Circulation 75,648 • Volume 20, No. 1 • Spring 2005   Issue PDF

Cause of Ventilator Failure is Unclear

Albert R. Davis, MD; Bruce Kleinman, MD; W. Scott Jellish, MD, PhD; Robert Clark

We would like to report an incident of anesthesia machine failure with possible patient safety implications. After the induction and intubation of an 8-year-old female and during the sterile skin preparation for orthopedic surgery, a loud warning buzzer was heard by the operating room staff, apparently emanating from the Dräger Narkomed 6000 (Dräger Medical, Inc., Telford, PA) anesthesia machine. A “walk-around” of the machine was required in order to isolate the buzzing noise arising from the vicinity of the red ventilator bypass button located below the machine’s power switch. Pressing the button caused the buzzing to stop and the Ventilator Override button to illuminate. The display screen of the machine did not appear to be affected by the problem, but during the walk-around the display was not visible to the anesthesiologist. Upon the anesthesiologist’s return to the head of the patient, examination of the machine’s display (absence of any ventilator information on the Narkomed LCD display and a lack of end-tidal CO2 on a separate monitor) revealed that the ventilator had stopped functioning. The case start was delayed, and the patient was hand ventilated until the machine was exchanged. The case subsequently proceeded without further incident. Later examination determined that the large sealed power supply, located at the rear of the base of the machine, had failed. This incident exposes, in our opinion, several design shortcomings in the Dräger Narkomed 6000 machine, some of which may pose threats to patient safety.

Power Supply Failure

It is disconcerting that the power supply failure in the present instance led to a failure of the ventilator. Given that the Divan ventilator (Dräger Medical, Inc., Telford, PA.) in the Narkomed 6000 is an electronically controlled, electrically powered piston ventilator, the continuous availability of electrical power must be assured. We think it is reasonable to expect that some sort of “fail-over” back-up system, such as exists in the computer, airline, and shipping industries, would be in place on this ventilator. Such was not the case in the present instance. When the power supply failed, the patient went without ventilation for somewhat less than one minute. We think this episode highlights what we believe is a design shortcoming of the Narkomed 6000 machine: there exists a linkage between the power supply and the reserve batteries such that a failure of the power supply itself also eliminates the ability of the back-up batteries to function. In other words, the machine is protected from external power loss, but not from internally originating power interruptions.

Anesthesia machines in general and the Divan ventilator, in particular, require stable and uninterrupted electrical power in order to function properly. Power supply design is modestly complex; electrical efficiency, heat dissipation, weight, shape, size, and cost must all be considered as the design criteria are set and achieved. The engineering, however, is well understood and the power generating industry is stable and mature. Even so, manufacturers understand that component failures do happen during routine use of complex equipment, and therefore manufacturers attempt to incorporate appropriate design safeguards.

It has been truthfully said that the basic functions of the anesthesia provider are to put air in the lungs and water in the veins. With that in mind, it would seem intuitive that the one function for which the anesthesia machine must have powerful redundancy is ventilation. Dräger has clearly considered the loss of external power in its Narkomed 6000 machine. However, our episode suggests that the potential failure of internal components did not receive the same level of attention. Specifically, we believe it is a machine design fault to have combined the back-up battery system and AC power supply in such a way that the failure of the AC power supply subsystem does not initiate both battery back-up and alarm systems in the same manner as does the loss of external AC power. Instead, the failure of the AC power supply caused a total power failure in the ventilator. This design seems to us to be “counterintuitive” and would suggest that Dräger engineers should readdress this design.

Location of the Ventilator Override Button

One drawback, in our opinion, of the Divan ventilator is that electrically controlled valves have the potential to allow it to reach a state, following a power failure, where manual patient ventilation is not possible. The Ventilator Override button is specifically intended to alleviate this scenario and did indeed do so in the present instance. The location of the button is a concern, however.

While Dräger has taken the trouble to build a bright red light into the button, the button is located low on the left side of the machine, beneath the power switch and behind both the flow sensor assembly and the inspiratory and expiratory valves. In general use, this button may or may not be visible to the operator, depending on his height and position relative to the anesthesia machine. In this instance, the button was not immediately visible to the anesthesiologist because the line of sight was obscured by the aforementioned items. The anesthesiologist had to walk completely around the machine and approach it from the left side (as viewed by an operator in the usual position) before seeing the red light.

