Acid-base disturbances Flashcards
(25 cards)
What is acidosis?
An abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia
What is alkalosis?
An abnormal process with decrease in the hydrogen ion concentration, resulting in alkalaemia
What 3 things is a blood gas analysis useful for?
(1) Determining the adequacy of oxygenation and ventilation, (2)Assessing respiratory function, (3) Determining acid-base balance
pH: D PaCO2: N HCO3: D
Primary metabolic acidosis
pH: D PaCO2: D HCO3: D
Metabolic acidosis with respiratory compensation
pH: D PaCO2: I HCO3: N
Primary respiratory acidosis
pH: D PaCO2: I HCO3: I
Respiratory acidosis with renal compensation
pH: D PaCO2: I HCO3: D
Mixed metabolic and respiratory acidosis
pH: I PaCO2: D HCO3: N
Primary respiratory alkalosis
pH: I PaCO2: D HCO3: D
Respiratory alkalosis with renal compensation
pH: I PaCO2: N HCO3: I
Primary metabolic alkalosis
pH: I PaCO2: I HCO3: I
Metabolic alkalosis with respiratory alkalosis
pH: I PaCO2: D HCO3: I
Mixed metabolic and respiratory alkalosis
Describe the process of interpreting an ABG
- Determine to adequacy of oxygenation: Normal range 80-100mmHg, direct evidence of hypoxaemia, if PaO2 lower than expected determine if there is a raised A-a gradient d/t VQ mismatch/shunting
- Review the pH: Normal range 7.35 - 7.45
- Determine the respiratory component: Normal range 35-45mmHg. PaCO2 > 45mmHg indicates a primary respiratory acidosis. Alkalaemia indicated respiratory compensation for a metabolic alkalosis. PaCO2 < 35mmHg Alkalaemia indicates primary respiratory alkalosis, acidaemia indicates respiratory compensation for a metabolic acidosis.
- Determine the metabolic component: Normal HCO3 range 22-26 mmol/L. HCO3 < 22 mmol/L, acidaemia indicates primary metabolic acidosis, alkalaemia indicates renal compensation for resp alkalosis. HCO3 > 26 mmol/L, alkalaemia indicates primary metabolic alkalosis, acidaemia indicates renal compensation or a respiratory acidosis.
How do you calculate the A-a gradient? What is a normal A-a gradient?
Assuming 100% humidity at sea level, A-a gradient = ((FiO2 x (760-47))-(PaCO2/0.8)) - PaO2
Normal gradient is <(age/4) + 4
pH: N PaCO2: >45mmHg HCO3: >26mmol/L
Dual primary processes involving a primary resp acidosis and a primary metabolic alkalosis
pH: N PaCO2: <22mmol/L
Dual primary process involving a primary respiratory alkalosis and a primary metabolic acidosis
What is metabolic acidosis?
An abnormal process leading to the increase o fixed acid in the blood, best determined by a fall in plasma bicarbonate to less than 22 mmol/L
How do you calculate the anion gap?
AG = [Na+] - [Cl-] + [HCO3-] mmol/L
What is a normal anion gap?
8-16 mmol/L
What are some causes of high anion gap (AG > 16) metabolic acidosis?
(1) Increased acid production: Ketoacidosis (eg. diabetes, alcoholic, starvation), lactic acidosis (serum lactate > 2.5mmol/L) type A: impaired tissue perfusion in cardiac arrest, shock, hypoxia, sepsis, type B: impaired carbohydrate metabolism in hepatic or renal failure, lymphoma, pancreatitis, drugs such as metformin (2) Decreased acid excretion , as in renal failure (3) Exogenous acid ingestion (methanol, ethylene glycol, iron, cyanide and salicylates
What are causes of a normal anion gap metabolic acidosis?
(1) Renal (renal tubular acidosis, carbonic anhydrase inhibitors), (2) Gastrointestinal (severe diarrhoea, small bowel fistula, drainage of pancreatic or biliary secretions) (3) Other (administration of synthetic amino acid solutions, ammonium chloride, recovery from ketoacidosis)
How do you predict the expected compensatory change in PaCO2 in metabolic acidosis?
1.5 x [HCO3] + 8mmHg (+/-2) mmHg
How do you predict the expected compensatory change in PaCO2 in metabolic alkalosis?
0.7 x [HCO3] + 20mmHg (+/- 5)
