In my class on metabolic acidosis ventilation today, I explained why the bicarb shrinks when anions like ketones accumulate in DKA.
Obviously, the clinician wants to avoid intubation in this population, but sometimes your hand is forced when they fail to compensate. One of the scariest roles you play in the world of critical care is that of the brain in the intubated patient with metabolic acidosis.
Knowing the cations and anions must remain equal, adding ketone bodies to the anion side of the equation would result in more anions than cations (and that can't happen), so the body will break down the bicarbonate into CO2 and water.
The body does not like elevated levels of dissolved CO2 in the plasma. This is why the majority of CO2 is actually transported as bicarbonate. For example, look at a patient with COPD; due to them retaining CO2, the body will naturally increase the bicarbonate levels. The graph below illustrates the CO2 dissociation curve. This is similar to the oxyhemoglobin dissociation curve, but unlike oxygen, CO2 can be transported in three different ways.
If the bicarbonate levels were to be diminished in the graph above, the net result would be an increased dissolved gas.
CO2 is one of the independent variables that influence pH. Knowing that pH is just the measurement of how often hydrogen from one water (H2O) molecule comes detached and floats onto another water molecule (H3O), we can conclude that this reaction happens more frequently when CO2 levels within a solution increase.
For example, if you were to just blow into a cup of water for a few minutes, you would change the pH of the water to be more acidic. CO2 makes water dissociate more frequently, and that is exactly what pH measures.
In my class, someone asked, “So if we blow off that extra CO2 and get back to the normal 35-45 range, why do we have to keep blowing off more?
In other words, if we blow off the excess CO2 from the breaking down of bicarb, why do we need even lower CO2 levels when bicarb is forced to shrink?
This is where it gets interesting...
There is a formula that helps a clinician determine what the PaCO2 should be for a given bicarbonate level (Winter's Formula).
When you have lower levels of bicarb, you have fewer vehicles to transport the CO2 and therefore end up with higher plasma levels of CO2. Higher plasma CO2 levels mean a decrease in pH; this is why you get Kussmaul respirations trying to blow off CO2 and decrease the partial pressure of CO2 in the plasma.
Kussmaul respirations are rapid, deep breaths that are the body's way of increasing alveolar ventilation. One of the most common misnomers is that if a clinician is forced to intubate a patient with severe metabolic acidosis, they should try to imitate the respiratory rate the patient was at before intubation.
Here's why that is bogus...
Trying to replicate a spontaneous rate with a positive pressure rate is nearly impossible. Lung compliance is drastically higher when a patient is participating in dropping their own diaphragm and lifting their own chest wall.
To truly mimic Kussmaul's respirations, the clinician should optimize tidal volume first and rate second. This will optimize minute volume and allow maximum compensation. For more information on these strategies, check out the podcast I did with Bryan Winchell.
Click the picture below to see the podcast.
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Stewart PA. Independent and dependent variables of acid-base control. Respir Physiol. 1978 Apr;33(1):9-26. doi: 10.1016/0034-5687(78)90079-8. PMID: 27857.
Pahal P, Hashmi MF, Sharma S. Chronic Obstructive Pulmonary Disease Compensatory Measures. [Updated 2023 Feb 19]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK525962/