Acid Base Balance - Pt 2 Flashcards
What do respiratory and renal disorders affect?
Resp. = PCO2
Renal = conc. of HCO3-
Describe respiratory acidosis
pH has fallen and is due to resp. change
PCO2 is increased
Results from reduced ventilation and retention of CO2
What are the acute causes for respiratory acidosis?
Drugs which depress medullary respiratory centres - barbiturates and opiates
Obstructions to major airways
What are the chronic causes for respiratory acidosis?
Lung disease - bronchitis, emphysema and asthma
PCO2 increased so increase secretion of H+ and HCO3-
Need to protect pH so increase HCO3-
Generation of new HCO3- but also increased reabsorption
What do acid conditions stimulate?
Renal glutaminase - more NH3 produced but it takes time
In chronic causes of respiratory acidosis - why does original disturbance not corrected?
Renal compensation for increase HCO3- protects pH but only restoration of ventilation can remove primary disturbance
So BG values are never normalised - kidney maintains high HCO3-
Describe respiratory alkalosis
Alkalosis of resp. origin so must be due to fall in PCO2 and can occur through hyperventilation and CO2 blow off
What are the acute causes of respiratory alkalosis?
Voluntary hyperventilation, aspirin and first ascent to altitude
What are the causes of chronic alkalosis?
Long term residence at altitude so PCO2 below 60mmHg stimulates peripheral chemoreceptors to increase ventilation
Ventilation must be normal to correct disturbance
What are alkaline condition dealt with?
HCO3- reabsorption mechanism
If PCO2 drops then less H+ available for excretion so less filtered HCO3- reabsorbed so HCO3- lost in urine
Describe metabolic acidosis
Metabolic origin - loss of HCO3-
Decreased HCO3- either from increased buffering or direct loss of HCO3-
PCO2 must be decreased
What are the causes of metabolic acidosis?
Increased H+ production - ketoacidosis or lactic acidosis
Failure to excrete dietary load of H+ in renal failure
Loss of HCO3- as in diarrhoea - failure to reabsorb intestinal HCO3-
Describe Kussmaul breathing
Degree of hyperventilation - increase depth of breathing so max 30l/min when pH falls to 7
Sign of renal failure or diabetic ketoacidosis
What is the problem in metabolic acidosis?
Normally kidneys correct disturbance by restoring HCO3- and getting rid of H+ ions
Source of H+ ions is the carbonic acids from CO2 but resp. compensation lowers PCO2 to protect pH
What would complete compensation lead to?
Would remove the drive to correct the original disturbance
Then there is no pressure to correct disturbance - further perturbation may push system too far so compensation may longer not be effective
What does decreased PCO2 lead to?
Total amount of H+ secretion by renal tubule will be less than normal but because of plasma HCO3- conc. and filtered HCO3- conc. is reduced
What does a reduced plasma HCO3- and filtered HCO3- load mean?
Less total H+ is needed for HCO3- reabsorption and therefore greater proportion is available for excretion to form titratable acid and NH4+
Less H+ secretion = Decreased HCO3- reabsorption and increase new HCO3-
What does increase metabolic H+ in body lead to?
Immediate buffering in ECF and then ICF
Resp. compensation within minutes
Renal correction takes longer - to develop response to increase H+ excretion and generates new HCO3-
What does respiratory compensation do in metabolic acidosis?
Delays the renal correction but protects the pH
Describe metabolic alkalosis
HCO3- must have increased and PCO2 will increase to protect the pH
What are the causes of metabolic alkalosis?
Increase H+ ion loss in vomiting
Increase H+ loss in aldosterone excess and liquorice ingestion
Excess administration of HCO3- - renal impairment
Massive blood transfusions
How can massive blood transfusions lead to metabolic alkalosis?
Bank blood contains citrate to prevent coagulation which is converted to HCO3- but need 8 units for this
What does increased filtered load of HCO3- lead to?
Exceeds the level of H+ secretion to reabsorb it, even in presence of increased PCO2
So excess lost in urine
Resp. delays renal correction but protects the pH
What is the summary of respiratory acidosis?
Increase H+ so decreases pH
Primary disturbance is increase PCO2
Compensation is increase HCO3- conc.
What is the summary of respiratory alkalosis?
Decrease H+ so increase pH
Primary disturbance is decrease PCO2
Compensation is decrease HCO3- conc.
What is the summary of metabolic acidosis?
Increase H+ so decrease pH
Primary disturbance is decrease HCO3- conc.
Compensation is decrease PCO2
What is the summary of metabolic alkalosis?
Decrease H+ so increase pH
Primary disturbance is increase HCO3- conc.
Compensation is increase PCO2
What is a decrease in pH caused by?
Decreased HCO3-
Increased PCO2
What is an increase in pH caused by?
Increased HCO3-
Decreased PCO2
Does chronic or acute resp. acidosis cause bigger decrease in pH
Acute
Because NH3 production takes 4-5 days to turn on and off
What can a haemorrhage lead to?
Lactic acidosis
What is happening in a patient with metabolic acidosis, respiratory acidosis, and lactic acidosis?
Diabetic ketoacidosis decreases pH
Chronic bronchitis decompensates
Haemorrhage loss means additional metabolic acidosis
Brings pH to brink of survival
High acidity causes hyperkalaemia as H+ ions are buffered intracellularly in exchange for K+
What would you do for a patient with metabolic and respiratory acidosis?
Insulin (and glucose if non-diabetic) which stimulates cellular uptake of K+
Hyperkalaemia - calcium resonium
Ca gluconate IV which increases excitability of heart
What does calcium resonium do?
Exchanges Ca2+ ions for K+ ions
What happens in a bad case of vomiting - hypovolaemia and metabolc alkalosis?
Loss of NaCl + H2O and HCl
Hypovolaemia - stimulates aldosterone to increase distal tubule Na reabsorption
In avid Na reabsorption Na is exchanged for H+
PCO2 helps drive H+ secretion and exacerbates metabolic alkalosis
What takes precedence between volume and metabolic correction?
Volume
What is given in bad case of vomiting - hypovolaemia and metabolic alkalosis?
Give NaCl to restore volume then alkalosis will be corrected
What does liquorice contain?
Glycyrrhizic acid which similar to aldosterone - so excess ingestion causes metabolic alkalosis
