Pin Index Safety System

Issues of Concern

Pin Index Configurations for Commonly Used Medical Gases

Each PISS has 6 possible pin placement sites, depending on the gas within the cylinder. The pins protrude from the gas tank yokes and fit corresponding holes in the cylinder valves, providing a foolproof way to connect the gas to the appropriate valve. Two pins are placed per cylinder, allowing for multiple medically used gases that have been added to the standard formats. The following are examples of pin locations for common medical gases:

• Oxygen: 2, 5
• Nitro oxide: 3, 5
• Air: 1, 5 (see Image. Pin Index Safety System)
• Carbon dioxide: 1, 6
• Nitrogen: 1, 4

These numbers describe the location of the pins in millimeters from the edge of the yoke,[3] with pin #1 being the farthest to the right when looking at the system straight on. The first pin is located 30° from horizontal on the gas tank yoke, and each subsequent pin is located 12° further down, creating a semicircle. These numbers and pin placements are standardized across countries.

Improvements of the Pin Index Safety System 

Multiple documented cases of failure of the PISS have led to quality improvements in the system. An early flaw was noted by Pauling in 1983 when a patient died after crossing gas pipelines, leading to the intraoperative delivery of 100% nitrous oxide instead of oxygen. In the weeks leading up to the case, a maintenance team in the hospital had stripped the gas line threads to replace valves but did not independently test the gases that could flow through each line after the work was complete.[5] This case highlights the importance of ensuring proper valve/pin placement. Incorrect setup of any valves, including those involved in filling cylinders, can result in system failure. 

A second case report in 1995 documented an incident in which a woman undergoing a laparoscopic cholecystectomy sustained an intra-abdominal fire after a trocar caught fire, leading to the ignition of the insufflating gas within the abdomen. Post-incident analysis revealed that the company providing the gas had a policy that canisters with greater than 7% CO2 were to be fitted with a CO2 pin index; however, this particular canister had a green stripe around the top to identify it as having a majority of O₂, not CO₂. This specific canister in the operating room was 14% CO₂ and 86% O₂, leading to the operating room fire. This incident highlighted the potential for human error in the PISS and the potential downfalls in labeling cylinders with mixed gases.[2] Subsequently, efforts were made to distinguish O₂ from CO₂ and improve oversight.

Clinical Significance

In each of the above cases, thorough investigations were conducted to identify the causes and implement preventive measures. Although the PISS was initially designed for smaller cylinders, its success led to adaptations for larger systems, with some countries mandating pin indexing for even larger cylinders. The crossing of pipelines in hospitals has resulted in more oversight by maintenance crews, and the mislabeling of canisters has since been addressed. Although this system has been designed to improve safety, it also relies upon clinical oversight of medical equipment, staff vigilance, and careful monitoring of anesthesia gas delivery to patients.

Nursing, Allied Health, and Interprofessional Team Interventions

Although the PISS is meant to be a foolproof system, instances have occured where the gas-specific pins have been removed, enabling misconnections and possibly unintended administration of improper gases to patients.[6] Therefore, clinicians using equipment with the PISS should always double check the connections to ensure proper gas delivery.