Peep increases FRC at Zeep in ards patients

INTRODUCTION. Alveolar recruitment is commonly computed by measuring the difference in volume at a given pressure between different pressure-volume (P-V) curves. For this purpose the expired volume from positive end expiratory pressure (PEEP) to zero end expiratory pressure (ZEEP) (PEEP related lung volume) is used as reference of the P-V curves on the volume axis. This method assumes that the absolute lung volume at ZEEP does not change with PEEP or recruitment maneuvers, even in presence of recruitment. However, this assumption has been proved only in postcardiac-surgery patients1. We hypothesized that in ALI/ARDS patients part of the PEEP recruited lung remains open at ZEEP. Aim of the study was to investigate this assumption in ARDS patients. METHODS. We studied five sedated and paralyzed ARDS patients (4 males, age 59±14, PaO2/FiO2 90 ± 12), ventilated in CPPV (PEEP 11 ± 4 cmH2O, FiO2 0.63±0.16). All patients were monitored with an arterial pressure catheter and a Swan-Ganz catheter. We recorded airflow, airway and esophageal pressures. In all patients we randomly applied three different level of PEEP (5,10 and 15 cmH2O), while maintaining the same ventilatory setting. At each PEEP level, after 30 minutes, we performed a P-V curve using the slow inflation method from PEEP to 50 cmH2O. PEEP related lung volume was measured recording the volume exhaled after disconnecting the patients from the ventilator during an end-expiratory occlusion maneuver. After waiting at least 5 seconds for complete exhalation we measured FRC by means of an helium dilution technique. PaO2 and shunt was also measured in all patients. RESULTS. The increasing levels of PEEP resulted in a significant improvement in PaO2 and shunt. Values of PEEP related lung volumes were 192±63 ml, 552±118 ml and 974±251 ml respectively at 5, 10 and 15 PEEP levels (p<0.01). For the same levels of PEEP the values of the FRC at ZEEP were 384±170 ml, 484 ± 207 ml and 555±190 ml respectively (p<0.01). CONCLUSION. In ARDS patients ventilation at different levels of PEEP may result in different absolute lung volumes at ZEEP. Assuming the same lung volumes at ZEEP would have resulted in underestimation of recruited volume of 100±65 ml between PEEP 5 and 10 cmH2O and of 70±39 ml between 10 and 15 cmH2O. these results imply that closing pressure of a portion of the recruited lung are lower than ZEEP. We suggest that FRC at ZEEP should be measured when computing alveolar recruitment. REFERENCES. 1) Valta P, Takala J, Eissa NT, Milic-Emili J. Does alveolar recruitment occur with positive end-expiratory pressure in Adult Respiratory Distress Syndrome patients? J Crit Care 1993;8:34-42.