9.3 What can I fix with ventilation?
Let’s review what we have discussed in previous chapters. We know that with mechanical ventilation, we often have to intubate patients who are having difficulty regulating their CO2 and O2 levels. Patients can be intubated due to ventilatory failure (increasing CO2, hypercarbia) or due to hypoxia (decreasing oxygen levels). We have discussed how changing CO2 levels in the body can affect the overall pH of the body, which we can measure using an ABG. Often, the decision to intubate a patient is directly due to the ABG results that are being seen.
In an ABG, we know that we have values for pH, pCO2, pO2 and HCO3. Which values can be affected by ventilation and which ones cannot?
Apply Your Learning
Think about how the body normally tries to fix acid-base imbalances. Which of the compensation mechanisms is related to breathing? Which value is not related to breathing? If you have difficulty answering, please refer to Chapter 8. Then, read on for the answer…
You guessed it! CO2 is directly affected by ventilation, while HCO3 cannot be changed by ventilation and must be regulated by the kidneys. Therefore, the pH can be affected in ventilation only by affecting the CO2 and not the “bicarb” (bicarbonate). In addition, the pO2 can also be directly impacted by affecting how much oxygen we are delivering effectively to the lungs.
Key Takeaway
For most patients, the goal is to normalize PaCO2 in the range of [latex]\text{35 - 45 mmHg}[/latex].
Iatrogenic Hyperventilation
In certain clinical situations, usually involving traumatic brain injury with increase intra-cranial pressure or other neurological pathology, you may encounter the term iatrogenic hyperventilation. This is intentional hyperventilation and the idea behind it is that reducing PaCO2 may result in cerebral vasoconstriction and reduced intracranial pressure (ICP). However this vasoconstriction also reduced cerebral blood flow resulting in potential hypoxic injury to the brain. For this reason, this medical strategy is considered controversial and, while it may provide a temporary solution to increased ICP, more robust research is needed to better guide the clinical use of iatrogenic hyperventilation.
Hyperventilation Article
Hyperventilation in neurological patients: from physiology to outcome evidence
Key Takeaway
While iatrogenic hyperventilation may provided a temporary solution to increased ICP, [latex]\text{PaCO}_2<25\text{ mmHg}[/latex] or during the first 24 hours after traumatic brain injury is not recommended. PaCO2 falls and pH rises resulting in a respiratory alkalosis. Low PaCO2 may lead to unwanted physiological side effects and should be used with caution.
Permissive Hypercapnia
Permissive Hypercapnia is a unique ventilatory strategy that moves away from routine clinical practices. By using small tidal volumes and lower ventilation pressures, this approach aims to minimize alveolar overdistention, reducing the risk for potential complications. The deliberate elevation of PaCO2 levels and the subsequent decline in pH leads to respiratory acidosis. While you may see this strategy used in clinical practice, current evidence shows that hypercapnia may lead to unwanted physiological side effects and should be used with caution.
Key Takeaway
Permissive Hypercapnia allows temporary respiratory acidosis to develop, with pH no less then [latex]7.15[/latex] to [latex]7.20[/latex], in clinical situations where tidal volume of [latex]6\text{ mL/Kg}[/latex] IBW or less must be maintained. Hypercapnia may lead to unwanted physiological side effects and should be used with caution.
“What can I fix with ventilation?” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
