Anesth Analg 2000;91:1306-1307
© 2000 International Anesthesia Research Society
LETTERS TO THE EDITOR
N2O Usage in Laparoscopic Cases
Clair S. Weenig, MD
University of California San Francisco San Francisco, CA
To the Editor: Diemunsch et al. (1) reported that intraperitoneal N2O (FIP N2O) equilibrates with the inspired N2O in pigs given CO2 pneumoperitoneum under general anesthesia. The author expressed concern regarding the potential of N2O-facilitated combustion. A combustion supporting FIP N2O of 29%, (FET N2O = 66%) was reached in 2 h. Avoidance of "this N2O pollution" was recommended.
I have measured the FIP N2O in 50 laparoscopic gynecology, intraperitoneal hernia repair and cholecystectomy patients. The FET N2O was 65%–70%, the duration of cases from 20 min to 3 h.
At case end, a sterile tubing was connected to the trocar stopcock before deflation of peritoneal gas. The tubing was then connected to a standard end-tidal gas monitor. Peritoneal gas was sampled for approximately 60 s, at which time a stable reading had been achieved.
The smallest FIP N2O observed was 1%–2%, the largest was 9%–10%. Small numbers were associated with shorter cases, large numbers with longer cases. The FIP N2O never exceed 10%, a concentration that does not facilitate combustion. Bowel lumen N2O was not measured and would require a different methodology.
The difference between observations in the pig model and clinical patients may be by a "washout" effect on the increase of FIP N2O caused by the loss of insufflating CO2 around additional trocar incisions used in patients. The seal around the trocar in the pig experiment may have been airtight. It is a common observation that large quantities of insufflating CO2 are required in patient cases to replace lost CO2. Interested parties can easily duplicate my study in the clinical patient setting. All that is required is a sterile IV extension tubing and 60 s of time. If FIP N2O concentrations stay at or below the 10% level during cases of 3 h, perhaps N2O ought not be abandoned.
Those who advocate abandonment of N2O usually advocate use of 100% O2. This same methodology could easily be used to measure the intraperitoneal concentration of O2, a gas that supports combustion at roughly one-half the concentration that does N2O and might increase the risk of combustion rather than decrease it.
References
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Diemunsch PA, Torp KD, Van Dorsselaer T, et al. Nitrous oxide fraction in the carbon dioxide pneumoperitoneum during laparoscopy under general inhaled anesthesia in pigs. Anesth Analg 2000; 90: 951–3.[Abstract/Free Full Text]
Response
Pierre A. Diemunsch, MD,
Sidiki Doumbia, MD, and
Joël Leroy, MD
Hôpitaux Universitaires de Strasbourg 67000 Strasbourg, France
In Response: As stated in our article (1), the aim of our study was to establish the time course of the N2O pollution in a CO2 pneumoperitoneum. This was done under precise experimental conditions, i.e., a 9-h pneumoperitoneum without external leaks and provision of fresh CO2 from the insufflator limited to the amount needed to compensate for peritoneal gas resorption, i.e., 3.5 ± 0.8 L/h, to maintain an intraperitoneal pressure of 12 mm Hg.
It seems unlikely that these experimental conditions, in which FIP N2O ultimately approaches FET N2O, could be met in a usual clinical situation. Conversely, as shown by Neuman et al. (2), time periods with no external leaks from the pneumoperitoneum can occur and last long enough for the FIP N2O to reach values above 30%, which may support combustion of bowel gases.
Even in the presence of such FIP N2O, the risk of fire is fortunately small because a bowel perforation has to occur to allow H2 and/or methane to reach the pneumoperitoneum. Indeed, the flammable colonic gases cross the intact bowel barrier poorly. Hunter et al. (3) could not detect methane and found hydrogen in very small amounts (0.016% to 0.075%) in the pneumoperitoneum during laparoscopic procedures lasting 30 min to 2 h. Similarly, in our experimental model, H2 and O2 were found as traces only (one search every 10 min during 9 h after the peritoneal insufflation).
