Acid-Base Balance Disorders & Anion Gap

What is the primary measure of blood acidity?

pH

How is pH determined?

By measuring free H+ in the blood

What is HCO3- useful for measuring?

Determination of A/B status

What is PCO2?

Partial pressure of CO2 gas dissolved in blood

What is PCO2 useful for?

To assess alveolar ventilation

What does PO2 reflect?

Dissolved O2, not O2 bound to HgB

What is PO2 used for?

Assessment of pulmonary gas exchange

When is PO2 of interpretive value?

Only when arterial blood is used

What is H+?

A nonvolatile acid

What is acidemia?

A decreased blood pH

What is alkalemia?

An increased blood pH

What is acidosis?

A condition in which acidemia tends to occur

What is alkalosis?

A condition in which alkalemia tends to occur

What is hypercapnia?

Excess CO2 in blood (increased Pco2) (AKA hypercarbia)

What is hypocapnia?

Deficiency of CO2 in blood (decreased PCO2) (AKA hypocarbia)

What is CO2 sometimes referred to as?

A volatile (respiratory acid)

What is hypoxemia?

Decreased dissolved O2 in blood (decreased PO2)

What's the difference between hypoxemia and hypoxia?

Hypoxia is decreased oxygen delivery whereas hypoxemia is decreased dissolved O2 in blood

How can acid-bases status be measured?

-Electrochemical analyzers

What do electrochemical analyzers do?

-Evaluates blood gas and electrolytes

-Measures the H+, PO2, and PCO2 (HCO3- is calculated)

-Selective membranes allow only H+, O2 and CO2 to pass from blood to react with specific electrodes

What may pH be affected by?

Changes in PCO2 (volatile)

How should a blood-gas sample be collected and run?

-Ideally a balanced heparin-containing syringe produced specifically for B/G analysis

-Any gas bubbles in the collection syringe should be expelled

-Syringe should be capped to prevent sample contact with air

-Immediate analysis is ideal- or ice bath for maximum 1 hour

What can cause erroneous blood-gas values?

-Exposure to air

-Excess heparin

-Delayed sample analysis

How does blood-gas change when the sample is exposed to air?

-Increased PO2

-Decreased PCO2

-Decreased H+ (Increased pH)

-Decreased bicarb (calculated value)

How does excess heparin change blood-gas values?

-Increase PO2

-Decreased PCO2

-Decreased H+ (Increased pH or WRI)

-Decreased bicarb (calculated value)

The PCO2 and PO2 of heparin reflect what?

The air/fluid boundary

How does delayed sample analysis change blood-gas values?

-Decrease PO2

-Increased H+ (Decreased pH)

What is an acid?

A substance that liberates hydrogen ions (protons) in solution

What are examples of acids?

-Ammonium ion (NH4+)

-Acetic acid (CH3COOH)

-Lactic acid (CH3CHOHCOOH)

What is a base?

A substance that binds/accepts hydrogen ions thus removing them from the solution

What are some examples of bases?

-Ammonia (NH3)

-Acetic anion (CH3COO-)

-Lactate anion (CH3CHOHCOO-)

What is a strong acid/base?

An acid/base that is completely ionized (dissociated) in aqueous solution

Can weak or strong acids/bases act as buffers?

Weak

What are some examples of strong acids?

-HCl

-H3PO4

-HNO3

-H2SO4 etc

What are some examples of strong bases?

-NaOH

-KOH

What are weak acids/bases?

-Acids/bases that are only partly ionized in a solution

-Titration necessary to determine pH

What is the relationship between H+ concentration in ECF versus that of other electrolytes?

H+ concentration in ECF is 1/1,000,000th that of other major electrolytes

One unit increase of pH represents ________ in H+

A 10-fold decrease

What can alterations in pH cause?

-Altered protein/enzyme configuration

-Profound effects on metabolic events

-Altered protein and enzyme function, interaction and reaction rates

What is intracellular pH critical for?

Cell viability, enzyme function, metabolic processes

When is hydrogen balance achieved?

When:

-H+ dietary input

-Metabolism

-Fecal base loss

-Output (urine)

Are balanced

What generates most of H+?

Metabolism

How many non-volatile acids are produced by a 30kg dog under basal conditions and normal diet?

About 30mEq/day (H+ eliminated via the urine)

How many volatile acids are produced by a 30kg dog under basal conditions and normal diet?

About 7000 mEq/day of CO2 (the CO2 is the dehydrated form of carbonic acid (H2CO3) and is eliminated via lungs

Is the magnitude of volatile or nonvolatile acid production greater?

Volatile

How are volatile acids (CO2) generated?

-Carbohydrate oxidation (CO2 + H2O)

-B-oxidation of fatty acids (CO2 + H2O)

How are non-volatile acids (H+) generated?

-Carbohydrate oxidation (lactic acid)

-B-oxidation of fatty acids (ketone bodies)

-Amino acid oxidation

-Nucleic acid oxidation

What are examples of endogenous acids?

-Ketoacids: ketosis, ketoacidotic diabetes mellitus

-Uremic acids (sulfates/citrate): azotemia

-Phosphatic acids: renal disease

-Lactic acids: Poor perfusion, anaerobic metabolism

What are some examples of exogenous acids?

-Oxalic acid: ethylene glycol toxicosis

-Salicylic acid: aspirin toxicosis

How is H+ balance achieved?

Through coordinated action of buffer systems

What specific processes control H+ balance?

