Measurement of changes in respiratory mechanics during partial liquid ventilation using jet pulses.

OBJECTIVE: To compare the changes in respiratory mechanics within the breathing cycle in healthy lungs between gas ventilation and partial liquid ventilation using a special forced-oscillation technique. DESIGN: Prospective animal trial. SETTINGS: Animal laboratory in a university setting. SUBJECTS: A total of 12 newborn piglets (age, <12 hrs; mean weight, 725 g). INTERVENTIONS: After intubation and instrumentation, lung mechanics of the anesthetized piglets were measured by forced-oscillation technique at the end of inspiration and the end of expiration. The measurements were performed during gas ventilation and 80 mins after instillation of 30 mL/kg perfluorocarbon PF 5080. MEASUREMENTS AND MAIN RESULTS: Brief flow pulses (width, 10 msec; peak flow, 16 L/min) were generated by a jet generator to measure the end-inspiratory and the end-expiratory respiratory input impedance in the frequency range of 4-32 Hz. The mechanical variables resistance, inertance, and compliance were determined by model fitting, using the method of least squares. At least in the lower frequency range, respiratory mechanics could be described adequately by an RIC single-compartment model in all piglets. During gas ventilation, the respiratory variables resistance and inertance did not differ significantly between end-inspiratory and end-expiratory measurements (mean [sd]: 4.2 [0.7] vs. 4.1 [0.6] kPa x L(-1) x sec, 30.0 [3.2] vs. 30.7 [3.1] Pa x L(-1) x sec2, respectively), whereas compliance decreased during inspiration from 14.8 (2.0) to 10.2 (2.4) mL x kPa(-1) x kg(-1) due to a slight lung overdistension. During partial liquid ventilation, the end-inspiratory respiratory mechanics was not different from the end-inspiratory respiratory mechanics measured during gas ventilation. However, in contrast to gas ventilation during partial liquid ventilation, compliance rose from 8.2 (1.0) to 13.0 (3.0) mL x kPa(-1) x kg(-1) during inspiration. During expiration, when perfluorocarbon came into the upper airways, both resistance and inertance increased considerably (mean with 95% confidence interval) by 34.3% (23.1%-45.8%) and 104.1% (96.0%-112.1%), respectively. CONCLUSIONS: The changes in the respiratory mechanics within the breathing cycle are considerably higher during partial liquid ventilation compared with gas ventilation. This dependence of lung mechanics from the pulmonary gas volume hampers the comparability of dynamic measurements during partial liquid ventilation, and the magnitude of these changes cannot be detected by conventional respiratory-mechanical analysis using time-averaged variables.

Comparison of two different CPAP systems by tidal breathing parameters.

OBJECTIVE: Comparison of tidal breathing and pressure fluctuation of the continuous positive airway pressure (CPAP) associated with the use of the valveless Infant Flow System versus the conventional constant-flow CPAP (Babylog 8000) in preterm infants. DESIGN: Randomized cross-over trial. SETTING: Neonatal intensive care unit level III. PATIENTS: Twenty infants; median (range): birth weight 1,035 g (640-4,110 g), actual weight 1,165 g (820-4,250 g), gestational age at birth 27 (26-40) weeks. INTERVENTIONS: After extubation two CPAP devices (Infant Flow System vs Babylog 8000) were applied in a random order to the same infant. Fluctuations of the applied pressure during the breathing cycle and tidal breathing parameters were measured by the flow-through technique. MAIN RESULTS: Using the Infant Flow System the mean (standard deviation) inspiratory flow [1.5 (0.1) vs 1.3 (0.1) l.min(-1).kg(-1), P<0.05] and tidal volume were significantly increased [5.3 (1.3) vs 4.7 (1.3) ml/kg(-1), P<0.05] compared to Babylog 8000. The fluctuations of the applied pressure of the Infant Flow System during the breathing cycle were significantly lower [0.1 (0.03) kPa vs 0.15 (0.08) kPa, P<0.05] compared to Babylog 8000. No differences were seen in the duration of inspiration and expiration and the time to peak tidal flow. In the Infant Flow System pressures during expiration remained stable whereas they increased during the use of Babylog 8000. CONCLUSIONS: Within-subject comparisons of tidal breathing parameters of the two CPAP devices Infant Flow System and Babylog 8000 show: (1) a significant influence of the system used; and (2) that the valveless Infant Flow System increases air flow and tidal volume with less fluctuations in CPAP pressures during the breathing cycle.

Influence of preterm onset of inspiration on tidal breathing parameters in infants with and without CLD.

The preterm onset of inspiration (POI) is a well-known breathing strategy in newborns to increase their end-expiratory lung volume. The aim of this study was to investigate to which extent POI is related to tidal breathing (TB) parameters in healthy neonates (n=54) and infants with chronic lung diseases (CLD, n=45) with same postconceptional age. Using the deadspace free flow-through technique, 10-60 consecutive breaths were evaluated during quiet sleep and POI was derived from the averaged flow-volume loop considering the end-expiratory flow level. Respiratory rate (RR), ventilation (VE) and peak flows were significantly higher in CLD infants compared with controls. The incidence of POI did not differ significantly between both patient groups. POI is strongly associated with TB parameters describing the shape of flow profiles or flow-volume loops. In contrast, TB parameters, which depend only on breathing depth and rate (e.g., RR, VT, VE), were not significantly associated. The study shows that in infants TB parameters describing the flow profile may reflect differences in breathing strategy rather than impaired respiratory functions.