Acid / Base Disorders

Introduction

• Most of the metabolic processes that occur within the body result in the production of acid.
•  About 22,000 mEq of hydrogen produced daily is volatile and can be exhaled during respiration. 
• 70-100 mEq of the hydrogen produced is non-volatile and requires another route of excretion; this is where the kidneys come in.
• The normal concentration of hydrogen molecules is about 40 mEq/L (pH=7.4).
• This concentration is maintained constant, despite variations in acid input, via three mechanisms:
  • Conversion of the acid into carbon dioxide
  • Most is buffered (using bicarbonate, proteins, hemoglobin, creatinine, and phosphate salts),
  • The rest is excreted 
• There are many systems used to assess the acid-base status of the body; many of these methods utilize the fact that the amount of bicarbonate within the body is readily measured.
• We will only focus on the most popular method of assessment.

Assessing the Acid-Base Status

• This system is based on Bicarbonate and CO2 and is derived from this basic equation:

CO2      CO2 + H2O    H2CO3    H + HCO3

• As much fun as the conversion of logs is…it’s kind of hard to use them on the floor.  Another relationship that is helpful is the fact that the pH at any given time is proportional to the ratio HCO₃ /pCO₂.
• How do we know that the above statement is true? 
  • Well if you take the normal pH of 7.4 and correspond it to a HCO₃/pCO₂ = 24/1.2, we get a ratio of about 20:1.  This 20:1 ratio defines the normal pH.  (if you plug the values into the HH equation you see that the log of 20 is the difference between the normal pH and the normal pK).

• By the way, where did the number 1.2 come from?
  • Arranging the values of a normal ABG into our equation we run into a slight problem, the units for these values are not interchangeable.
  • To solve this problem we multiply the normal pCO₂ (40 mmHg) by a constant (.03) to get 1.2.  This constant is dependent on Henry’s law (see above).

What exactly is measured? 

• To obtain the values used in these equations we use the Arterial Blood Gases (ABG) and the serum electrolytes. 
  • The ABG gives us three variable the pH, the pCO₂ (both measured by an electrode) and the HCO₃ (derived by calculation).
  •  The serum electrolytes give us the CO₂.
• Although the terms CO₂ and HCO₃ tend to be used interchangeably_, there is a difference between the two. 
  • CO₂ represents three variables [CO_{2 ,} H₂CO₃,  HCO₃]
• Remember back when we said that bicarbonate is measured…actually what is measured is the total CO₂ content which for our purposes represents the sum of  HCO₃ and pCO_2. 
  • Interpretation: The measurement of CO₂ is actually a measurement of all the different forms in which CO₂ can exist in the body; these forms include that of HCO₃.

Acid / Base Disorders

• In the ratio HCO₃ /pCO₂, the numerator is controlled by metabolic/renal processes and the denominator is controlled by lung function.
• If this ratio is >20 we have a situation of alkalosis (pH is >7.4)
• If this ratio is <20 we have a situation of acidosis (pH is <7.4)
• Please note that the terms alkalosis and acidosis refer to a process while alkalemia and acidemia refers to the actual pH of the blood.
• There are four types of acid-base disturbances
  • If the HCO₃ decreases and the pH decreases = Metabolic Acidosis
  • If the HCO₃ increases and the pH increases  = Metabolic Alkalosis
  • If the pCO₂ increases and the pH decreases = Respiratory Acidosis
  • If the pCO₂ decreases and the pH increases = Respiratory Alkalosis
• The body seeks to maintain equilibrium, so if there is a change in one component of the ratio the other component of the ratio will also shift in a similar direction.  This is referred to as compensation.
• There is however, a limit to the degree of compensation possible, if the amount of compensation surpasses the expected value, we must then conclude that there is a mixed disturbance.

Calculating the values for compensation:

• Metabolic Acidosis:
  • Expected pCO₂ = 1.5 (HCO₃) + 8 + 2
  • 1.2-1.5 mmHg decrease in pCO₂ for each mEq/L decrease in HCO₃
• Metabolic Alkalosis:
  •  .3-.5 mmHg increase in pCO₂ for each mEq/L increase in HCO₃
• Respiratory Acidosis:
  • Acute: 1-2 mEq/L increase in HCO₃ for each 10 mmHg increase in pCO₂
  • Chronic: 3-4 mEq/L increase in HCO₃ for each 10 mmHg increase in pCO₂
• Respiratory Alkalosis:
  • Acute: 1-3 mEq/l decrease in HCO₃ for each 10mmHg decrease in pCO₂
  • Chronic: 5-6 mEq/L decrease in HCO₃ for each 10 mmHg decrease in pCO₂

Metabolic Acidosis

§         There are three main causes of metabolic acidosis:

• Loss of bicarbonate via loss of lower GI secretion of bicarbonate (most common)
• Gain of Acid (lactic acid, uremic acid, ketoacids, or HCl derived acid)
• Failure to excrete non-volatile acid (renal tubular acidosis)

§         The anion gap is useful in the diagnosis is metabolic acidosis.   

