Mechanical ventilation of pediatric patients, especially infants and neonates, is challenging because small changes in tidal volume can result in unintended hyper- or hypoventilation. Barotrauma and/or volutrauma are possible if the patient’s lungs are exposed to excessive pressure or volume.
Various strategies for safely and effectively ventilating the pediatric patient during general anesthesia have been successfully used, although all have had limitations. Mapleson D circuits were popular for pediatric patients at one time and continue to be used, especially in neonates having thoracic procedures and during patient transport. The primary drawbacks of these circuits are the need to manage fresh gas flow carefully, and inefficient delivery of anesthetic vapors. Circle anesthesia systems are now most commonly used for the efficiency of anesthetic vapor delivery, but it is difficult to deliver tidal volume accurately. In a circle system, the volume delivered by the ventilator into the circuit is not the same volume that reaches the patient. The fresh gas flow and the compliance of the circuit both affect the volume delivered to the patient. Furthermore, it is difficult to determine the volume delivered to the patient accurately, since exhaled tidal volume is typically measured at the expiratory valve. In that location, the flow sensor measures exhaled gas plus the gas compressed in the circuit during the previous inspiration, and therefore over-estimates the delivered tidal volume. Earlier anesthesia machine ventilators provided only volume-controlled ventilation (VCV) and often required empiric adjustment based on observation of the patient’s chest movement to deliver appropriate tidal volumes to small pediatric patients.1 Since the volume delivered by the ventilator is limited to the set volume during VCV, any leak of gas around an uncuffed endotracheal tube also reduces the delivered tidal volume.
These complexities of volume delivery when using a traditional circle system led to the adoption of pressure-controlled ventilation (PCV) by many pediatric anesthesiologists as the preferred mode of mechanical ventilation. During PCV, the volume delivered to the patient depends predominantly on the patient’s lung compliance and is independent of circuit compliance and fresh gas flow.2,3 Furthermore, with PCV, the ventilator is able to deliver its maximum volume, so that leaks in the circuit, or around the endotracheal tube, do not reduce tidal volume substantially unless the leak is large. Finally, the maximum pressure to which the patient’s lungs will be exposed is set by the user, reducing the potential for barotrauma. The important limitation of PCV is the variability of tidal volume that occurs due to changes in lung compliance. Unless the exhaled volume is carefully monitored during PCV with a sensor at the airway, fluctuations in tidal volume may not be detected.4
New anesthesia ventilators, of both bellows and piston type, have been designed to accurately deliver the set tidal volume to the patient’s airway. Their new features include compensation for the compliance of the breathing system and for changes in fresh gas flow, such that the volume set to be delivered is actually delivered to the patient’s airway. These new ventilators may facilitate the use of VCV for pediatric patients if the desired tidal volume can be set and delivered to the patient with confidence.
The goal of this study was to evaluate the accuracy of tidal volume delivery to the airway during VCV using a variety of anesthesia ventilators, including both traditional and new designs. Various conditions of set tidal volume, lung compliance, and circuit configuration were tested to identify the spectrum of performance of these ventilators. The primary question we wish to address is whether these new ventilators are capable of sufficiently accurate volume delivery that VCV can be used with confidence in children under anesthesia.
METHODS
The study was performed entirely in the lab using a mechanical test lung; hence, no IRB approval was required. The goal of the protocol was to document the accuracy of tidal volume delivery by anesthesia ventilators using varied conditions of breathing circuit compliance and lung compliance. Each ventilator was connected to a mechanical test lung using a commercially available circle anesthesia circuit.
Equipment and Experimental Setup
The test lung consists of both infant and adult mechanical lungs (MI Instruments, Grand Rapids, MI) and can simulate a wide range of lung/thorax compliances and airway resistances. For the purpose of this study, the airway resistance was set to 20 cm H2O/L · s for all measurements. The infant test lung was used for tests with set tidal volumes of 100 and 200 mL. The adult test lung was used for tests with set tidal volumes of 500 mL.
The breathing circuit used was an expandable circuit (Ped Circle, Vital Signs, Totowa, NJ) that can be set from a length of 107 cm when fully contracted to a fully extended length of 274 cm.
