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TECHNOLOGY, COMPUTING, AND SIMULATION

Guidelines for Inspiratory Flow Setting When Measuring the Pressure-Volume Relationship

Anesthesia Department, Hospital das Clinicas, Faculdade de Medicina da Universidade de Sao Paulo, Sao Paulo, Brazil

Address correspondence to Fábio Ely Bensenor, MD, PhD, Rua Mauá, 934/936, Sao Paulo, SP 01028–000, Brazil. Address e-mail to bensenor{at}aol.com

Acquisition of pressure-volume (PV) curves to improve ventilation strategy is time consuming when using static methods. Low-flow techniques use less time, but compliance values can be decreased by the resistance to flow in airways and tracheal tube (P-t). In this study, we determined the impact of three flows on the resistive component of airway pressure during anesthesia. We studied 10 ASA status P1/P2 patients with normal respiratory function. Airway and esophageal pressures were measured while volume-control ventilated with 6, 12, and 30 L/min continuous flows. PV curves, lower inflection point, respiratory system, and chest wall compliances at 250, 500, 750, and 1000 mL tidal volume were established before and after removing P-t. Data were submitted to analysis of variance. The inflection point was lower for the lower flow when comparing 6 and 12 with 30 L/min (P < 0.001). No difference was found between 6 and 12 L/min. Removal of P-t showed a difference only for 30 L/min (P = 0.004). Higher flows generated lower compliances. P-t subtraction reduced compliances only for 30 L/min. Chest wall compliances showed no difference between flows. We concluded that flows ≤12 L/min minimize P-t during intraoperative PV curves acquisition. Compliances suggest 6 L/min as the most adequate flow.

IMPLICATIONS: We suggest guidelines for inspiratory flow setting when measuring the pressure-volume relationship during anesthesia based on the comparison among three different continuous flow values, aiming at better intraoperative respiratory settings in patients with normal respiratory function.