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

5. Outline Why is everyone using VBG’s anyway? Basic Review
Lists that you have to remember (D’oh!)
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. What’s bad about ABG’s ABG’s 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)
N=218 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 = ( )
pCO2 = 6.0 ( ) mmHg
HCO3 = 1.5 ( ) mEq/L

12. Can be used to follow trends
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
58% response rate to their survey
0.05 ph units, 6.6mmHg PCO2, 3.5mEq/L HCO3

13. Since 2002
Arterial Blood Gas Analysis: Are Its Values Needed for the Management of Diabetic Ketoacidosis?
Ann Emerg Med. 2005;45:
Good correlation between arterial and venous pH and HCO3
The case for venous rather than arterial blood gases in diabetic ketoacidosis
Emerg Med Australas Feb;18(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
Equal to [H+] μM
pH>7.8 or <6.8 is incompatible w/life

17. 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
BCABS: open ended, continuous removal of organic acid is possible with exhalation of CO2. Primary contributor
Hemoglobin can buffer large amounts of acid. Venous blood would be at a ph of 4.5 if not for Hgb

19. Lungs Changes in pH sensed by chemoreceptors Drop in pH Increase in pH
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-
>6hrs of Alkalemia
Active excretion of HCO3-
Retention of H+
Predominantly in the form of NH4 (ammonium)

21. Normal ABG parameters pH 7.40 PCO2 40 mmHg [HCO3] 24 mM
Anion Gap =

22. Terminology Acidemia: blood pH < 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 > 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
i.e. a patient who hyperventilates (lowers PaCO2) solely as compensation for MAc does not have a RAlk, the latter being a primary disorder that, alone, would lead to alkalemia. In simple, uncomplicated MAc the patient will never develop alkalemia.

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

26. Lists you have to remember….
D’oh…

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
Otherwise known as chloride responsive and unresponsive

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/Addison’s/ 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 <10 mEq/L
Vomiting/Gastric Suction
Diuretics
Ion-deficient baby formula
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 >10mEq/L
Primary aldosteronism
Exogenous steroids
Adenocarcinoma
Bartter’s Syndrome
Cushing’s disease
Ectopic adrenocorticotropic hormone
Exogenous mineralocorticoids are Licorice and chewing tobacco

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 : 1
Acute Resp Alkalosis : 2
Chronic Resp Acidosis : 3
Chronic Resp Alkalosis : 4

36. Metabolic Compensation
Compensation PaCO2 : HCO3-
Metabolic Acidosis : 10
Metabolic Alkalosis : 7.5

37. “The Corey Slovis approach to acid-base abnormalities”
A no approach
for non-nephrologist

38. Slovis 6-step approach to ABG
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

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 (>7.45, <7.35)
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 = 1:1

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
HCO = 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 = expected pCO2

53. Examples of rule of 15 1) HCO3=20, pCO2=35 pH= 7.35
Pure wide gap metabolic acidosis with an appropriate 2ndary resp alkalosis
2) HCO3=10, pCO2=20 pH= 7.32
pCO2 is too low. Superimposed primary resp alkalosis
3) HCO3=10, pCO2=32 pH= 7.14
pCO2 is too high. Superimposed primary resp acidosis

54. 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

55. Examples of delta gap AG=20 HCO3=19 AG=22 HCO3=8 AG=26 HCO3=20
∆AG = 5 and ∆HCO3 = 5
No hidden process
AG=22 HCO3=8
∆AG = 7 and ∆HCO3 = 16
Bicarb too low = additional normal AG metabolic acidosis
AG=26 HCO3=20
∆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

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 = 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 con’t 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 = pO2 = 100
HCO3 =15
Is this DKA???

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? 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 = 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 Con’t Metabolic Alkalosis w/ Resp alkalosis (10:7.5)
? Saline responsive or resistant
Resistant
DDx?
Primary aldosteronism
Exogenous steroids
Adenocarcinoma
Bartter’s Syndrome
Cushing’s disease
Ectopic adrenocorticotropic hormone
Why is the K+ so low?
Mineralcorticoid excess results in sodium reabsorption. To maintain neutrality the kidney must pump out H+ and K+

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 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

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 = 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? What is the Osmolar Gap?
Serum Osmolarity = 487 mOsm
Serum EtOH < 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: ABG: 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……
Does anyone know what the Figge Correction is

93. Case #10 Cont
Does a low serum albumin affect the measurement of the anion gap?
Yes!
If albumin <40 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 Are you concerned about her? 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
PO2 = mm Hg
PCO2 = mm Hg,
HCO3 = 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 Renal Failure
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!