L10 - Acid-Base Balance Flashcards
What is the relationship between [H+] and pH?
pH is the negative log of hydrogen ion concentration
What concentrations of H+ are incompatible with life?
> 120nmol/L and <20nmol/L
What is the major way hydrogen ions are produced in the body?
Oxidation of ingested amino acids, as they contain sulphur
What is acidosis and alkalosis, in terms of pH and [H+]?
Acidosis - high [H+], low pH
Alkalosis - low [H+], high pH
How are excess H+ ions dealt with?
Excreted in urine or temporarily removed via buffers
What are the three main parameters measured to assess acid-base balance?
[H+] - hydrogen concentration
[HCO3] - bicarbonate
PCO2 - partial pressure of carbon dioxide
What are the main buffer systems?
What is the most important in the ECF?
Bicarbonate, proteins (e.g. haemoglobins), phosphate system
Bicarbonate is the main system
What three factors will cause [H+] to increase?
Adding H+, removing bicarbonate, increasing PCO2
What three factors will cause [H+] to decrease?
Removing H+, adding bicarbonate, decreasing PCO2
What 2 organs are responsible for maintaining acid-base balance?
How are they able to do this?
Kidneys - excrete H+, reabsorb bicarbonate
Lungs - remove CO2
In metabolic acidosis and alkalosis, what is the main organ involved?
Kidneys
In metabolic acidosis and alkalosis, what is the main test parameter involved?
How are the levels of this affected in each condition?
Bicarbonate (HCO3)
Decreases in metabolic acidosis
Increases in metabolic alkalosis
In respiratory acidosis and alkalosis, what is the main organ involved?
Lungs
In respiratory acidosis and alkalosis, what is the main test parameter involved?
How are the levels of this affected in each condition?
Partial pressure of CO2 (PCO2)
Increases in respiratory acidosis
Decreases in respiratory alkalosis
What is renal compensation?
When lung function is compromised, kidneys increase or decrease the amount of H+ excreted, increasing/lowering bicarbonate levels
What is respiratory compensation?
When renal function is compromised, the lungs can remove CO2 through hyperventilation, or reduce loss through hypoventilation
How does the bicarbonate buffer system control acid-base balance, and what enzyme is involved in this process?
H+ associates with bicarbonate to form carbonic acid (H2CO3). Carbonic anhydrase aids in the breakdown of carbonic acid to CO2 and water
Which is faster to take effect, respiratory or renal compensation
Respiratory compensation
Define complete compensation
When the compensation mechanism returns [H+] to within reference range, but the PCO2 and bicarbonate are abnormal
Define partial compensation
When compensation has occurred, but the [H+] has not returned to within reference range
What is metabolic acidosis?
Accumulation of H+, resulting in decreased bicarbonate
Give three causes of metabolic acidosis
Renal disease
Diabetic ketoacidosis
Lactic acidosis
What is the anion gap and how is it calculated?
The anion gap is an estimation of the concentration of unmeasured ions; it is the difference between the anions and the cations
[Na+] - [(Cl-) + (HCO3-)]
What is knowledge of the anion gap useful for?
Finding out the cause of a metabolic acidosis when it is not apparent from clinical history
In chemical terms, a normal anion gap in metabolic acidosis is due to…
Chloride substituting for the lost bicarbonate, maintaining electrochemical balance
What can a normal anion gap in metabolic acidosis indicate?
Bicarbonate is being lost e.g. diarrhoea
In chemical terms, a raised anion gap in metabolic acidosis is due to…
Acids (anions) being produced which replace the bicarbonate being used up to buffer the H+ ions
What can a raised anion gap in metabolic acidosis indicate?
There is excess production of organic acids e.g. lactic acid in lactic acidosis, ketoacids in diabetic ketoacidosis
What type of compensation can be seen in metabolic acidosis?
Hyperventilation, to lower PCO2
Kussmaul breathing is the type of respiratory pattern seen (deep, rapid, gasping breaths)
What can occur as a result of metabolic acidosis?
Increased neuromuscular irritability
Arrhythmias progressing to cardiac arrest (more likely if there is also hyperkalaemia)
Depression of consciousness can progress to coma and death
What acid-base disorder can result in hyperkalaemia?
Metabolic acidosis
What acid-base disorder can result in hypokalaemia?
Metabolic alkalosis
What is metabolic alkalosis?
A decrease in [H+] resulting in elevated bicarbonate levels
Give three causes of metabolic alkalosis
Loss of H+ through vomiting
Absorbable alkali ingestion e.g. sodium bicarbonate
Potassium deficiency
What cause of metabolic alkalosis results in acidic urine?
How does this occur?
Potassium deficiency - H+, rather than potassium, are exchanged for reabsorption of Na in kidneys
What can occur as a result of metabolic alkalosis?
Confusion and coma
Decrease in unbound calcium - causes muscle cramps, tetany (muscle spasms) and paraesthesia (pins and needles)
Which acid base disorder is associated with hypocalcaemia?
Metabolic alkalosis
What is respiratory acidosis?
H+ ion excess due to insufficient gas exchange, resulting in increased PCO2
What type of compensation can be seen in metabolic alkalosis?
Hypoventilation
What type of compensation can be seen in respiratory acidosis?
Kidney excretes more H+ and reabsorbs bicarbonate
Only in chronic as it is slow to develop
What can cause acute respiratory acidosis?
Choking
Bronchopneumonia
Acute exacerbation of asthma
What can cause chronic respiratory acidosis?
Chronic bronchitis
Emphysema
COAD/COPD
What is respiratory alkalosis?
Decrease in [H+] due to excessive gas exchange, resulting in lowered PCO2
What type of compensation can be seen in respiratory alkalosis?
Kidney excretes less H+ and doesn’t absorb as much bicarbonate
This is only if the cause is chronic (usually acute)
Give three causes of respiratory alkalosis?
Hysteric hyperventilation
Raised intracranial pressure
Hypoxia (lack of oxygen)
What is a mixed acid-base disorder?
A combination of more than one disorder
In terms of mixed acid-base disorders, why must care be taken if the patient displays signs of compensation?
The patient may have antagonistic disorders, where one disorder can mimic the compensatory response
Give an example of antagonistic mixed acid-base disorders, where one mimics the compensatory response
COPD (respiratory acidosis) causing an increase in PCO2, along-side thiazide induced potassium deficiency (metabolic alkalosis) causing an increase in bicarbonate
What is PO2 and what does it show?
Partial pressure of oxygen (PO2) is proportional to the total unbound oxygen in blood
What is oxygen saturation?
Percentage of haemoglobin that is bound to oxygen
Define ventilation
The mechanical process of moving air into and out of the respiratory tract
What is respiratory failure?
Blood PO2 < 8kPa in patient breathing room air at rest
What is seen in type 1 respiratory failure?
Hypoxia with CO2 retention
What is seen type 2 respiratory failure?
Hypoxia without CO2 retention (normal levels)
What contributes to type 1 respiratory failure?
Impaired diffusion and some ventilation / perfusion imbalances e.g. lobar pneumonia
What contributes to type 2 respiratory failure?
Some ventilation/perfusion imbalances e.g. chronic bronchitis or bronchial pneumonia
What results in impaired diffusion?
Fluid (oedema) or thickened alveolar walls (fibrosis) inhibit oxygen diffusion
CO2 is less affected so may be within normal range
What occurs in lobar pneumonia to contribute to a ventilation/perfusion imbalance?
Some blood in the lungs does not reach functional alveoli; this increase in CO2 stimulates hyperventilation, making the functional alveoli work harder
