A22-year-old man is hypotensive and tachycardic after a shotgun wound to the left shoulder. his blood pressure is initially 80/40 mm hg. after initial fluid resuscitation his blood pressure increases to 122/84 mm hg. his heart rate is now 100 beats per minute and his respiratory rate is 28 breaths per minute. a tube thoracostomy is performed for decreased left chest breath sounds with the return of a small amount of blood and no air leak. after chest tube insertion, the most appropriate next step is: