Anesth Analg 2000;91:1145-1150
© 2000 International Anesthesia Research Society
PEDIATRIC ANESTHESIA
Pressure Control Ventilation: Three Anesthesia Ventilators Compared Using an Infant Lung Model
Stephen A. Stayer, MD,
Sabrina T. Bent, MD,
Barbara S. Skjonsby, RN,
Anna Frolov, MS, and
Dean B. Andropoulos, MD
Department of Anesthesiology, Baylor College of Medicine, Texas Children’s Hospital, Houston, Texas
Address correspondence and reprint requests to Stephen Stayer, MD, Department of Anesthesiology, Texas Children’s Hospital, 6621 Fannin, Suite 310, Mailcode 2-1495, Houston, TX 77030. Address e-mail to sstayer{at}bcm.tmc.edu
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Abstract
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We compared three ventilators—Servo 900C (Siemens Medical Systems, Danvers, MA), Aestiva 3000 (Datex-Ohmeda, Madison, WI), and NAD 6000 (North American Dräger, Telford, PA)—set to deliver pressure control ventilation using an infant test lung model. Ventilator settings were selected to test "near-maximum" settings that would be used for a neonatal patient (peak inspiratory pressure [PIP] 30 cm H2O) or older child (PIP 60 cm H2O). When adjusted for set inspiratory pressure and compliance, the average tidal volume (Vt) produced by the NAD 6000 was 5.8 mL less than the Servo 900C (P = 0.103), and the average Vt produced by the Aestiva 3000 was 18.9 mL less than the Servo 900C (P < 0.001). The Servo 900C generated increased peak pressures, tending to overshoot the set maximum inflating pressures, especially during rapid respiratory rates with decreased inspiratory times. The Aestiva 3000 did not achieve the set PIP during testing conditions of decreased inspiratory times, and the NAD 6000 was not greatly affected by changes in inspiratory time. All three ventilators measured expiratory Vt to be larger than the actual Vt delivered to the lung; however, the NAD 6000 was more accurate.
Implications: There are differences in performance of ventilators when set to deliver pressure control ventilation to an infant test lung model.
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Introduction
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Pressure limited or pressure control ventilation (PCV) is frequently applied to infants and children needing mechanical ventilatory support, particularly when their pulmonary pathology dictates the need for rapid respiratory rates or high inflating pressures. There are several advantages to this mode of ventilation. Limiting the peak inflating pressure delivered by the ventilator will limit the transalveolar pressure produced, thereby reducing ventilator-induced lung injury (1). The decelerating flow used to produce PCV is thought to improve the distribution of gas flow (2). When compared with volume control ventilation, there is a more rapid improvement in lung compliance and oxygenation (3). Lastly, setting anesthesia ventilators to deliver accurate small tidal volumes (Vt) is difficult because of the proportionately large compression volume loss in the ventilator and circuit (4).
Studies of adult patients with lung disease have proven a critical care ventilator, the Servo 900C, to be more effective than traditional anesthesia ventilators (5,6). Recently, Ohmeda-Datex and North American Dräger have released ventilators that may be used in volume control or pressure control mode of ventilation with a standard anesthesia circle system. The purpose of this study was to evaluate the accuracy of pressure delivery and to compare the flows and volumes achieved by the Servo 900C (Siemens Medical Systems, Danvers, MA), Aestiva 3000 (Datex-Ohmeda, Madison, WI), and NAD 6000 (North American Dräger, Telford, PA) ventilators when used for PCV in an infant test lung model.
