Among patients diagnosed with exacerbations of COPD, this study found significant association between log2BNP and in-hospital mortality, a composite outcome of in-hospital mortality or mechanical ventilation, and mechanical ventilation, after adjusting for baseline demographics, comorbidities and other hematological data; results of the subgroup analysis were stable; and this suggests that BNP can be used as an important biomarker of prognosis in patients with COPD exacerbations.
Table 2: Univariate associations with in-hospital mortality
Variables
|
All (n=300)
|
P Value
|
Sex, n (%)
|
|
|
female
|
110 (36.67)
|
1.0
|
male
|
190 (63.33)
|
0.573
|
Age, mean in years (SD)
|
76.88 (9.53)
|
0.191
|
Smoking status, n (%)
|
|
|
Never
|
104 (34.67)
|
1.0
|
Ex-smoker
|
156 (52.00)
|
0.301
|
Current smoking
|
40 (13.33)
|
0.050
|
drinking status, n (%)
|
|
|
Never
|
193 (64.33)
|
1.0
|
Ex-drinker
|
90 (30.00)
|
0.362
|
Current drinker
|
17 (5.67)
|
0.374
|
Past medical history
|
|
|
Hypertension, n (%)
|
79 (26.33)
|
0.701
|
Diabetes, n (%)
|
35 (11.67)
|
0.311
|
Cerebral vascular disease, n (%)
|
38 (12.67)
|
0.154
|
Renal dysfunction, n (%)
|
4 (1.33)
|
NA
|
Malignancy, n (%)
|
7 (2.33)
|
0.430
|
Atrial fibrillation, n (%)
|
10 (3.33)
|
0.062
|
Lung function, mean (SD)
|
|
|
FVC (litres)
|
2.95 (0.71)
|
0.659
|
FEV1(litres)
|
2.22 (0.63)
|
0.551
|
FEV1(%predicted)
|
70.39 (19.75)
|
0.204
|
FEV1/FVC (%)
|
56.38 (10.41)
|
0.421
|
Clinical scores
|
|
|
CURB65, increment by 1
|
1.24 (0.69)
|
0.017
|
BAP, increment by 1
|
2.33 (0.73)
|
0.121
|
BMI
|
24.15 (4.14)
|
0.480
|
PaCO2
|
49.47 (17.67)
|
0.993
|
PaO2
|
79.74 (26.08)
|
0.017
|
pH
|
7.39 (0.08)
|
0.532
|
Neutrophil count, ×109/l
|
6.70 (4.25)
|
0.189
|
CRP, mg/l
|
37.56(67.09)
|
0.535
|
Creatinine, umol/l
|
78.26 (43.24)
|
0.732
|
Urea, mmol/l
|
6.07 (3.96)
|
0.166
|
Albumin, g/l
|
37.90 (4.96)
|
0.125
|
D-dimer, mg/l
|
1.17 (1.67)
|
0.412
|
BNP, pg/ml
|
207.82 (378.35)
|
0.002
|
Abbreviations: CRP, C reactive protein; BMI, body mass index; PaO2, arterial partial pressure of O2; PaCO2, arterial partial pressure of CO2; BNP, plasma B-type natriuretic peptide; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity.
A previous prospective study by Chang et al. found that elevated NT-pro-BNP levels (>220 pmol/L) were associated with increased 30-day mortality, and this finding is consistent with that of our study. [10] Another study by Hai et al. found that NT-pro-BNP level (≥551.35 ng/l) was associated with in-hospital mortality and 1-year mortality in patients with exacerbations of COPD, but in their study, about 35.9% of patients had congestive heart failure. [11] A systematic review and meta-analysis also found that elevated NT-pro-BNP values were associated with all-cause in-hospital mortality in patients with and without exacerbation of COPD. [17]
Table 3. Hazard ratios for log2BNP and in-hospital mortality, composite of in-hospital mortality or invasive ventilation, invasive ventilation
|
No. (%)
|
Unadjusted model
|
Age-and sex-adjusted model
|
Fully adjusted model*
|
|
HR (95% CI) P-value
|
HR (95% CI) P-value
|
HR (95% CI) P-value
|
Primary outcome
|
|
|
|
|
In-hospital mortality
|
16(5.33)
|
1.85 (1.39, 2.47) <0.001
|
2.00 (1.44, 2.77) <0.001
|
3.65 (2.54, 5.26) <0.001
|
Secondary outcomes
|
|
|
|
|
Mortality or invasive ventilation
|
31 (10.33)
|
1.45 (1.20, 1.75) <0.001
|
1.50 (1.23, 1.82) <0.001
|
1.43 (1.11, 1.84) 0.006
|
Invasive ventilation
|
28 (9.33)
|
1.51 (1.24, 1.85) <0.001
|
1.57 (1.27, 1.93) <0.001
|
1.50 (1.14, 1.97) 0.004
|
*fully adjusted model includes the following covariates: age, sex, smoking status, drinking status, past medical history, CURB65, PaO2.