Three different anesthesia ventilators were studied, but one of the ventilators was studied with two different software configurations that influence tidal volume delivery. From a functional perspective, four different ventilators were studied: two with and two without circuit compliance compensation.
Anesthesia Ventilators Without Breathing Circuit Compliance Compensation
- Smartvent 7900 (Datex-Ohmeda, Madison, WI): A bellows-driven ventilator configured with a flow sensor at the inspiratory valve, which is used to control the volume delivered by the ventilator such that the set tidal volume is delivered to that flow sensor.
- Avance (GE Healthcare, Madison, WI): A newer version of the Smartvent, a bellows-driven ventilator with a similar inspiratory flow sensor design. The Avance has an optional flow sensor (pediatric or adult) that may be placed at the junction of the endotracheal tube and wye of the breathing circuit to measure tidal volume at the airway. This sensor, if used, has no effect on the functioning of the ventilator.
Anesthesia Ventilators with Breathing Circuit Compliance Compensation
- Aisys (GE Healthcare): A version of the Avance configured with optional software that measures circuit compliance during the preuse test. The ventilator uses this information to compensate for the circuit compliance during volume-mode ventilation with the goal of delivering the set tidal volume to the airway.
- Apollo (Draeger Medical, Telford, PA): A piston-driven ventilator whose preuse system check measures the breathing circuit compliance and then compensates for this compliance, again with the goal of delivering the set tidal volume to the airway.
For the ventilators capable of measuring and compensating for circuit compliance, a preuse test was performed, so that the ventilator could measure the circuit compliance before each set of measurements.
The volume set to be delivered by the ventilator was compared with the volume actually delivered to the patient’s airway. A calibrated screen pneumotachometer (Hans Rudolph, KS City, MO) and variable reluctance differential pressure sensor (Model MP45, Validyne Engineering, Northridge, CA) were placed between the breathing circuit and the test lung and used to measure the volume and pressure delivered at a location equivalent to a patient’s airway (at the endotracheal tube) using custom software (Measurement Computing, Norton, MA). The pneumotachometer and pressure sensor were calibrated before each set of measurements on a given day. The pneumotachometer was located at the airway for some measurements and for others at the expiratory limb of the circuit, next to the anesthesia machine, for comparison to the exhaled volume monitor used clinically. The pneumotachometer was our “gold standard” for measuring the tidal volume delivered at the airway as well as at the expiratory valve. Figure 1 is a drawing of our experimental setup.
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Figure 1.:
Experimental setup. Note extendable/contractable breathing circuit and test lung with adjustable lung compliance. Tidal volume or inspiratory pressure was set on the anesthesia machine ventilator. The pneumotachograph was our gold standard for volume measurement.
Protocols
Protocol 1: Changes in Delivered Volume During PCV
For this and all subsequent protocols, the respiratory rate was set at 20 breaths per minute with an inspiratory to expiratory time ratio of 1:2. The fresh gas flow was set at 2 L/min of air plus 0.2 L/min of oxygen. The oxygen flow was the minimum allowed by some of the anesthesia machines and was kept consistent for all measurements. Three measurements of volume were done at each setting after allowing the ventilator to equilibrate by ventilating the test lung at the experimental settings for at least 20 s or until the tidal volumes appeared stable. Reported numbers are averages of the three measurements with standard deviations.
Protocol 1 mimicked a typical ventilation scenario using PCV with the Ohmeda Smartvent 7900 to document the variation in delivered volume that can occur as lung compliance changes. Inspiratory pressure was set to 25 cm H2O and volume delivered to the airway and to the breathing circuit were measured (using the screen pneumotachometer described above) at lung compliance settings of 0.005, 0.008, and 0.01 L/cm H2O. Volume measurements indicated by the flow sensor of the anesthesia machine mounted at the expiratory valve were also recorded. Both fully extended and completely contracted configurations of the circuit were tested.
