TIDAL VOLUME, BREATHING FREQUENCY, AND OXYGEN-CONSUMPTION AT DIFFERENT PRESSURE SUPPORT LEVELS IN THE EARLY STAGE OF WEANING IN PATIENTS WITHOUT CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

The objective of this study was to evaluate the influence of different PSV levels on V(T), F, VO2 in the early weaning phase of patients without chronic obstructive pulmonary disease. These parameters were tested for the predictive power for the success of the weaning. Patients on SIMV were studied during the first weaning attempt with PSV. Depending on their ventilatory support demands after 24 h they were divided into responders (patients breathing on CPAP) and nonresponders (patients being on a more invasive ventilatory mode). 14 ICU patients without pre-existing pulmonary disease being ventilated for at least 3 days entered the study. 2 of them could be studied a second time after failing the first weaning attempt. Beside the level of ventilatory support no other changes (drugs, nutrition) were allowed. VO2, V(T), F were measured by a computer controlled, metabolic unit connected to the expiratory port of a Siemens Servo Ventilator. In addition, airway pressures, arterial pressure and heart rate were recorded. The measurements were performed at PSV of 5, 10 and 20 cmH2O. Arterial blood-gases were drawn at the end of each 60 min lasting PS period. Responders and nonresponders could be separated by the response of VO2, V(T) and F to a change in PS 10 to PS 20 cmH2O. Patients who significantly increased V(T) and significantly decreased F did not fulfil our weaning criteria. Our responders did not show a significant change in these two parameters, but a significant increase in VO2 at PS 20 cmH2O could be observed. In patients offering a steady increase in V(T) and decrease in F up to high levels of PS the contribution of respiratory muscles to the volume displacement seemed to be poor. Patients who could be weaned used their respiratory muscles to control their V(T) at higher levels of PS.