Clinical Aspects of Acid Base Balance

Question 1

When do acid base disturbances occur?

Answer 1

• problem with ventilation
• problem with renal function
• overwhelming acid or base load body cannot handle

Question 2

Describe the step-wise approach to assessing arterial blood gases

Answer 2

• step 1: assess pO2 and oxygenation
• step 2: assess pH, acidaemia or alkalaemia?
• step 3: determine primary problem
• step 4: is compensation occuring

Question 3

Step 1: assessing pO2 and oxygenation

Answer 3

• use PaO2/FiO2 ratio (P/F ratio)
• if >50 = healthy
• if <40 = acute lung injury
• if <26.7 = acute respiratory distress syndrome

Question 4

What is a sign that compensation is occuring?

Answer 4

• if pCO2 and bicarbonate values are moving in the same direction
• if they are moving in the same direction suspect a mixed disorder

Question 5

What would you do if your patient has no arterial blood gas results available?

Answer 5

• using urea and electrolyte measurements from venous blood
• sum of routinely measured cations in venous blood minus routinely measured anions
• anion gap is early indicator of metabolic acidaemia

Question 6

What is the normal anion gap?

Answer 6

16

Question 7

What are common causes of acidic metabolic problems?

Answer 7

• overwhelming acid load
• increases anion gap
• from bodies own production (endogenous)
• ingestion (exogenous)
• failure of kidneys to excrete/regenerate bicarbonate

Question 8

What is the main way that the body’s endogenous production of acid cause metabolic problems?

Answer 8

• hypoperfusion conditions
• eg. whole body: shock (cardiogenic, septic, hypovolaemic, anaphylactic shock)
• eg. part of the body: femoral artery embolism
• results in increased anaerobic metabolism with subsequent increased production of lactic acid
  = lactic acidaemia
• uncontrolled diabetes mellitus
• alcoholic ketoacidosis
• starvation

Question 9

Describe the production of lactic acid

Answer 9

• product of anaerobic metabolism
• in healthy individuals it is metabolised in the liver and therefore there is no net production of it
• however metabolism requires oxygen
• therefore the production of it increases when O2 falls

Question 10

Other than hypoperfusion, what other causes are there of lactic acidaemia?

Answer 10

• severe acute hypoxia
• severe convulsions (resp arrest)
• strenuous exercise (dehydration)

Question 11

What point of care testing would you do for a patient with suspected DKA?

Answer 11

• check venous capillary blood ketones

- if over 3mmol/L then at risk and need urgent care

Question 12

What exogenous sources are there of acid?

Answer 12

• methanol (industrial solvent, windscreen wash)

- ethylene glycol (anti-freeze)

Question 13

What are some renal causes of metabolic acidosis?

Answer 13

• renal failure (acute and chronic which increases anion gap)
• renal tubular acidosis (normal gap)

Less commonly:

• diarrhoea
• renal tubular acidosis

Question 14

What are the GI causes of metabolic acidosis?

Answer 14

• normal anion gap
• much of gut below pylorus secretes bicarbonate into gut lumen
• for every bicarb ion into gut a H+ ion enters ECF
• diarrhoea and volume depletion
  (stimulates RAAS retaining chloride)

Question 15

What is the clinical presentation of metabolic acidosis?

Answer 15

Kussmaul respiration: laboured deep, rapid, pattern of breathing

Question 16

What are the 2 processes involved in metabolic alkalosis?

Answer 16

• an initiating process (loss of H+ ions from the gut above the pylorus through vomiting)
• a maintaining process (volume and chloride depletion activates RAAS, retains Na+ and water, forces kidneys to lose more potassium, takes in bicarbonate too)