Understanding Bicarb on a CMP

They say, it’s all about the base about the base about the base…

Bicarbonate is the “base” and is made in the kidney. It takes hours to days to actually generate (and compensate) bicarb.

When there is a “funky” acid in the body, this will cause the bicarbonate to precipitously drop. It’s this low bicarb that gives us an elevated anion gap when you do the math and apply the formula. (Na+ - (Chloride + Bicarb.)  A positive anion gap is greater than 16. We are not concerned if it is low or normal. We just want to know if it is elevated. Now, a positive anion gap implies a positive “funky” acid in the body.  This “funky” acid is any acid that is exogenous to the body that is causing us to be sick. So, a positive anion gap equals a positive “funky” acid, which equals a metabolic acidosis. Once again, a positive anion gap equals positive “funky” acid that we need to identify and diagnose which equals a metabolic acidosis. It’s critical to the treatment. Because we treat DKA with IVF and an insulin drip. We treat renal failure very differently. It's the clinical understanding that is key. This is something we stress at our CME conferences - clinical understanding and application. For more details click here.

 Now, carbon dioxide in the body is an acid. So, under normal circumstances, if you are sitting down reading this atricle and you decide to take 50 deep breaths in and out, what you will do is blow off carbon dioxide, which is an acid. If you are losing acid, you will make your body become more alkalotic and thus put your body into a respiratory alkalosis. Your lips will tingle and hands have parenthesias. 

If you are in a metabolic acidosis, (implied by a positive anion gap and a “funky” acid in the body) your body is going to freak out because it has an acid within the body that it cannot tolerate. The body will compensate from a respiratory perspective by breathing fast. This rapid breathing from a metabolic acidosis is known as KUSSMAL breathing. KUSSMAL, coincidentally, happens to be the mnemonic for the main causes of a metabolic acidosis where K is ketones, U is uremia, S is sepsis, S is salicylates, M is methanol or other heavy alcohols, A is aldehyde or all others (such as iron or isoniazid) and L is lactic acidosis.

Talking about those in a little more detail, K in ketones has to do with diabetic ketoacidosis or a starvation ketoacidosis, which could occur in someone who has fasted for a prolonged period of time or even someone who has been on the Adkins diet or low carbohydrate diet. Uremia refers to someone in renal failure with a backup of all their toxins in the body that cannot be properly filtered by the kidneys. S for sepsis has to do with an overwhelming body infection which we talked about work up for under white blood cell count, to include a good temperature, a CBC, blood cultures x2, and a chest x-ray. S is for salicylates, which is an aspirin overdose or a salicylic acid. These patients will have tinnitus (or an abnormal ringing in the ears), and they will appear pretty sick. When a grandma comes in saying she took some Tylenol for her headache, do not assume that she really knows what she is talking about and it really was Tylenol. If it is an over the counter medication, assume that it could be aspirin and anybody with an unexplained metabolic acidosis should have a salicylate level. M is for ethanol or other heavy alcohols, and this could be isopropyl alcohol, methanol, ethanol or any of the heavy alcohols. And, if you recall under sodium, these heavy alcohols will cause a high osmolality. And, if you are working up a patient with hyponatremia, remember that the first step is to check serum osmolality. If the sodium is low and the osmolality is high, a heavy alcohol could be the cause of this. It is from this work up that we would check an osmolar gap, which is really beyond the scope of this text and more for an advanced toxicology class.

Aldehyde is kind of an antiquated chemical but with this, you should think of all others such as iron or isoniazid in a patient on treatment for tuberculosis or being treated for a positive PPD. It is isoniazid that really drains B6 stores. So, people who are taking INH (or isoniazid) will be on B6 and it so happens that if someone has a bad overdose for INH, the treatment would be B6. L is lactic acidosis, which could be found in someone who had prolonged hypoxia to the muscles or could be from Glucophage. Glucophage is in the biguanide class. It works in the liver for diabetes and patients who are on Glucophage are more prone to lactic acidosis. There is concern for people on Glucophage who need I.V. contrast studies, and patients who need an I.V. contrast for an angiogram or pulmonary CT scan to rule out pulmonary embolism. If they are on Glucophage and need a contrast study, the work up is to check renal functions. If these tests are normal, you would go ahead and do the study and hold the Glucophage. Two days later you would recheck renal functions. If they are normal, the patient can restart the Glucophage. If the patient’s renal functions are abnormal in the first place such as with a creatinine of 1.8, the study would be held and dye would not be given until the patient was hydrated enough that the creatinine came down to normal.

 When we talk about acid base from a basic perspective, we need to talk about a fast response and slow response. The fast response is carbon dioxide or change in the patient’s breathing patterns. A patient can alter how they breathe relatively quickly. It is kind of like switching a light switch and having the light bulb turn on quickly. A slow response has to do more with flicking a light switch and it being like an iron that needs a little bit of time to get heated up. Whenever there is an acid base problem, there will be a compensatory response by the body.

 Metabolic problems refer to problems with bicarbonate where respiratory problems refer to problems with carbon dioxide.

 A metabolic acidosis, once again, suggests a positive anion gap, a “funky” acid that we need to find, and KUSSMAL breathing, which is a hyper-ventilatory breathing in an attempt to blow off carbon dioxide. We must first get a good history, then find the acid, check a ketone level, check renal functions, do a septic work up, check a salicylate level, and then look for a heavy alcohol, either by serum osmolality or an osmolar gap. Then, see if the patient has taken iron, isoniazid and/or check a lactic acid level. The KUSSMAL breathing from a metabolic acidosis is referred to as a respiratory compensation for the metabolic acidosis. We will talk about acid base in greater detail in a later blog/article. 

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