Simulation of pressure support for spontaneous breathing trials in neonates
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Endotracheal tubes used for neonates are not as resistant to breathing as originally anticipated; therefore, spontaneous breathing trials (SBTs) with continuous positive airway pressure (CPAP), without pressure support (PS), are recommended. However, PS extubation criteria have predetermined pressure values for each endotracheal tube diameter (PS 10 cmH2O with 3.0- and 3.5-mm tubes or PS 8 cmH2O with 4.0-mm tubes). This study aimed to assess the validity of these SBT criteria for neonates, using an artificial lung simulator, ASL 5000™ lung simulator, and a SERVO-i Universal™ ventilator (minute volume, 240–360 mL/kg/min; tidal volume, 30 mL; respiratory rate, 24–36/min; lung compliance, 0.5 mL/cmH2O/kg; resistance, 40 cmH2O/L/s) in an intensive care unit. We simulated a spontaneous breathing test in a 3-kg neonate after cardiac surgery with 3.0–3.5-mm endotracheal tubes. We measured the work of breathing (WOB), trigger work, and parameters of pressure support ventilation (PSV), T-piece breathing, or ASL 5000™ alone.
WOB displayed respiratory rate dependency under intubation. PS compensating tube resistance fluctuated with respiratory rate. At a respiratory rate of 24/min, the endotracheal tube did not greatly influence WOB under PSV and the regression line of WOB converged with the WOB of ASL 5000™ alone under PS 1 cmH2O; however, at 36/min, endotracheal tube was resistant to breathing under PSV because trigger work increased exponentially with PS ≤ 9 cmH2O. The regression line of WOB under PSV converged with the WOB of T-piece breathing under PS 1 cmH2O. Furthermore, PS compensating endotracheal tube resistance was 6 cmH2O. The WOB of ASL 5000™ alone approached that of respiratory distress syndrome (RDS); however, the pressure of patient effort was normal physiological range at PS 10 cmH2O. PS equalizing WOB under PSV with that after extubation depended on the respiratory rate and upper airway resistance. If WOB after extubation equaled that of T-piece breathing, the PS was 0 cmH2O regardless of the respiratory rates. If WOB after extubation approximated to that of ASL 5000™ alone, the PS depended on the respiratory rate.
SBT strategies should be selected per neonatal respiratory rates and upper airway resistance.
KeywordsAirway extubation Mechanical ventilators Neonate Pulmonary ventilation Ventilator weaning Work of breathing
Analysis of variance
Continuous positive airway pressure
Airway opening pressure
Positive end-expiratory pressure
Peak inspiratory pressure
Pressure of muscle
Pressure support ventilation
Spontaneous breathing trial
Work of breathing
Mechanical ventilation weaning has become a common procedure in the neonatal intensive care unit. Extubation failure reportedly increases morbidity, length of hospital stay, and mortality . Spontaneous breathing trials (SBTs) with pressure support (PS) are better than continuous positive airway pressure (CPAP) for adults because successful SBT rates with PS are higher than CPAP without an increase in the reintubation rate [2, 3]. There are both pros and cons to apply pressure support for spontaneous breathing trials in infant. Endotracheal tubes used in neonates are not as resistant to breathing as was originally anticipated [4, 5, 6]; therefore, spontaneous breathing trials (SBTs) with CPAP, without PS, have been recommended [7, 8]. However, SBT with PS is reportedly useful and the positive predictive value of successful extubation is 93% . The PS criteria for SBTs are set at 10 cmH2O with 3.0- and 3.5-mm tubes or at 8 cmH2O with 4.0- and 4.5-mm tubes [9, 10, 11, 12]. There are obvious discrepancies between the two theories [7, 9]. It is difficult to clinically evaluate work of breathing. This study aimed to assess the validity of these criteria for neonates.
We conducted a lung simulation study using a high-end lung simulator to investigate the effect of reductions in PS and increase in the respiratory rate on SBTs.
