title: 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.
abstract: 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 ([f(2)/Vt(2)]/[f(1)/Vt(1)]). 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.
authors.affiliation: Department of Surgery, Division of Trauma & Surgical Critical Care, Vanderbilt, University Medical Center, Nashville, Tennessee, USA.
authors.forename: Raeanna C
authors.initials: RC
authors.lastname: Adams
authors.forename: Oliver L
authors.initials: OL
authors.lastname: Gunter
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Wisler
authors.forename: Melissa L
authors.initials: ML
authors.lastname: Whitmill
authors.forename: James
authors.initials: J
authors.lastname: Cipolla
authors.forename: David E
authors.initials: DE
authors.lastname: Lindsey
authors.forename: Christy
authors.initials: C
authors.lastname: Stehly
authors.forename: Steven M
authors.initials: SM
authors.lastname: Steinberg
authors.forename: Charles H
authors.initials: CH
authors.lastname: Cook
authors.forename: Stanislaw P A
authors.initials: SP
authors.lastname: Stawicki
mesh_terms: D018806:APACHE; D016009:Chi-Square Distribution; D005260:Female; D006801:Humans; D007902:Length of Stay / Q000706:statistics & numerical data; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D011237:Predictive Value of Tests; D011379:Prognosis; D012121:Respiration, Artificial; D012129:Respiratory Function Tests; D056152:Respiratory Rate / Q000502:physiology*; D012189:Retrospective Studies; D013990:Tidal Volume / Q000502:physiology*; D013997:Time Factors; D017211:Treatment Failure; D015300:Ventilator Weaning