Blood gases

Question 1

Discuss PF ratio

Answer 1

The ratio of inspired air to arterial o2

should be greater than 200mmhg

Question 2

Discuss Assessing o2 content

Answer 2

Determined by HB concentration and saturation

Assess function of HB is the measured saturation what you would expect for the Pao2 if not there is a shift in the curve

Measure your P50 – partial pressure at which o2 sat is 50%
increase means a rightward shift (hyperthermia, hypercapnia, acidosis, increase 23 DPG)
decrease means a leftward shift (hypothermia, hypocapnia, alkalsosi, decrease 23 DPG

Question 3

Discuss Anion Gap

Answer 3

NA - (CL+HCO2)

normal 8-16

Question 4

Discuss causes of high anion gap metabolic acidosis (KUSMAL)

Answer 4

Ketoacidosis 
Ureamia 
salicylates
methanol 
alcoholic ketoacidosis 
lactic acidossis

Question 5

Discuss causes of normal anion gap metabolic acidosis

Answer 5

Gi hco3 loss
-diarrhoea
-pancreatic or biliary drainage 
-urinary diversion 
Renal bicarb loss 
-type 2 rneal tubular acidosis 
-ketoacidosis

Question 6

Discuss lactic acidosis

Answer 6

Without clear evidence to the contrary lactic acidosis should be assumed to be secondary to inadequate tissue perfusion

Question 7

Discuss classes of lactic acidosis

Answer 7

Type 1 (inadequate tissue perfusion) 
-shock 
-severe hypoxaemia 
-aneamia 
-post convulsions 
-sever exercise 
sepsis 

Type B (no evidence of inadequate perfusion) 
B1 -- associated with udnerlying disease (leukaemia, lymphoma) 
B2- Drugs 
-metformin
-salicylates
-sorbitol
-paracetamol 
B3 - inborn metabolic errors

Question 8

Discuss respiratory compensation for metabolic acidosis

Answer 8

Complete respiratory compensation for acidosis does not occur and normal PH in a patient with metabolic acidosis should raise suspicion of co-current alkolosis

Question 9

Discuss metabolic alkalosis

Answer 9

Usually initiated by increase loss of acid from the stomach or kidney. Excretion of high hco3 is usually so high that metabolic acid is not sustained

Sustained metabolic acidosis is usually due to volume deficit. In volume deficit kidney re-absorbs NA over other homeostatic mechanisms. HCO3 is combined in reabsorption of NA through carbonic acid

Another mechanisms is hyperadrenocorticoidism (conns, cushings)

Question 10

Discuss respiratory alkalosis

Answer 10

Can be caused by hypoxia or non hypoxic respiratory center stimulation

Non hypoxic include

• anxiety
• stress
• pregnancy
• sepsis
• salicylates
• hepatic cirrhosis

Can cause tetany probably due to direct increase in neuromuscular excitability rather then modest decrease in ionized calcium, sever may also cause confusion or loss of consciousness

Question 11

Discuss Mixed

Answer 11

Common causes

• salicylate
• sepsis
• DKA
• alcohol (metabolic acidosis with hyperventilation from DT)