In sum, the location, appearance, and use of the button do not cause the user to have a high degree of confidence or certainty in its function. It is nearly hidden from sight and ambiguously named and labeled, and the fact that it turns bright red from a light built into the switch makes it somewhat startling to use the first time. The button is, essentially, the last line of defense against electrical failure of the Divan ventilator. The Narkomed 6000 operator’s manual states that the button “is provided for use in the unlikely event of a fault which does not allow the clinician to ventilate a patient in normal Manual/Spontaneous Mode or safe state.” Good ergonomic design suggests that a rarely used button would be best placed away from the primary lines of sight and reach of a machine operator. However, in terms of patient safety, it is our opinion that a rarely used button intended as a last ditch response to a potentially life-threatening situation should be highly visible, unambiguously labeled, and easily accessible. We believe that future Narkomed anesthesia machines equipped with the current iteration of the Divan ventilator should continue to feature the Ventilator Override button, but that button should be redesigned and relocated to satisfy both ergonomic and safety considerations.

Ventilator Visibility

One of the more controversial aspects of the Narkomed 6000 machine is the placement of the Divan ventilator. The Divan ventilator is located beneath a metal panel which serves as the “table top” for the anesthesia machine, and is thus completely hidden from view. When the Narkomed 6000 machine was first introduced, many experienced anesthesiologists were taken aback at the absence of a visible bellows, particularly in light of the years of controversy over the “hanging vs. standing bellows” issue. Indeed, when this ventilator failed, no specific visible indication that the event had occurred was noted except for the buzzer sound and the red light in the ventilator override button. An inexperienced operator, or one distracted by other simultaneous problems, might have taken significantly longer to notice the failure that occurred in this case. The placement of the Divan ventilator out of the operator’s sight contributed to our delay in realizing that ventilator failure had occurred. While there may (or may not) be an increase in reliability afforded by placing the ventilator in a metal cabinet rather than a clear plastic column, the loss of visibility of the ventilator is, in our opinion, a high cost trade-off. We suggest that Dräger reconsider this design and find a way to combine reliable function, safe location and visibility in future machines.

On-Screen Ventilator Failure Alarm/Notification

The LCD display screen of the Narkomed 6000 is one of the machine’s most attractive features. The flexibility of such a display is unparalleled; it is intuitive and informative. One must wonder then why there was not a very large, very noticeable alert on the display indicating that there had been a major failure. Interestingly, exactly this type of alarm does appear if an external power failure occurs or the back-up battery system is engaged for any other reason.

The diagnostic information available to our technical staff after the incident reveals that the system was aware that there had been a power supply failure. We propose making this vital information accessible to the clinician. We would like to suggest that Dräger add an intrusive, informative, and possibly even instructive on-screen alarm in case of any significant detectable internal or external power failure. This software upgrade should be installed in the field as soon as possible.

Power Supply Failure History

In the publication of a power supply failure in the same model anesthesia machine in September 2003,1 Dräger was reported to have redesigned and replaced the power supply in all affected machines in the US and Canada. The earlier published failure was tracked to a specific design flaw allowing contact between a capacitor can and a circuit board tracing. Analysis of the power supply in the current instance is pending, but the circumstances of our power supply failure vary significantly from the earlier reported incident, in which there was a “pop” and smoke emanated from the power supply. The machine serial number in the present instance is within the range of all the serial numbers of machines affected by the earlier power supply recall (Dräger correspondence), but this specific machine was not affected by the recall.

Incident Tracking

Ventilator failure in an anesthesia machine is a potentially devastating device mishap. Although it does not appear that FDA incident tracking is mandated by this incident (there was no death or injury), one could consider that such tracking might give Dräger the ability to cast a wider net as the company attempts to discover whether the power supply/ventilator failure presented here is indeed an isolated incident or is the sentinel event in a chain of potential adverse outcomes.

Albert R. Davis, MD
Bruce Kleinman, MD
W. Scott Jellish, MD, PhD
Loyola University Medical Center
Maywood, IL


  1. Usher AG, Cave DA, Finegan BA. Critical incident with Narkomed 6000 anesthesia system. Anesthesiology 2003;99:762.


Dräger Medical is grateful for the opportunity to respond to the concerns identified by the clinical staff at Loyola University Medical Center. Dräger Medical considers patient safety to be paramount, and Dräger Medical’s history of equipment innovation is evidence of this commitment to patient safety.

