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Comparison of NAD 6000 and Servo 900C Ventilators in an Infant Lung Model

Stayer, Stephen A. MD; Bent, Sabrina T. MD; Campos, Carlos J. MD; Skjonsby, Barbara S. RN; Andropoulos, Dean B. MD

doi: 10.1213/00000539-200002000-00014

We compared the ability of the NAD 6000 (North American Dräger, Telford, PA) and the Servo 900C (Siemens-Elema AB, Solna, Sweden) anesthesia ventilators to maintain precise delivery of small tidal volumes (Vt) and positive end-expiratory pressure using an infant test lung model. A variety of ventilator and lung model settings were selected to test clinical conditions simulating normal and extremely compromised lung function. Differences in ventilator output were analyzed by using an independent t-test with P <0.05 considered significant. With the ventilators set to deliver a Vt of 30 mL, the actual delivered Vt was significantly better for the NAD 6000 (25 ± 2 mL) compared with the Servo 900C (18 ± 3 mL), P <0.001. When the ventilators were set to deliver 100 mL Vt, their delivered Vt were not significantly different, NAD 6000 (66 ± 19 mL) and Servo 900C (60 ± 12 mL), P = 0.09. The exhaled Vt read by the anesthesia machines was significantly closer to the delivered Vt for the NAD 6000 (11 ± 9 mL) compared with the Servo 900C (37 ± 11 mL), P < 0.001. Both ventilators maintained the end expiratory pressure delivered to the test lung within 2 cm H2O of the set positive end-expiratory pressure on average. As the conditions changed requiring the ventilator to develop a higher peak inflating pressure, both ventilators showed a decrease in Vt delivered, which was proportionate to the tubing compression volume loss.

Implications The NAD 6000 (North American Dräger, Telford, PA) and Servo 900C (Siemens-Elema AB, Solna, Sweden) are able to precisely deliver small Tidal Volumes. They both decreased in performance when tested under extreme conditions. Earlier studies of traditional anesthesia ventilators suggest that the NAD 6000 and Servo 900C are superior pediatric ventilators.

Department of Anesthesiology, Baylor College of Medicine, Texas Children’s Hospital, Houston, Texas

September 27, 1999.

Address correspondence and reprint requests to Stephen Stayer, MD, Department of Anesthesiology, Texas Children’s Hospital, 6621 Fannin, Ste 310, Mailcode 2-1495, Houston, TX 77030. Address e-mail to

Controlled ventilation using the ventilator built into an anesthesia machine with a circle system is commonly used to provide positive pressure ventilation in children during general anesthesia. These ventilators and circuits have a large compression volume, and therefore, the ventilator must be set to deliver a tidal volume (Vt) that is substantially larger than the desired Vt for infants. Commonly, these ventilators must be set to generate a Vt up to 6 times as large as the desired Vt (1–4). Anesthesiologists who care for children have learned to manipulate these ventilators to provide adequate ventilation for most children (1). However, as an infant’s lung compliance changes, the delivered Vt could change significantly unless the ventilator settings are adjusted. For this reason, anesthesiologists who care for children with lung disease typically use a ventilator designed for critically ill patients or one that has been adapted to deliver anesthetic gases, such as the Siemens Servo 900C (Siemens-Elema AB, Solna, Sweden) (4). This type of anesthesia delivery system has a much smaller compression volume, compensates for changes in delivered Vt, and does not allow rebreathing of gases. North American Dräger (Telford, PA) has recently released a new anesthesia machine, the Narkomed 6000 (NAD 6000), which incorporates a ventilator with decreased compression volumes and compensation for changes in the compliance of the circuit and patient, yet maintains the advantages of carbon dioxide (CO2) scavenging, rebreathing of gases, and convenient spontaneous or manually assisted ventilation. The purpose of this study was to compare the ventilator incorporated in the NAD 6000 with the Servo 900C under conditions that commonly occur in pediatric anesthesia and to determine how both ventilators perform at extremes of inspiratory pressure and flow requirements.

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To compare the ventilators incorporated in the NAD 6000 and the Servo 900C, a variety of ventilator settings commonly used to ventilate infants in the presence or absence of lung disease were tested while conditions varied in an infant test lung model. The NAD 6000 and the Servo 900C were set in the volume control mode, and ventilator settings were matched throughout the study. The ventilator output was evaluated using an infant test lung model, Bio-Tek VT-2 (Bio-Tek Instruments, Inc., Winooski, VT). This model can simulate normal infant lung compliance (0.003 L/cm H2O) and low infant lung compliance (0.001 L/cm H2O). Measurements were made with either no resistor or a tight resistor (200 cm H2O · L−1 · s−1) added to the circuit to simulate increased infant airway resistance. The Bio-Tek test lung measures pressures with an accuracy of ±2% and volumes with an accuracy of ±4%. All measurements were performed at a barometric pressure of 760 mm Hg, relative humidity of 40%, fraction of inspired oxygen of 1.0, and ambient temperature of 26°C. However, the NAD 6000 warms delivered gases to a maximum of 32°C.

