Assessment of Acid-Base Balance

Question 1

what pH range and [H+] does the body need to be within

Answer 1

pH - 7.35-7.45
[H+] - 35-45nmol/l

Question 2

what processes produce acid in the body

Answer 2

cellular respiration (CO2 and water which make carbonic acid)
metabolic processes (ketones, lactate etc.)

Question 3

what things help to remove acid from the body

Answer 3

buffers
lungs
kidney

Question 4

what things help to remove acid from the body

Answer 4

buffers
lungs
kidneys

Question 5

hypoventilation will cause

Answer 5

resp. acidosis

Question 6

hyperventilation will cause

Answer 6

resp. alkalosis

Question 7

what changes in H+ and HCO would result in metabolic acidosis and what would cause these changes

Answer 7

high H+ (overproduction or impaired secretion)
low HCO3- (unusual losses)

Question 8

what changes in H+ and HCO would result in metabolic alkalosis and what would cause these changes

Answer 8

low H+ (projectile vomiting)
high HCO3- (unusual ingestion e.g. IV sodium bicarb treatment)

Question 9

how does the body compensate in respiratory acidosis

Answer 9

increased retention of HCO3- by kidneys

Question 10

how does the body compensate in respiratory acidosis

Answer 10

increased retention of HCO3- by kidneys
this is a slow process

Question 11

how does the body compensate in respiratory alkalosis

Answer 11

decreased retention of HCO3- by the kidneys
this is a small and usually marginal process

Question 12

how does the body compensate in metabolic acidosis

Answer 12

increased resp. rate to decrease CO2
increased retention of HCO3 by kidneys

Question 13

how does the body compensate in metabolic alkalosis

Answer 13

decreased resp. rate to increase CO2
decreased retention of HCO3 by kidneys
this is usually minimal

Question 14

what is deep fast breathing called and when does it occur

Answer 14

kusmal breathing
when resp. rate increases to compensate for metabolic acidosis

Question 15

what practical things should be considered before does ABGs

Answer 15

would a venous sample do?
consider usual local anaesthetic
consider appropriate site (usually radial, likely femoral in arrest)
analyse ASAP

Question 16

how long is an ABG sample stable

Answer 16

10 minutes
60 minutes on ice

Question 17

what are the six steps for interpreting blood gases

Answer 17

1. is the patient adequately oxygenated (does this suggest a respiratory cause?)
2. what is their pH (acidosis or alkalosis)
3. is there CO2 disturbance (resp?)
4. is there HCO3 disturbance (metabolic?)
5. what is the primary disturbance (usually this will match the underlying cause/symptoms and direction of change)
6. is there compensation (does the timing of clinical picture fit - e.g. very acute can’t have metabolic compensation)
7. if it doesn’t make sense (did you sample wrong? ask for help)

Question 18

what is base excess

Answer 18

the amount of H+ per litre of blood required to return [H+] to the reference range at a reference range pCO2

Question 19

when will a base excess be positive or negative

Answer 19

positive - metabolic alkalosis
negative - metabolic acidosis

Question 20

what is the standard bicarb

Answer 20

what the bicarbonate would be if pCo2 were reference range

Question 21

what will the standard bicarb be in
purely resp disorder
purely metabolic disorder
mixed resp-met disorder

Answer 21

ref. range standard bicarb
approx equivalent to acutal bicarb
significant difference with actual bicarb

Question 22

if someone has low CO2 and low bicarb but high H what do they have

Answer 22

high pH = acidosis
low bicarb = metabolic
low CO2 would cause alkalosis and HCO3 compensation wouldn’t cause acidosis
so metabolic acidosis with respiratory compensation

Question 23

what is the anion gap

Answer 23

difference between most abundant cations and anions
[Na+] - [Cl-] - [HCO3 -]

Question 24

what causes of metabolic acidosis would you expect the anion gap to be normal

Answer 24

severe diarrhoea
high interstinal fistula output
renal tubular acidosis

Question 25

what causes of metabolic acidosis would you expect the anion gap to be high

Answer 25

diabetic ketoacidosis
lactic acidosis
aspirin overdose
alcohol poisoning
renal failure

Question 26

chronic metabolic acidosis can have what effect on bone

Answer 26

buffering by bone
leading to decalcification

Question 27

which compensatory mechanisms are usually marginal (i.e. you wouldn’t expect them to cause alkalosis from acidosis or vice versa)

Answer 27

metabolic compensation for respiratory alkalosis
respiratory and metabolic compensation for metabolic alkalosis

the alkalosis ones - you are unlikely to get acidotic as a result of compensation to alkalosis

Question 28

causes of acute respiratory acidosis

Answer 28

airway obstruction
cardio-pulmonary arrest
COPD exacerbation
pneumonia
opiate toxicity
guillain-barre syndrome
myasthenia gravis

Question 29

causes of chronic respiratory acidosis

Answer 29

COPD
obstructive obesity
pulmonary fibrosis
MND
myopathy

Question 30

effects of respiratory acidosis on body

Answer 30

hypercapnia causing:
shortness of breath
neurological symptoms - anxiety, coma, headaches
cardiovascular - systemic vasodilation

Question 31

causes of acute respiratory alkalosis

Answer 31

asthma, COPD exacerbation,PE
pain
panic attack
iatrogenic (e.g. overventilation when under GA)
altitude sickness
innappropriate stimulation of respiratory centre in brain stem (head injury etc.)

Question 32

chronic cause of respiratory alkalosis

Answer 32

pregnancy
but physiological and compensated by mild metabolic acidosis

Question 33

effects of respiratory alkalosis

Answer 33

acute hypocapnia - lightheadedness, confusion, syncope, fits, paraesthesia
cardiovascular - increased heart rate, vasoconstriction, angina in those with CVD

Question 34

causes of metabolic alkalosis

Answer 34

loss of H+
- vomiting
- hypokalaemia (e.g. secondary to loop diuretic)
- primary hyperaldosteronism

gain of HCO3-
- sodium bicarbonate infusion (common cause in CKD)