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Impact of Underassisted Ventilation on Diaphragm Function and Structure in a Porcine Model. BACKGROUND: Long-term controlled mechanical ventilation in the intensive care unit induces ventilator-induced diaphragm dysfunction (VIDD). The transition from controlled mechanical ventilation to assisted mechanical ventilation is a challenge that requires clinicians to balance overassistance and underassistance. While the effects of overassistance on the diaphragm are well known, the authors aimed to assess the impact of underassistance on diaphragm function and structure in a piglet model with preexisting VIDD (after long-term controlled mechanical ventilation) or without VIDD (short-term controlled mechanical ventilation). METHODS: Twenty-two Large White female piglets were anesthetized, ventilated, and separated into two groups: a VIDD group (n = 10) with long-term 72-h controlled mechanical ventilation, and a no-VIDD group (n = 12) with short-term 2-h controlled mechanical ventilation. After sedation reduction at the end of the controlled mechanical ventilation period, each piglet was switched to underassisted ventilation for 2 h. Diaphragm function (supramaximal diaphragm pressure-generating capacity assessed by negative tracheal pressure after transvenous phrenic nerve stimulation) and diaphragm structure (mini-invasive in vivo biopsies) were assessed before and after underassisted ventilation. RESULTS: In the VIDD group, supramaximal diaphragm pressure-generating capacity decreased by 22% from (mean ± SD) 69.9 ± 12.7 to 54.9 ± 19.7 cm H 2 O ( P = 0.04) after 72 h of controlled mechanical ventilation evidencing VIDD, then dropped by a further 29% from 54.9 ± 19.7 to 38.9 ± 15.5 cm H 2 O ( P < 0.01) after 2 h of underassisted ventilation. Diaphragm pressure-generating capacity remains stable from 55.3 ± 22.7 to 58.2 ± 24 cm H 2 O ( P = 0.24) in the no-VIDD group. Diaphragm structure showed that sarcomeric injuries increase from 13 ± 10% to 24 ± 19% ( P < 0.01) and lipid droplets decrease from 14 ± 8% to 11 ± 6% ( P = 0.03) of the total micrograph area after 2 h of underassisted ventilation in the VIDD group. Sarcomeric injuries and lipid droplets accounted, respectively, for 17 ± 16% and 2 ± 3% of the total micrograph area after underassisted ventilation in the no-VIDD group. CONCLUSIONS: In this porcine model, a short 2-h exposure of underassisted ventilation induces impairment of diaphragm function with damage to the diaphragm structure in intensive care unit condition with preexisting VIDD.