Respiratory adverse events can cause significant injury to patients undergoing general anesthesia (1). In this context, iatrogenic causes such as human error and failure of the gas delivery system are important to avoid. Adverse outcomes associated with gas delivery equipment are rare but can result in severe patient injury (2). The use of gas delivery systems with built-in mechanisms to prevent the delivery of hypoxic gas mixtures helps to reduce the incidence of respiratory adverse events. In the Ohmeda Excel 210 SE anesthetic machine, this is accomplished by the use of a linkage system between the oxygen and nitrous oxide flowmeters. We describe a case in which the chain-link mechanism failed, resulting in the delivery of a hypoxic mixture to an anesthetized patient.
A healthy 5-yr-old child was scheduled for dental extraction under general anesthesia. After the institution of pulse oximetry, electrocardiogram, and blood pressure monitoring, anesthesia was induced by inhalation of sevoflurane in a nitrous oxide and oxygen mixture.
During the course of the induction, the flow of oxygen was reduced to deliver a gas mixture containing 70% nitrous oxide in 30% oxygen. The chain-link mechanism usually comes into action when the inspired oxygen concentration (Fio2) reaches 0.25. However, on this occasion, the Fio2 was inadvertently reduced to <0.3, and a give and a click sensation was felt on turning the oxygen flowmeter knob. This immediately alerted us to inspect the flowmeters, and we discovered that the oxygen flow rate had decreased to <0.2 L/min while the nitrous oxide continued to flow at 5 L/min. When the oxygen flow rate was increased in an attempt to increase the Fio2, it was impossible to deliver more than 25% oxygen, because the engagement of the chain-link mechanism resulted in the simultaneous increase of nitrous oxide flow. Any attempts to reduce the nitrous oxide flow rate resulted in the simultaneous reduction in oxygen flow rate, thus retaining the Fio2 at 0.25.
We then tried to forcibly increase the oxygen flow rate while simultaneously reducing the nitrous oxide flow rate. A give and a click sensation was felt once more, after which the chain-link mechanism started working as it should, preventing the delivery of <25% oxygen in the gas mixture.
The oxygen saturation remained at 99% throughout this period, and the oxygen sensor alarm was not activated in view of the speed at which this problem was noted and rectified. The rest of the procedure continued uneventfully.
Ensuring the safety of patients undergoing general anesthesia is an important part of the anesthesiologist’s job. Clinical vigilance and the use of mechanical safety devices are integral in this aspect of anesthetic patient care.
Respiratory events constituted the most important class of injury to patients undergoing anesthesia in the American Society of Anesthesiologists Closed Claims study (1). Failure of anesthetic gas delivery systems is a potentially important source of patient injury from respiratory events. Although problems associated with gas delivery systems are relatively rare, they can result in severe damage to patients (2). The sequelae from hypoxia in such situations can be devastating. Gas delivery equipment includes the anesthetic machine, breathing circuits, supplemental oxygen delivery tubing, gas supply tanks, vaporizers, and the mechanical ventilator (2).
The Ohmeda Excel 210 SE anesthetic machine has a chain-link mechanism that prevents the delivery of a hypoxic gas mixture when nitrous oxide is administered with oxygen. The linkage consists of a chain that connects a sprocket on the nitrous oxide flow control knob to one on the oxygen flow control knob. A stop screw on the collar of the oxygen flow control knob engages and disengages the linkage (Fig. 1). When the linkage is engaged, the oxygen and nitrous oxide flowmeters will move simultaneously should the oxygen flow rate be reduced. This is usually set to occur at an oxygen concentration of 25%(3). A loosening of the stop screw can result in failure of the linkage mechanism and the delivery of a hypoxic mixture.
Subsequent servicing of our anesthetic machine by the Ohmeda representative revealed that the stop screw had loosened, thus causing the problem described. The suggested reason for this loosening was wear and tear. This fault had developed despite 6-mo servicing of this 3-yr-old machine. The most recent check was 8 days before this incident, and this problem was not detected.
There have been reports of inseparable linkage between the oxygen and nitrous oxide control knobs with the Ohmeda Excel anesthetic machines (4,5). In this situation, it was impossible to turn one knob in isolation of the other. We have not found any other reports describing the delivery of a severely hypoxic gas mixture, as in our scenario.
In conclusion, clinical vigilance remains vital in the safeguarding of patients against adverse outcomes during general anesthesia. The mechanisms that are built into modern anesthetic machines to prevent the delivery of hypoxic mixtures are not infallible and cannot be a substitute for the attentiveness of the attending anesthesiologist.
1. Caplan RA, Posner KL, Ward RJ, Cheny FW. Adverse respiratory events in anesthesia: a closed claims analysis. Anesthesiology 1990; 72: 828–33.
2. Caplan RA, Vistica MF, Posner KL, Cheney FW. Adverse anesthetic outcomes from gas delivery equipment. Anesthesiology 1997; 87: 741–8.
3. Ohmeda Excel 210 anesthetic machine operation manual.
4. Kidd AG, Hall I. Fault with an Ohmeda Excel 210 anaesthetic machine [letter]. Anaesthesia 1994; 49: 83.
5. Lohmann G. Fault with an Ohmeda Excel 410 machine [letter]. Anaesthesia 1991; 46: 695.