Crossover study of proportional assist versus assist control ventilation

Novel forms of ventilation in neonates: Neurally adjusted ventilatory assist and proportional assist ventilation

Patient-triggered modes of ventilation are currently the standard of practice in the care of term and preterm infants. Maintaining spontaneous breathing during mechanical ventilation promotes earlier weaning and possibly reduces ventilator-induced diaphragmatic dysfunction. A further development of assisted ventilation provides support in proportion to the respiratory effort and enables the patient to have full control of their ventilatory cycle. In this paper we will review the literature on two of these modes of ventilation: neurally adjusted ventilatory assist (NAVA) and proportional assist ventilation (PAV), propose future studies and suggest clinical applications of these modes.

Early origins of respiratory disease

Chronic respiratory morbidity is unfortunately common in childhood, particularly in those born very prematurely or with congenital anomalies affecting pulmonary development and those with sickle cell disease. Our research group, therefore, has focused on the early origins of chronic respiratory disease. This has included assessing antenatal diagnostic techniques and potentially therapeutic interventions in infants with congenital diaphragmatic hernia. Undertaking physiological studies, we have increased the understanding of the premature baby's response to resuscitation and evaluated interventions in the delivery suite. Mechanical ventilation modes have been optimised and randomised controlled trials (RCTs) with short- and long-term outcomes undertaken. Our studies highlighted respiratory syncytial virus lower respiratory tract infections (LRTIs) and other respiratory viral LRTIs had an adverse impact on respiratory outcomes of prematurely born infants, who we demonstrated have a functional and genetic predisposition to respiratory viral LRTIs. We have described the long-term respiratory outcomes for children with sickle cell disease and importantly identified influencing factors. In conclusion, it is essential to undertake long term follow up of infants at high risk of chronic respiratory morbidity if effective preventative strategies are to be developed.

Eligibility Criteria and Representativeness of Randomized Clinical Trials That Include Infants Born Extremely Premature: A Systematic Review

OBJECTIVE

To assess the eligibility criteria and trial characteristics among contemporary (2010-2019) randomized clinical trials (RCTs) that included infants born extremely preterm (<28 weeks of gestation) and to evaluate whether eligibility criteria result in underrepresentation of high-risk subgroups (eg, infants born at <24 weeks of gestation).

STUDY DESIGN

PubMed and Scopus were searched January 1, 2010, to December 31, 2019, with no language restrictions. RCTs with mean or median gestational ages at birth of <28 weeks of gestation were included. The study followed the PRISMA guidelines; outcomes were registered prospectively. Data extraction was performed independently by multiple observers. Study quality was evaluated using a modified Jadad scale.

RESULTS

Among RCTs (n = 201), 32 552 infants were included. Study participant characteristics, interventions, and outcomes were highly variable. A total of 1603 eligibility criteria were identified; rationales were provided for 18.8% (n = 301) of criteria. Fifty-five RCTs (27.4%) included infants <24 weeks of gestation; 454 (1.4%) infants were identified as <24 weeks of gestation.

CONCLUSIONS

The present study identifies sources of variability across RCTs that included infants born extremely preterm and reinforces the critical need for consistent and transparent policies governing eligibility criteria.

Proportional assist and neurally adjusted ventilation: Clinical knowledge and future trials in newborn infants

Different types of patient triggered ventilator modes have become the mainstay of ventilation in term and preterm newborn infants. Maintaining spontaneous breathing has allowed for earlier weaning and the additive effects of respiratory efforts combined with pre-set mechanical inflations have reduced mean airway pressures, both of which are important components in trying to avoid lung injury and promote normal lung development. New sophisticated modes of assisted ventilation have been developed during the last decades where the control of ventilator support is turned over to the patient. The ventilator detects the respiratory effort and adjusts ventilatory assistance proportionally to each phase of the respiratory cycle, thus enabling the patient to have full control of the start, the duration and the amount of ventilatory assistance. In this paper we will review the literature on the ventilatory modes of proportional assist ventilation and neurally adjusted ventilatory assistance, examine the different ways the signals are analyzed, propose future studies, and suggest ways to apply these modes in the clinical environment.

