The Science Journal of the American Association for Respiratory Care
Inspired carbon dioxide has often been administered to increase pulmonary vascular resistance and limit pulmonary blood flow in patients with single ventricle physiology where the potential for excessive pulmonary blood flow exists.1 At our institution current practice is to deliver inspired carbon dioxide to patients requiring mechanical ventilation via the Servo 900C. Steady state can be easily achieved by blending CO2 with air/O2 mixture prebellows into the low-flow inlet. Actual inspired CO2 is then continuously measured at the ventilator outlet (pre-humidifier, inspiratory limb). Spontaneous breathing does not alter percent of CO2 delivery. With the addition of the Siemens Servo 300, which does not have a bellows, to our ventilator fleet, we evaluated the effectiveness and accuracy of blending inspired CO2 at the outlet of the ventilator on the inspiratory limb. With spontaneous breathing on the Servo 300, the patient's respiratory rate varies breath to breath, causing fluctuations in the level of inspired CO2. These fluctuations may lead to hemodynamic instability of the patient.
Methods: Bench studies on the Servo 300 were done with and without simulated spontaneous breathing. CO2 of 10 mm Hg was added at the outport of the ventilator. Actual inspired CO2 was monitored continuously (pre-humidifier) on the inspiratory limb with an adequate circuit volume for mixing to occur. The Nellcor 6000 CO2 monitor was used to measure inspired CO2. The FIO2 was set at .21 on the air/O2 blender.
|Mode||Vt||Set RR||Total RR||Pressure Support||Measured CO2|
|PRVC||100 cc||20||20||N/A||10 mm Hg|
|PRVC||100 cc||20||24||N/A||7-9mm Hg|
|PRVC||100 cc||20||48||N/A||4-6mm Hg|
|PRVC||200 cc||20||20||N/A||10 mm Hg|
|PRVC||200 cc||20||24||N/A||4-6mm Hg|
|SIMV||100 cc||20||20||0||10 mm Hg|
|SIMV||100 cc||20||24||10 cm||5-8 mm Hg|
|SIMV||100 cc||20||48||10 cm||2-5 mm Hg|
|SIMV||200 cc||20||20||0||10 mm Hg|
|SIMV||200 cc||20||24||0||3-6mm Hg|
|SIMV||200 cc||20||24||10 cm||2-6mm Hg|
Conclusion: Delivery of inspired carbon dioxide can be safely and effectively delivered through the Servo 300 ventilator for patients without spontaneous breathing. In spontaneously breathing patients, the Servo 900C would be preferred, to prevent the variability of inspired CO2 levels.
1. Jobes, D.L., et al.