Continuous negative abdominal pressure-augments PEEP, reduces lung injury

Abstract

Introduction: In supine patients with ARDS, the lung usually partitions into dorsal atelectasis and ventral aeration. Conventional positive-pressure ventilation directs tidal volume (VT) preferentially to ventral aerated areas. Attempts to 'recruit' atelectasis with positive pressure often overinflate aerated regions. This occurs due to a large vertical gradient of pleural pressure (Ppl) in ARDS. Prone positioning is the only effective means to lessen this Ppl gradient. Objectives: To test 2 hypotheses-That Continuous Negative Abdominal Pressure (CNAP), by selectively recruiting basal atelectasis, would: (1) improve lung function in a stable lung injury model; (2) attenuate ventilator-induced lung injury in a model of progressive lung injury. Methods: An established model of ventilator-induced lung injury (VILI) was used (anesthetized pig) and all animals were monitored using esophageal manometry (Pes, pleural pressure), Electrical Impedance Tomography (EIT, regional ventilation) and pulmonary artery catheter. 2 series of experiments were performed. Series 1-Stable Lung Injury. 7 pigs were subject to maximal lung recruitment using PEEP (20 cmH2O) followed with progressive derecruit-ment by lowering PEEP in serial increments of 2 cmH2O; full physiologic assessment was performed at each decrement of PEEP. This series of assessments was performed twice in each animal: Once with CNAP (-5 cmH2O) and once with no CNAP (the order was randomized). Series 2-Progressive Lung Injury. Animals were randomized to 'CNAP' or 'no CNAP' (5 per group) and both groups were exposed to standardized injurious ventilation: VT 20 mL/kg, low PEEP, and the same expiratory transpulmonary pressure (PL,-3 cmH2O) for 4 h. Results: Series 1-Stable Lung Injury. CNAP considerably reduced the vertical gradient of Ppl (measured by direct instrumentation): At PEEP = 4: 11.3 ± 3.5 vs. 6.6 ± 2.5 cmH2O, PEEP vs. PEEP + CNAP (P < 0.01). Application of CNAP consistently resulted in better oxygenation, respiratory system compliance, and more homogeneous ventilation at descending PEEP values from 12 to 4 cmH2O. At PEEP = 4, the P/F ratio (mmHg) was 67 ± 5 (no CNAP) vs. 263 ± 36 (with CNAP, P < 0.01). At most levels of PEEP, oxygenation with added CNAP was greater despite a lower global PL (Fig. 92). Dynamic CT demonstrated that addition of CNAP (-5 cmH2O) to a PEEP level of 10 cmH2O yielded the same amount of aeration as a PEEP of 18 cmH2O. Series 2-Progressive Lung Injury. Ventilation with no CNAP resulted in progressive lung injury; use of CNAP reduced the progression of lung injury resulting in greater oxygenation, respiratory system compliance, and homogeneity of ventilation (Figs. 93 and 94) Conclusion CNAP selectively recruited basal atelectasis and homogenized ventilation by increasing regional PL in dependent lung. CNAP is not only 'equal but opposite' to positive pressure; instead, at comparable PL it improves lung function and lessens lung injury. BPK, TY & DE-CNAP device patent applied.