<prompt> Generally accepted indications for mechanical ventilatory support includePaO2 of less than 70 kPa and PaCO2 of greater than 50 kPa while breathing room airAlveolar-arterial oxygen tension difference of 150 kPa while breathing 100 O2Vital capacity of 40-60 mLkgRespiratory rate greater than 35 breathsmin  <response>: Anticipation and early aggressive treatment of pulmonary insufficiency by mechanical ventilatory support are critical in managing the seriously ill patient. Readily measured changes that can be used to determine either the need for intubation or the appropriate time for weaning from mechanical respiratory support include arterial blood gas levels, dead space-tidal volume ratio VD VT , alveolar-arterial oxygen tension difference , vital capacity, and respiratory rate. Indications for mechanical ventilation include a respiratory rate over 35 breathsmin, vital capacity less than 15 mLkg, greater than 350 kPa after 15 min on 100 oxygen, VD VT greater than 0.6, PaO2 less than 60 kPa, and PaCO2 greater than 60 kPa. 