Volume guarantee and ventilator-induced lung injury: Goldilock's rules apply

Adjusting ventilator settings to avoid air trapping in extremely premature infants reduces the need for tracheostomy and length of stay

Despite the improving understanding of how lung mechanics and tidal volume requirements evolve during the evolution of bronchopulmonary dysplasia (BPD), clinical management continues to be heterogeneous and inconsistent at many institutions. Recent reports have examined the use of high tidal-volume low respiratory rate strategies in these patients once disease has been well established to help facilitate their eventual extubation and improve their long-term neurodevelopmental outcomes. In this retrospective observational research study, we describe how intentional adjustment of ventilator settings based on patient lung mechanics by an interdisciplinary BPD team improved the care of the at-risk population of infants, reduced the need for tracheostomies, as well as length of stay over a period of over 3 years. The team aimed to establish consistency in the management of these children using a high tidal volume, low-rate approach, and titrating PEEP to address the autoPEEP and bronchomalacia that is frequently observed in this patient population.

Evaluating peak inspiratory pressures and tidal volume in premature neonates on NAVA ventilation

Neurally adjusted ventilatory assist (NAVA) ventilation allows patients to determine their peak inspiratory pressure and tidal volume on a breath-by-breath basis. Apprehension exists about premature neonates' ability to self-regulate breath size. This study describes peak pressure and tidal volume distribution of neonates on NAVA and non-invasive NAVA. This is a retrospective study of stored ventilator data with exploratory analysis. Summary statistics were calculated. Distributional assessment of peak pressure and tidal volume were evaluated, overall and per NAVA level. Over 1 million breaths were evaluated from 56 subjects. Mean peak pressure was 16.4 ± 6.4 in the NAVA group, and 15.8 ± 6.4 in the NIV-NAVA group (t test, p < 0.001). Mean tidal volume was 3.5 ± 2.7 ml/kg.Conclusion:In neonates on NAVA, most pressures and volumes were within or lower than recommended ranges with pressure-limited or volume-guarantee ventilation. What is known: • Limiting peak inspiratory pressures or tidal volumes are the main strategies to minimize ventilator-induced lung injury in neonates. Neurally adjusted ventilatory assist allows neonates to regulate their own peak inspiratory pressures and tidal volumes on a breath-to-breath basis using neural feedback. What is new: • When neonates chose the size of their breaths based on neural feedback, the majority of peak inspiratory pressures and tidal volumes were within or lower than the recommended peak inspiratory pressure or tidal volume ranges with pressure-limited or volume guarantee ventilation.

Setting the Ventilator in the NICU

Success in providing respiratory support to the neonate requires a clear understanding of the context in which it is being applied. Perhaps more than for any other age group, the array of different situations in which ventilation is applied to the newborn infant is extremely broad, with in each case different pathophysiological disturbances and often the need to use a specific approach to apply ventilation optimally. Table 42.1 provides a list of the more common situations in which conventional ventilation is used in the neonate and includes some considerations regarding ventilator settings for each situation. For each situation, a suggested mode of ventilation is indicated, along with target ranges for positive end-expiratory pressure (PEEP) and tidal volume (V_T). Further discussion of the physiological rationale and available evidence for ventilator settings is set out below.

Volume-targeted ventilation is more suitable than pressure-limited ventilation for preterm infants: a systematic review and meta-analysis

OBJECTIVE

To assess the effect of volume-targeted ventilation (VTV) compared with pressure-limited ventilation (PLV) in preterm infants.

METHOD

We searched the Cochrane Library (Issue 3, 2013), PubMed (1966 to 5 March 2013), China National Knowledge Infrastructure (CNKI) and periodical databases (1979 to 5 March 2013). We selected randomised controlled trials (RCTs) and quasi-RCTs of VTV versus PLV as active interventions in preterm infants. We performed meta-analyses using the Cochrane statistical package RevMan 5.0.

RESULTS

Eighteen trials met our inclusion criteria. There was no evidence that VTV modes reduced the incidence of death (relative risk (RR) 0.73, 95% CI 0.51 to 1.05). The use of VTV modes resulted in a reduction in the incidence of bronchopulmonary dysplasia (BPD) (RR 0.61, 95% CI 0.46 to 0.82) and duration of mechanical ventilation (mean difference (MD) -2.0 days, 95% CI -3.14 to -0.86). VTV modes also resulted in reductions in intraventricular haemorrhage (IVH) (RR 0.65, 95% CI 0.42 to 0.99), grade 3/4 IVH (RR 0.55, 95% CI 0.39 to 0.79), periventricular leukomalacia (PVL) (RR 0.33, 95% CI 0.15 to 0.72), pneumothorax (RR 0.52, 95% CI 0.29 to 0.93), failure of primary mode of ventilation (RR 0.64, 95% CI 0.43 to 0.94), hypocarbia (RR 0.56, 95% CI 0.33 to 0.96), mean airway pressure (MD -0.54 cmH2O, 95% CI -1.05 to -0.02) and days of supplemental oxygen administration (MD -1.68 days, 95% CI -2.47 to -0.88).

