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 BUN =11 pH = 7.30
Cl =100 Glucose =38 pCO2 = 30
K =6.0 pO2 = 100
HCO3 =15
Case #2
• 36yo M presents with altered LOC. He is markedly agitated, febrile and hyperventilating
Na =140 pH = 7.32
Cl =100 pCO2 = 20
K =3.8 pO2 = 80
HCO3 =10
• Two immediate things you have to think about?
Case #3
• 84yo F found down in her apartment with altered mental status
Na =140 pH = 7.16
Cl =108 pCO2 = 64
K =3.2 pO2 = 80
HCO3 =22
DDx?
Case #4
• 48yo known diabetic presents with 4d hx of abdominal pains, vomiting and severe diarrhea
• Not eating so stopped insulin
Na =130 BUN =14 pH = 7.30
Cl =105 Glucose =29 pCO2 = 30
K =4.8 pO2 = 100
HCO3 =15
Is this DKA?
Case #5
• 22yo F presents with retrosternal chest pain and describes SOB during her MCAT exam
Na =131 BUN = 4.0 pH = 7.40
Cl =96 Glucose = 7.8 pCO2 = 40
K =4.0 pO2 = 100
HCO3 =24
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 pH = 7.25
Cl =108 pCO2 = 25
K =5.0 pO2 = 100
HCO3 =10
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 pH = 7.52
Cl =98 pCO2 = 30
K =2.9 pO2 = 62
HCO3 =21
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 pH = 7.58 Urine Cl = 74 mmol/L
Cl =86 pCO2 = 49
K =1.9 pO2 = 84
HCO3 =45
· 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 BUN = 5.9 pH = 6.49
Cl =108 Glucose = 6.0 pCO2 = 62
K =5.4 Cr = 174 pO2 = 100
HCO3 =5
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 pH = 7.10
Cl = 85 pCO2 = 14
K = 4.2 pO2 = 103
Glucose = 2.88 mmol/L HCO3 = 4
BUN = 8 mmol/L
Creatinine = 78 umol/L
Lactate = 20.3 mmol/l
Hgb = 62 g/L
Albumin = 20g/L
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
• 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?