Volume-Targeted Ventilation

Original Article Volume-Targeted Ventilation

Martin Keszler, MD exchange is the primary goal of ventilator support and must be achieved with a minimum of lung injury and least possible degree of hemodynamic impairment, while avoiding injury to distant organs, such as the brain. Reduction of the work of breathing Recognition that volume, not pressure, is the key factor in ventilator- (WOB) is another important objective, especially in the extremely induced lung injury and awareness of the association of hypocarbia and premature infants who now constitute the bulk of infants requiring brain injury foster the desire to better control delivered tidal volume. mechanical ventilation. Recently, microprocessor-based modifications of pressure-limited, time- cycled ventilators were developed to combine advantages of pressure-limited ventilation with the ability to deliver a more consistent tidal volume. Each Volutrauma Vs Barotrauma of the modes has advantages and disadvantages, with limited clinical data There is now overwhelming evidence that excessive tidal volume, available to judge their effectiveness. The Volume Guarantee mode has been rather than high inspiratory pressure, is the primary determinant studied most thoroughly and is the only one that provides automatic of lung injury.1,2 As a result, most clinicians now monitor the weaning of peak pressure in response to improving lung compliance and delivered tidal volume (VT) when using pressure-limited patient respiratory effort. More consistent tidal volume, fewer excessively ventilation. The critical importance of distributing this tidal large breaths, lower peak pressure, less hypocarbia and lower levels of volume evenly into an optimally aerated lung has not been as inflammatory cytokines have been documented. It remains to be seen if widely appreciated (Figure 1). It is critical to recognize that these short-term benefits will translate into shorter duration of ventilation optimizing lung volume improves lung compliance. If inflating or reduced incidence of chronic lung disease. pressure is not promptly reduced in response, inadvertent Journal of Perinatology (2005) 25, S19–S22. doi:10.1038/sj.jp.7211313 hyperventilation, lung overexpansion and hemodynamic impairment will inevitably occur.

INTRODUCTION Importance of Adequate Peep The availability of a dizzying number of ventilator modes, In practical terms, optimization of lung inflation, referred to as techniques and devices characterizes the state of the art in neonatal ‘‘open lung concept,’’ is achieved by applying adequate positive ventilatory support at the outset of the 21st century. Unfortunately, end-expiratory pressure (PEEP). For a variety of reasons, including the technical advances that resulted in development of devices with poorly conceived animal studies where moderate to high levels of complex capabilities have outpaced our understanding of how and PEEP were applied to animals with normal lungs, resulting in when to use them optimally. Although our understanding of all the significant hemodynamic impairment, there is widespread fear of complexities of available modes is incomplete, several important using adequate levels of PEEP. This ‘‘PEEP-o-phobia’’ has been general principles of respiratory support need to be emphasized in difficult to overcome and may be one of the most important order to optimize the use of any of these modalities. obstacles to optimization of ventilatory support. It is important to understand that there is no single ‘‘safe’’ PEEP level. Rather, optimal end-expiratory pressure must be tailored to the degree of Goals of Mechanical Ventilation lung injury (i.e. lung compliance). For infants without lung disease and thus normal lung compliance, PEEP of 3 cmH Ois First, it is important to understand the goals of mechanical 2 probably appropriate and PEEP of 5 cmH O may result in ventilation. Maintenance of adequate, not necessarily normal, gas 2 overexpansion of the lungs with impairment of venous return, elevated cerebral venous and systemic venous pressures and decreased cardiac output. On the other hand, severely atelectatic, Division of Neonatology, Department of Pediatrics, Georgetown University, Washington, DC, USA. poorly compliant lungs may require PEEP levels as high as 8 or

Disclosure: Research support was from Discovery Laboratories, Draeger Inc., Pﬁzer Inc. 10 cmH2O to achieve adequate lung volume and improve Consultant: Bunnel Inc., Draeger Inc. Speakers bureau: INO Therapeutics. ventilation/perfusion ratio. Because we seldom ventilate infants

Address correspondence and reprint requests to Martin Keszler, MD, Georgetown University with healthy lungs, PEEP of <5 cmH2O should be the exception, Hospital, 3800 Reservoir Rd. N.W., Washington, DC 20007, USA. rather than the rule.

Figure 1. Transverse CT view of lungs with severe RDS demonstrating nonuniform aeration of the lungs seen in lower left corner. This situation is schematically represented in the upper panel. The resulting pressure-volume relationship is seen in the lower right panel, illustrating low functional residual capacity and poor lung compliance. The vertical dotted lines show that the peak pressure is below the critical opening pressure (COP) of most of the alveoli and the end-expiratory pressure is below the critical closing pressure of most of the lung. Both peak inspiratory pressure and the end-expiratory pressure need to be increased to recruit alveoli and then maintain the recruitment during expiration. Failure to achieve more uniform aeration of the lungs (‘‘open lung’’) results in the tidal volume going into a small portion of the lungs, resulting in Volutrauma even when the tidal volume is in a normal range.

