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© 2001 International Anesthesia Research Society
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Abstract |
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We compared the ventilation and pulmonary mechanics produced by a new anesthesia ventilator (NAD 6000) using a circle system with that produced by a critical care ventilator (Servo 900C) using a nonrebreathing circuit in infants with congenital heart disease. Twenty patients, aged 1 day to 7 mo, weighing 2.1 to 4.6 kg, were studied. The NAD 6000 had improved alveolar ventilation: PaCO2 43 ± 8 vs 47 ± 5 mm Hg (P = 0.005), end-tidal CO2 34 ± 7 vs 37 ± 5 mm Hg (P = 0.042); larger inspired tidal volumes 12.9 ± 2.8 vs 11.3 ± 2.2 mL/kg (P < 0.001), but with higher mean airway pressures 9.7 ± 1.6 vs 8.6 ± 1.3 cm H2O (P < 0.001). These differences in ventilation and airway pressures were not clinically significant. Although there were differences in observed ventilatory variables, both machines provided adequate ventilation when set in the volume control mode.
Implications: We compared two ventilators for use in infants. Twenty infants undergoing surgery for congenital heart defects were randomized to receive ventilation first with one ventilator, then with the other. Although there were differences in observed ventilatory variables, both machines provided adequate ventilation when set in the volume control mode.
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Introduction |
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The ability to precisely control ventilation in patients with congenital heart disease is essential for optimal hemodynamic management. However, providing volume ventilation even for healthy infants by using standard anesthesia ventilators is difficult because of the large compression volume of these devices. Commonly, these ventilators must be set to generate a tidal volume (VT) up to 6 times as large as the desired VT (1–4). Anesthesiologists who care for children have learned to adjust 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 (1). For this reason, anesthesiologists sometimes use a ventilator designed for critically ill patients when caring for children with severe heart or lung disease, or use an intensive care unit ventilator that has been adapted to deliver anesthetic gases, such as the Siemens Servo 900C (2). Previous studies in adults with lung disease have found that a critical care type of ventilator provides superior gas exchange when compared with standard anesthesia ventilators (5,6). North American Dräger has recently released a new anesthesia machine, the Narkomed 6000 (NAD 6000), which incorporates a ventilator that can compensate for changes in the compliance of the patient’s lungs and breathing circuit, yet maintains the advantages of CO2 scavenging, rebreathing of gases, and convenient spontaneous or manually assisted ventilation. Our purpose was to compare ventilation and pulmonary mechanics produced by a critical care ventilator, the Servo 900C (Siemens-Elema AB, Solna, Sweden), to an anesthesia ventilator NAD 6000 (North American Dräger, Telford, PA) in infants, weighing <5 kg, scheduled for heart surgery.
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Methods |
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After IRB approval and parental informed consent, patients weighing <5 kg scheduled for congenital heart surgery were enrolled. General anesthesia was induced with fentanyl 10–25 µg/kg, midazolam 0.1–0.3 mg/kg, and pancuronium 0.2 mg/kg. All patients were nasally intubated with a 3.0- or 3.5-mm uncuffed endotracheal tube (ETT). A leak test was performed and the next larger size ETT was placed if there was an audible leak at <15 cm H2O. Before surgery, patients were randomized to receive ventilation with the Servo 900C or NAD 6000 as the initial ventilator.
The Servo 900C stores the respiratory gas mixture in a constant pressure bellows. With inspiration, a scissors valve opens that 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 expired VT displayed on the anesthesia machine. The NAD machine uses a piston-driven bellows for the ventilator with pressure transducers located adjacent to the piston and distal to the inspiration valve. An ultrasonic flow sensor is external to the ventilator, proximal to the expiration valve. The same ventilator circuit tubing (Anamed Co., Las Vegas, NV) was used for both ventilators. Even though the NAD 6000 uses a circle rebreathing system and the Servo 900C has no rebreathing of gases, this same ventilator circuit was attached to the inspiratory outlet and expiratory inlet of each ventilator without the need for modification of the circuit.
The ventilator was set in the volume control mode, with the attending anesthesiologist choosing the initial ventilator settings. After 10 min of ventilation, the patient’s heart rate, blood pressure, central venous pressure, pulse oximetry saturation, and end-tidal CO2 were recorded. The exhaled VT as measured by the anesthesia machine was recorded. An arterial blood gas analysis was performed and pulmonary function tests were measured by using a neonatal pulmonary monitor, Bicore CP-100 (Bear Medical Systems, Inc., Palm Springs, CA). This device directly measures airway pressure and flow and calculates the following variables for each breath: inspired VT in milliliters, expired VT in milliliters, peak inspiratory pressure (PIP) in centimeters of water, mean airway pressure in centimeters of water, positive end-expiratory pressure (PEEP) in centimeters of water, peak inspiratory flow rate (PIFR) in milliliters/second, peak expiratory flow rate (PEFR) in milliliters/second, dynamic compliance in milliliters/centimeters of water, and airway resistance in centimeters of water/liter/second. Once the data had been recorded, the ventilator was changed, and the new ventilator was set exactly the same as the initial ventilator.
