MECHANICAL CONDITIONS OF REBOUND OF THE INTRACRANIAL-PRESSURE AFTER APERIOD OF CEREBRAL COMPRESSION

Brain compression of precisely defined degree and duration was produce d by means of expansion of a supratentorial epidural balloon in anaest hetized and mechanically ventilated dogs. After deflation of the ballo on there was a rebound of the intracranial pressure (ICP) to values in the range 30-89 mm Hg when critical thresholds of time and compressio n had been exceeded during the period of compression. A time versus ce rebral perfusion pressure (CPP) graph indicated a CPP threshold of abo ut 50 mm Hg and time threshold of about 6 min. Within these limits the critical CPP varied as a function of time. The ICP rebound had an app roximately exponential time course with an initial rapid rise levellin g off towards a final plateau in about 30 min. The final value was dep endent on the time of compression but independent of the CPP. A transt entorialpressure gradient was induced during the compression phase and reestablished after the decompression when a rebound of ICP developed . Hydrostatic compression by means of infusion of fluid into the subar achnoid space was followed by a rebound of ICP in only 2/21 of the exp eriments in spite of time and CPP parameters which crossed the critica l thresholds defined in the balloon compression experiments. These res ults support the hypothesis that the rebound phenomenon is related to an ischaemic mechanism. The difference in incidence of rebound in ball oon compression and hydrostatic compression can probably be accounted for by a greater depth of ischaemia in the former case because of a di rect compressive effect on the vascular bed by the balloon.