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

Airway management in infants and children

Anaesthesiologists, paediatricians, paediatric intensivists and emergency physicians are routinely challenged with airway management in children and infants. There are important differences from adult airway management as a result of specific features of paediatric anatomy and physiology, which are more relevant the younger the child. In addition, a number of inherited and acquired pathological syndromes have significant impact on airway management in this age group. Several new devices--e.g. different types of laryngeal mask airways in various sizes, small fibre-endoscopes--have been introduced into clinical practice with the intention of improving airway management in this age group. Important new studies have gathered evidence about risks and benefits of certain confounding variables for airway problems and specific techniques for solving them. Airway-related morbidity and mortality in children and infants during the perioperative period are still high, and only a thorough risk determination prior to and continuous attention during the procedure can reduce these risks. Appropriate preparation of the available equipment and frequent training in management algorithms for all personnel involved appear to be very important.

The influence of flow rate on breathing circuit compliance and tidal volume delivered to patients in mechanical ventilation

Assessment of the gas volume that actually reaches the airways during mechanical ventilation appears to be a difficult task because of the presence of the breathing circuit. Most ventilators measure tidal volume at ventilator level making the determination of circuit compliance a critical factor in estimating the actual tidal volume. Tubing compliance can be measured in several ways and its value, being strongly dependent on the compressibility of the gas, may significantly differ depending on the measurement procedure. This paper addresses the dependence of the circuit compliance on the flow rate, and a theoretical hypothesis on the pneumatic behaviour of the breathing circuit is formulated and experimentally validated, with several tests conducted in vitro on an infant analogue. The dependence of the compliance on the inspiratory flow is experimentally assessed, and differences of about 20% on the measured value in the common flow range utilized in infant ventilation have been found, with consequent estimation errors of the volume delivered. Experimental tests show that the correct value of the tidal volume actually delivered to the patient can be reliably estimated from measurements performed at the ventilator level if the circuit compliance is determined with the same flow rates that will be utilized in mechanical ventilation.

The temperature and humidity of inspired gases in infants using a pediatric circle system: effects of high and low-flow anesthesia

BACKGROUND

The effects of low-flow anesthesia on the temperature and humidity of the inspired gas in infants during mechanical ventilation is unknown. This study was designed to evaluate the temperature and humidity of the inspired gas in infants using a pediatric circle absorber system with high and low fresh gas flow (HFGF and LFGF) anesthesia.

METHODS

Twenty infants participated in this observational, sequential, cross-over study. Each infant was mechanically ventilated with a Kion Anesthesia Workstation, using a pediatric anesthesia circle circuit with both HFGF (6 l.min(-1)) and LFGF (0.6 l.min(-1)) technique. Airway temperature was recorded continuously at 16 sites throughout the breathing circuit. The relative humidity of the inspired gas was measured at the elbow connector adjacent to the CO2 sampling line.

RESULTS

The mean airway temperatures of the inspired gas and the changes in mean airway temperatures throughout the breathing circuit during HFGF and LFGF did not differ significantly. The mean relative humidity of the inspired gas at steady state using a LFGF technique, 33.7 +/- 3.6%, was approximately threefold greater than it was with a HFGF technique, 11.9 +/- 5.1% (P < 0.05).

CONCLUSIONS

Low-flow anesthesia with a pediatric circle system in infants neither increases the temperature of the inspired gas, nor achieves the minimum humidity of 50% reported to prevent ciliary damage, although the humidity during LFGF did increase threefold compared with HFGF. To maintain the temperature and humidity of the inspired gas during mechanical ventilation in infants, passive or active gas humidification should be used.

Volume ventilation of infants with congenital heart disease: a comparison of Dräger, NAD 6000 and Siemens, Servo 900C ventilators

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: PaCO(2) 43 +/- 8 vs 47 +/- 5 mm Hg (P = 0.005), end-tidal CO(2) 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 H(2)O (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.

