Lecture 28: Acid-base homeostasis 2 Flashcards
How do we determine the acid-base status of a patient?
Gases (pCO2, pO2)
Metabolites (Glucose, Lactate)
Electrolytes (Sodium, Potassium, Chloride, Calcium)
pH
Actual bicarbonate
Co-oximetry (Total Hb, O2 saturation, OxyHb, COHb, MetHb)
Derived parameters (Base excess, Standard bicarbonate, Anion gap)
Give 3 derived parameters used in investigating acid-base homeostasis:
1) standard bicarbonate
2) base excess
3) anion gap
What is standard bicarbonate?
a calculated hypothetical value that predicts the expected bicarbonate concentration in a blood sample if the pCO2 is normal
What assumption does the calculation of standard bicarbonate make?
there are no contributions from any respiratory disturbance
What does an abnormal standard bicarbonate tell us?
there is a metabolic component to the disorder
What is base excess?
a calculated parameter telling us the amount of acid or alkali required to titrate blood pH to 7.40
What is anion gap?
the difference between the sum of measured anions and cations
What does an increased anion gap indicate?
there are significant amounts of unmeasured anions present e.g ketones, lactate, salicylate
How does having air bubbles in blood gas samples affect readings? (3)
1) pO2 is affected
2) pH increases
3) pCO2 decreases
Does respiratory acidosis present with high or low pCO2?
high
Does respiratory alkalosis present with high or low pCO2?
Low
Does metabolic acidosis present with high or low HCO3-?
Low
Does metabolic alkalosis present with high or low HCO3-?
high
Give 2 key general symptoms of acidosis:
1) shortness of breath
2) coughing
Give 2 key general symptoms of alkalosis:
1) hand tremor
2) numbness or tingling in the face, hands or feat
Does metabolic acidosis present with high or low pCO2?
normal pCO2 but low if there is compensation
Does metabolic alkalosis present with high or low pCO2?
normal pCO2 if there is compensation
Give three causes of metabolic acidosis:
1) increased acid formation (ketoacidosis, lactic acidosis, poisoning)
2) decreased acid excretion (uraemia/ renal failure, renal tubular acidosis type 1)
3) loss of bicarbonate (diarrhoea, fistula, renal tubular acidosis type 2)
What two causes of metabolic acidosis are associated with an increased anion gap?
1) increased acid formation
2) decreased acid excretion
Give two compensation mechanisms used in metabolic acidosis:
1) hyperventilation to blow off CO2
2) increased rate of bicarbonate regeneration
Give 4 symptoms of metabolic acidosis:
1) nausea
2) vomiting
3) anorexia
4) Kussmaul breathing
What is Kussmaul breathing?
rapid, deep, laboured breathing
What drug is used to manage metabolic acidosis?
sodium bicarbonate (if pH is under 7)
Give 3 causes of metabolic alkalosis:
1) loss of H+ through vomiting
2) administration of bicarbonate (iatrogenic)
3) potassium depletion
How can potassium depletion cause metabolic alkalosis? (2)
1) in the kidneys, excretion of H+ takes place to spare K+ at aldosterone-controlled renal transporters
2) in red blood cells, K+ is transported out of the cell and H+ is transported in to maintain electroneutrality, decreasing [H+]
Give one way that the body responds (compensates) to metabolic alkalosis:
reduced respiratory rate to retain CO2
How is metabolic alkalosis managed?
IV fluids (normal saline)
Give two causes respiratory acidosis?
1) defective respiratory function (COPD, asthma, pneumothorax)
2) defective control of respiration either by CNS depression or CNS disease
How do the kidneys respond to respiratory alkalosis?
they maximise bicarbonate regeneration and maximise H+ excretion
Give 3 causes of respiratory alkalosis:
1) pulmonary issues (PEs, pneumonia, asthma)
2) central issues (head injury, stroke, anxiety hyperventilation)
3) iatrogenic causes (excessive mechanical ventilation)
How do the kidneys compensate during respiratory alkalosis?
they decrease renal regeneration of bicarbonate
Give a key symptom of SEVERE respiratory alkalosis:
increased protein binding of Ca2+ leads to hypocalcaemia (headaches, lethargy, delirium, seizures)
Give 2 management techniques for respiratory alkalosis:
1) paper bag rebreathing
2) sedation and mechanical ventilation
Give the three crucial physiological pathways the kidney facilitates in acid-base homeostasis:
1) reabsorption of bicarbonate in the proximal tubule
2) excretion of H+ in the distal tubule
3) regeneration of bicarbonate in the distal tubule