•  Changes in the anion gap indicate whether the disorder is of the Anion Gap variety or the Non-Anion Gap variety.

§         What is the anion gap?

• The anion gap = the measured cations – measured anions.
• For our purposes, cations refers only to Na⁺  and anions refers to Cl^- and HCO₃
• So the Urinary anion Gap = Na⁺ - (Cl^- + HCO₃) [normal value is 12-15 mEq].

§         When hydrogen ions (from organic acid) are added to the ECF the bicarbonate concentration will drop as it buffers the acid.  This drop in bicarbonate will increase the anion gap. 

§         Similarly if hydrogen ions (from a Cl- derived acid) was added to ECF the bicarbonate concentration will decrease, HOWEVER there will be a concomitant increase in the Cl- concentration and the anion gap will remain unchanged.

§         Differential Diagnosis of Anion Gap Acidosis:

• Uremic Acidosis
•  Lactic Acidosis
•  Diabetic Ketoacidosis (DKA)
•  Poisoning (methanol, ASA, ethylene glycol, or paraldehyde ingestion)

§         Differential Diagnosis of Non-Anion Gap Acidosis:

• Diarrhea
• Renal Tubular Acidosis (in one variation there is a loss of bicarbonate)
•  Carbonic anhydrase inhibitors (permit loss of bicarbonate into urine)
•  Uretero sigmoidostomy (procedure in which there is an anastamosis between the GU and the GI tract allowing urine to exist in the GI)
•  NH₄Cl administration

Approach to Diagnosis 

Here is a series of questions useful for determining the disease process:

What is it?

What type?

Is compensation present?

Is it appropriate?

If it isn’t appropriate, what else is happening?

What is the anion gap?

Is it an acute or chronic process? (Refers only to respiratory processes)

What is delta/delta? (helps to determine if there is a tertiary process, more later)

Case 1: 

 Patient has severe diarrhea for a long time, resulting in a loss of Bicarbonate

ABG: pH= 7.2, HCO₃ = 12, pCO₂ =26 

What is it? Acidosis

What type? Metabolic

Is compensation present? yes

Is it appropriate? Yes (see section on calculating appropriate compensatory values)

What is the anion gap? Normal

Diagnosis: Metabolic Acidosis with compensation

Case 2: 

A 60 year old ex-coal miner and heavy smoker with COPD also begin to experience diarrhea.

ABG: pH = 7.2, HCO₃ = 12, pCO₂ 40

What is it? acidosis

What type? metabolic

Is compensation present? No

Is that appropriate? No

If it isn’t appropriate, what else is happening? A second process (COPD) is interfering with compensation.

Diagnosis: Mixed Disturbance – Primary Metabolic Acidosis with a Secondary respiratory Acidosis. 

Case 3:  

A diabetic patient stops taking insulin.

ABG: pH = 7.2, HCO₃ = 12, pCO₂ 26

What is it? acidosis

What type? metabolic

Is compensation present? Yes

Is it appropriate? yes

What is the anion gap? 22

Diagnosis: Anion Gap Metabolic Acidosis (due to DKA) with compensation 

• Sometimes in a mixed disturbance everything may be normal, BUT if there is an increase in the anion gap one must conclude metabolic acidosis is present.
• Although the pH is the first thing you look, it may be normal in some mixed disturbances so other abnormal values have to be used for diagnosis.

Case 4: 

Septic Patient develops renal failure

ABG: pH = 7.4, HCO₃ = 12, pCO₂ 18

What is it? acidosis

What type? Metabolic

What is the anion gap? 22

Is compensation present? Yes

Is it appropriate? NO

If it isn’t appropriate, what else is happening? Respiratory alkalosis is also present.

Diagnosis: Metabolic Acidosis w/ concurrent Respiratory Alkalosis 

Note: Compensation is NEVER complete (of course there is an exception that will be stated in the second lecture), so if there is a normal value present we know that there is a mixed disturbance occurring.

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