Validation of a nitrogen washout system to measure functional residual capacity in premature infants with hyaline membrane disease

Validation of multiple-breath washout equipment for infants and young children

INTRODUCTION

The new ATS/ERS consensus report recommends in vitro validation of multiple-breath inert gas washout (MBW) equipment based on a lung model with simulated physiologic conditions. We aimed to assess accuracy of two MBW setups for infants and young children using this model, and to compare functional residual capacity (FRC) from helium MBW (FRC(MBW)) with FRC from plethysmography (FRC(pleth)) in vivo.

METHODS

The MBW setups were based on ultrasonic flow meter technology. Sulfur hexafluoride and helium were used as tracer gases. We measured FRC in vitro for specific model settings with and without carbon dioxide and calculated differences of measured to generated FRC. For in vivo evaluation, difference between FRC(MBW) and FRC(pleth) was calculated in 20 healthy children, median age 6.1 years. Coefficient of variation (CV) was calculated per FRC.

RESULTS

In the infant model (51 runs, FRC 80-300 ml), mean (SD) relative difference between generated and measured FRCs was 0.7 (4.7) %, median CV was 4.4% for measured FRCs. In the young child model, one setting (8 runs, FRC 400 ml) showed a relative difference of up to 13%. For the remaining FRCs (42 runs, FRC 600-1,400 ml), mean (SD) relative difference was -2.0 (3.4) %; median CV was 1.4% for measured FRCs. In vivo FRC(pleth) exceeded FRC(MBW) values by 37% on average.

CONCLUSIONS

Both setups measure lung volumes in the intended age group reliably and reproducibly. Characteristics of different techniques should be considered when measuring lung volumes in vivo.

Reproducibility of multiple breath washout indices in the unsedated preterm neonate

Multiple breath inert gas washout (MBW) is gaining popularity for measurements of resting lung volume and ventilation inhomogeneity. Test reproducibility is an important determinant of the clinical applicability of diagnostic tests. The between-test reproducibility of variables derived from MBW tests in newborn infants is unknown. We aimed to determine the within-test repeatability and short-term between-test reproducibility of MBW variables in unsedated preterm infants. We hypothesized that measurements obtained within a 3-day interval in clinically stable preterm infants would be reproducible and suitable for use as an objective clinical outcome measurement. In this cross-sectional observational study, clinically stable hospitalized preterm infants whose parents had given informed consent for MBW studies were tested twice within 72 hr during quiet, unsedated sleep. Functional residual capacity (FRC), lung clearance index (LCI), and the first and second to zeroeth moment ratios (M(1):M(0); M(2):M(0)) were computed from MBW traces obtained using a mainstream ultrasonic flowmeter and 4% sulphur hexafluoride (MBW(SF6)). Within-test repeatability and between-test reproducibility were determined. Within-test repeatability (expressed as a coefficient of variability (C(v))) for differences between two and four replicate measurements on the same test occasion, were 9.3% (FRC), 9.0% (LCI), 7.6% (M(1):M(0)), and 15.6% (M(2):M(0)), respectively. The within-test C(v)'s were not statistically different to the between-tests C(v)'s, which were 7.7% (FRC), 10.3% (LCI), 6.1% (M(1):M(0)), and 13.0% (M(2):M(0)), respectively. Among unsedated preterm infants, between-test reproducibility over a 3-day interval was similar to within-test repeatability. The wide limits of agreement may limit the application of these measures to detect a clinically significant change in condition in small preterm infants.

Functional residual capacity measurement by heptafluoropropane in ventilated newborn lungs: in vitro and in vivo validation

OBJECTIVE

Heptafluoropropane is an inert gas commercially used as propellant for inhalers. Since heptafluoropropane can be detected in low concentrations, it could also be used as a tracer gas to measure functional residual capacity and ventilation homogeneity. The aim of the present study was to validate functional residual capacity measurements by heptafluoropropane wash-in/wash-out (0.8%) during mechanical ventilation in small, surfactant-depleted lungs using a newborn piglet model.

DESIGN

Prospective laboratory and animal trial.

SETTING

Animal laboratory in a university setting.

SUBJECTS

Sixteen newborn piglets (age<12 hrs, median weight 1390 g [705-4200 g]) before and after surfactant depletion (Pao2<100 torr in Fio2=1.0) by lung lavage.

INTERVENTIONS

Heptafluoropropane was measured with a new infrared mainstream sensor connected with the flow sensor of the Dräger Babylog 8000. Accuracy and precision of the measurement technique were tested in a mechanical lung model with a volume range from 11 to 35 mL. Reproducibility of the method and its sensitivity to detect changes of functional residual capacity were assessed in vivo by variation of ventilatory variables.

