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THE Datex-Ohmeda 7900 SmartVent (Madison, WI) is a microprocessor-controlled ventilator that is supplied with the Aestiva 3000 (Datex-Ohmeda) and the Excell 210 SE anesthesia machine (Datex-Ohmeda). It allows ventilation in two ventilatory modes: volume and pressure control. The ventilator is therefore capable of handling a wide range of patients, from neonates to patients with challenging procedures, such as thoracic and neurosurgical. With a wide range of control and alarm settings, it can compensate for small leaks in the breathing circuit and compression losses. Recently, we had three cases in which the Datex-Ohmeda SmartVent failed to maintain the ventilatory settings, requiring swift intervention.

Case 1

A 60-yr-old man with American Society of Anesthesiologists physical status class III and cervical spinal stenosis was scheduled to undergo a C3–C6 posterior cervical fusion. After an uneventful intravenous induction of anesthesia and orotracheal intubation, the SmartVent ventilator was set to deliver a tidal volume of 850 ml, with a respiratory rate set at 10 min. The gas flows were set at a 1-l/min combination of air and oxygen. The ventilator was attached to an Excell 210 SE and a Fisher & Paykel MR-720 Anesthesia Humidifier (Panmure, Auckland, New Zealand) in the breathing circuit.

Anesthesia was maintained with a continuous infusion of propofol, rocuronium, and fentanyl. Concentrations of isoflurane were maintained below 0.5% end-tidal because of a request from the electrophysiologist who was monitoring the somatosensory evoked potentials. Adequate muscle relaxation was maintained during the procedure. The patient was turned to the prone position. A few minutes after positioning the patient, we noticed that the patient's chest was not rising adequately. The tidal volume was found to be only 425 ml. Peak inspiratory pressures decreased from mid 20s to high teens (cm H2O). The ventilator displayed the following alarms: expiratory reverse flow and unable to drive bellows. The bellows was not empty and the bag–vent switch was set at the vent position.

Despite changing the flow sensors, the same alarms were still present. Heavy condensation was found at this point on the expiratory valve dome. The patient was disconnected from the circuit and underwent ventilation with an Airlife Resuscitator Bag (Allegiance Healthcare, McGaw Park, IL), and the expiratory valve was dissembled and dried. The ventilator was now delivering a tidal volume of 500 ml. We then changed the ventilation mode to pressure control of 24 cm H2O from volume control of 850 ml, which solved the problem of inadequate ventilation previously experienced.

The case proceeded uneventfully for the next 6 h with the ventilator set in the pressure mode. The patient emerged from anesthesia without complications.

Case 2

A 38-yr-old woman with American Society of Anesthesiologists physical status class II and a diagnosis of endometriosis was scheduled to undergo laparoscopic ablation of endometriosis with a carbon dioxide laser and hysteroscopy. The breathing circuit had a heat and moisture exchanger (Intersurgical Inc., Liverpool, NY). Maintenance was achieved by using a 3-l/min fresh gas flow of 33% oxygen in the balance of nitrous oxide, and isoflurane was titrated to hemodynamic response. A tidal volume of 650 ml was set on the Datex-Ohmeda Smart Vent with a respiratory rate of 10 breaths/min. The ventilator was attached to an Excell 210SE.

Approximately halfway into the procedure, the following alarms were displayed by the ventilator: expiratory reverse flow, tidal volume not achieved, low minute ventilation (Ve), and check sensor. The tidal volume being delivered was 420 ml. Peak inspiratory pressure decreased from low 20s to mid teens (cm H2O). The ventilator was changed to pressure control mode from volume control mode. Despite this change, the following alarms were displayed while in the pressure control mode: low Ve, expiratory reverse flow, and check sensors. Inspection of the expiratory and inspiratory sensors revealed that they were both dry. However, we changed the expiratory sensor, and this solved the problem. The procedure was finished with the ventilator set in the volume mode without further problems.

Case 3

A 52-yr-old woman with American Society of Anesthesiologists physical status class II and a history of pseudomeningocele who had previously undergone a suboccipital decompression with duraplasty presented for a wound revision of duraplasty. After an uneventful induction of anesthesia and orotracheal intubation, a tidal volume of 750 ml was set on the Datex-Ohmeda 7900 SmartVent with a respiratory rate of 10 breaths/min. The patient's anesthetic was maintained with desflurane in a mixture of oxygen and nitrous oxide at a total flow of 2 l/min. The breathing circuit had a heat and moisture exchanger inserted.

