Introduction
Inhalation injury (InI) occurs from the inhalation of smoke from a fire in an enclosed space and is the cause of inflammation [1]. Local damage to the tracheobronchial tree and lung parenchyma caused by toxic smoke compounds and inflammation are the causes of obstructive bronchitis [2], acute respiratory distress syndrome (ARDS) [3–6] and acute respiratory failure (ARF). The most common (70%) cause of ARF in patients with IT is bronchial obstruction. Less frequent (12%) cause is parenchymal pulmonary insufficiency [7, 8]. ARF can develop immediately after IT or 3-5 days later [7, 9] and necessitate mechanical ventilation. It is difficult to predict when and which patients will need mechanical ventilation due to delayed manifestation of ARF [10]. According to the literature, up to 30% of intubations in patients with burns at pre-hospital [11, 12] and in-hospital [10] stage turned out to be unreasonable.
To date, there are no objective criteria for diagnosis and stratification of patients depending on severity of inhalation injury. Searching for changes in serum and sputum biomarkers indicating the risk of complications in IT is being continued. Perhaps, data on biomarkers will be valuable in deciding on the need for respiratory support [13].
The purpose of the study was to study biomarkers of inflammation in ARF in patients with InI.
Material and methods
A retrospective study included 34 patients with InI who admitted to the intensive care unit of the Sklifosovsky Research Institute for Emergency Care between 2019 and 2021.
Inclusion criteria: patients over 18 years old, InI with previous respiratory tract burn, no skin burns. There were no exclusion criteria. Continuous sampling method was applied.
All patients admitted within 2 hours after injury. ARF with indications for invasive respiratory support developed in 17 (50%) out of 34 patients (on the first day after injury in 10 (59%) patients, within 2-5 days in 7 (41%) patients). Among 17 patients, ARF was caused by bronchial obstructive syndrome in 11 (65%) cases, parenchymal insufficiency (ARDS) — in 3 (18%) patients, mixed nature — in 4 (17%) patients.
Invasive mechanical ventilation was initiated for arterial oxygen saturation ≤ 86%, oxygenation index (PaO2/FiO2) ≤200 or severe bronchial obstructive syndrome with PaCO2 > 50 mm Hg. At admission, 15 (44%) out of 34 patients had PaO2/FiO2 index > 300. Of these, 4 (27%) patients subsequently required mechanical ventilation. Among 22 (56%) patients with PaO2/FiO2 < 300, mechanical ventilation was required in 13 (59 %) cases.
Duration of mechanical ventilation in survivors was 21 (16; 27) days (range 13-56), in dead patients — 43 (26; 66) days (range 17-143).
Age of patients with and without ARF was similar (54 (47; 71) (range 21-89) vs. 72 (58; 79) (range 33-87) years, respectively, p=0.131; Mann-Whitney U-test).
Upon admission, all patients underwent bronchoscopy to confirm InI and establish degree of tracheobronchial burns according to visual classification proposed by Skripal A.Yu.: grade 1 — catarrhal, grade 2 — erosive, grade 3 — ulcerative, grade 4 — necrotic form [14]. Severity of tracheobronchial burns was similar in patients with and without ARF: grade 1 in none patient without ARF and 2 patients with ARF (p = 0.485; Fisher's exact test), grade 2 — 9 and 3 patients (p=0.071; Fisher's exact test), grade 3 — 8 and 12 (p=0.296; Fisher's exact test) patients, respectively.
To study biomarkers, venous blood sampling was performed within 24 hours after admission. We analyzed leukocyte count (Le), C-reactive protein (CRP), procalcitonin (PCT), presepsin (PSP), interleukins 6 (IL-6) and 10 (IL-10). Leukocytes were counted on Advia 2120 hematological analyzer. CRP was analyzed using Atellica NEPH 630 analyzer (Siemens, Germany), PCT using VIdas immunochemiluminescent analyzer (bioMerieux, France), PSP using Pathfast chemiluminescent enzyme immunoassay analyzer (LSI Medience Corporation, Japan). Cytokines (IL-6 and IL-10) were analyzed by enzyme immunoassay (JSC Vector-Best, Russia).
Reference values: Le — 4–9×109/l, CRP — < 3.0 mg/l; PCT — < 0.05 ng/ml; PSP — < 337.0 pg/ml, IL-6 — < 10.0 pg/ml, IL-10 — < 20.0 pg/ml. Le, CRP, PCT and PSP were studied in all patients, IL-6 and IL-10 — in 21 patients.
Speaking about exceeding of reference values, we meant exceeding the upper limit of reference interval.
Statistical analysis was performed using Statistica 13.3 TIBCO Software Inc. Data are presented as absolute values (n) and percentages (%) with 95% confidence interval [95% CI]. Most of continuous data had abnormal distribution. Descriptive statistics is presented by medians (Me), interquartile ranges (Q1; Q3), maximum (max) and minimum (min) values. Comparative statistics is presented by non-parametric tests. Quantitative variables were compared using two-sided Fisher's exact test, continuous independent variables — using Mann-Whitney U-test. We analyzed odds ratio (OR) with 95% CI. Sensitivity, specificity, positive and negative predictive values with 95% CI were calculated.
