Time course evolution of ventilatory response to inspiratory unloading in patients

Introduction

Inspiratory unloading decreases ventilatory drive and work of breathing in patients undergoing mechanical ventilation. We examined the time course of this effect in patients receiving permanent ventilatory support, provided by pressure support ventilation (PSV), or intermittent ventilatory support, provided by biphasic positive airway pressure (BIPAP).

Methods

Two sets of measurements were taken from 8 COPD patients durign the period of weaning from mechanical ventilation: First, during spontaneous breathing, then during the first 15 respiratory cycles following the onset of PSV; second, during BIPAP set in such a manner that one spontaneous breath took place between two pressure-assisted breaths. The following variables were measured: VT, PO.1, mean transdiaphragmatic pressure (Pdi), inspiratory work (Wi), and diaphragmatic electrical activity (EMG-di).

Results

The first breath following the onset of PSV was associated with an increase in VT and a drop in Pdi and Wi performed per liter, with unchanged values of PO.l, Wi performed per breath, and EMG-di. The same phenomena were observed for the assisted breath of BIPAP as compared to the preceding spontaneous breath. During the subsequent breaths of PSV; PO.l Wi, and EMG-di decreased progressively up to the sixth to eighth breath, and VT returned to pre-PSV values.

Conclusion

The decrease in ventilatory drive associated with PSV takes place from the first breath onwards, but requires 6 to 8 breaths to be fully achieved. This transient period could explain the characteristics of the pressure-assisted breaths intermixed with spontaneous breaths during BIPAP ventilation: an increased VT for an unchanged work per breath, suggesting an enhanced inspiratory efficiency.