Protocol 2: Accuracy of Volume Delivered to the Airway During VCV by Different Ventilators
This protocol was designed to determine the degree to which different modern anesthesia ventilators deliver volume accurately to the airway during VCV. We used both contracted and extended breathing circuits to connect the different anesthesia machine ventilators to the test lung. Tidal volumes were measured at different ventilator settings and lung compliances using the screen pneumotachometer described above. Table 1 indicates the experimental conditions under which each ventilator was evaluated.
Table 1: Experimental Conditions for Protocol 2
The Smartvent 7900 measures tidal volume at the expiratory valve. The tidal volume data measured by the optional airway flow sensor (mentioned above) on the Avance machine (without compliance compensation) can be viewed on the Avance monitor. The data are presented as inspiratory and expiratory volumes, which during our data collection were nearly the same. We compared the displayed inspiratory airway volume with the volume that was actually delivered to the airway as measured by our pneumotachometer (Table 3). The adult flow sensor was used for tidal volumes of 500 and 200 mL, since its specifications report it to be accurate to volumes of 200 mL. The pediatric flow sensor was used for tidal volume tests of 100 mL. We did not use the pediatric flow sensor for measurements at tidal volumes of 200 mL. For all ventilators, our gold standard was the volume delivered to the airway measured by the pneumotachometer located at the patient’s airway.
Table 3: Tidal Volume Reported by Ventilators Compared with Tidal Volume Delivered to the Airway (Protocol 2)
Protocol 3: Mismatched Circuit Compliance During VCV with Compliance Compensation
This protocol was performed in the same way as protocol 2 above except that after the preuse machine check, the breathing circuit compliance was changed by switching the circuit configuration (extended to contracted or vice versa). These data were compared with delivered volume measurements obtained in Protocol 2 when the circuit configuration matched the configuration used during the preuse checkout. We made the mismatched measurements under just one experimental condition: set tidal volume 100 mL with lung compliance of 0.005 L/cm H2O. Only the ventilators with compliance compensation were tested in this protocol.
RESULTS
Protocol 1: Changes in Delivered Volume During PCV
For each circuit configuration (extended or contracted), we found that tidal volume measured at the airway decreased in proportion to lung compliance (Table 2). For example, reducing lung compliance by 50% from 0.01 to 0.005 L/cm H2O led to a similar decrease in delivered tidal volume. However, if lung compliance was not changed, volume delivered to the airway was independent of circuit configuration, underscoring the fact that during PCV, circuit compliance does not affect delivered volume. Furthermore, the tidal volume reported by the anesthesia machine (measured at the expiratory valve) over-estimated the tidal volume delivered to the airway, and the magnitude of the over-estimation was greater for the extended circuit than the contracted circuit.
Table 2: Pressure-Controlled Ventilation (Protocol 1)
Protocol 2: Accuracy of Volume Delivered to the Airway During VCV by Different Ventilators
The data obtained using the Ohmeda Smartvent 7900 with a test-lung compliance of 0.0025 L/cm H2O and set tidal volume of 100 mL exemplify the limitations of a ventilator and circle system without compliance compensation for delivering the set tidal volume to the airway (Fig. 2). The tidal volume displayed by the anesthesia machine (measured by the Smartvent’s flow sensor located at the expiratory limb between the breathing circuit and the machine) is a measurement of the volume of gas delivered to the circuit that can be compared with our pneumotachometer data measured at the expiratory limb. The Smartvent is designed to deliver the set tidal volume into the breathing circuit as controlled by the inspiratory flow sensor. The measured volume delivered into the circuit was slightly larger than the set tidal volume. However, the tidal volume actually delivered to the airway is lower than the set tidal volume of 100 mL (Fig. 2). When the compliance of the breathing circuit is increased by maximally extending the circuit, the tidal volume delivered to the airway decreases even further when compared with the contracted circuit, since there is no mechanism for the ventilator to sense or respond to this change in circuit compliance.
Figure 2.:
Volume-controlled ventilation (VCV) using Smartvent 7900 ventilator without breathing circuit compliance compensation. Tidal volume (Vt) measured at the airway is 75% and 55% of the set Vt for contracted and extended breathing circuits respectively. The anesthesia machine expiratory flow sensor over-estimated the delivered Vt by 32%–50%. Accurate volume refers to the Vt measured by our pneumotachometer, placed either at the airway or at the expiratory limb close to the ventilator. Set Vt refers to the Vt set on the Smartvent 7900 before starting ventilation of the test lung.
