Acutely ill patient - acid/base balance

Question 1

Which organs are involved in acid base balance?

Answer 1

Kidneys, lungs and liver

Question 2

What do the kidneys lungs and liver do in health?

Answer 2

Kidneys - remove acid, regenerate bicarb

Lungs - regulate the removal of acid via CO2 by varying the RR

Liver - removes and recycles lactate

Question 3

What determines blood pH?

Answer 3

Ratio of HCO3- to CO2

Question 4

What does the kidney buffering system control?

Answer 4

Protons and bicarb excretion or reabsorption

The conversion of ammonia to ammonium

Question 5

What does the lung buffering system control?

Answer 5

Carbon dioxide in the blood

Question 6

What diseases increase the risk of acid-base balance?

Answer 6

CKD (eGFR<30), resp disease and liver disease.

Question 7

What is a dangerously high/low pH?

Answer 7

High >7.6

Low <7.1

Question 8

What is the Schwartz-Bartter approach?

Answer 8

•Accepts the Bronsted-Lowry definition of
–acids as proton donors
–bases as proton acceptors
–approach utilises the anion gap calculation to classify acid-base disturbances
•The hydrogen ion concentration is a function of the ratio between the PCO2 and the serum bicarbonate

Question 9

What is the Stewart approach?

Answer 9

• Termed strong ion difference, based on principle that the serum bicarb does not alter pH.

Favoured by intensivists and anaesthetists

Question 10

What is the clinical presentation of an acid-base disturbance?

Answer 10

•acutely ill patients consider whether there may be an underlying acid-base disturbance
•Serum bicarbonate and chloride are not standard components of all U&E reports
•Severe acidaemia (pH <7.1)
–cardiac function and vascular tone
•Severe alkalaemia (pH >7.6)
–irritability of cardiac and skeletal muscle

Question 11

What conditions are associated with acid-base disorders?

Answer 11

•vomiting/diarrhoea
• shock
– cardiogenic
– septic
• hypovolaemia
• acute kidney injury
• respiratory failure
• altered neurological status
– coma
– seizures
• decompensated diabetes
• prolonged and excessive infusions of 0.9% sodium chloride

Question 12

What investigations should you perform if you suspect an acid-base imbalance?

Answer 12

Urea, creatinine and electrolytes. Bicarb, cholride and ABG (inc lactate).

Question 13

What is the step-by-step pathway to identify the cause?

Answer 13

•pH to determine whether
–acidaemia or
–alkalaemia
•Change in bicarbonate and base excess = metabolic process
•Change in PCO2 = respiratory process

•Determine whether
–simple disorder i.e. either metabolic or respiratory process alone
•Evidence of compensatory changes in either bicarbonate or PCO2
– mixed disorder i.e. a combination of a metabolic and respiratory process occurring together

Question 14

What is the effect of metabolic acidosis on bicarb and what is the compensation?

Answer 14

Effect: low bicarb

Compensation: lower PCO2

Question 15

What is the effect of metabolic alkalosis on bicarb and what is the compensation?

Answer 15

Effect: High bicarb

Compensation: Raised PCO2

Question 16

What is the effect of respiratory acidosis on bicarb and what is the compensation?

Answer 16

Effect: high PCO2

Compensation: High bicarb

Question 17

What is the effect of respiratory alkalosis on bicarb and what is the compensation?

Answer 17

Effect: Low PCO2

Compensation: Low bicarb

Question 18

What is the anion gap?

Answer 18

•determined by negative charge on serum proteins
–particularly albumin

• anion gap = unmeasured anions – unmeasured cations

[Na+] – ([HCO3–] + [Cl–])

• normal anion gap = 5-11 mmol/l

Question 19

What does an increased anion gap indicate?

Answer 19

Acidosis

Question 20

What does a decreased anion gap indicate?

Answer 20

Alkalosis.

Question 21

What are the causes of high anion gap acidosis?

Methanol

Uraemia

Diabetes

Paraldehyde

Alcohol

Lactate

Ethelyne glycol

Salicylate

Answer 21

Ketoacidosis, lactic, drug toxicity, kidney disease

Question 22

What is ethylene glycol toxicity?

Answer 22

Metabolism generates glycolate - toxic to tissues. Elevated osmolal gap >25. Calcium oxalate crystals in urine indicative.

Treatment: alcohol infusion - inhibit alcohol dehydrogenase, fomepizole - inhibit alcohol dehydrogenase, dialysis

Question 23

What is normal anion gap acidosis (hyperchloraemic)?

Answer 23

GI bicarb loss, renal bicarb loss and infusion of 0.9% saline

Question 24

What is the managment of acid-base balance problems?

Answer 24

•Correcting
–fluid and electrolyte balance (e.g. hypovolaemia, salt and water deficit
–underlying cause for the acid-base disorder (ketoacidosis, acute kidney injury, sepsis)
•specific therapy to correct the HCO3– or PCO2
–if the acid-base disorder is affecting organ function or
–pH is <7.1 or >7.6