As part of my fundamentals of Anesthesiology and Critical Care Series I have posted 3 tutorials on the Role of CO2 /HCO3 in Acid Base Balance. These are entirely new tutorials (not part of the previous acid base series – that I have not finished yet! There is some overlap and updated facts and figures) and I have put a lot of work into getting the message of why the respiratory system is so important in acid base. Tutorial 1 is the basics of acid base. Tutorial 2 discusses respiratory acidosis, acute and chronic, and respiratory alkalosis. Tutorial 3 discusses respiratory compensation for acute metabolic acidosis. Although I cover the respiratory component in great depth, I also explain what metabolic acidosis is, what causes it and briefly discuss the anion gap, expected bicarbonate, base deficit and base deficit gap. I guarantee that you will learn something.
Traditionally rules of thumb regarding the changes in PaCO2 and Bicarbonate in acid base balance have utilized mmHg. Unfortunately, in large tracts of the world, particularly in Europe, blood gases are reported in the SI unit kPa. This tutorial is for those people. I cover various acid base abnormalities – pH vs PaCO2, acute and chronic respiratory acidosis, respiratory alkalosis, metabolic acidosis and alkalosis and go through the various acid base rules of thumb using kPa, with examples. I guarantee you’ll learn something.
Rules:
Rule 1 H+ vs pH: a 1nmol/L increase in [H+} results in a 0.01 fall in pH
Rule 2 PaCO2 in Apnea: In apnea the PaCO2 rises by 1.5kPa in the first minute and by 0.5kPa per minute thereafter (this reduces progressively over time to 0.2-3kPa)
Rule 3 PaCO2 vs pH: For every 1kPa increase in the PaCO2 the pH falls by 0.06
Rule 4 PaCO2 vs HCO3 in Acute Respiratory Failure: For every 1kPa increase in the PaCO2, the HCO3 rises by 1mmol/L
Rule 5 PaCO2 vs HCO3 in Chronic Respiratory Failure: For every 1kPa increase in the PaCO2, the HCO3 rises by 3mmol/L and the Chloride falls by an equal value.
Rule 6 PaCO2 vs HCO3 in Acute Respiratory Alkalosis: For every 1kPa increase in the PaCO2, the HCO3 falls by 2mmol/L
Rule 7 PaCO2 versus Base Deficit in Acute Metabolic Acidosis: For every 1mmol/L increase in the Base Deficit (-BE e.g. from -1 to -2), the PaCO2 falls by 0.13kPa e.g. if the BD is -10 the PaCO2 will fall by 1.3kPa from 5.3 to 4
Rule 8 PaCO2 vs HCO3 in Chronic Metabolic Alkalosis (in ICU): For every 1mmol/L increase in the Base Excess (or HCO3) the PaCO2 increase by 0.13kPa e.g. if the BE is +10 then the PaCO2 will increase from 5.3 to 6.6
To truly understand acid base chemistry, it is imperative that you have a grasp of ionization theory. Although this might appear a little nerdy, it is quite straightforward and will also provide you with a basis for understanding the basic pharmacology of local anesthetics and opioids. Particles that disintegrate into component parts that carry charge are known as ions. If that charge is positive they are cations and if it is negative they are anions. Measurement of charge is known as valency, Most electrolytes in the body are univalent – Na, Cl, K, HCO3 – and their valency is quantifiably identical to their molarity (i.e. 140 mmol/L of Na+ = 1mEq/L). Some, however, are divalent – Calcium and Magnesium and Phosphorous. Ionized particles are a major component of acid base chemistry. They may be derived from mineral salts – Na, Cl, K, PO4, Mg, Ca or organic molecules – Lactate, Ketones, Metabolic Junk Products – manufactured in the body. Weak anionic acids are also manufactured – Bicarbonate and Albumin.
The relative quantities of different particles is governed by MASS CONSERVATION. Regardless of the source and quantity of anions and cations ELECTRICAL NEUTRALITY must always hold. Where there is imbalance between anions and cations the electrochemical void is filled by hydrogen or hydroxyl (derived from water dissociation) and acid base abnormalities ensue.
What makes ionized particles “strong” or “weak” acids or bases is determined by the pKa – the Ion Dissociation constant. This is the pH at which the particle is 50% dissociated or associated. As all electrochemical activity in the body occurs withing the physiological range of pH – 6.8 to about 7.65 – whether a ionic particle’s pKa is below or above, essentially 7.4, determines whether it is an acid or a base. For example – Lactic Acid has a pKa of 3.1 – at that point is is 50% associated (LA-H) and 50% dissociated (La-). At the environmental pH falls, for example towards 1, for example in the stomach, the chemical associates more (Lactic Acid). As the pH rises towards 7.4 it dissociates more (Lactate). At all physiologic ranges of pH Lactate is fully dissociated. Likewise, chemicals that have a pKa above the physiologic range pH (i.e greater than 7.6) are bases – and they become more associated at higher pH ranges. Sodium Hydroxide has a pKa of greater than12, which means that at pH 12 it is 50% associated, at pH 15 it is close to 100% associated. At physiologic range pH it is fully dissociated. Particles that are fully dissociated at all physiologic ranges of pH – cations such as Na+, K+, Mg2+ and Ca2+ and anions such as Cl-, Lactate- and Beta-Hydroxybutyrate, are known as STRONG IONS – they never bind to other ions (to create salts), hydroxyl or hydrogen in the body. Particles that are partially dissociated, whose pKa is closer to 7.4 – Bicarbonate, Albumin, Phosphate, Hemoglobin, are WEAK ACIDS and as they pick up more hydrogen ions at lower pH levels, they act as buffers.
Metabolic acid base balance is governed by the relative charge distribution (mEq/L) of STRONG IONS – known as the STRONG ION DIFFERENCE (SID) and the availability of weak acid buffers (ATOT). If the SID reduces, there is excess anion and metabolic acidosis. If the SID increases, there is excess cation or deficient anion and metabolic alkalosis.
ORtoICU 