07-11-22 – Acid-Base Homeostasis – Respiratory Flashcards
Learning outcomes
- Describe how PCO2 affects blood pH
- Calculate blood pH from given values
- Draw a Davenport diagram to indicate how pH and [HCO3-] alter with changes to PCO2 and changes to non-volatile acid or base
- List the primary causes of acid-base disturbances
- Describe, with the use of a Davenport diagram, respiratory acidosis and its compensation
- Describe, with the use of a Davenport diagram, respiratory alkalosis and its compensation
- Describe, with the use of a Davenport diagram, metabolic acidosis and its compensation
- Describe, with the use of a Davenport diagram, metabolic alkalosis and its compensation
What is an acid?
What is a base?
What Is pH a measure of?
Why was the pH scale made?
What is the formula for pH?
What is the H+ concentration at pH 7?
- An acid is defined as any chemical substance that can donate a proton, H+
- A base (alkali) is defined as any chemical substance that can accept a proton, H+
- pH is a measure of H+ concentration
- Because H+ concentration can vary over a large range in solutions, the pH scale was created
- pH = -log10 [H+]
- [H+] of 10-7M = pH 7.0
What is the calculation of plasma pH defined by?
What does this equation look at?
What does this equation look like (in picture)?
What are normal values for each variable?
What will this equation equal to?
- The calculation of plasma pH is defined by Henderson-Hasselbalch equation
- In this case, this equation looks at the reaction of carbonic acid dissociating into bicarbonate and hydrogen ions
- The pH of this equation will be about 7.4
What is pKa?
What is carbonic acid?
What are its constituent parts?
What is the pKa of Carbonic acid?
What % of carbonic acid will be ionised and unionised below and above a pH of 6.1?
What form is primarily it in at pH 7.4?
How do we shift the dissociation reaction to the left or right?
- pKa (aka the dissociation constant) is the pH at which 50% of a compound is ionised and 50% is unionised
- Carbonic acid (H2CO3) is a weak acid that almost instantaneously dissociates into its constituent parts (HCO3- and H+ ions)
- The pKa of carbonic acid is 6.1
- This means above pH 6.1, there will be more than half of carbonic acid ionised
- Below a pH of 6.1, there will be more than half of carbonic acid unionised
- This means at pH 7.4, more of carbonic acid will be ionised than unionised
- To shift the dissociation reaction left towards formation of H2CO3, we increase the concentrations of H+ and or HCO3-
- To shift the dissociation reaction right towards the formation of H+ and HCO3-, we increase H2CO3 concentration
How are the absolute levels of bicarbonate changed by changes to respiration?
What are respiratory and metabolic acid-base disturbances (in picture)?
What may cause metabolic acidosis?
- Absolute levels of bicarbonate can be changed by changes to respiration because the by-products of aerobic respiration (H2O+CO2) can be used to create carbonic acid (H2CO3)
- This means more CO2 (PCO2) generates more H2CO3, which will dissociate into more HCO3- and H+ ions
- An example of metabolic is ketoacidosis due to uncontrolled diabetes, where acids start to build in the bloodstream due to lack of insulin
What is considered acidosis? What is considered alkalosis? Describe the diagram
- pH < 7.35 acidosis
- pH >7.45 alkalosis
- Describing the diagram:
- Start is physiological pH, bicarbonate concentration, and PCO2
- From start to A shows an increase in PCO2 and decrease in pH (acidosis)
- From start to B shows a decrease in PCO2 and an increase in pH (alkalosis)
- From start to C shows plasma pH change when non-volatile acid is added/base is decreased – the PCO2 is static, meaning this is not a respiratory acid-base disturbance, and either acid is being generated from another course (such as ketoacidosis) or there is less bicarbonate ion production
- From start to D shows plasma pH change when non-volatile acid is removed/base is increased – the PCO2 is static, meaning this is not a respiratory acid-base disturbance, and either acid is being removed or there is more bicarbonate ion production
What are the 4 different causes of acid-base disturbances?
Which ones are respiratory and metabolic disorders?
What are 2 ways acidosis and alkalosis can be caused?
- 4 different causes of acid-base disturbances:
1) Increased CO2
2) Decreased CO2
3) Increased non-volatile acid/decreased base
4) Increased base/decreased non-volatile acid - Where primary change is to the CO2 levels - respiratory disorders
- Where primary change is to bicarbonate levels - metabolic disorders
- An acidosis can be caused by:
1) Rise in PCO2
2) Fall in HCO3 - An alkalosis can be caused by:
1) Fall in PCO2
2) Rise in HCO3
Describe the acid-base disorders flow chart
1) 1st Step: examine the pH.
* A pH less than 7.4 indicates acidosis, whereas a pH greater than 7.4 indicates alkalosis.
2) 2nd Step: examine the plasma PCO2 and HCO3− concentration.
* The normal values for PCO2 is about 40 mm Hg, and for HCO3− is 24 mEq/L.
* Expected values for a simple respiratory acidosis would be reduced plasma pH, increased PCO2, and increased plasma HCO3− concentration after partial renal compensation.
* Expected values for a simple metabolic acidosis would be a low pH, a low plasma HCO3− concentration, and a reduction in PCO2 after partial respiratory compensation.
3) The lungs and kidneys may try to return any disturbance towards normal – compensation
* The ways in which the two systems compensate are
* the respiratory system alters ventilation – this happens quickly
* the kidneys alter excretion of bicarbonate – this takes 2-3 days
What is respiratory acidosis a result of?
What 3 things cause increased PCO2?
How does increase PCO2 lead to acidosis according to the Henderson-Hasselbalch equation?
What is the compensation for this?
What is the renal compensation for this?
What are 5 conditions/drugs that cause respiratory acidosis?
What is the Davenport diagram?
