Effects of changes in trunk inclination on ventilatory efficiency in ARDS patients: quasi-experimental study

Abstract

Background: Trunk inclination from semirecumbent head-upright to supine-flat positioning reduces driving pressure and increases respiratory system compliance in patients with acute respiratory distress syndrome (ARDS). These effects are associated with an improved ventilatory ratio and reduction in the partial pressure of carbon dioxide (PaCO₂). However, these physiological effects have not been completely studied, and their mechanisms have not yet been elucidated. Therefore, this study aimed to evaluate the effects of a change in trunk inclination from semirecumbent (45°) to supine-flat (10°) on physiological dead space and ventilation distribution in different lung regions.

Results: Twenty-two ARDS patients on pressure-controlled ventilation underwent three 60-min steps in which trunk inclination was changed from 45° (baseline) to 10° (intervention) and back to 45° (control) in the last step. Tunk inclination from a semirecumbent (45°) to a supine-flat (10°) position resulted in a higher tidal volume [371 (± 76) vs. 433 (± 84) mL (P < 0.001)] and respiratory system compliance [34 (± 10) to 41 (± 12) mL/cmH₂O (P < 0.001)]. The CO₂ exhaled per minute improved from 191 mL/min (± 34) to 227 mL/min (± 38) (P < 0.001). Accordingly, Bohr's dead space ratio decreased from 0.49 (± 0.07) to 0.41 (± 0.06) (p < 0.001), and PaCO₂ decreased from 43 (± 5) to 36 (± 4) mmHg (p < 0.001). In addition, the impedance ratio, which divides the ventilation activity of the ventral region by the dorsal region ventilation activity in tidal images, dropped from 1.27 (0.83-1.78) to 0.86 (0.51-1.33) (p < 0.001). These results, calculated from functional EIT images, indicated further ventilation activity in the dorsal lung regions. These effects rapidly reversed once the patient was repositioned at 45°.

Conclusions: A change in trunk inclination from a semirecumbent (45 degrees) to a supine-flat position (10 degrees) improved Bohr's dead space ratio and reduced PaCO₂ in patients with ARDS. This effect is associated with an increase in tidal volume and respiratory system compliance, along with further favourable impedance ventilation distribution toward the dorsal lung regions. This study highlights the importance of considering trunk inclination as a modifiable determinant of physiological parameters. The angle of trunk inclination is essential information that must be reported in ARDS patients.

Keywords: Acute respiratory distress syndrome; Body position; Respiratory dead space; Tidal volume.

Fig. 1

Study protocol. Graphic representation of the study. Each step was performed for 60 min. Mechanical ventilation, arterial blood gases, volumetric capnography and electric impedance tomography were recorded in each step

Fig. 2

Effects of trunk postural change on PaCO2 and Bohr’s dead space. A PaCO₂ and B Bohr’s dead space (VD_Bohr/V_T). Twenty-two ARDS patients on pressure-controlled ventilation underwent three 60-min steps in which the trunk inclination was changed from 45° (baseline) to 10° (intervention) and back to 45° (control). Scatter-box-violin plot summary values. The box depicts the 25th to 75th percentiles [IQR], the error bars denote the 10th to 90th percentiles, and the horizontal bar shows the median. Intergroup difference. Post hoc Bonferroni P values: *p < 0.05: STEP II (°10) vs. STEP I (45°); **p < 0.05: STEP III (45°) vs. STEP II (10°)

Fig. 3

Effects of trunk postural change on the alveolar ventilation ratio (VT_alv/V_T) and impedance ratio (IR). A VT_alv/V_T and B impedance ratio (IR). Scatter-box-violin plot summary values. The box depicts the 25th to 75th percentiles [IQR], the error bars denote the 10th to 90th percentiles, and the horizontal bar shows the median. Intergroup difference. Post hoc Bonferroni P values: *p < 0.05 step II (°10) vs. step I (45°); **p < 0.05 step III (45°) vs. step II (10°)

Fig. 4

Effects of trunk postural change on regional end-expiratory lung impedance (EELI) and tidal variation of impedance (VTI). Ventral and dorsal EELI (A and B) and dorsal and ventral VTI (C and D). Scatter-box-violin plot summary values. The box depicts the 25th to 75th percentiles [IQR], the error bars denote the 10th to 90th percentiles, and the horizontal bar shows the median. Intergroup difference. Post hoc Bonferroni P values: *p < 0.05 step II (°10) vs. step I (45°); **p < 0.05 step III (45°) vs. step II (10°)