METABOLIC ACIDOSIS

METABOLIC ACIDOSIS

by a future nephrologist

This is from reading the book Nephrology Secrets by Dr Lerma et al. 

I was a cardiothoracic anaesthesia medical officer from year 2015 till 2018. There were a lot of things I learned during my time there. However, over time, the knowledge disappeared as it was not used. Now, I have chosen Nephrology as my future as it is the closest to ICU, as it is the case for Pulmonology. Metabolic acidosis belongs to nephrology as much as respiratory acidosis belongs to Pulmonology. 

Now, what is the point of me talking about all of this stuff?

Respiratory acidosis can worsen metabolic acidosis. This book by Dr. Lerma explains it well. 

Look at the equilibrium here below: 

H+   +  HCO3-   <--->  CO2  + H20

Equilibrium means both sides would be the same amount at any particular time. The bicarbonate buffer system relies upon bicarbonate to neutralise H+.  For that to happen, the above balance must always maintain a rightward shift, i.e., the CO2 should always be at a low level to allow HCO3 + H+ to occur. 

If CO2 is high, HCO3 + H+ cannot occur. This will cause acidosis to get worse as acids are pouring in, and CO2 will remain high. 

So, if you have high PaCO2 due to respiratory issues, assisting ventilation is required to correct this. 

On the other hand, if high PvCO2 is due to poor perfusion, then perfusion should be improved with optimisation of fluid status, i.e. keep the patient euvolemic, and if required, we need to start vasopressor or inotropes to maintain perfusion pressure. 

PvO2 will be high in both cases. This reminds us of that PvCO2 cannot be used to diagnose Type 2 Respiratory Failure, in the absence of clinical correlation, such as a severe exacerbation of COPD. Many clinicians would agree that ABG is not required in this clinical context. 

What is the practical way of diagnosing tissue hypoperfusion?

A gap of >10 mmHg between PaCO2 and brachial PvO2 suggests tissue hypoperfusion. 

Remember, brachial PvO2 not any vein! 

What is a normal anion gap?

From a nephrology point of view, the normal anion gap is in the range between 6-12. 

However, I need to quote Anion Gap • LITFL Medical Blog • CCC Acid-Base

• The normal anion gap varies with different assays, but is typically 4 to 12mmol/L (if measured by ion selective electrode; 8 to 16 if measured by older technique of flame photometry)

How to know which measurement technique was used? It is very simple, just refer to the given reference range. 

What is an anion gap? 

Firstly, we need to define what is an anion. We are often hear the word normal anion gap and high anion gap. Obviously, although it may not be obvious too many people including me, high anion gap means we have 'new' anions. These new anions cause metabolic acidosis. 

We rarely know precisely the new anions. Therefore, the presence of new anions in plasma can be detected by calculating the PAnion gap. 

Electroneutrality requires that:  

Sodium + unmeasured cations (UC)  =  Chloride + Bicarbonate. + unmeasured anions 

The cation is + charge

Anion is - charge.

Actually, almost everything is measured in the automatic blood gases sample calculation. However when we do the equation we dont directly calculate K, Ca, Phosphate and others. In other words, K and Ca are unmeasured cations and Phosphate is an unmeasured anion. 

Let us rearrange this the equation.

Sodium minus Chloride minus Bicarbonate = UA minus UC = Anion Gap

Normally this gap is 6-12. Therefore, any increase in the gap is either due to increase in UA (such as lactate) or a decrease in UC (such as globulin)

The major UCA is albumin about 0.3 negative charge for every g/L. Therefore, when Albumin is 40g/L, it contributes to 12 negative charge or 12 mmol/L roughly equal to AG. 

Therefore, Anion gap should be corrected to Albumin level. For every decrease of Serum Albumin of 10g/L, we should minus 3 mmol/L from anion gap. 

Seriously, I dont understand this one. So again I have to refer to Anion Gap • LITFL Medical Blog • CCC Acid-Base

• a normally high anion gap acidosis in a patient with hypoalbuminaemia may appear as a normal anion gap acidosis.

Now it is clear to me why we have to correct for albumin. We don't want to miss high anion gap Metabolic acidosis!