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[Effects of neurally adjusted ventilatory assist on patient-ventilator synchrony in patients with acute respiratory distress syndrome].

OBJECTIVE
To observe the effect of neurally adjusted ventilatory assist (NAVA) on patient-ventilator synchrony in patients with acute respiratory distress syndrome (ARDS).


METHODS
Eighteen patients with ARDS were enrolled in the study. Each patient underwent both an incremental pressure support ventilation (PSV) and NAVA run randomly in 4 steps. The PSV level was gradually increased 5 cm H2O (1 cm H2O = 0.098 kPa) every 5 min from 5 cm H2O until to 20 cm H2O. Incremental NAVA was individually set in steps of 0.2 - 1.0 cm H2O/microV every 5 min to determine the NAVA level predicted to give an airway pressure in each step equivalent to that of PSV. Parameters in patient-ventilator synchrony such as ventilation trigger, ventilation switching and ventilation maintenance were observed at the same time during PSV(PSV1-PSV4) and NAVA(NAVA1-NAVA4).


RESULTS
(1) Ventilation trigger:(1) trigger delay: with progressive increases in PSV, the trigger delays increased significantly (P < 0.05). With increasing NAVA, the trigger delay did not increase significantly (P > 0.05) and each of them was shorter than that of PSV in each corresponding step (P < 0.05). (2) Ineffective trigger: At PSV1, the ineffective triggers accounted for 2. 3% of the neural respiratory rate (NRR), but the ineffective triggers added up to 22% in PSV4 significantly (P < 0.05). All neural efforts were triggered and there were not ineffective triggers at all level in NAVA. (2) Off cycle delay: the off cycle delays in PSV2 - PSV4 increased significantly when compared with PSV1 (P < 0.05). With increasing NAVA, the off cycle delays did not increase significantly (P > 0.05) and each of them was shorter than that of PSV in each corresponding step (P < 0.05). (3) Magnitude of assist [tidal volume (VT)]: The VT [(361 +/- 69) ml] in NAVA1 was equivalent to the VT[(361 +/- 121) ml] in PSV1(P > 0.05). The VT [(417 +/- 71) ml, (427 +/- 80) ml, respectively] in NAVA3-NAVA4 was significantly lower than the VT[(604 +/- 141) ml, (675 +/- 108) ml, respectively] in PSV3-PSV4 (P < 0.05). (4)Respiratory muscle loading: Increasing NAVA and PSV level reduced the magnitude of diaphragm electrical activity (EAdi) and the pressure-time products of esophagus (PTPes) (P < 0.05), but there was no significant difference in each corresponding step of PSV and NAVA(P > 0.05).


CONCLUSIONS
Compared with PSV, the ventilation cycle and the magnitude of assist in NAVA matched the patient's breathing pattern. NAVA improved patient-ventilator synchrony in patients with ARDS.