Tests were performed with the ventilators set to deliver Vt (set-Vt) of 30 and 100 mL. Using the 30-mL set-Vt, we adjusted the positive end-expiratory pressure (PEEP) to deliver 3, 10, or 15 cm H2O; the inspiratory time was set at 25%, 33%, or 50% of the ventilatory cycle; and the respiratory rate was adjusted to deliver 20, 40, or 60 breaths/min. Using the 100-mL set-Vt, we adjusted the PEEP to deliver 0, 10, or 20 cm H2O; the inspiratory time was set at 25% or 50% of the ventilatory cycle; and the respiratory rate was adjusted to 10, 20, or 40 breaths/min. Each of the ventilators were cycled at the above settings while connected to the test lung at a compliance of 0.001 L/cm H2O and then repeated with the test lung changed to a compliance of 0.003 L/cm H2O, with no added resistance. All the ventilator settings were also performed with the Rp200 resistor (200 cm H2O · L−1 · s−1) added to the circuit at a compliance of 0.003 L/cm H2O. The same disposable pediatric circuit (King System Corp, Noblesville, IN) was used in testing each ventilator.

Before the study, each ventilator was calibrated using the protocol recommended by the manufacturer. Additionally, the accuracy of the test lung was checked against a 100-mL calibration volume syringe (Hans Rudolph Inc, Kansas City, MO) before and after each ventilator was tested. The compliance of the ventilator circuit was determined independently of the ventilator at 26° and 32°C.

Peak inflating pressure (PIP) and exhaled Vt (exp-Vt) was recorded from the manometer and flow transducers in the anesthesia machines. The Servo 900C stores the respiratory gas mixture in a constant-pressure bellows. With inspiration, a scissors valve opens and regulates the inspiratory gas flow. Gas flow is measured between the bellows and the inspiration valve, and pressure is measured just distal to the valve on the inspiration side. This information is used for Servo regulation of the Vt generated by the ventilator. Gas flow returning from the patient and airway pressure are also measured on the expiration side proximal to the expiration valve, with exp-Vt and pressures displayed on the anesthesia machine. The NAD machine uses a piston-driven bellows for the ventilator with pressure transducers adjacent to the piston and distal to the inspiration valve. An ultrasonic flow sensor is external to the ventilator, proximal to the expiration valve. Before use, the ventilator measures the compliance of the breathing system and patient circuit during an automated self-test. With the patient-end of the breathing circuit occluded, the piston advances incrementally to pressurize the circuit. The compliance is determined by the relationship between the volume displacement of the piston and the pressure that results. During clinical use in volume mode, the piston is controlled by a combination of the volume displacement of the piston and the resulting pressure. It is designed to deliver sufficient volume into the circuit to compensate for the breathing system and circuit compliance, therefore insuring that the set volume is delivered to the patient’s airway regardless of the patient’s lung compliance. The following variables were recorded from the test lung model: peak lung pressure, end expiratory pressure, and delivered Vt (lung-Vt). The test lung reports these measurements after receiving four consistent ventilatory cycles. Each measurement was repeated twice, and the average test data are presented. Measurements that revealed greater than a 20% difference were discarded.

The following variables were compared for each ventilator at each Vt: difference between the machine set-Vt and the delivered lung-Vt, difference between exp-Vt as measured by the anesthesia machine and the delivered lung-Vt, and difference between set PEEP and delivered end-expiratory pressure. Comparison plots were made of the delivered lung-Vt versus the PIP and of the differences between the machine set-Vt and delivered lung-Vt versus the ventilator PIP. These differences were analyzed by using an independent t-test (SigmaStat 2.03; SPSS Inc., Chicago, IL), with P <0.05 considered significant. Data were presented as mean ± SD.

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Both mechanical ventilators were able to consistently deliver small Vt ventilation under a variety of conditions. Eighty-one different conditions were evaluated using the 30-mL set-Vt, and 36 conditions tested with 100-mL set-Vt. All repeated measurements were within 20% variance, and none was discarded. With the set-Vt of 30 mL, the actual delivered lung-Vt was significantly greater for the NAD 6000 compared with the Servo 900C. Also, the exp-Vt measured by the NAD 6000 was significantly closer to the actual lung-Vt delivered to the test lung. (Table 1). When the ventilators were adjusted to 100-mL set-Vt, the lung-Vt delivered did not significantly differ; however, the exp-Vt measured by the NAD 6000 was significantly closer to the actual lung-Vt delivered. Both ventilators maintained the end expiratory pressure delivered to the test lung within 2 cm H2O of the set PEEP on average. (Table 1).

