Patient characteristics
The mean patient age was 64 ± 15 years, and 37.8% of the patients were female. The proportion of patients with septic AKI was 58.6%. When the patients were divided into 3 groups by initial lactate levels, the proportion of septic AKI, and SOFA and APACHE II scores were significantly higher in the moderate and high lactate groups than in the low lactate group. Other baseline characteristics are presented in Table 1.
Table 1
Baseline characteristics of the patients
|
|
Lactate groups
|
|
Variables
|
All
(n = 1,661)
|
Low
(n = 317)
|
Moderate
(n = 802)
|
High
(n = 542)
|
P
|
Age (years)
|
63.6 ± 15.1
|
64.1 ± 15.8
|
64.2 ± 15.2
|
62.7 ± 14.7
|
0.144
|
Female (%)
|
37.8
|
36.0
|
35.0
|
41.4
|
0.041
|
Weight (kg)
|
65.3 ± 13.0
|
62.3 ± 13.7
|
62.2 ± 12.6
|
62.4 ± 13.1
|
0.542
|
Septic acute kidney injury (%)
|
58.6
|
54.6
|
55.8
|
63.3†
|
0.005
|
Target dose (ml/kg/hr)
|
41.0 ± 12.8
|
39.8 ± 12.9
|
39.8 ± 12.4
|
42.8 ± 13.4†
|
< 0.001
|
Blood flow rate (ml/min)
|
113.4 ± 24.8
|
114.1 ± 25.3
|
112.7 ± 24.0
|
113.6 ± 25.3
|
0.674
|
Target ultrafiltration (ml/d)
|
0 (–500–0)
|
0 (–500–0)
|
0 (–500–0)
|
0 (–500–0)
|
0.761
|
Mechanical ventilation (%)
|
77.2
|
62.8
|
76.2‡
|
84.9‡
|
< 0.001
|
Use of ≥ 3 vasopressors (%)
|
17.5
|
6.6
|
13.9†
|
26.0‡
|
< 0.001
|
ICU type (%)
|
|
|
|
|
0.110
|
Medical ICU
|
60.9
|
58.7
|
61.1
|
61.7
|
|
Surgical ICU
|
18.8
|
21.8
|
17.7
|
18.6
|
|
Cardiopulmonary ICU
|
1.5
|
2.2
|
2.1
|
0.6
|
|
Emergency ICU
|
18.5
|
16.7
|
18.9
|
19.1
|
|
COVID-19 ICU
|
0.2
|
0.6
|
0.3
|
0.0
|
|
SOFA score
|
12.0 ± 3.7
|
9.9 ± 3.4
|
11.8 ± 3.6‡
|
13.1 ± 3.5‡
|
< 0.001
|
APACHE II score
|
26.5 ± 7.8
|
23.1 ± 7.3
|
26.1 ± 7.7‡
|
28.6 ± 7.6‡
|
< 0.001
|
CCI score
|
2 (1–3)
|
2 (1–3)
|
2 (1–3)
|
2 (1–3)
|
0.868
|
ICU, intensive care unit; COVID-19, COrona VIrus Disease-19; SOFA, Sequential Organ Failure Assessment; APACHE, Acute Physiology and Chronic Health Evaluation; CCI, Charlson comorbidity index.
*P < 0.05; †P < 0.01; ‡P < 0.001 compared with the low lactate group.
Relationship between serum lactate and mortality
During a median follow-up period of 10 days (interquartile range, 3–28 days), 1,167 patients (70.3%) died. The mortality incidence was 26.7 deaths per 1,000 person-days. When univariable Cox regression model was applied, several variables, such as age, septic AKI, mechanical ventilation, use of ≥ 3 vasopressors, and scores of SOFA, APACHE II, and CCI were related to all-cause mortality in the present cohort (Table 2), and these were used in subsequent multivariable regression models for the adjustment.
