Acid-Base Balance

Chapter 75 Acid-Base Balance



Thomas J. Meyer, MD




Cases Considered



1. Sepsis with hypotension

2. NG suctioning in a patient with ileus

3. Confusion and tachypnea in the ED



Speaking Intelligently


When I’m asked to evaluate a patient for an acid-base disturbance, my most important first step is to make sure that I have a simultaneous ABG and basic metabolic panel available. The patient’s bicarbonate level alone is insufficient to define the acid-base status. Once the pH is known, I can calculate if the patient is acidotic or alkalotic and the extent of compensation for this abnormality.



Clinical Thinking


I generally follow seven steps when approaching an acid-base problem:


1. Get the right information: Obtain an ABG and metabolic panel.

2. Do the numbers make sense? The following is a method to check that the numbers are consistent:



H+ concentration = 24 × PCO2/measured HCO3


Remember the logarithmic relationship:





















H+ concentration equals pH
30   7.50
40   7.40
50   7.30

If the numbers do not make sense, then repeat the labs.


3. Based on the pH, is the patient acidemic or alkalemic? This is the primary disorder or process causing the illness.

4. Can you explain the pH by the Pco2? Remember the relationship:

The acute change in PCO2 of 10 mmHg causes a reciprocal change in pH of 0.08.


5. ALWAYS calculate an anion gap:



Anion gap = Na+ – (HCO3 + Cl)


Normal anion gap = 12 ± 2.


All causes of an increased anion gap are abnormal!


6. If there is a metabolic acidosis, is the Pco2 compensation appropriate? (Remember, in a metabolic acidosis, the respiratory rate will increase, resulting in a lowering of the PCO2, in order to drive the pH back toward normal.)



Predicted PCO2 = HCO3 (1.5) + 8 (variability ± 2)


7A. If there is an increase in anion gap, what is the net change or “delta gap”? For each unit of anion gap increase from 12, there is a unit decrease in HCO3. Therefore, one can determine what the patient’s HCO3 level was prior to becoming ill. This allows the determination of an acid-base disturbance that may be initially occult. For example, if a patient has an anion gap of 20, and a measured HCO3 of 24:



Delta gap = 20 – 12 = 8


The patient’s HCO3 prior to getting ill was:




24 + 8 = 32


The patient had a metabolic alkalosis prior to getting ill with a metabolic acidosis.


7B. If the patient has a metabolic alkalosis, what represents a normal respiratory compensation? In other words, can we predict how high a PCO2 should rise in the setting of a significant metabolic alkalosis? The change in PCO2 is 0.7 times the change in the HCO3. If the HCO3 is 42, assuming normal HCO3 is 24, the HCO3 change is:



42 – 24 = 18


The PCO2 change, therefore, is 0.7 × 18 = 12.6. Assuming a starting PCO2 of 40, one would predict that the PCO2 would rise to 52.6 if the HCO3 was 42.

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Mar 20, 2017 | Posted by in GENERAL SURGERY | Comments Off on Acid-Base Balance

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