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   Table of Contents      
CASE REPORT
Year : 2019  |  Volume : 5  |  Issue : 2  |  Page : 93-95

Faulty nitrous oxide–oxygen interlock or hypoxic device leading to higher concentration of oxygen delivery


1 Department of Anaesthesia, All India Institute of Medical Sciences, Rishikesh, Uttarakhand, India
2 Department of Anaesthesia, Maulana Azad Medical College and Lok Nayak Hospital, New Delhi, India

Date of Web Publication20-Aug-2019

Correspondence Address:
Bhavna Gupta
Department of Anaesthesia, AIIMS, Rishikesh, Uttarakhand
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mamcjms.mamcjms_20_19

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  Abstract 


A “pre-use check” to ensure the correct functioning of anesthetic equipment is essential to patient safety. The anesthesiologist has a primary responsibility of checking the machine and equipment before administration of anesthesia. We report a case where pre-use check would have failed to check delivery of high oxygen had oxygen analyzer not been used. Incorporation of safety features in anesthesia machines and ensuring that a proper check of the machine is performed before use on a patient ensures patient safety.

Keywords: Hypoxic device, nitrous oxide–oxygen interlock, oxygen analyzer


How to cite this article:
Gupta B, Chaudhary K, Manchanda G, Dali JS, Arya M. Faulty nitrous oxide–oxygen interlock or hypoxic device leading to higher concentration of oxygen delivery. MAMC J Med Sci 2019;5:93-5

How to cite this URL:
Gupta B, Chaudhary K, Manchanda G, Dali JS, Arya M. Faulty nitrous oxide–oxygen interlock or hypoxic device leading to higher concentration of oxygen delivery. MAMC J Med Sci [serial online] 2019 [cited 2019 Nov 22];5:93-5. Available from: http://www.mamcjms.in/text.asp?2019/5/2/93/264777




  Introduction Top


A “pre-use check” to ensure the correct functioning of anesthetic equipment is essential to patient safety. The anesthesiologist has a primary responsibility of checking the machine and equipment before administration of anesthesia. We report a case where pre-use check would have failed to check erroneously high oxygen (O2) administration, had O2 analyzer not been used. Incorporation of safety features in anesthesia machines and ensuring that a proper check of the machine is performed before use on a patient ensures patient safety.


  Case Report Top


A 48-year-old male presented for tonsillectomy in our operation theater. During pre-use check, it was observed that the oxygen analyzer attached to the expiratory limb of our anesthesia workstation (Meditec GalaxyPLUS anesthesia workstation, England) read 98% to 100% with 100% O2 and 90% to 100% with 50:50 O2:nitrous oxide (N2O) mixture. With N2O:O2 mixture in 2:1 ratio, the analyzer showed readings varying between 85% and 90% [[Figure 1]]. Flow meters were checked for smooth operation of floats and visually inspected for any damage in the flow tubes. The attempts to deliver hypoxic gas mixture by reducing oxygen flow also reduced the flow of N2O through N2O flow meter and the ratio between N2O and O2 was maintained as 2:1 at all flow rates, ensuring minimum mandatory oxygen supply. Functional status of the oxygen analyzer was confirmed after recalibration at room air and testing on another anesthesia workstation (Penlon Prima anesthesia workstation, UK) at 100% O2, 50:50, and 2:1 N2O:O2, and read correctly as 100%, 50%, and 33%, respectively. The use of another oxygen analyzer also revealed similar readings as seen with the first one on the first or faulty anesthesia workstation (Meditec GalaxyPLUS anesthesia workstation, England). The pipeline and cylinder pressures were checked and found to be in normal range. Considering safety of the patient concerned, another workstation was used for conducting anesthesia.
Figure 1 Oxygen analyzer suggesting erroneous readings.

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The company mechanic was called to identify the surprising cause of erroneous high delivery of O2 when the oxygen:nitrous oxide flow meters were showing ratio of 50:50 and 2:1. He observed that the floats of oxygen and nitrous oxide rotameter were moving freely and were not faulty. Although the nitrous oxide did not flow in flow meter after disconnecting nitrous oxide pipeline or cylinder supply, the rise in flow meter was normal with link 25 mechanism when cylinder/pipeline supply was on. The mechanic informed of the fault at the level of nitrous oxide interlock and internal tubing of both nitrous oxide and oxygen. Faulty nitrous interlock allowed oxygen to flow through nitrous oxide flow meter resulting in erroneously high oxygen delivery.


