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Title: (V)ABG interpretation


1
(V)ABG interpretation
  • Kristian Hecht PGY-3 EM
  • With thanks to Marc, Mark and Dr. Rigby

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5
Outline
  • Why is everyone using VBGs anyway?
  • Basic Review
  • Lists that you have to remember (Doh!)
  • Calculations you can do at the bedside
  • A BS-free approach to the ABG
  • Cases
  • Special circumstances

6
Basic Review
7
Why do we care about ABGs?
  • Aids in diagnosis
  • Provides clues about clinically unrecognized
    disorders
  • May indicate what treatments are needed
  • Helps assess progress of illness or therapy

8
Whats bad about ABGs
  • ABGs are invasive
  • Painful, even with lido!
  • Have potential complications
  • Local hematoma
  • Arterial dissection
  • thrombosis (rarely)
  • Technically difficult, esp. in kids and elderly,
    thus, several attempts may be required.

9
ABG Vs. VBG
  • 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?

10
  • Canadian prospective observational study in the
    ED (CJEM January 2002 Vol 4, No 1)
  • N218 pts
  • Pts requiring ABG simultaneous venous sampling
  • Correlation coefficients and mean differences
    were calculated
  • Also 45 academic ED physicians were surveyed to
    determine the minimal clinically important
    difference in each variable

11
  • The mean differences (95 CI) in arterial and
    venous samples were
  • pH 0.036 (0.030 - 0.042)
  • pCO2 6.0 (5.0 - 7.0) mmHg
  • HCO3 1.5 (1.3 - 1.7) mEq/L

12
  • These differences were considered greater than
    the minimum clinically significant differences
    identified in the survey
  • Concluded that although highly correlated, the
    differences between them preclude using them
    interchangeably
  • Can be used to follow trends

13
Since 2002
  • Arterial Blood Gas Analysis Are Its Values
    Needed for the Management of Diabetic
    Ketoacidosis?
  • Ann Emerg Med. 200545550-551
  • Good correlation between arterial and venous pH
    and HCO3
  • The case for venous rather than arterial blood
    gases in diabetic ketoacidosis
  • Emerg Med Australas. 2006 Feb18(1)64-7
  • Review article analyzing the validity of venous
    BG sampling in DKA
  • In patients with DKA the weighted average
    difference between arterial and venous pH was
    0.02 pH units
  • Venous HCO3- was 1.88 higher than arterial

14
Principles of Acid-Base
  • Normal serum pH is maintained within a very
    narrow range of 7.36-7.44
  • Equal to H 447 - 355µM
  • pHgt7.8 or lt6.8 is incompatible w/life

15
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Principles of Acid-Base
  • pH is maintained by 3 systems
  • Physiologic buffers
  • Lungs
  • Kidneys
  • Disorders in any of these systems leads to
    alterations in blood pH

18
Physiologic Buffers
  • 1) Bicarbonate-carbonic acid buffer system
  • H HCO3- ? H2CO3 ? H2O CO2
  • 2) Intracellular blood protein buffers
  • Hemoglobin
  • w/o this venous blood would be pH 4.5
  • 3) Bone
  • Reservoir of bicarb and phosphate

19
Lungs
  • Changes in pH sensed by chemoreceptors
  • Peripherally (carotid bodies)
  • Centrally (medulla oblongata)
  • Drop in pH
  • Increased minute ventilation
  • Lowers PaCO2
  • Increase in pH
  • Decreased ventilatory effort
  • Increases PaCO2

20
Kidneys
  • Play no role in acute compensation
  • 6-12hrs Acidosis
  • Active excretion of H
  • Retention of HCO3-
  • gt6hrs of Alkalemia
  • Active excretion of HCO3-
  • Retention of H

21
Normal ABG parameters
  • pH 7.40
  • PCO2 40 mmHg
  • HCO3 24 mM
  • Anion Gap 12 - 15

22
Terminology
  • Acidemia blood pH lt 7.35
  • Acidosis a physiologic process that, occurring
    alone, tends to cause acidemia
  • e.g. metabolic acidosis from increased ketoacid
    production in DKA
  • If the patient also has an alkalosis at the same
    time, the resulting blood pH may be low, normal
    or high

23
Terminology
  • Alkalemia blood pH gt 7.45
  • Alkalosis a primary physiologic process that,
    occurring alone, tends to cause alkalemia
  • i.e. respiratory alkalosis from
    hyperventilation
  • If the patient also has an acidosis at the same
    time, the resulting blood pH may be high, normal
    or low.

