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4 min read

Mentally Predict ABG Results

Mentally Predict ABG Results
What’s a kiloPascal, anywho…

It is a sophisticated display when the seasoned intensivist or veteran RT predicts the next arterial blood gas results after conjuring ventilator settings based on current ABG results.

Although it may appear as magic to some and confusing to others, it is actually easy math.

Oye, math…

Learn this post for your Valentine 💘:

Recall Cardiac Output = heart rate x stroke volume in 🫀cardiac hemodynamics?

Well, pulmonary mechanics formulas draw a parallel...

  • Minute ventilation (V̇E) is analogous to cardiac output (CO).

  • Heart rate (HR) is analagous to respiratory rate (RR).

  • Stroke volume (SV) is analagous to tidal volume (Vt).

So now we have:

🫁
V̇E = RR x Vt

This is the alveolar ventilation equation.

The key here is to calculate and adjust for the parameters of minute ventilation. Minute ventilation is termed V̇ (Or spoken as “V dot”). Sometimes this is labeled as V̇E. The goal here is to achieve the desired arterial CO2 (paCO2), and thus, correct the deranged acid-base situation.

First, understand the relationship between dissolved blood CO₂ and ventilation which is

PCO2 ∝ 1/V̇E
or in English, the partial pressure of arterial CO2 is inversely proportional to minute ventilation

In plain speak, that means the lower the respiratory rate or the tidal volume (products of the minute ventilation), then there is more arterial CO2 that will not leave the blood.

🤔 So, we want to perform magic remember?

You will need to calculate for V̇E using this simple equation:

🫁
V̇Enew = V̇Ecurrent x (PaCO2current / PaCO2desired)
This can be applied to new ventilator parameters: RRnew = RRcurrent x (PaCO2current / PaCO2desired) Vtnew = Vtcurrent x (PaCO2current / PaCO2desired)

Your 3am Case 🕵️‍♂️

59 year old female w/stage 3 COPD pt on baseline supplemental 4L of home O2. Currently intubated (emergently in ER) for acute on chronic hypoxic hypercapnic respiratory failure after failing BIPAP. Influenza results pending. Concern about shift on CXR.

Drips: On propofol drip at 35 micrograms/kg/min.

Current mechanical ventilator settings: Assist control / volume control (ACVC) 18 bpm RR/350cc Vt/50% FIO2/5 mmHg PEEP. Only breathing 2breaths over set rate.

Current ABG pH 7.225 / paCO2 89.6 mmHg / paO2 158 mmHg

Calculate the current minute ventilation.

V̇Ecurrent = RRcurrent x Vtcurrent

V̇Ecurrent = 20 x 350. I placed 20 instead of 18 because she is breathing 2 breaths over set rate of 18. That is a total RR of 20. V̇Ecurrent = 7.0 LPM.

Note: That is fairly a low minute ventilation for an acute on chronic respiratory acidosis derangement.

What do you notice about the set PEEP?

It is very low for a chronically hypercapnic pt. Now in some cases this will be okay. Best thing to do is to check/order the intrinsic PEEP (or autoPEEP) maneuver on the ventilator with discussion with the Respiratory Therapist.

Think dynamic hyperinflation.

Literature recommends setting external PEEP to approximately 70-80% of the measured intrinsic PEEP. This essentially ties into the waterfall theory of Dr Lodato’s.

Benefits of matching intrinsic PEEP include

  • Reduction of inspiratory effort, thus improved pt comfort
  • Improve ventilator trigger
  • Decrease pt-ventilator dyssynchronies
  • Improving ventilator gas exchange
❗
Remember desired or “target” PaCO2 is usually ~40mmHg if normal ventilating pt. In chronic hypercapnic pts, the target goal should be higher: sometimes 50-55 mmHg.

Calculate the new minute ventilation to achieve the desired paCO2.

V̇Enew = V̇Ecurrent x (PaCO2current / PaCO2desired)

I’m going to pick a target goal of 60 mmHg for the desired PaCO2 as the pt is chronic hypercapnic (CO2 retainer).

So, V̇Enew = 7.0LPM x (89.6 / 60) 👉 10.45 LPM

Calculate the new respiratory rate.

Apply to the ventilator parameters for either RR or Vt. One method is to divide the minute ventilation by the set Vt.

So that would be 10.45/350 = RR of 29.8.

So look at that magic. The new rate is 29 breaths /min!

Doctor’s order

The goal would be to raise the RR to 29 bpm for a desired or target PaCO2 of 60mmHg. This would raise the V̇E to 10.45 LPM. 👉Remember to check ABG in 1 hour of changes and always keep close eye on the patient and ventilator.

There it is. A very simple equation that appears as a sophisticated and cerebral, yet if done properly, it can expedite pt care and potentially lead to more rapid liberation from ventilation. Or…

One more question while we are on this:

Name 3 pulmonary physiology considerations when adjusting minute ventilation on the ventilator

  1. Rapid correction of respiratory acidosis can lead to post-hypercapnic respiratory alkalosis. This occurs usually in chronic CO2 retainers (i.e., chronic hypoxic hypercapnic respiratory failure which could be due to advanced COPD).
  2. Use caution with high respiratory rates. This can cause auto-PEEP phenomena in some pts especially those with obstructive lung diseases. If unattended, these can lead to high Ppeak pressure alarms 🚨, pt-ventilator dyssynchrony, and eventually pneumothorax 🩻
  3. Breath stacking can occur with high minute ventilation from dead space ventilation. This can cause further hypoxia and ventilator alarms 🚨. One solution could be disconnecting pt from ventilator circuit and manually insufflating with a bag-valve mask.

References

Jubran A. Setting positive end-expiratory pressure in the severely obstructive patient. Current Opinion in Critical Care. 2024;30(1):89-96. https://doi.org/10.1097/mcc.0000000000001131

Tobin & Lodato. PEEP, autoPEEP, and Waterfalls. CHEST. 1989. https://doi.org/10.1378/chest.96.3.449

Mithoefer JC, Bossman OG, Thibeault DW, Mead GD. The clinical estimation of alveolar ventilation. Am Rev Respir Dis. 1968;98(5):868-71.

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