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Methods
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The NAD 6000, Aestiva 3000, and Servo 900C were set in the pressure 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 lung compliance (0.003 L/cm H2O) and low lung compliance (0.001 L/cm H2O) for a newborn infant. Measurements were made with the Rp20 connector added to the circuit, which adds 20 cm H2O · L-1 · s-1, simulating normal infant airway resistance. The Bio-Tek test lung measures pressures with an accuracy of ±2%, flow with an accuracy of ±5%, and volumes with an accuracy of ±4%. All measurements were performed at a barometric pressure of 760 mm Hg, relative humidity of 40%, FIO2 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 peak inflating pressures (PIPs) of 30 and 60 cm H2O. Using the PIP of 30, positive end-expiratory pressure (PEEP) was adjusted to deliver 0, 10, or 15 cm H2O; the I:E ratio was set at 1:3 or 1:1; and the respiratory rate was adjusted to deliver 20, 40, or 60 breaths/min. Using the PIP of 60, the PEEP was adjusted to deliver 0, 10, or 20 cm H2O; the inspiratory:expiratory (I:E) ratio was set at 1:3 or 1:1; and the respiratory rate was adjusted to 10, 20, or 40 breaths/min. Each of the ventilators was 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. The same disposable pediatric circuit (King System Corp., Noblesville, IN) was used to test each ventilator; both the NAD 6000 and Aestiva 3000 use a circle system and the Servo 900C uses a nonrebreathing system.
Prior to the study, each ventilator was calibrated using the protocol recommended by the manufacturer. Additionally, the accuracy of the flow sensor which the test lung uses to measure flow and volume, 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 independent of the ventilator at 26°C and 32°C. Measurements were made at a fresh gas flow rate of 1 L/min for both the NAD 6000 and Aestiva 3000; fresh gas flow cannot be adjusted with the Servo 900C.
PIP and exhaled Vt (exp Vt) were recorded from the manometer and flow transducers in the anesthesia machines. The following variables were recorded using the test lung sensors: peak airway pressure, end-expiratory pressure, inspiratory flow, 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.
These variables were compared for each ventilator: difference between the machine set PIP and the delivered lung PIP, difference between set PEEP and delivered lung PEEP, difference in flow generated by each ventilator, and the difference between the Vt delivered to the lung and the tidal volume measured by the anesthesia machine (exp Vt). Comparison plots were made of the Vt delivered to the lung versus the inspiratory pressure, and the difference between measured Vt and actual Vt versus inspiratory pressure. Differences between ventilators in pressures and flows delivered to the lung were analyzed using one-way analysis of variance with post-hoc group comparison using Tukey’s test. One-sample t-tests were used to compare mean pressures generated by the ventilators to the preset value. The change in lung Vt produced by changing I:E ratio (see Table 3) was analyzed with an independent samples t-test. Differences in delivered lung Vt between ventilators were analyzed using a multiple regression technique. The dependent variable was lung Vt; independent variables were the ventilator, inspiratory pressure, and lung compliance. Differences between measured Vt and actual Vt were also analyzed using a multiple regression technique. The dependent variable was measured Vt minus actual Vt; independent variables were the ventilator, inspiratory pressure, and lung compliance. Data analysis was performed using SigmaStat 2.03 (SPSS Inc., Chicago, IL) and Microsoft Excel 97 (Microsoft Corp., Redmond, WA) with a significance level set to 0.05.
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Table 3. Comparison of Changing I:E Ratio: Difference Between PIP Measured in Infant Lung Model and Set PIP (cm H2O)
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Results
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Forty-eight conditions were repeatedly measured with each ventilator. Of the 144 tests performed, three were discarded because of a greater than 20% discrepancy between the repeated tests; two from the NAD 6000 and one from the Servo 900C. We found differences between ventilators in the Vt delivered to the test lung at different pressure settings. Adjusted for set inspiratory pressure and lung compliance, the average Vt produced by the NAD 6000 was 5.8 mL less than the Servo 900C, P = 0.103; and the average Vt produced by the Aestiva 3000 was 18.9 mL less than the Servo 900C, P < 0.001. See the Appendix for details (Fig. 1).
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Figure 1. Comparison of the delivered tidal volume (Vt) for the Servo 900C (open circles), the NAD 6000 (black dot), and the Aestiva 3000 (open triangles). The x-axis represents the inspiratory pressure (peak pressure - end expiratory pressure). Dotted lines represent the tidal volumes delivered to the test lung when set for normal compliance; solid lines represent low compliance. There are significant differences in the Vt produced by the Aestiva 3000 versus NAD 6000 and Servo 900C, P < 0.001.
