Evaluation of a new weaning index based on ventilatory endurance and the efficiency of gas exchange
We hypothesized that the ventilatory capacity needed to wean from mechanical ventilation (mv) depends on two variables: ventilatory endurance and the efficiency of gas exchange. We also hypothesized that these variables could be assessed from data readily available at the bedside, including tidal volume (VT) on mv and during spontaneous breathing (sb), ventilator peak inspiratory pressure (Ppk), and patient negative inspiratory pressure (NIP). Ventilatory endurance was evaluated using a modified pressure-time index: PTI = TI/Ttot x Pbreath/NIP, where Pbreath = Ppk x VT(sb)/VT(mv). Defining VE40 as the minute ventilation needed to bring Pa(CO2) to 40 mm Hg, the efficiency of gas exchange was evaluated by calculating VE40/VT(sb) = (VE x Pa(CO2))mv/VT(sb) x 40. Because high levels of inspiratory effort might cause patients to reduce VT(sb) and thereby compromise CO2 elimination, we devised a weaning index (WI) that combines ventilatory endurance and the efficiency of gas exchange: WI = PTI x (VE40/VT(sb)). The study population comprised 38 patients with chronic obstructive pulmonary disease, adult respiratory distress syndrome, pneumonia, neuromuscular disease, and miscellaneous other conditions. They had been mechanically ventilated more than 3 days and were considered by clinical criteria to be ready for weaning. Of 46 weaning trials, 19 were successful, 2 were partially successful, and 25 failed. PTI and VE40/VT(sb) were higher in patients who failed (p < 0.05), but neither variable alone had sufficient sensitivity or specificity to predict the outcome of weaning trials accurately. All 24 trials in which WI was > 4 min-1 failed, whereas 18 of 19 trials in which WI was < 4 min-1 succeeded. We conclude that WI successfully predicts the outcome of weaning trials and also provides valuable insight into mechanisms responsible for weaning failure.
Jabour, ER; Rabil, DM; Truwit, JD; Rochester, DF
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