Respiratory response and inspiratory effort during pressure support ventilation in COPD patients

S Nava, C Bruschi, F Rubini, A Palo, G Iotti, A Braschi
Intensive Care Medicine 1995, 21 (11): 871-9

OBJECTIVE: Pressure Support Ventilation (PSV) is now widely used in the process of weaning patients from mechanical ventilation. The aim of this study was to evaluate the effects of various levels of PS on respiratory pattern and diaphragmatic efforts in patients affected by chronic obstructive pulmonary disease (COPD).

SETTING: Intermediate intensive care unit.

PATIENTS: We studied ten patients undergoing PSV and recovering from an episode of acute respiratory failure due to exacerbation 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 > or = 7.32 and an SaO2 > 93%. Then, PS was set at 5 cmH2O above (PSb + 5) and below (PSb-5) this starting level. Ventilatory pattern, transdiaphragmatic pressure (Pdi), the pressure-time product of the diaphragm (PTPdi), the integrated EMG of the diaphragm, static PEEP (PEEPi, stat), dynamic PEEP (PEEPi, dyn), and the static compliance and resistance of the total respiratory system were recorded.

RESULTS: Minute ventilation did not significantly change with variations in the level of PS, while Vt significantly increased 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 respiratory frequency with longer expiratory time. The best values of blood gases were obtained at PSb, while at PSb-5, PaCO2 markedly increased. During PSb and PSb + 5 and to a lesser extent during PSb-5, most of the patients made several inspiratory efforts that were not efficient enough to trigger the ventilator to inspire; thus, the PTPdi "wasted" during these inefficient efforts was increased, especially during PS + 5. The application of an external PEEP (PEEPe) of 75% of the static intrinsic PEEP during PSb caused a significant reduction in the occurrence 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 significantly reduced (by 59% and 31% of control, respectively, p < 0.001) with the application of PEEPe.

CONCLUSION: We conclude that in COPD patients, different levels of PSV may induce different respiratory patterns and gas exchange. PS levels capable of obtaining a satisfactory equilibrium 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 diaphragm without altering gas exchange.

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