Reported Problem

Dräger Medical was contacted regarding the reported condition by a biomed from the facility. The biomed reported that during a case the Divan ventilator was noted to lose power and the display information went blank. An audible alarm was noted and the monitor screen for the anesthesia machine continued to function. The user depressed the ventilator override button and manually ventilated the patient on the machine until the machine was replaced. Upon replacing the anesthesia machine, the biomed performance tested the device and was unable to reproduce the reported occurrence. The biomed replaced the power supply to be cautious.


The power supply was returned to Dräger Medical for evaluation. Performance testing found the power supply to function normally.


The fact that the rest of the system remained active indicates that this was likely not a power supply failure. This was confirmed when the power supply was returned, and the failure analysis found the power supply to function normally.

It would appear from the report that a condition may have arisen in which an alarm was posted by the Divan. The Divan ventilator does have an internal audible speaker that is activated during an atypical hardware or software condition.

The Divan Ventilator incorporates safety software that monitors the function of the ventilator during the power-on self-tests, as well as during normal use. In the event that the safety software monitors a fault condition, the Divan may post an error message on the Divan Control Panel display and an audible alarm. Upon posting the message, the Divan ventilator may configure a warm start or configure itself into Safe State. Safe State automatically configures the Divan for manual or spontaneous ventilation and the next power-on self-test cannot be aborted. A warm start by the Divan would cause the ventilator to momentarily reset, power off, and then on. The reset will take approximately five to ten seconds and then the Divan ventilator will resume ventilation at the user set parameters.

The ventilator override button was designed in the event that an unforeseen condition occurs that the safety software does not recognize. Activating this button removes power from the ventilator forcing it into Safe State which allows for manual or spontaneous ventilation.

Power Supply Failure

The author’s concerns about the design of the power supply are unfounded. The functionality and behavior that the author is proposing is indeed how the system works. The Narkomed 6000 is designed to primarily function on AC power. In the absence of AC power, the anesthesia machine will function on back-up battery power for a minimum of 30 minutes with a fully charged battery. The anesthesia machine will inform the user that AC power is no longer present by a visual message and audible alarm.

The author expresses concern about the dependence of the ventilator on electrical power. The dependence of the NM6000 series anesthesia machine on electrical power is no different than previous anesthesia machines manufactured by Dräger Medical, which include the Narkomed product line of anesthesia machines. The Narkomed products also require AC electrical power to function during automatic ventilation.

On-Screen Ventilator Failure Alarm/Notification

Current software released for the NM6000 anesthesia machine does incorporate such features as discussed into the alarm structure. In an AC power failure condition, the device posts an advisory alarm message, “AC Power Fail,” plus a single audible tone. A dialog box is also displayed on the screen with a warning tone. Upon activation of back-up battery power, the device will function for a minimum of 30 minutes from a full charge. Prior to depletion of battery power and machine shut down, a battery low message will be posted indicating that approximately 10 minutes of power are remaining. Prior to use, the power on self-test will post appropriate indications to the user regarding the power status to the device. The system will post a message indicating the status of the device as functional, conditionally functional, or non-functional.

Power Supply Failure History

The power supply returned from Loyola was not within the suspect population for the recall that has occurred. The power supply recall was issued for 151 machines. All facilities with machines that were affected by the recall were properly informed and had their power supplies replaced. The recall started November 01, 2002, and was completed February 28, 2003.

Ventilator Override Location

Figure 1: General Location of Ventilator Override Switch

Customer input and design determined the current location of the ventilator override switch to the ventilator on the machine. The switch is clearly labeled (see Figure 1), and the defined use is provided to the user in the Operator’s Manual.

Ventilator Visibility

Dräger Medical recognizes that the location of the ventilator is a significant change for the user versus past designs. As such, Dräger Medical offers as an option, a top cover for the ventilator with a transparent window to allow the user to easily visualize movement of the piston ventilator.

Incident Tracking

Dräger Medical complies with post market surveillance and the medical device reporting regulations.

We appreciate all feedback regarding the features and functionality of Dräger anesthesia workstations. This feedback is important in helping to identify potential performance issues and also helps to incorporate new features that our customers require. Additionally, anesthesia machines are not all the same, and patient safety may be managed through various means and systems. As anesthesia machines are required to perform more complex functions, machine features and usage are always changing. As a result, it is recommended that all staff undergo initial and periodic refresher training to ensure a high level of proficiency for all types of anesthesia machines. Dräger Medical makes its personnel available to help in the continuing education of its customers.

Robert Clark
CareArea Director
Perioperative Care
Dräger Medical, Inc.