Differential Effects of Intra-Abdominal Hypertension and ARDS on Respiratory Mechanics in a Porcine Model

Abstract

Background and Objectives: Intra-abdominal hypertension (IAH) and acute respiratory distress syndrome (ARDS) are common concerns in intensive care unit patients with acute respiratory failure (ARF). Although both conditions lead to impairment of global respiratory parameters, their underlying mechanisms differ substantially. Therefore, a separate assessment of the different respiratory compartments should reveal differences in respiratory mechanics. Materials and Methods: We prospectively investigated alterations in lung and chest wall mechanics in 18 mechanically ventilated pigs exposed to varying levels of intra-abdominal pressures (IAP) and ARDS. The animals were divided into three groups: group A (IAP 10 mmHg, no ARDS), B (IAP 20 mmHg, no ARDS), and C (IAP 10 mmHg, with ARDS). Following induction of IAP (by inflating an intra-abdominal balloon) and ARDS (by saline lung lavage and injurious ventilation), respiratory mechanics were monitored for six hours. Statistical analysis was performed using one-way ANOVA to compare the alterations within each group. Results: After six hours of ventilation, end-expiratory lung volume (EELV) decreased across all groups, while airway and thoracic pressures increased. Significant differences were noted between group (B) and (C) regarding alterations in transpulmonary pressure (TPP) (2.7 ± 0.6 vs. 11.3 ± 2.1 cmH₂O, p < 0.001), elastance of the lung (E_L) (8.9 ± 1.9 vs. 29.9 ± 5.9 cmH₂O/mL, p = 0.003), and elastance of the chest wall (E_CW) (32.8 ± 3.2 vs. 4.4 ± 1.8 cmH₂O/mL, p < 0.001). However, global respiratory parameters such as EELV/kg bodyweight (-6.1 ± 1.3 vs. -11.0 ± 2.5 mL/kg), driving pressure (12.5 ± 0.9 vs. 13.2 ± 2.3 cmH₂O), and compliance of the respiratory system (-21.7 ± 2.8 vs. -19.5 ± 3.4 mL/cmH₂O) did not show significant differences among the groups. Conclusions: Separate measurements of lung and chest wall mechanics in pigs with IAH or ARDS reveals significant differences in TPP, E_L, and E_CW, whereas global respiratory parameters do not differ significantly. Therefore, assessing the compartments of the respiratory system separately could aid in identifying the underlying cause of ARF.

Keywords: acute respiratory distress syndrome; compliance; elastance; intra-abdominal hypertension; respiratory mechanics; strain; transpulmonary pressure.

Figure 2

Alterations of EELV/kg bodyweight over six hours of ventilation. EELV: end-expiratory lung volume, h: hours, KG: kilogram bodyweight, kg: kilogram, ml: milliliter.

Figure 3

Alterations of driving pressure (ΔP) over six hours of ventilation. ΔP: driving pressure, cmH₂O: centimeter of water, h: hours.

Figure 4

Alterations of transpulmonary pressure (TPP) over six hours of ventilation. cmH₂O: centimeter of water, h: hours, TPP: transpulmonary pressure.

Figure 5

Alterations of compliance of the respiratory system (C_stat) over six hours of ventilation. cmH₂O: centimeter of water, C_stat: static compliance of the respiratory system, h: hours, ml: milliliter.

Figure 6

Alterations of elastance of the lung (E_{stat L}) over six hours of ventilation. cmH₂O: centimeter of water, E_{stat L}: static elastance of the lung, h: hours, l: liter.

Figure 7

Alterations of elastance of the chest wall (E_{stat CW}) over six hours of ventilation. cmH₂O: centimeter of water, E_{stat CW}: static elastance of the chest wall, h: hours, l: liter.

Figure 1

Timeline of the experimental protocol. †: euthanasia, ARDS: acute respiratory distress syndrome, cmH₂O: centimeter of water, FiO₂: inspiratory oxygen fraction, H: hour, IAP: intraabdominal pressure, kg: kilogram, ml: milliliter, mmHg: millimeter mercury, PEEP: positive end-expiratory pressure.