Binomial 95% CI was determined using Clopper-Pearson method. Differences were significant at p-value <0.05; p-value 0.05–0.10 was considered as statistical trend [15].
Results
Reference values of Le were exceeded in 27 out of 34 patients, CRP — in 29 out of 34, PCT — in 22 out of 34, PSP — in 3 out of 21 patients. Serum IL-6 exceeded reference values in 8 out of 21 patients, IL-10 — in 2 patients (Figure).
Percentage of patients with indicators exceeding the reference values.
ARF occurred in patients with normal parameters and in those with abnormal indicators. Analysis of odds ratio indicates significantly higher probability of ARF only in patients with elevated PCT. Moreover, there was statistical trend of association between ARF and elevated IL-6 (Table 1).
Table 1. Incidence of acute respiratory failure in patients with normal and elevated values of biomarkers
|
Variable |
Value |
Acute respiratory failure, n |
OR [95% CI] |
p-value (Fisher’s exact test) |
|||
|
yes |
no |
||||||
|
n |
% |
n |
% |
||||
|
Le, ×109/L |
>9 |
15 |
56 [35; 75] |
12 |
44 [25; 65] |
3.13 [0.51; 19.04] |
0.398 |
|
≤9 |
2 |
29 [3; 71] |
5 |
71 [29; 96] |
|||
|
CRP, mg/l |
>3.0 |
16 |
55 [36; 74] |
13 |
44 [26; 64] |
4.92 [0.49; 49.61] |
0.335 |
|
≤3.0 |
1 |
20 [0; 72] |
4 |
80 [28; 99] |
|||
|
PCT, ng/ml |
>0.05 |
15 |
68 [45; 86] |
7 |
32 [14; 55] |
10.71 [1.84; 62.46] |
0.010 |
|
≤0.05 |
2 |
17 [2; 48] |
10 |
83 [52; 98] |
|||
|
PSP, pg/ml |
>337.0 |
3 |
100 [29; 100] |
0 |
0 [0; 71] |
2.43 [0.20; 29.66] |
0.227 |
|
≤337.0 |
14 |
45 [27; 64] |
17 |
55 [36; 73] |
|||
|
IL-6, pg/ml |
>10.0 |
7 |
88 [47; 100] |
1 |
13 [0; 53] |
11.20 [1.04; 120.36] |
0.067 |
|
≤10.0 |
5 |
38 [14; 68] |
8 |
62 [32; 86] |
|||
|
IL-10, pg/ml |
>20.0 |
2 |
100 [16; 100] |
0 |
0 [0; 84] |
0.90 [0.05; 16.59] |
0.486 |
|
≤20.0 |
10 |
53 [29; 6] |
9 |
47 [24; 71] |
|||
Note. Le — leukocyte count; CRP — C-reactive protein; PCT — procalcitonin; PSP — presepsin; IL — interleukin.
We analyzed sensitivity, specificity, positive and negative predictive values for each indicator regarding ARF. The cut-off value was upper limit of reference interval for each indicator, true positive result — development of ARF in elevated parameters (Table 2). Only Le, CRP and PCT had high sensitivity. There was a relationship of ARF with elevated PCT only, and no correlation with Le and CRP was found. However, these indicators exceeded reference values in most patients with ARF. Thus, we decided to determine the risk of ARF using their combinations (Table 3). ARF was not associated with excess of reference values by one or two indicators. The probability of ARF was 7.8 times higher when Le, CRP and PCT (significant difference) exceeded the upper limit simultaneously.
Table 2. Sensitivity, specificity, positive and negative predictive values of increased parameters regarding acute respiratory failure
|
Variable |
n |
Sensitivity, % |
Specificity, % |
Positive predictive value, % |
Negative predictive value, % |
|
Le |
34 |
88 [72; 98] |
29 [14; 39] |
56 [45; 62] |
71 [33; 95] |
|
CRP |
34 |
94 [80; 100] |
24 [9; 29] |
55 [47; 58] |
80 [32; 99] |
|
PCT |
34 |
88 [69; 98] |
59 [40; 68] |
68 [54; 76] |
83 [57; 97] |
|
PSP |
34 |
18 [6; 18] |
100 [88; 100] |
100 [33; 100] |
55 [48; 55] |
|
IL-6 |
21 |
58 [36; 66] |
89 [59; 99] |
88 [54; 99] |
62 [41; 69] |
|
IL-10 |
21 |
17 [4; 17] |
100 [82; 100] |
100 [21; 100] |
47 [39; 47] |
Note. Le — leukocyte count; CRP — C-reactive protein; PCT — procalcitonin; PSP — presepsin; IL — interleukin.
