Cortical thermal clearance as a predictor of imminent neurological deterioration

Cerebral blood flow (CBF) is vital for the perfusion of brain tissue. It is frequently deranged in acute neurosurgical disorders, particularly subarachnoid haemorrhage and head injury. Despite its importance, in clinical practice the routine measurement of CBF is uncommon, as changes in CBF can occur abruptly. However, a method of CBF monitoring may be potentially useful, particularly if warning could be obtained of impending ischaemia before neurological deterioration. Measurement of tissue thermal clearance has been used as an estimate of local tissue blood flow since 1933. Its history is full of controversy, mostly centred around quantification. The ability of perfused tissues to clear heat is, as a first approximation, the sum of two components: a fixed component related to the constituents of the tissue, primarily the water content, and a variable convective component, related to the local blood flow. The mathematical relationship between flow and the observed increment in thermal clearance is still debatable. Here, the history of thermal clearance is reviewed, and the results of our work with a relatively simple device are described. It consisted of an implantable probe, designed to measure the thermal clearance of the cortical surface in arbitrary clearance units (CU), ranging from 27 CU (cadaveric) to 69 CU (well perfused brain). Pre- and postoperative studies showed that the system was capable of following changes in blood flow rapidly. The cortical thermal clearance (CTC) was monitored postoperatively in 24 patients after aneurysm surgery. Most remained clinically stable and had thermal clearances over 50 CU. In others, however, it was seen that a low-or falling-thermal clearance was associated with development of a neurological deficit. Analysis using receiver operating characteristics curves established that the method had a sensitivity of 0.86 and a specificity of 0.82 in the detection of a contralateral ischaemic motor deficit. No patient in whom the CTC remained above 50 CU ever developed a new neurological deficit, whereas all patients with a CTC below 35 did. The evidence-historical, mathematical, practical, and theoretical-that CTC is closely related to local blood flow is discussed. Changes in thermal clearance have been observed prior to the development of ischaemic neurological deterioration. Detection of imminent ischaemia may become increasingly important as means of improving cortical blood flow become more widely available. Whether such early detection- and subsequent treatment-of ischaemia will result in better patient outcome remains to be established. I believe it will.