Acid-base homeostasis - respiratory Flashcards
An acid is defined as
any chemical substance that can donate a proton, H+
A base (alkali) is defined as
any chemical substance that can accept a proton, H+
pH =
-log10 [H+]
pKa is defined as
the pH at which 50% is ionised and 50% is unionised in the reaction
The pKa for carbonic acid/bicarbonate is
6.1
Normal pH is
7.4
The absolute levels of bicarbonate can be changed by changes to breathing - Increased CO2 leads to
more H2CO3 and vice versa
acidosis
pH < 7.35
alkalosis
pH >7.45
Causes of Acid-Base Disturbances
– Increased CO2
– Decreased CO2
– Increased non-volatile acid/decreased base
– Increased base/decreased non-volatile acid
respiratory disorders
Where primary change is to the CO2 levels
metabolic disorders
Where primary change is to bicarbonate levels
• An acidosis can be caused by:
– Rise in PCO2
– Fall in HCO3-
• An alkalosis can be caused by:
– Fall in PCO2
– Rise in HCO3-
Respiratory Acidosis, Results from an increase in PCO2 caused by:
– Hypoventilation (less CO2 being blown off)
– Ventilation-perfusion mismatch
– Reduced lung diffusing capacity
• From Henderson-Hasselbalch equation, an increase in PCO2 causes
an increase in H+, so a lowering of pH, Thus, plasma HCO - levels increase to compensate for increased H+ concentration 3
• Renal compensation in respiratory acidosis
increased HCO - reabsorption and increased HCO - 33
production – raises pH towards normal
• Causes of respiratory acidosis
– COPD – Blocked airway – foreign body or tumour – Lung collapse – Injury to chest wall – Drugs reducing respiratory drive, eg morphine, barbiturates, general anaesthetics
respiratory alkalosis
• Results from a decrease in PCO2 generally caused by
– alveolar hyperventilation (more CO2 being blown off)
This causes a decrease in H+ concentration and thus a rise in pH
renal compensation in respiratory alkalosis
– reduced HCO - reabsorption, and reduced HCO - production 33
– Thus plasma HCO - levels fall, compensating for lower H+, moving pH back 3
towards normal
• Causes of respiratory alkalosis
– Increased ventilation, from hypoxic drive in pneumonia, diffuse interstitial lung diseases, high
altitude, mechanical ventilation
– Hyperventilation – brainstem damage, infection driving fever
Metabolic Acidosis
- Results from an excess of H+ in the body,
- This reduces HCO3 levels (shifts equation to the left)
- Addition of acid decreases pH, ventilation unaffected so PCO2 initially normal
respiratory compensation in metabolic acidosis
– the lower pH is detected by peripheral chemoreceptors, causes an increase in ventilation which lowers PCO2
– the bicarbonate equation is driven further to the left, lowering H+ and HCO - concentration further 3
– The decrease in H+ concentration moves pH towards normal
– Respiratory compensation cannot fully correct the pH, HCO3 and H+, so excess H+ needs to be
removed or HCO - restored (by slow renal comp)
causes of metabolic acidosis
– Loss of HCO -, eg from gut in diarrhoea
3
– exogenous acid overloading (aspirin overdose)
– endogenous acid production (ketogenesis)
– Failure to secrete H+, eg in renal failure
Metabolic Alkalosis
- Results from an increase in HCO - concentration or a fall in H+ 3
- Removing H+ from equation drives reaction to right, increases HCO - 3
- Lowering of H+ raises pH, with PCO2 initially normal
Respiratory compensation in metabolic alkalosis
– increase in pH detected by peripheral chemoreceptors – decreases ventilation which raises PCO2
– the equation is driven further to right, increasing H+ and HCO - 3
– Increase in H+ moves pH towards normal
– Respiratory compensation is often small (or even absent) – ventilation cannot reduce enough to
correct imbalance
– Renal response is to secrete less H+
causes of metabolic alkalosis
– Vomiting - loss of HCl from stomach
– Ingestion of alkali substances
– Potassium depletion (eg diuretics)
The Acid-Base Nomogram
ABGs can be analysed using the acid–base nomogram
• By plotting the PaCO2 and H+/pH values on the ABG nomogram, most ABGs can be analysed
• If the plotted point lies outside the designated areas, this implies a mixed disturbance