After 10 min of ventilation with the replacement ventilator, the pulmonary function tests, arterial blood gas, and vital signs were recorded again. A paired t-test was used for statistical comparison of pulmonary mechanics, blood gases, and patient vital signs with P < 0.05 considered significant. Data were presented as mean ± SD.
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Results |
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Twenty patients, aged 1 day to 7 mo, weighing 2.1 to 4.6 kg were studied after the induction of anesthesia and before surgery. Patient diagnoses were listed in Table 1. Eleven patients were randomized to be ventilated first with the Servo 900C and nine patients by the NAD 6000. The ventilator settings, as chosen by the anesthesiologist, were VT 20.6 ± 3.0 mL/kg; respiratory rate 25.5 ± 4.8 breaths/min; PEEP 3.6 ± 0.6 cm H2O; and inspiratory:expiratory ratio of 1:2. Three patients were tracheally intubated with a 3.0-mm ETT, 16 with 3.5 mm, and one with a 4.0-mm tube. The leak test revealed an audible escape of gas at 31 cm H2O on average, and >15 cm H2O for all patients. There were no differences in vital signs among groups (see Table 2). Both noninvasive monitoring and arterial blood gas analysis revealed that the NAD 6000 produced better ventilation (see Table 2). The NAD 6000 produced larger inspired and exhaled VT, with higher peak and mean airway pressures (see Table 3). Both ventilators measured the exhaled VT to be more than the actual VT delivered to the patient (see Table 3). The PIFR was higher, and the PEFR lower when these infants were ventilated with the NAD 6000 compared with ventilation with the Servo 900C (see Table 2).
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Discussion |
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This study shows that volume ventilation for infants provided by the NAD 6000, using a circle system, compared favorably to a critical care ventilator without a rebreathing circuit. The NAD 6000 produced higher inflating pressures and larger VT than the Servo 900C. Even though there were statistically significant differences among the groups, we believe that these differences in ventilation and airway pressures are clinically insignificant.
Precise control of ventilation is critically important in the management of patients with congenital heart disease. Many of these patients have a communication between the systemic and pulmonary circulations. Because pulmonary vascular resistance is significantly affected by changes in pH and PCO2, alterations in ventilation will have a profound effect on the balance between systemic and pulmonary blood flow. Like the patients in this study, infants with congenital heart disease have altered lung compliance and airway resistance1 (7–9). In addition, surgery and cardiopulmonary bypass cause significant changes in lung compliance or airway resistance secondary to changes in lung water, pulmonary blood flow, or pulmonary arterial pressure. One advantage of volume-cycled modes of ventilation is that the ventilator maintains delivery of the preset VT. If a patient’s pulmonary compliance or resistance worsens, airway pressures will increase, but the preset VT is delivered.
When used for infants, traditional anesthesia ventilators poorly compensate for changes in lung compliance or resistance, and the delivered VT can be significantly affected by changes in these variables. Badgwell et al. (1) evaluated the use of the Ohmeda 7800 ventilator in infants. They found that this ventilator must be set to a very large VT for adequate chest excursion, and that infants weighing less than 5 kg required a set VT between 40 and 275 mL/kg. Badgwell et al. (1) found that for every breath delivered by the ventilator, the majority of the volume is taken up by the circuit as compressed gas with only a small portion delivered to the patient. Although it is possible to set the ventilator to deliver an adequate volume, if the patient’s lung compliance worsens, the inspiratory pressure will increase and more of the total volume will remain in the circuit, leaving less for the patient. In an earlier study using an infant lung model, we have shown that both the Servo 900C and NAD 6000 maintain the VT delivered until conditions become extreme, PIP more than 50 cm H2O (10).
The difference in ventilation produced by the NAD 6000 is likely attributable to compliance compensation performed by this ventilator. 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, a piston-driven bellows 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 to deliver sufficient volume into the circuit to compensate for the breathing system and circuit compliance. From our previous in vitro study, we would have predicted the NAD 6000 to deliver a VT to the patient that is 75 to 100 percent of the VT set on the ventilator, and would have predicted the Servo 900C to deliver a VT to the patient that is 50 to 80 percent of that set on the ventilator (10). In this in vivo study, we found that, on average, the NAD 6000 delivered a VT to the infants that was 63 percent of the set VT, and the VT delivered by the Servo 900C was 55 percent of that set on the ventilator. Compliance compensation performed by the NAD 6000 improved VT delivery; however, it did not completely compensate for compression volume loss. This difference in set VT and delivered VT is significantly better than that reported by Badgwell et al. (1) using the Ohmeda 7800 ventilator for infants. We found that both of these ventilators measured the exhaled VT to be almost the same as the set VT. (See Table 3) This again differs from our earlier study using an infant lung model in which we found the NAD 6000 to more accurately reflect the actual delivered VT (10).
Despite improvements in ventilator design and performance, when ventilating infants, these ventilators are imprecise in the VT delivered and inaccurate in the measurement of the delivered VT. Both ventilators were set to deliver 20.6 mL/kg on average, and the exhaled VT measured by flow transducers within the ventilator was 20.0 mL/kg for the NAD 6000, and 19.3 for the Servo 900C. However, the actual VT delivered to the infants was 12.9 for the NAD 6000, and 11.3 for the Servo 900C. Because of these limitations in ventilator performance, anesthesiologists must continue to rely on chest excursion, PIP, exhaled CO2 measurement, or blood gas analysis to determine appropriate ventilation in infants.