Proportional assist ventilation (PAV) versus neurally adjusted ventilator assist (NAVA): effect on oxygenation in infants with evolving or established bronchopulmonary dysplasia

Both proportional assist ventilation (PAV) and neurally adjusted ventilatory assist (NAVA) provide pressure support synchronised throughout the respiratory cycle proportional to the patient's respiratory demand. Our aim was to compare the effect of these two modes on oxygenation in infants with evolving or established bronchopulmonary dysplasia. Two-hour periods of PAV and NAVA were delivered in random order to 18 infants born less than 32 weeks of gestation. Quasi oxygenation indices ("OI") and alveolar-arterial ("A-a") oxygen gradients at the end of each period on PAV, NAVA and baseline ventilation were calculated using capillary blood samples. The mean "OI" was not significantly different on PAV compared to NAVA (7.8 (standard deviation (SD) 3.2) versus 8.1 (SD 3.4), respectively, p = 0.70, but lower on both than on baseline ventilation (mean baseline "OI" 11.0 (SD 5.0)), p = 0.002, 0.004, respectively). The "A-a" oxygen gradient was higher on PAV and baseline ventilation than on NAVA (20.8 (SD 12.3) and 22.9 (SD 11.8) versus 18.5 (SD 10.8) kPa, p = 0.015, < 0.001, respectively).Conclusion: Both NAVA and PAV improved oxygenation compared to conventional ventilation. There was no significant difference in the mean "OI" between the two modes, but the mean "A-a" gradient was better on NAVA.What is Known:• Proportional assist ventilation (PAV) and neurally adjusted ventilatory assist (NAVA) can improve the oxygenation index (OI) in prematurely born infants.• Both PAV and NAVA can provide support proportional to respiratory drive or demand throughout the respiratory cycle.What is New:• In infants with evolving or established BPD, using capillary blood samples, both PAV and NAVA compared to baseline ventilation resulted in improvement in the "OI", but there was no significant difference in the "OI" on PAV compared to NAVA.• The "alveolar-arterial" oxygen gradient was better on NAVA compared to PAV.

Advances in treating bronchopulmonary dysplasia

Introduction: Bronchopulmonary dysplasia (BPD) is a common long-term adverse complication of very premature delivery. Affected infants can suffer chronic respiratory morbidities including lung function abnormalities and reduced exercise capacity even as young adults. Many studies have investigated possible preventative strategies; however, it is equally important to identify optimum management strategies for infants with evolving or established BPD. Areas covered: Respiratory support modalities and established and novel pharmacological treatments. Expert opinion: Respiratory support modalities including proportional assist ventilation and neurally adjusted ventilatory assist are associated with short term improvements in oxygenation indices. Such modalities need to be investigated in appropriate RCTs. Many pharmacological treatments are routinely used with a limited evidence base, for example diuretics. Stem cell therapies in small case series are associated with promising results. More research is required before it is possible to determine if such therapies should be investigated in large RCTs with long-term outcomes.

Neurally adjusted ventilatory assist compared to other forms of triggered ventilation for neonatal respiratory support

BACKGROUND

Effective synchronisation of infant respiratory effort with mechanical ventilation may allow adequate gas exchange to occur at lower peak airway pressures, potentially reducing barotrauma and volutrauma and development of air leaks and bronchopulmonary dysplasia. During neurally adjusted ventilatory assist ventilation (NAVA), respiratory support is initiated upon detection of an electrical signal from the diaphragm muscle, and pressure is provided in proportion to and synchronous with electrical activity of the diaphragm (EADi). Compared to other modes of triggered ventilation, this may provide advantages in improving synchrony.

OBJECTIVES

Primary• To determine whether NAVA, when used as a primary or rescue mode of ventilation, results in reduced rates of bronchopulmonary dysplasia (BPD) or death among term and preterm newborn infants compared to other forms of triggered ventilation• To assess the safety of NAVA by determining whether it leads to greater risk of intraventricular haemorrhage (IVH), periventricular leukomalacia, or air leaks when compared to other forms of triggered ventilation Secondary• To determine whether benefits of NAVA differ by gestational age (term or preterm)• To determine whether outcomes of cross-over trials performed during the first two weeks of life include peak pressure requirements, episodes of hypocarbia or hypercarbia, oxygenation index, and the work of breathing SEARCH METHODS: We performed searches of the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cohrane Library; MEDLINE via Ovid SP (January 1966 to March 2017); Embase via Ovid SP (January 1980 to March 2017); the Cumulative Index to Nursing and Allied Health Literature (CINAHL) via EBSCO host (1982 to March 2017); and the Web of Science (1985 to 2017). We searched abstracts from annual meetings of the Pediatric Academic Societies (PAS) (2000 to 2016); meetings of the European Society of Pediatric Research (published in Pediatric Research); and meetings of the Perinatal Society of Australia and New Zealand (PSANZ) (2005 to 2016). We also searched clinical trials databases to March 2017.

