Clinical evaluation of a new closed loop ventilation mode: adaptive supportive ventilation (ASV)

Introduction

ASV (Galileo, Hamilton Medical, Inc.) is a mode of mechanical ventilation (MV) with a closed loop algorithm to determine and adjust ventilator settings. ASV may adjust mandatory breath rate, I : E ratio, and inspiratory pressure of mandatory breaths. Target minute ventilation (V_E) is determined by ideal body weight (IBW) and clinician selected Percent Vent (% Min Vol). Galileo assesses the pt by measuring dynamic compliance and expiratory time constant. With IBW and %Min Vol settings, optimal settings for rate, T_i, V_T, and V_E are determined. We compared MV with ASV to MV with physician determined vent settings during the initiation of MV.

Methods

Ninteen post-operative pts requiring MV were studied. Vent settings by physician were noted and each pt was placed on these settings or ASV randomly. IBW was determined from standardized tables and %MinVol was set to 100%. PEEP and FiO₂ were determined by staff and held constant. ABG's and cardiopulmonary variables (f, V_T, V_E, T_I, PIP, P_aw HR, MAP, and VCO₂) were measured and recorded after 30 min on each mode. Data compared using student's t-test.

Results

19 pts (14 male) were studied. Initial `test breaths' during ASV were well tolerated. Mean IBW was 88.8 Kg. Mean age was 54.3 years. Table 1 reveals set and measured ventilator parameters for both study periods. PIP and V_T were lower during ASV. Respiratory rate was higher during ASV. V_E, T_I, and P_aw were unchanged between study periods. Mean values for PEEP and FiO₂, were 7.3 and 0.48, respectively. Table 2 reveals ABG measurements, CO₂ production, and V_D/V_T ratio. There were no clinically relevant differences in ABG's or VCO₂ between study periods. V_D/V_T was lower during ASV. No pt suffered any adverse events from derangments in ventilation or acid-base balance. One pt with ARDS receiving 17 cmH₂O PEEP was hypoxemic during ASV (PaO₂ 57.2). Table 3 reveals heart rate and mean arterial pressure during each study period. There were no clinical changes to any measured vital sign between the two study periods.

Discussion/conclusion

Upon initiation of mechanical ventilation, the precise V_E requirement of the pt may not be known. Clinicians use rough estimates and clinical experience to determine V_E, respiratory rate, V_T and T_i. Determination of vent settings made by the machine have been suggested (Intern Care Med 1996, 22:199). Our results suggest that ASV as startup mode of ventilation is acceptable and comparable to physician determined ventilator settings. Gas exchange during ASV is equivalent to physician determined ventilation. V_T during ASV is more consistent with `lung protective' strategy (7.8 ml/kg) than was conventional V_T (9.7 ml/kg). Mechanical ventilation with ASV is more efficient as evidenced by lower V_D/V_T and may be safer as a result of lower V_T and PIP.

Table 1

Table 2

Table 3