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New Techniques in Neonatal Ventilation

Volume-targeted ventilation can be a powerful tool in preventing neonatal lung injury

Vol. 20 • Issue 3 • Page 14

Ventilation Today

Editors's Note: This is part one of a two-part article. Part two will discuss pressure-targeted ventilation strategies.

For more than a quarter of a century, neonatal respiratory failure was treated with mechanical ventilation using time-cycled, pressure-limited intermittent mandatory ventilation (IMV). A series of technological developments, including the advent of microprocessor-based mechanical ventilation, has greatly expanded the realm of ventilation for even the smallest of preterm infants. Over the last decade, neonatologists have begun using volume-targeted ventilation. Strategies employing volume-targeted mechanical ventilation are increasing in popularity.

Pressure-targeted ventilation delivers a volume of gas at a fixed peak pressure set by the clinician. The volume of gas reaching the lungs is a function of pulmonary compliance, and to a lesser extent, resistance. At the same pressure, less gas will reach the lungs when compliance is poor. At the onset of inspiration, there is a rapid acceleration of gas flow resulting in peak pressure and maximum tidal volume occurring early in inspiration.

In contrast, volume-targeted breaths provide a continuous flow of gas throughout inspiration, producing the characteristic "square wave" of flow vs. time. Peak pressure and volume delivery occur at the end of inspiration, resulting in slower and more uniform inflation of the lung. (See Figure.) The pressure is varied to deliver the desired volume of gas. An example of the value of volume-targeting is seen following surfactant administration, where lung compliance changes rapidly.

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In pressure-targeting, as compliance improves, tidal volume may increase rapidly if the clinician is not mindful to adjust the peak
inspiratory pressure. With volume-targeting, as compliance improves, the peak inspiratory pressure is automatically weaned to maintain consistent tidal volume delivery. This appears to decrease volutrauma and thoracic air leaks.1

True volume-targeted ventilation is difficult to achieve in the newborn because cuffed endotracheal tubes are not used. Some degree of leak will occur around the endotracheal tube. Most of this occurs during inspiration. When the lungs are stiff, there may also be significant compressible volume loss within the ventilator circuit. Thus, it is mandatory to measure the delivered gas volume as close to the airway as possible.

Volume-targeted modes

Similarities exist between pressure-targeted and volume-targeted ventilation. Both can be provided as IMV, synchronized intermittent mandatory ventilation (SIMV), alone or with pressure support ventilation (PSV) or assist control (A/C). A/C is generally recommended for acute illness, while SIMV/PSV is recommended during weaning. Targeted tidal volumes for neonates < 1000 g are between 4 and 7 mL/kg and for neonates >1000g, 5 to 8 mL/kg.2 Verification of appropriate tidal volume delivery can be done by observing pulmonary mechanics on real-time graphic monitoring, an essential component of present day mechanical ventilators.

Hybrid volume-targeted ventilation

In addition to traditional volume-targeting, there are several hybrid forms of volume-targeted ventilation, which aim to combine the benefits of both pressure and volume-targeted ventilation. These vary by manufacturer and device.

• Volume-guarantee (VG) ventilation functions like pressure-limited ventilation, but it allows the clinician to select a target tidal volume and to limit the peak inspiratory pressure. It does this by measuring the exhaled tidal volume of the previous breath and adjusting pressure on the subsequent breath to meet the set volume. It does not utilize a square flow waveform, however.3

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Figure: Pressure and flow vs. time for volume-targeted ventilation

• Pressure-regulated volume control (PRVC) ventilation offers a variable flow rate of pressure-control ventilation with volume targeting. In this modality, like VG, the clinician sets the target tidal volume and maximum peak inspiratory pressure. The biggest difference between the two is that PRVC uses test breaths to calculate compliance then uses the lowest possible pressure to achieve the desired tidal volume.1

• Volume-assured pressure support (VAPS) differs from VG and PRVC in that it makes an intra-breath adjustment to assure that the patient receives at least a minimum tidal volume. The neonate initiates a breath, and the ventilator determines if the desired tidal volume is met.

Based upon this measurement, VAPS either terminates the breath (as a pressure support breath) or transitions it to a volume-targeted breath by prolonging inspiration, increasing inspiratory pressure, or doing both, creating a square flow waveform.1

• Volume-support (VS) ventilation is weaning modality in spontaneously breathing neonates with adequate respiratory drive. VS increases pressure delivered in a stepwise fashion until the set tidal volume is achieved.1

• Pressure augmentation (PA) is similar to VAPS, but differs in that it ensures a minimum tidal volume by augmenting the neonate's breath with increased flow and does not limit the maximum tidal volume.1

Limitations of volume-targeted ventilation

One drawback of volume-targeting is related to the use of uncuffed endotracheal tubes. Because most of the leak occurs during inspiration, differences between measured inspiratory and expiratory tidal volumes will occur and can be confusing to inexperienced clinicians. During volume-targeted ventilation, the inspiratory time is set by the flow rate. If set too low, there may be excessive hysteresis on the pressure-volume loop and a prolonged inspiratory time. If set too high, the inspiratory time may be too short to deliver the desired tidal volume, leading to air hunger. Use of an inspiratory hold can alleviate this.2

Clinical evidence

The Cochrane Collaboration published recently a systematic review comparing volume-targeted (VTV) to pressure-limited (PLV) ventilation. The investigators extensively evaluated the literature and ultimately found 12 trials that met criteria to include in their analysis. While no differences in mortality were found, there was a 27 percent reduction in the risk of death or bronchopulmonary dysplasia (defined as supplemental oxygen requirement at 36 weeks gestation) with volume-targeted ventilation. Additionally, the review found that VTV decreased incidence of pneumothorax, hypocarbia, duration of ventilation, and even neurologic injury (severe intraventricular hemorrhage or periventricular leukomalacia). Lastly, no differences were apparent in long-term neurologic outcomes (although only two studies evaluated this).4

The review concluded that the use of VTV should no longer be considered investigational. Indeed, there were clearly demonstrated benefits of VTV compared to PLV with no evidence of harm. VTV offers new choices to clinicians and enhanced benefits to babies. It even appears preferable in certain clinical circumstances, especially those characterized by changing lung compliance.

References

1. Donn S, Boon W. Mechanical ventilation of the neonate: should we target volume or pressure? Respiratory Care. 2009. 54(9):1236-43.

2. Donn S, Sinha S. Manual of Neonatal Respiratory Care. 2006, Mosby-Elsevier: Philadelphia. 206-209.

3. Keszler M, Abubakar K. Volume guarantee ventilation. Clinics in perinatology. 2007. 34(1):107-16.

4. Wheeler K, et al. Volume-targeted versus pressure-limited ventilation in the neonate. Cochrane Database of Systematic Reviews, 2010(11):1-84.

Stephen W. Patrick, MD, MPH, is a post-doctoral fellow in neonatal-perinatal medicine at C.S. Mott Children's Hospital and a Robert Wood Johnson Foundation Clinical Scholar at the University of Michigan Health System, Ann Arbor. Steven M. Donn, MD, FAAP, is a professor of pediatrics at the University of Michigan Medical School and a member of the division of neonatal-perinatal medicine at C.S. Mott Children's Hospital.




     

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