Elapsed time from the onset of stroke symptoms to the point of revascularization is the key determinant of the optimal outcome of acute ischemic stroke. Pharmacological treatment is less effective if the thrombus occluding the artery is big enough and mechanical thrombectomy is required to gain recanalization. Mechanical thrombectomy can be done only in comprehensive stroke centres. There are 5 comprehensive stroke centres in Finland which causes regional inequality when it comes to reaching mechanical thrombectomy in a reasonable time limit.

The aims of the study is to measure the effect of dispatching a helicopter emergency medical services unit on the treatment delays of a stroke patient with large vessel occlusion. The HEMS unit is dispatched to both: getting patient directly to the comprehensive stroke centre as well as to interfacility tranfers.

Inclusion Criteria:

- all adult patients transferred for mechanical thrombectomy from the South Ostrobothnia Hospital District to the Tampere University Hospital

Exclusion Criteria:

• The neurologist on call at the Tampere University hospital deems the patient unable to benefit from mechanical thrombectomy

Either verified large vessel occlusion with computed tomography angiography in Seinäjoki central hospital or positive prehospital stroke severity scale for large vessel occlusion (Finnish Prehospital Stroke Scale: FPSS) and eligibility for mechanical thrombectomy