[Positive end-expiratory pressure adjustment in parenchimal respiratory failure: static pressure-volume loop or transpulmonary pressure?].

OBJECTIVE: The aim of the study was compare the prognostic value, efficacy and safety ofpositive end-expiratory pressure (PEEP) adjustment in conformity with lower inflection point of static "pressure-volume" loop (LIP) or end-expiratory esophageal pressure (EEEP) in parenchymal respiratory failure.

METHODS: We included in the study 56 patients (39 males) at age 47 +/- 17.8 years with parenchymal respiratory failure (PaO2/FiO2 < 250 mmHg, bilateral infiltrates on chest X-ray or lung CT scan, no signs of left ventricular failure), who were mecAanically ventilated for less than 48 hours. All patients were sedated and paralyzed. We measured intra- Sabdominal pressure, PaO2/FiO2, PaCO2, alveolar dead space (Vdalv), plotted static "pressure-volume" loop by low flow technique in range of 0 to 40 mbar, recording LIP Then we placed nasogastric tube with balloon for esophageal pressure measurement and measured esophageal pressure at PEEP range from 8 to 20 mbar (with 2 mbar steps) and recorded plateau pressure (Pplat), transpulmonary plateau pressure (Ptp plat), transpulmonary pressure at PEEP level (Ptp PEEP), static compliance of respiratory system (Cstat), lung compliance (Clung), chest wall compliance (Ccw) at every step. Also by volumetric capnography technique we measured end-tidal carbon dioxide concentration (EtCO2), minute volume of exhaled carbon dioxide (VCO2) volume of exhaled carbon dioxide by single breath (VtCO2) and calculate VC2/EtCO2 as a surrogate marker of pulmnonary perfusion. After that we set PEEP at EEEP level (at zero end-expirato- my ranspulmonary pressure) and recorded changes of PaO2/FiO2 and Vdalv.

RESULTS: LIP value was 5 (6-10) mbar and it was less than empirically set PEEP in most of patients before enrollment and had no prognostic value for PEEP setting. EEEP level was 14 (12-18.25) mbar and it was higher than LIP in 96.4% patients. Distribution of EEEP values was close to normal unlike LIP Chest wall compliance was less than normal (100 ml/mbar) in 46% of patients. EEEP has correlation with body mass index (rho 0.554, p=0.002). We did not find any correlation between intra-abdominal pres- sure (IAP) and EEEP (p=0.376) or IAP and LIP (p=0.464). PEEP levels higher than 14 mbar led to significant decrease in Cstat and Clung (p<0.001). We observed significant decrease in VCO at PEEP levels more than 16 mbar, i.e., more than EEEP median. PEEP levels more than 16 mbar decreased VCO2/EtCO2, (decreased pulmonary perfusion) from 7.47 (6.54-8.7) at PEEP 14 mbar to 7.32 (6.35-8.76) at PEEP 20 mbar (p=0.004). PEEP setting at EEEP level increased PaO/FiO2 from 205 (154-235) to 280 (208-358) mmHg (p<0.001), did hot change Vdalv (p=0.093) and decreased Cstat and Clung in the most of patients (64.3%).

CONCLUSION: L1P was lower than empirically set PEEP in most patients and did not help to optimize gas exchange. PEEP setting at EEEP level in patients with parenchimal respiratory failure increases PaO/FiO, (reflects opening of collapsed alveoli), decreases volume of expired carbon dioxide and decreases lung compliance (reflects overdistenion of opened alveoli). VCO2/EtCO2 ratio decreases (decreased pulmonary perfusion) at PEEP levels more than 16 mbar, which was more than EEEP.