Changing the lung compliance most affected the tidal volume delivered to the airway by ventilators without breathing-circuit compliance compensation (Fig. 3a). With a lung compliance of 0.0025 L/cm H2O, which is comparable to that of a neonate, the tidal volume actually delivered to the airway ranged from 45.6% to 100.3% of the set tidal volume depending on the type of ventilator used. The piston-driven ventilator (Apollo) and the bellows-driven ventilator (Aisys with compensation software) that use breathing-circuit compliance compensation delivered from 95.5% to 106.2% of their set tidal volumes to the airway, varying for different lung compliances and set tidal volumes (Fig. 3b). The tidal volumes delivered had standard deviations ranging from 0 to 0.15 mL for the Apollo and from 0 to 0.26 mL for the Aisys. The two bellows-driven ventilators without compliance compensation (Smartvent 7900 and Avance, without compensation software) delivered tidal volume to the airway ranging from 45.6% to 109.3% of the set value (depending on lung compliance and set tidal volume). The precision of their tidal volume delivery was also good, with standard deviations of 0.06 to 1.03 mL for the Smartvent and 0.06 to 0.35 mL for the Avance. The peak inspiratory pressures for all the tests ranged from 21 to 53 cm H2O and depended only on the lung compliance and tidal volume delivered, not on the anesthesia ventilator.
Figure 3.:
Volume-controlled ventilation summary. (a) The percent of the set tidal volume actually delivered to the airway during volume-controlled ventilation (VCV) by anesthesia machine ventilators without breathing-circuit compliance compensation. As lung compliance decreased to 0.005 and 0.0025 L/cm H2O, tidal volume delivered to the airway decreased to 45%–90% of set tidal volume. (b) The same data measured during VCV with anesthesia ventilators that use breathing- circuit compliance compensation. The percent of set tidal volume delivered to the airway was close to 100%.
The anesthesia machine ventilators we tested that use breathing-circuit compliance compensation (GE Aisys and Draeger Apollo) reported the tidal volume delivered to the airway with an error of <9% over all the conditions we tested, as long as the appropriate preuse circuit compliance test was performed. However, as shown in Figure 2 and Table 3, the tidal volume reported by the Smartvent 7900 differs by up to 50% from the actual volume delivered to the airway. This is particularly true with small tidal volumes (≤200 mL). Table 3 also reports data collected using the Avance (without compliance compensation) and its optional flow sensor placed at the airway. The table shows the percentage of the displayed inspiratory airway volume that was actually delivered to the airway as measured by our pneumotachometer.
Protocol 3: Mismatched Circuit Compliance During VCV with Compliance Compensation
Figure 3 shows that without compliance compensation, the configuration of the breathing circuit (extended or contracted) can have a major effect on the tidal volume delivered to the airway when the lung compliance and tidal volume are low (but within normal range for an infant or neonate). The anesthesia machine ventilators with compliance compensation avoid this issue only if the breathing circuit compliance check is performed after each change in circuit configuration (Fig. 4). When we tested a circuit with greater compliance than measured by the anesthesia machine, delivered tidal volume decreased by 23%. If the compliance was lower, then delivered tidal volume increased by 25%. Of note, the tidal volume reported by the ventilator’s display did not correlate with the tidal volume delivered to the airway if the breathing circuit compliance was changed after the compliance test had already been done for the Apollo (Fig. 4) and Aisys ventilators.
Figure 4.:
Impact of Mismatched circuit compliance (Apollo Ventilator). These measurements were done with the Apollo ventilator, test-lung compliance of 0.005 L/cm H2O, and set tidal volume of 100 mL. “Circuit contracted after compliance test” means that the compliance test was performed with an extended circuit but then a contracted circuit was used to ventilate the test lung and make measurements. “Circuit extended after compliance test” means the same thing but with the compliance test done for a contracted circuit and then an extended circuit used for ventilation measurements. “Contracted circuit” means that both the compliance test and the measurements were performed using the same contracted breathing circuit. “Extended circuit” means that both the compliance test and the measurements were performed using the same extended breathing circuit.
