AN AUTOMATED TECHNIQUE FOR DETERMINATION OF THE VOLUME PRESSURE RATIO IN THE LUNGS OF MECHANICALLY VENTILATED PATIENTS AT THE BEDSIDE

  • 1 April 1989
    • journal article
    • research article
    • Vol. 38  (4) , 214-219
Abstract
The volume/pressure (V/P) ratio in the lungs has been reported to be useful in the adjustment of mechanical ventilation equipment to suit individual pulmonary mechanics. Most of the techniques used so far (e.g. the super-syringe technique) need an apneic period of approximately 60-120 s, in which the pulmonary gas volume is reduced by the continuing oxygen uptake. Thus, a bias of 200-400 ml in volume is superimposed on the record. In contrast to the super-syringe technique, a new automatic procedure has been developed for which no apneic period is needed. This technique is called the PEEP wave technique; it is based on the imbalance between inspiratory and expiratory volumes after a sudden change in PEEP. The software of a Drager Evita respirator was adapted to allow automatic application of a special sequence of respiratory cycles with stepwise increase and decreases in PEEP between two preselectable borderline levels. The equipment is switched to the next PEEP level when the difference between two consecutive expiratory tidal volumes is less than 15 ml. After the highest level of PEEP is reached the procedure is reversed until PEEP returns to its initial value. Constant inspiratory tidal volumes (Vti) are achieved by a high pressure servo valve (HPSV) under conditions of chocked flow, resulting in inspiratory tidal volumes which are independent of back-pressure. Thus, only the difference in expiratory tidal volumes (Vte) before and after PEEP change is necessary to determine gain and loss in lung volume (.DELTA. FRC). All parameters [end-inspiratory (Pei) and end-expiratory (Pee) airway pressure, Vte, .DELTA. FRC, and quasi-static compliance] recorded during a PEEP wave maneuver are transfered to a computer for further numerical and graphic analysis via serial interface. The graphic representation of the results is contained in four bivariant tableaux showing .DELTA. FRC versus PEEP (fig. 5a), a calculated V/P relation characterized by Pei/Vte + .SIGMA. .DELTA.FRC (fig. 5b), quasi-static compliance (C) versus PEEP (Fig. 5c) and the incremental compliance (CINC) as .DELTA. FRC/.DELTA. PEEP versus PEEP (Fig. 5d). In contrast to V/P ratios recorded with a super-syringe technique, no trapped volume and no hysteresis was detectable with our new method. Errors induced by oxygen uptake during apnea are avoided, since ventilation is continued throughout the PEEP wave maneuver.