Midline catheters for blood gas and acid/base monitoring in critical patients: A prospective observational study.

Background: Arterial lines and central venous catheter (CVC) allow to monitor patients' acid-base status and gas exchange. Their placement and maintenance may however be burdened by severe complications. Midline Catheters (MC) are peripheral venous accesses that are less invasive and easier to insert compared to CVC and arterial lines.

Methods: A prospective observational study was performed including stabilized critical patients with clinical indication to midline positioning before intensive care unit (ICU) discharge. The primary aim was to assess if venous sampling from MCs can be a reliable alternative to CVC for pH and CO monitoring. The secondary aim was to evaluate the correlation between samplings from MC, CVC and arterial line with regards to pH, carbon dioxide tension (pCO), lactates and electrolytes. Three samples from CVC, arterial line and MC were collected simultaneously. Agreement and correlation of the studied parameters between different sampling sites were explored.

Results: 40 patients were included in the analysis. A good agreement for pH and pCO was recorded between MC and CVC: mean differences were 0.001 (95% CI -0.006 to 0.007) and 0.7 (-0.1 to 1.5), percentage error 0.4% and 11.2%, respectively. Correlation between MC and both central venous and arterial samples for pH, pCO, lactates and electrolytes was found to be moderate-to-strong (Pearson's coefficient range 0.59-0.99,  < 0.001 for all these parameters).

Conclusions: In stabilized critical patients, midline catheters represent a reliable alternative to CVC and arterial lines to monitor acid-base disturbances, CO levels and electrolytes. The present findings add to the known advantages of MC, which might be considered a first-line vascular access for non-critical or stabilized patients who do not require infusion of vesicant or irritant drugs.