DISCUSSION
Our study aimed to evaluate different anesthesia machine ventilators with respect to their ability to provide accurate VCV to pediatric patients. There are two major ways that ventilators attempt to improve the accuracy of delivered tidal volume: measuring the volume delivered to the circuit using an inspiratory flow sensor and controlling the ventilator using that sensor, or using an algorithm that compensates for the compliance of the breathing circuit. The first approach is implemented by the Ohmeda Smartvent 7900 and the GE Avance (without the optional compliance compensation software). The volume delivered to the airway by the Smartvent 7900 and the Avance differed from the set tidal volume by up to 55%, particularly at lung compliances and tidal volumes similar to those expected in neonates and small infants. Normal lung compliance in infants (aged 1–12 mo) has been estimated to be 0.0012–0.0018 L/cm H2O/kg, which would be about 0.009 to 0.014 L/cm H2O in an 8-kg child.5,6 In neonates, normal lung compliance is thought to be about 0.003 L/cm H2O and low lung compliance about 0.001 L/cm H2O.3
Also of note, the exhaled volume sensor typically located at the expiratory valve over-estimates the volume the patient receives (Fig. 2 and Table 3). If the clinician were to want accurate tidal volume information, he or she could use the optional adult or pediatric airway flow-sensor with the Avance designed to measure tidal volume at the airway. The clinician might also use this strategy by adding to any anesthesia machine a separate flow sensor at the airway. Using such a flow sensor, the clinician could adjust the ventilator settings guided by the tidal volume delivered to the airway during VCV or PCV.
Two newer anesthesia machine ventilators tested (the GE Aisys and the Draeger Apollo) use breathing-circuit compliance compensation to deliver tidal volume accurately. One is bellows-driven (the Aisys, which is similar to the Avance machine, but with compliance compensation software installed) and the other is piston-driven (the Apollo). Both of these anesthesia machine ventilators successfully delivered the set tidal volume to the airway, within an accuracy of about ±5%, even at low lung compliances and low tidal volumes. We did not find a significant difference between the bellows-driven or piston-driven results. In addition, the machines using compliance compensation did accurately report the tidal volume delivered to the airway by using the compliance of the circuit to correct the exhaled volume measurement for the volume “stored” in the breathing circuit during the previous inspiration.
As in other studies, our data confirmed that during VCV, the compliance of the breathing circuit has a large impact on tidal volume delivered.1,7,8 For the circuit tested in this study, a change from a contracted breathing circuit to an extended breathing circuit reduced the tidal volume delivered to the airway by about 20%. This decrease in tidal volume is avoided if a compliance test is done after each change in breathing circuit configuration when using the anesthesia machine ventilators that use compliance compensation. If the machine check is performed before a case and then the breathing circuit configuration is altered during the case (e.g., by extending the tubing), the compliance of the breathing circuit changes, but the anesthesia machine continues to compensate for the tested compliance of the circuit (as it was during the machine check). To reestablish accurate volume delivery, the ventilator would need to be disconnected from the patient to repeat the compliance test. When using a ventilator with no compliance compensation, the ventilation settings would also likely need to be adjusted after extending or contracting the breathing circuit during VCV to maintain constant volume delivery. One would expect to create a sudden increase in tidal volume delivered to the airway if the breathing circuit were changed from the extended to the contracted configuration in the middle of an anesthetic (using ventilators with or without compliance compensation). In our experiments, the increase in tidal volume in such a situation was about 25% (Fig. 4), and this was accompanied by an increase in peak airway pressure of about 4 cm H2O. However, it is unlikely that the breathing circuit would be reduced in length during an anesthetic.