  Discussion Top


Pre-use check-up includes checking gas supply (pipeline pressure and cylinder pressure, oxygen flush, and auxiliary oxygen), vaporizers (interlock, fill level, and safety filler), emergency equipment (resuscitation bag), breathing circuit (fully assembled, connected, CO2 absorbent), suction apparatus, various tests to check machine integrity, and last, but not the least, checking oxygen concentration at common gas outlet using oxygen analyzer. Continuous monitoring of inspired oxygen is a mandatory norm in low-flow anesthesia so as to avoid delivering hypoxic mixtures to the patient. The oxygen analyzer is an essential component to measure adulteration of oxygen supply. Most analyzers require routine calibration once daily to read 21% when sampling room air. It is the only monitor to detect oxygen delivery errors downstream the flow control valves. All other oxygen safety devices are located upstream from the flow control valves. After calibrations, common gas outlet is flushed with 100% oxygen and analyzer should read more than 90%; the same is checked with varying percentage of N2O. Most modern machines incorporate an antihypoxia device in the form of a link between the oxygen and N2O controls, such that a hypoxic mixture cannot be generated. The antihypoxia device ensures that at a certain set percentage of concentration of oxygen, both flows [N2O and oxygen] either increase or decrease in proportion to the oxygen.[1]

The failure devices may decrease the supply of all other gases in proportion to the fall in oxygen supply pressure or may totally cutoff other gases, which might also include air. In case the failure device is faulty, there can be delivery of 100% N2O (in the absence of hypoxic guard, N2O can be administered without opening oxygen flow control valve due to variations in the arrangement of O2 flow meter in relation to others).[2] Usually, these devices only link oxygen and N2O. Anti hypoxia device was found to be faulty in anesthesia workstation that led to inadvertent flow of 100% oxygen irrespective of N2O, with some mixing of N2O and O2. In reported text, 100% O2 can flow even in the presence of N2O, in the presence of low pipeline O2 pressure, but the same was checked multiple times and the pressure was found to be adequate (55–60 psi).[1]

There have been case reports in literature in which fault was found at the diaphragm of hypoxic device, thereby leading to flow of O2 via nitrous oxide flow meter; authors have suggested that flow via N2O flow meter continued to flow even after disconnecting cylinder or pipeline supply of N2O.[3] However, in our case, N2O did not flow after disconnecting N2O pipeline or cylinder supply, and diaphragm was not ruptured; internal fault was found in the nitrous interlock device and internal tubing of both N2O and O2, thereby allowing mixture of O2 and N2O to flow through nitrous interlock but only in the presence of N2O supply. The fault got corrected after replacement of antihypoxia device and tubing. Hyperoxia mixtures are known to have adverse effects if used for prolonged period of time, owing to generation of O2 free radicals, more requirement of anesthetic agents, opioids, and increasing chances of awareness. Also one may not be able to provide entonox to provide analgesia. Finally, if equipments fail, they should be removed from operation theater and there should be sufficient back-up equipment to cover up for the same. The equipment removed from the service must be clearly marked with a sign to that effect, so that it is not put back into service by well-meaning support personnel or even other practitioners; the sign should include the date, time, person discovering the fault, and details of the problem. The responsible personnel must be notified so that they can remove the equipment, make an entry in the log, and initiate repair. To conclude, O2 analyzer use is mandatory in machine preuse check not only in low flow, but also should be religiously used before conduct of any case in the operation theater, to avoid untoward consequences in the instance of hyperoxia leading to increased sympathetic responses, requirement of anesthetic agents and opioids, and awareness. In case fault is found, the same should be rectified.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Dorsh JA, Dorsh SE. Understanding anesthesia equipment. In: The Anesthesia Machine. 5th ed. Philadelphia: Lippincott Williams and Wilkins 2008. p. 84.  Back to cited text no. 1
    
2.
Goneppanavar U, Prabhu M. Anaesthesia machine: checklist, hazards, scavenging. Indian J Anaesth 2013;57:533-40.  Back to cited text no. 2
[PUBMED]  [Full text]  
3.
Bhar D, Bhar KS, Sarathi HP. Intraoperative awareness due to malfunctioning of anaesthesia machine: a rare incident. Sri Lankan J Anaesthesiol 2013;21:76-8.  Back to cited text no. 3
    


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