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Year : 2016  |  Volume : 9  |  Issue : 4  |  Page : 626-628

Pin index safety system and color coding: is it enough?

1 Department of Anesthesia, R.N.T. Medical College, Udaipur, Rajasthan, India
2 Department of Anesthesia, J.I.P.M.E.R., Pondicherry, Tamil Nadu, India

Date of Submission07-Nov-2015
Date of Acceptance12-Jun-2016
Date of Web Publication12-Jan-2017

Correspondence Address:
Sneha Arun Betkekar
Department of Anesthesia, R.N.T. Medical College, Udaipur, Rajasthan 313001
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1687-7934.198257

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Despite a number of preventive mechanisms, inadvertent administration of nitrous oxide in place of oxygen can lead to fatal hypoxemia. Here we report two cases of hypoxia that occurred when we switched to the emergency cylinder for oxygen supply after exhaustion of the main oxygen cylinder. The urgency shown by the anesthetist and operating room staff to restore the main oxygen supply prevented any fatalities from occurring in our case. We found that there was incorrect painting of the nitrous oxide cylinder with the color code of oxygen. Further, damaged pins on the yoke assembly allowed the attachment of the faulty E cylinder to the machine. Even though such errors are made by the supplier we suggest that all equipment including the cylinder be thoroughly checked by the anesthetist. This also highlights the role of respiratory gas monitoring in the prevention of such mishaps.

Keywords: color coding, cylinder, hypoxia, nitrous oxide, pin index safety system, respiratory gas monitoring

How to cite this article:
Naithani U, Betkekar SA, Verma D, Gehlot RK, Sundararaj R. Pin index safety system and color coding: is it enough?. Ain-Shams J Anaesthesiol 2016;9:626-8

How to cite this URL:
Naithani U, Betkekar SA, Verma D, Gehlot RK, Sundararaj R. Pin index safety system and color coding: is it enough?. Ain-Shams J Anaesthesiol [serial online] 2016 [cited 2023 Dec 11];9:626-8. Available from:

  Introduction Top

Mishaps with the use of nitrous oxide are rare and are usually associated with technical errors in its administration, such as crossed gas supply lines, failure of pin index safety system (PISS), error in color coding, over-ride of the N2O/O2 fail-safe mechanism, etc. [1],[2],[3],[4]. Reporting of such cases is infrequent as excessive nitrous administration leads to fatalities resulting in legal action and media attention [5]. It is necessary to report such incidents to create awareness, improve equipment safety features, and prevent further incidents.

Here we report two hypoxic events that occurred as a result of wrong color coding and failure of PISS, which led to the administration of 100% nitrous oxide in place of oxygen.

  Case report Top

Case 1

A 27-year-old woman, with unremarkable preanesthetic evaluation, underwent diagnostic laparoscopy for primary infertility under general anesthesia, which was induced with thiopentone 250 mg intravenously and succinylcholine 100 mg intravenously, and she was ventilated using a bag and mask with 100% oxygen on Bains circuit. Ten minutes into the procedure the main oxygen supply cylinder (H cylinder) was depleted and we had to shift to the emergency oxygen cylinder for maintenance of oxygen supply. Immediately after changing to the emergency cylinder the patient rapidly desaturated to a SpO2 of 30% and became cyanosed. Her heart rate increased to 160/min and ECG showed ST depression with intermittent ventricular premature beats. The patient was promptly intubated. Meanwhile, the main oxygen supply cylinder was replaced with a new cylinder. We switched back to the main supply and immediate resolution of hypoxia occurred. For ECG changes 3 ml (60 mg) lignocaine intravenously (inj. xylocard) was given as a bolus, followed by an infusion of esmolol and xylocard at 2 mg/min each. At this time the patient developed seizures, which were successfully treated with anticonvulsants (inj. phenytoin 300 mg intravenously), inj. hydrocortisone 100 mg intravenously, and inj. dexamethasone 8 mg intravenously. ECG changes reverted to a normal sinus rhythm with a heart rate of 115/min. Blood pressure remained within normal limits throughout the episode. The patient was unconscious and was shifted to the ICU for mechanical ventilation for the next 24 h. Suspicion regarding the contents of the cylinder did arise, but on visual examination the color indicated that the cylinder was of oxygen ([Figure 1]a and [Figure 1]b). We assumed that laryngospasm might have been the cause for the sudden desaturation as the patient was being ventilated with a bag and mask. The following day the patient was extubated and completely recovered without any residual signs and symptoms.
Figure 1: (a, b) Nitrous valve with pin index 3,5 attached to cylinder with color code of oxygen.

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Cases were continued on the same anesthesia workstation on the main oxygen cylinder without any complications.

Case 2

Seven days later the same anesthesia workstation was used for a similar procedure in a 30-year-old woman, with an unremarkable preanesthetic evaluation. Induction of anesthesia and ventilation was performed as mentioned above. Midway through the procedure the main cylinder was exhausted and we moved to the emergency oxygen cylinder for supply. The patient developed immediate hypoxia to a SpO2 of 80%. This raised doubts regarding the gas filled in the emergency oxygen cylinder. We immediately switched to Ambu bag ventilation and the SpO2 increased to 98%. The only clinical consequences of this hypoxia was tachycardia with a heart rate of 120/min, which resolved with improvement in the saturation. When we again used bag and mask ventilation with the emergency cylinder the patient desaturated. By this time there was a return of spontaneous respiration and when kept on room air the patient maintained 100% saturation.