Rationale for Volume-Targeted Ventilation pressure-limited time-cycled mode that adjusts inspiratory pressure Recognition that volume, rather than pressure, is the critical to target a set VT, based on compliance calculation from the determinant of ventilator-induced lung injury has focused pressure plateau of an initial volume-controlled breath. The 1,2 breath-to-breath change in peak inspiratory pressure (PIP) is attention on the need to better control delivered VT. Furthermore, published literature strongly supports the role of hypocarbia in the limited to 3 cmH2O to avoid overshoot. The main problem with the development of brain injury.3–6 Despite awareness of its dangers, PRVC mode is major inaccuracy of VT measurement performed at inadvertent hyperventilation remains a common problem in the ventilator end of the circuit, rather than at the airway opening. clinical practice.7 Unfortunately, traditional volume-controlled A compensation scheme attempts to correct for loss of volume in the circuit, but the accuracy remains insufficient for use in small ventilation is not feasible in small newborns because of 8,9 neonates. Additionally, inspiratory VT is used to adjust PIP, unpredictable loss of VT to gas compression in the circuit, stretching of the tubing and variable leak around uncuffed making the device susceptible to large error in the presence of endotracheal tubes. Therefore, microprocessor-based modifications significant endotracheal tube (ETT) leak. of pressure-limited, time-cycled ventilation were developed to try to The volume assured pressure support (VAPS) mode on the Bird combine the advantages of pressure-limited ventilation with the VIP Gold (Viasys Healthcare, Palm Springs, CA) is a hybrid mode, which seeks to ensure that the desired VT is reached. Each breath ability to deliver a more consistent VT. starts as a pressure-limited breath, but if the set VT is not reached, the breath converts to flow cycled mode by prolonging inspiratory time with a passive increase in PIP. This may result in prolonged Types of Volume-Targeted Ventilation inspiratory time leading to expiratory asynchrony. Targeting of Three devices widely used in neonatal ventilation offer some form tidal volume is also based on inspiratory VT and therefore is of volume-targeted ventilation. Each of the available modes has susceptible to error in the presence of significant ETT leak. More advantages and disadvantages. Clinical data validating the importantly, there is no provision for automatically lowering performance of these modes are limited. inspiratory pressure as lung compliance improves. The focus is on Pressure-regulated volume control (PRVC) available on the ensuring a large enough VT, but no provision is made to avoid Servo 300 and Servo-I (Maquet Critical Care, Bridgewater, NJ) is a inadvertent hyperventilation and allow for automatic weaning. The

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new Avea ventilator by Viasys shares the basic features of VAPS, but adds a volume limit function that will terminate inspiration if the upper limit of VT is exceeded. This added function should reduce the risk of volutrauma and hyperventilation, but still does not lead to automatic weaning of inspiratory pressure. The Draeger Babylog (Draeger, Inc., Lubeck, Germany) Volume Guarantee (VG) option regulates inspiratory pressure using exhaled VT measurement to minimize artifact due to ETT leak. The operator chooses a target VT and selects a pressure limit up to which the ventilator operating pressure (the working pressure) may be adjusted. The microprocessor compares the VT of the previous breath and adjusts the working pressure to achieve the set VT. The algorithm limits the amount of pressure increase from one breath Figure 2. Volume guarantee combined with A/C reduced, but did not eliminate breaths that fell outside the target range of 4–6 ml/kg when to the next, in order to avoid overcorrection leading to excessive V . T compared to A/C alone. This, and the fact that the exhaled VT of the prior breath is used, means that with rapid changes in compliance or patient inspiratory effort, several breaths may be needed to reach target VT. If the delivered VT exceeds 130% of the previous breath, the setting lower VT target, the work of breathing could be progressively microprocessor opens the expiratory valve, terminating any shifted from the ventilator to the infant.14 In a small randomized additional pressure delivery in order to minimize the risk of clinical trial, we showed that the proportion of breaths outside the 3 excessive VT. By design, the algorithm is geared toward slower target range of 4 to 6 cm /kg and incidence of hypocarbia could be 15 adjustment for low VT and more rapid adjustment for excessive, significantly reduced with the use of VG, combined with A/C potentially dangerous VT. However, with continuous flow of fresh (Figure 2). We also showed that the combining VG with A/C, rather gas in the circuit, the infant can still generate a large breath on his than SIMV, results in more consistent VT, higher and more stable own. Separate algorithms control triggered and untriggered oxygen saturation, less tachypnea and tachycardia and requires mechanical breaths. The automatic reduction of inspiratory lower working pressure to achieve the same VT and minute pressure in response to improving lung compliance and increased ventilation.16 Finally, a recent study from Italy demonstrated lower patient effort makes VG a self-weaning mode. Since weaning occurs levels of proinflammatory cytokines in tracheal aspirates of preterm in real-time, rather than intermittently in response to blood gases, infants ventilated with VG combined with pressure support, VG has the potential to achieve faster weaning from mechanical compared to pressure support ventilation alone.17 ventilation. It remains to be seen if these short-term benefits will translate into major outcome benefits such as shorter duration of ventilation or decreased risk of intraventricular hemorrhage/periventricular leukomalacia or reduced incidence of BPD. The confirmation of Available Clinical Studies of Volume-Targeted these potential benefits will require completion of adequately Ventilation powered clinical trials. The only published study of PRVC in newborn infants demonstrated feasibility of the approach. The control group was ventilated with unsynchronized IMV (BP-200 or Sechrist Acknowledgements ventilators). The authors appeared to rely more on clinical Some of the research referred to in this paper was partially supported by a assessment of adequacy of chest rise than VT measurement, with research grant from Draeger Inc. 3 target VT settings of 9 to 11 cm /kg. Duration of ventilation and incidence of BPD were not different from controls. The rate of IVH, grade II or higher, was reduced with PRVC.10 A recent clinical trial References (available only in abstract form) also did not demonstrate a 1. Dreyfuss D, Saumon G. Ventilator-induced lung injury: lessons from difference in time to extubation or rate of BPD with PRVC assist/ 11 experimental studies. Am J Respir Crit Care Med 1998;157:294–323. control (A/C) mode compared to simple SIMV. 2. Clark RH, Slutsky AS, Gertsmann DR. Lung protective strategies of Several small, short-term clinical trials demonstrated feasibility ventilation in the neonate: what are they? Pediatrics 2000;105(1):112–4. and apparent safety of VG combined with A/C or SIMV. The 3. Graziani LJ, Spitzer AR, Mitchell DG, et al. Mechanical ventilation in working pressure was equal or lower with VG than without and the preterm infants. Neurosonographic and developmental studies. Pediatrics 12–14 incidence of excessively large VT was reduced substantially. By 1992;90:515–22.