Immediately after induction and intubation, difficulty with ventilation was noted by alarms displayed in the ventilator. The alarm read: expiratory reverse flow, unable to drive bellows. The ventilator was changed to the pressure control mode without improvement. Inspection of the sensors revealed no condensation on them. The patient underwent ventilation with an Ambu bag while the sensors were changed without improvement.

As a result, the machine was removed from the room and replaced. The case proceeded uneventfully, and the patient awakened without complication.

Mechanical ventilation during anesthesia has evolved from simple oxygen-powered breathing devices to complex microprocessor-controlled systems capable of providing a variety of ventilatory techniques. The 7900 Datex-Ohmeda ventilator incorporates the latest technological advances to provide ventilatory control of the anesthetized patient. It has the capability of delivering both pressure and volume control ventilation. Sensors in the breathing circuit allow for compensation of compression loses, fresh gas contribution, and small leakage in the breathing circuit. Positive end-expiratory pressure is electronically regulated. The software for the ventilator is upgraded, with the latest version being 3.3 (installed at our institution). It is a useful ventilator for neonates and children, and the controls are easy to use and understand.

The expiratory flow sensor measures expiratory flow (used for volume monitoring and alarms). Electrical connections of the sensors to the ventilator are monitored. Each sensor also contains calibration data stored at the time of manufacture. If the data cannot be read, the system shows “Flow Sensor Failure.” The flow sensors use a change in internal diameter to generate a pressure decrease that is proportional to the flow through the sensor. Clear tubes connect to pressure transducers inside the anesthesia machine. During volume control mode, the ventilator calculates the flow per second that will supply the required tidal volume. It applies current to the flow valve needed to supply this flow. The inspiratory flow sensor measures the actual volume, and the valve current is adjusted until the actual volume equals the desired tidal volume. It can compensate for air leaks proximal to the inspiratory flow sensor. It does not compensate for losses distal to this sensor. Volume compensation stops if the sensor fails. 1 

The flow sensors on the 7900 Datex-Ohmeda ventilator seem to be very sensitive to humidity. The presence of water in the flow sensor above a certain threshold causes failure of the sensor and results in general ventilator failure and inadequate tidal volumes. Even when the sensor seems dry, humidity from previous use can find its way inside the anesthesia machine via  the flow sensors to the interface board of the ventilator and can cause the ventilator to fail.

The above cases all illustrate failure of one of the sensors, interface boards of the sensors, or both with the rest of the mechanical and electronic components of the ventilator. The use of a heated humidified circuit worsens the situation because of its constant water output, allowing humidity to track into the ventilator interface and control boards. Before these incidents, the use of heated humidifiers was common at our institution. Although some of the incidents do not involve the use of heated humidifiers, each of these anesthesia machines had been used with a heated humidifier earlier that day or the day before. Because the sensor is sensitive to humidity, we suspected this as the most logical cause in all of the cases.

On all instances, the Datex-Ohmeda service representative found no problem with the ventilator when he tested it. The test by the service representative consisted of a test lung, total fresh gas flow of 2 l/min, and no humidification.

Usually, the first sensor to fail is the expiratory sensor because of the presence of higher humidity in exhaled gases. One of the first alarms to appear was “Exp. Reverse Flow.” If not acted on quickly, this can interfere with the patient's ventilation. We contacted the manufacturer and were surprised to find that they were aware of the problem. This is important because they do not provide any warnings about this problem in the instruction manual for this ventilator. We believe that new guidelines should be set on the preoperative checklist of this ventilator until a new sensor that is less sensitive to water is designed.

Increased failure rate with the use of heated humidifiers has prompted us to evaluate our policy regarding their use. We have previously demonstrated that the heat and moisture exchanger does not prevent temperature decreases but may possibly maintain humidity in outpatients undergoing laparoscopic procedures. 2Heat loss from the respiratory system represents only a small fraction of the total heat lost during anesthesia and surgery in adults. Airway temperature may increase 2–8°C when using a heat and moisture exchanger, and heat conserved by these filters represents 5.51–7.2% of the estimated total metabolic heat production during anesthesia. 3It seems that heated humidifiers offer little advantage over moisture exchangers. 4A moisture exchanger may also prevent some of the humidity from reaching the flow sensor.

