BACKGROUND: Indexes of heart rate variability (HRV) appear to reflect severity and may have prognostic value in patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD). We hypothesized that AECOPD without adequate treatment response would demonstrate impaired cardiac autonomic regulation and changes in HRV after emergency department (ED) treatment. METHODS: A prospective study of measuring HRV in admitted and discharged patients with AECOPD shortly after ED arrival and again 24 h after treatment. RESULTS: Total consecutive 33 patients (18 admitted and 15 discharged, age 77.1 +/- 1.6 years) were enrolled. Among admitted patients, high frequency in normalized unit (HF%) was significantly lower (P < 0.001) while Ratio of LF to HF (LF/HF ratio) was significantly higher (P < 0.001) than discharged. 24 h after treatment, admitted patients had a significantly larger increase in HF% (P < 0.002) and larger decrease in LF/HF ratio (P < 0.05) than discharged. ROC curve analysis show the relative potential of the DeltaHF% and DeltaLF/HF% in the discrimination of groups. The area under the ROC curve between the 2 groups was 0.807 (P < 0.01) and 0.722 (P < 0.05), respectively. The best cut-off value for the admission between groups was DeltaHF% >7.1 and DeltaLF/HF% <==-0.39. CONCLUSION: Patients with AECOPD requiring admission after ED treatment had a greater increase in HF% and greater decrease in LF/HF ratio compared to those discharged. Our study demonstrates patient with DeltaHF% was >7.1 or a DeltaLF/HF% <==-0.39 require admission despite 24 h of ED treatment.