SELECTION CRITERIA

We included randomised and quasi-randomised clinical trials including cross-over trials comparing NAVA with other modes of triggered ventilation (assist control ventilation (ACV),synchronous intermittent mandatory ventilation plus pressure support (SIMV ± PS), pressure support ventilation (PSV), or proportional assist ventilation (PAV)) used in neonates.

DATA COLLECTION AND ANALYSIS

Primary outcomes of interest from randomised controlled trials were all-cause mortality, bronchopulmonary dysplasia (BPD; defined as oxygen requirement at 28 days), and a combined outcome of all-cause mortality or BPD. Secondary outcomes were duration of mechanical ventilation, incidence of air leak, incidence of IVH or periventricular leukomalacia, and survival with an oxygen requirement at 36 weeks' postmenstrual age.Outcomes of interest from cross-over trials were maximum fraction of inspired oxygen, mean peak inspiratory pressure, episodes of hypocarbia, and episodes of hypercarbia measured across the time period of each arm of the cross-over. We planned to assess work of breathing; oxygenation index, and thoraco-abdominal asynchrony at the end of the time period of each arm of the cross-over study.

MAIN RESULTS

We included one randomised controlled study comparing NAVA versus patient-triggered time-cycled pressure-limited ventilation. This study found no significant difference in duration of mechanical ventilation, nor in rates of BPD, pneumothorax, or IVH.

AUTHORS' CONCLUSIONS

Risks and benefits of NAVA compared to other forms of ventilation for neonates are uncertain. Well-designed trials are required to evaluate this new form of triggered ventilation.

Crossover study of assist control ventilation and neurally adjusted ventilatory assist

Some studies of infants with acute respiratory distress have demonstrated that neurally adjusted ventilator assist (NAVA) had better short-term results compared to non-triggered or other triggered models. We determined if very prematurely born infants with evolving or established bronchopulmonary dysplasia (BPD) had a lower oxygenation index (OI) on NAVA compared to assist control ventilation (ACV). Infants were studied for 1 h each on each mode. At the end of each hour, blood gas analysis was performed and the OI calculated. The inspired oxygen concentration (FiO₂), the peak inflation (PIP) and mean airway pressures (MAP) and compliance were averaged from the last 5 min on each mode. Nine infants, median gestational age of 25 (range 22-27) weeks, were studied at a median postnatal age of 20 (range 8-84) days. The mean OI after 1 h on NAVA was 7.9 compared to 11.1 on ACV (p = 0.0007). The FiO₂ (0.36 versus 0.45, p = 0.007), PIP (16.7 versus 20.1 cm H₂O, p = 0.017) and MAP (9.2 versus 10.5 cm H₂O, p = 0.004) were lower on NAVA. Compliance was higher on NAVA (0.62 versus 0.50 ml/cmH₂O/kg, p = 0.005).

CONCLUSION

NAVA compared to ACV improved oxygenation in prematurely born infants with evolving or established BPD. What is Known: • Neurally assist ventilator adjust (NAVA) uses the electrical activity of the diaphragm to servo control the applied pressure. • In infants with acute RDS, use of NAVA was associated with lower peak inflation pressures and higher tidal volumes. What is New: • This study uniquely reports infants with evolving or established BPD, and their results were compared on 1 h each of NAVA and assist controlled ventilation. • On NAVA, infants had superior (lower) oxygen indices, lower inspired oxygen concentrations and peak and mean airway pressures and higher compliance.

In vitro assessment of the effect of proportional assist ventilation on the work of breathing

During proportional assist ventilation, elastic and resistive unloading can be delivered to reduce the work of breathing (WOB). Our aim was to determine the effects of different levels of elastic and resistive unloading on the WOB in lung models designed to mimic certain neonatal respiratory disorders. Two dynamic lung models were used, one with a compliance of 0.4 ml/cm H2O to mimic an infant with respiratory distress syndrome and one with a resistance of 300 cm H2O/l/s to mimic an infant with bronchopulmonary dypslasia. Pressure volume curves were constructed at each unloading level. Elastic unloading in the low compliance model was highly effective in reducing the WOB measured in the lung model; the effective compliance increased from 0.4 ml/cm H2O at baseline to 4.1 ml/cm H2O at maximum possible elastic unloading (2.0 cm H2O/ml). Maximum possible resistive unloading (200 cm H2O/l/s) in the high-resistance model only reduced the effective resistance from 300 to 204 cm H2O/l/s. At maximum resistive unloading, oscillations appeared in the airway pressure waveform.

