To the Editor:—
As new technology enters the operating room, integrated computers “automatically” manage functions and parameters normally controlled by the anesthesiologist. While our background and training in pharmacology allows us to fully understand the implications of using a new drug, our relative lack of training in mechanical and electrical engineering forces us to “assume” new technology brought to the operating room is safe. In this letter I would like to report an episode of incompatibility between the automated systems of a Datex-Ohmeda D-Tec “Plus” Desflurane vaporizer (Datex-Ohmeda, Inc., Madison, WI) and a Draeger Medical Julian anesthesia machine (Draeger Medical, Inc., Telford, PA) that resulted in the intraoperative failure of the vaporizer.
The episode occurred after the uneventful induction and intubation of an otherwise healthy 36-yr-old woman presenting for a laparoscopic tubal ligation. The Julian ventilator was programmed for volume control ventilation with a tidal volume of 600 ml, a respiratory rate of 8 breaths/min and an I:E ratio of 1:2.5 with a 50% oxygen in air mixture at 2 l total flow. The Desflurane vaporizer was initially set at 8% and then reduced on reaching an expired concentration of 6%. Approximately 20 min into the case (10 min of mechanical ventilation), the Desflurane vaporizer alarm labeled “no output” went off, and the Desflurane concentration in the anesthesia circuit began to fall. The Desflurane vaporizer was immediately turned off and isoflurane was added to the inspired gas flow to ensure adequate anesthetic depth. After several seconds the Desflurane vaporizer automatically reset, and a second attempt was made to use the vaporizer. Within several minutes the same no output error occurred. To rule out the possibility that the failure was caused by obstruction at the vaporizer attachment site, the vaporizer was removed and repositioned in another site. Once again the vaporizer would work for approximately 5 min before alarming no output. To determine if this was a faulty vaporizer, the vaporizer was exchanged for a new one, however, the problem persisted. It was hypothesized that the no output condition might be secondary to the current ventilator settings. Despite changes to the tidal volume, flow rate, % inspiratory pause, and I:E ratio the Desflurane vaporizer continued to fail within minutes of being started. By this time the surgery was coming to a close, so the use of Desflurane was abandoned and the surgery was completed using isoflurane. The patient had an uneventful wake-up with no recollection of interoperative events.
In an effort to determine if the vaporizer failure was caused by the mode of ventilation, the next patient in the room was ventilated using pressure control ventilation with settings that would result in ventilatory parameters similar to those used in the previous case. The patient was once again started on Desflurane, however, this time the vaporizer worked without any problem. To validate our finding an attempt was made mid case to switch to volume control ventilation. The no output alarm was triggered within 30 s of switching ventilation modes. After reinitiating pressure control ventilation and allowing the Desflurane vaporizer to reset, the remainder of the surgery proceeded without incident.
To better understand what had occurred, technical representatives from both Draeger and Datex-Ohmeda were contacted. The companies supplied the following information that helps to clarify the cause of the failure of the vaporizer in volume control ventilation mode. The new Draeger Julian ventilator is capable of operating in three different modes, spontaneous ventilation (or hand-bagging with pop-off valve), volume control ventilation, and pressure control ventilation. The ventilator controls are entered via a digital interface. When in volume control mode, the interface allows the anesthesiologist to set the tidal volume he wishes the patient to receive and automatically compensates for changes in fresh gas flow. The ventilator compensates for fresh gas flow by decoupling the fresh gas flow from the ventilator circuit during the inspiratory phase of ventilation. As a result, during the inspiratory phase of the ventilatory cycle, there is no fresh gas flow into the patient circuit. A Draeger representative explained that the way decoupling works is that all the fresh gas is delivered during expiration. The fresh gas flow set by the anesthesiologist is actually the average flow over the course of the respiratory cycle. The actual gas flow is zero during inspiration, and higher than the set flow during expiration, so the average is what that user has set. For example, at an I:E ratio of 1:2 and a set gas flow of 4 l/min, actual gas flow is 6 l/min during expiration and zero during inspiration, thus averaging 4 l/min. This results in a period of time where there is no flow past the vaporizer.
The Desflurane vaporizer made by Datex-Ohmeda has an imbedded circuit board that senses output from the vaporizer and turns off the vaporizer if no output is detected. According to the technical personnel at Datex-Ohmeda, the vaporizer monitors the power source, temperature, tilt, fluid level, and pressure differentials between the input and output ports. If any of these parameters deviate from “normal” the vaporizer shuts down. The engineers I spoke with reported that the changes made between the original Tec 6 Desflurane vaporizer, used during the certification of the Julian ventilator, and the Tec-D Desflurane Plus vaporizer were made to help increase the functional life expectancy of the vaporizer and minimize maintenance requirements. However, the changes were felt to have minimal impact on the overall function of the vaporizer and therefore did not require FDA recertification. There was agreement among representatives from both companies that the circumstances pointed to the vaporizer sensing the decoupling of flow in volume control mode. When pressure control or spontaneous ventilation modes are used on the Julien ventilator, fresh gas flow is continuous, as there is no need to compensate for fresh gas flow during inspiration. In these modes, the vaporizer senses continuous flow and no error is triggered. This would explain the uneventful nature of the second case where pressure control ventilation was used.
This case serves to emphasize the importance of fully understanding the equipment we use every day. Even seemingly minor upgrades in a previously approved piece of equipment can result in unforeseen problems. If a drug company were to present a new drug, we would want to know its mechanism of action, route of delivery, metabolism, excretion, side effects, indications, and contraindications before administering it to a patient. However, we routinely accept new pieces of technology into the operating room that we do not fully understand, and as this case points out, Food and Drug Administration approval does not guarantee compatibility with other pieces of equipment. Since identifying this problem to Datex-Ohmeda, they have recalled, modified, and replaced our Desflurane vaporizers so that they are now compatible. As technology in the operating room continues to become “smarter,” understanding the mechanisms behind the interfaces is imperative.
