Ventilatory Parameters and Respiratory Mechanics

Pressure-controlled, assist-control (A/C) ventilation was used on all patients to
unload the respiratory pump effectively during both an assisted and controlled
breath, thereby minimizing respiratory muscle activity. Ventilatory variables
collected included inspired oxygen fraction, respiratory rate, tidal volume
(VT), peak inspiratory airway pressure (P_max), and positive
end-expiratory pressure (PEEP), with the following parameters calculated:
∆P_aw (dynamic driving pressure, defined as P_max – PEEP
in the pressure-controlled ventilation mode), dynamic lung-thorax compliance
(LTC_dyn, defined as VT/∆P_aw),^{22 (link)}VR,^{9 (link)} and MP^{10 (link)} using the simplified
formula proposed by Becher and colleagues.^{23 (link)}To account for lung dimensions on which MP acts, referred to as ‘specific MP’, we
normalized total power to LTC_dyn (LTC_dyn-MP), a measure of
mechanical ventilation stress intensity. Dynamic compliance with its temporal
changes is a surrogate of actual ventilated lung volume, accounting for the
force required to overcome the respiratory system’s resistance and elastance
(equals the dynamic driving pressure), which is crucial during ventilator
weaning.^{17 (link),18 (link)} In creating the Power index_rs,^{17 (link),18 (link)} isocapnia
was simulated by adjusting LTC_dyn-MP values for complementary
P_aCO₂ to account for individual ventilator settings. A
detailed description of the calculated variables and indexes is provided in the
online supplement (see Supplementary file 1).