To the Editor:

—Here we report on a malfunction of the volume monitor sensor on an Ohmeda Modulus CD series anesthesia machine. This failure occurred after the overhead surgical lights were directed toward the sensor.

A 29-yr-old black man was scheduled for a cystoscopy, ureteral stent placement, and lymphocele drainage approximately 3 months after he underwent a cadaveric renal transplant. After satisfactory induction of anesthesia and tracheal intubation, the patient was mechanically ventilated with 50% nitrous oxide/oxygen and 3% desflurane. Both an Ohmeda 7850 ventilator and an Ohmeda modulus CD series anesthesia machine were used (Datex-Ohmeda, Madison, WI). The patient was then placed in the lithotomy position. Bilateral breath sounds were confirmed by auscultation, end-tidal carbon dioxide reading was noted to be 35 mmHg, and the endotracheal tube cuff was palpated in the sternal notch. The SpO2was 97%, the tidal volume was noted to be 700 ml, respiratory rate was 10/min, and the peak inspiratory pressure was 20 cm H2O, both before and after positioning.

After preparing the patient, the surgical lights (Skytron [IN3022EC], Grand Rapids, MI) at high intensity (5 on a scale of 1–5) were directed toward the head of the operating room table and anesthesia machine, away from the operative site. Within 1 min the spirometer of the anesthesia machine indicated “apnea volume” with no tidal volume or respiratory rate indicated on the monitor. Auscultation of the chest showed clear bilateral breath sounds, and an end-tidal carbon dioxide reading of 34 mmHg with a normal capnogram was noted. The chest was rising symmetrically, the peak inspiratory pressures were unchanged at 20 cm H2O, the SpO2was 97%, and water condensation was noted in the endotracheal tube. The volume monitor vanes were rotating. Because the tidal volume monitor operated partially on optical readings, we thought perhaps that the overhead light shining on the volume monitor of the anesthesia machine was distorting the reading, much like fluorescent lights interfere with pulse oximeters. 1–3Within 1 min after redirecting the overhead lights, the spirometer recorded our initial tidal volume and respiratory rate readings.


Volume measurements, including tidal volume, expired minute volume, and respiratory rate, are measured by both an optical and mechanical sensor placed on the expiratory limb of the breathing circuit. In the Ohmeda Modulus CD, a series of three vanes located within a transparent cartridge constitute the mechanical portion of the volume detector. Two of the vanes are stationary and are positioned before and after a rotating vane. The stationary vanes are shaped much like six-spoked wheels. As gas moves through the cartridge, the stationary vanes channel the gas to move the middle rotating vane. A pair of optical sensors on a plastic clip, each of which consists of an infrared light emitting diode and photosensitive detector, converts the motion of the rotating vane into electrical signals. As the vane in the transparent cartridge spins, it momentarily blocks the path of the infrared light traveling to the optical detector. Each time the vanes pass an optical detector, an electrical pulse is sensed by the monitor's microprocessor. The microprocessor counts the pulses to determine the gas flow direction, volume, and respiratory rate. 4 

The orientation of the photodetector on the transparent cartridge is crucial for this described malfunction to occur. During this incident, the open part of the clip was directed upward with a greater exposure to the surgical light. The spirometer failure will not occur with the photodetectors directed downward toward the floor. Unfortunately, this is not the “neutral” position the sensor takes because of traction from the attached electrical cord. In addition, the intensity of the overhead surgical lights is important. In this report, the lights were set at the highest intensity. The malfunction did not occur when the lights were dimmed to a lower intensity (< 2 on a scale of 1–5).

Our observation demonstrates that intense lighting, such as overhead surgical lighting, can affect the Ohmeda Modulus CD series volume monitor sensor and lead to erroneous data. No warnings were noted in the Ohmeda Modulus CD series manual regarding this possible malfunction. Such failures are especially likely to occur more frequently as video-assisted surgical procedures become more commonplace and the overhead operating light(s) are directed toward the head of the table to provide the anesthesiologist's otherwise dark work area with light. This reported failure occurs with this design of spirometry sensor (Ohmeda 7850 ventilator); it would not occur with the newer models of Ohmeda spirometry (e.g. , the 7900 ventilator).

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