In a 5-4 case, the United States Supreme Court ruled that a warrant is not generally needed to draw blood from an unconscious person suspected of DUI. In the case of Mitchell v. Wisconsin, the Court's Opinion stated that police are almost always allowed to order a blood draw from an unconscious driver without a warrant when they have probable case to believe the person was driving drunk.
A breathalyzer does not actually measure blood alcohol content or concentration, which would require a blood sample. There are big differences between the small handheld breathalyzers used in the field, and the larger devices at police stations. Small breathalyzers may justify an arrest, but may not be admissible evidence in court. So how does it work?