A69-year-old female was involved in a motor vehicle crash. she is semiconscious with a blood pressure of 80/50 mm hg and a heart rate of 74 beats/min that is weak. her daughter, who was uninjured in the crash, tells you that her mother has a history of hypertension and takes beta-blockers. considering the fact that this patient is probably in shock, what is the most likely explanation for the absence of tachycardia?