Dynamic Changes in Respiratory Frequency/Tidal Volume May Predict Failures of Ventilatory Liberation in Patients on Prolonged Mechanical Ventilation and Normal Preliberation Respiratory Frequency/Tidal Volume Values
Rapid shallow breathing index (RSBI, respiratory frequency [f] divided by tidal volume [Vt]) has been used to prognosticate liberation from mechanical ventilation (LMV). We hypothesize that dynamic changes in RSBI predict failed LMV better than isolated RSBI measurements. We
conducted a retrospective study of patients who were mechanically ventilated (MV) for longer than 72 hours. Failed LMV was defined as need for reinstitution of MV within 48 hours post-LMV. Ventilatory frequency (f) and Vt (liters) were serially recorded. The instantaneous RSBI (i-RSBI) was
defined as f/Vt. Dynamic f/Vt ratio (d-RSBI) was defined as the ratio between two consecutive i-RSBI (f/Vt) measurements ([f2/Vt2]/[f1/Vt1]). RSBI Product (RSB-P) was defined as (i-RSBI × d-RSBI). Data from 32 patients were analyzed (Acute
Physiology and Chronic Health Evaluation II 13.4, male 69%, mean age 57 years). Mean length of stay was 19.5 days (11.5 ventilator; 14.1 intensive care unit days). For LMV failures, mean time to reinstitution of invasive MV was 20.8 hours. All patients had pre-LMV i-RSBI less than 100. Failed
LMVs had higher i-RSBI values (68.9, n = 18) than successful LMVs (44.2, n = 23, P < 0.01). Failures had higher d-RSBI (1.48) than successful LMVs (1.05, P < 0.04). The RSB-P was higher for failed LMVs (118) than for successful LMVs (48.8, P < 0.01) with
failures having larger proportion of pre-LMV d-RSBI values greater than 1.5 (39.0 vs 10.7%, P < 0.03). Pre-LMV RSB-P may offer early prediction of failed LMV in patients on MV for longer than 72 hours despite normal pre-LMV i-RSBI. Divergence between RSB-P for successful and
failed LMVs occurred earlier than i-RSBI divergence with a greater proportion of pre-LMV d-RSBI greater than 1.5 among failures.
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Document Type: Research Article
Department of Surgery, Division of Trauma & Surgical Critical Care, Vanderbilt, University Medical Center, Nashville, Tennessee, USA
Department of Surgery, Division of Critical Care, Trauma, and Burn, The Ohio State University Medical Center, Columbus, Ohio, USA
St. Luke's, Regional Level I Trauma Center, Bethlehem, Pennsylvania, USA
Publication date: January 1, 2012
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