The simultaneous occurrence of 1) a high FIP N2O, 2) a bowel perforation, and 3) an electric spark, certainly represents an unlikely event (because of the relative scarcity of the two first points), but the risk is not nil. More important may be the risk for a N2O-containing CO2 pneumoperitoneum to worsen the consequences of a gas embolization. We are currently working on this subject.
Avoidance of N2O pollution in the CO2 pneumoperitoneum could be achieved by giving anesthesia without N2O. However, the solution we suggest for a constant N2O-free pneumoperitoneum is to set up a constant pneumoperitoneal leak whose continuous compensation by fresh CO2 from insufflator to maintain the pre-set pressure; this would prevent any significant N2O accumulation. The two questions related to this solution are the definition of a significant FIP N2O and the flow of the leak necessary to avoid such a FIP N2O. Preliminary results indicate that a leak of 24 L/hour is enough to maintain FIP N2O less than 10%.
Most laparoscopic procedures are performed by using a much higher total pneumoperitoneal turnover, and the usual total gas output from the insufflator in our clinical conditions can range from 100 to 300 L/hour. This washout is neither steady nor constant. It occurs mainly during limited periods of time, essentially when the surgeon moves instruments in and out through the trocars, or during the extraction of surgical samples. All maneuvers of this kind cause the gas-tight septa of the trocars to open. Leaks around the trocars are less important in our practice. Between these high-flow washout periods, other phases without leaks may occur and promote N2O accumulation. In the Neuman et al. (2) series (19 female patients), when no external leak occurs, FIP N2O reaches 19.9% ± 4.8%, 30.3% ± 6.8%, and 36.1% ± 6.9% after 10 min, 20 min, and 30 min, respectively. This means that during the course of a laparoscopic procedure, the FIP N2O may increase relatively fast and evolves alternately with the washout phases. One sole measurement at the end of the procedure may conceal the possible "peaks and valleys" in FIP N2O and does not reflect the associated risk periods that may have occurred during the surgery. However, short procedures (20 min in all) may not allow a gas-tight pneumoperitoneum time enough to enable the FIP N2O to rise.
For all the above mentioned reasons, we advocate the setting of a constant and measured pneumoperitoneal leak rather than the abandonment of the N2O as part of the anesthesia and lung ventilation with oxygen-enriched air. Some surgeons open one of the trocar stopcocks from time to time or even leave it open all the time for the smoke from electrocautery to be vented. This unmeasured venting may also prevent N2O accumulation, but it is unpredictable and, according to our recent experiments, seems much too important for the aimed objectives. A much oversized gas turnover unduly increases the heat loss associated with laparoscopic procedures.
Finally, our end-tidal gas monitors based on infrared technology are not efficient enough to perform the above-mentioned measurement, i.e., to provide simultaneous readings of CO2, N2O, O2, CH4 and H2 in a range of concentrations from 0% to 100% (the initial value of FIP CO2). The devices used in the literature for similar protocols are mass spectrometers, alone or coupled with a gas chromatograph as in our study. For our research protocols in progress, we are currently using a micro gas chromatograph, allowing reliable constant monitoring of the pneumoperitoneum composition in the operating theater, just as the end-tidal gas monitor does for the respiratory gas mixture.
References
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Diemunsch PA, Torp K, Van Dorsselaer, T et al. Nitrous oxyde fraction in the carbon dioxide pneumoperitoneum during laparoscopy under general inhaled anesthesia in pigs. Anesth Analg 2000; 90: 951–3.
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Neuman GG, Sidebotham G, Negoianu E, et al. Laparoscopy explosion hazards with nitrous oxide. Anesthesiology 1993; 78: 857–9.
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Hunter JG, Staheli J, Oddsdottir M, et al. Nitrous oxide pneumoperitoneum revisited: is there a risk of combustion? Surg Endosc 1995; 9: 501–4.[Medline]
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