A simple automated method for measuring pressure-volume curves during mechanical ventilation

Measurement of respiratory compliance is advocated for assessing the severi ty of acute respiratory failure (ARF). Recently, the administration of an a utomated constant flow of 15 L/min was proposed as a method easier to imple ment at the bedside than supersyringe or inspiratory occlusions methods. Ho wever, pressure-volume (P-V) curves were shifted to the right because of th e resistive properties of the respiratory system. The aim of this study was to compare the P-V curves obtained using two constant flows-3 and 9 L/min- during volume-controlled mechanical ventilation with those obtained with th e supersyringe and the inspiratory occlusions methods. Fourteen paralyzed p atients with ARF were studied. The supersyringe and the inspiratory occlusi ons methods were performed according to usual recommendations. The new auto mated method was performed during volume-controlled mechanical ventilation by setting the inspiratory:expiratory ratio at 80%, the respiratory frequen cy at 5 breaths/min, and the tidal volume at 500 or 1,500 ml. These peculia r ventilatory settings were equivalent to administering a constant flow of 3 or 9 L/min during a 9.6-s inspiration. Esophageal and airway pressures we re recorded. P-V curves obtained by the 3-L/min constant-flow method were i dentical to those obtained by the reference methods, whereas the P-V curve obtained by the 9-L/min constant flow was slightly shifted to the right. Th e slopes of the P-V curves and the lower inflection points were not differe nt between all methods, indicating that the resistive component induced by administering a constant flow equal to or less than 9 L/min is not of clini cal relevance. Because the 3-L/min constant-flow method is not artifacted b y the resistive properties of the respiratory system and does not require a ny other equipment than a ventilator, it is an easy-to-implement, inexpensi ve, safe, and reliable method for measuring the thoracopulmonary P-V curve at the bedside.