Describe normal to point A on the Davenport diagram
- Respiratory acidosis is a result from an increase in PCO2
- An increase in PCO2 can be due to:
1) Hypoventilation (less CO2 being blown off)
2) Ventilation-perfusion mismatch
3) Reduced lung diffusing capacity
- From Henderson-Hasselbalch equation, an increase in PCO2 causes an increase in H+, so a lowering of pH
- Thus, plasma HCO3 - levels increase to compensate for increased H+ concentration
- Renal compensation – increased HCO3 - reabsorption and increased HCO3 - production – raises pH towards normal
- 5 conditions/drugs that cause respiratory acidosis:
1) COPD
2) Blocked airway – foreign body or tumour
3) Lung collapse
4) Injury to chest wall
5) Drugs reducing respiratory drive, eg morphine, barbiturates, general anaesthetics
- The Davenport Diagram is a graphical tool to Interpret acid-base Issues and is the diagram used in the photo
- Normal to point A on Davenport diagram
- Lower pH and an increase in bicarbonate.
- No respiratory compensation. Kidneys secrete more bicarbonate to buffer the extra H+ (indicated by the shift A→A2) The kidneys excrete more acid.
- Note that the PCO2 has not changed during compensation.
What is respiratory alkalosis a result of?
What is a cause of decreased PCO2?
How does this affect pH?
What is the renal compensation for this?
What are the 2 variables that lead to respiratory alkalosis?
What are 4 conditions that increase ventilation?
What are 2 conditions that cause hyperventilation?
Describe normal to point B on the Davenport diagram
- Respiratory alkalosis is a result of decreased PCO2
- Decreased PCO2 is generally caused by alveolar hyperventilation (more CO2 being blown off)
- This causes a decrease in H+ concentration and thus a rise in pH
- Renal compensation - reduced HCO3- reabsorption, and reduced HCO3 - production
- This leads to plasma HCO3- levels falling and compensating for lower H+, which moves pH back towards normal
- 2 causes of respiratory alkalosis:
1) Increased ventilation (taking deeper breathes)
* Caused by:
* Hypoxic drive-in pneumonia
* Diffuse interstitial lung diseases
* High altitude
* Mechanical ventilation
2) Hyperventilation
* Caused by:
* Brainstem damage
* Infection driving fever
- Normal to point B on Davenport diagram
- Increased pH and a decrease in bicarbonate.
- No respiratory compensation.
- Kidneys secrete less H+ and excrete more bicarbonate indicated by the shift B→B2 Note that the PCO2 has not changed during compensation
What is metabolic acidosis a result of?
What does this affect HCO3- levels?
How are PCO2 levels affected?
What are the 4 respiratory compensations for metabolic acidosis?
What are the 4 causes of metabolic acidosis?
Describe normal to point C on the Davenport diagram
- Metabolic acidosis is a result from an excess of H+ in the body
- This reduces HCO3- levels (shifts equation to the left)
- Addition of acid decreases pH but ventilation is unaffected, so PCO2 initially normal
- 4 respiratory compensations for metabolic acidosis:
1) The lower pH is detected by peripheral chemoreceptors, causes an increase in ventilation which lowers PCO2
2) The bicarbonate equation is driven further to the left, lowering H+ and HCO3 - concentration further
3) The decrease in H+ concentration moves pH towards normal
4) Respiratory compensation cannot fully correct the pH, HCO3 and H+, so excess H+ needs to be removed or HCO3 - restored (by slow renal comp)
- 4 causes of metabolic acidosis:
1) Loss of HCO3 - , eg from gut in diarrhoea
2) Exogenous acid overloading (aspirin overdose)
3) Endogenous acid production (ketogenesis)
4) Failure to secrete H+, eg in renal failure
- Normal to point C on the Davenport diagram
- Fall in pH due to addition of H+ or loss of base.
- PCO2 initially normal. Low pH stimulates an increase in ventilation. Shift from C → C2 (seconds to minutes).
- PCO2 falls. Full metabolic (renal) compensation is not shown (hours to days).
What is metabolic alkalosis the result of?
How does removing H+ from the equation affect the reaction?
How does this affect pH and PCO2?
What are 5 respiratory compensations of metabolic alkalosis?
What are 3 causes of metabolic alkalosis?
Describe normal to point D on the Davenport diagram
- Metabolic alkalosis is the result of an increase in HCO3- concentration or a fall in H+
- Removing H+ from equation drives the reaction to right, which increases HCO3-
- Lowering of the H+ raises pH, with PCO2 initially normal
- 5 respiratory compensations of metabolic alkalosis:
1) Increase in pH detected by peripheral chemoreceptors – this decreases ventilation which raises PCO2
2) The equation is driven further to right, increasing H+ and HCO3
3) Increase in H+ moves pH towards normal
4) Respiratory compensation is often small (or even absent) – ventilation cannot reduce enough to correct imbalance
5) Renal response is to secrete less H+
- 3 causes of metabolic alkalosis?
1) Vomiting - loss of HCl from stomach
2) Ingestion of alkali substances
3) Potassium depletion (eg diuretics)
- Normal to point D on the Davenport diagram:
- Rise in pH due to loss of H+ or addition of base.
- PCO2 initially normal. Raised pH depresses respiration.
- Shift from D → D2 (seconds to minutes). PCO2 rises. Full metabolic (renal) compensation is not shown (hours to days)
What are ABGs?
How can they be analysed using an acid-base (ABG) nomogram?
Describe the different disturbances that can be seen on an ABG nomogram?
- ABGs are arterial blood gases
- ABGs can be analysed using an acid-base (ABG) nomogram
- By plotting the PaCO2 and H+/pH values on the ABG nomogram, most ABGs can be analysed
- If the plotted point lies outside the designated areas, this implies a mixed disturbance
