Effect of trunk upward verticalization on pulmonary vascular resistance in ARDS

Dear Editor,

Bouchant et al. examined the effects of progressive verticalization on lung mechanics and hemodynamics in patients with Acute Respiratory Distress Syndrome (ARDS) [1]. We commended the authors for this high-quality research. Several of their findings are particularly noteworthy and warrant further discussion.

Changes in trunk inclination affect respiratory mechanics, oxygenation, ventilation distribution, and ventilatory efficiency in patients with acute respiratory failure [2]. When patients are transitioned from a flat supine position to a semi-recumbent position, the driving pressure increases, respiratory system compliance decreases, and ventilatory efficiency for carbon dioxide (CO₂) removal decreases [3,4,5,6].

Marrazzo et al. observed that the PEEP level optimized in the flat supine position led to overdistension when patients were moved to a semi-recumbent position [7]. Therefore, the change in trunk inclination from a flat supine position to a semi-recumbent position could generate a similar effect as an increase in PEEP levels.

In this way, both the effect of PEEP setting and trunk inclination on respiratory mechanics could depend on lung recruitment potential [8]. These studies revealed that in patients with low recruitment potential, an increase in PEEP results in minimal lung volume expansion, elevated airway pressure, increased pulmonary vascular resistance, and impaired right ventricular function [8, 9].

In a comprehensive hemodynamic assessment using pulmonary artery catheterization, Bouchant et al. investigated the effects of progressive verticalization in 30 ARDS patients by optimizing the PEEP level in a 30° semi-recumbent position and maintaining it unchanged throughout the study [1]. The study demonstrated a consistent increase in pulmonary vascular resistance as the verticalization angle progressed from supine (0°) to upright (90°). Specifically, the pulmonary vascular resistance increased from 181 (143–266) dyn·s·cm⁻⁵ at 0° to 287 (241–429) dyn·s·cm⁻⁵ at 90°, indicating a clear association between vertical positioning and vascular resistance. Likewise, cardiac output steadily declined from 6.5 (4.8–8.0) L/min at 0° to 4.8 (3.2–5.8) L/min at 90°, requiring increasing vasopressor support to maintain adequate perfusion. Interestingly, the end-expiratory lung volume (EELV) increased from 24 mL/kg (15–30 mL/kg) at 0° to 34 mL/kg (27–37 mL/kg) at 90°, while respiratory system compliance decreased from 43 mL/cmH₂O (32–49 mL/cmH₂O) at 0° to 25 mL/cmH₂O (21–37 mL/cmH₂O) at 90°, being the elasticity of the chest wall the most affected component of postural changes. Likewise, PaCO₂ levels progressively increased from 45 (37–50) mm Hg at 0° to 51 (42–60) mm Hg at 90°, reflecting a concurrent deterioration in gas exchange. In summary, upward verticalization without modification of the PEEP level was associated with increased pulmonary vascular resistance, reduced cardiac output, impaired CO₂ clearance, and decreased respiratory system compliance. Thus, alveolar overdistension could partially explain these findings.

In this context, progressive trunk verticalization likely induces airway pressure changes comparable to those observed with increased PEEP in poorly recruitable lungs, where PEEP elevation minimally expands lung volume but disproportionately increases airway pressure [8, 9]. This imbalance leads to alveolar overdistension, capillary compression, reduction in cross-sectional vascular area, and increased pulmonary vascular resistance [10]. Therefore, under conditions of low lung recruitment, upward verticalization may similarly exacerbate these effects, where the expansion of already aerated alveoli, rather than the recruitment of previously collapsed units, can lead to alveolar overdistension, capillary compression, and hemodynamic impairments.

Based on these observations, the following question arises: Can lung recruitment assessment be a key factor in understanding hemodynamic, lung mechanics, and gas exchange responses to trunk inclination changes in ARDS?

The relationship between recruitment potential and the physiological effects of trunk verticalization may provide valuable insights for individualizing patient positioning strategies. Understanding this association could help anticipate both pulmonary and systemic hemodynamic responses, potentially preventing adverse outcomes during positional interventions in ARDS patients.

In conclusion, the elegant study by Bouchant et al. provides valuable insights into hemodynamic compromise associated with upward verticalization. It also paves the way for future research on whether lung recruitment can explain the adverse effects associated with trunk elevation. These observations reinforce the need to investigate trunk inclination settings as a critical factor for optimizing PEEP in clinical practice and research.