CO2-reactivity of regional cerebral blood flow as estimated by using 99mTc-HMPAO

The purpose of this study was to directly assess the regional perfusion reserve by CO2 stimuli using 99mTc-HMPAO. Twenty-one investigations were carried out, each of them consisting of two consecutive injections of 300 MBq 99mTc-HMAPO at different end-tidal concentrations of CO2. These differences were achieved by hyperventilation and by rebreathing of expired air and amounted to 2.3 vol% on average. This mean difference in CO2-concentration produced an overall 31% difference in 99mTc-HMPAO uptake in the total of the investigations. The individual increase of uptake depended on the increment in CO2-concentration obtained in the respective patient within a range from 3 to 7 vol%, but not on the level of activity uptake relative to the amount injected. There was no significant difference in the response of activity uptake to the CO2-concentration between different cortical territories. The global response per % CO2 was 9.7±3.9% in patients with carotid stenoses > 75% (N=9), 17.5±8% in patients without extracranial stenoses, but with multivessel coronary disease (N=6), and it exceeded 30% in 2 normal volunteers. 99mTc-HMPAO and SPECT allow to determine a direct parameter of regional perfusion reserve by making use of different arterial CO2-concentrations. Preliminary data suggest that this parameter differs in populations with and without cerebrovascular disease. (orig./MG)