Cumulative effect of repetitive ischemia: pathophysiological findings.

The cumulative effect of ischemia on the brain was investigated in cats using a repetitive transient global ischemia model. The cats were submitted to three series of repetitive ischemia of 5.0, 7.5 and 10.0-minute durations at 1-hour intervals by intrathoracic clamping of the innominate and subclavian arteries. Pathophysiological changes during and after the ischemic episodes were evaluated by monitoring the electroencephalograms (EEG), cerebral blood flow (CBF), specific gravity and 31P-MR spectroscopy (MRS). Transient 5.0, 7.5, and 10.0-minute ischemias appeared to produce a slightly more severe energy failure on the 31P MRS measurement in the animals that had previously experienced an ischemic injury than those that had not. Additionally, repetition of ischemic episodes at 1-hour intervals led to a progressive lengthening of the duration of the spontaneous electrocortical suppression that followed each ischemic episode. However, preischemic hypoxia (5% O2 for 5 minutes) resulted in minor changes in the levels of phosphocreatine and intracellular inorganic phosphate on the MRS measurement, otherwise the EEG activity declined progressively. This shut-down response of the EEG can be concluded to serve in preserving the energy state of the brain although it is not capable of preventing the development of postischemic brain edema and neuronal death.