We used an IngMar ASL 5000™ artificial lung simulator (version 3.4, 3.5; IngMar Medical, Pittsburgh, PA) with a built-in cylinder with a 17.8-cm diameter. The ASL 5000™ is a popular lung stimulator, which can imitate different breathing conditions and can measure various ventilation parameters including WOB, trigger work (TW), pressure of effort (pressure of muscle [Pmus]), maximum pressure drop during trigger, and positive end-expiratory pressure (PEEP). Respiratory parameters are automatically displayed on the control panel. We regarded the ASL 5000™ as a model of the lower respiratory tract (i.e., the upper respiratory tract was not included). We set the ASL 5000™ to reflect a 3-kg neonate after cardiac surgery to simulate SBTs with compliance at 0.5 mL/cmH2O/kg  and resistance at 40 cmH2O/L/s. The reference values for healthy neonate compliance and resistance are 1.5–2.0 mL/cmH2O/kg and 20–40 cmH2O/L/s, respectively [13, 14]. The ASL 5000™ was set to the constant VT mode under computer control, with a tidal volume of 30 mL (10 mL/kg) and a minute volume of 720–1080 mL/min, which corresponds to a respiratory rate (f) of 24–36/min. Endotracheal tubes with an inside diameter of 3.0 and 3.5 (Mallinckrodt™; Hi-Contour Oral/Nasal Tracheal Tube Cuffed Murphy Eye, Dublin, Ireland) were clinically curved and cuffed to prevent gas leakage. A 22/19-mm adapter with a built-in duct (diameter, 9 mm) was attached because the port of the ASL 5000™ was too large to attach an endotracheal tube. A ventilator (SERVO-i Universal™, version 3.0.1; Maquet, Danvers, MA) was set at PSV: PEEP, 4 cmH2O; FIO2, 0.4; inspiration time was set at 45% of respiration; and bias flow of 0.5 L/min was continuously delivered to the respiratory circuit. Trigger sensitivity was set to 5 to detect bias flow deviation of 0.25 L/min at the expiratory channel. The ventilator was connected to the artificial lung by means of a respiratory circuit (Smooth-Bor™; Smooth-Bor Plastics, Laguna Hills, CA). No respiratory humidifier or heat/moisture exchanger was used.
The following work and pressure parameters were measured under three breathing settings: (1) ASL 5000™ alone, (2) T-piece breathing, and (3) PSV. The parameters were measured under two control settings: the respiratory rate control setting and the PS control setting. At first, the parameters of all three breathing settings were measured in the respiratory rate control setting. In the respiratory rate control setting, the respiratory rate was increased from 24 to 36/min. The parameters under PSV were measured with a fixed PS of 10 cmH2O and 8 cmH2O in the respiratory rate control setting. Then, the PS control setting was used under PSV alone. The parameters were measured under the PS control setting with a fixed respiratory rate of 24 and 36/min. Under the PS control setting, PS was decreased from 14 to 0 cmH2O.
Definition of respiratory variables
WOB was measured at a stable tidal volume, and the mean and standard deviation values were determined from 10 breaths to account for instability.
Peak inspiratory pressure (PIP) is the pressure which is delivered by ventilator. Dynamic distending pressure of T-piece breathing is equivalent of Pmus of T-piece breathing, because T-piece breathing is not under pressure support.
Ten successive breaths per condition were measured. We used two-way analysis of variance (ANOVA) with Tukey’s multiple-comparison test for statistical analyses. WOB and TW were analyzed by linear or non-linear regression analysis as appropriately. All statistical analyses were performed using GraphPad Prism (GraphPad Software, Inc., La Jolla, CA). A p value < 0.05 was considered statistically significant.
Effect of respiratory rate on patient effort
Effect of pressure support on patient effort
Effect of flow rate on patient effort
Therefore, the PS compensating tube resistance fluctuates with the respiratory rate.