CONCLUSIONS

Preterm infants ventilated using VTV modes had reduced duration of mechanical ventilation, incidence of BPD, failure of primary mode of ventilation, hypocarbia, grade 3/4 IVH, pneumothorax and PVL compared with preterm infants ventilated using PLV modes. There was no evidence that infants ventilated with VTV modes had reduced death compared to infants ventilated using PLV modes.

Impact of volume guarantee on synchronized ventilation in preterm infants: a randomized controlled trial

PURPOSE

The aim of this randomized controlled trial was to assess whether the addition of volume guarantee (VG) to triggered ventilation decreases the duration of ventilation in very low birth weight (VLBW) infants with respiratory distress syndrome (RDS).

METHODS

Infants were randomized into two groups to initially receive either assist/control (A/C) or A/C plus VG ventilation and then weaned with synchronized intermittent mandatory ventilation (SIMV) or SIMV plus VG.

RESULTS

Forty-five infants were included in the study. The demographic and clinical characteristics, values of tidal volume (VT), peak inspiratory pressure (PIP), fraction of inspired oxygen, carbon dioxide tension, and pH were similar for all participating infants initially. During the follow-up, the VT levels were more stable, and the PIP levels were significantly decreasing in the VG group. Although the duration of ventilation was shorter in the VG group, this trend was not statistically significant. The incidences of death and bronchopulmonary dysplasia (BPD) were not significantly different, but the combined outcome of death or BPD was lower in the VG group. Although the VG group experienced less frequent BPD, periventricular leukomalacia, and intraventricular hemorrhage, these differences were not statistically different.

CONCLUSION

The VG option, when combined with A/C (in the acute phase of RDS) and SIMV (in the weaning), reduced VT variability, and may have shortened the duration of ventilation in VLBW infants. Overall mortality and BPD rates did not change, but their combined outcome was significantly improved in infants treated with VG modes as compared to those treated with synchronized pressure-limited modes alone.

State of the art in conventional mechanical ventilation

Despite a shift to noninvasive respiratory support, mechanical ventilation remains an essential tool in the care of critically ill neonates. The availability of a variety of technologically advanced devices with a host of available modes and confusing terminology presents a daunting challenge to the practicing neonatologist. Many of the available modes have not been adequately evaluated in newborn infants and there is paucity of information on the relative merits of those modes that have been studied. This review examines the special challenges of ventilating the extremely low birth weight infants that now constitute an increasing proportion of ventilated infants, attempts to provide a simple functional classification of ventilator modes and addresses the key aspects of synchronized ventilation modes. The rationale for volume-targeted ventilation is presented, the available modes are described and the importance of the open-lung strategy is emphasized. The available literature on volume-targeted modalities is reviewed in detail and general recommendations for their clinical application are provided. Volume guarantee has been studied most extensively and shown to reduce excessively large tidal volumes, decrease incidence of inadvertent hyperventilation, reduce duration of mechanical ventilation and reduce pro-inflammatory cytokines. It remains to be seen whether the demonstrated short-term benefits translate into significant reduction in chronic lung disease. Avoidance of mechanical ventilation by means of early continuous positive airway pressure with or without surfactant administration may still be the most effective way to reduce the risk of lung injury. For babies who do require mechanical ventilation, the combination of volume-targeted ventilation, combined with the open-lung strategy appears to offer the best chance of reducing the risk of bronchopulmonary dysplasia.

The impact of instrumental dead-space in volume-targeted ventilation of the extremely low birth weight (ELBW) infant

BACKGROUND

Volume-targeted ventilation is increasingly used in neonatal ventilation to reduce the risk of volutrauma and inadvertent hyperventilation. However, normative data for appropriate tidal volume (V(T)) settings are lacking, especially in extremely low birth weight (ELBW) infants in whom the added dead space (DS) of the flow sensor may be important.

OBJECTIVE

To quantify the effect of instrumental dead-space (IDS) on ventilation and to obtain normative data for initial V(T) associated with normocapnia in ELBW infants ventilated with volume guarantee (VG) ventilation.

DESIGN/METHODS

Set and measured V(T), respiratory rate (RR) and arterial blood gas values (ABG) were extracted from charts of babies <800 g born between January 2003 and August 2005, who were ventilated with VG. Data were collected at the time of each ABG during the 1st 48 hr of life. Theoretical alveolar minute ventilation (AMV) was calculated as (V(T) - DS) x RR. IDS was measured by filling with water a 2.5 mm endotracheal tube cut to 10 cm with attached hub of the inline suction catheter and flow sensor. We added 0.5 mL/kg to this value to account for distal tracheal/mainstem bronchi DS (anatomical dead space). Descriptive statistics and linear regression were used for analysis.

