Flashcards in Acid Base Balance II Deck (35):
If renal or respiratory function is abnormal, or any acid or base load overwhelms the body, a change in pH occurs.
What are these individually called?
Decreased pH = Acidosis
Increased pH = alkalosis
Respiratory disorders effect what in the acid base balance?
Renal disorders effect what in the acid base balance?
What is respiratory acidosis?
pH has fallen and it is due to a respiratory change, so Pco2 must have increased.
Respiratory acidosis results from reduced ventilation and therefore retention of CO2.
What are the acute causes of respiratory acidosis?
Drugs which depress the medullay respiratory centres, such as barbiturates and opiates.
Obstructions of major airways
what are the chronic causes of respiratory acidosis?
What is the bodies response to respiratory acidosis?
Need to protect pH so need to increase bicarbonate to keep Pco2/[HCO3-] ratio.
The increase in Pco2 will increase secretion of H+ and increase HCO3-
Acid conditions stimulate renal glutaminase so get more NH3 produced, BUT it takes time
So there is incraesed generation of new HCO3- as well as increased reabsorption, because having generated more HCO3- the increased Pco2 will also increase the ability to reabsorb it
What is the downside to renal compensation of respiratory acidosis?
Increased HCO3- protects the pH, it does not correct the original disturbance.
ONLY restoration of normal ventilation can remove the primary disturbance.
This means that in chronic respiratory acidosis blood gas values are never normalised. The underlysing disease process prevents the correction of ventilation, but because the kidney maintains high [HCO3-] the pH is protected.
What is respiratory alkalosis?
Alkalosis of respiratory origin so must be due to a fall in Pco2 and this can only occur through increased ventilation and CO2 blow off.
What are the causes of acute respiratory alkalosis?
First ascent to altitude
What are the chronic causes of respiratory alkalosis?
Long term residense at altitude.
Decreased Po2 to
How is pH protected in respiratory alkalosis?
[HCO3-] should decrease.
Alkaline conditions are dealt with by the HCO3- reabsorptive mechanism.
If decreased Pco2, less H+ is available for secretion, therefore less of the filtered load of HCO3- is reabsorbed so HCO3- is lost in the urine.
(Ventilation must be normalised to correct the disturbance)
What is metabolic acidosis?
An acidosis of metabolic origin must be due to a decreased bicarbonate.
So decreased [HCO3-], either due to increased buffering of H+ or direct loss of HCO3-.
To protect the pH, Pco2 must be decreased
What are the causes of metabolic acidosis?
1. Increased H+ production, as in ketoacidosis of a diabetic or in lactic acidosis
2. Failure to excrete the normal dietary load of H+ as in renal failure
3. Loss of HCO3- as in diarrhoea
i.e. failure to reabsorb intestinal HCO3-
What effect does metabolic acidosis cause?
Stimulates ventilation so that Pco2 falls.
The increase in ventilation is in depth rather than rate, may be very striking, reaching a max 30l/min compared to normal 5-6l/min when the arterial pH falls to 7.
This degree of hyperventilation = Kussmaul breathing = an established clinical sign of renal failure or diabetic ketoacidosis. Very serious.
What problem does respiratory compensation of metabolic acidosis cause?
Normally the kdineys correct the disturbance by restoring [HCO3-] and getting rid of H+ ions
The source of H+ ions is the CO2 in the peritubular capillaries but respiratory compensation lowers the Pco2 to protect the pH.
This lowers the driving force for renal compensation through renal glutaminase.
Respiratory compensation DELAYS the renal correction, BUT PROTECTS THE pH, much more important
Why do compensatory mechanisms not restore pH exactly back to original?
Complete compensation would remove the drive to correct the original disturbance. Survival value of this is that if there were no pressure to correct initial disturbance, a further perturbation may push the system so far that compensation can no longer be effective
How does renal compensation in metabolic acidosis take place when respiratory compensation has taken place?
Decreased Pco2 due to respiratory compensation.
Lowers H+ but also lowers bicarbonate even more (lowered bicarbonate original problem).