At our institution, the mentioned problems were isolated to one machine. The manufacturer replaced the interface and control board of the ventilator. They seemed to think that water had found its way to the board because of their sensor design and our constant use of humidified circuits.

We propose that as a part of the preoperative checklist, the sensor be inspected. If presence of condensation is found (fig. 1), this sensor should be removed from the machine and replaced with a dry sensor (a color image of fig. 1is available in the Web Enhancement). The old sensor can be sterilized and dried between cases. We also propose that the manufacturer issue a warning about using heated humidified circuits on these ventilators because they increase the probability of humidity tracking into the control board of the ventilator. All of our cases were handled appropriately, and potentially devastating consequences were prevented by quick intervention.

Fig. 1. Datex-Ohmeda Flow Sensor with water condensation.

Fig. 1. Datex-Ohmeda Flow Sensor with water condensation.

Close modal
Datex-Ohmeda 7900 Smart Vent Software Version 3.X Operation Manual Parts 1 and 2, published February 2, 1999, pages 8-1 through 8-15
Goldberg ME, Jan R, Gregg CE, Berko R, Marr AT, Larijani GE: The heat and moisture exchanger does not preserve body temperature or reduce recovery time in outpatients undergoing surgery and anesthesia. A nesthesiology 1988; 68: 122–3
Bickler PE, Sessler DL: Efficiency of airway heat and moisture exchangers in anesthetized patients. Anesth Analg 1990; 71: 415–8
Rathgeber J, Weyland W, Bettka T, Zuchner K, Kettler D: Is reduction of intraoperative heat loss and management of hypothermic patients with anesthetic gas climate control advisable? Heat and humidity exchangers vs. active humidifiers in a functional lung model. Anaesthetist 1996; 45: 807–13

In Reply:—

We appreciate the opportunity to respond to the article “Ventilatory Failures with the Datex-Ohmeda 7900 SmartVent.” The authors point out some issues of topical importance in the ever-changing worlds of both anesthesia practice and the design and manufacture of anesthesia equipment. In addition, the authors have correctly provided a brief explanation of the internal workings of the 7900 SmartVent, the function of the flow sensors, and the interaction of active humidification on the ventilator functions.

The 7900 SmartVent is designed around a flow sensor technology that provides ventilation to a broad spectrum of patients and addresses specific pressure control ventilation requirements. These specific requirements include an extremely short response time necessary for precise pressure control ventilation. Such precision prevents pressure overshoot. To accomplish these goals, the 7900 SmartVent depends on flow-sensor interface tubing that has an extremely narrow diameter and is constructed from nondistensible material. As a result, although humidity is not of concern, the presence of condensed water within the flow sensor, where the flow sensor interface tubing is attached, may migrate into and along the tubing. This water may migrate as far as the sensor interface board on the 7900 SmartVent. The cases described in the accompanying article most probably are the result of such events.

The fact that these events have prompted the authors to evaluate their policy regarding the use of active humidifiers is a tribute to the dynamic nature of their particular anesthesia practice. As the practice of anesthesia has trended from fresh gas flows of 3 l or more toward flows measured in milliliters, Datex-Ohmeda has continued to improve the 7900 SmartVent in general, as well as the flow sensors and software used in the ventilator. This evolution has produced flow sensors that exhibit a much greater tolerance to excessive humidity and discourage the formation of water at the openings to the flow sensor interface tubing.

Another concern raised by the authors was the Reverse Flow alarm that prompted the dismantling and drying of the expiratory valve. Although this alarm may be a false-positive alarm, which is most likely in the case presented, Datex-Ohmeda decided to err on the side of safety by presenting this alarm and asking the user to evaluate the situation.

We agree with the authors that additional user-level information is needed that addresses active humidification, humidity, and water. We are exploring alternate methods, beyond operations manual updates, to both heighten the awareness of and educate the user in these areas of concern. The presentation of cases in Anesthesiology is an excellent forum for both.

Michael Mitton,

Datex-Ohmeda, North America, Madison, Wisconsin.