The Science Journal of the American Association for Respiratory Care
BACKGROUND: Static pressure volume curves (SPVC) performed on mechanically ventilated patients with parenchymal lung injury may be a useful method to evaluate the critical lung opening pressures as well as potentially overdistending pressures/volumes. Indeed recent data (Amato - New Engl J Med Feb 1998) suggest that setting ventilator parameters according to these measurements reduces mortality. Unfortunately, SPVC measurements require a completely passive patient (requiring sedation or paralytics) and are combursome, time consuming and require patient removal from the ventilator. An alternative approach is the slow flow pressure volume curve technique (SFPVC). This measurement uses only a single lung inflation and, because inspiratory flow is very low (eg <10L/min), flow resistive pressures are minimized such that the resultant pressure volume plot mimics the SPVC. We evaluated two automated SFPVC methods incorporated into the software of a mechanical ventilator. PATIENT POPULATION: Mechanically ventilated adult patients that presented with ARDS (P/F ratios < 200) and bilateral infiltrates on chest x-ray were eligible for study.
Methods: Using a large volume graduated syringe (2.0L) equipped with a manometer and filled with 100% oxygen, patients received step-wise injections of volumes with measurement of the corresponding pressures (max 45cmH20) and a standard SPVC was plotted. Patients were then evaluated using the two automated SFPVC methods. The first (Bear 1000 t/es, Thermo Respiratory Group, Palm Springs, CA), allows the user to freeze and display the SFPVC using a clinician triggered volume assist breath (flow rate 10 L/min, FiO2 1.0, PEEP 0, and tidal volume set to the max volume from the SPVC). The user can also elect to use tracheal (ie distal endotracheal tube) pressure to minimize flow resistive pressures. Markers indicate upper and lower inflection points and can be adjusted if necessary. Numeric values are displayed. The second (Venturi ventilator, Cardiopulmonary Corp, Milford, CT), has a feature known as Pflex, which functions independently of the set mode. Activation causes the suspension of normal ventilation and 2 consecutive slow flow breaths are delivered. The SFPVC plot is displayed. This device automatically subtracts endotracheal tube resistance effects in an attempt to eliminate flow resistive pressures. Markers indicating upper and lower inflection points can be adjusted if necessary. Numeric values are also displayed.
Results: Eleven patients had technically satisfactory SPVC and SFPVC tracings recorded (5 from the Bear 1000 t/es, 3 from the Venturi and 3 from both ventilators). Visual inspection of the compliance slope and the lower inflection points in 10 of the 11 patients revealed virtually identical SPVC and SFPVC tracings. In one patient, the SFPVC tracing on both ventilators was displaced slightly to the right of the SPVC tracing. CONCLUSION: These preliminary data suggest that automated SFPVC techniques may be acceptable substitutes for standard SPVC. Although patients must still have no spontaneous activity during SFPVC measurements, the fact that the technique requires only one to two inhalations and can be done in less than 1 minute make the procedure a much easier clinical measurement to perform.