Table 3. Incidence of acute respiratory failure depending on the number of elevated indicators
|
Number of variables exceeding the upper limit |
Acute respiratory failure |
OR [95% CI] |
p, (Fisher’s exact test) |
|||
|
no |
yes |
|||||
|
n |
% |
n |
% |
|||
|
1 |
5 |
100 [48; 100] |
0 |
0 [0; 52] |
0,35 [0,04; 3,58] |
0,137 |
|
2 |
8 |
61 [32; 86] |
5 |
39 [13; 68] |
0,47 [0,11; 1,93] |
0,481 |
|
3 |
4 |
25 [7; 52] |
12 |
75 [48; 93] |
7,80 [1,69; 36,06] |
0,015 |
We analyzed changes of sensitivity, specificity, positive and negative predictive values regarding ARF when one, two or three indicators (Le, CRP and PCT) exceeded reference intervals (Table 4). Only simultaneous excess of reference values by three indicators (Le, CRP and PCT) demonstrated the highest sensitivity, specificity, positive and negative predictive values regarding ARF.
Table 4. Sensitivity, specificity, positive and negative predictive values regarding acute respiratory failure depending on the number of elevated indicators
|
Number of variables exceeding the upper limit |
Sensitivity, % |
Specificity, % |
Positive predictive value, % |
Negative predictive value, % |
|
1 |
0 [0; 15] |
70 [70; 85] |
0 [0; 51] |
41 [41; 50] |
|
2 |
29 [13; 48] |
53 [37; 72] |
39 [17; 63] |
43 [30; 58] |
|
3 |
71 [51; 84] |
77 [57; 90] |
75 [54; 90] |
72 [54; 85] |
Discussion
Damage to the bronchopulmonary system following smoke inhalation can lead to dangerous complication (ARF) [2, 5]. Blood biomarkers are not well studied in patients with ARF and isolated InI. We have selected 6 biomarkers whose changes are associated with ARF in patients with InI [16–18].
Leukocytosis in patients with trauma is a natural response to active release of pro-inflammatory mediators and one of the four clinical signs of systemic inflammatory response [19, 20]. CRP is considered to be a marker of acute inflammation and tissue damage [21]. The same is true for patients with burns [22]. PCT is important serum marker of systemic inflammation and sepsis [23, 24]. Elevated serum PCT in patients with ARDS correlates with severity of lung damage [25]. Elevated PSP is associated with sepsis in patients with burns and not associated with ARDS [16, 17]. We found more common ARF in patients with isolated InI whose Le, CRP and PCTexceeded the upper limit of reference values compared to those with normal Le, CRP and PCT. We associate these findings with development of systemic inflammation immediately after trauma.
Experimental study revealed that elevated cytokines are a marker of lung damage in patients with InI [26]. However, there controversial data on changes in interleukin levels in patients undergoing mechanical ventilation [27–29]. Serum IL-6 is increased in patients with risk of ARDS at the early stage [30, 31]. This was confirmed in experimental study on animals when ARDS developed after InI. Elevated serum IL-6 was detected after 2 hours [31]. In patients with InI and skin burns ≤ 5% of body surface area, IL-6 exceeded the reference values in most ones with ARDS [27]. In our study, serum IL-6 within the first day after injury exceeded the upper limit only in 38% of patients, and ARF was more common in these ones compared to patients with normal values.
IL-10 is an anti-inflammatory cytokine [32]. In one study, serum IL-10 was lower in patients with ARDS than in patients without ARDS [33]. In our study, serum IL-10 was normal in 90% of patients with and without ARF.
To date, biomarkers and their values have not been determined to predict ARF in patients with InI. However, combination of clinical data and biological markers can increase sensitivity and specificity of testing [34]. In our study, excess of reference values simultaneously by three indicators (Le, CRP and PCT) on the first day after injury was associated with higher risk of ARF compared to elevation of only one or two indicators.
In this study, we considered patients with ARF regardless of its cause (bronchial obstruction following damage to tracheobronchial mucous membrane or ARDS following damage to lung parenchyma, as well as their combination) [8, 35, 36]. Damage to tracheobronchial epithelium following InI and lesion of lung parenchyma lead to cascade of inflammatory reactions and changes in serum inflammatory markers [37]. Our study showed that their elevation can be detected several hours or days before the first signs of ARF.
Further studies with analysis of inflammatory biomarkers depending on the cause of ARF are required to determine their diagnostic levels associated with high risk of respiratory failure.
Conclusion
1. Leukocyte count, C-reactive protein and procalcitonin exceeded the upper limit in more than 65% of patients on the first day after injury, serum presepsin, interleukins 6 and 10 — less than in 38% of patients.
2. The risk of acute respiratory failure is associated only with elevation of procalcitonin and not associated with excess of leukocytes, C-reactive protein, presepsin, interleukins 6 and 10.
3. Leukocyte count, C-reactive protein and procalcitonin had the highest sensitivity regarding acute respiratory failure.
4. The probability of acute respiratory failure was 7.8 times higher when the upper limit was exceeded simultaneously by three indicators (leukocyte count, C-reactive protein and procalcitonin) compared to elevation of only one or two of these indicators.
The authors declare no conflicts of interest.