Dynamic ventilatory characteristics during weaning in postoperative critically ill patients

Postoperative patients occasionally require more than 48 h of mechanical ventilation. This study examined whether there were distinct differences in dynamic respiratory variables between patients who successfully weaned from mechanical ventilation and those who failed. Forty general and thoracic surgery patients underwent a standardized weaning sequence: 25 min of synchronous intermittent mandatory ventilation (SIMV) at 8 bpm plus 5 cm H2O pressure support ventilation (PSV), then SIMV at 4 bpm plus 5 cm H2O PSV, followed by continuous positive airway pressure (CPAP) plus 5 cm H2O PSV and, finally, CPAP without PSV. Twenty-eight patients successfully weaned and 12 failed. During SIMV at 4 bpm plus 5 cm H2O PSV, the spontaneous respiratory rate to spontaneous tidal volume ratio (sRR/sV(T)) and total and spontaneous respiratory rates were higher (P < 0.01) in the failure group. sRR/sV(T) values (threshold 65 bpm/L, sensitivity 1.00, specificity 0.82) and sRR values (threshold 12 bpm, sensitivity 0.95, specificity 0.84) were distinctive. During CPAP plus 5 cm H2O of PSV, respiratory rate, minute ventilation, patient work of breathing, and P-0.1 were higher (P < 0.01) in those who failed. P-0.1 (threshold 4.5 cm H2O, sensitivity 1.00, specificity 1.00), patient work of breathing (threshold 1.3 J/L, sensitivity 0.92, and specificity 0.98), and the sRR/sV, ratio (threshold 65 bpm/L, sensitivity 0.90, specificity 0.80) were distinctive. Most unique was the analysis of spontaneous breaths during low SIMV rates. This appears to permit an early determination of whether weaning would succeed.