The clamping of the aorta excludes the systemic circulation, by definition, thus causing an ischemia. When a long cross-clamping period (longer than 25 min) or a drop in distal aortic pressure below 50–60 mmHg is anticipated, the use of an intraoperative shunt may prevent complications such as paraplegia.
The constriction of the aorta precludes systemic circulation by definition and results in ischemia. If there's a prolonged cross-clamping period (over 25 minutes) or a drop in distal aortic pressure to less than 50–60 mmHg, using an intraoperative shunt can prevent complications such as paraplegia.