24
Terminology
  • Primary acid-base disorder One of the four
    acid-base disturbances that is manifested by an
    initial change in HCO3- or PaCO2.
  • Compensation The change in HCO3- or PaCO2 that
    results from the primary event. Compensatory
    changes are not classified by the terms used for
    the four primary acid-base disturbances.
  • You cannot overcompensate for an Acid-Base
    disturbance

25
Acid-Base Disorders
  • Respiratory disorders
  • Alter the serum PaCO2
  • Metabolic disorders
  • Alter the serum HCO3-

26
Lists you have to remember.
  • Doh

27
Respiratory Disorders
  • ACIDOSIS
  • Hypoventilation
  • Pulmonary pathology
  • Airway obstruction
  • Decreased respiratory drive
  • ALKALOSIS
  • ? minute ventilation
  • CNS disease
  • Hypoxemia
  • Anxiety
  • Toxic states
  • Hepatic insufficiency
  • Assisted ventilation

28
Metabolic Disorders
  • ACIDOSIS
  • Anion gap metabolic acidosis
  • Non-AG metabolic acidosis
  • ALKALOSIS
  • Saline responsive
  • Saline resistant

29
Anion Gap Metabolic Acidosis
  • Addition of exogenous acids
  • or
  • Creation of endogenous acids
  • Cat Mudpiles
  • Carbon monoxide/cyanide
  • Alcohol/AKA
  • Toluene
  • Methanol
  • Uremia
  • DKA
  • Paraldehyde
  • INH/Iron
  • Lactic Acidosis
  • Ethylene glycol
  • Salicylates

30
Normal AG Metabolic Acidosis
  • Excessive loss of HCO3-
  • or
  • Inability to excrete H
  • Hard ups
  • Hyperalimentation/ Hyperventilation
  • Acids/Addisons/ Acetazolamide
  • RTA
  • Diarrhea/Dehydration/ Diuretics
  • Uterosigmoidostomy
  • Pancreatic fistula or drainage
  • Saline (large amounts)

31
Saline-responsive metabolic alkalosis
  • Volume contracted
  • Contraction of the ECF around the constant plasma
    HCO3-
  • Relative Excess
  • Urinary chloride level lt10 mEq/L
  1. Vomiting/Gastric Suction
  2. Diuretics
  3. Ion-deficient baby formula
  4. Colonic adenomas

32
Saline-resistant metabolic alkalosis
  • Associated with mineralcorticoid excess
  • Leads to ? Na reabsorption
  • Secretion of K and H to maintain neutrality
  • Urinary chloride gt10mEq/L
  1. Primary aldosteronism
  2. Exogenous steroids
  3. Adenocarcinoma
  4. Bartters Syndrome
  5. Cushings disease
  6. Ectopic adrenocorticotropic hormone

33
Calculations that can help you
34
Henderson-Hasselbalch equation
  • Check validity of laboratory measurements
    obtained
  • H 24 x PaCO2 HCO3 40 nEq/L
  • HCO3 calculated on ABG with HH eqn
  • HCO3 measued on Chem 6

35
Respiratory Compensation
  • Compensation PaCO2 HCO3-
  • Acute Resp Acidosis 10 1
  • Acute Resp Alkalosis 10 2
  • Chronic Resp Acidosis 10 3
  • Chronic Resp Alkalosis 10 4

36
Metabolic Compensation
  • Compensation PaCO2 HCO3-
  • Metabolic Acidosis 10 10
  • Metabolic Alkalosis 10 7.5

37
The Corey Slovis approach to acid-base
abnormalities
  • A no bull_at_ approach
  • for non-nephrologist

38
Slovis 6-step approach to ABG
  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

39
Check the numbers
  • Need both Chem 6 and blood gas
  • Know your normal values
  • Does the blood gas make sense?
  • Are there any immediate hints to the diagnosis

40
The ABG rules
  • 1) Is it an Acidosis or Alkalosis
  • Look at the pH (gt7.45, lt7.35)
  • 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 11

41
Calculate the AG
  • Na HCO3- Cl
  • Normal 5-12
  • Upper limit of normal is 15

42
Unmeasured ions
Mg2 Ca2 K
Albumin PO43- Acetate
HCO3-
Na
Cl-
43
Anion Gap
HCO3-
Na
Cl-
44
Anion Gap
  • For example give me an M
  • Methanol intoxication
  • Methanol oxidized to formic acid
  • Formate- H HCO3- ? Formate- ? CO2 H2O

