Man revived after Sydney car crash

Dominica Sanda
AAP

A man who suffered a cardiac arrest behind the wheel and crashed into oncoming peak-hour Sydney traffic has been brought back to life thanks to the quick action of bystanders.

The 45-year-old lost consciousness after suffering a heart attack and went into cardiac arrest while driving in Baulkham Hills on Wednesday morning, a NSW Ambulance spokesman told AAP.

After crashing into oncoming traffic, a group of bystanders, including an orthopaedic surgeon, rushed to the man's aid.

Ambulance chief inspector Brian Parsell said their prompt action helped paramedics to successfully revive the man once they arrived on the scene.

"This patient was clinically dead - his heart had stopped and he was not breathing when we arrived on scene," Insp Parsell said in a statement.

"While en route to hospital the patient became increasingly responsive and started to wake up, which was absolutely terrific."

Out-of-hospital heart attacks have a mortality rate of more than 90 per cent but the combination of CPR and defibrillation meant the patient was able to be revived, Insp Parsell said.

"The signs are certainly very promising for this patient," he said.

The man remains in a critical condition at Westmead Hospital.