Guidelines for inspiratory flow setting when measuring the pressure-volume relationship.
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 < or =12 L/min minimize P-t during intraoperative PV curves acquisition. Compliances suggest 6 L/min as the most adequate flow.


CONCLUSIONS
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.
Bensenor
Fábio E
FE
Anesthesia Department, Hospital das Clinicas, Faculdade de Medicina da Universidade de Sao Paulo, Brazil. bensenor@aol.com
Vieira
Joaquim E
JE
Auler
Jose Otávio C
JO
D000328:Adult; D000399:Air Pressure; D000758:Anesthesia / Q000592:standards*; D000768:Anesthesia, General; D004947:Esophagus / Q000502:physiology; D005260:Female; D006801:Humans; D007442:Intubation, Intratracheal; D008168:Lung / Q000601:surgery; D008170:Lung Compliance / Q000502:physiology; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000592:standards*; D012121:Respiration, Artificial / Q000295:instrumentation / Q000592:standards*; D012129:Respiratory Function Tests; D015656:Respiratory Mechanics / Q000502:physiology*; D013990:Tidal Volume / Q000502:physiology