Standards applied to the design of medical equipment have a variety of purposes, with patient safety being paramount. In this issue of the APSF Newsletter, we have two examples where patient safety was threatened using equipment where the standards intended to protect patients proved to be inadequate.
Bond and co-authors describe an incident where a sevoflurane vaporizer was misfilled with isoflurane. The result was a mixture of isoflurane and sevoflurane delivered to the patient with the potential for overdosage of the isoflurane. Agent-specific filling systems were adopted many years ago to prevent this type of error. Although the cause of the misfilling is not proven, the authors demonstrate that it is entirely possible to place a keyed sevoflurane filler cap on an isoflurane bottle. In this case, neither the standards for a keyed collar on the bottle nor the color coding were sufficient to prevent misfilling of the sevoflurane vaporizer. The authors are to be congratulated for believing the information provided by the agent monitor in the absence of an obvious explanation and making appropriate changes to patient care.
Kruppiah and co-authors provide a case report of a patient receiving 100% carbon dioxide via an AMBU bag connected directly to a carbon dioxide cylinder. Medical gas cylinders are keyed with a pin index system so that they can only be connected to specific yokes. In this case, the pin index system worked as designed, but the mounted flowmeter (which was labelled Carbon Dioxide) included a green nipple for connecting gas supply tubing. The green color of the nipple indicates an oxygen source, and the design allowed for commonly used oxygen supply tubing to be connected to the flowmeter. Again, the standards in place intended to prevent patient injury from accidental CO2 administration were insufficient to protect the patient.
Medical equipment design standards will not protect patients 100% of the time. Human error remains a factor in causing patient injury. There are numerous examples of properly connected gas tubing yet patient injury from contamination of gas supply lines or compressed gas cylinders filled with the wrong gas. The risk of this problem is one of the motivations for requiring an oxygen analyzer for every anesthetic to insure that oxygen is indeed being delivered. Some standards are however more robust than others. The Diameter Index Safety System (DISS) is designed to insure that gas supply lines are connected properly between the wall supply and the gas delivery device, e.g., anesthesia machine or ventilator. This standard uses the diameters of both the internal nozzle and external screw connector to insure that gas connections are proper. DISS is a very robust standard in that it is not possible to misconnect gases when using this standard. An older standard, the Pin Index Safety Standard (PISS) is designed for connecting high-pressure medical gas cylinders to a regulator or other ancillary equipment but has failed to prevent patient injury numerous times.
When standards fail to protect patients, we are reminded of the importance of monitoring devices, and vigilance by care providers, to identify when there are problems and intervene before there is harm. Both of the reports here demonstrate the importance of human intervention. That said, we should seek to make our standards as robust as possible to minimize the risk of harm. These case reports show that both the standards for anesthetic agent key filling and flowmeters that mount to compressed gas cylinders could be improved. It also reminds us all that no standard or monitor takes the place of a vigilant health care professional.
Dr. Feldman is professor of Clinical Anesthesiology Children’s Hospital of Philadelphia, Perelman School of Medicine, University of Pennsylvania, and chair of the APSF Committee on Technology
Dr. Feldman has received consulting compensation from Micropore, Inc., and Dräger Medical.