Assessing fluid responsiveness in mechanically ventilated patients with intra-abdominal hypertension: a two-center, prospective, observational study

BACKGROUND: Intra-abdominal hypertension (IAH) may reduce the diagnostic accuracy of the passive leg raising (PLR) test for predicting fluid responsiveness, with unclear mechanisms. The reliability of the end-expiratory occlusion (EEO) test and mini-fluid challenge in IAH remains unknown. This study explored the mechanisms underlying PLR impairment and assessed the accuracy of EEO and mini-fluid challenge in detecting fluid responsiveness in patients with and without IAH. METHODS: In this prospective study in two intensive care units (ICUs), we included ventilated patients with IAH ("IAH + "; intra-abdominal pressure [IAP] ≥ 12 mmHg) and without ("IAH-"), all monitored via transpulmonary thermodilution and receiving a 500-mL fluid challenge. Patients consecutively underwent a 1‑minute PLR, a 15‑second EEO, and a 1‑minute mini-fluid challenge of 100 mL, with cardiac index (CI) changes recorded during each maneuver. Following the mini-fluid challenge, the remaining 400 mL were infused over 14 min, and a ≥ 15% increase in CI was used to define fluid responders. The transmural pressure of the inferior vena cava was estimated by the central venous pressure (CVP) - IAP gradient. RESULTS: We included 88 patients, 44 IAH- (25 fluid responders and 19 non-responders) and 44 IAH + (22 responders and 22 non-responders). Baseline IAP was 9 ± 2 mmHg in IAH- and 17 ± 3 mmHg in IAH + (p < 0.001). In IAH- responders, CI increased by 19 ± 11% during PLR and 31 ± 17% after volume expansion, with PLR positive in 24/25 responders. In IAH + responders, CI increased by 6 ± 7% during PLR (p < 0.001 vs. IAH-) and 30 ± 18% after volume expansion (p = 0.907 vs. IAH-). The AUROC of the PLR for detecting fluid responsiveness was 0.96 (0.87-1.00) in IAH- and 0.71 (0.56-0.87) in IAH + (p = 0.009 vs. IAH-). Among IAH + , there were 16 false negatives and 6 true positives for PLR, both with negative baseline CVP-IAP gradients. During PLR, the gradient reversed in true-positives (from -2.4 ± 4.0 to + 2.2 ± 2.7 mmHg, p = 0.014), whereas it remained negative in false-negatives (from -6.3 ± 3.7 to -1.4 ± 3.4 mmHg, p < 0.001). AUROC between IAH- and IAH + was similar for either the EEO test (0.95 [0.87-1.00] vs. 0.89 [0.80-0.97], p = 0.332) or mini-fluid challenge (0.94 [0.87-1.00] vs. 0.90 [0.79-1.00], p = 0.514). CONCLUSION: In patients with IAH, the limited diagnostic value of PLR for fluid responsiveness may be related to persistently negative CVP-IAP gradients in false-negative cases, whereas EEO and mini-fluid challenge remain reliable alternatives.