Comparison of NAD 6000 and servo 900C ventilators in an infant lung model

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 (V(t)) 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 V(t) of 30 mL, the actual delivered V(t) 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 V(t), their delivered V(t) were not significantly different, NAD 6000 (66 +/- 19 mL) and Servo 900C (60 +/- 12 mL), P = 0.09. The exhaled V(t) read by the anesthesia machines was significantly closer to the delivered V(t) 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 H(2)O 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 V(t) 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.

Pressure-limited ventilation of infants with low-compliance lungs: the efficacy of an adult circle system versus two free-standing intensive care unit ventilator systems using an in vitro model

We compared the efficacy of a Drager Narkomed GS (North American Drager, Telford, PA) equipped with an adult circle system with two free-standing infant ventilator systems (Servo 300; Siemens Medical Systems, Danvers, MA and Babylog 8000; North American Drager) to deliver minute ventilation (VE) using pressure-limited ventilation to a test lung set to low compliance. To simulate a wide variety of potential patterns of ventilation, VE was measured at peak inspiratory pressures (PIP) of 20, 30, 40, and 50 cm H2O and at respiratory rates (RR) of 20, 30, 40, and 50 breaths/min. Each measurement was made three times; the average was used for data analysis using the multiple regression technique. Delivered V(E) was positively correlated with both PIP (P = 0.001) and RR (P = 0.001). Only minimal differences in VE were observed between the circle and the two free-standing systems. At lower RR and PIP, the Babylog 8000 system delivered slightly higher VE than the circle system, whereas at higher RR and PIP, the Babylog 8000 delivered slightly lower VE than the circle system; these differences in VE were not statistically significant (P = 0.45). The Servo 300 delivered slightly higher VE than the circle system in all test conditions, but these differences were not statistically significant (P = 0.09). None of the differences in delivered VE between the Servo 300 and the circle system are of clinical importance.

IMPLICATIONS

Our laboratory investigation suggests that pressure-limited ventilation delivered by a standard adult circle system compares favorably with that of freestanding infant ventilators used in pressure-limited mode. Changing from an adult circle system to a free-standing pressure-limited ventilator may not substantially improve ventilation of a low-compliance infant lung; the efficacy of such a practice should be investigated.

An adult system versus a Bain system: comparative ability to deliver minute ventilation to an infant lung model with pressure-limited ventilation

We compared the efficacy of an adult circle system versus a Bain system to deliver minute ventilation (V(E)) to an infant test lung model using pressure-limited ventilation. To simulate a wide variety of potential infant clinical states, V(E) was measured with two compliances: at peak inspiratory pressures (PIP) of 20, 30, 40, and 50 cm H2O and at respiratory rates (RR) of 20, 30, 40, and 50 breaths/min. Each measurement was made three times, and their average was used for analysis. Data were analyzed using the multiple regression technique. In both normal and low-compliance lung models, V(E) was nearly identical between adult circle and Bain systems (P = 0.67 for normal compliance model, P = 0.89 for low-compliance model). V(E) positively correlated with RR (P < 0.001), PIP (P < 0.001), and lung compliance (P < 0.001). Very high PIP or RR were required to deliver V(E) to the low-compliance lung model. The adult circle system is equivalent to the Bain system in its ability to ventilate an infant test lung over a wide range of RR, PIP, and two compliances during pressure-limited ventilation. V(E) is dependent of PIP, RR, and lung compliance. With low-compliance lungs, both systems require a high PIP. We conclude that both anesthetic systems deliver ventilation over a wide range of respiratory variables during pressure-limited ventilation in infants.

IMPLICATIONS

We obtained results from this infant test lung study that indicate that either an adult circle breathing system or the Bain system can reliably deliver ventilation over a wide range of respiratory variables during pressure-limited ventilation in infants.

Evaluation of a new operating room ventilator with volume-controlled ventilation: the Ohmeda 7900

Changes in fresh gas flow (FGF) during volume-controlled ventilation with the circle system have clinically important effects on the ventilatory variables of children. Current operating room ventilators allow a portion of the FGF to be added to the delivered tidal volume. The Ohmeda 7900 (Madison, WI) ventilator was designed to compensate for changes in FGF. We compared this ventilator with a standard ventilator, the Ohmeda 7000. Twenty patients (13-56 kg) undergoing dental or lower extremity surgery were studied. A side-by-side comparison of the two ventilators was performed using each patient as his or her own control. Beginning with the 7900 ventilator, FGF was set at 3.0 L/min, and the inspiratory to expiratory ratio was set at 1:2. Respiratory rate and tidal volume were adjusted to achieve an ETCO2 of 30-40 mm Hg. After a 10-min period of stabilization, inspired minute ventilation (VI), expired minute ventilation (VE), and ETCO2 were measured. FGF was then increased to 6.0 L/min, and the measurements were repeated after 10 min; FGF was then decreased to 1.5 L/min, and measurements were repeated after 10 min. The patient was then ventilated with an Ohmeda 7000 ventilator, and the sequence was repeated. The Ohmeda 7000 ventilator demonstrated significant changes in VI, VE, plateau pressure, and ETCO2, with changes in FGF (P = 0.0039-0.0001). The Ohmeda 7900 ventilator demonstrated compensation for changes in FGF; there were no significant changes in VI, VE, and ETCO2. We conclude that the Ohmeda 7900 ventilator provides stable ventilatory variables regardless of alterations in FGF (1.5-6.0 L/min).

IMPLICATIONS

In this study, we compared the effects of changing fresh gas flow on volume-controlled ventilation using two operating room ventilators (Ohmeda 7000 and Ohmeda 7900). The Ohmeda 7900, but not the Ohmeda 7000, provided stable ventilatory variables with fresh gas flows between 1.5 and 6.0 L/min.

A comparison of three modes of ventilation with the use of an adult circle system in an infant lung model

We examined the efficiency of an adult circle system with adult bellows to deliver minute ventilation (VE) to an infant test lung model. A Narkomed 2B system (North American Drager, Telford, PA) using three modes of ventilator setup were used: A = time-cycled, volume-controlled using bellows excursion to control delivered volume; B = time-cycled, pressure-controlled using inspiratory pressure limit adjustment to control delivered volume; C = time-cycled, pressure-controlled using the inspiratory flow adjustment to control delivered volume. VE was measured with two compliances (normal and low) and four endotracheal tube (ETT) sizes (2.5-, 3.0-, 3.5-, and 4.0-mm inner diameter). VE was measured at peak inspiratory pressures (PIP) of 20, 30, 40 or 50 cm H2O while respiratory rate (RR) was held constant at 20 breaths/min. VE was measured as RR was set at 20, 30, 40, or 50 breaths/min while target PIP was held constant at 20 cm H2O. Data were analyzed using the multiple regression technique. With the low compliance model, VE was nearly identical regardless of the ventilator setup. With the normal compliance model, minor differences in VE were observed, especially at the highest RR and PIP. VE was dependent on RR, PIP, and lung compliance. Overall, the ventilator setup resulted in minor changes in VE. Very high PIPs were required to deliver VE to the low compliance model. ETT size did not affect VE when lung compliance was low; however, smaller ETT size was a factor when test lung compliance was normal, decreasing delivered VE at higher PIP and RR. We conclude that with a Narkomed 2B adult circle system VE is dependent on PIP, RR, and lung compliance, but not on mode of ventilator setup.

IMPLICATIONS

The results of this laboratory investigation indicate that when an adult circle system is used during infant anesthesia, the ventilation delivered depends primarily on the respiratory rate, peak inspiratory pressure, and the compliance of the lung being ventilated, rather than on the specific mode of ventilator setup.