Measurements of cerebral blood flow in the pig by the 133Xe clearance technique. Interpretation of the initial part of clearance curves.

The 133Xe clearance technique is used to measure cerebral blood flow in the pig. Previous publications have shown that the two-compartmental model is inadequate for interpreting the experimental data. The present study deals with certain phenomena seen during the initial part of the clearance curves, which can lead to correct estimation of the blood flow rate. Two types of initial peaks can be distinguished: one is a very fast type, which is interpreted as being due to transit of the tracer via large vessels and is referred to in the literature as the 'arterial peak'; the second is a slower 'tissue peak,' seen in animal experiments at high CO2 levels and in clinical patients with cerebrovascular instability. In animal preparations discrepancies were found between flow measurements according to the 133Xe clearance technique and those made with the aid of an electromagnetic flow probe situated around the carotid artery. A hysteresis effect was also found in the relation between respired CO2 level and cerebral blood flow. All these findings can be interpreted in terms of arteriovenous shunting, i.e., non-nutrient flow of blood, which is not necessarily due to anatomical anastomosis but can also be explained by the assumption that the clearance is partly limited by diffusion. The possibility of diffusion-limited clearance can have important clinical implications. Further research may provide a better basis for analysis of clearance curves to detect cerebral perfusion deficiencies that give rise to false flow calculations in present techniques.