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Uptake of Nitrous Oxide By CORP~ESPONDENCE 447 The basic modification allows a greater versatil- Stethoscope ity of this common piece of equipment in everyday practice. The Luer connector itself may prove modification invaluable in an emergency situation. To the Editor: Jean-Francois Hardy MI) I would like to present a modification of one's Resident in Anaesthesia personal stethoscope, consisting of the insertion of Universit6 de Montrdal a Luer connector between the ear set and the 5000 est, rue B~langer stethoscope head (Figure). This arrangement was Montr6al, Qu6bec, HIT 1C8 devised to provide an adequate and comfortable ear set, always immediately available, for use with REFERENCE oesophageal stethoscopes. Also, when access is 1 Scuderi PE, McLeskey CH, Comer PB. Emergency possible, the stethoscope head can be left taped to percutaneous transtraeheal ventilation during the chest and monitored as needed. This improves anesthesia using readily available equipment. oesophageal and precordial chest auscultation, in- Anesth Analg 1982; 61: 867-70. asmuch as the ear set and stethoscope head are of a higher quality than that of the equipment usually available in the operating room. Another benefit of this arrangement is that one may vary the length of the tubing at will to suit specific monitoring needs or Uptake of nitrous personal preference. Lastly, in emergency situations, the male end of oxide by man the Luer connector is truly instantly available to set up a device for percutaneous transtracheal ventila- To the Editor: tion similar to that described previously, t The I wish to comment on the recent article by Virtue et connector can be fitted to the fresh gas hose from the al. ~ concerning the uptake of nitrous oxide in man. anaesthesia machine, or to a length of standard The rate of uptake of nitrous oxide in man was first oxygen tubing, and attached to the hub of a 14 described in 1954 by Severinghaus who estimated gauge intravenous catheter inserted into the trachea. uptake to be above 1000 ml in the first minute with a Ventilation is carried out by intermittently activa- rapid reduction to a level of 150 ml-min-~ within the ting the flush system on the anaesthesia machine. next few minutes and approximated by the power function 1000 x t-q. 2 This rate of uptake was measured at the mouthpiece and not at the alveolar membrane, however, so that the effect of wash-in into the functional residual capacity of the lung was overlooked. The functional residual capacity of the lung in a given patient is a relatively fixed non- collapsible space and the wash-in of any anaesthetic gas, such as nitrous oxide, takes place according to a simple exponential function. This wash-in should not be considered as body uptake. However, true body uptake of inhalation anaesthetics takes place across the alveolar membrane and closely follows Fiek's principle of diffusion which, in this case, indicates that the uptake depends on the inspired concentration-mixed venous concentration differ- FIGURE 1 Modified stethoscope. Note that the male end ence, on the existence of a membrane factor, and on of the Luer connector is on the side of the stethoscope head. cardiac output, s 448 CANADIAN ANAESTHETISTS ~ SOCIETY JOURNAL Recently, many advocates of closed circuit Our previous papers s'gA~ were intended to shed anaesthesia have tried to provide inhalation anaes- light on this subject and to draw attention to the fact thetics according to a power function curve 3'4 with that one should not forget the existence of the FRC the priming volume of anaesthetic agents into the and the alveolar membrane when one considers the circuit and FRC being considered separately. How- uptake of inhalation anaesthetic in order to avoid ever, the power function already includes the over-estimation of the clinical significance of sec- amounts of inhalation anesthetics required to fill the ond gas effect and diffusion hypoxia. I strongly FRC. believe that only the inhalation anaesthetics that In the past, some aspects of the uptake of cross the alveolar membrane should be considered inhalation anaesthetics, such as the second gas as contributing to body uptake, since FRC wash-in effect6'7 and diffusion hypoxia, s have been based and uptake (alveolar) have totally different kinetics. on the premise that large quantities of nitrous oxide Besides my major objection to the definition of are taken up by the body from the start of admin- body uptake, I also want to raise several minor istration of an anaesthetic gas or given up equally as questions concerning the article: rapidly at the end. The point at issue, then, is the 1. The authors state that "The mass spectrometer rate of uptake of nitrous oxide in man. Is nitrous was calibrated for volume and concentration of all