PCV offers the advantage of limiting barotrauma in the face of changing lung compliance. However, as lung compliance varies, the actual tidal volume delivered to the patient changes in ways not measured by most current anesthesia machines,8,9 which our data confirm. In addition, the actual transmural and transalveolar airway pressure may be much less than the pressure the anesthesiologist sets on the ventilator depending on the patient’s intraabdominal or intrathoracic (pleural) pressure. Furthermore, some anesthesia machine ventilators are not capable of delivering the set inspiratory pressures, especially with short inspiratory times, and this goes undetected by the anesthesia machines.3,10 These limitations prevent the anesthesiologist from knowing exactly how much ventilation the patient is receiving during PCV. Anesthesiologists rely on capnography and observation of chest wall movement to monitor and adjust ventilation. Capnography is limited by the end-tidal to arterial Pco2 gradient, which increases as tidal volume decreases, thus obscuring the hypoventilation that may occur.
Our measurements of PCV with the Smartvent 7900 confirmed that, as one would expect, the tidal volume delivered to the airway did not depend on the breathing circuit configuration, only on the inspiratory pressure and lung compliance. However, the anesthesia machine monitor over-estimated the tidal volume delivered to the airway by 7%–40% depending on the lung compliance, with a lower lung compliance resulting in a less accurate reported tidal volume. Also, the anesthesia machine reported higher tidal volumes (and therefore was less accurate) when the breathing circuit was extended. The lowest lung compliance we tested using pressure control (0.005 L/cm H2O) was still higher than normal neonatal lung compliance. Our PCV measurements confirm the difficulty of determining the tidal volume received by a small pediatric patient, especially if lung compliance changes.
One limitation of our study was that we did not collect PCV data for the newer anesthesia machine ventilators because PCV is already well established and our study goal was to explore the feasibility of VCV in pediatric anesthesia. The major limitations of pressure control are unlikely to change in newer ventilators; these disadvantages were mentioned above and include uncontrolled changes in tidal volume with changes in lung compliance as well as inability to measure true transmural airway pressure. Another limitation of our study is that we did not simulate tidal volumes <100 mL. As a result, the data do not support conclusions about the utility of VCV at tidal volumes <100 mL. Nevertheless, the advantages of compliance compensation documented in this study should apply at tidal volumes within the specifications of the ventilator.
In addition to barotrauma due to high airway pressure, both atelectotrauma, related to repeated recruitment of alveoli in the absence of positive end-expiratory pressure, as well as over-distention of alveoli at larger tidal volumes have been identified as potentially injurious to the lung in patients who have, or are at risk for, respiratory distress syndrome.11 Trials of “lung-protective” ventilatory strategies have used volume-based variables (rather than PCV) in an effort to limit the inspired tidal volume.12–15 Evidence is accumulating that careful control of tidal volume may improve postoperative pulmonary function in patients at risk.
Our study indicates that new anesthesia ventilators designed to compensate for circuit compliance are capable of accurate volume delivery to the airway. Coupled with improved monitoring technology, it is possible to use VCV in virtually all patients. PCV has been used successfully for some time, but interest in the benefits of careful volume control makes the choice of VCV clinically compelling for some patients. Now that we can accurately control tidal volumes, the primary disadvantage of VCV is the fluctuation in airway pressure that results from changes in lung compliance. Air leaks around uncuffed endotracheal tubes can reduce the delivered tidal volume so that VCV may not be the best choice if there is any significant leak of gas around an uncuffed endotracheal tube. Modern anesthesia ventilators allow for a pressure limit to be set that will define the maximum pressure possible. The pressure limit setting can help to protect the patient from transient airway pressure increases, but it will limit the tidal volume if set below the limit required to deliver the set volume.
It remains to be seen whether VCV makes a clinical and/or outcome difference in pediatric patients under anesthesia. Anesthesia ventilators with compliance compensation allow clinicians to confidently use VCV for pediatric patients. Future studies could address the impact of using VCV in pediatric patients, especially when lung compliance is reduced and delivered volume is likely to fluctuate if PCV is used.
ACKNOWLEDGMENTS
We thank Mr. Kurt Goebel for his technical assistance with the anesthesia machines as well as for his kindness in sharing space with us for our tests.
We also thank GE Medical and Draeger Medical for providing equipment and technical information without which this project could not have been completed.
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