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4. Fujimoto S, Togari H, Yamaguchi N, Mizutani F, Suzuki S, Sobajima H. ventilation in neonates: a prospective, randomised study. Intens Care Med Hypocarbia and cystic periventricular leukomalacia in premature infants. 1997;23(9):975–81. Arch Dis Child 1994;71:F107–10. 11. D’Angio CT, Chess PR, Kovacs SJ, et al. Assist-control ventilation in infants 5. Wiswell TE, Graziani LJ, Kornhauser MS, et al. Effects of hypocarbia 500–1250 grams’ birthweight does not decrease time to extubation when on the development of cystic periventricular leukomalacia in premature compared to synchronized intermittent mandatory ventilation: a rando- infants treated with high-frequency jet ventilation. Pediatrics 1996;98: mized controlled trial. Pediatr Res 2003;53:367A. 918–924. 12. Abubakar KM, Keszler M. Patient–ventilator interactions in newer modes of 6. Okumura A, Hayakawa F, Kato T, et al. Hypocarbia in preterm infants with mechanical ventilation. Pediatr Pulmonol 2001;32:71–5. periventricular leukomalacia: the relation between hypocarbia and 13. Cheema IU, Ahluwalia JS. Feasibility of tidal volume-guided ventilation in mechanical ventilation. Pediatrics 2001;107(3):469–75. newborn infants: a randomized, crossover trial using the volume guarantee 7. Luyt K, Wright D, Baumer JH. Randomised study comparing extent of modality. Pediatrics 2001;107:1323–8. hypocarbia in preterm infants during conventional and patient triggered 14. Herrera CM, Gerhardt T, Claure N, et al. Effects of volume-guaranteed ventilation. Arch Dis Child Fetal Neonatal Ed 2001;84:F14–7. synchronized intermittent mandatory ventilation in preterm infants 8. Cannon ML, Cornell J, Tripp-Hamel DS, et al. Tidal volumes for recovering from respiratory failure. Pediatrics 2002;110:529–33. ventilated infants should be determined with a pneumotachometer 15. Keszler M, Abubakar KM. Volume Guarantee: stability of tidal volume and placed at the endotracheal tube. Am J Respir Crit Care Med 2000;162: the incidence of hypocarbia. Pediatr Pulmonol 2004;38:240–5. 2109–12. 16. Abubakar KM, Keszler M. Volume Guarantee is more effective when 9. Castle RA, Dunne CJ, Mok Q, Wade AM, Stocks J. Accuracy of displayed combined with Assist/Control ventilation than with synchronized values of tidal volume in the pediatric intensive care unit. Crit Care Med intermittent mandatory ventilation (SIMV). Pediatr Res 2004;55:532A. 2002;30:2566–74. 17. Lista G, Colnaghi M, Castoldi F, et al. Impact of targeted-volume ventilation 10. Piotrowski A, Sobala W, Kawczynski P. Patient-initiated, pressure-regulated, on lung inﬂammatory response in preterm infants with respiratory distress volume-controlled ventilation compared with intermittent mandatory syndrome (RDS). Pediatr Pulmonol 2004;37(6):510–4.

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