Decreased respiratory rate variability during mechanical ventilation is associated with increased mortality.

OBJECTIVE: Patients on ventilatory support often experience significant changes in respiratory rate. Our aim was to determine the possible association between respiratory rate variability (RRV) and outcomes in these patients. DESIGN: A longitudinal, prospective, observational study of patients mechanically ventilated for at least 12 h performed in a medical-surgical intensive care unit. Patients were enrolled within 24 h of the initiation of ventilatory support. We measured airway signals continuously for the duration of ventilatory support and calculated expiratory flow frequency spectra at 2.5-min intervals. We assessed RRV using the amplitude ratio of the flow spectrum's first harmonic to the zero frequency component. Measures of the amplitude ratio were averaged over the total monitored time. Patients with time-averaged amplitude ratios <40 % were classified as high RRV and those ≥40 % as low RRV. All-cause mortality rates were assessed at 28 and 180 days from enrollment with a Cox proportional hazards model adjusted for disease acuity by the simplified acute physiology score II. RESULTS: We enrolled 178 patients, of whom 47 had high RRV and 131 low RRV. Both groups had similar disease acuity upon enrollment. The 28- and 180-day mortality rates were greater for low RRV patients with hazard ratios of 4.81 (95 % CI 1.85-12.65, p = 0.001) and 2.26 (95 % CI 1.21-4.20, p = 0.01), respectively. Independent predictors of 28-day mortality were low RRV, i.v. vasopressin, and SAPS II. CONCLUSIONS: Decreased RRV during ventilatory support is associated with increased mortality. The mechanisms responsible for this finding remain to be determined.