MEASUREMENTS AND MAIN RESULTS

In vitro the absolute error of functional residual capacity was <1 mL (relative errors<3%) with a coefficient of variation<4%. The coefficient of variation of consecutive in vivo measurements was only slightly higher (<5.1%). Measurement of heptafluoropropane concentrations in blood showed no significant accumulation for repeated functional residual capacity measurements within short time periods. After lung lavage, the functional residual capacity decreased from 20.9 mL/kg to 14.5 mL/kg (p<.05) despite increased ventilatory pressures, and lung clearance index (p<.001) and moment ratios (p<.01) increased significantly due to uneven alveolar ventilation. In healthy lungs, the increase in peak inflation pressure and positive end-expiratory pressure by 3-4 cm H2O had only a moderate effect on functional residual capacity (20.9+/-8.6 vs. 26.0+/-11.9 mL/kg, p=.17) and no effect on ventilatory homogeneity, whereas in surfactant-depleted lungs the functional residual capacity increased from 14.5+/-6.7 mL/kg to 29.9+/-12.6 mL/kg (p<.001) and lung clearance index and moment ratios decreased significantly (p < .01).

CONCLUSIONS

Heptafluoropropane is a suitable tracer gas for precise functional residual capacity measurements tested in vitro and allows for reproducible measurements in ventilated small lungs without any adverse effects on mechanical ventilation. The sensitivity of the method is sufficiently high to demonstrate the effect of changes in ventilatory settings on the functional residual capacity and ventilation homogeneity.

Effects of short vs. prolonged mechanical ventilation on antioxidant systems in piglet diaphragm

OBJECTIVE

Prolonged controlled mechanical ventilation (MV) is known to induce diaphragmatic oxidative stress that seems to be an important factor reducing force-generating capacity. To better understand the cellular mechanisms involved, this work examined the effect of short vs. prolonged MV on antioxidant defense in the diaphragm.

DESIGN AND SETTING

Prospective, randomized, controlled animal study in a university laboratory.

METHODS

Eleven piglets (15-20 kg) were assigned to one of two groups: a long-MV group (n=6) ventilated for 3 days or a short-MV group (n=5) ventilated for 3 h. Force frequency curves of the transdiaphragmatic pressure (Pdi) were obtained in vivo by phrenic nerve pacing. Oxidative stress was evaluated by thiobarbituric reactive substance (TBARs) content and the enzymatic antioxidant activity of both superoxide dismutase (SOD) and glutathione peroxidase (GPx) in samples of diaphragm.

RESULTS

Pdi decreased in the long-MV group by 30-35% over the 3 days at all frequencies compared to the short-MV group. Diaphragm TBARs content was significantly higher and SOD activity lower in long-MV animals than in short-MV animals after 72 h. GPx activity tended to be lower in diaphragms from long-MV animals, but this difference was not significant.

CONCLUSIONS

This study shows that 3 days of MV in piglets is associated with a decrease in antioxidant activity which could emphasize oxidative stress and both contribute to the diaphragm dysfunction caused by MV.

Functional residual capacity determines the effect of inhaled nitric oxide on intrapulmonary shunt and gas exchange in a piglet model of lung injury

OBJECTIVE: Hypoxemic respiratory failure in the newborn infant due to severe parenchymal lung disease is caused by large, intrapulmonary, right-to-left shunting. Nitric oxide (NO) has been shown to reduce shunting and improve oxygenation-however inconsistently-in a variety of neonatal lung diseases such as meconium aspiration pneumonitis, bacterial pneumonia, surfactant deficiency, and others. The aim of this study was to determine whether lung expansion, as determined by functional residual capacity (FRC), by means of increasing levels for positive end-expiratory pressure (PEEP) would augment the effect of NO on reducing right-to-left shunting. DESIGN: Prospective, randomized, controlled, animal laboratory investigation. SETTING: University laboratory. SUBJECTS: Newborn piglets (n = 8), anesthetized and mechanically ventilated. INTERVENTIONS: The piglets were made surfactant deficient by repeated airway lavage aiming at a Pao(2) of 45 mm Hg (6 kPa) while using an Fio(2) of.6. Two hours after lavage, different PEEP levels of 4, 6, 8, 10, and 12 cm H(2)O (.4,.6,.8, 1.0, and 1.2 kPa) were used in a random order, keeping tidal volumes strictly at 8 mL/kg. All measurements were made with or without NO at 10 ppm in a random order, thus each animal served as its own control. A nitrogen washout method was used to measure FRC and alveolar volume, in addition to tidal volume, and compliance and resistance of the respiratory system. MEASUREMENTS AND MAIN RESULTS: Improvement in oxygenation and reduction of right-to-left shunting was optimal while achieving FRC values comparable with those values before airway lavage (approximately 25 mL/kg) while using PEEP levels of 6 to 8 cm H(2)O (.6 to.8 kPa). Further lung expansion did not augment the NO effect. In addition, alveolar volume and compliance of the respiratory system were positively influenced by NO, resulting in a small, but significant, decrease in Paco(2). CONCLUSION: We conclude that improvement in oxygenation by the administration of inhaled NO can be optimized by achieving FRC values comparable with those of the undiseased lung before airway lavage.