45
Add Formic acid
Anion Gap
HCO3-
Na
Cl-
46
Formate-
Anion Gap
HCO3-
Na
Cl-
47
Narrow AG?
  • Sure, add more unmeasured cations, as carbonate
    or chloride
  • e.g. FeCl2 MgCl2

48
Add MgCl2
Anion Gap
HCO3-
Na
Cl-
49
Mg2
Anion Gap
HCO3-
Na
Cl-
50
Rule of 15
HCO3- 15 pCO2 pH (last 2 digits)
51
Rule of 15
  • Used in acidosis
  • Derived from the Henderson Hasselbalch equation
  • It predicts what resp compensation will do to the
    pCO2 and the pH
  • If the Rule is broken then another process other
    than just resp compensation exists

52
Rule of 15
  • 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

53
Examples of rule of 15
  • 1) HCO320, pCO235 pH 7.35
  • Pure wide gap metabolic acidosis with an
    appropriate 2ndary resp alkalosis
  • 2) HCO310, pCO220 pH 7.32
  • pCO2 is too low. Superimposed primary resp
    alkalosis
  • 3) HCO310, pCO232 pH 7.14
  • pCO2 is too high. Superimposed primary resp
    acidosis

54
Delta Gap
  • Checks for hidden metabolic process
  • Based on the 11 concept that
    ?AG ?HCO3
  • Upper limit of AG 15
  • Normal HCO3 24
  • Bicarb too high metabolic alkalosis
  • Bicarb too low Non-gap metabolic acidosis

55
Examples of delta gap
  • AG20 HCO319
  • ?AG 5 and ?HCO3 5
  • No hidden process
  • AG22 HCO38
  • ?AG 7 and ?HCO3 16
  • Bicarb too low additional normal AG metabolic
    acidosis
  • AG26 HCO320
  • ?AG 11 and ?HCO3 4
  • Bicarb too high superimposed metabolic
    alkalosis

56
Osmolar Gap
  • Use if an unexplained anion gap acidosis
  • 2Na BUN Glucose calculated gap
  • OG Measured calculated
  • Upper limit of normal is 10
  • If higher consider toxic alcohols

57
Intermission
  • http//www.youtube.com/watch?vRcL6DwSufMI

58
Confused???? Lets hit the cases
59
Case 1
  • 19yo male presents with 2 week hx of abdominal
    pains and blurred vision
  • Na 135 BUN 30 pH 7.30
  • Cl 100 Glucose 38 pCO2 30
  • K 6.0 pO2 100
  • HCO3 15

60
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

61
Case 1
  • Anion Gap Metabolic acidosis with appropriate
    resp compensation
  • DDx MUDPILES
  • Diagnosis DKA

62
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

63
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

64
Case 2 cont
  • Anion gap metabolic acidosis
  • And Resp alkalosis
  • Two immediate things you have to think about?
  • ASA overdose
  • Sepsis

65
Case 3
  • 84yo F found down in her apartment with altered
    mental status
  • Na 140 pH 7.16
  • Cl 104 pCO2 64
  • K 3.2 pO2 80
  • HCO3 28

66
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

67
Case 3
  • Pure respiratory acidosis
  • DDx
  • Pulmonary pathology
  • Airway obstruction
  • Decreased respiratory drive

68
Case 4
  • 48yo known diabetic presents with 4d hx of
    abdominal pains, vomiting and severe diarrhea
  • Not eating so stopped insulin
  • Na 130 BUN 40 pH 7.30
  • Cl 105 Glucose 29 pCO2 30
  • K 4.8 pO2 100
  • HCO3 15

69
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

70
Case 4 cont
  • Is this DKA?
  • No!
  • Non-AG Metabolic Acidosis
  • DDx HARDUPS
  • Most likely secondary to severe diarrhea

71
Case 5
  • 22yo F presents with retrosternal chest pain and
    describes SOB during her biology exam
  • Na 135 BUN 9 pH 7.46
  • Cl 101 Glucose 7.8 pCO2 35
  • K 4.0 pO2 100
  • HCO3 23

72
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

73
Case 5 cont
  • Respiratory alkalosis
  • DDx
  • CNS disease
  • Hypoxemia
  • Anxiety
  • Toxic states
  • Hepatic insufficiency
  • Assisted ventilation

74
Case 5 cont
  • Respiratory alkalosis
  • DDx
  • CNS disease
  • Hypoxemia
  • Anxiety
  • Toxic states
  • Hepatic insufficiency
  • Assisted ventilation

