In-vitro and in-vivo comparative analysis of four tissue pH monitoring systems

The purpose of this investigation was to identify the type of pH-reference electrode combination that is the most suitable and reliable in clinical applications involving long-term postoperative monitoring of microvascular reconstructive transplants and diagnosis of compartment syndrome. Four types of pH-sensing devices were chosen for the study: a standard glass pH electrode, a polymer-based pH electrode, an ISFET pH sensor, and a fiberoptic pH sensor. Various combinations of electrodes were tested in vitro for typically four days. The glass and polymer electrodes maintained stable pH readings, averaging drifts of 0.14 +/- 0.07 and 0.14 +/- 0.08 pH units per 96 hours, respectively. The fiberoptic sensors displayed an average drift of 0.20 +/- 0.15 pH units per 96 hours. ISFET sensors displayed nearly linear drifts, averaging 1.36 +/- 0.54 pH units per 96 hours. When placed in healthy animal tissue, glass and polymer electrode pH readings followed the arterial blood pH values, measured by a blood gas analyzer. In compromised tissue, both glass and polymer electrodes recorded falling pH levels correctly, indicating ischemic conditions. Generally, ISFET sensors in healthy tissue produced pH readings that did not correlate well with arterial blood pH values. Fiberoptic sensors monitored healthy-tissue pH correctly; however, mechanical disturbances of the fiberoptic sensor and occasional discontinuation of computer operation would shift the pH output significantly (at times by 0.2 pH units), resulting in subsequent inaccurate pH readings. The glass electrode was the only sensor evaluated clinically. It correctly indicated tissue viability in all clinical cases, involving both healthy and ischemic tissue. The authors conclude that the glass pH electrode has the best combination of characteristics for clinical tissue pH measurements.