Upper airway resistance is similarly dynamic, depending on nasal breathing, mouth breathing, or respiratory support . Nasal airway resistance accounts for approximately two thirds of total upper airway resistance, and the resistance is comparable to that of the 3.0–3.5-mm tube . However, the glottis and larynx contribute to less than 10% of total upper airway resistance . PS equalizing WOB under PSV with that after extubation depended on the respiratory rate and upper airway resistance. If WOB after extubation equaled that of T-piece breathing, the PS was 0 cmH2O regardless of the respiratory rates. If WOB after extubation approximated to that of ASL 5000™ alone, the PS depended on the respiratory rate. Minimum PS is adequate for neonates in a better condition, requiring a lower respiratory rate; however, PS compensating tube resistance may be necessary for neonates in marginal respiratory conditions, requiring higher respiratory rates. SBTs with PS 10 cmH2O are so potent that patient effort is decreased to normal physiological range under respiratory distress syndrome (RDS)-like conditions regardless of tube size [5, 22, 23]. Extubation is not recommended for neonates intolerant to SBTs even at PS 10 cmH2O. At a respiratory rate of 36/min with 3.0–3.5-mm tubes, the pressure of patient effort exceeded the physiological range under PS 8 cmH2O even when WOB under PSV was lower than that after extubation. Furthermore, it is necessary to evaluate patient effort to assess SBTs. Tachypnea and flow starvation may impose non-physiological stress on the lungs .
Furthermore, the Reynolds number at mean flow was < 1760 and was > 2000 at peak flow under intubation. Therefore, gas flow became turbulent at peak flow and then decelerated markedly to a laminar flow, regardless of tube size, because the lower critical Reynolds number is approximately 1760, below which turbulent structures cannot be sustained by any induced disturbance . The pressure gradient of turbulent flow produces fluid flow less efficiently than that of laminar flow [26, 27, 28], which may have affected WOB and pressure parameters. The upper critical value of the Reynolds number for transition from a laminar to a turbulent flow cannot be generalized even when at 2000 in clinical practice. Hof et al.  reported that “Most pipe flows are turbulent in practice even at modest flow rates”. The inlet diameter of ASL 5000™ was sufficiently large, such that inspiratory flow constantly remained laminar with an increase in respiratory rate because the Reynolds number was constantly < 1760. These results potentially explain why WOB of ASL 5000™ alone did not increase with an increase in respiratory rate in addition to the difference in inlet diameter and absence of tube length resistance.
The ASL 5000™ is an artificial lung model that excludes the upper respiratory tract. In our study, we could not determine the pressure at which WOB would be equivalent to WOB after extubation. The tidal volume of the ASL 5000™ can be set in increments of 10 mL. We considered the physiological tidal volume to be 5–8 mL/kg. A tidal volume of 20 mL cannot cover 8 mL/kg (24 mL for a 3-kg infant); therefore, we chose to use a tidal volume of 30 mL. The upper limit of respiratory rate was determined by dividing the physiological minute volume of 1080 mL/min by the tidal volume of 30 mL. We added PEEP at the minimum required value of 4 cmH2O for neonates, which was lower than that generally used during SBTs. However, there was no atelectasis or tidal recruitment in the ASL 5000™; therefore, the PEEP value of 4 cmH2O seemed to have minimal impact on this study. Gas is heated and humidified in a clinical setting, but humidified gas could not be used for the ASL 5000™. Water vapor has a lower density and viscosity than oxygen or nitrogen; kinetic viscosity (η/ρ [m2/s]) increased from 15.1 × 10−6 to 16.6 × 10−6 with heating and humidification of dry air at 20 °C to relative humidity of 100% at 37 °C, and the Reynolds number decreased by approximately 9%. Therefore, heating and humidification may not greatly affect fluid characteristics [17, 26, 29].
WOB displayed respiratory rate dependency under intubation. We should judge which strategy is appropriate for neonates in various respiratory conditions.
This study was supported by a research grant from Yokohama City University, Yokohama, Japan. The authors have disclosed that they do not have any conflicts of interest.
Availability of data and materials
The datasets generated and analysed during the current study are available from the corresponding author upon reasonable request.
YY and YM participated in the design of the study and helped to draft the manuscript. TM conceived the simulation study and helped to draft the manuscript. MO and OY participated in the design of the study and helped to draft the manuscript. TG helped to draft the manuscript. All authors read and approved the final manuscript.
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Consent for publication
The authors declare that they have no competing interests.
