Work of Breathing during CPAP and PSV Imposed by the New Generation Mechanical Ventilators: A Lung Model Study

Abstract

BACKGROUND: Numerous mechanical ventilators are presently available—some have never been evaluated for imposed work of breathing (WOBᵢ), and some have not been evaluated for WOBᵢ subsequent to being upgraded by the manufacturer. In this study, we evaluated the WOBᵢ of a free-standing CPAP system at 0 and 10 cm H₂O CPAP and of 9 mechanical ventilators at 0 cm H₂O PSV, 0 cm H₂O CPAP; 0 cm H₂O PSV, 10 cm H₂O CPAP; 10 cm H₂O PSV, 0 cm H₂O CPAP; and 10 cm H₂O PSV, 10 cm H₂O CPAP. METHODS: All evaluations were performed using a 2-chamber lung model powered by a Bear 5 ventilator set at Vᴛ 300 mL, ${\dot{V}}_{\max }20$ L/min; Vᴛ 400 mL, ${\dot{V}}_{\max }40$ L/min; and Vᴛ 500 mL, < L/min. RESULTS: Minimal WOBᵢ was measured at 0 cm H₂O PSV, 0 cm H₂O CPAP; 10 cm H₂O PSV, 0 cm H₂O CPAP; and 10 cm H₂O PSV, 10 cm H₂O CPAP. Markedly elevated WOBᵢ was measured (with most systems) at 0 cm H₂O PSV, 10 cm H₂O CPAP. All demand systems produced low levels of PSV (1.5-5 cm H₂O), even when the PSV lev-el was set at 0 cm H₂O. CONCLUSIONS: If properly maintained, most ventilator demand-valve systems impose minimal WOB that can be essentially eliminated by using 10 cm H₂O PSV. The addition of CPAP increases WOBᵢ, as does the use of a bubble-through humidifier. The free-standing CPAP system outperformed most ventilator demand-valve systems when a PEEP valve with minimal flow resistance was used and appropriate continuous flows were set.