The terms ventilation (the exchange of pCO2) and oxygenation (pO2) are probably very familiar to you. Remember, pCO2 and pO2 are values in an ABG reading. Let’s relate these concepts to settings on the ventilator and learn which specific setting affects ventilation or oxygenation.

Ventilation refers specifically to the movement of air in and out of the lungs. In terms of blood gas effects, ventilation directly refers to the removal of CO2. Therefore, what ventilator settings would affect this? What settings directly impact the amount of air going in and out of the lungs? If you guessed the tidal volume, you are correct! The size of breath will directly impact the amount of air going in and CO2 coming out of the lungs.

There is another setting that will impact the amount of CO2 clearance. What do the chemoreceptors in the brain trigger if the CO2 levels start to rise? If you answered the respiratory rate, then you are remembering correctly! The respiratory rate also has a direct impact on the amount of CO2 leaving the lungs over time. If you breathe faster, you are getting rid of CO2 more often, and this will help drive CO2 levels down.

What about oxygenation? Which ventilator settings directly impact the patient’s oxygenation status? This concept should not be new to you. We have talked extensively about FiO2 and PEEP as working together to deliver oxygen into the body. The FiO2 can be increased to deliver higher amounts of oxygen to the lungs, while PEEP expands the lung units and increases the surface area of the lung to the capillary vasculature of the lung. Please note that this is a slight simplification of these concepts. There is some crossover between the ventilator settings that affect oxygenation and ventilation. However, for beginning learners it is better to keep oxygenation and ventilation and the settings that affect them separate in their minds without crossover. With practice they will be able to see the big picture and even make predictions on what order to use ventilation settings.  (See Figure 4^[1])

Take a look at the following table, which summarizes the mechanical ventilation settings that affect oxygen versus ventilation. Keep in mind that VT can also be affected by other settings on a ventilator.

Oxygenation Issues: FiO2 or Positive End Expiratory Pressure (PEEP)

When it comes to changing either FiO2 or PEEP, you need to think about the impacts of either of these values on the body. Remember, high levels of oxygen can cause lung damage, and we are always targeting the lowest FiO2 to maintain SpO2 > 92% and a pO2 at normal ranges (80 – 100 mmHg). FiO2s of higher than 50% can lead to oxygen damage to the lungs.

For PEEP, a normal starting range is 4-6 cmH2O —you will rarely decrease the PEEP below that number. Conversely, increasing PEEP too high can start to negatively impact the body. PEEP can negatively impact the compliance of the lungs—just like balloons that are already inflated with pressure could lose elasticity and not be able to inflate as easily. High PEEPs will also increase the pressure in the alveoli that, when added to the additional volume or pressure applied with every breath, could increase your patient beyond safe pressure levels and put the lungs at risk of barotrauma. We need to maintain pressures of less than 35cmH2O —and, ideally, the lower, the better.

PEEP also increases the pressure in the thorax (chest) of the patient. Other than the lungs, this cavity also houses the heart and important vessels like the aorta and vena cava. Increased intrathoracic pressure will increase the pressure on these vessels as well. This pressure could squeeze the heart, decreasing the blood flow back into the heart and the pumping effectiveness of the heart. The medical terms for these situations are venous return and ejection fraction of the heart. Both conditions will show an impact on the blood pressure of the patient.

Ventilation Issues:  Respiratory Rate (RR) or Tidal Volume (VT)

If the pH is abnormal and you are going to try to normalize it by increasing or decreasing the pCO2 level, you know VT and RR are both options to impact the amount of CO2 being exhaled every breath—but which change is most correct?

The answer depends on your current your ventilator settings. Remember the safe ranges for respiratory rate – 10-16 breaths per minute.

Now think about your tidal volume ranges. This parameter is much more definitive with its allowances. The safe tidal volume ranges for medical providers with basic ventilator knowledge is 6 – 8 mL/Kg. Using the knowledge you have of ideal body weight (IBW) and calculated safe tidal volume ranges, you would compare the tidal volume your patient is getting to your calculated ranges. Do you have room to move to correct the problem? If you do, this would be an option to take. However, if you are at either limit, then changing the tidal volume would not be an option to correct the problem.

If you are at the low ends of your safe ranges for both, then either RR or VT can be adjusted. If you are at the higher ends of either one, then use the other setting instead. If you are at the high end of both RR and VT, remember that the VT is typically a hard limit, while RR can still be adjusted carefully. If you are already at the low ends of VT and RR and you need to move your patient even lower, consider a different mode of ventilation or weaning the patient (Sault College, 2022).

Equations to utilize when changing ventilator parameters

Making Changes in FIO2

[latex]\text{Desired Ve} = \frac{\text{Ve (current)} \cdot \text{PaCO}_{2} \text{(Current)}}{\text{PaCO}_{2} \text{(desired)}}[/latex]

Remember VA and PaCO2 are inversely proportional.

Ventilator-Associated Events (VAE)

Studies suggest that most VAEs are caused by pneumonia, fluid overload, ARDS, and atelectasis. VAEs are associated with a doubling of the risk of death compared to patients without VAEs and compared to patients who meet traditional VAP criteria.

Risk factors for VAEs include sedation with benzodiazepines or propofol, volume overload, high tidal-volume ventilation, high inspiratory driving pressures, blood transfusions and patient transport.

Potential strategies to prevent VAEs include minimizing sedation, paired daily spontaneous awakening and breathing trials, early mobility, conservative fluid management, conservative transfusion thresholds, oral care with chlorhexidine, stress ulcer prophylaxis, and low tidal-volume ventilation.