. Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): a physiological method of increasing apnoea time in patients with difficult airways. Anaesthesia. 2015;70:323–329.
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