CONCLUSION

Our results suggest that elastic unloading will be helpful in respiratory conditions characterised by a low compliance, but resistive unloading as currently delivered is unlikely to be of major clinical benefit.

WHAT IS KNOWN

• During PAV, the ventilator can provide elastic and resistive unloading. What is New: • Elastic unloading was highly effective in reducing the work of breathing. • Maximum resistive unloading only partially reduced the effective resistance.

Invasive and non-invasive ventilation for prematurely born infants - current practice in neonatal ventilation

Non-invasive techniques, include nasal continuous positive airways pressure (nCPAP), nasal intermittent positive pressure ventilation (NIPPV) and heated, humidified, high flow cannula (HHFNC). Randomised controlled trials (RCTs) of nCPAP versus ventilation have given mixed results, but one demonstrated fewer respiratory problems during infancy. Meta-analysis demonstrated NIPPV rather than nCPAP provided better support post extubation. After extubation or initial support HHFNC has similar efficacy to CPAP. Invasive techniques include those that synchronise inflations with the patient's respiratory efforts. Assist control/ synchronised intermittent mandatory ventilation compared to non triggered modes only reduce the duration of ventilation. Further data are required to determine the efficacy of proportional assist ventilation and neurally adjusted ventilatory assist. Other techniques aim to minimise volutrauma. RCTs of volume targeted ventilation demonstrated reductions in BPD and respiratory medication usage at follow-up. Prophylactic high frequency oscillatory ventilation does not reduce BPD, but is associated with superior lung function at school age.

Proportional assist versus assist control ventilation in premature infants

During proportional assist ventilation (PAV), the applied pressure is servo-controlled based on continuous input from the infant's breathing. In addition, elastic and resistive unloading can be employed to compensate for the abnormalities in the infant's lung mechanics. The aim of this study was to test the hypothesis that in very prematurely born infants remaining ventilated beyond the first week, PAV compared to assist control ventilation (ACV) would be associated with superior oxygenation. A randomised crossover study was undertaken. Infants were studied for 4 hours each on PAV and ACV in random order; at the end of each 4-h period, the oxygenation index (OI) was calculated. Eight infants, median gestational age of 25 (range 24-33) weeks, were studied at a median of 19 (range 10-105) days. It had been intended to study 18 infants but as all the infants had superior oxygenation on PAV (p = 0.0039), the study was terminated after recruitment of eight infants. The median inspired oxygen concentration (p = 0.049), mean airway pressure (p = 0.012) and OI (p = 0.012) were all lower on PAV.

CONCLUSION

These results suggest that PAV compared to ACV is advantageous in improving oxygenation for prematurely born infants with evolving or established BPD.

WHAT IS KNOWN

During proportional assist ventilation (PAV), the applied pressure is servo controlled throughout each spontaneous breath. Elastic and resistive unloading can compensate for the infant's abnormalities in lung mechanics.

WHAT IS NEW

In a randomised crossover study, infants with evolving/established BPD were studied on PAV and ACV each for 4 h. The oxygenation index was significantly lower on PAV in all infants studied.

Updates on Functional Characterization of Bronchopulmonary Dysplasia - The Contribution of Lung Function Testing

Bronchopulmonary dysplasia (BPD) is a chronic lung disease that predominantly affects prematurely born infants. Initially, BPD was described in infants who had suffered severe respiratory failure and required high pressure, mechanical ventilation with high concentrations of supplementary oxygen. Now, it also occurs in very prematurely born infants who initially had minimal or even no signs of lung disease. These differences impact the nature of the lung function abnormalities suffered by “BPD” infants, which are also influenced by the criteria used to diagnose BPD and the oxygen saturation level used to determine the supplementary oxygen requirement. Key also to interpreting lung function data in this population is whether appropriate lung function tests have been used and in an adequately sized population to make meaningful conclusions. It should also be emphasized that BPD is a poor predictor of long-term respiratory morbidity. Bearing in mind those caveats, studies have consistently demonstrated that infants who develop BPD have low compliance and functional residual capacities and raised resistances in the neonatal period. There is, however, no agreement with regard to which early lung function measurement predicts the development of BPD, likely reflecting different techniques were used in different populations in often underpowered studies. During infancy, lung function generally improves, but importantly airflow limitation persists and small airway function appears to decline. Improvements in lung function following administration of diuretics or bronchodilators have not translated into long-term improvements in respiratory outcomes. By contrast, early differences in lung function related to different ventilation modes have led to investigation and demonstration that prophylactic, neonatal high-frequency oscillation appears to protect small airway function.