Blood Gas Abnormalities ✅ Flashcards by Mollie O

What is the normal blood pH?

7.34-7.44

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What is the normal blood CO2?

4.6-6

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What happens to the blood CO2 in respiratory acidosis?

It is high

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What happens to the blood CO2 in uncompensated metabolic acidosis?

It is normal

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What happens to the blood CO2 in compensated metabolic acidosis?

It is low

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What is the normal blood HCO3?

22-26mmol/L

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What happens to the blood HCO3 in metabolic acidosis?

It is low

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What happens to the blood HCO3 in uncompensated respiratory acidosis?

It is normal

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What happens to blood HCO3 in compensated respiratory acidosis?

It is high

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What is the normal base excess?

-2 to +2 mmol/L

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What happens to base excess in uncompensated respiratory acidosis?

It is normal

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What happens to base excess in uncompensated respiratory acidosis?

It is high

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What happens to base excess in metabolic acidosis?

It is low

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What is pH defined as?

A decimal logarithm of the reciprocal of the hydrogen ion activity or concentration in a solution

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How is pH maintained in the range of 7.34-7.44?

By several buffering systems

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What is the main buffering system that maintains blood pH in range?

The carbonic acid-bicarbonate system

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What is the equation for the carbonic acid-carbonate system

HCO3- + H+ ↔ H2CO3 ↔ H2O + CO2

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What equation describes the relationship between pH, HCO3, and CO2?

The Henderson-Hasselbach equation

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What is the Henderson-Hasselbach equation?

pH = 6.1 + log10 ( HCO3- / 0.03xpCO2)

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What causes respiratory alkalosis?

Hyperventilation

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How does hyperventilation cause respiratory alkalosis?

Because in hyperventilation, CO2 is blown off and pH increases

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What causes respiratory acidosis?

Hypoventilation

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How does hypoventilation cause respiratory acidosis?

Because CO2 is retained and the blood becomes acidotic

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Do respiratory changes have a fast or slow effect on the blood pH?

Fast

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What is the importance of HCO3- in the. blood?

It is the base and a buffer of hydrogen ions

Where do hydrogen ions in the body come from?

They are a product of normal cellular metabolism

What causes metabolic acidosis?

Excess production of hydrogen ions

How does excess production of hydrogen ions cause metabolic acidosis?

The buffering effect of HCO3 is overcome, and the blood becomes acidotic

Give 4 examples of causes of excess acid?

- Lactic acid - Ketone production - Acid administration - Bicarbonate loss

When might there be excess lactic acid?

In shock

When might there be excess ketones?

Diabetic ketoacidosis

When might there be acid administration?

Salicylic acid in aspirin positioning

When might there be excess bicarbonate loss?

From the gut in gastroenteritis, or urine in renal tubular disease

What organ can compensate for respiratory acidosis in chronic respiratory failure?

Kidneys

How can the kidneys compensate for respiratory acidosis in chronic respiratory failure?

Increasing the amount of HCO3 in the blood and extracellular fluid

What will be found on blood gas in compensated respiratory acidosis?

Normal pH | High CO2 and HCO3

How long does compensation in respiratory acidosis take?

Several days

What organ can compensate for metabolic acidosis?

Lungs

How can the respiratory system compensate for metabolic acidosis

Can blow off CO2 to normalise pH

What mediates respiratory compensation for metabolic acidosis?

The carotid chemoreceptors

What is found on blood gas in compensated metabolic acidosis?

Normal pH | Low CO2 and HCO3

What is the main limitation of the Henderson-Hasselbach approach to considering pH?

Buffers other than HCO3 exist

Give 2 buffers of pH other than HCO3?

- Albumin | - Haemoglobin

What is the result of HCO3 not being the only buffer of pH?

HCO3 and CO2 are not independent

What happens to hydrogen ions when there is a rise in CO2?

There is a rise in hydrogen ions

Why is there a rise in hydrogen ions when there is a rise in CO2?

CO2 + H2O ↔ H+ + HCO3-

What happens to HCO3 when there is a rise in CO2?

There is a rise in HCO3

Why is there a rise in HCO3 when there is a rise in CO2?

CO2 + H2O ↔ H+ + HCO3, and there are other buffers for H+, so not all the HCO3-reforms with H+

What is the result of HCO3 rising with an increase in CO2?

Respiratory acidosis could be mistaken for metabolic alkalosis

How can respiratory acidosis be differentiated from metabolic alkalosis?

Base excess

How is base excess determined?

By equilibrating the sample to a normal pCO2 (5.33kPa), then titrating it to pH 7.4 = the number of mmol/L needed to do this is the base excess

What is base excess a measure of?

How acidotic or alkaloid the sample is without any contribution of CO2

How is the anion gap calculated?

(Na + K) - (Cl + HCO3)

What is the purpose of calculation of the anion gap?

It allows classification of a metabolic acidosis into those with normal or increased anion gap

What is the anion gap a measure of?

The concentration of unmeasured anions

Give 3 examples of unmeasured anions?

- Plasma proteins - Ketones - Lactate

What theory is the anion gap based on?

Electrical neutrality

What is the theory of electrical neutrality?

The sum of positive ions must equal the sum of negative ions

What is considered to be an increased anion gap?

>16mmol/L

What does an increased anion gap suggest?

The presence of unmeasured organic acid

What does a normal anion gap suggest?

Bicarbonate loss and/or increase in chloride concentration

Give 9 causes of metabolic acidosis with increased anion gap

- Diabetic ketoacidosis - Alcohol poisioning - Starvation - Inborn errors of metabolism - Hyperosmolar non-ketotic coma - Lactic acidosis - Methanol - Ethylene glycol - Salicylate

Give 8 causes of metabolic acidosis with a normal anion gap

- Diarrhoea - Parenteral nutrition - Carbonic anhydrase inhibitors - Dilutional acidosis - Ingestion of HCl or other acid - Renal tubular acidosis - Ileostomy - Sodium chloride administration

Who is hyperchloraemic acidosis common in?

Patients given normal-saline containing fluids

Why is hyperchloraemic acidosis common in patients given normal-saline containing fluids?

- Normal saline is acidic, and has little buffering capacity | - Volume expansion causes plasma bicarbonate dilution

What is the pH of normal saline?

5-6

What is the acidic pH of normal saline due to?

'Grotthuss mechanism'

What is the Grothuss mechanism?

Ions high dissociated when dissolved in water, i.e. Na and Cl ions, cause disruption of the ionic bonding of H2O, leading to greater dissociation and generation of H+

Why is it important to recognise hyperchloraemia as a cause of acidosis?

To avoid administration of fluids when further fluid therapy is unnecessary and will exacerbate the problem

What findings on bloods should indicate hyperchloraemia as the cause of acidosis?

- Normal anion gap - Raised chloride concentration - Low bicarbonate