[A theoretical study on a method for estimating dynamic intrinsic positive end-expiratory pressure in invasive mechanical ventilation].

OBJECTIVE: To explore a simple method for measuring the dynamic intrinsic positive end-expiratory pressure (PEEPi) during invasive mechanical ventilation.

METHODS: A 60-year-old male patient was admitted to the critical care medicine department of Dongying People's Hospital in September 2020. He underwent invasive mechanical ventilation treatment for respiratory failure due to head and chest trauma, and incomplete expiratory flow occurred during the treatment. The expiratory flow-time curve of this patient was served as the research object. The expiratory flow-time curve of the patient was observed, the start time of exhalation was taken as T0, the time before the initiation of inspiratory action (inspiratory force) was taken as T1 , and the time when expiratory flow was reduced to zero by inspiratory drive (inspiratory force continued) was taken as T2 . Taking T1 as the starting point, the follow-up tracing line was drawn according to the evolution trending of the natural expiratory curve before the T1 point, until the expiratory flow reached to 0, which was called T3 point. According to the time phase, the intrapulmonary pressure at the time just from expiratory to inspiratory (T1 point) was called PEEPi1 . When the expiratory flow was reduced to 0 (T2 point), the intrapulmonary pressure with the inhaling power being removed hypothetically was called PEEPi2 . And it was equal to positive end-expiratory pressure (PEEP) set in the ventilator at T3 point. The area under the expiratory flow-time curve (expiratory volume) between T0 and T1 was called S1 . And it was S2 between T0 and T2 , S3 between T0 and T3 . After sedation, in the volume controlled ventilation mode, approximately one-third of the tidal volume was selected, and the static compliance of patient's respiratory system called "C" was measured using the inspiratory pause method. PEEPi1 and PEEP2 were calculated according to the formula "C = ΔV/ΔP". Here, ΔV was the change in alveolar volume during a certain period of time, and ΔP represented the change in intrapulmonary pressure during the same time period. This estimation method had obtained a National Invention Patent of China (ZL 2020 1 0391736.1).

RESULTS: (1) PEEPi1 : according to the formula "C = ΔV/ΔP", the expiratory volume span from T1 to T3 was "S3 -S1 ", and the intrapulmonary pressure decreased span was "PEEPi1 -PEEP". So, C = (S3 -S1 )/(PEEPi1 -PEEP), PEEPi1 = PEEP+(S3 -S1 )/C. (2)PEEPi2 : the expiratory volume span from T2 to T3 was "S3 -S2 ", and the intrapulmonary pressure decreased span was "PEEPi2 -PEEP". So, C = (S3 -S2 )/(PEEPi2 -PEEP), PEEPi2 = PEEP+(S3 -S2 )/C.

CONCLUSIONS: For patients with incomplete expiratory during invasive mechanical ventilation, the expiratory flow-time curve extension method can theoretically be used to estimate the dynamic PEEPi in real time.