We Have No Beds: Ventilator Series
Noninvasive Ventilation - Part 2
Written by Dr. Ashley Binder
Remember our 60 yo male with COPD who has been on NIV for about an hour, he was looking a little less sick, let’s check back in with him. (If you missed the last post click here).
Our initial settings for the BiPAP: rate 8, IPAP 10/EPAP5, FiO2 50%.
Your ABG returns. pH 7.21/PaCO2 100/PaO2 120/HCO3 40
You note the acidosis and the elevated PaCO2. Your patient still has acute on chronic hypercapnic respiratory failure. Do you want to change your settings…? The answer is YES!
Ventilation problem? (PaCO2 out of whack?)
If the PaCO2 is too high there is hypoventilation. Too low and there is hyperventilation. We are used to adjusting minute ventilation on a standard ventilator in assist control/volume control mode by changing the tidal volume or respiratory rate. However, BiPAP is more analogous to pressure support mode. Recall, on a standard ventilator in pressure support mode, we can adjust PEEP and pressure support (the pressure above PEEP we want generate).
Similarly, with BiPAP, your settings include IPAP (inspiratory positive airway pressure) and EPAP (expiratory positive airway pressure). The difference between these two pressures is referred to as ΔP. Increasing the ΔP increases the tidal volume and thus the minute ventilation. One significant difference in nomenclature should be addressed here. Whereas ΔP on an intubated patient using pressure support is identical to the level of pressure support (i.e. setting PS to 10cmH2O means ΔP = 10cmH2O, because there is a baseline PEEP), ΔP with BiPAP = IPAP – EPAP due to the manufacturer referring to IPAP with an assumed baseline of 0cmH2O.
We can also change the rate on some machines, but remember, this is only a back up rate, and you will have to set it higher than what the patient is doing on their own to be effective. Additionally, with BiPAP, the patient is required to make some effort in order for significant tidal volumes to be delivered, and the relationship between an increase in ΔP and increased in TV is infrequently linear, so do not be surprised if you make significant changes to IPAP without the expected increased in TV.
Let’s try this with your patient:
The PaCO2 of 100 means he is still hypoventilating, so lets help him out. We need to increase the ΔP to increase his tidal volume. Increasing his IPAP from 10 to 14 will increase his ΔP from 5 to 9. We can leave his rate alone because he is spontaneously breathing at 25 bpm.
Oxygenation issues? (PaO2 ridiculous?)
Look at the PaO2 (goal of at least 60mmHg): is it too high or low? You can also watch the O2 saturation monitor, your goal is typically >93% but with COPD you can have a slightly lower goal, 88-93%. Try to keep saturations <100% to avoid hyperoxemia. To change the oxygenation of a ventilated patient you change the FiO2 and PEEP. How do we do this on the BiPAP?
FiO2 is the same on the BiPAP as it is on a ventilator. PEEP (positive end expiratory pressure) is analogous to the EPAP.
Let’s try this with your patient:
The PaO2 of 120 means there is hyperoxemia, we should avoid this as much as possible. The EPAP is at 5 and any lower than 4 risks rebreathing CO2 so let’s keep it simple and adjust the FiO2 from 50% to 35%.
Our new BiPAP settings are rate 8, IPAP 14/EPAP5, FiO2 35%. 1 hour later you check an ABG: pH 7.36/CO2 70/O2 75/HCO3 38. Success!!
The patient has shown improvement with the setting changes. His ABG is not back to normal but it is finally moving in the right direction. He is now looking more relaxed, and the medical management you have provided with bronchodilators and steroids is kicking in. He is very grateful to you for not intubating him.
Time to call the MICU for admission: the indication is hypercapneic respiratory failure in need of continuous BiPAP.