ABSTRACT. The combined system of ventilator circuit, endotracheal tube, and lung commonly imposes a resistive load on spontaneous breathing efforts. It is possible to compensate for this positive resistance by a device generating a "negative ventilator resistance" (NVR), i.e. delivering a positive pressure during inspiration and a negative pressure during expiration in constant proportion to the instantaneous flow of the spontaneous breathing. The concept of NVR implies that there must not be any phase lag between flow and pressure signals. In eight anesthetized, intubated, spontaneously breathing rabbits (mean body wt 3570 g, range 2900-4600 g), challenged either by aerosolized histamine or an extrapulmonary resistive load, lung mechanical data were calculated from esophageal pressure and flow signals. Each animal served as its own control with and without NVR. In a total of 39 experiments, NVR was applied in amounts between 1 and 15 k P a s/L. During both types of additional resistive load, NVR immediately reduced the resistive work of breathing. There was a strong linear correlation between the amount of NVR applied and the decrease in total resistance, where the total resistance equals the resistive load on the animal's respiratory muscles (sum of the resistances of all components of the combined respirator-lung system): r = 0.93, p < 0.001.The relationship between NVR and the drop in resistive work per mL of tidal volume was similar: r = 0.85, p < 0.001. Throughout the experiments, NVR operated in perfect synchronization with the animal's spontaneous breathing activity. (28: 79-82, 1990) Abbreviations CPAP, continuous positive airway pressure NVR, negative ventilator resistance P,,, esophageal pressure P,, pressure at the mouthpiece level R,,, extrapulmonary resistive load V. flow resistance) on the patient's spontaneous breathing efforts. That is, the pressure at the endotracheal tube's connector (P,) drops during inspiration and rises during expiration as a function of V. If a ventilator's resistance equals 0, there is no upward or downward deviation in P, during spontaneous breathing. Therefore, with NVR, the apparatus delivers a positive P, during inspiration and a negative P, during expiration in a constant proportion to V. This proportion, P,/V, has units of resistance and determines the degree of resistive unloading of spontaneous breathing.The aim of our study was to test the applicability of NVR in reducing the resistive work of breathing in an animal model of airway obstruction.
MATERIALS AND METHODS
Description of the ventilator. The ventilator (4) uses a constantflow of 12 L/min of humidified and warmed gas, which enters a venturi system (Fig. 1). The main outlet of this system is partly (but never totally) obstructed by a metal plate moved by a bobbin. The distance of the plate from the main outlet determines the magnitude and direction of the V through the lateral outlet of the venturi system. In other words, the jet flow within this system enables the unit to provide positive and negative ...