Regional lung volume changes with noninvasive positive pressure ventilation in healthy adults

Abstract

Noninvasive assessments of lung volume distribution often require inhaled contrast and are limited by low regional resolution. We aimed to examine a noncontrast imaging method of spatial lung volume displacement, adapted to assess changes with noninvasive positive pressure ventilation (NIPPV). This study evaluated regional lung volume displacement in nine healthy volunteers (6 males and 3 females; ages 29-55 yr; body mass index 20.2-31.3 kg/m²) using X-ray velocimetry (XV). Participants were assessed during tidal breathing and, also with 15 cmH₂O inspiratory and 5 cmH₂O expiratory pressures in a supine position. Regional specific ventilation (SV) was measured during tidal breathing and NIPPV. Mean specific ventilation (MSV, mL/mL), low-volume region (LVR; % < 0.1 mL/mL), and high-volume region (HVR; % > 0.3 mL/mL) were calculated as output variables. Images were segmented into lobar as well as central and peripheral zones. Two-way ANOVA and paired t tests were used to determine regional differences within individuals and the effect of NIPPV. NIPPV increased MSV in both peripheral (P = 0.01) and central (P = 0.02) lung regions compared with tidal breathing. High-volume regions increased in both peripheral (P = 0.04) and central regions (P = 0.04) during NIPPV. This study demonstrates that noncontrast imaging techniques can assess regional lung ventilation and redistribution of lung volumes on NIPPV. Heterogeneous responses to NIPPV may be associated with a distinct distribution of ventilation, and further work is needed to ascertain differential responses to NIPPV due to lung pathology among those with respiratory disease.NEW & NOTEWORTHY Noninvasive positive pressure ventilation (NIPPV) is a commonly utilized intervention for acute and chronic respiratory failure. In this study, we use functional lung imaging to describe changes in regional lung ventilation and redistribution of lung volume with NIPPV. These results offer insight into the regional effects of NIPPV on volume expansion with the use of functional imaging.

Keywords: X-ray velocimetry; airway physiology; functional lung imaging; lung volume distribution; specific ventilation.

Figure 1.. Changes in flow between Tidal Breathing and NIPPV.

The upper trace shows the flow rate (L/min) over time (s) during tidal breathing and the application of non-invasive positive airway pressure (NIPPV). Tidal breathing is characterized by regular fluctuations in flow, while NIPPV results in increased amplitude and frequency of these fluctuations. The lower trace indicates the fluoroscopy acquisition status (ON/OFF) with corresponding rotational angles (degrees) at specific time intervals, synchronized with the flow measurements. Fluoroscopy was performed at various rotational angles (−72°, −36°, 0°, 36°, 72°) to capture the regional distribution of lung volume expansion.

Figure 2.. Schematic Illustration of Regional Average Specific Ventilation.

Representation of Specific Ventilation (SV) using a volume analogy and its spatial representation in the lungs. The Left Panel depicts a single SV value as the ratio of inspired tidal volume (V_Insp – V_Exp) to expired volume (V_Exp) using a cubic display of the three-dimensional changes from one 8 mm x 8 mm x 8 mm x 8 mm voxel. The Right Panel provides a 3D visualization of the left lower lung segment, illustrating the average SV (SV) calculated for the entire region of interest. SV for the region is the average tidal volume (V_T) as a ration of the volume at functional residual capacity (FRC).

Figure 3.. A single cross-sectional CT slice showing the normalized specific ventilation (SV) distribution in the lungs of one subject.

This is visualized using heat maps (refer to the SV color legend). The left panel represents quiet tidal breathing during spontaneous breathing, while the right panel illustrates ventilation under non-invasive positive pressure ventilation (NIPPV). Each lung is segmented into central and peripheral regions, as outlined by the wireframes in both images and detailed in the manuscript and supplemental materials. The SV distribution during tidal breathing appears more heterogeneous, with noticeable differences between the left and right lungs, whereas NIPPV results in a more uniform distribution. Additionally, regional differences in SV between central and peripheral lung regions are evident. Although this image represents a single cross-sectional slice, the findings are volumetric, reflecting the summation of sequential slices across the entire lung volume. Both conditions, tidal breathing and NIPPV, are aligned and registered to the same CT image for direct comparison.