3D airway model to assess airway dead space

High flow therapy works partly by washout of airway dead space, the volume of which has not been quantified in newborns. This observational study aimed to quantify airway dead space in infants and to compare efficacy of washout between high flow devices in three-dimensional (3D) printed airway models of infants weighing 2.5–3.8 kg. Nasopharyngeal airway dead space volume was 1.5–2.0 mL/kg in newborns. A single cannula device produced lower carbon dioxide (CO2) levels than a dual cannula device (33.7, 31.2, 23.1, 15.9, 10.9 and 6.3 mm Hg vs 36.8, 35.5, 32.1, 26.8, 23.1 and 18.8 mm Hg at flow rates of 1, 2, 3, 4, 6 and 8 L/min, respectively; p<0.0001 at all flow rates). Airway pressure was 1 mm Hg at all flow rates with the single cannula but increased at higher flow rates with the dual cannula.Relative nasopharyngeal airway dead space volume is increased in newborns. In 3D-printed airway models, a single cannula high flow device produces improved CO2 washout with lower airway pressure.

Effects of inspiratory pause on CO2 elimination and arterial Pco2 in acute lung injury

A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO2 elimination. We hypothesized that a postinspiratory pause could enhance CO2 elimination and reduce PaCO2 by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% postinspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without postinspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO2 was recorded every 5 min to measure tidal CO2 elimination (VtCO2), airway dead space (VDaw), and slope of the alveolar plateau. PaO2, PaCO2, and physiological and alveolar dead space (VDphys, VDalv) were determined at the end of each 30-min period. The postinspiratory pause, 0.7 ± 0.2 s, induced on average <0.5 cmH2O of intrinsic positive end-expiratory pressure (PEEP). During Tp20, VtCO2 increased immediately by 28 ± 10% (14 ± 5 ml per breath compared with 11 ± 4 for Tp0) and then decreased without reaching the initial value within 30 min. The addition of a postinspiratory pause significantly decreased VDaw by 14% and VDphys by 11% with no change in VDalv. During Tp20, the slope of the alveolar plateau initially fell to 65 ± 10% of baseline value and continued to decrease. Tp20 induced a 10 ± 3% decrease in PaCO2 at 30 min (from 55 ± 10 to 49 ± 9 mmHg, P < 0.001) with no significant variation in PaO2. Postinspiratory pause has a significant influence on CO2 elimination when small tidal volumes are used during mechanical ventilation for ALI.

A method of reconstruction of clinical gas-analyzer signals corrupted by positive-pressure ventilation

The use of sidestream infrared and paramagnetic clinical gas analyzers is widespread in anesthesiology and respiratory medicine. For most clinical applications, these instruments are entirely satisfactory. However, their ability to measure breath-by-breath volumetric gas fluxes, as required for measurement of airway dead space, oxygen uptake, and so on, is usually inferior to that of the mass spectrometer, and this is thought to be due, in part, to their slower response times. We describe how volumetric gas analysis with the Datex Ultima analyzer, although reasonably accurate for spontaneous ventilation, gives very inaccurate results in conditions of positive-pressure ventilation. We show that this problem is a property of the gas sampling system rather than the technique of gas analysis itself. We examine the source of this error and describe how cyclic changes in airway pressure result in variations in the flow rate of the gas within the sampling catheter. This results in the phenomenon of “time distortion,” and the resultant gas concentration signal becomes a nonlinear time series. This corrupted signal cannot be aligned or integrated with the measured flow signal. We describe a method to correct for this effect. With the use of this method, measurements required for breath-by-breath gas-exchange models can be made easily and reliably in the clinical setting.