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Most pressures delivered to the test lung had a statistically significant change from the preset values for all three ventilators (Table 1). The Servo 900C generated increased peak pressures, tending to overshoot the set maximum inflating pressures. When set to deliver 0 PEEP, the Aestiva 3000 delivered 5.6 cm H2O on average. However, when set to deliver PEEP at 10, 15, or 20 cm H2O, the Aestiva 3000 more precisely delivered the set PEEP (Table 1). The Servo 900C and NAD 6000 generated larger flow on inspiration than the Aestiva 3000 (Table 2). When the ventilators were set with rapid rates and shortened inspiratory times, the Aestiva 3000 did not achieve the set PIP. The Aestiva 3000 was better able to deliver the set PIP when the inspiratory time was prolonged. At rapid respiratory rates with decreased inspiratory times, the Servo 900C produced a PIP larger than the set PIP, and also was better able to deliver the set PIP when the inspiratory time was prolonged. The NAD 6000 was not affected by changes in inspiratory time (Table 3). On average, all three ventilators measured the exp Vt to be larger than the actual Vt delivered to the lung. However, there were statistically significant differences of the measured exp Vt between ventilators. Adjusted for set inspiratory pressure and lung compliance, the average exp Vt measured by the Aestiva 3000 was 4.8 mL larger than that measured by the NAD 6000, P = 0.003; and the average exp Vt measured by the Servo 900C was 17.1 mL larger than that measured by the NAD 6000, P < 0.001. See the Appendix for details (Fig. 2).
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Figure 2. Comparison of the difference between measured tidal volume (exp Vt) and tidal volume delivered to the test lung (lung Vt) for the Servo 900C (open circles), the NAD 6000 (black dot), and the Aestiva 3000 (open triangles). The x-axis represents the inspiratory pressure (peak pressure - end expiratory pressure). Dashed line represents the volume of gas compressed in the ventilator circuit at each pressure setting. There are significant differences between each ventilator, P < 0.001.
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Discussion
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This study was designed to evaluate "near-maximum" settings that would be used to ventilate a neonatal patient (PIP 30 cm H2O) or older child (PIP 60 cm H2O). Under these conditions, we found that the NAD 6000 and Servo 900C develop larger flows during inspiration and delivered larger Vt to an infant lung model than the Aestiva 3000 when the same settings are used for PCV. The Aestiva 3000 most accurately delivered the set pressures, and the NAD 6000 most accurately measured the actual Vt delivered.
PCV has several theoretical advantages for use in neonatal patients or in children with lung disease. Ventilators set for PCV deliver a "square wave" of pressure to the patient’s airway. The Vt that is delivered depends on resistance to gas flow, total respiratory compliance, and inspiratory time. Pressure develops rapidly due to very high flow at the initiation of inspiration followed by rapid flow deceleration. This decelerating flow is thought to enhance the distribution of ventilation among alveolar units with heterogeneous ventilatory time constants (2), and to improve pulmonary compliance (3). When using PCV, the peak pressure is limited, thus limiting the pressure that develops within the alveolus. Increased transalveolar pressures, and the associated alveolar overdistention, will injure both normal lung and exacerbate the lung injury of acute respiratory distress syndrome (1,7–10).
One disadvantage to the use of PCV is that alveolar ventilation will be altered by changes in lung compliance or resistance. Volume-cycled modes of ventilation ensure that the ventilator delivers the preset Vt. If a patient’s pulmonary compliance or resistance worsens, airway pressures will increase. However, this advantage is negated when an adult anesthesia ventilator is used to deliver volume control ventilation to infants. An adult anesthesia ventilator must be set to deliver a large Vt (25–300 mL/kg) because the large compression volume consumes most of this ventilation, and a small Vt is delivered to the infant (4). If the infant’s lung compliance or resistance changes, there will be minimal alteration in the effectively delivered Vt. When a ventilator is used in PCV mode, pressure remains constant and changes in lung compliance or resistance will be reflected by changes in Vt produced. To assess the impact of changes in lung compliance on ventilation, it is essential to have an accurate indication of the delivered Vt. In this study, all three ventilators tested measured the exp Vt to be larger than the actual Vt delivered to the test lung model. This difference was proportionate to the compression volume of the ventilator circuit (Fig. 2). The NAD 6000 had the most accurate measurement of exp Vt with a 13% difference between delivered and measured Vt. The Aestiva 3000 had an 18% difference and the Servo 900C a 30% difference. The NAD 6000 provides a more accurate volume measurement because this ventilator determines the circuit compliance and uses this value to adjust the exhaled volume measurement for Vt less than 200 mL.
