Patient-Determined Inspiratory Flow during Assisted Mechanical Ventilation

Abstract

BACKGROUND: We wished to determine whether providing patients the ca- pability to exceed the set inspiratory flow during mechanical ventilation is as- sociated with a decrease in work-of-breathing indices. METHODS: A Bird 8400-ST ventilator, modified to assure a minimum but variable tidal volume (V_T) and to allow patient-determined inspiratory flow (V_pd), was used to compare assisted mechanical ventilation (AMV) with a preset inspiratory flow (V_set) to AMV with V_pd. Patients (n = 18) were studied during 3 periods of ventilation- V_set-1, V_pd, and V_set-2. The set respiratory rate (f) equaled half the actual assist f. F_DO2, minimum V_T, and set f were held constant. Minute ventilation (V_E), f, arterial blood-gas (ABG) evaluations, end-tidal CO₂ (P_etCO2), O₂ consumption, CO₂ production, cardiac output, heart rate, systemic and pulmonary arteri- al pressure, pressure-time product (PTP), intrinsic positive end-expiratory pres- sure (PEEP_i), esophageal pressure (P_es), peak and mean airway pressure, peak inspiratory flow (V_max), respiratory drive (P_0.1), patient (WOB_pt) and venti- lator (WOB_vent) work of breathing, and duty cycle (t₁/t_tot) were measured. RESULTS: V_max and P_etCO2 were significantly higher with V_pd than with either V_set-1 or V_set-2, and V_E, f, P_es and PTP were lower with V_pd than with V_set-2. There were no significant differences in the hemodynamic, ABG, or metabolic data. Although not statistically significant, tendencies were noted toward a reduction in PEEP;, P0.1, and WOBpt, and toward an increase in WOBvent with Vpd. CON- CLUSION: If allowed, patients exceed the Vset during AMV, reducing indices of patient work