Comparing midline versus central line venous blood gas oxygen saturations: a novel pilot study

Introduction: Midlines (ML) are long peripheral intravenous catheters inserted above the antecubital space via the basilic, cephalic, or brachial veins. These catheters provide safe and comfortable mid-term vascular access for critically-ill patients. Central venous oxygen saturations (ScvO2) obtained from central venous catheters (CVCs) are often used to approximate cardiac oxygen delivery. Given the proximity of MLs to the axillary vein, we aimed to compare the equivalence of midline oxygen (MO2) saturation and ScvO2. If correlated, MO2 could provide additional physiologic information, while decreasing the need for invasive CVCs. Methods: A prospective observational study in adult patients with a CVC in the internal jugular or subclavian veins, who were planned to have a ML placed or with one already in situ. Venous oxygen saturations were compared between both catheters using a point-of-care blood gas analysis on iSTAT (Abbott ©) machines and analyzed within a 15-minute period. Results: A total of n= 24 patients were enrolled. The relationship between ScvO2 and MO2 was evaluated by the Wilcoxon Signed Rank Test and Bland-Altman Plot Analysis. Subjects were 67% male and 62.5% Black. The mean age was 57.9 years with an average SOFA score of 8.3 and APACHE II of 19.1. Patients were most commonly admitted for acute respiratory failure (45.8%) and septic shock (25%). The overall mean ScvO2 and MO2 were 66.2% ± 10.6% and 60% ± 16.6% respectively (p = 0.003), with a mean bias of 6.25% ± 9.59%. A Bland-Altman analysis demonstrated that more than 50% of ScvO2 and MO2 had a discordance of at least 5%; a clinically relevant difference. In a subgroup analysis, samples comparing right sided MLs to CVCs (n=13) revealed a mean ScvO2 of 66.2% ± 10.3% and mean MO2 of 62.1% ± 16.15% (p=0.17). Conclusions: MO2 as a potential correlate of ScvO2 is an intriguing concept and its clinical utility remains to be seen. Our preliminary findings showed an approximate aggregate 6.2% difference in mean MO2 vs ScvO2, however with significant divergence in >50% of subjects. A right sided ML is an interesting variable as the anatomic location may affect the relationship to the ScvO2, in addition to concurrent vasopressor use and various shock states. Further investigation is actively ongoing.