Lecture 5: Acid-base disturbances in poisoning Flashcards
What is the normal plasma pH
7.36 - 7.46
What organs regulate acid-base disturbances
Kidneys and lungs
How is acid-base balance maintained
removal of acids accumulated from normal metabolism
What is used as a buffer in ECF to neutralise metabolically produced acids
Bicarbonate ions
How is acid lost from the body
- exhalation of CO2
- renal tubular H+ secretion
What causes disturbances in acid-base balance
Altered respiratory or metabolic processes
Metabolic acidosis effect
- Decreased pH
- Decreased ECF [HCO3-]
Metabolic alkalosis effect
- Increased pH
- Increased ECF [HCO3-]
Respiratory acidosis effect
- Decreased pH
- Increased pCO2
Respiratory alkalosis effect
- Increased pH
- Decreased pCO2
What are the measurements seen in Metabolic acidosis
- pH <7.36 (below physiological pH)
- Plasma [HCO3-] <21mmol/L
- low pCO2
Why is low pCO2 seen in metabolic acidosis
Patients are excessively breathing to remove XS CO2
(compensatory hyperventilation)
Anion gap equation
Anion gap = Sum measured cations - Sum measured anions
What does the anion gap show
Lactate and anions in plasma that are not normally measured
Why does the anion gap increase in metabolic acidosis
Due to a loss of bicarbonate, not accumulation of acid
What is renal tubular acidosis
urinary loss of bicarbonate when renal tubules are damaged
How is renal tubular acidosis compensated for by the body
retention of chloride ions to retain tubular electroneutrality
Two types of lactic acidosis
Type A - hypoxic
Type B - non-hypoxic
How does lactic acidosis occur
renal failure due to retention of acid from protein metabolism as kidneys cannot clear them
When does Type A lactic acidosis occur
When hypoxia causes impaired oxidative metabolism of pyruvate
In absence of O2, lactate converted to pyruvate to allow anaerobic respiration
What are the causes of Type A lactic acidosis (3)
- Cardiorespiratory depression
- Repeated convulsions
- Impaired O2-carrying capacity of blood
What causes Type B lactic acidosis (2)
- When normal oxidative metabolism of pyruvate cannot occur due to inhibition of mitochondrial respiratory chain
- Liver failure so lactate cannot be processed leading to accumulation
What measurements are seen in Metabolic alkalosis
- pH >7.46
- plasma bicarbonate conc. >35 mmol/L
How does metabolic alkalosis occur in poisoning
occurs due to severe volume depletion and arises as part of the renal compensatory mechanism
What measurements are seen in respiratory acidosis
- pH <7.36
- pCO2 > 6KPa = retention of CO2
- normal plasma bicarbonate
What is the normal plasma bicarbonate range
21 - 25 mmol/L
What normally causes respiratory acidosis
hypoventilation due to poisoning with drug that reduces CNS drive of ventilation
What measurements are seen in respiratory alkalosis
- pH >7.46
- pCO2 < 4.5 KPa = low CO2
- normal plasma bicarbonate
What causes respiratory alkalosis
Poisoning with drugs that stimulate CNS respiratory centre
What two conditions show in aspirin poisoning on an ABG and why
Respiratory alkalosis = stimulation of respiratory centre
Metabolic acidosis = accumulation of salicylic acid
