Different Low Constant Flows Can Equally Determine the Lower Inflection Point in Acute Respiratory Distress Syndrome Patients Ana Maria Casati Nogueira Gama, Eduardo Correa Meyer, Ana Maria Araújo Silva Gaudêncio, Michelle Andrade Grunauer, Marcelo Britto Passos Amato, Carlos Roberto Ribeiro de Carvalho, and Carmen Sílvia Valente Barbas Respiratory Intensive Care Unit, Pulmonary Division of the School of Medicine, University of São Paulo, São Paulo, Brazil Abstract: Among the possible techniques to obtain the pressure–volume (P × V) curve at the bedside the low constant flow (CF) is the easiest and quickest one. How- ever, the best value for CF to perform a good semi- static P × V curve is still to be determined. The purpose of this study was to evaluate the influence of 4 different CFs (1, 2, 5, and 10 L/min) on determination of lower inflection point of the P × V curve (L-Pflex) and upper inflection point of the P × V curve (U-Pflex) on the maximum slope and on the inspiratory work of breathing (up to volume of 1.35 L; inspiratory work L/cm H 2 O), comparing the vol- ume estimated from the CF with the measured volume obtained by the respiratory inductive plethysmograph. The design was a prospective study, and the setting was an adult medical intensive care unit of a university hospital. There were 7 acute respiratory distress syndrome (ARDS) patients, less than 5 days of installation, after the standard- ization of lung volume history received sequentially from 4 different low inspiratory CFs in 2 trials. The P × V curve lasted from 73 ± 1.6 s (1 L/min) to 8.8 ± 0.69 s ( 10 L/min). The L-Pflex differed in the 2 performed trials (p  0.04). There was no difference of L-Pflex among the 4 CFs com- paring the 3 methods (p  0.072) used for its calculation as well as comparing the estimated and the measured volume (p  0.456). The maximum slope decreased significantly while increasing the flow from 1 to 10 L/min just in the estimated volume (p  0.03). The inspiratory work did not increase with the increment of the flow either in the esti- mated volume (p  0.217) or in the measured volume (p  0.149). The U-Pflex differed among the trials (p  0.003) and the methods used for its calculation (p < 0.01). Constant flows from 1 to 10 L/min can equally determine L-Pflex in ARDS patients and is an easy and quick way to obtain the L-Pflex in order to optimize positive end expi- ratory pressure (PEEP) in ARDS patients. Key Words: Pressure–volume curves—Constant flow technique— Pflex—Acute respiratory distress syndrome—Positive end expiratory pressure—Mechanical ventilation. The titration of the ideal positive end expiratory pressure (PEEP) by the lower inflection point of the pressure–volume (P × V) curve of the respiratory system (L-Pflex) associated with low tidal ventilation led to an improvement in survival in acute respira- tory distress syndrome (ARDS) patients (1). How- ever, the measurement of the P × V curve at the bedside is complex. We have to standardize the lung volume history. We have to take into account the necessary time to perform the entire P × V curve because of the presence of artifacts that can modify the thoracopulmonary compliance: oxygen con- sumption (VO 2 ), temperature, humidification, and gas compression (2,3). Among the possible tech- niques to obtain the P × V curve at the bedside (supersyringe method, static inspiratory occlusion method, and PEEP-compliance method), the low constant flow (CF) is the easiest and quickest one (3–5). However, we do not completely understand the influence of different levels of low constant flows on the determination of the L- Pflex and U-Pflex on the inspiratory work and on the shape of the curve in Received July 2001. Partially presented at the 1999 International Conference of the American Thoracic Society, April 23–28, San Diego, California. Address correspondence and reprint requests to Dr. Carmen Sílvia Valente Barbas, UTI-Respiratória-Disciplina de Pneumo- logia, Hospital das Clínicas da Faculdade de Medicina da Univer- sidade de São Paulo, Av. Dr. Eneas de Carvalho Aguiar, 255, 7 andar sala 7079, CEP: 05403-900, São Paulo, Brazil. E-mail: cbarbas@attglobal.net Artificial Organs 25(11):882–889, Blackwell Science, Inc. © 2001 International Society for Artificial Organs 882