We used a BICORE CP-100 neonatal monitor to measure the pulmonary mechanics produced by each ventilator. This device uses a variable orifice pneumotachograph to measure flow and pressure between the Y of the ventilator circuit and the ETT to accurately measure neonatal pulmonary mechanics (11,12). Although the ventilator settings were matched, the NAD 6000 produced a higher PIFR, and a lower PEFR that was consistent for all patients. This difference is likely attributable to differences in mechanical design of the ventilators.
The Servo 900C uses a standard nonrebreathing ventilator circuit and stores gas in a constant pressure bellows; with inspiration, a scissors valve opens that regulates the inspiratory gas flow. The NAD machine develops flow from a piston-driven bellows. When the infants were ventilated with the NAD 6000, we mea-sured higher peak pressures and larger VT. The higher PIFR may have contributed to the increased PIP. During exhalation, the Servo 900C has a scissors valve that opens, allowing passive exhalation. The orifice provided by this scissors valve is electronically controlled to maintain pressure in the circuit at the set PEEP. The electronically regulated piston on the NAD 6000 is controlled during exhalation to ensure that it is retracted before the next breath. Therefore, the rate at which the piston retracts is determined by the total displacement of the piston for each breath and the duration of the expiratory phase. The piston will need to retract more rapidly for a large VT and short expiratory time than for a small VT and long expiratory time. The rate of piston retraction will affect PEFR. These differences in ventilator design did not affect the measured PEEP, and appear to be clinically insignificant.
In summary, we compared the ventilation and pulmonary mechanics produced by a new anesthesia ventilator (NAD 6000) using a circle system with that produced by a critical care ventilator (Servo 900C) using a nonrebreathing circuit in infants with congenital heart disease. Both ventilators provided adequate ventilation when set in a volume control mode.
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Acknowledgments |
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The authors thank Jill Gouvion, RRT, for her assistance in data collection and pulmonary function measurements and Jeff Feldman, MD, for his editorial assistance with this manuscript.
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Footnotes |
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1 Motoyama EK, Tanaka T, Fricker FJ, et al. Peripheral airway obstruction in children with congenital heart disease and pulmonary hypertension (PAH) [abstract]. Am Rev Respir Dis 1986;133:A10.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- Badgwell JM, Swan J, Foster AC. Volume-controlled ventilation is made possible in infants by using compliant breathing circuits with large compression volume. Anesth Analg 1996; 82: 719–23.[Abstract]
- Steven JM, Cohen DE, Scaalabassi RJ. Anesthesia equipment and monitoring. In: Mototyama EK, Davis PJ, eds. Smith’s anesthesia for infants and children. St. Louis, MO: Mosby, 1996: 229–79.
- Stevenson GW, Tobin MJ, Horn BJ, et al. The effect of circuit compliance on delivered ventilation with use of an adult circle system for time cycled volume controlled ventilation using an infant lung model. Paediatr Anaesth 1998; 8: 139–44.[Medline]
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Tobin MJ, Stevenson GW, Babette JH, et al. A comparison of three modes of ventilation with the use of an adult circle system in an infant lung model. Anesth Analg 1998; 87: 766–71.
[Abstract/Free Full Text] - Marks JD, Schapera A, Kraemer RW, Katz JA. Pressure and flow limitations of anesthesia ventilators. Anesthesiology 1989; 71: 403–8.[Medline]
- Schapera A, Marks JD, Minagi H, et al. Perioperative pulmonary function in acute respiratory failure: effect of ventilator type and gas mixture. Anesthesiology 1989; 71: 396–402.[Medline]
- Howlett G. Lung mechanics in normal infants and infants with congenital heart disease. Arch Dis Child 1972; 47: 707–15.
- Schindler MB, Bohn DJ, Bryan AC, et al. Increased respiratory system resistance and bronchial smooth muscle hypertrophy in children with acute postoperative pulmonary hypertension. Am J Respir Crit Care Med 1995; 152: 1347–52.[Abstract]
- Bancalari E, Jesse MJ, Gelband H, et al. Lung mechanics in congenital heart disease with increased and decreased pulmonary blood flow. J Pediatr 1977; 90: 192.[Web of Science][Medline]
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Stayer SA, Bent ST, Campos CJ, et al. Comparison of NAD 6000 and Servo 900C ventilators in an infant lung model. Anesth Analg 2000; 90: 315–21.
[Abstract/Free Full Text] - Roske K, Foitzik B, Wauer RR, Schmalisch G. Accuracy of volume measurements in mechanically ventilated newborns: a comparative study of commercial devices. J Clin Monit Comput 1998; 14: 413–20.[Web of Science][Medline]
- Jackson EA, Coates AL, Gappa M, Stocks J. In vitro assessment of infant pulmonary function equipment. Pediatr Pulmonol 1995; 19: 205–13.[Medline]
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