RESULTS

The measured IDS was 2.7 mL. Mean combined DS (instrumental + anatomical) was 3.01 mL. There were 344 paired observations of V(T) and ABG with PaCO(2) in the normocapnic range in 38 infants (mean birth weight 625 g +/- 115 g SD, range 400-790 g) during the study period. The mean pH was 7.30 +/- 0.06 (SD), mean PaCO(2) 43.4 +/- 5.4 Torr. The mean target V(T) was 3.11 +/- 0.64 mL and the measured V(T) was 3.17 +/- 0.73 mL. Despite normocapnia, 47% of the V(T) were equal to or less than estimated DS. Mean theoretical AMV was only 8.7 mL/kg/min. The V(T)/kg needed for normocapnia was inversely related to weight (r = -0.70, P < 0.01), indicating some effect of the fixed IDS. Mean V(T)/kg of infants <500 g was 5.9 +/- 0.3 mL, compared to 4.7 +/- 0.5 mL for those >700 g (P < 0.001).

CONCLUSIONS

Effective alveolar ventilation occurs with V(T) at or below calculated DS. This can be explained by the fact that at the high flow rates seen in these tiny infants who have extremely short inspiratory times, fresh gas penetrates through the dead space gas, rather than pushing it ahead. Therefore there is no need to forego synchronized and volume targeted ventilation because of dead space concerns. In infants <800 g, initial V(T) of 5-6 mL/kg was associated with normocapnia when using assist/control or pressure support ventilation.

Volume targeted ventilation (volume guarantee) in the weaning phase of premature newborn infants

OBJECTIVE

Several options are currently available in neonatal mechanical ventilation: complete breathing synchronization (patient triggered ventilation, synchronized intermittent positive pressure ventilation--SIPPV); positive pressure flow-cycled ventilation (pressure support ventilation, PSV); and volume targeted positive pressure ventilation (volume guarantee, VG). The software algorithm for the guarantee volume attempts to deliver a tidal volume (Vt) as close as possible to what has been selected by the clinician as the target volume. Main objectives of the present study were to compare patient-ventilator interactions and Vt variability in premature infants recovering from respiratory distress syndrome (RDS) who were weaned by various ventilator modes (SIMV/PSV + VG/SIPPV + VG and SIMV + VG).

METHODS

This was a short-term crossover trial in which each infant served as his/her own control. Ten premature infants born before the 32nd week of gestation in the recovery phase of RDS were enrolled in the study. All recruited infants started ventilation with SIPPV and in the weaning phase were switched to synchronized intermittent mandatory ventilation (SIMV). Baseline data were collected during an initial 20-min period of monitoring with the infant receiving SIMV alone, then they were switched to SIPPV + VG for a 20-min period and then switched back to SIMV for 15 min. Next, they were switched to PSV + VG for the study period and switched back to SIMV for a further 15 min. Finally, they were switched to SIMV + VG and, at the end of monitoring, they were again switched back to SIMV alone.

RESULTS

Each mode combined with VG discharged comparable Vts, which were very close to the target volume. Among the VG-combined modes, mean variability of Vt from preset Vt was significantly different. Variability from the target value was significantly lower in SIPPV and PSV modes than in SIMV (P < 0.0001 and P < 0.04 respectively). SIPPV + VG showed greater stability of Vt, fewer large breaths, lower respiratory rate, and allowed for lower peak inspiratory pressure than what was delivered by the ventilator during other modes. No significant changes in blood gases were observed after each of the study periods.

CONCLUSIONS

With regards to the weaning phase, among combined modes, both of the ones in which every breath is supported (SIPPV/PSV) are likely to be the most effective in the delivery of stable Vt using a low working pressure, thus, at least in the short term, likely more gentle for the neonatal lung. In summary, we can suggest that the VG option, when combined with traditional, patient triggered ventilation, adheres very closely to the proposed theoretical algorithm, achieving highly effective ventilation.

Volume guarantee ventilation

Recognition that volume, not pressure, is the key factor in ventilator-induced lung injury and the association of hypocarbia with neonatal brain injury demonstrate the importance of better control delivered tidal volume. New microprocessor-based ventilator modalities combine advantages of pressure-limited ventilation with the ability to deliver a more consistent tidal volume. This article discusses automatic weaning of peak inspiratory pressure in response to changing lung compliance and respiratory effort. More consistent tidal volume, fewer excessively large breaths, lower peak pressure, less hypocapnia, shorter duration of mechanical ventilation, and lower levels of inflammatory cytokines have been documented in short-term clinical trials. It remains to be seen if these short-term benefits ultimately lead to a reduced incidence of chronic lung disease.

Volume-targeted ventilation

Recognition that volume, not pressure, is the key factor in ventilator-induced lung injury and the association of hypocarbia and brain injury dictate the need to better control delivered tidal volume. Volume-controlled ventilation, though much improved, still suffers from loss of volume due to endotracheal tube leak and gas compression in the circuit. Recent microprocessor-based modifications of pressure-limited, time-cycled ventilators combine advantages of pressure-limited ventilation with the ability to deliver a more consistent tidal volume. Each of the modes has advantages and disadvantages, with limited data available to judge their effectiveness. The Volume Guarantee mode, studied most thoroughly, provides automatic weaning of peak pressure in response to improving lung compliance and respiratory effort. More consistent tidal volume, fewer excessively large breaths, lower peak pressure, less hypocarbia and lower levels of inflammatory cytokines have been documented. It remains to be seen if these short-term benefits translate into shorter duration of ventilation or reduced incidence of chronic lung disease.