Less H+ needed for bicarbonate reabsorption as less bicarbonate will be filtered.
More H+ available for excretion through NH4+
What is metabolic alkalosis?
[HCO3-] must have increased and Pco2 will have to increase to protect the pH
What causes metabolic alkalosis?
1. Increased H+ ion loss -> vomiting loss of gastric secretions
2. Increased renal H+ loss
-> Aldosterone excess, excess liquorice ingestion
3. Excess administration of HCO3- is unlikely to produce a metabolic alkalsis in subjects with normal renal function, but may do so if renal function impaired.
4. Massive blood transfusions
How can massive blood transfusions cause metabolic alkalosis?
Can lead to metabolic alkalosis because bank blood contains citrate to prevent coagulation, which is converted to HCO3- but need at least 8 units to have this effect.
The greatly increased filtered load of HCO3- exceeds the level of H+ secretion to reabsorb it, even in the presence of increased Pco2, so the excess is lost in the urine
How does respiratory compensation effect renal compensation in metabolic ketoacidosis?
Once again respiratory compensation delays renal correction, but protects the pH.
What is the H+, pH, Primary disturbance and compensation in Respiratory acidosis?
H+ = Increased
pH = Decreased
Primary Disturbance = Increased Pco2
Compensation = Increased [HCO3-]
What is the H+, pH, primary disturbance and compensation in Respiratory alkalosis?
H+ = decreased
pH = Increased
Primary disturbance = Decreased Pco2
Compensation = Decreased [HCO3-]
What is the H+, pH, primary disturbance and compensation in metabolic acidosis?
H+ = Increased
pH = decreased
Primary disturbance = Decreased [HCO3-]
Compensation = Decreased Pco2
What is the H+, pH, primary disturbance and compensation in metabolic alkalosis?
H+ = decreased
pH = increased
Primary disturbance = increased [HCO3-]
Compensation = increased Pco2
When looking at ABGs why do you need to take all the values into account?
Need to distinguish between cause and effect.
e.g. a decrease in pH (acidosis) is caused by either decreased HCO3- or increased Pco2
For a given Pco2, there is a smaller decrease in pH in chronic respiratory acidosis than in acute respiratory acidosis.
Why is this?
NH3 production takes 4-5 days to be fully turned on. So initially can only raise [HCO3-] by titratable acid, so limited.
With time, can use NH3 production which has a considerable capacity to raise [HCO3-].
How long do renal compensations take to kick in compared to respiratory compensations?
Respiratory compensations can take minutes
Renal compensations can take 4-5 days
(Turning on or off NH3 production takes time)
What electrolyte should you always be aware of in acidosis?
H+ taken into cells to buffer in exchange for K+
In hyperkalaemia due to acidosis what treatments can you consider?
Insulin (+ glucose if non-diabetic), stimulates cellular uptake of K+
Calcium resonium, either oral or per rectum, exchanges Ca2+ for K+ ions.
Ca gluconate (IV) -> decreased excitablility of the heart, stabilizes cardiac muscle cell membranes
Consider a bad case of vomiting:
Loss of NaCl and H2O -> hypovolaemia
Loss of HCl -> metabolic alkalosis
Why might the kidneys response cause further alkalosis?
Hypovolaemia will stimulate aldosterone to increase distal tubule Na+ reabsorption.
Under conditions of avid Na+ reabsorption (and due to loss of Cl-), the main ion exchanged for Na+ is H+
The respiratory compensation for the metabolic alkalosis i.e. increased Pco2 helps drive the H+ secretion and exacerbates the metabolic alkalosis by adding yet more HCO3- to the plasma
Why in vomiting and diarrhoea, although you lose acid and alkali, you become alkalotic?
Decrease in ECF volume takes precedence over correction of metabolic alkalosis
Increased aldosterone -> increased reabsorption of Na+ and therefore increased loss of H+ and reabsorption of HCO3-
What is the anion gap?
The difference between the sum of the principle cations (Na+ and K+) and the principal anions in the plasma (Cl- and HCO3-)