Table 1. Summary of studies identified from a PRISMA-guided search of transcutaneous gas monitoring in free flaps.

Study reference	No. of flaps	Monitoring modality	Value measured	Summary	Flap loss n (%)
Abbreviations: p CO 2 , carbon dioxide partial pressure; p O 2 , oxygen partial pressure. a Free as well as pedicled flaps.
Schiltz et al 4	32	VisiSens	Transcutaneous p O 2 based on tissue flux of O 2 using ratiometric fluorescence imaging	Increased O 2 flux was associated with increased venous congestion and flap failure	2 (6.7)
Trignano et al 5	21 a	Licox Recon	Measures tissue p O 2 and temperature through a Clark-type polarographic micro-catheter positioned in the tip of a flexible probe with an oxygen sensing area	Low p O 2 combined with exam findings were indicative of venous congestion and flap failure	1 (5.5)
Geis et al 6	9	Luminescent lifetime imaging	Transcutaneous p O 2 measurements using dynamic phosphorescence imaging and sensors	Drops in p O 2 below 10 mmHg acutely (within 4 hours) required surgical intervention for flap salvage	0 (0)
Schrey et a. 7	13	Licox Polarographic Probe	Tissue p O 2 was measured using a needle probe in Hartrampf perfusion zone 1 of the flap	Needle probes monitoring p O 2 are reliable to evaluate a single portion of the flap but are inadequate at measuring blood flow throughout the entirety of a flap	1 (7.7)
Hashimoto et al 8	27	Polarographic probes	Transcutaneous p O 2 and p CO 2 were measured using probe contains a Clark-type electrode and a pH-sensitive glass electrode, respectively	Physical exam is a necessary adjunct. p O 2 alone is inadequate to determine venous congestion; a congested flap with a p CO 2 more than 90 mm Hg requires further treatment	1 (3.7)
Kamolz et al 9	60	Licox micropolarographic probe	p O 2 was measured with a microcatheter in some centralized portion of the flap	Failing flaps demonstrate a tissue p O 2 decrease of 10 mm Hg within a half-hour, or a tissue p O 2 drop below 10 mm Hg	2 (3.3)
Total	162				7 (4.3)