EFFECTS OF ASSISTED VENTILATION ON THE WORK OF BREATHING - VOLUME-CONTROLLED VERSUS PRESSURE-CONTROLLED VENTILATION

During assisted ventilation, the same tidal volume can be delivered in different ways, with the possibility for the physician to vary the ve ntilatory target (pressure or volume) and the peak flow setting. We co mpared the effects on the respiratory work rate of assisted ventilatio n, delivered either with a square wave flow pattern (assist control ve ntilation [ACV]) or with a decelerating flow pattern and a constant pr essure (assisted pressure-control ventilation [APCV]). In the first pa rt of the study where seven patients were studied, inspiratory time an d tidal volume were similar in the two modes of ventilation. High and moderate levels of tidal volume (VT) were studied (12 ml/kg and 8 ml/k g, respectively). To obtain moderate VT, inspiratory time was kept con stant and, therefore, mean inspiratory flow was reduced. At high VT, n o difference between ACV and APCV was noted for breathing pattern, res piratory drive indexes, respiratory muscle work, or arterial blood gas es. All patients exhibited respiratory alkalosis. At moderate VT, norm al pH was achieved. In this situation significantly lower levels were observed during APCV than during ACV for the power of breathing (10 +/ - 2 versus 19 +/- 5 J/min, p < 0.05), transdiaphragmatic pressure swin g (7 +/- 1 versus 11 +/- 2 cm H2O, p < 0.05), and pressure-time index (252 +/- 43 versus 484 +/- 114 cm H2O.s, p < 0.05), even though breath ing pattern and gas exchange were similar. In the second part of the s tudy where six additional patients were studied, tidal volume was kept constant at a moderate level (8 ml/kg), and we studied the effect of shortening inspiratory time and increasing mean inspiratory flow. At m oderate VT and high inspiratory flow, no significant differences could be found between ACV and APCV, and although pressure-time index tende d to be lower during APCV, absolute levels of effort were of small mag nitude (56 +/- 55 versus 76 +/- 55 cm H2O.s). We conclude that at mode rate VT and low flow rates only, inspiratory assistance delivered at a constant pressure reduces the respiratory work rate more effectively than assist control ventilation.