Table 2
Factors related to all-cause mortality
Variables
|
Hazard ratio (95% CI)
|
P
|
Age (per 1 year)
|
1.006 (0.999–1.012)
|
0.099
|
Female (vs. male)
|
0.970 (0.861–1.093)
|
0.970
|
Body weight (per 1 kilogram)
|
0.998 (0.994–1.003)
|
0.431
|
Septic AKI (vs. non-septic AKI)
|
1.337 (1.187–1.506)
|
< 0.001
|
Target dose (per 10 ml/kg/hr)
|
1.020 (0.977–1.065)
|
0.368
|
Blood flow rate (per 10 ml/min)
|
1.013 (0.990–1.037)
|
0.278
|
Target ultrafiltration (per 1 L/day)
|
1.008 (0.932–1.090)
|
0.884
|
Mechanical ventilation (vs. none)
|
1.493 (1.288–1.730)
|
< 0.001
|
≥ 3 vasopressors (vs. < 3)
|
1.593 (1.382–1.837)
|
< 0.001
|
SOFA score (per 1 score)
|
1.132 (1.113–1.151)
|
< 0.001
|
APACHE II score (per 1 score)
|
1.055 (1.047–1.063)
|
< 0.001
|
CCI score (per 1 score)
|
1.014 (1.013–1.066)
|
< 0.001
|
CI, confidence interval; AKI, acute kidney injury; SOFA, Sequential Organ Failure Assessment; APACHE, Acute Physiology and Chronic Health Evaluation; CCI, Charlson Comorbidity Index.
The nonlinear relationship analysis displayed a positive relationship between serum lactates and the mortality risk (Fig. 1). Figure 2 shows Kaplan–Meier survival curves of the 3 lactate groups, and the survival rates among the groups were different (P < 0.001) (Fig. 2). Table 3 shows the ORs of mortality outcomes according to lactate levels. The risk of mortality outcomes increased depending on the increase in lactate levels independent of the effects of multiple variables.
Table 3. Risk of mortality according to lactate levels
Outcomes
|
Lactate levels
|
Unadjusted OR (95% CI)
|
P
|
Adjusted OR (95% CI)*
|
P
|
CRRT mortality
|
Low
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Moderate
|
2.11 (1.596–2.798)
|
< 0.001
|
1.48 (1.099–2.004)
|
0.010
|
|
High
|
7.45 (5.567–10.102)
|
< 0.001
|
4.45 (3.231–6.135)
|
< 0.001
|
ICU mortality
|
Low
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Moderate
|
2.25 (1.707–2.961)
|
< 0.001
|
1.55 (1.154–2.093)
|
0.004
|
|
High
|
7.38 (5.474–9.942)
|
< 0.001
|
4.22 (3.048–5.830)
|
< 0.001
|
In-hospital mortality
|
Low
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Moderate
|
2.29 (1.744–3.009)
|
< 0.001
|
1.64 (1.222–2.207)
|
0.001
|
|
High
|
6.99 (5.137–9.515)
|
< 0.001
|
4.18 (2.985–5.847)
|
< 0.001
|
*Adjusted for age, sex, body weight, septic acute kidney injury, mechanical ventilation, vasopressors, and scores of SOFA, APACHE II, and CCI.
OR, odds ratio; CI, confidence interval; CRRT, continuous renal replacement therapy; ICU, intensive care unit.
Relationship between lactate clearance and mortality
The median value of lactate clearance was 0% (interquartile range, − 33% to + 33%). When a nonlinear relationship was applied, the risk or mortality seemed to be negatively related to the lactate clearance value (Fig. 4). Patients were categorized into two groups: the well (clearance > 0%) and less (clearance ≤ 0%) clearance groups. Figure 5 shows the Kaplan–Meier survival curves of the two groups, and their survival rates were different (P < 0.001). The well clearance group was associated with lower mortality rates than the less clearance group despite the adjustment for multiple variables (Table 4).
Table 4. Risk of mortality according to lactate clearance
Outcomes
|
Lactate clearance
|
Unadjusted OR (95% CI)
|
P
|
Adjusted OR (95% CI)*
|
P
|
CRRT mortality
|
Less clearance
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Well clearance
|
0.43 (0.340–0.551)
|
< 0.001
|
0.41 (0.315–0.533)
|
< 0.001
|
ICU mortality
|
Less clearance
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Well clearance
|
0.44 (0.343–0.560)
|
< 0.001
|
0.42 (0.317–0.544)
|
< 0.001
|
In-hospital mortality
|
Less clearance
|
1 (Reference)
|
|
1 (Reference)
|
|
|
Well clearance
|
0.51 (0.396–0.664)
|
< 0.001
|
0.51 (0.382–0.668)
|
< 0.001
|
*Adjusted for age, sex, body weight, septic acute kidney injury, mechanical ventilation, vasopressors, and scores of SOFA, APACHE II, and CCI.
OR, odds ratio; CI, confidence interval; CRRT, continuous renal replacement therapy; ICU, intensive care unit.