脳血行再建術における血行力学的脳虚血の定量的重症度評価

Abstract

Hemodynamic cerebral ischemia can be stratified into Stage I and Stage II (Misery perfusion). According to vasodilatory and metabolic compensation toward reduction of CPP, Stage I ischemia is defined as both preservation of resting CBF and reduction of vascular reserve. Stage II ischemia is defined as reduction of resting CBF, loss of vascular reserve and elevation of oxygen extraction fraction (OEF). The vasodilatory response to acetazolamide, a carbonic anhydrase inhibitor, provides an effective parameter of cerebrovascular reserve. Recently, quantitative measurement of CBF and cerebrovascular reserve (VR): (acetazolamide-activated CBF / resting CBF — 1)× 100% was introduced in clinical practice and hemodynamic cerebral ischemia can be quantitatively stratified into Stage I and Stage II. Both 123I-IMP-microsphere method and 123I-IMP-ARG method indicate acceptable accuracy, and can be commonly used as quantitative brain perfusion SPECT imaging. In 123I-IMP-ARG methods, Stage I ischemia is defined as follows: resting CBF more than 34 ml/100 g/min (this value corresponds to 80% of mean CBF of normal subjects) or VR from 10% to 30%, Stage II ischemia is defined as follows: rCBF less than 34 ml/100 g/min and VR less than 10%. Hemodynamically normal subjects are involved in VR more than 30% (Stage 0).Twenty-four patients with hemodynamic stroke were selected for a recent EC-IC bypass study. In this study, EC-IC Bypass surgery was performed in patients with Stage II and I (nearly II) ischemia using 123I-IMP-ARG methods. In the affected areas, the mean values of resting CBF and VR after surgery (35.0+/−7.2 ml/100 g/min, 32.9+/−15.2%) were significantly improved compared with the values before surgery (29.6+/−5.3 ml, −4.5+/−12.4%). In the unaffected areas, the mean values of resting CBF and VR after surgery (38.8+/−7.9 ml/100 g/min, 40.8+/−15.8%) were not significantly different than those values before surgery. The stage after surgery (Stage 0 in 16, Stage I in 8, Stage II in 0) was statistically improved in comparison with the stage before surgery (Stage 0 in 0, Stage I in 7, Stage II in 17). Stage II hemodynamic cerebral ischemia defined by quantitative brain perfusion SPECT imaging was successfully reversed to Stage I or 0 by EC-IC Bypass surgery. A Japanese EC-IC Bypass trial (JET Study) is being carried out to clarify the surgical benefits for Stage II ischemia, and quantitative measurement of CBF and VR has been recognized to be essential for the inclusion criteria for this trial.99mTc-HMPAO or 99mTc-ECD-Patlak plot method could be another option for quantitative brain perfusion SPECT imaging. However, these methods can indicate acceptable values of resting CBF but could not demonstrate accurate values of VR activated by acetazolamide challenge. First-pass extraction of these radiotracers is generally lower in the relative high-flow range. Therefore the quantitative CBF value tends to be underestimated in the area with normally vasodilatory response under acetazolamide-activated condition. Linearization corrections should be modified in these methods, especially using acetazolamide activation.Hemodynamic cerebral ischemia can be stratified into Stage I and Stage II (Misery perfusion) and the stratification may be important to determine future risk of stroke. The characteristics and kinetics of brain perfusion radiotracers should be considered in quantitative stratification of hemodynamic cerebral ischemia using brain perfusion SPECT.