[Effect of different levels of pressure support on physiological variables and patient-ventilator synchronicity during noninvasive pressure support ventilation in patients with respiratory failure].

OBJECTIVE

To evaluate the effects of different levels of pressure support (PS) during noninvasive pressure support ventilation (NPPV) on physiological variables, patient-ventilator synchronicity in patients with acute exacerbation chronic obstructive pulmonary disease (AECOPD) and hypercapnic respiratory failure.

METHODS

Fifteen hypercapnic respiratory failure AECOPD patients requiring NPPV in respiratory intensive care unit (RICU) were enrolled. Three different PS levels of 5, 10, 15 cm H2O (1 cm H2O=0.098 kPa) were applied randomly in all the 15 patients. At each PS level the patient was ventilated for 30 minutes, whereas physiological variables, ineffective effort (IE) expressed as IE index were recorded within the last 2 minutes.

RESULTS

The IE index, tidal volume (VT), minute ventilation (VE), coefficient of variation of VT, ventilator inspiratory time (TI), expiratory time (TE), leak volume at 15 cm H2O PS level were significantly higher than those at 5 cm H2O level [IE index: (33.8+/-9.1)% vs. (8.0+/-6.0)%, VT (ml): 626+/-203 vs. 339+/-115, VE (L/min): 11.1+/-4.7 vs. 7.7+/-2.7, coefficient of variation of VT: (32.6+/-15.5)% vs. (11.3+/-6.9)%, TI (s): 1.14+/-0.31 vs. 0.76+/-0.15, TE (s): 2.49+/-0.44 vs. 1.87+/-0.28, leak volume (L/min): 8.28+/-4.86 vs. 2.22+/-1.58, all P<0.05], while the ventilator respiratory rate [(17+/-3) breaths/min] was lower at 15 cm H2O PS level than that at 5 cm H2O level [(23+/-3) breaths/min, P<0.05]. At 5 cm H2O level, the IE index was positively correlated with TI (r=0.62, P<0.05). Furthermore, the increase in IE index followed by PS level increase was correlated negatively with the change of VT (Delta VT) and positively with that of the TI (Delta TI, R2=0.88, P=0.000).

CONCLUSIONS

(1) The IE at low assistance is associated with prolonged TI. (2) High PS level can significantly increase VE and VT, and reduce the ventilator respiratory rate. However, high PS level also lead to higher IE than low level. The factors favoring a higher increase in IE index as PS level increased include a less increase in VT and a higher increase in TI. However, an increase in leak volume is not associated with an elevated incidence of IE. (3) IE is high at high PS level during NPPV, even the Shape-signal is used as a trigger mechanism.