13. Acid Base Regulation Flashcards
What is the base excess (BE)?
- Concentration of bases compared with the ‘expected concentration’
- Normal = 0
- Excess = +ve
- Deficit = -ve
What is the haematocrit?
Proportion of the blood volume occupied by red blood cells (around 0.45)
What is the FHbCO and FHbO2?
- FHbCO - proportion of Hb bound to CO (usually <1%)
* FHbO2 - proportion of Hb bound to oxygen (>95%)
What is FMetHb?
- Proportion of Hb that is in the ferric methaemoglobin state (usually <1%)
- Important to know what kind of inspiratory gases the patient has been breathing and pressure e.g. supplemental oxygen
Why is it important to maintain pH in the body?
• Changes can alter structure of proteins
- impair their function
- impeded biological processes
• Affect drug metabolism and clotting
What are the normal pHs of fluids in the body?
- Intracellular - 7.0
- Extracellular - 7.4
- Arterial - 7.4
- Venous - 7.34
- Stomach - 2.4
What do the following describe: • Alkalaemia • Acidaemia • Alkalosis • Acidosis
- Alkalaemia - higher than normal pH of blood
- Acidaemia - lower than normal pH of blood
- Alkalosis - describes circumstances decrease [H+] (increase pH)
- Acidosis - describes circumstances that will increase [H+] (decrease pH)
(can have have acidaemia but be in a state of alkalosis)
What usually causes respiratory acidosis/alkalosis?
- Problems with lungs
* e.g. emphysema or COPD
What usually causes metabolic acidosis/alkalosis?
- Problems with another part of the body other than the lungs
- e.g. diabetic ketoacidosis
What is the difference between a weak and strong acid?
Ha <=> (H+) + (A-)
• Strong acid - mostly dissociated (forward)
• Weak acid - more conjugated acid (backward)
What is a base?
- Anionic (negative) compound that binds
- Reversibly binds to free H+ ions
- Can be referred to broadly as being a buffer e.g. HCO3-
What units do we use for measurements of ions?
- Equivalence per litre (Eq/L)
- NOT mmol/L
- As they are charged e.g. proton has equivalence of 1 and calcium ion has 2
Why do we use pH to measure acidity rather than [H+]?
- Very low concentration of H+ ions compared to other ions in the blood
- Logarithmic scale
- pH increase of 1 represents 10x decrease in [H+]
What was the Pitts and Swan experiment?
- Strong acid injected into a dog
- Expected decrease in pH and death
- Buffering capacity of blood caused immediate reaction to imbalance
What are the sources of respiratory of metabolic acids?
- Respiratory acid - CO2 forms carbonic acid
- Metabolic acid - e.g. lactic acid
(respiratory acid is much greater than metabolic acid - 99:1)
What is the Henderson and Henderson-Hasselbalch equation used to calculate?
- Henderson - dissociation constant (Ka)
* Henderson-Hasselbalch - combines the Sorensen equation (pH = -log10[H+]) and Henderson equation
How is the respiratory and metabolic acid component assessed?
- Respiratory acid - PaCO2
* Metabolic acid - Base Excess (BE)
What causes a rise and fall in base excess?
• Rise - increase in renal excretion of acid - ingestion/administration of base - vomiting (loss of stomach acid) • Fall - overproduction of metabolic acids - ingestion of acid - reduction/failure of acid excretion by kidney - excessive loss of alkali during diarrhoea (leading to metabolic acidosis)
What are the basic guidelines for O2 kPa
- Normal: >10 kPa
- Mild hypoxaemia: 8-10 kPa
- Moderate hypoxaemia: 6-8 kPa
- Severe hypoxaemia: <6 kPa
What causes a rapid and slow compensatory response?
- Rapid - changes in ventilation to change CO2 and pH
- Slow - changes in HCO3- and H+ retention/secretion in the kidneys
(issues with lungs fixed slowly by the kidney, issues with the kidneys and metabolism in the rest of the body fixed quickly by the lungs)
What does it mean when the pH is uncompensated, partially compensated and fully compensated?
- Uncompensated - acidosis/alkalosis deranges pH
- Partially compensated - pH returning to normal but not within normal range
- Fully compensated - pH in normal range (CO2 and BE abnormal)
What causes uncompensated respiratory acidosis and how is it compensated?
- Hypoventilation
- Build up of CO2 in blood (increased H+)
• Acute phase:
- CO2 in erythrocytes + H2O to form bicarbonate (moves out via AE1)
- more plasma bicarbonate - equilibrium shifts left, less H+
• Chronic phase:
- increased reabsorption of bicarbonate in the kidneys
(Partially compensated - low pH, high PCO2, high BE)
- pH eventually returns to normal
- BE and PCO2 will still be high
What causes uncompensated respiratory alkalosis and how is it compensated?
- Hyperventilation
- Reduces PACO2 - reduced [H+]
• No acute phase
• Chronic phase:
- reduced reabsorption of bicarbonate in the renal nephrons
- increase bicarbonate secretion into the collecting ducts
(Partially compensated - high pH, low PCO2, low BE)
- pH eventually returns to normal (as more carbonic acid dissociates)
- BE and PCO2 will still be low
What causes uncompensated metabolic acidosis and how is it compensated?
- Diarrhoea
- Bicarbonate lost
- Low BE
- More carbonic acid dissociates to compensate for the lost bicarbonate
- More H+ produced too - pH decreases = uncompensated metabolic acidosis
- Increased ventilation - reduces PACO2 and increases the diffusion gradient
- More CO2 leaves the blood
(Partially compensated - low pH, low PCO2, low BE)
- Decrease in PCO2 - (carbonic acid) equilibrium shifts left so lost CO2 can be replaced
- More H+ taken up
- Increased pH to normal - compensated
- BE and PCO2 are low
What causes uncompensated metabolic alkalosis and how is it compensated?
- Vomiting
- Loss of stomach acid
- More carbonic acid dissociates to compensate for the lost H+
- More bicarbonate produced too - high BE, pH increases = uncompensated metabolic alkalosis
- Reduced ventilation - more CO2 stays in the blood
- Increases PaCO2
(Partially compensated - high pH, high PCO2, high BE)
- (Carbonic acid) equilibrium shifts right so more H+ is produced
- Decreased pH to normal
- BE and PCO2 are high
Why should the rest of the blood components be looked at, even if pH is normal?
- Could be acidosis/alkalosis in a state of full compensation
- However you cannot tell what is the cause and compensatory mechanism
What 4 things to you look at when assessing blood acid/base contents?
- Type of imbalance - acidosis/alkalaemia/normal
- Aetiology - respiratory/metabolic/both/normal
- Compensation
- Oxygenation - hypoxaemia/normoxaemia/hyperoxaemia
