ABG Interpretation
ABG shorthand notation:
pH / PaC02 / PaO2 / calculated HCO3
What is NORMAL?
7.36-7.44 / 37-43 / >60 / 22-26
Anion Gap 12-15
Osmolar Gap 0
Delta Gap 0
When you find your ABG is ABNORMAL, have an approach to interpretation
1. Is patient acidemic or alkalemic?
2. Is primary disorder respiratory or metabolic?
3. If respiratory, is it acute or chronic?
4. If metabolic acidosis is there an anion gap? is there an osmolar gap?
5. Is there appropriate compensation? if no, what is the second disorder?
6. IN THE SETTING OF AG METABOLIC ACIDOSIS, is there another problem? what
is the delta gap?
1. Is patient acidemic or alkalemic?
less than 7.36 - acidemic
more than 7.44 - alkalemic
2. Is primary disorder respiratory or metabolic?
acidemic and CO2 high = resp
acidemic and HCO3 low = metabolic
alkalemic and CO2 low = resp
alkalemic and HCO3 high = metabolic
3. If respiratory, is it acute or chronic?
chronic resp acidosis - COPD, IPF
chronic resp alkalosis (hyperventilation) - anxiety, living at high altitude, anemia, CHF,
asthma, pregnancy, liver disease, R to L shunts, prolonged ventilation
4. If metabolic acidosis is there an anion gap? is there an osmolar gap?
AG = Na - Cl - HCO3 (normal is 12-15)
correct for albumin (every decrease of 10 in albumin, increase the gap 3)
OG = measured serum osm - (Na x 2, Glu, BUN)
5. Is there appropriate compensation? if no, what is the second disorder?
(you can not OVER compensate)
PROBLEM
acute resp acid
acute resp alk
chronic resp acid
chronic resp alk
CO2
up 10
down 10
up 10
down 10
HCO3
up 1
down 2
up 3
down 5
met acidosis
met alk
down 10
up 10
down 10
up 7.5
6. IN THE SETTING OF AG METABOLIC ACIDOSIS, is there another problem?
you CAN NOT have more than one respiratory problem
you CAN have more than one metabolic problem:
AG Met acid + Met Alk
AG Met acid + non AG Met acid
you CAN have a respiratory problem and a metabolic problem (and even a secondary
metabolic problem on top of that)
look at delta gap - anion gap delta versus bicarb delta
normally in AG Met Acidosis, the number of anions above 14 should equal the number of
bicarb below 24
(calculated anion gap - 14) versus (24 - measured HCO3)
for example
1. calculated anion gap of 23 is an anion delta of 9 (measured 23 - normal 14 = delta of 9)
2. measured bicarb of 15 is a bicarb delta of 9 ( normal 24 - measured 15 = delta of 9 )
normally the anion delta is equal to the bicarb delta
if they are NOT equal, there is a second problem...
if Bicarb delta is less than the anion gap delta = metabolic alkalosis as well
if Bicarb delta is more than the anion gap delta = non AG metabolic acidosis as well
Anion Gap Acidosis
addition of exogenous acids or creation of endogenous acids
Methanol/ Metformin
Uremia
Diabetic/starvation/alcoholic Ketoacidosis
Paraldehyde / Phenformin
Ischemia / iron / isoniazid
Lactic acidosis
Ethylene glycol (antifreeze)
Cyanide / Carbon Monoxide
ASA
Toluene
Solvents
Pearls
most common: ketoacidosis, lactic acidosis, ASA (IF ruled out investigate toxic alcohols)
ASA not only results in AG met acidosis but also Resp alkalosis
a low glucose does not rule out ketoacidosis: look out for starvation, dehydration, alcohol
consider cyanide and carbon monoxide in inhalation and burn victims
don’t be fooled by normal Osat and PaO2 - carbon monoxide needs carboxyhemoglobin
isoniazid (aka inh) antidote is vit b6...think about it in refractory seizures (esp high risk TB
populations)
reduced arterial-venous oxygen sat difference (<10%) suggests cyanide toxicity
cyanide antidote is induced methemaglobinemia
ethylene glycol or methanol antidote is etoh or Fomepizole
if you suspect Iron OD, get an AXR - pills may show up on the flat plate!
ethylene glycol in urine will fluoresce using a Wood’s lamp - easy bed side test!
calcium oxalate crystals in urine are sign of possible ethylene glycol ingestion
Non-anion gap acidosis
inability to excrete H+ or loss of HCO3
Fistulae (pancreatic - duodenal, ureter - enteric)
diarrhea
carbonic anhydrase inhibitors
diuretics
saline (large amounts)
RTA
parenteral nutrition
Pearls
most common loss of bicarb by GI or kidneys
- diarrhea
- tubular acidosis or renal failure
-
Metabolic Alkalosis - Saline Responsive
-either loss of H+ or contraction (volume contraction around constant HCO3)
-Urine Cl <10
Gastric suction
Vomiting
Diuretics
give Saline, gets better
Metabolic Alkalosis - Non Saline Responsive
-retention of HCO3 associated with mineralcorticoid excess
-Urine Cl>20
Hyper aldosteronism
Exogenous steroids
adenocarcinomoa
Bartter’s syndrome
Cushing’s syndrome
VBG vs ABG?
correlation does not equal accuracy
studies show excellent correlation of ph, pco2, calc hco3 between abg and vbg
but the accuracy was the problem!-the differences between the two were outside the
range noted to be sig by emerg docs
“Can peripheral venous blood gas replace arterial blood gases in ED patients” cjem Jan
2002
however, most notable delta is in the post arrest patient
that being said, most problems in the ED can be sorted out using vbg. It’s easier to get
and less painful for the patient.
perhaps the only time for abg is when you cant get venous access or when u want to
qualify for home o2 or in post arrest patients.