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.
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.
(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.
Base Excess of -6 (aka a Base Deficit of 6)
What this means is that you’d have to add 6 mmol/l of base to get to a pH of 7.4
i.e. the patient is acidotic
Base Excess of 4
i.e you’d have to add 4 mmol/l of acid to get to a pH of 7.4
i.e. the patient is alkalotic
Normal -2 to +2
If it is outside of this range it means there is an imbalance between Cations and Anions
An Artertial or Venous Blood Gas will do, but VBGs are much easier to get, as a a phlebotomist can draw a VBG, but at all the hospitals I’ve worked at, MDs are the one’s (with a few exceptions) who must acquire an ABG
this is a concentration, so getting one as proximal to the blood gas as possible is important as this can fluctuate
Complete Metabolic Panel
Urine Lytes (Na+, K+, Cl-)
2) Assess the Blood pH of the VBG or ABG
If the blood pH is <7.35, the patient is acidotic
At this point we don’t know if it’s metabolic, respiratory or mixed, but their primary problem is acidosis
If the blood pH is >7.45 the patient is alkalotic
Again, we don’t know if it’s metabolic, respiratory or mixed, but their primary problem is alkalosis
3) Evaluate the blood gas CO2
If the blood gas C02 is > 45 they have a respiratory acidosis
If the blood gas C02 is < 35 they have a respiratory alkalosis
4) Calculate the Strong Ion Difference
A strong ion is a salt that completes dissociates when put into solution
e.g. NaCL -> Na+ + Cl-
Examples of strong ions include: Na+ K+ Ca2+ Mg2+ Cl-
The difference between your strong ions is a major determinant of Acid/Base status
Sodium and Chloride are the major strong ions that determine acid/base (see figure below)
To maintain electroneutrality, the body must either lose a negative charge or gain a positive charge
The Strong Ion Difference that has clinical utility is the [Na+] - [Cl-]
Thus, SID = [Na] - [Cl]
Under normal conditions, this = 38
Thus, 38 is a good determinant of Acid/Base status
If the SID is less than 38 -> think acidosis
You can think of this as a relative increase in chloride
This is often referred to as a “SID Acidosis”
If the SID is greater than 38 -> think alkalosis
you can think of this as a relative increase in sodium
This is often referred to as a “SID Alkalosis”
Causes of SID Acidosis (SID < 38)
The three most common causes of a “SID Acidosis” are:
Rental Tubular Acidoses
Any fluid that has a SID of < 24 can cause an acidosis
2 liters of NS (which has a SID of 0) in <24 hours is enough to cause acidosis
This is fascinating topic and worthy of a separate monograph, but in the meantime if you want more reading check out Scott Weingart’s post
Renal Tubular Acidosis
To work a suspected RTA up further, evaluate your Urine Studies
Calculate a Urine Anion Gap (Urine Na+ + K+ – Cl-)
If the UAG is negative, consider other causes as unlikely to be an RTA
The UAG is a proxy for the ability of the kidney to secrete acid because it is hard to detect ammonium
Chloride is excreted proportionally to NH4+ to maintain electroneutrality
Thus, if the kidney can’t excrete acid i.e. in an RTA, the urine Chloride will be low, and the UAG will be positive
If UAG is negative, then the kidney is secreting NH4+ and Cl- and you don’t have an RTA