H2 clearance measurement of blood flow: a review of technique and polarographic principles.

H2 clearance is a powerful method for monitoring blood flow. Simple and inexpensive to implement, the method allows multiple in situ determinations of blood flow from any tissue in which a small electrode can be implanted. There is, however, evidence to suggest that H2 clearance is neither as accurate nor as local a measure of blood flow as generally supposed. Both in theory and practice, it probably cannot accurately determine blood flow rates greater than 100 ml/100 gm/min or localize blood flow to tissue volumes of less than 5 ml. Moreover, its experimental application is complicated by many technical problems hitherto largely ignored by workers in the field. Some of these problems arise from the limitations of the steady state polarographic technique used to measure tissue H2 concentrations. Other problems stem from the failure to consider possible sources of error in H2 clearance monitoring; these induce interference with the H2 signal by spurious electrode and tissue currents, and contributions from tissue ascorbate and O2. Nevertheless, with the appropriate safeguards and qualifications, H2 clearance is a valid and important approach to measuring blood