Table 1

Table 1

As the conditions changed that required the ventilator to develop a higher pressure to deliver Vt, both ventilators showed some degradation in lung-Vt (Figures 1 and 2). A variety of conditions contributed to the development of higher inflating pressures: low test lung model compliance, added resistance, shortened inspiratory time, and increased PEEP. The circuit compliance was measured to be between 0.631 and 0.654 mL/cm H2O (average 0.641 mL/cm H2O) at different volumes and was not affected by the change in temperature from 26° to 32°C. At the 30-mL testing, both ventilators closely maintained the lung-Vt delivered to the test lung as the PIP increased (Figure 1). When the ventilators were adjusted to deliver 100 mL set-Vt, the NAD 6000 delivered a lung-Vt that more closely approximated the set-Vt at lower inflating pressures. However, when inflating pressures exceeded 50 cm H2O, the lung-Vt delivered decreased to about 50 mL and was similar for both ventilators (Figure 2). As the inflating pressures increased and the lung-Vt decreased, the NAD 6000 measured decreased exp-Vt, but the Servo 900C continued to read an exp-Vt close to the machine set-Vt (Figure 3).

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

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This study demonstrates that the NAD 6000 anesthesia ventilator compares favorably with the Servo 900C and that both ventilators performed well under a variety of conditions. During this study, a test lung model was adjusted to create conditions that would simulate normal infant lung compliance and resistance, as well as poor compliance and increased resistance. Under these conditions, the ventilators were set to deliver a set-Vt of 30 or 100 mL at rapid rates (up to 60 breaths/minute) with shortened inspiratory times and escalating levels of PEEP. When using 30-mL set-Vt, these ventilators maintained a precise delivery of lung-Vt and PEEP to the test lung, despite worsening conditions in an infant test lung model. Both ventilators were minimally affected by the loss in Vt from gas compression in the ventilator circuit (Figure 1).

Some of the ventilator settings tested simulate the most severe conditions that may occur clinically. They were selected to determine if either ventilator failed to deliver the set-Vt, and when such failure would occur. When the ventilators were adjusted to deliver 100-mL set-Vt, and the inspiratory pressures exceeded 50 mm Hg, there was a diminution of delivered lung-Vt. The decrease in lung-Vt was proportionate to the loss of Vt from the compression of gas in the ventilator circuit. Both ventilators continued to deliver lung-Vt at approximately 50% of the set-Vt, and they did not significantly differ from each other (Figure 2).

Even though we were able to stress these machines to a point of relative failure, they both performed extremely well compared with traditional anesthesia ventilators. Badgwell et al. (1) evaluated the Ohmeda 7800 (Datex-Ohmeda, Madison, WI) and found the compliance compression volume to vary between 7.58 and 9.38 mL/cm H2O, depending on the type of respiratory circuit used. Using these figures, the wasted Vt would be between 152 and 188 mL for every breath when a PIP of 20 cm H2O is reached. If the delivery of a 30-mL Vt breath required a PIP of 20 cm H2O, the ventilator must be set to deliver between 180 and 220 mL. If the infant’s lung compliance worsened, requiring a PIP of 25 to deliver this breath, the ventilator would have to be reset to deliver between 220 and 265 mL. Without adjustment, the delivered Vt would be less than 8 mL. A 20% change in PIP would lead to a 75% loss in delivered ventilation. In Badgwell et al.’s (1) study, the authors state that they were able to provide adequate ventilation for all but one infant of the 80 infants studied. However, it is clear from their data that the ratio of compression volume to delivered Vt is very high, and therefore, changes in lung compliance will have a profound affect on delivered Vt in infants.

Stevenson et al. (2) and Tobin et al. (3) have also proven this relationship between delivered Vt and PIP with the Narkomed 2B (North American Dräger) anesthesia ventilator. These authors compared a variety of circuits and three modes of setting this ventilator using an infant lung model with normal and low compliance. The Narkomed 2B ventilator was set to reach a target PIP of 20, 30, 40, and 50 cm H2O. They found that the type of circuit did not significantly affect the delivered minute volume; however, minute volume was greatly affected by PIP and lung compliance. When the infant lung model was set to low compliance, a PIP of 50 cm H2O was required to generate the same volumes as a PIP of 20 cm H2O delivered when the test lung was set to normal compliance.

