Tag Archives: electrical neutrality

Metabolic Acidosis – What it is, Diagnosis and Tools

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This is Tutorial 4 in the Acid Base Series – on the topic of Metabolic Acidosis. The tutorial is based on a single blood gas – a random sample that was handed to me in the ICU recently. Blood Gas Used in This Tutorial: pH 7.19 PaCO2 32mmHg (4.1kPa) HCO3- 13.1 BE – 16.5 AG 20 Na+ 126 K+ 3.1 Cl- 96 Lactate- 7.2 Ketones- 0.6mmol/L Albumin 21g/L Creatinine 3.3mg/dl (293mmol/l)

Metabolic Acidosis is characterized by an increase in the relative ratio of strong anions to strong cations in the plasma. The PaCO2 and the Bicarbonate fall in a predictable manner. It is possible to compute the effectiveness of respiratory compensation for metabolic acidosis by using the Winters equation.

To understand the mechanism of metabolic acidosis – caused by accumulation of mineral (Chloride) and organic (Lactate, Ketones, Metabolic Junk Products) anions – one needs to apply the law of Electrical Neutrality. All of the positive charges must equal all of the negative charges. As Bicarbonate is consumed in the process of buffering metabolic acidosis, the change in the Bicarbonate level (downwards) can be used to quantify the degree of acidosis. This is important because the pH may be within the normal range due to respiratory compensation. Be aware that the HCO3- quantum that is displayed on a blood gas is derived from the pH and PCO2 by the Henderson Hasselbalch equation.

Unfortunately, because respiratory abnormalities may complicate the diagnosis of metabolic acidosis, and pH and PCO2 are altered by changes in temperature, the precision of a single reading of PCO2 and HCO3- may be poor. Consequently, the Standard Base Excess was developed to excise the respiratory component from the change in bicarbonate. Again it is a derived variable and may be imprecise. Nevertheless, BE (or 1-BE the Base Deficit BD) is a terrific scanning tool to identify the presence of a metabolic acidosis (BD) or alkalosis (BE). It is defined as the amount of strong cation (BD) or strong anion (BE) required to bring the pH back to 7.4 when the temperature is 37 degrees Celcius and the the PaCO2 is 40mmHg or 5.3kPa.

The Base Deficit does not indicate the source of the acidosis, but it can be recalculated to remove the impact of the [Na+], the [Cl-], the body water and the serum Albumin (and the Lactate) to determine the Base Deficit Gap – indicative of the quantity of Unmeasured Anions (UMA, Ketones, if not measured, and Renal Acids (metabolic junk products – MJP).

Traditionally clinicians use the Anion Gap to determine whether a patient has a Hyperchloremic Acidosis (no gap) from a UMA acidosis. I find this quite a dated concept. If the [Cl-] exceeds 105 and the plasma Sodium is normal, the patient has a Hypercloremic acidosis. We can easily measure Ketones and Lactate. The AG is imprecise and should be adjusted for the Albumin level, which tends to hover around 25g per liter in critically ill patients (narrowing the Gap and alkalinizing the patient). I do think if you are calculating the AG that you must include the K+ on the Cation side, the Lactate on the Anion side and adjust the Albumin.

The Strong Ion Gap is a more advanced, more precise and more cumbersome version of the AG. Regardless of the approach, one eventually ends up with a quantify of unidentifiable anions (SIG) that may be of medley origin (metabolism, poisoning etc). It is my opinion that it is useful to tease out all of the different acidifying and alkalinizing processes (the Fencl approach) to determine what is going on with the patient. All of these calculations can be done in seconds with smartphone apps and spreadsheets.

I guarantee you will learn something. @ccmtutorials http://www.ccm-tutorials.com

The Ripple of Ions – Ionization and the pKa

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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.

I guarantee you’ll learn something. @ccmtutorials http://www.ccmtutorials.org