Stolz et al. found that BNP was not associated with 6-month or 2-year mortality in patients with exacerbations of COPD, but BNP accurately predicted the need for ICU treatment, and the need for mechanical ventilation was the most common reasons for ICU admission. [12] This is confirmed in our study, we also found that the association between log2BNP and invasive mechanical ventilation was strong in multivariate analysis. This was consistent with the finding of previous studies in which BNP was an independent predictor of higher noninvasive ventilation requirement, mechanical ventilation use, and noninvasive ventilation failure in patients with AECOPD with preserved left ventricular function, but they did not assess the association of BNP and in-hospital mortality. [13] Another previous retrospective study by Muhammad et al. demonstrated that log-transformed NT-pro-BNP levels were positively associated with echocardiographically estimated right ventricular systolic pressure; however, NT-pro-BNP was not associated with the need for noninvasive positive pressure ventilation/mechanical ventilation or with in-hospital mortality in patients with AECOPD without underlying left ventricular dysfunction. [18]
Plasma brain natriuretic peptide and NT-pro-BNP are cardiac neurohormones that are synthesized and released from ventricular myocytes in response to myocardial stretch. [4] It was established that NT-pro-BNP is a diagnostic and prognostic biomarker of congestive heart failure, [5] and it has also been found to be a significant and independent predictor of death and cardiovascular events in stable coronary artery disease.[6] In our study, we had excluded patients with a past medical history of coronary ischemic heart disease; however, we had not obtained the echocardiographic data; therefore, a small number of patients with undiagnosed heart failure may have been included. The meta-analysis performed by Pavasini et al. suggested that having a history of heart failure did not influence the relationship between elevated NT-pro-BNP levels and long-term mortality. [18]
The pathophysiological processes underlying the elevation of BNP level in patients with exacerbations of COPD are unknown. Hypoxia often occurs in severe COPD, especially in patients with AECOPD. In our research, oxygen tension on arterial blood gas measurement was strongly associated with in-hospital mortality (P=0.008), composite outcome of in-hospital mortality or mechanical ventilation(P=0.009), and mechanical ventilation(P=0.014) in univariate analysis. Hypoxia can cause pulmonary hypertension and right ventricular dysfunction by pulmonary vasoconstriction. [7] Concentration of NT pro-BNP was significantly higher in those with a higher pulmonary hypertension than in those with a lower pulmonary hypertension, and they found that NT pro-BNP was an excellent discriminator of right-ventricular impairment but not of cor pulmonale. [8] Previous studies have found that BNP and NT-pro-BNP levels were elevated in patients with pulmonary thromboembolism, and BNP may be a useful determinant of the short-term outcome. [9]
Coronary artery ischemia may be more likely to occur in patients with AECOPD. Gavin et al found that exacerbations of COPD increase the risk of myocardial infarction. [14] This finding may be related to hypoxic ischemia of myocardium caused by hypoxia. Hypoxic myocardial ischemia leads to a decrease in myocardial contractility, which increases the burden on the heart and increases BNP or NT-pro-BNP secretion. In our study, 12 patients had acute myocardial infarction during hospitalization and were excluded.
There are some limitations to our research. First, our study was observational; no causal inference can be obtained, and relationships should be interpreted as associations. Second, COPD specific diagnostic codes for the identification of patients were adopted. Spirometry data of some patients were not available completely. However, we reviewed the electronic patient files and medical data intelligence platform of Jinan Central Hospital and found that they were hospitalized many times and inhaled maintenance drugs of COPD for a long time; in subgroup analysis, irrespective of whether data on the lung function were missing or not, the results were consistent. Third, although our study was a retrospective single-center trial, it was closer to real-world clinical practice, and the patient treatments by clinicians were not influenced by the study. Considering that no multicenter study has been performed so far, a multi-center prospective study is urgently needed to further explore the prognostic efficiency of BNP for AECOPD.
Table 4 Unadjusted hazard ratios for log2BNP and in-hospital mortality, composite of in-hospital mortality or invasive ventilation, invasive ventilation by subgroups
|
No. of patients
|
Mortality
|
P interaction
|
Mortality or invasive ventilation
|
P interaction
|
Invasive ventilation
|
P interaction
|
HR (95% CI)
|
HR (95% CI)
|
HR (95% CI)
|
Sex
|
|
|
0.051
|
|
0.377
|
|
0.173
|
female
|
110
|
1.30 (0.83, 2.06)
|
|
1.45 (1.05, 1.98)
|
|
1.43(1.03, 1.99)
|
|
male
|
190
|
2.60 (1.64, 4.11)
|
|
1.54 (1.18, 2.01)
|
|
1.71(1.27, 2.31)
|
|
FEV1 (% predicted)
|
|
|
0.915
|
|
0.676
|
|
0.989
|
missing
|
142
|
1.97 (1.32, 2.94)
|
|
1.41 (1.12, 1.79)
|
|
1.52 (1.17, 1.98)
|
|
No missing
|
158
|
2.133 (1.19, 3.82)
|
|
1.54 (1.02, 2.32)
|
|
1.54 (1.02, 2.32)
|
|
Age, years
|
|
|
0.218
|
|
0.809
|
|
0.794
|
low
|
134
|
2.43 (1.10, 5.35)
|
|
1.37 (1.08, 1.74)
|
|
1.41(1.09, 1.83)
|
|
high
|
166
|
1.77 (1.26, 2.48)
|
|
1.60 (1.21, 2.12)
|
|
1.68 (1.25, 2.27)
|
|