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 www.ccm-tutorials.com
Contents:
00:14 Introduction
00:46 Review of Previous Tutorial and Preview
02:00 Clinical Scenario - Larry - Acute Complex Metabolic Acidosis
03:48 The Value of Bicarbonate
04:20 Henderson and Buffers
05:30 Electrochemical Balance in Extracellular Fluid
06:43 "Anions are Acids, Cations are Bases"
08:00 Bicarbonate is the Major Extracellular Buffer
09:00 Systems for Assessing Metabolic Acidosis
09:52 The Boston Approach
11:00 Compensation for Metabolic Acidosis (Winters)
13:38 The Copenhagen Approach (Base Excess, Base Deficit)
17:30 Electrical Neutrality
17:50 The Anion Gap
23:10 The Anion Gap Only Really Diagnoses Hyperchloremia
24:12 Correcting the Anion Gap For Albumin
26:07 A Modern Comprehensive Anion Gap
26:53 What Does The Gap Mean?
27:53 A Simple Approach to Thinking About Metabolic Acidosis
29:00 The Strong Ion Gap and the Base Deficit Gap
34:55 Smartphone Apps and Acid Base
36:14 Review