-Chemical: hemoglobin, bicarb, phosphate, ammonia, protein

-Pulmonary: Volatile acid (CO2) eliminated by lungs

-Hepatic: Bicarb + NH4+ --> urea, glutamate formation

-Renal: Non-volatile acid H+ eliminated in urine, bicarb

What produces CO2?

Cells continuously

Is CO2 an acid?

No, but when dissolved in water, it forms carbonic acid: H2CO3

What is another name for CO2?

Respiratory acid or volatile acid

What organ excretes CO2?

The lungs

What is the normal ration of HCO3- to CO2?

20:1

What is the metabolic component of blood pH regulation?

Bicarb

What is the respiratory component of pH regulation?

pCO2

What is included in total CO2?

Bicarbonate, carbonic acid and dissolved CO2

How is total CO2 run?

Red top tube, clot, separate serum, chemistry assay

How is actual bicarb measured?

Calculated by blood gas anaylsis

How do you measure actual bicarb?

-Heparinized syringe, air-tight, blood gas instrument, ion-selective electrodes

-Calculated using Henderson-Hasselbalch equation

Does respiratory or metabolic acidosis/alkalosis occur faster?

Respiratory

How can you determine metabolic acidosis/alkalosis?

Serum electrolytes +/- blood gas analysis

How can you determine respiratory acidosis/alkalosis?

Blood gas analysis

What are the 6 steps to interpreting blood gas?

1. Evaluate pH: Is there acidemia? Alkalemia? Or WRI?

2. Evaluate HCO3-

3. Evaluate pCO2

4. What is the primary problem: which analyte is changed in direction consistent with pH change, pCO2 (respiratory) or HCO3 (metabolic)

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

6. Correlate lab results with history, clinical and physical findings of patient

What is the anion gap?

Tool for detecting presence of anions that are not normally in blood in significant concentrations

When are anion gaps most useful?

When metabolic acidosis is present

What is the basic principle of an anion gap?

Cations (+) = Anions (-)

What is a cation?

A positively charged atom or molecule

What is measured cation charge?

Na+ and K+

What is unmeasured cation charge?

fCa2+, fMg2+, globulins+

What is total cation charge?

mC+ + uC+

What is an anion?

Negatively charged atom or molecule

What is measured anion charge?

Cl- and HCO3-

What is unmeasured anion charge?

PO4 3-, proteins, org. anions-

What is total anion charge?

mA- + uA-

Clinically significant changes in anion gap are usually due to what?

Changes in anions

What happens to the anion gap in a healthy animal?

WRI

What happens to the anion gap in normochloremic metabolic acidosis?

Increased

What happens to the anion gap in hypochloremic metabolic acidosis?

Increased

What happens to the anion gap in hypochloremic metabolic alkalosis?

WRI

What happens to the anion gap in an animal with hyponatremia and hypochloremia?

AG WRI

What happens to the anion gap in hyproteinemia?

Decreased

What conditions cause anion gap acidosis and what is the unmeasured anion?

-Lactic acidosis (lactate)

-Ketoacidosis (ketone bodies)

-Renal failure (phosphate, sulfate, citrate)

-Glycolate, oxalate

-Formate

-Metaldehyde poisoning (?)

What are the 2 types of metabolic acidosis?

Titration type and secretory type

What causes titration metabolic acidosis?

Excess hydrogen production:

-Lactic acidosis

-Ketoacidosis

-Ethylene glycol catabolism

Impaired renal excretion of H+:

-Renal failure

-Uroperitoneum

-Distal renal tubular acidosis

What causes secretory type metabolic acidosis?

-Diarrhea

-Loss of saliva (ruminants)

-Proximal renal tubular acidosis

What is type 1 renal tubular acidosis?

Distal tubular acidosis

What is type 2 renal tubular acidosis?

Proximal tubular acidosis

What is titration type metabolic acidosis?

Addition of an acid to the body, which is buffered (titrated) by HCO3

What can cause titration type metabolic acidosis?

-DKA

-Azotemia/uremia

-Lactic acidosis

-Exogenous toxins: salicylate, methanol, ethylene glycol, paraldehyde

What causes secretion type metabolic acidosis?

-Loss of HCO3: diarrhea, loss of saliva in large animals, renal tubular acidosis, vomiting duodenal content (bile-stained vomitus)

-Hyperchloremia: Loss of fluid, renal retention Na+ accompanied by Cl-

How does the body compensate for metabolic acidosis?

-Increase ventilation to reduce pCO2 (starts within minutes, lasts a few days)

What causes respiratory acidosis?

Any disorder that interferes with normal effective ventilation:

-Acute upper airway obstruction, pneumonia, pneumothorax, COPD

-Drugs or diseases that inhibit medullary respiratory center

-Really anything that causes hypoventilation or severe impairment of gas exchange

How much more quickly does CO2 diffuse than oxygen?

CO2 diffuses 20x more rapidly

Is bicarb/carbonic acid pair capable of buffering respiratory acidosis?

No

What is the compensation for respiratory acidosis?

-Intracellular buffers almost exclusively

-Bicarb/carbonic acid pair are incapable of buffering respiratory acidosis

-Renal retention of HCO3: takes 3-4 days, is linked with H+ excretion; therefore, see compensation only in chronic cases

-Acute hypercapnia/

What must you perform in order to diagnose respiratory acidosis?

Blood-gas

What is commonly seen in patients with chronic respiratory acidosis?

Hypochloremia and hyperbicrbonatremia