Functional residual capacity (FRC) and lung clearance index (LCI) in mechanically ventilated infants: application in the newborn with congenital diaphragmatic hernia (CDH)

Ventilator strategies in congenital diaphragmatic hernia

This review focuses on contemporary mechanical ventilator practices used in the initial management of neonates born with congenital diaphragmatic hernia (CDH). Both conventional and non-conventional ventilation modes in CDH are reviewed. Special emphasis is placed on the rationale for gentle ventilation and the current evidence-based clinical practice guidelines that are recommended for supporting these fragile infants. The interplay between CDH lung hypoplasia and other key cardiopulmonary elements of the disease, namely a reduced pulmonary vascular bed, abnormal pulmonary vascular remodeling, and left ventricular hypoplasia, are discussed. Finally, we provide insights into future avenues for mechanical ventilator research in CDH.

Mechanical ventilation in special populations

Optimal respiratory support can only be achieved if the ventilator strategy utilized for each individual patient at any given point in the evolution of their disease process is tailored to the underlying pathophysiology. The critically ill newborn infant requires individualized patient care when it comes to mechanical ventilation. This can only occur if the clinician has a good understanding of the different pathophysiologies of a variety of conditions that can lead to respiratory failure. In this chapter we describe the key pathophysiological features of bronchopulmonary dysplasia, meconium aspiration syndrome and lung hypoplasia syndromes with emphasis on congenital diaphragmatic hernia. We review available evidence to guide management an provide specific recommendations for pathophysiologically-based mechanical ventilation support.

A new method of ventilation inhomogeneity assessment based on a simulation study using clinical data on congenital diaphragmatic hernia cases

Congenital Diaphragmatic Hernia (CDH) is a diaphragm defect associated with lung hypoplasia and ventilation inhomogeneity (VI). The affected neonates are usually born with respiratory failure and require mechanical ventilation after birth. However, significant interindividual VI differences make ventilation difficult. So far, there are no clinical methods of VI assessment that could be applied to optimize ventilation at the bedside. A new VI index is a ratio of time constants T₁/T₂ of gas flows in both lungs. Pressure-controlled ventilation simulations were conducted using an infant hybrid (numerical-physical) respiratory simulator connected to a ventilator. The parameters of the respiratory system model and ventilator settings were based on retrospective clinical data taken from three neonates (2, 2.6, 3.6 kg) treated in the Paediatric Teaching Clinical Hospital of the Medical University of Warsaw. We searched for relationships between respiratory system impedance (Z) and ventilation parameters: work of breathing (WOB), peak inspiratory pressure (PIP), and mean airway pressure (MAP). The study showed the increased VI described by the T₁/T₂ index value highly correlated with elevated Z, WOB, PIP and MAP (0.8-0.9, the Spearman correlation coefficients were significant at P < 0.001). It indicates that the T₁/T₂ index may help to improve the ventilation therapy of CDH neonates.

Growth and morbidity in infants with Congenital Diaphragmatic Hernia according to initial lung volume: A pilot study

Background In congenital diaphragmatic hernia (CDH) survivors, failure to thrive is a well-known complication, ascribed to several factors. The impact of lung volume on growth of CDH survivors is poorly explored. Our aim was to evaluate if, in CDH survivors, lung volume (LV) after extubation correlates with growth at 12 and 24 months of life. Methods LV (measured as functional residual capacity-FRC) was evaluated by multibreath washout traces with an ultrasonic flowmeter and helium gas dilution technique, shortly after extubation. All CDH survivors are enrolled in a dedicated follow-up program. For the purpose of this study, we analyzed the correlation between FRC obtained shortly after extubation and anthropometric measurements at 12 and 24 months of age. We also compared growth between infants with normal lungs and those with hypoplasic lungs according to FRC values. A p < 0.05 was considered as statistically significant. Results We included in the study 22 CDH survivors who had FRC analyzed after extubation and auxological follow-up at 12 and 24 months of age. We found a significant correlation between FRC and weight Z-score at 12 months, weight Z-score at 24 months and height Z-score at 24 months. We also demonstrated that CDH infants with hypoplasic lungs had a significantly lower weight at 12 months and at 24 months and a significantly lower height at 24 months, when compared to infants with normal lungs. Conclusion We analyzed the predictive value of bedside measured lung volumes in a homogeneous cohort of CDH infants and demonstrated a significant correlation between FRC and growth at 12 and 24 months of age. An earlier identification of patients that will require an aggressive nutritional support (such as those with pulmonary hypoplasia) may help reducing the burden of failure to thrive.