On further investigation we found that there was cooling of the outer surface of the oxygen cylinder suggesting that the gas filled within the cylinder was nitrous oxide. The valve attached to the cylinder was that of nitrous oxide with pin index 3,5 and the cylinder was erroneously painted as per the color code of an oxygen cylinder (black body with white shoulder) ([Figure 1]a and [Figure 1]b). Furthermore, the pins on the anesthesia machine yoke assembly were damaged ([Figure 2]), which caused the PISS to be bypassed and led to the attachment of nitrous valve in the oxygen yoke. The supplier was notified of the event and the cylinder was sent back for evaluation. They confirmed that the cylinder contained nitrous oxide. Respiratory gas monitoring was not available on our machine; therefore, 100% nitrous oxide administration in place of oxygen could not be detected in both cases and hypoxia occurred.
Figure 2: Damaged pins on the machine yoke assembly.

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  Discussion Top

With widespread use of cylinders containing different gases, a potential hazard is connection of a cylinder to equipment intended for a different gas. To deal with this problem, color coding and a noninterchangeable PISS was developed. Under these two systems the oxygen cylinder has a black body with a white shoulder and pin index of 2, 5, and the nitrous oxide cylinder has a blue body and shoulder with pin index 3,5 [6].

Despite these preventive measures errors leading to wrong attachment of cylinders do occur occasionally because of incorrect painting of cylinders. In addition, the PISS can be over-ridden by inexactness of total pin length, backward seating of the pin into its mount on the yoke rather than the seat hole on the head of the cylinder, placing more than one washer on the nipple of the yoke, and physical damage to the pins [1],[3],[4].

In developing countries like India, cylinders are recycled often [7]. This leads to wearing out of the cylinders on repeated use. Recently a case occurred in Tamil Nadu, India, where an oxygen cylinder was filled with nitrous oxide leading to the death of the patient, the responsibility for which was placed on the supplier [8]. In our case the cylinder was sent for refilling and was subsequently incorrectly painted. Even our anesthesia machine was not serviced frequently; hence, the damaged pins went unidentified and the nitrous oxide valve could be attached with some manipulation.

In our institute there was no central pipeline supply at the time; we relied on H cylinders as our main oxygen source and E cylinders as back-up. In such a system the probability of errors occurring is more as the cylinders must be changed often [1]. The nontechnical OT assistant identified the gas cylinder as that of oxygen based on the color code of the cylinder (white shoulder with black body) and attached it to the workstation. Being a nontechnical person, he did not verify the valve attached to the cylinder, which was that of nitrous oxide.

Anesthesia workstations are equipped with several devices to prevent the delivery of hypoxic gas mixtures (low pressure alarms, fail-safe mechanism, etc.). But most of these mechanisms detect the quantity of gas as opposed to the quality of the gas. As long as a positive flow or pressure of gas is detected through the oxygen portal the machine will assume it is oxygen [4],[9]. Continuous inline oxygen monitoring tells us the concentration of oxygen in the inspired gas but this is not fool-proof as the operator might forget to attach it into the circuit or turn it on [4]. Modern anesthesia workstations are provided with built-in oxygen analyzers that detect the concentration of oxygen in the fresh gas flow. European standards, American Society of Anesthesiologists, and Association of Anesthetists of Great Britain and Ireland standards endorse the use of an oxygen analyzer for every anesthetic [5],[10]. The facility of respiratory gas monitoring was not available to us at the time.

  Conclusion Top

Even though the color coding error was made by the supplier, the anesthetist should perform a thorough check of the equipment before each use as the ultimate responsibility lies on the operator if such incidents occur. In addition, respiratory gas monitoring should be available for every anesthetic to prevent such mishaps.

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Conflicts of interest

None of the authors associated with this case report have any conflicts of interest to declare.

  References Top

Goebel WM. Failure of nitrous oxide and oxygen pin-indexing. Anesth Prog 1980; 28:188–191.  Back to cited text no. 1
Freeley TW, Hedley White J. Bulk oxygen and nitrous oxide delivery system. Anesthesiology 1976; 44:301–305  Back to cited text no. 2
Hogg CE. Pin indexing failures. Anesthesiology 1973; 38:85–87.  Back to cited text no. 3
Goho C, Kittle P. Override of an N2O/O2 machine fail-safe mechanism: case report. Pediatr Dent 1991; 13:234–235.  Back to cited text no. 4
Herff H, Paal P, von Goedecke A, Lindner KH, Keller C, Wenzel V. Fatal errors in nitrous oxide delivery. Anaesthesia 2007; 62:1202–1206.  Back to cited text no. 5
Dorsch JA, Dorsch SE. Understanding anesthesia equipment. 5th ed. New Delhi, India: Wolters Kluwer (India) Pvt. Ltd; 2008.  Back to cited text no. 6
Oak S, Patel R, Singh K, Dewoolkar L, Shetty S. Pin index system: safety mechanism? Internet J Anesthesiol 2006; 14:1–6.  Back to cited text no. 7
The Hindu. Madurai, Tamil Nadu. 21 March 2013. Available at: [Accessed 1st November 2015]  Back to cited text no. 8
Mazze RI. Therapeutic misadventures with oxygen delivery systems: the need for continuous in-line oxygen monitors. Anesth Analg 1972; 51:787–792.  Back to cited text no. 9
American Society of Anesthesiologists. Committee of origins. Standards for Basic Anesthetic Monitoring. 2011. Available at: [Accessed 31 October 2015]  Back to cited text no. 10


  [Figure 1], [Figure 2]


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