06-03-23 - Clinical aspects of Acid-Base control Flashcards
Learning outcomes
- To be able to apply a simple stepwise method to the clinical interpretation of arterial blood gas results
- To discuss metabolic causes of acid base abnormalities
What is 1kPa equal to?
When is the body producing acid?
What are 3 ways the pH is maintained?
What are 2 ways acid is excreted from the body?
- 1kPa = 7.5mmHg
- The body is constantly producing acid – volatile acid from CO2 and non-volatile acid usually from the metabolism of proteins
- 3 ways the pH is maintained:
1) Proteins
2) Haemoglobin
3) Carbonic acid / bicarbonate - 2 ways acid is excreted from the body:
1) Lungs – excretes CO2 which would form volatile acid
2) Kidneys – excretes non-volatile acids
What are 3 cases when acid-base disturbances occur?
- 3 cases when acid-base disturbances occur:
1) There is a problem with ventilation
2) There is a problem with renal function
3) Overwhelming acid or base load the body can’t handle
Describe the normal arterial and venous values for (in picture):
* pH
* PO2
* PCO2
* Bicarbonate (standard)
What is the equation for acid base balance (in picture).
How is standard bicarbonate calculated?
What does standard bicarbonate reflect?
- Equation for acid base balance (in picture)
- Standard bicarbonate is calculated from the actual bicarbonate but assuming 370C and a paCO2 of 5.3kPa
- Standard bicarbonate reflects the metabolic component of acid base balance
How is base excess calculated?
What is the definition of base excess?
What is its normal range?
When does it become more negative?
- Base excess Is the value calculated from blood pH & pCO2
- Base excess is defined as Defined as the amount of acid required to restore a litre of blood to its normal pH at a PCO2 of 5.3.
- Normal range -2 - +2mmol/L
- It becomes more negative in the case of metabolic acidosis, as bicarbonate is use to buffer the excess acid
What are the 4 steps to analysing ABGs?
- 4 steps to analysing ABGs:
1) Assess pO2 & oxygenation
2) Assess pH, acidaemia or alkalaemia?
3) Determine the primary problem (think about the patient)
4) Is compensation occurring?
Step 1: assess pO2 & oxygenation.
What is P/F ratio? What is FiO2?
What is the P/F ratio for patients that are:
1) Healthy
2) Acute lung with injury
3) ARDS
What do we have to consider about the patient?
What is the Oxygen saturation (SpO2) target ratio for someone with Type 2 respiratory failure?
- Step 1: assess pO2 & oxygenation
- PaO2 / FiO2 ratio is known as the P/F ratio (kpa divided by inspired fraction of oxygen)
- FiO2 is the % of air breathed that is oxygen e.g normal air is 20% oxygen, so FiO2 = 0.2
- P/F ratio for patients that are:
1) Healthy - >50 (PaO2 about 12, and FiO2 about 0.2, so P/F around 60)
2) Acute lung with injury - <40
3) ARDS <26.7 - We have to consider the situation the patient is in e.g breathing an oxygen mask at 30% will change the FiO2 to 30% therefor altering the P/F ration
- Those with type 2 respiratory failure (hypoxic and hypercapnic) have an oxygen saturation (SpO2) target of 88-92%
- They are running off hypoxic drive, so we have to be cautious of this when giving oxygen, as they can become over oxygenated
- The most common cause of type 2 respiratory failure is COPD
Step 4: Is compensation happening?
What is compensation?
How is pH linked to HCO3- and PCO2?
What will the body never do?
When is there respiratory compensation?
How can we tell if compensation is occurring?
- Step 4: Is compensation happening?
- Compensation is the altering in function of the respiratory or renal system to change the secondary variable in an attempt to minimise an acid – base imbalance
- pH correlates with HCO3/pCO2
- The body will never overcompensate
- In metabolic problems the compensation is respiratory
- If pCO2 and bicarb are moving in the same direction compensation is likely to be occurring.
- If they are moving in different directions suspect a mixed disorder
What is the anion gap?
How is it calculated?
What is the formula for it?
What are the typical values for each of the anions and cations in the formula?
When are these ions measured?
- The anion gap Is the sum of routinely measured cations in venous blood minus routinely measured anions
- This is calculated from venous blood
- Formula for anion gap - ([Na+] + [K+])- ([Cl- ] +[HCO3 - ])
- Some equations dont have potassium, as it tends to be small
- Typical values for each of the anions and cations in the formula:
- Anions:
1) Chloride - 104
2) Bicarbonate - 24 - Cations:
1) K+ - 4.5
2) Na+ - 140 - These ions are routinely measure in routine urinary electrolytes
What is a normal value for anion gap?
