Respiratory Acid-Base balance

complete - not well understood - need to look at Davenport diagram

what is an acid defined as?

•any chemical substance that can donate a proton, H⁺

what is a base defined as?

•any chemical substance that can accept a proton, H⁺

what is the calculation for pH?

pH = -log₁₀ [H⁺]

what equation defined plasma pH?

Henderson-Hasselbalch equation

what is the equation for plasma pH?

pH = pKa + log₁₀ [HCO₃^-]

        [CO₂]

how is plasma [CO2] calculated?

PCO2 x solubility constant

what is pKa defined as?

the pH at which 50% is ionised and 50% is unionised in the reaction

how does an increase in H+ affect this reaction? H₂CO₃  ↔  HCO₃^-  +  H⁺

shifts it to the left

how does an decrease in H+ affect this reaction? H₂CO₃  ↔  HCO₃^-  +  H⁺

the equation is driven to the right

what is the pKa for carbonic acid/bicarbonate?

6.1

what is the is the normal plasma pH?

7.4

what can the absolute levels of bicarbonate be changed by?

changes to breathing

what does increased CO2 lead to for bicarbonate?

increased CO2 leads to more H2CO3

what does increased pH lead to for levels of HCO3^-?

increase

what pH indicates acidosis?

<7.35

what pH indicates alkalosis?

>7.45

what are causes of acid-base disturbances?

•Increased CO₂

•Decreased CO₂

•Increased non-volatile acid/decreased base

•Increased base/decreased non-volatile acid

what are the expected values for simple respiratory acidosis?

reduced plasma pH, increased PCO₂ , and increased plasma HCO ₃ ⁻ concentration after partial renal compensation.

what are the expected values for simple metabolic acidosis?

low pH, a low plasma HCO₃⁻ concentration, and a reduction in PCO₂ after partial respiratory compensation.

what is the cause when the primary change is to CO2 levels?

respiratory disorders

what is the cause when the primary change is to bicarbonate levels?

metabolic disorders

what can acidosis can be caused by?

•Rise in PCO₂

•Fall in HCO₃^-

what can alkalosis can be caused by?

•Fall in PCO₂

•Rise in HCO₃^-

what could mechanisms can cause an increase in PCO2?

•Hypoventilation (less CO₂ being blown off)

•Ventilation-perfusion mismatch

•Reduced lung diffusing capacity

what happens to HCO3^- levels when there is an increase in PCO2?

an increase in PCO₂ causes an increase in H⁺, so a lowering of pH, plasma HCO₃^- levels increase to compensate for increased H⁺concentration

what is the renal compensation in response to respiratory acidosis?

increased HCO₃^- reabsorption and increased HCO₃^- production – raises pH towards normal

what are the 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

what could mechanisms can cause an decrease in PCO2?

alveolar hyperventilation (more CO₂ being blown off)

what is renal compensation in response to respiratory alkalosis?

• reduced HCO₃^- reabsorption

• reduced HCO₃^- production

• plasma HCO₃^- levels fall compensating for lower H⁺, moving pH back towards normal

what can cause respiratory alkalosis?

•Increased ventilation, from hypoxic drive in pneumonia, diffuse interstitial lung diseases, high altitude, mechanical ventilation

•Hyperventilation – brainstem damage, infection driving fever

what causes and results from 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

what is the respiratory compensation to metabolic acidosis?

• lower pH is detected by peripheral chemoreceptors, causes an increase in ventilation which lowers PCO₂

• bicarbonate equation is driven further to the left, lowering H⁺ and HCO₃^-concentration further

•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 HCO₃^- restored (by slow renal comp)

what are the causes of metabolic acidosis?

•Loss of HCO₃^-, eg from gut in diarrhoea

•exogenous acid overloading (aspirin overdose)

•endogenous acid production (ketogenesis)

•Failure to secrete H⁺, eg in renal failure

what causes and results of metabolic alkalosis?

•Results from an increase in HCO₃^- concentration or a fall in H⁺

•Removing H⁺ from equation drives reaction to right, increases HCO₃^-

•Lowering of H⁺ raises pH, with PCO₂ initially normal

what is the respiratory compensation for metabolic alkalosis?

•increase in pH detected by peripheral chemoreceptors – decreases ventilation which raises PCO₂

•the equation is driven further to right, increasing H⁺ and HCO₃^-

•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⁺

what are the causes of metabolic alkalosis?

•Vomiting - loss of HCl from stomach

•Ingestion of alkali substances

•Potassium depletion (eg diuretics)

how can acid-base gradients be analysed with the acid-base nomogram?

by plotting the PaCO2 and H+/pH values on the nomogram

what does it mean if the plotted point lies outside the designated areas on the acid base nomogram?

it implies a mixed disturbance