9.7: 9.7 How much do I change ventilation settings by?

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We usually choose one value to change at a time and then test the ABGs and SpO₂ again to assess the changes. But, how much do we change each setting by to see changes?

Remember the factors affecting alveolar ventilation are: respiratory rate, tidal volume and dead space.

$V_A=(V_T-V_{Dphys})\times RR$

Arterial partial pressure of CO₂ (PaCO₂) is considered the best indicator of ventilation. As we previously learned, there is an inverse relationship between alveolar ventilation and PaCO_2.

As alveolar ventilation increases, PaCO₂ decreases.

For most patients, the goal is to normalize PaCO₂ in the range of $\text{35 - 45 mmHg}$ .

Assuming, there is no change in a patient’s CO₂ production and dead space, and we have full control of ventilation, the following equation can be used to quantify the change in ventilation parameters, to target a specific PaCO₂. This equation is especially useful as we navigate this chapter and practice making adjustments to ventilation.

Example

$\text{Known PaCO}_2=50\text{ mmHg,}$
$\text{Known }V_E=7\text{ Lpm}$

$\text{Desired PaCO}_2=40\text{ mmHg}$

What $V_E$ is required to achieve a PaCO₂ of $40\text{ mmHg}$ ?

Solution

$\begin{align*}\small{\text{Desired }V_E}&=\scriptsize{\frac{\text{Known PaCO}_2\times\text{Known }V_E}{\text{Desired PaCO}_2}}\\\small{\text{Desired }V_E}&=\scriptsize{\frac{50\times 7}{40}}\\\small{\text{Desired }V_E}&=\small{8.75\text{ Lpm}}\end{align*}$

$\text{Known PaCO}_2\times V_E=\text{Desired PaCO}_2\times V_E$

Note that this equation provides us with the value of minute volume required, however, to make ventilation adjustments on the ventilator, we must change either tidal volume $(V_T)$ or respiratory rate $(RR)$ , as most ventilators and modes don’t have a minute volume setting. Which one do we choose: tidal volume or respiratory rate?

Changing Tidal Volume

Keep in mind that under normal circumstances we are targeting a tidal volume of $\text{[math]}$ . If the known $V_T$ is $d650351991a99ca00b4dfe5aeaa150ee.png$ , we may consider increasing the tidal volume as long as plateau pressure does not increase above $30\text{ cmH}_2\text{O}$ .

Changing Respiratory Rate

If tidal volume is already on the high side, at $74c7efffc3dcc1c4ffb3f811881d3467.png$ , and/or plateau pressure is greater than $30\text{ cmH}_2\text{O}$ , perhaps a better choice is to increase the respiratory rate to achieve desired minute volume. Whenever rate is increased, care must be taken to allow patient sufficient expiratory time to avoid air trapping.

Key Takeaway

Strategies to Increase/Decrease $V_T$

In Volume Control Ventilation, set $V_T$ . Consider other factors that might affect $V_T$ depending on ventilator features for volume control (Flow and Inspiratory time may have to be adjusted to achieve desired $V_T$ ).

In Pressure Control Ventilation, target tidal volume can be achieved (increased or decreased) by adjusting $\Delta P$ or inspiratory time.

Strategies to Increase/Decrease $RR$

Change the set $RR$ .

Change Inspiratory time or $\text{I:E}$ ratio.

With practice and experience, ventilation adjustments are often done by using simple logic in clinical practice, often when other factors impact acid-base balance, including CO₂ production and dead space. A good approach to stepwise changes is the following:

Table 9.7.1: Guidelines for Stepwise Changes
Setting	Guideline of each stepwise change	Limits and Cautionary Ranges
$RR$	$\pm 2\text{ bpm}$	$\small{\text{10 - 24 bpm (higher if needed, with caution)}}$
$V_T$	$e996756bf4d36cf4a765eb7a8f889c94.png$	$867bc0e924d686184314591c39ffc837.png$
$\text{FiO}_2$	$\pm 0.1\text{ or }10\%\text{ O}_2$	$\small{\text{Max 1.0 (use caution over 0.5)}}$
$\text{PEEP}$	$\pm\text{1 - 2 cmH}_2\text{O}$	$\small{\text{5 - 12 cmH}_2\text{O (higher if needed, with caution)}}$

Let’s return once more to the example patient.

Example – Patient | 7.31/57/68/24

Vent settings: $RR=\text{16 bpm}$ , $V_T=\text{420 mL}$ , $\text{PEEP}=5$ , $\text{FiO}_2=0.5$

Known information: $\text{IBW}=52\text{ Kg}$ . When calculated using the safe $V_T$ range of $a0c9f04e47afaa9cac0c10ef9afd0a89.png$ equals a safe tidal volume range of $\text{312 - 416 mLs}$ for this patient.

When answering the previous questions, you already decided that you needed to fix the pH by blowing off more CO₂. Though the two options would be increasing the RR or increasing the tidal volume (think of the analogy of the rising water and needing to bail either faster or with a bigger bucket), you determined you cannot increase the $V_T$ since the patient is at the maximum tidal volume of $74c7efffc3dcc1c4ffb3f811881d3467.png$ . Therefore, the only change you can make is increasing the $RR$ to decrease the pCO₂ and correct the pH. A practitioner would most likely choose to increase the $RR\text{ +2 bpm}$ and set it at $18\text{ bpm}$ .

In addition, you already know that your patient has mild hypoxia, and you decided that although you can increase either PEEP or FiO₂ to fix this issue, since we are already at an FiO₂ of $0.5$ , it might be worthwhile to just increase the PEEP as long as the $\text{SpO}_2>92\%$ . A practitioner would most likely increase the PEEP to $7\text{ cmH}_2\text{O}$ . If the SpO₂ is less than $92\%$ , the FiO₂ might be increased to $0.6$ for about 30 minutes and weaned as soon as the PEEP change starts to impact the patient.

You have just learned the typical amounts each setting is adjusted to impact a change on an ABG. Sometimes, ABGs will show mild imbalances, while other times, the issues are quite significant. If the numbers on the ABG are profoundly off, it might be worthwhile to do two steps of changes.

When completing two stages of changes, the same rules still apply—only some changes will be appropriate for your patient. What this approach looks like in practice is changing two settings (if able) or doing two-step changes to the same settings. For example, a significant issue would be if your pH is less than $7.3$ or greater than $7.5$ and your pO₂ is less than $55$ . In these cases, if the $RR$ was the only change you could make, you might consider changing it to $20\text{ bpm}(16+2+2=20\text{ bpm})$ , and you would definitely increase your $\text{FiO}_2\text{ +0.1}$ and consider your $\text{PEEP +2}$ as well. Remember, PEEP is slower to work, and the hope is you would bring the FiO₂ back down as soon as PEEP starts to work.