75
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

76
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

77
Case 6 cont
  • You explain to her that this is
  • obviously.
  • Wide gap metabolic acidosis with appropriate
    respiratory compensation
  • DDx MUDPILES
  • Delta gap indicating an additional non-AG
    metabolic acidosis
  • DDx HARDUPS

78
Case 7
  • 35-year-old man with renal insufficiency admitted
    to hospital with pneumonia and the following lab
    values
  • Na 145 pH 7.52
  • Cl 98 pCO2 30
  • K 2.9 pO2 62
  • HCO3 21

79
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

80
Case 7
  • Three separate acid-base disorders !!!
  • 1) Acute respiratory alkalosis
  • Acute hyperventilation due to pneumonia
  • 2) Concomitant metabolic acidosis
  • From renal disease
  • 3) Hypokalemic metabolic alkalosis
  • From excessive diuretic therapy
  • The result of all this acid-base abnormality?
    Blood gas values that are indistinguishable from
    those of simple acute respiratory alkalosis.

81
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

82
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

83
Case 8 Cont
  • Metabolic Alkalosis w/ Resp alkalosis (107.5)
  • ? Saline responsive or resistant
  • Resistant
  • DDx?
  • Primary aldosteronism
  • Exogenous steroids
  • Adenocarcinoma
  • Bartters Syndrome
  • Cushings disease
  • Ectopic adrenocorticotropic hormone
  • Why is the K so low?

84
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 36C, 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

85
Case 9
  • Labs
  • Na 153 BUN 5.9 pH 6.49
  • Cl 108 Glucose 6.0 pCO2 62
  • K 5.4 Cr 174 pO2 100
  • HCO3 5

86
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

87
Case 9 Cont
  • What would be appropriate resp compensation for
    this metabolic acidosis?
  • HCO3 x 1.5 8 expected pCO2
  • pCO2 should 16
  • Acid-Base abnormality?
  • Severe AG metabolic acidosis
  • Secondary severe Resp Acidosis

88
Case 9 Cont
  • Anything else you would like?
  • Serum Osmolarity 487 mOsm
  • Serum EtOH lt 2.2mmol/L
  • What is the Osmolar Gap?
  • 169 mEq/L
  • Diagnosis?
  • Severe methanol intoxication
  • Serum methanol 37mmol/L
  • Patient died

89
Case 10
  • 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

90
Case 10 Cont
  • Labs
  • 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
  • ABG
  • pH 7.10
  • pCO2 14 mmHg
  • pO2 103 mmHg
  • HCO3 4 mmol/l

91
Approach
  • Check the numbers
  • Apply the ABG rules
  • Calculate the AG
  • If Acidosis apply the rule of 15 (/- 2)
  • If Acidosis apply the delta gap (/- 4)
  • Check the osmolar gap

92
Case 10 Cont
  • WG metabolic acidosis with appropriate
    respiratory compensation
  • Likely lactic acidosis
  • Is there a secondary metabolic process?
  • ?AG 27 and ?HCO3 20
  • But

93
Case 10 Cont
  • Does a low serum albumin affect the measurement
    of the anion gap?
  • Yes!
  • If albumin lt40 g/l for every decline of 10 g/l
    subtract 4 from the normal value of the AG
  • Therefore the ?AG ?HCO3 and it is a pure WG
    metabolic acidosis

94
Case 11
  • 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

95
Case 11 Cont
  • ABG
  • pH 7.36
  • PO2 90
  • PCO2 45
  • HCO3 22
  • Are you concerned about her?

96
Case 11
  • Physiologic changes of pregnancy
  • Physiological hyperventilation results in
    respiratory alkalosis with compensatory renal
    excretion of bicarbonate
  • These changes alter normal ABG values
  • pH 7.4-7.45
  • PO2 95-105 mm Hg
  • PCO2 28-32 mm Hg,
  • HCO3 18-21 mEq/L.

97
Case 11 Cont
  • Even though the ABG does not at first glance
    appear worrisome
  • A pCO2 of 45 at this stage in pregnancy likely
    represents a significant degree of CO2 retention
  • Potentially impending resp failure!!!

98
Name that Acidosis
  • Distinctive Breath
  • DKA
  • Renal Failure
  • Uremia
  • Refractory Seizures
  • INH
  • Xray diagnosis
  • Iron ingestion
  • Blindness
  • Methanol
  • 1 Resp Alkalosis
  • ASA
  • GI Bleed
  • Lactic acidosis
  • U/A diagnosis
  • Ethylene glycol

99
Thanks!
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