The Role of Lung Function Testing in Newborn Infants With Congenital Thoracic Arterial Anomalies

Introduction: Congenital thoracic arterial anomalies (CTAAs), such as complete or incomplete vascular rings, pulmonary artery sling, and innominate artery compression syndrome, may cause severe tracheomalacia and upper airway obstruction. An obstructive ventilatory pattern at lung function testing (LFT) has been suggested in the presence of CTAA. The severity of obstruction may be evaluated by LFT. Little is known about the use of LFT in newborn infants with CTAA. The aim of our study is to evaluate the role of LFT in CTAA diagnosis. Methods: This is a retrospective study, conducted between February 2016 and July 2020. All CTAA cases for whom LFT was performed preoperatively were considered for inclusion. Tidal volume (Vt), respiratory rate, and the ratio of time to reach the peak tidal expiratory flow over total expiratory time (tPTEF/tE) were assessed and compared to existing normative data. Demographics and CTAA characteristics were also collected. Results: Thirty cases were included. All infants with CTAA showed a significantly reduced Vt and tPTEF/tE, compared to existing normative data suggesting an obstructive pattern. No significant differences were found for LFT between cases with a tracheal obstruction <50% compared to those with tracheal obstruction ≥50%, or between cases with and without symptoms. Sixteen infants (53.3%) had respiratory symptoms related to CTAA. Of these, only two cases had also dysphagia. Conclusion: LFT values were significantly reduced in cases with CTAA before surgery. LFT represents a potential feasible and non-invasive useful tool to guide diagnosis in the suspect of CTAA.

Mechanical ventilation strategies

Although only a small proportion of full term and late preterm infants require invasive respiratory support, they are not immune from ventilator-associated lung injury. The process of lung damage from mechanical ventilation is multifactorial and cannot be linked to any single variable. Atelectrauma and volutrauma have been identified as the most important and potentially preventable elements of lung injury. Respiratory support strategies for full term and late preterm infants have not been as thoroughly studied as those for preterm infants; consequently, a strong evidence base on which to make recommendations is lacking. The choice of modalities of support and ventilation strategies should be guided by the specific underlying pathophysiologic considerations and the ventilatory approach must be individualized for each patient based on the predominant pathophysiology at the time.

Functional residual capacity and lung clearance index in infants treated for esophageal atresia and tracheoesophageal fistula

BACKGROUND

Newborn babies with esophageal atresia/tracheoesophageal fistula (EA/TEF) are prone to respiratory tract disorders. Functional residual capacity (FRC) and lung clearance index (LCI) are commonly considered useful and sensitive tools to investigate lung function and early detecting airways diseases. The aim of the present study is to report the first series of EA/TEF infants prospectively evaluated for FRC and LCI.

METHODS

Prospective observational cohort study of all patients treated for EA/TEF. Lung volume and ventilation inhomogeneity were measured by helium gas dilution technique using an ultrasonic flow meter. Babies were studied both in assisted controlled ventilation (sedated) and in spontaneous breathing (quiet sleep). Three consecutive FRC and LCI measurements were collected for each test at three different time points: before surgery (T0), 24hours after surgery (T1) and after extubation (T2).

RESULTS

16 EA newborns were eligible for the study between December 2011 and July 2013. Three were excluded because of technical problems. At T0 FRC values were in the normal range regardless the presence of TEF but worsened afterwards at T1, with a subsequent recovering after extubation; a significant improvement after surgery was observed concerning LCI while no differences were found in tidal volume.

CONCLUSION

Helium gas dilution technique is a suitable method to measure the effect of surgery on lung physiology, even in ventilated infants with EA. The changes observed could be related to the ventilatory management and lung compression during surgical procedure.

Lung volume assessments in normal and surfactant depleted lungs: agreement between bedside techniques and CT imaging

Background

Bedside assessment of lung volume in clinical practice is crucial to adapt ventilation strategy. We compared bedside measures of lung volume by helium multiple-breath washout technique (EELV_MBW,He) and effective lung volume based on capnodynamics (ELV) to those assessed from spiral chest CT scans (EELV_CT) under different PEEP levels in control and surfactant-depleted lungs.

Methods

Lung volume was assessed in anaesthetized mechanically ventilated rabbits successively by measuring i) ELV by analyzing CO₂ elimination traces during the application of periods of 5 consecutive alterations in inspiratory/expiratory ratio (1:2 to 1.5:1), ii) measuring EELV_MBW,He by using helium as a tracer gas, and iii) EELV_CT from CT scan images by computing the normalized lung density. All measurements were performed at PEEP of 0, 3 and 9 cmH₂O in random order under control condition and following surfactant depletion by whole lung lavage.

Results

Variables obtained with all techniques followed sensitively the lung volume changes with PEEP. Excellent correlation and close agreement was observed between EELV_MBW,He and EELV_CT (r = 0.93, p < 0.0001). ELV overestimated EELV_MBW,He and EELV_CT in normal lungs, whereas this difference was not evidenced following surfactant depletion. These findings resulted in somewhat diminished but still significant correlations between ELV and EELV_CT (r = 0.58, p < 0.001) or EELV_MBW,He (0.76, p < 0.001) and moderate agreements.

Conclusions

Lung volume assessed with bedside techniques allow the monitoring of the changes in the lung aeration with PEEP both in normal lungs and in a model of acute lung injury. Under stable pulmonary haemodynamic condition, ELV allows continuous lung volume monitoring, whereas EELV_MBW,He offers a more accurate estimation, but intermittently.