The segmented thermal clearance probe is a non-invasive instrument which measures heat thermal clearance, a variable directly proportional to superficial blood flow, with a depth sensitivity theoretically proportional to the sensor diameter. We compared an 18 mm sensor and a recently developed 57 mm sensor with the reference technique of xenon washout. 2. The theoretical depth sensitivity of the sensors was assessed using Perspex spacers. Ninety-five per cent of sensitivity had been lost at the respective depths of 3.5 mm and 7.1 mm for the 18 mm and 57 mm sensors. 3. A comparison was made between heat thermal clearance reading and 133Xe decay curves using the two probes for 15 min after injection of 133Xe at 2 mm and 6 mm depths in the anterior thigh in 41 subjects. The 57 mm sensor showed similar correlation with xenon washout at 2 mm injection depth (r = 0.89) and 6 mm injection depth (r = 0.86), whereas the 18 mm sensor showed greater correlation at 2 mm (r = 0.92) than at 6 mm (r = 0.62). 4. The 57 mm sensor was compared with xenon washout at 6 mm in eight insulin-treated diabetic subjects. The relationship was similar to that in non-diabetic subjects (r = 0.79), with no significant difference in slope or intercept. 5. It is concluded that the 57 mm thermal clearance probe may be used to study blood flow at the depth of insulin injection (6 mm) in diabetic and non-diabetic subjects.
