Continuous tissue CO₂ monitoring for early microcirculatory assessment across a broad ICU population : a descriptive and prognostic study

Microcirculatory dysfunction is common in critical care, but has been primarily described in severely ill patients. While its prognostic significance is well established in septic shock, it might be associated with poor outcome in other critical conditions. The tissue-to-arterial CO₂ gradient (PtaCO₂) provides a straightforward, non-invasive and dynamic approach to microcirculatory assessment, but its relevance beyond acute circulatory failure remains unclear. We aimed to determine whether the evolution of PtaCO₂ over the first 48 h predicts 28-day mortality in a broad intensive care unit (ICU) population. This prospective, descriptive study included 94 adult patients between November 2022 and September 2023, within 24 h of admission to a mixed surgical ICU, regardless of admission diagnosis. PtaCO₂ was measured using a transcutaneous CO₂ sensor and arterial blood gas analysis at regular intervals during the first 48 h. The primary endpoint was 28-day mortality. Mean age was 61 ± 16 years, mean SOFA score at 24 h was 5.7 ± 4. Admission diagnoses included acute brain injury (42%), sepsis/septic shock (33%), non-septic shock (16%), and acute respiratory failure (9%). Admission PtaCO₂ did not differ between survivors and non survivors (15.8 [10.5–21.3] vs. 17.9 [14.4–20.4] mmHg, p = 0.31). In contrast, its evolution between day 1 and day 2 (∆ (d2−d1) PtaCO₂) was associated with mortality, independently from macrohemodynamics. Microcirculatory dysfunction improved in survivors (∆ (d2−d1) PtaCO₂: -3.4 [-8 - -0.6] mmHg), whereas it worsened in nonsurvivors (+ 0.4 [-3.4-6.6] mmHg, p < 0.05), with higher PtaCO₂ on day 2 in nonsurvivors (16.5 [11.7–20.9] vs. 11.9 [8.8–16] mmHg, p < 0.05). Based on these findings, we designed a simple and exploratory prognostic score—SkinScore—based on the average 48-hour and day 2 PtaCO₂. SkinScore effectively stratified mortality risk and independently predicted death (AUC 0.90 [0.82–0.96]). Integrating Skinscore into SOFA significantly improved prognostic performance (AUC 0.90 [0.80–0.97] vs. 0.81 [0.68–0.91], p < 0.05). Early and dynamic microcirculatory monitoring via tissue capnometry predicts outcome in a diverse and moderately ill ICU population. Our findings indicate that many ICU patients may exhibit microperfusion abnormalities associated with prognosis, supporting broader use of this non-invasive, continuous method as a microcirculatory marker.