Anaesthesia machines are supplied through a central medical gas pipeline system (MGPS) or cylinders. Type E cylinders are used as backup when the pipeline system fails.[1,2] Ensuring a filled type E oxygen cylinder attached to the anaesthesia machine is an element of the pre-use daily equipment checklist in the operating rooms (ORs). Various safety features ensure that correct gas is delivered to the patient through the cylinders, namely colour coding and label, pin index system, etc. Pin index is unique to each gas and prevents connection of wrong cylinder to the anaesthesia machine.
We report our recent encounter with a cylinder, colour coded (black body and white shoulder) and labelled for oxygen but with a pin index and label on valve for nitrous oxide (N2O).
In an OR, the backup oxygen cylinder was empty. Hence, another oxygen cylinder (as identified by ‘O2’ marking and colour coding) was attempted to attach to the hanger yoke assembly. However, it could not be attached. Upon close inspection of the cylinder, the holes on the cylinder corresponded to pin index of N2O, (3,5) and N2O was engraved on the valve [Figure 1a and b]. This cylinder could be successfully attached to the nitrous oxide hanger yoke assembly without any leak [Figure 1c]. Also, there was doubt regarding the actual contents of the cylinder. The anaesthetic gas monitor (AGM) of the anaesthesia workstation (Datex Ohmeda 9100c NXT, GE Healthcare, United Kingdom) was utilized after disconnecting the N2O and air pipelines. The oxygen pipeline was not disconnected since N2O cannot be turned on without oxygen. A test lung was connected to the breathing circuit, and gas flow was turned on (O2:N2O 50:50; 5 L/min each). The N2O flowmeter was supplied by the spurious cylinder and oxygen flowmeter by the pipeline. As depicted by the AGM, both the inspiratory and expiratory gases had just oxygen and no N2O, indicating that the actual content of the cylinder in question was oxygen. The faulty oxygen cylinder was labelled appropriately for caution and segregated, and another oxygen cylinder was attached as backup for the day. The event was reported to the hospital authorities for suitable action towards the supplier/manufacturer.
Despite the technical advances and incorporation of safety features, potentially catastrophic events continue to happen every now and then. Pauling and Ball reviewed case reports pertaining to the delivery of anoxic gas mixtures to the patients, both prior to and after the obligatory safety checklist. The safety features are prone to disregard which may arise out of unawareness, disinterest or accident. This can occur at any point in the process, manufacturer, supplier or end user.
Pin index system has been instrumental in preventing erroneous connection of cylinder. The pin index system is also used to fill the correct gas in the cylinder. However, this system is not infallible. It can be bypassed by using an adapter while filling the gas or using multiple washers (Bodok seal) while attaching to the hanger yoke assembly of anaesthesia machine. The same seemed to have happened in our case as well, where oxygen was filled in the cylinder through a N2O valve. Had it been the other way around with N2O filled in an oxygen cylinder, the outcome could have been catastrophic.
Ours was a clear instance of threading a N2O valve onto a shoulder and body which was colour coded and labelled for oxygen. This could have been the result of amateurish work in an unsupervised environment at the point of manufacturing. Sensitization, upskilling, training and reeducating at this level of the supply chain are mandatory to avoid such mishaps.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
We acknowledge the role of Dr Sadik Mohammed, Additional Professor, Anaesthesiology and Critical Care at AIIMS Jodhpur, for his contribution in the editing of the manuscript.
1. Sinclair CM, Thadsad MK, Barker I. Modern anaesthetic machines. Contin Educ Anaesth Crit Care Pain 2006;6:75–8.
2. Mohanty CR, Ahmad SR, Bellapukonda S, Sahoo S. Medical gas pipeline system disruption during tropical cyclone 'Fani'-a clear and present danger of tropical cyclones:Are we prepared?. Indian J Anaesth 2020;64:439–41.
3. Goneppanavar U, Prabhu M. Anaesthesia machine:Checklist, hazards, scavenging. Indian J Anaesth 2013;57:533–40.
4. Srivastava U. Anaesthesia gas supply:Gas cylinders. Indian J Anaesth 2013;57:500–6.
5. Pauling M, Ball CM. Delivery of anoxic gas mixtures in anaesthesia:Case report and review of the struggle towards safer standards of care. Anaesth Intensive Care 2017;45:21–8.
6. Lin ES. Anaesthetic equipment Smith T, Pinnock C, Lin T. Fundamentals of Anaesthesia 3rd
ed Cambridge University Press 2009 828–63.