Tag Archives: metabolic alkalosis

kPa “RULES” – Part 2: The “Rules of Acid Base”

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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

@ccmtutorials http://www.ccmtutorials.org

ACID BASE 1 – The Power of HYDROGEN

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This is the first tutorial in a new series on acid base balance. This is not a beginners course – although I will attempt to cover everything the bedside clinician should know, particularly in the ICU. I have been teaching and writing about acid base for more than 25 years and I find it disappointing how many clinicians fail to understand even the basics of physical chemistry that underpin this topic.

This course is built on the foundation of physical and electrochemistry (all acid base reactions occur in water, all ionizing processes must be accounted for electrical neutrality must always hold.

The first tutorial is titled “The Power of Hydrogen” and it looks at the chemistry of water, the tendency for water to dissociate into moieties that display hydrogen ions and hydroxyl ions, and how temperature impacts that dissociation equilibrium. It is imperative that you understand that there are effectively no free protons (hydrogen ions) in the extracellular fluid. When we measure [H+] or its corollary, pH, we are measuring hydrogen ion ACTIVITY not hydrogen ion concentration. I explain the origin of pH and how pH varies with temperature despite the aqueous solution remaining chemically neutral. I explain the history of acid base, starting with O’Shaughnessy and then moving on to Arrhenius and Bronsted and Lowry. It is easier to understand acid base if one utilizes the Arrhenius theory, but the concepts are fully consistent with the BL approach, because water is amphiprotic (it can act as a “proton donor” or “proton acceptor.”

I explain how blood gas machines measure pH and why pH (and PCO2) should almost always be measured at 37 degrees Celsius. At the end of the tutorial I explain the terms acidosis and alkalosis, respiratory and metabolic. @ccmtutorials http://www.ccmtutorials.org