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ABG Analysis Handout and Questions
Slovis 6-step approach to ABGs
1) Check the numbers
2) Apply the ABG rules
3) Calculate the AG
4) If Acidosis apply the rule of 15 (+/- 2)
5) If Acidosis apply the delta gap (+/- 4)
6) Check the osmolar gap
ABG RULES
1) Is it an Acidosis or Alkalosis
• Look at the pH
2) Is it Respiratory or Metabolic
• Metabolic = pCO2 + pH ∆ in same direction
• Resp = pCO2 + pH ∆ in opposite direction
3) Is it a pure respiratory acidosis?
↑pCO2 : ↓pH = 1:1
RULE of 15
HCO3 + 15 = pCO2 and pH (last 2 digits)
• Creates a new set point for the pCO2
• pCO2 appropriate = normal compensation
• pCO2 too low = superimposed primary resp alkalosis
• pCO2 too high = superimposed primary resp acidosis
• Note: as HCO3 falls below 10 you need to use the formula HCO3 x 1.5 + 8 =
expected pCO2
Delta Gap
• Checks for “hidden” metabolic process
• Based on the 1:1 concept that ↑AG = ↓HCO3
• Upper limit of AG = 15
• Normal HCO3 = 24
• Bicarb too high = metabolic alkalosis
• Bicarb too low = Non-gap metabolic acidosis
Case #1
• 19yo male presents with 2 week hx of abdominal pains and blurred vision
Na =135
Cl =100
K =6.0
HCO3 =15
BUN =11
Glucose =38
pH = 7.30
pCO2 = 30
pO2 = 100
Case #2
• 36yo M presents with altered LOC. He is markedly agitated, febrile and hyperventilating
Na =140
Cl =100
K =3.8
HCO3 =10
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pH = 7.32
pCO2 = 20
pO2 = 80
Two immediate things you have to think about?
Case #3
• 84yo F found down in her apartment with altered mental status
Na =140
Cl =108
K =3.2
HCO3 =22
pH = 7.16
pCO2 = 64
pO2 = 80
DDx?
Case #4
• 48yo known diabetic presents with 4d hx of abdominal pains, vomiting and severe diarrhea
• Not eating so stopped insulin
Na =130
Cl =105
K =4.8
HCO3 =15
BUN =14
Glucose =29
pH = 7.30
pCO2 = 30
pO2 = 100
Is this DKA?
Case #5
• 22yo F presents with retrosternal chest pain and describes SOB during her MCAT exam
Na =131
Cl =96
K =4.0
HCO3 =24
BUN = 4.0
Glucose = 7.8
pH = 7.40
pCO2 = 40
pO2 = 100
Case #6
• You are about to place the ETT in a crashing patient when the RT shoves the following ABG
into your face with no patient history at all…
Na =138
Cl =108
K =5.0
HCO3 =10
pH = 7.25
pCO2 = 25
pO2 = 100
What are the issues in intubating this patient?
What vent settings are required?
Case #7
• 35-year-old man with renal insufficiency admitted to hospital with pneumonia and the
following lab values
• Medications: Lasix
Na =145
Cl =98
K =2.9
HCO3 =21
pH = 7.52
pCO2 = 30
pO2 = 62
Case #8
• Elderly man from nursing home with hx of RA
• Profound weakness and areflexia + poor oral intake for days
• Current meds:
• Sleeping pills PRN
• Prednisone 45mg daily
Na =145
Cl =86
K =1.9
HCO3 =45

pH = 7.58
pCO2 = 49
pO2 = 84
Urine Cl = 74 mmol/L
Why is the K so low?
Case #9
• EMS called for 38yo male increasingly agitated and incoherent
• paramedics noted he appeared "drunk" but normal vital signs and 02 Sats
• BP 110/70, HR 72, T 36°C, RR 24, Sat 97% RA
• Thirty minutes later:
• GCS fell to 9 (E2/M4/V3)
• RR ↑ 30 breaths/min
• No focal neurologic signs
• Physical examination was otherwise unremarkable
• PEA arrest requiring resuscitation with epi
Na =153
Cl =108
K =5.4
HCO3 =5
BUN = 5.9
Glucose = 6.0
Cr = 174
pH = 6.49
pCO2 = 62
pO2 = 100
Anything else you would like?
Case #10
• You are attending to a 67yo female who presents in acute respiratory distress
• A venous blood gas has already been sent and returns with the following values
• pH = 7.35, pCO2 = 49.6, HCO3 = 23.3
• The RT asks if you really need to poke grandma again for an arterial gas?
Can you use a venous gas to replace an ABG in the ED?
What are the mean differences between arterial and venous samples?
Are they clinically significant?
Case #11
• 60yo male seriously ill on arrival to ED
• Vomiting dark brown fluid ‘every hour or two’ for about a day plus several episodes of
melena
• Past history of alcoholism, cirrhosis, portal hypertension
• Examination:
• Jaundiced, sweaty, clammy and tachypnoeic
• BP 98/50, pulse 120/min
• Peripheries were cool
• Abdomen soft and nontender
• Signs of chronic liver disease present
Na = 131
Cl = 85
K = 4.2
Glucose = 2.88 mmol/L
BUN = 8 mmol/L
Creatinine = 78 umol/L
Lactate = 20.3 mmol/l
Hgb = 62 g/L
Albumin = 20g/L
pH = 7.10
pCO2 = 14
pO2 = 103
HCO3 = 4
Is there a secondary metabolic process?
Does a low serum albumin affect the measurement of the anion gap?
Case #12
• 87yo Female from nursing home is febrile, confused and tachycardic
• You are concerned about sepsis but a lactate has not been ordered in her initial bloodwork
• You decide that you will calculate her AG as a screening tool and only order the lactate if it
is elevated
Is the AG a reliable screen for lactic acidosis in the ED?
Case#13
• 28yo F known asthmatic and 8 months pregnant presents with increasing SOB over 24hrs
• She has been taking her inhalers with no effect
• Exam
• In resp distress, diaphoretic, and looking very tired
• Auscultation reveals no wheezing
ABG
pH = 7.32
PO2 = 90
PCO2 = 45
HCO3 = 22
Are you concerned about her?
Case #14
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A 54yo M presents in acute COPD exacerbation
You intubate the patient for respiratory failure
The end tidal CO2 reads 50mmHg but the ABG says Pa CO2 is 75mmHg
Which one is correct and why?