oxide uptake above 1000 rnl in the first minute and gases used." The mass spectrometer per se is not is body uptake approximated by a function of the capable of measuring gas volume; the authors square root of time? Virtue et al. agree with us that a should have stated that a hot wire was used for large volume of inhalation anaesthetic is required to measurement of the total volume of gas and not that fill the functional residual capacity of the lung of the individual gases. initially;9 this volume represents most of the initial 2. "The total nitrogen exhaled after breathing portion of the t-~ curve. This volume should not be nitrogen-free mixtures (estimate of Functional considered as uptake. Virtue et al. then state that the Residual Capacity) was used with the measurement uptake, adjusted for a 70 kg subject inhaling 75 per of the end-tidal nitrous oxide to compute the nitrous cent nitrous oxide, agreed well with absorption of oxide volume in the pulmonary stores on a breath- about 1000 ml of nitrous oxide during the first to-breath basis." This indicates that the nitrous minute and decreased approximately according to oxide experiment was carried out separately from the square root of time in minutes. The major the FRC measurement. How can one be sure that the change in rate occurring in the square root of time same volume gas exchange occurred during the two function is at the beginning. The rate can be sets of experiments with spontaneous respiration? considered near constant after the initial first few 3. During spontaneous respiration, the alveolar minutes of rapid change, depending on how accu- nitrous oxide concentration cannot be assumed to rate an approximation is expected. Since a major stay constant outside the alveolar membrane part of the initial t-t curve represents the anaesthetic because there is no assurance the anaesthetic supply volume required to fill the functional residual will exceed the uptake at each respiratory cycle. capacity, we should not use the square root of time Finally, I would like to re-emphasize that we function to describe or approximate the rate of body should consider only the amounts of anaesthetic gas uptake. In fact, the first minute uptake is less than crossing the alveolar membrane to account for body the second minute, which is as it should be, since uptake. one should not expect the maximum rate of uptake until the FRC wash-in is nearly complete. When an Chung-Yuan Lin rat anaesthetic gas mixture is washed into the FRC, it Department of Anesthesiology must be exchanged with gases there. During this University of Chicago Hospitals & Clinics exchange, some of the anaesthetic gas mixture will Chicago, Illinois 60637 be absorbed from the lungs, but only to a limited degree, because the anaesthetic gas has not reached its peak inspiratory concentration due to dilution by the pre-existing gases in the lung. CORRESPONDENCE 449 REFERENCES and the hot-wire flowmeter are at the same physical lace. 1 Virtue R, Sherrill DL, Swanson GD. Uptake of tion in the middle of the tube. Thus, there is no phase de- nitrous oxide by man. Can Anaesth Sac J 1982; 29" lay between the flow signal and the gas fraction signal. An on-line computer processes these signals directly 424-7. every 20 ms to yield breath-to-breath gas exchange data. 2 Severinghaas JW. Rate of uptake of nitrous oxide in Our method ~ for estimating alveolar gas exchange man. J Clin Invest 1954; 33: 1183-9. (gas exchange from the lung to blood) utilizes a version of 3 AIdrete JA, Lowe HJ, Virtue RW. Low flow and the Auchincloss algorithm, 6 which is also used by other closed system anesthesia. New York, Grune and groups, z It determines the uptake of the mouth and subtracts an estimate of nitrous oxide taken into the Stratton, 1979. pulmonary stores (FRC) using approximately one per 4 Lowe HJ, Ernst EA. The quantitative practice of cent argon as a tracer gas. We and others have carried anesthesia. Baltimore, William and Wilkins, chapter out an extensive analysis of these methods. 7"~ 5 pp 67-97, 1981. Dr. Lin has suggested that our "nitrous oxide experi- 5 Lin CY. Nitrous oxide uptake in adults (Abstract). ment was carried out separately from the FRC measure- ment." However, this was not the case. The subjects, Anesthesiology 1982; 57: $372. breathing room air, were switched onto the oxygen 6 El)stein RM, Rackow H, Salanitre E, et al. In- nitrous oxide mixture at time zero. Nitrogen wash-out fluence of 1he concentration effect on the uptake was then measured simultaneously with the nitrous oxide anesthetic mixture: The Second gas effect. Anes- wash-in, while the room air argon concentration was thesiology 1964; 25: 364-71. maintained. Real time measurements were combined off-line to yield our nitrous oxide uptake estimates.
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