A means of assessing whether a patient receiving mechanical ventilation is capable of spontaneous, unsupported breathing. A variety of criteria are used. The patient must have (1) objective evidence that the disease requiring mechanical ventilation is improving; (2) a body temperature less than 38.5° C; (3) a hemoglobin level > 8; (4) received no sedative or paralytic drugs during the preceding 24 hours; (5) no evidence of left ventricular failure, cardiac rhythm disturbances, or unstable coronary artery disease; (6) oxygen saturation over 92% while receiving less than 50% inspired oxygen; and (7) no evidence of elevated intracranial pressures.