Study design and participants
This study was a single-center prospective observational study, which has been registered on the UMIN Clinical Trials Registry (registry number: UMIN 000045966). Adult patients (> 18 years old), who were diagnosed with sepsis and orally intubated in the ICU of the University of Tokyo Hospital between September 2019 and March 2020, were included. Sepsis was diagnosed according to Sepsis-3 [17]. We excluded the following patients because of the inaccuracy of IC measurement: patients complicated with pneumothorax, those treated with extracorporeal membrane oxygenation (ECMO) therapy, those ventilated with more than 85% fraction of inspired oxygen (FIO2), those with changes in ventilator settings including FIO2 during IC measurement, those intubated from nasal or tracheostomy, and those isolated for high risk of airborne infection. Moreover, we excluded patients who declared do not attempt resuscitation and those without informed consent. During the study period, 66 patients with sepsis were mechanically ventilated in our ICU, and 34 patients were finally enrolled in this study (Fig. 1). IC measurement was initiated within 24 h after oral tracheal intubation. VCO2 and VO2 values were measured for 2 h continuously.
This study was conducted according to the amended Declaration of Helsinki, and the Institutional Review Board of the University of Tokyo approved this study (2018094NI). Informed consent was obtained from all participants or their legal representatives.
Indirect calorimetry
For IC, CCM Express (MGC Diagnostics, Saint Paul, Minnesota) [18] was used. Warm-up and calibration were conducted according to the specifications of the manufacturer. IC measures the difference between inspiratory and expiratory VCO2 and VO2 using the breath-by-breath analysis method. It uses a pneumotach flowmeter connected near the endotracheal tube. Inspiratory and expiratory gases were collected through a sampling line connected to this flowmeter. VCO2 is measured using an infrared analyzer, while VO2 is measured using a galvanic fuel cell. Patient ventilation is measured at the endotracheal tube. Therefore, considering any bias flow provided by the ventilator is not needed [19, 20].
Data collection
The following patient characteristics and clinical data were collected from the medical records: age, sex, past medical history, height, weight, catecholamine use, continuous renal replacement therapy use, induction medication, sedation, Richmond Agitation-Sedation Scale (RASS), source of infection, thiamine administration, and positive results of the culture. Blood gas analysis, including blood lactate levels, was performed during intubation and after IC measurement. Furthermore, ventilator settings and vital signs during IC measurement were obtained. The Acute Physiology and Chronic Health Evaluation (APACHE) II score, Sequential Organ Failure Assessment (SOFA) score, and catecholamine index were calculated.
VCO2, VO2, and respiratory quotient (RQ) were measured using IC. Before the analysis, VCO2, VO2, and RQ data obtained using IC were modified to minimize the possible artifacts in the following process (Additional file 1: Fig. S1). First, the values considered out of the physiological range (VCO2 < 70 mL/min or > 800 mL/min; VO2 < 100 mL/min or > 1000 mL/min; and RQ < 0.67 or > 1.3) were excluded [21,22,23]. Second, the average values every 5 min (24 points for 2 h) in each of VCO2, VO2, and RQ were obtained. The outlier values outside the mean ± 2 standard deviation of 24 points and points before and after the outlier values were excluded. Finally, a linear regression line was obtained from the remaining points, and each slope was defined as VCO2, VO2 and RQ slopes.
For temporal changes in lactate levels, the percentage of change was measured hourly (%/hr); the percentage of changes in blood lactate level was obtained by dividing the hours from two time points (during intubation and the end of IC measurement).
Outcomes
The primary outcome was 28-day survival, and its association with VCO2 and VO2 slopes was evaluated. We further evaluated the additional information provided by VCO2 and VO2 slopes along with lactate temporal changes.
Statistical analysis
Continuous variables were presented as median (interquartile range), and categorical variables were presented as percentages. Categorical data were compared using the chi-square test or Fisher’s exact test as appropriate. Multivariate logistic regression analysis was performed to examine the associations of VCO2 and VO2 slopes with 28-day mortality adjusted from the predefined confounding factors of APACHE II score and lactate temporal changes. Predictive performance of each parameter for 28-day mortality was evaluated by receiver operating characteristic (ROC) analysis, and the cutoff values were determined with Youden’s index. All statistical analyses were performed using JMP Pro (version 15.1.0; SAS Institute Inc., Cary, NC, US). Two-tailed p values of less than 0.05 were used to denote statistical significance for all tests.