A ventilator that will maintain minute ventilation despite changing pulmonary conditions has even greater utility for children because changes in delivered ventilation are often difficult to detect in infants. Capnography is often less accurate because of a leak around the uncuffed endotracheal tube or because the fresh gas flow washout of the small volume of CO2 sampled (4,5). The measured exhaled Vt may be grossly inaccurate. The spirometer located at the end of the expiratory limb will overestimate the patient’s Vt because it reflects the patient’s exhaled Vt and the compression volume in the breathing circuit. If there is a leak around the endotracheal tube, the spirometer will underestimate the exhaled gas. Without reliable ETCO2 monitoring or exp-Vt measurements, the pediatric anesthesiologist must rely on chest expansion and PIP to make ventilator adjustments. This technique is commonly used by most pediatric anesthesiologists. However, drapes that cover the chest make the assessment of chest excursion more difficult, and because of large compression volumes, only profound changes in lung compliance are reflected as changes in PIP.

At our institution, we use the Servo 900C anesthesia machine to provide positive pressure ventilation for children undergoing cardiac surgery. This machine was chosen after traditional anesthesia machines and ventilators failed to provide adequate mechanical ventilation. Other studies have also proven this ventilator to be more effective than traditional anesthesia ventilators (6,7). Infants with congenital heart disease may have poor lung compliance, and they often have significant changes in lung compliance secondary to changes in lung water from cardiopulmonary bypass and from alterations in pulmonary blood flow. Although the Siemens anesthetic delivery system works well for providing positive pressure ventilation, there are some disadvantages to this anesthesia machine. The ventilator controls are complex; only one anesthetic vaporizer may be attached; there is no rebreathing of gases for the typical system; and manually assisted ventilation is difficult to deliver. The NAD 6000 uses a standard circle system with rebreathing of anesthetic gases and has an adjustable pressure limit valve for manually assisted ventilation. This system is the most common breathing system used in the United States and is familiar to most anesthesiologists. It enables lower fresh gas flows and, thus, conserves heat, humidity, and anesthetics. Also, the user interface for ventilator settings is simple and intuitive, and manually assisted ventilation is easily delivered.

Both these ventilators contain transducers that continually measure flow and pressures. In the feedback control loop used by the Servo 900C, inspiratory flow information is compared with the preset values and results in correction signals to the control valves of the ventilator. The NAD 6000 is designed to measure and compensate for the entire circuit compliance when set in volume mode, and at lower airway pressures, we observed the NAD 6000 delivered lung-Vt closer to the set-Vt. This is likely a result of the design difference in which the NAD 6000 measures and compensates for volume changes in the ventilator circuit and the Servo 900C does not. At very high inflating pressures, >50 cm H2O, we observed a decrease in lung-Vt proportionate to the loss of volume from gas compression in the ventilator tubing and similar for both ventilators. There are two possible reasons for this reduction in delivered lung-Vt. First, when the infant lung model was set to simulate poor compliance or high resistance, the flow of gas on inspiration may not have been sufficient to achieve the Vt during the set inspiratory time. If this were a significant factor, we would expect to have observed better ventilator performance as the inspiratory time was prolonged. The second factor, and probably most significant, is that a significant portion of the Vt is lost in the gas compression in the ventilator tubing at high inflating pressures.

Most ventilators measure the volume of gas that is compressed in the circuit during inspiration and that gas which returns from the patient as the expired Vt. However, the NAD 6000 spirometer uses the circuit compliance measurement to adjust the exhaled volume measurement and reports a more accurate indication of true delivered volume to the user. The Servo 900C measures all volume that passes through the expiratory limb, and the displayed expiratory Vt will reflect the exhaled gas plus the gas compressed in the circuit. These design differences explain why the NAD 6000 more accurately reflects the true delivered Vt and why the difference between exhaled and delivered Vt increased for the Servo 900C as airway pressure increased (Figure 3).

In summary, we tested the NAD 6000 and Servo 900C ventilators using an infant test lung model set to simulate the ventilation of infants with normal lungs and with lung disease. This lung model simulates static conditions in a laboratory, and does not evaluate in vivo pulmonary changes, such as shunt or dead space ventilation. Both ventilators were able to deliver precise, small Vt and PEEP. When working under extreme conditions, the Vt delivered by both ventilators were approximately one half of the set-Vt. This performance is significantly better than studies of traditional anesthesia volume ventilators.

The authors would like to thank David Walding, BSBE, for his technical assistance with the calibration and use of the infant lung model.

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