Respiratory Acid-Base Balance Flashcards
Learning outcomes
• Describe how PCO2 affects blood pH
• Calculate blood pH from given values
• Draw a Davenport diagram to indicate how pH and [HCO -] alter with changes to 3
PCO2 and changes to non-volatile acid or base
• List the primary causes of acid-base disturbances
• Describe, with the use of a Davenport diagram, respiratory acidosis and its compensation
• Describe, with the use of a Davenport diagram, respiratory alkalosis and its compensation
• Describe, with the use of a Davenport diagram, metabolic acidosis and its compensation
• Describe, with the use of a Davenport diagram, metabolic alkalosis and its compensation
Discuss pH and Buffers
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+
Because H+ concentration can vary over a large range in solutions, the pH scale was created
pH = -log10 [H+]
So a [H+] of 10-7M = pH 7.0
How do you calculate plasma pH?
• Defined by Henderson-Hasselbalch equation, so
pH = pKa + log([HCO -] OVER [CO2])
where, bicarbonate is in mmol/L (mM)
and [CO2] is calculated as PCO2 x solubility constant
What is pKa
The dissociation constant
• pKa is defined as the pH at which 50% is ionised and 50% is unionised in the reaction
• For bicarbonate this would be:
H2CO3 HCO3- + H+
At a ratio of 1:1 but this can shift if:
If H+ rises, the equation is driven to the left If H+ falls, the equation is driven to the right
Discuss the normal plasma pH, pKa and shit
• The pKa for carbonic acid/bicarbonate is 6.1
• Normal pH is 7.4
• At pH = 7.4 is there more H CO or more HCO - ?
- More H2CO3 as less H+ - pH 7.4
The effect of the respiratory system on acid-base balance
• The absolute levels of bicarbonate can be changed by changes to breathing
H2O + CO2 → H2CO3
• Increased CO2 leads to more H2CO3 and vice versa
Discuss the difference between the impact of resp and metabolic acid-base disturbances on H2CO3 levels
Resp will impact CO2 + H2O
Metabolic will effect H+ + HCO3-
In
CO2 + H2O H2CO3 H+ + HCO3-
Define acidosis and alkalosis
- pH < 7.35 acidosis
* pH >7.45 alkalosis
What are some causes of acid-base disturbances in plasma
– Increased CO2
– Decreased CO2
– Increased non-volatile acid/decreased base
– Increased base/decreased non-volatile acid
• Where primary change is to the CO2 levels - respiratory disorders
• Where primary change is to bicarbonate levels - metabolic disorders
• An acidosis can be caused by:
– Rise in PCO2
– Fall in HCO3-
• An alkalosis can be caused by:
– Fall in PCO2
– Rise in HCO3-
Discuss 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 HCO3- levels increase to compensate for increased H+ concentration
• Renal compensation – increased HCO3- reabsorption and increased HCO3-
production – raises pH towards normal
• Causes
– COPD
– Blocked airway – foreign body or tumour
– Lung collapse
– Injury to chest wall
– Drugs reducing respiratory drive, eg morphine, barbiturates, general anaesthetics
What is a graphical tool to interpret acid-base issues
Davenport diagram (have a look)
Discuss 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
– reduced HCO3- reabsorption, and reduced HCO3- production
– Thus plasma HCO3- levels fall, compensating for lower H+, moving pH back
towards normal
• Causes
– Increased ventilation, from hypoxic drive in pneumonia, diffuse interstitial lung diseases, high
altitude, mechanical ventilation
– Hyperventilation – brainstem damage, infection driving fever
Discuss 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
– 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 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)
• Causes
– 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
Discuss 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
• Respiratory compensation
– 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+
• Causes
– Vomiting - loss of HCl from stomach
– Ingestion of alkali substances
– Potassium depletion (eg diuretics)
Discuss 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
