Acid-Base Balance - Part 1: Intro Concepts and Anion Gap

What is the primary measure of blood acidity?

pH

How is blood pH obtained?

By measuring the concentration of free H+ in the blood

What is pCO2?

Partial pressure of CO2 gas dissolved in bood

What is pCO2 used to assess?

Alevolar ventilation

Are pCO2 and tCO2 the same?

NO!

tCO2 is an approximate measure of HCO3-

What does pO2 reflect?

Dissolved O2, not O2 bound to Hgb

What is pO2 used to assess?

Pulmonary gas exchange

What type of blood needs to be used for pO2?

Arterial blood

What is acidemia?

Decreased blood pH (increased H+)

What is alkalemia?

Increased blood pH (decreased H+)

What is acidosis?

Condition in which acidemia tends to occur

What is alkalosis?

Condition in which alkalemia tends to occur

What is hypercapnia?

Excess CO2 in the blood (increased pCO2)

What is hypocapnia?

Deficiency of CO2 in the blood (Decreased pCO2)

What is hypoxemia?

Decreased dissolved O2 in the blood (decreased pO2)

What is hypoxia?

Decreased oxygen delivery

What is measured for acid-base values?

Concentration of H+

pO2

pCO2

What is calculated for acid-base values?

HCO3-

What can pH be affected by?

Changed in pCO2

What blood sample should be collected to measure acid-base?

Heparinized whole blood sample

When should acid-base tests be run after blood collection?

Immediately

Or no later than 1 hour later if it's in an ice bath

What can cause erroneous blood-gas values?

Exposure to air

Excess heparin

Delayed sample analysis

What happens to blood-gas values when a blood sample is exposed to air?

Increased pO2, pH

Decreased pCO2, H+ concentration, HCO3-

What happens to blood-gas values when there is excess heparin in the sample?

Dilution effects

Increased pO2 and maybe pH (or WRI)

Decreased pCO2, H+ concentration, HCO3-

What happens to blood-gas values when the sample is run late?

Decreased pO2, pH

Increased H+ concentration

Why does running a sample late cause the blood-gas values changes seen?

(Decreased pO2, Increased H+)

O2 is used in glycolysis, glycolysis leads to the production of H+

What is an acid?

Substance that liberates H+ in a solution (gives up H+)

What are examples of acids?

Ammonium ion (NH4+)

Acetic acid

Lactic acid

What is a base?

Substance that binds/accepts H+ thus removing them from the solution

What are examples of bases?

Ammonia (NH3)

Acetic anion

Lactate anion

What are strong acids/bases?

Acids and bases that completely ionize (dissociate) in aqueous solutions

What can strong acids/bases not function as?

Buffers

What are examples of strong acids?

HCl, H3PO4, HNO3

What are examples of strong bases?

NaOH, KOH

What are weak acids/bases?

Acids and bases that only partially ionize (dissociate) in a solution

What is an important function of weak acids/bases?

To act as buffers

When H+ concentration decreases, what happens to pH?

pH increases

Why is H+ concentration expressed in log form?

Because it is in such small amounts, especially compared to other electrolytes

What can alterations in pH cause?

Altered protein/enzyme configuration

Profound effects on metabolic events

Altered protein and enzyme function, interaction, and reaction rates

Is the pH higher intracellularly or extracellularly?

Extracellularly

What is the pH intracellularly?

6.0-7.4

Why is intracellular pH so important?

It is critical for cell viability, enzyme function, and metabolic processes

Where is most of H+ generated?

From metabolism

When is H+ balance achieved?

When dietary input and metabolism is equal to fecal base loss and output (urine)

What is produced in greater quantities: volatile or nonvolatile acid?

Volatile acid

What is an example of a volatile acid?

CO2

What are examples of endogenous acids?

Ketoacids

Uremic acids

Phosphatic acids

Lactic acids

What are examples of exogenous acids?

Oxalic acid (from ethylene glycol toxicosis)

Salicylic acid (from aspirin toxicosis)

What is the major extracellular buffer of an acid load?

Bicarbonate

What classes of buffer systems help balance H+?

Chemical

Pulmonary

Hepatic

Renal

How is CO2 produced?

Continuously in cells

When does CO2 become an acid?

When it is dissolved in water and forms carbonic acid (H2CO3)

What causes the formation of carbonic acid?

CO2 dissolving in water

How is CO2 excreted?

Through the lungs

What is the normal ratio of HCO3- to CO2?

20:1

What is the metabolic component of blood pH regulation?

HCO3-

What is the respiratory component of blood pH regulation?

CO2

What does tCO2 measure?

Bicarbonate, carbonic acid, and dissolved CO2

What is tCO2?

Amount of CO2 gas that can be extracted from serum

What is tCO2 essentially equivalent to?

HCO3-

How much of tCO2 is HCO3-?

95%

How is tCO2 measured?

Collect blood in red top tube and allow it to clot

Separate serum

Perform chemical assay

How is the actual bicarbonate concentration determined?

It is calculated by a blood gas analysis

What are the primary acid-base disorders?

Metabolic acidosis

Metabolic alkalosis

Respiratory acidosis

Respiratory alkalosis

What acid-base disorders take time to develop?

Metabolic acidosis and alkalosis

How can metabolic acid-base disorders be determined?

With serum electrolytes, with or without a blood gas analysis

Which acid-base disorders develop rapidly?

Respiratory acidosis/alkalosis

What are respiratory acid-base disorders dependent on?

Blood-gas analysis

What are the 6 steps to a blood-gas analysis interpretation?

1. Evaluate pH

2. Evaluate HCO3-

3. Evaluate pCO2

4. Determine the primary problem: which analyte is changed in a direction consistent with the pH change?

5. Evaluate if there are combined abnormalities; determine if there is a compensatory response

6. Correlate results with history, clinical and physical findings

What is the anion a tool for?

Detecting the presence of anions that are not normally in the blood in significant concentrations

When is the anion gap most useful?

When metabolic acidosis is present

What are the measured cations in the anion gap?

K+ and Na+

What are the unmeasured cations in the anion gap?

Ca2+ and Mg2+

What are the measured anions in the anion gap?

Cl- and HCO3-

What are the unmeasured anions in the anion gap?

Phosphate, proteins, and organic anions

How is the anion gap calculated?

Taking Na+ and K+ minus Cl- and HCO3-

What is the normal anion gap?

16-20 mmol/L

When anions change in the anion gap, what must happen?

Corresponding change in cations OR inverse change in another anion

Why does a change in an anion require an inverse change in a different anion or corresponding change in a cation?

To maintain electroneutrality

What are clinically significant changes in the anion gap usually due to?

Changes in anions

What are examples of conditions that cause an increased anion gap acidosis?

Lactic acidosis

Ketoacidosis

Renal failure

Ethylene glycol poisoning

Methanol poisoning (antifreeze)

Metaldehyde poisoning (snail bait)

What are the two possible types of metabolic acidosis?

Titration type

Secretory type

What happens in titration type metabolic acidosis?

The anion gap increases due to the addition of an acid and use of HCO3- as a buffer

What happens in secretory type metabolic acidosis?

The anion gap remains within the reference interval.

Sodium and bicarbonate are lost.

No acid is added