Recently Stevenson et al. (11) compared two critical care ventilators, Servo 300 (Siemens Medical Systems, Danvers, MA) and Babylog 8000 (North American Dräger, Telford, PA), with an adult anesthesia ventilator, Dräger Narkomed GS (North American Dräger). These authors found no significant differences in minute ventilation produced when the ventilator settings were matched in the PCV mode. Even though our study had a similar design, we found statistically significant differences between ventilators in the Vt generated. These differences in outcome are probably related to the differences in ventilators studied. The anesthesia ventilator studied by Stevenson (Dräger Narkomed GS) can generate very high flow on inspiration, if set properly, and will rapidly develop the set PIP. A ventilator with limited flow may not meet the set PIP during a short period of inspiration.
The three ventilators we evaluated have differences in mechanical design. The Servo 900C uses a standard nonrebreathing ventilator circuit, stores gas in a constant pressure bellows, and with inspiration a scissor valve opens that regulates the inspiratory gas flow. The NAD 6000 uses a circle system with a piston-driven bellows. When pressure mode is selected, the piston retracts fully. At the onset of inspiration, the piston moves forward until the set pressure is attained and will continue to move forward as needed to sustain that pressure throughout the inspiratory cycle. The 7900 SmartVent incorporated in the Aestiva 3000 makes use of a circle system. This ventilator provides pressure control using a closed-loop, adaptive control system that modulates the flow rate of drive gas that moves the ventilator bellows every 4 ms, based on feedback from the airway pressure transducer. These design differences account for the variation in gas flow generated. Compared with the Aestiva 3000, the NAD 6000 and Servo 900C produced greater flow on inspiration (Table 2). Because of this decreased flow rate, the Aestiva 3000 did not reach the set PIP when the inspiratory time was short (Table 3). At each pressure setting tested, the NAD 6000 and Servo 900C produced greater Vt. This difference in delivered Vt is probably due to both increased flows delivered and greater pressures generated by these ventilators.
Even though ventilator settings were matched in this study, the pressures produced varied. We used a one-sample t-test to compare the pressure delivered to the test lung with the preset value. This analysis determines whether the deviation from the set pressure observed in the experiment is a random or a consistent effect for each ventilator. Using this type of analysis, pressures that are reproduced consistently (small standard deviation) will show statistically significant differences even for small inaccuracies. However, although many of the statistically significant differences in Table 1 would not be clinically important, some of the pressure differences do have clinical relevance. Throughout the study, the Servo 900C tended to "overshoot" the set PIP, especially when the inspiratory time was short (Tables 1 and 3). Both the NAD 6000 and the Servo 900C produced no PEEP when the PEEP was set at zero. The Aestiva 3000 uses a diaphragm-type exhalation valve to ensure the bellows refills during exhalation. This type of exhalation valve generally produces 2–4 cm H2O PEEP, and we measured an average of 5.6 cm H2O PEEP when the Aestiva 3000 was set for minimal PEEP. However, at higher PEEP settings, the Aestiva 3000 more accurately produced the set PEEP, whereas both the Servo 900C and NAD 6000 did not develop the set PEEP (Table 1). Since the Vt generated in PCV is related to the change in pressure, PIP minus PEEP, the higher PEEP produced by the Aestiva 3000 would decrease this difference and produce smaller Vt.
In summary, this study shows statistically significant differences in performance of three anesthesia ventilators when set to deliver PCV to an infant test lung model. When rapid rates at increased pressures are required in PCV, the Servo 900C produces inspiratory pressures that are larger than the set pressure and the Aestiva 3000 produces pressures that are smaller. These differences in pressures produced differences in Vt generated by each ventilator. It is unclear whether the differences found during this in vitro testing reflect clinically important differences.
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Appendix
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Acknowledgments
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The authors thank David Walding, BSBE, for his technical assistance with the calibration and use of the infant lung model.
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References
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Accepted for publication July 11, 2000.
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Abstract
Introduction
