Modifying ventilator settings based on arterial carbon dioxide (PaCO2) levels

Now that we’ve set our patient on acceptable initial settings, it’s time to determine whether those initial settings provide adequate ventilation and oxygenation.

First, we’ll assess for adequate ventilation (we’ll assess for adequate oxygenation in the next article).

One of the primary ways to assess the appropriateness of the initial settings is to analyze an arterial blood gas (ABG). An ABG test should be routinely ordered after the initial placement of the patient on a ventilator and each time any significant changes to settings have been made, in order to assess whether or not the ventilator settings are appropriate.

Remember, ventilation is the removal of carbon dioxide (CO₂), and we can assess if our settings are providing adequate removal of CO₂ by analyzing the arterial carbon dioxide levels, or P_aCO₂ , on the ABG.

Let’s suppose—for a hypothetical patient on a tidal volume (V_T) of 400 mL and a respiratory rate (RR) of 15 breaths / min (i.e., a minute ventilation of 6 L / minute)—that the ABG results came back and the P_aCO₂ was 70 mmHg. Well, because an acceptable P_aCO₂ is generally anywhere between 35 and 45 mmHg, we would have to conclude that our set minute ventilation is not adequately removing CO₂. Our patient is being hypoventilated or under-ventilated.

So, what would you recommend? Well, the answer should be clear—we’ll need to remove ­­­­­more CO₂. And this increase in ventilation can only be achieved by increasing the amount of minute ventilation—in other words, increasing the tidal volume, respiratory rate, or both.

So let’s say we’ve made the adjustment to increase minute ventilation by increasing the tidal volume to 500, and we ordered another ABG. Now the P_aCO₂ is 30 mmHg. What would you do?

Now, the P_aCO₂ falls below the acceptable P_aCO₂ range (35–45 mmHg), and our patient is being hyperventilated, or over-ventilated. In other words, our settings are causing this patient to remove too much CO₂. So, we should decrease the minute ventilation, which means we should decrease the tidal volume, respiratory rate, or both. This time, we’ll adjust the RR—we’ll reduce it from 15 to 10 breaths / min.

And, after reducing the minute ventilation, the ABG comes back and the P_aCO₂ is 41 mmHg, which is well within our acceptable range for P_aCO₂, meaning our minute ventilation setting of a V_T of 500 mL and RR of 10 breaths / min, produced a desirable ventilation of CO₂ and no further change is necessary.

So now that we know how to assess for adequate ventilation and the controls that would need to be adjusted, let’s turn our attention to assess for adequate oxygenation.