Acid-Base Pearls Part 1

The Strong Ion Approach to Acid-Base Disorders


David M. Miller


May 23, 2020

Pearls on how to approach Acid-Base issues commonly seen on inpatient medicine


Overview & Learning Objectives
Approach to the Patient With An Acid/Base Disturbance

Acid-Base Basics
Systematic Approach to the Work Up

Overview & Learning Objectives

The objective of this lecture is to provide a few clinical pearls on how to approach Acid-Base issues commonly seen on inpatient medicine. There is a slide component, which is used for didactic purposes, as well as a monograph section for background (see below). In part 1, we will introduce a methodology to approach clinical scenarios where Acid-Base Disturbances are key elements.

There are many excellent resources on this subject and the principles found in this lecture are an amalgam of several superb papers and monographs. Although I learned a great deal about Acid-Base physiology in Medical School and Residency, the EMCRIT podcasts by Scott D Weingart, MD were amongst some of the most valuable in my education. Please check them out.

Other resources include:
- Clinical review: Acid–base abnormalities in the intensive care unit by Lewis Kaplan and Spiros Frangos
- Physiological Approach to Assessment of Acid–Base Disturbances by Kenrick Berend, M.D., Ph.D., Aiko P.J. de Vries, M.D., Ph.D., and Rijk O.B. Gans, M.D., Ph.D
- Why Is Saline So Acidic (and Does It Really Matter?) by Benjamin AJ Reddi
- Integration of Acid–Base and Electrolyte Disorders by Julian Seifter MD

Approach to the Patient With An Acid/Base Disturbance

  1. History and Physical Exam
  2. Determination of the Primary Acid-Base Disorder and Secondary Response
  3. Evaluation of the Metabolic Component of the Acid-Base Disorder
  4. Evaluation for the presence of Mixed Metabolic Acid-Base Disturbances
  5. Consideration of the Serum (or Plasma) Osmolal Gap
  6. Evaluation of the Respiratory Component of an Acid-Base Disorder
  7. Interpretation of Acid-Base Disorders in the Clinical Context
Physiological Range of pH

The Range of pH that is compatible with life is 6.8 to 7.8 (a hydrogen ion concentration of 160 to 16 nanomoles per liter). Whether or not Acidosis or Alkalosis actually affects the body negatively has not been firmly established. Probably not between 6.8-7.8.

Rather, it is more likely that that the underlying problem that results in the acid/base distrubance (e.g. toxic substances, or hypoperfusion) causes the pathology, not the pH itself.

For example, does a hyperchloremic acidosis actually negatively affects one’s health? Unclear.

Does low pH decrease cardiac or catecholamine function? Still unclear.

Basics of the ABG
Evaluating the ABG
pH / PCO2/ PO2
Nl 7.4 / 40 / 80-100
A-aO2 Gradient

PAO2 (Alveolar (calculated)) – PaO2 (the ABG O2)

A-a O2 Gradient = [ (FiO2) × O2 Pressure) - (PaCO2/0.8) ] – PaO2 from ABG

Normal A-a O2 Gradient Estimate = (Age/4) + 4


On Room Air, the PAO2 simplifies to:

150 - PCO2 x 1.25

(Given that Room Air is about 21% O2, the PAO2 = 0.21(760-47) ~ 150)

Thus, with an ABG in hand, use the following formula if on Room Air

A-a O2 Gradient = (150 - PCO2 x 1.25) - PaO2

Base Deficit/Base Excess

The Base Deficit (or Base Excess) is the amount of base or acid needed to bring the sample back to a pH of 7.4, after normalizing for a pCO2 of 40.

Thankfully, the Blood Gas analyzer performs an analsys by which it gets rid of any respiratory component that is contributing to the acid/base status. Thus, the Base Deficit (or Excess) will tell you if the patient has a metabolic acidosis or alkalosis.

For example:

1) Acquire the Appropriate Intial Studies
Blood Tests
Urine Studies
2) Assess the Blood pH of the VBG or ABG
3) Evaluate the blood gas CO2
4) Calculate the Strong Ion Difference
Causes of SID Acidosis (SID < 38)
Fluid Administration
Renal Tubular Acidosis
Causes of SID Alkalosis (SID > 38)
Nasogastric Suction
Volume Depletion
5) Evaluate the Lactate
Causes of Hyperlactatemia ( > 2)
Infection (probably the most common in clinical medicine)
If patient not infected, consider, other states of hypoperfusion:
Bowel Ischemia
Hepatic failure
Use of b‐agonists
6) Calculate the Strong Ion Gap (SIG)
If the SIG >2, there is a Metabolic Acidosis present
If the SIG < 0, it means there are more cation (this is rare!)
7) Assess for Compensatory Mechanisms
8) Evaluate for an Osmolar Gap
This site represents our opinions only. See our full Disclaimer and Terms of Use Agreement