Approximate entropy of respiratory rate and tidal volume during weaning from mechanical ventilation

Objective: To determine the effects of respiratory failure on respiratory rate pattern and tidal volume pattern. Design: Prospective, clinical study. Setting: Cardiovascular intensive care unit. Patients: Ten patients within 12 hrs of cardiac surgery, and 21 patients who required prolonged (>7 days) mechanical ventilation. Interventions: Patients were placed on spontaneous ventilation for weaning trials. Measurements and Main Results: During spontaneous ventilation, each breath's instantaneous respiratory rate and tidal volume were recorded for later analysis. Approximate entropy (ApEn) was calculated for respiratory rate and tidal volume series of the terminal 1000 breaths on each spontaneous ventilation trial in series of 100, 300, and 1000 breaths. Ten patients (controls) were studied and extubated within 12 hrs of cardiac surgery. The other 21 patients were studied during attempts to wean them from mechanical ventilation. These patients passed (Group V-Pass) 59 and failed (Group V-Fail) 14 weaning trials. Mean tidal volume did not vary between groups, but respiratory rate increased progressively from the control group to Group V-Pass to Group V-Fail (p < .017). Conversely, aproximate entropy of respiratory rate (ApEn-RR) did not vary among the three groups at any time series length, but aproximate entropy of tidal volume (ApEn-VT) increased from the control group to Group V-P (p < .017) to Group V-F (p < .017) at all time series lengths. ApEn-VT was very specific but only moderately sensitive at identifying respiratory failure. Conclusion: Respiratory failure causes tidal volume patterns to become increasingly irregular, but increasing respiratory rate has no effect on respiratory rate pattern.