Many patients present to the ED with hypercarbic respiratory failure (i.e. COPD exacerbation, obesity hypoventilation syndrome etc.). Typically, our first line of treatment is the use of BiPAP, where we set an inspiratory pressure (IPAP) and an expiratory pressure (EPAP). The difference between these two pressures (as well as patient effort) determines the tidal volumes (and consequently minute ventilation) a patient receives in our attempts to help the patient “blow off CO2.”
If you are having trouble with continued hypercarbia despite the use of BiPAP, another NIPPV mode that can be trialed is Average Volume-Assured Pressure Support (AVAPS). With BiPAP the patient receives the same fixed IPAP no matter what, even if their tidal volumes are lower than what is needed. With AVAPS, the ventilator will self-titrate the IPAP and increase (or decrease) the IPAP to reach the tidal volumes that you set, increasing the odds the patient will achieve the minute ventilation you are trying to achieve.
(AVAPS is essentially a non-invasive version of PRVC)
Initial settings (ask your RTs for help!):
The target tidal volume is set to 8 ml/kg of ideal weight
The maximal IPAP value is generally fixed at 20-25 cm H2O
The minimal IPAP value equals to EPAP + 4 cm H2O
The value of the minimal inspiratory pressure is no less than 8 cmH2O and commonly higher
The respiratory rate is set at 2-3 BPM below the resting respiratory rate
