The investigators will perform a prospective observational study to evaluate the triggers for, expected physiological and actual physiological response to fluid bolus therapy in the intensive care unit. Specifically, the investigators will assess which of the the physiological trigger(s) intensive care doctors use when deciding to give a fluid bolus; the expected physiological response to the fluid bolus by intensive care doctors; and, to what degree the patients actual physiological response 1 hour after a fluid bolus correlate with expectations.