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- 1.Blackwood B, Murray M, Chisakuta A, Cardwell CR, O’Halloran P (2013) Protocolized versus non-protocolized weaning for reducing the duration of invasive mechanical ventilation in critically ill paediatric patients. Cochrane Database Syst Rev:CD009082Google Scholar
- 2.Burns KEA, Soliman I, Adhikari NKJ, Zwein A, Wong JTY, Builes CG, Pellegrini JA, Chen L, Rittayamai N, Sklar M, Brochard LJ, Friedrich JO (2017) Trials directly comparing alternative spontaneous breathing trial techniques: a systematic review and meta-analysis. Crit Care 21:127CrossRefGoogle Scholar
- 3.Oulette DR, Patel S, Girard TD, Morris PE, Schmidt GA, Truwit JD, Alhazzani W, Burns SM, Epstein SK, Esteban A, Fan E, Ferre M, Fraser GL, Gong MN, Hough CL, Mehta S, Nanchal R, Pawlik AJ, Schweickert SCN, Strom T, Kress JP (2017) Liberation from mechanical ventilation in critically ill adults: an official American College of Chest Physicians / American Thoracic Society Clinical Practice Guideline. Chest 151:166–180CrossRefGoogle Scholar
- 4.Newth CJ, Venkataraman S, Willson DF, Meert KL, Harrison R, Dean JM, Pollack M, Zimmerman J, Anand KJ, Carcillo JA, Nicholson CE, Eunice Shriver Kennedy National Institute of Child Health and Human Development Collaborative Pediatric Critical Care Research Network (2009) Weaning and extubation readiness in pediatric patients. Pediatr Crit Care Med 10:1–11CrossRefGoogle Scholar
- 9.Faustino EV, Gedeit R, LA SAJA, Wypij D, Curley MA, Randomized Evaluation of Sedation Titration for Respiratory Failure (RESTORE) Study Investigators (2017) Accuracy of an extubation readiness test in predicting successful extubation in children with acute respiratory failure from lower respiratory tract disease. Crit Care Med 45:94–102CrossRefGoogle Scholar
- 11.Randolph AG, Wypij D, Venkataraman ST, Hanson JH, Gedeit RG, Meert KL, Luckett PM, Forbes P, Lilley M, Thompson J, Cheifetz IM, Hibberd P, Wetzel R, Cox PN, Arnold JH, Pediatric Acute Lung Injury and Sepsis Investigators (PALISI) Network (2002) Effect of mechanical ventilator weaning protocols on respiratory outcomes in infants and children: a randomized controlled trial. JAMA 288:2561–2568CrossRefGoogle Scholar
- 13.Charles JC, Lerman J, Anderson B (2009) A practice of anesthesia for infants and children, 4th edn. Saunders Elsevier, Philadelphia, pp 7–24Google Scholar
- 14.Goldsmith JP, Karotkin E, Suresh G, Keszler M (2010) Assisted ventilation of the neonate, 5th edn. Saunders Elsevier, St. Louis, pp 186–199 306–320Google Scholar
- 15.Dubois AB, Fenn WB, Rahn H (1964) Resistance to breathing. In: Handbook of physiology, section 3, vol 1. The Williams and Willkins Company, Baltimore, pp 451–462Google Scholar
- 16.Kaye GW, Laby TH (1920) Table of physical and chemical constants and some mathematical functions, 4th edn. Longmans, Green & CO, LondonGoogle Scholar
- 17.Kaye GW, Laby TH (2016) Tables of Physical & Chemical Contents. Available at: http://www.kayelaby.npl.co.uk. Accessed 1 Dec 2016
- 19.Chatburn RL, Mireles-Cabodevilla E (2013) Basics principles of ventilator design. In: Tobin MJ (ed) Principles and practice of mechanical ventilation, 3rd edn. McGraw-Hill, New York, pp 65–97Google Scholar
- 21.Goldsmith JP, Karotkin E, Suresh G, Keszler M (2010) Assisted ventilation of the neonate, 5th edn. Saunders Elsevier, St. Louis, pp 19–46Google Scholar
- 27.Reynolds O (1883) An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels. Philos Trans R Soc Lond Ser B Biol Sci 174:935–982Google Scholar
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