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LETTERS TO THE EDITOR

Nitric Oxide Delivery System

Division of Anesthesiology and Critical Care MD Anderson Cancer Center Houston, TX 77030

To the Editor:

I would like to thank Dr. Patrizia Ceccarelli and the co-authors for a most interesting article regarding the use of an anesthesia machine as a nitric oxide delivery device. Their study was most thorough and interesting; however, by their own admission, this was a bench study and they only looked at nitric oxide concentrations. Nitric oxide can break down the presence of oxygen to form nitrogen dioxide. If water is present, nitric dioxide forms nitric acid (1). The complete nitric oxide delivery system includes "A nitric oxide administration apparatus, a nitric oxide gas analyzer, and a nitrogen dioxide gas analyzer" (2). To use an anesthesia machine as a nitric dioxide delivery device in clinical use requires the addition of nitrogen dioxide gas analysis. There is another safety point in that the diameter index safety system (DISS) fittings that prevent cross connects to the compressed gas supplied to the anesthesia machine were circumvented. This is one of the major safety features of anesthesia machines (3).

Although the article by Dr. Ceccarelli and co-authors is an excellent and interesting bench experiment, it should not be put into clinical practice without the necessary safety devices.

References

1. Sladen RN. Nitric oxide in anesthesia: when to say "no." IARS 2000 Review Course Lectures. 2000;35:78–85.
2. Guidance for Industry and for FDA Reviewers. Guidance Document for Premarket Notification Submissions for Nitric Oxide Delivery Apparatus, Nitric Oxide Analyzer and Nitrogen Dioxide Analyzer, U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health. Documents Issued on: January 24, 2000
3. Dorsch JA, Dorsch SE. Understanding anesthesia equipment. 4th ed. Baltimore: Williams and Wilkins, 1998:91.

Response

Massachusetts General HospitalBoston, MA

In Response:

Dr. Supkis’ letter underscores the importance of safe delivery of nitric oxide (NO) by anesthesia machines. We have previously investigated methods of promoting the safe and precise administration of inhaled NO to patients receiving mechanical ventilation (1–3). Our current study (4) tested the accuracy and precision of NO delivery rather than its safety. Dr. Supkis’ letter provides an opportunity to highlight these important issues.

Nitrogen dioxide (NO₂) is formed from NO and O₂ in proportion to the fraction of inspired O₂ (FIO₂), the residence time of NO in O₂, and the square of the NO concentration (3). Hence, the first step to minimize NO₂ formation is to limit the NO inspired concentration and the FIO₂. With a combination of 20 ppm NO and/or an FIO₂ less than 0.8, the generation of NO₂ is minimal (3). The presence of soda lime in the anesthesia breathing circuit further limits the amount of NO₂ delivered to the patient (5). Monitoring inspired NO₂ concentration adds safety, and the INOvent delivery system (Datex-Ohmeda, Madison, WI) described in our study provides continuous inspired NO₂ analysis. When NO is delivered through the N₂O flowmeter, a commonly used slow-response electrochemical NO analyzer can provide NO₂ analysis as well (1). Even though the formation of nitric acid is a theoretical danger of NO inhalation, its formation in relevant concentrations, or the mechanical damage to breathing circuits and ventilators, has not been reported.

We agree that circumventing the Diameter Index Safety System of an anesthesia machine when delivering NO through the N₂O flowmeter is potentially dangerous. Using "home made" NO delivery systems was necessary during initial experimental NO use, when commercially developed devices were not available (6). Since inhaled NO has been approved by the Food and Drug Administration for clinical use, our hospital has adopted a policy of delivering NO only through Food and Drug Administration-approved commercial devices, such as the one described in our paper. We make the same suggestion to our readers.

References

1. Hess D, Ritz R, Branson RD. Delivery systems for inhaled nitric oxide. In: Hess D, Hurford WE, eds. Respir Care Clin N Am 1997; 3: 371–410.[Medline]
2. Imanaka H, Hess D, Kirmse M, et al. Inaccuracies of nitric oxide delivery systems during adult mechanical ventilation. Anesthesiology 1997; 86: 676–88.[Web of Science][Medline]
3. Nishimura M, Hess D, Kacmarek RM, et al. Nitrogen dioxide production during mechanical ventilation with nitric oxide in adults. Anesthesiology 1995; 82: 1246–54.[Web of Science][Medline]
4. Ceccarelli P, Bigatello LM, Hess D, et al. Inhaled nitric oxide delivery by anesthesia machines. Anesth Analg 2000; 90: 482–8.[Abstract/Free Full Text]
5. Weimann J, Hagenah JU, Motsch J. Reduction in nitrogen dioxide concentration by soda lime preparations during simulated nitric oxide inhalation. Br J Anaesth 1997; 79: 641–4.[Abstract/Free Full Text]
6. Bigatello LM, Hurford WE, Kacmarek RM, et al. Prolonged inhalation of low concentrations of nitric oxide in patients with severe adult respiratory distress syndrome: effects on pulmonary hemodynamics and oxygenation. Anesthesiology 1994; 80: 761–70.[Web of Science][Medline]

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