A patient on PC-SIMV with no PEEP has the following ventilatory parameters and ABG's: set pressure = 20 cm H2O; Vt = 400 ml; set rate = 8 bpm; spontaneous rate = 25 bpm; spontaneous Vt = 225; FiO2 = 0.4. pH = 7.28; pCO2 = 58 mm Hg; pO2 = 89 mm Hg. THe patient is at IBW of 140 lbs (64 kg). Based on ABG results, how do you interpret these data and what changes are appropriate?
The patient has a respiratory acidosis. In addition, his spontaneous breathing rate is hgh and his spontaneous Vt is low, indicating a high WOB. His Pset is within safe limits. The CO2 can be reduced by increasing the pressure to increase the Vt or by increasing the set rate. Increasing the Pset to 29 cm H2O theoretically would increase his Vt to 860ml (about 14 ml/kg) and reduce the pCO2 to 40 mm Hg. This is a high Vt, although the Pset is still safe. On the other hand, increasing the rate to 12 bpm would also reduce the pCO2 to 40mm Hg. In addition, using PSV to overcome the WOB required to move air through the resistance of the ETT would reduce the patient's spontaneous WOB. PSV could also be used to reduce the PaCO2, instead of changing the mandatory rate or volume.