Why do we use anion gap?
What can an increase in anion gap indicate?
- Normal anion gap = 16
- We use the anion gap as it helps differentiate the cause of a metabolic acidaemia
- An increased anion gap signals the presence of a metabolic acidaemia – metabolic acidaemias increase the anion gap
How are metabolic problems often caused?
How will it change the anion gap?
What are 3 events that can lead to an overwhelming acid load?
- Metabolic problems commonly occur from an overwhelming acid load
- This will lead to an increased anion gap
- 3 events that can lead to an overwhelming acid load:
1) Bodies own production (endogenous)
2) Ingestion (exogenous source)
3) Failure of excretion/ regeneration bicarb by the kidneys
What conditions will lead to excess acid production in the body?
How will this affect the anion gap?
What will this result in?
- Any condition causing hypoperfusion will lead to excess acid production in the body (increased anion gap)
- These conditions include:
1) Of the whole body: shock (cardiogenic, septic, hypovolaemic, anaphylactic)
2) Or part of the body: femoral artery embolism
* In atrial fibrillation, there can be formation of thrombi which can move around the body and potentially cause stroke or ischaemia distal to the embolism
- These will result in increased anaerobic metabolism with subsequent increased production of lactic acid, leading to lactic acidaemia
What is lactic acid a product of?
What happens to lactate in a healthy person?
What does this process require?
When will production of lactate increase?
- Lactic acid is a product of anaerobic metabolism
- In health lactate is metabolised in liver and overall, there is no NET production of acid
- This process needs oxygen
- Production of lactate increases when O2 delivery falls and consumption of lactate by the liver falls
What are 3 other causes of lactic acidaemia?
When is lactate level a cause for concern?
- 3 other causes of lactic acidaemia:
1) Severe acute hypoxia
2) Severe convulsions (respiratory arrest)
3) Strenuous exercise (dehydration) - Lactate >2mmol/L is a marker of concern – have to think is there any reason the patient might be in shock?
How does a normal ionogram differ with the addition of lactic acid?
- How a normal ionogram differs with the addition of lactic acid:
- Bicarbonate falls, as it has been used to buffer H+ ions
- There is also an addition of lactate in the unmeasured anions
What is another situation in which the body will increase production of acid?
How will this affect the anion gap?
What are 3 conditions this can occur in?
What will this lead to?
What can be a sign of ketoacidosis?
How can we test for ketones?
Describe the interpretation of ketones (in picture)?
- The body can increase production of acid in Situations of ↓ insulin & ↑ glucagon
- This will lead to an increased anion gap
- 3 conditions this can occur in:
1) Uncontrolled diabetes mellitus (severe, lifethreatening)
2) Alcoholic ketoacidosis (common clinically)
3) Starvation ketoacidosis (mild) - In these cases, it will lead to ketoacidosis
- A sign of ketoacidosis is when a patients breath smells like nail polish
- There is a point of care testing kit for ketones
- Interpretation of ketones (in picture)
How can we monitor the response to treatment in ketoacidosis?
What blood vessels should be used for this?
Why is this?
What is the venous blood range we look for (in photo)?
- The method of choice for monitoring response to ketoacidosis treatment is bedside measurement of capillary blood ketones
- Venous blood should be used rather than arterial (unless respiratory problems dictate otherwise) in blood gas analysers
- This is because it is less painful, easier and, carries less risk
- Venous blood range we look for (in photo)
What is the anion gap like in Gastrointestinal causes of metabolic acidosis?
Describe the 5 steps in the development of Gastrointestinal metabolic acidosis.
How does this affect the normal ionogram?
- Gastrointestinal causes of metabolic acidosis lead to a normal anion gap
- 5 steps in the development of Gastrointestinal metabolic acidosis:
1) Much of gut below pylorus secretes bicarbonate into gut lumen
2) For every bicarb ion into gut a H+ ion enters ECF
3) In diarrhoea, this process increases
4) This volume depletion can lead to the renin/angiotensin/aldosterone axis being stimulated, which leads to the retaining of chloride
5) This will result in metabolic acidaemia
- In the ionogram, this will lead to chloride being retained while bicarbonate falls
- There are no additional unmeasured anions (in picture)
How common is metabolic alkalosis? Why is this?
What 2 processes have to happen for metabolic alkalosis to occur?
- Metabolic alkalosis (alkalaemia is the Least common of the acid base disturbances
- This is because the kidney is very good at excreting excess bicarbonate
- 2 processes that have to happen for metabolic alkalosis to occur:
1) An initiating process
2) A maintaining process
Describe the maintenance of metabolic alkalosis flow chart
Why we must consider clinical context scenario
