The collision was reported at 11:28 a.m. on northbound I-5 near 116th Street NE, said trooper Mark Francis of the Washington State Patrol.
The man was driving a 2000 Subaru Legacy in the far right lane when the car swerved over the shoulder and across the on-ramp, Francis said. The Subaru then struck a tree head-on.
The man's injuries were described as life-threatening. Three dogs in the car also were injured and were taken to an animal hospital.
Investigators believe a medical problem caused the crash, according to a State Patrol memo.
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