RESPIRATORY RESPONSE AND INSPIRATORY EFFORT DURING PRESSURE SUPPORT VENTILATION IN COPD PATIENTS

Objective: Pressure Support Ventilation (PSV) is now widely used in th e process of weaning patients from mechanical ventilation. The aim of this study was to evaluate the effects of various levels of PS on resp iratory pattern and diaphragmatic efforts in patients affected by chro nic obstructive pulmonary disease (COPD). Setting: Intermediate intens ive care unit. Patients: We studied ten patients undergoing PSV and re covering from an episode of acute respiratory failure due to exacerbat ion of COPD. Methods: Three levels of PSV were studied, starting from the lowest (PSb) one at which it was possible to obtain an adequate Vt with a pH greater than or equal to 7.32 and an SaO(2) > 93%. Then, PS was set at 5 cmH(2)O above (PSb + 5) and below (PSb - 5) this startin g level. Ventilatory pattern, transdiaphragmatic pressure (Pdi), the p ressure-time product of the diaphragm (PTPdi), the integrated EMG of t he diaphragm, static PEEP (PEEPi, stat), dynamic PEEP (PEEPi, dyn), an d the static compliance and resistance of the total respiratory system were recorded. Results: Minute ventilation did not significantly chan ge with variations in the level of PS, while Vt significantly increase d with PS (PS - 5 = 6.3 +/- 0.5 ml/kg vs. PSb = 10.1 +/- 0.9 [p < 0.01 ] and vs. PS + 5 = 11.7 +/- 0.6 [p < 0.01]), producing a reduction in frequency with longer expiratory time. The best values of blood gases were obtained at PSb, while at PSb - 5, PaCO2 markedly increased. Duri ng PSb and PSb + 5 and to a lesser extent during PSb - 5, most of the patients made several inspiratory efforts that were not efficient enou gh to trigger the ventilator to inspire; thus, the PTPdi ''wasted'' du ring these inefficient efforts was increased, especially during PS + 5 . The application of an external PEEP (PEEPe) of 75% of the static int rinsic PEEP during PSb caused a significant reduction in the occurrenc e of these inefficient efforts (p < 0.05). Minute ventilation remained constant, but Vt decreased, together with Te, leaving the blood gases unaltered. The PTPdi per breath and the dynamic PEEPi were also signi ficantly reduced (by 59% and 31% of control, respectively, p < 0.001) with the application of PEEPe. Conclusion: We conclude that in COPD pa tients, different levels of PSV may induce different respiratory patte rns and gas exchange. PS levels capable of obtaining a satisfactory eq uilibrium in blood gases may result in ineffective respiratory efforts if external PEEP is not applied. The addition of PEEPe, not exceeding dynamic intrinsic PEEP, may also reduce the metabolic work of the dia phragm without altering gas exchange.