Interpreting ABGs

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Interpreting ABGs

• Suneel Kumar MD

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Arterial Blood Gases

• Written in following manner
• pH/PaCO2/PaO2/HCO3
• pH arterial blood pH
• PaCO2 arterial pressure of CO2
• PaO2 arterial pressure of O2
• HCO3 serum bicarbonate concentration

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Oxygenation

• Hypoxia reduced oxygen pressure in the alveolus
  (i.e. PAO2)
• Hypoxemia reduced oxygen pressure in arterial
  blood (i.e. PaO2)

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Hypoxia with Low PaO2

• Alveolar diffusion impairment
• Decreased alveolar PO2
• Decreased FiO2
• Hypoventilation
• High altitude
• R ? L shunt
• V/Q mismatch

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Hypoxia with Normal PaO2

• Alterations in hemoglobin
• Anemic hypoxia
• Carbon monoxide poisoning
• Methemoglobinemia
• Histotoxic hypoxia
• Cyanide
• Hypoperfusion hypoxia or stagnant hypoxia

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AlveolarArterial Gradient

• Indirect measurement of V/Q abnormalities
• Normal A-a gradient is 10 mmHg
• Rises with age
• Rises by 5-7 mmHg for every 0.10 rise in FiO2,
  from loss of hypoxic vasoconstriction in the lungs

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AlveolarArterial Gradient

• A-a gradient PAO2 PaO2
• PAO2 alveolar PO2 (calculated)
• PaO2 arterial PO2 (measured)

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AlveolarArterial Gradient

• PAO2 PIO2 (PaCO2/RQ)
• PAO2 alveolar PO2
• PIO2 PO2 in inspired gas
• PaCO2 arterial PCO2
• RQ respiratory quotient

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AlveolarArterial Gradient

• PIO2 FiO2 (PB PH2O)
• PB barometric pressure (760 mmHg)
• PH2O partial pressure of water vapor (47 mmHg)
• RQ VCO2/VO2
• RQ defines the exchange of O2 and CO2 across the
  alveolar-capillary interface (0.8)

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AlveolarArterial Gradient

• PAO2 FiO2 (PB PH2O) (PaCO2/RQ)
• Or
• PAO2 FiO2 (713) (PaCO2/0.8)

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AlveolarArterial Gradient

• For room air
• PAO2 150 (PaCO2/0.8)
• And assume a normal PaCO2 (40)
• PAO2 100

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Acid-Base

• Acidosis or alkalosis any disorder that causes
  an alteration in pH
• Acidemia or alkalemia alteration in blood pH
  may be result of one or more disorders.

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Six Simple Steps

1. Is there acidemia or alkalemia?
2. Is the primary disturbance respiratory or
   metabolic?
3. Is the respiratory problem acute or chronic?
4. For metabolic, what is the anion gap?
5. Are there any other processes in anion gap
   acidosis?
6. Is the respiratory compensation adequate?

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Henderson-Hasselbach Equation

• pH pK log HCO3/PaCO2 x K
• (K dissociation constant of CO2)
• Or
• H 24 x PaCO2/HCO3

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Henderson-Hasselbach Equation

• pH
• 7.20
• 7.30
• 7.40
• 7.50
• 7.60

• H
• 60
• 50
• 40
• 30
• 20

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Step 1Acidemia or Alkalemia?

• Normal arterial pH is 7.40 0.02
• pH lt 7.38 ? acidemia
• pH gt 7.42 ? alkalemia

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Step 2Primary Disturbance

• Anything that alters HCO3 is a metabolic process
• Anything that alters PaCO2 is a respiratory
  process

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Step 2Primary Disturbance

• If 6pH, there is either 5PaCO2 or 6HCO3
• If 5pH, there is either 6PaCO2 or 5HCO3

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Step 4For Metabolic, Anion Gap?

• Anion gap Na - (Cl- HCO3-)
• Normal is lt 12

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Increased Anion Gap

• Ingestion of drugs or toxins
• Ethanol
• Methanol
• Ethylene glycol
• Paraldehyde
• Toluene
• Ammonium chloride
• Salicylates

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Increased Anion Gap

• Ketoacidosis
• DKA
• Alcoholic
• Starvation
• Lactic acidosis
• Renal failure

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Step 4For Metabolic, Anion Gap?

• If AG, calculate Osm gap
• Calc Osm (2 x Na) (glucose/18) (BUN/2.8)
  (EtOH/4.6)
• Osm gap measured Osm calc Osm
• Normal lt 10 mOsm/kg

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Nongap Metabolic Acidosis

• Administration of acid or acid-producing
  substances
• Hyperalimentation
• Nonbicarbonate-containing IVF

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Nongap Metabolic Acidosis

• GI loss of HCO3
• Diarrhea
• Pancreatic fistulas
• Renal loss of HCO3
• Distal (type I) RTA
• Distal (type IV) RTA
• Proximal (type II) RTA

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Nongap Metabolic Acidosis

• Calculate urine anion gap
• Urine AG (Na K) Cl-
• Positive gap indicates renal impaired NH4
  excretion
• Negative gap indicates normal NH4 excretion and
  nonrenal cause

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Nongap Metabolic Acidosis

• Urine Cl- lt 10 mEq/l is chloride responsive and
  accompanied by contraction alkalosis and is
  saline responsive
• Urine Cl- gt 20 mEq/l is chloride resistant, and
  treatment is aimed at underlying disorder

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Step 5 Any other process with elevated AG?

• Calculate rgap, or gap-gap
• rGap Measured AG Normal AG (12)

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Step 5 Any other process with elevated AG?

• Add rgap to measured HCO3
• If normal (22-26), no other metabolic problems
• If lt 22, then concomitant metabolic acidosis
• If gt 26, then concomitant metabolic alkalosis

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Step 6 Adequate respiratory compensation?

• Winters Formula
• Expected PaCO2 (1.5 x HCO3) 8 2
• If measured PaCO2 is higher, then concomitant
  respiratory acidosis
• If measured PaCO2 is lower, then concomitant
  respiratory alkalosis

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Step 6 Adequate respiratory compensation?

• In metabolic alkalosis, Winters formula does not
  predict the respiratory response
• PaCO2 will rise gt 40 mmHg, but not exceed 50-55
  mmHg
• For respiratory compensation, pH will remain gt
  7.42

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Clues to a Mixed Disorder

• Normal pH with abnormal PaCO2 or HCO3
• PaCO2 and HCO3 move in opposite directions
• pH changes in opposite direction for a known
  primary disorder

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Case 1

• A 24 year old student on the 6 year undergraduate
  plan is brought to the ER cyanotic and profoundly
  weak. His roommate has just returned from a
  semester in Africa. The patient had been observed
  admiring his roommate's authentic African blowgun
  and had scraped his finger on the tip of one of
  the poison darts (curare).

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Case 1
138
100
26
7.08/80/37
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Case 1

• What is the anion gap?
• AG 138 (100 26)
• AG 12

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Case 1

• Acute respiratory acidosis

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Case 2

• A 42 year old diabetic female who has been on
  insulin since the age of 13 presents with a 4 day
  history of dysuria which has progressed to severe
  right flank pain. She has a temperature of
  38.8ºC, a WBC of 14,000, and is disoriented.

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Case 2
135
99
12
4.8
7.23/25/113
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Case 2

• What is the A-a gradient?
• A-a 150 25/0.8 113 6
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 135 (99 12) 24

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Case 2

• What is the rgap?
• rGap 24 12 12
• rGap HCO3 12 12 24
• No other metabolic abnormalities
• Is the respiratory compensation appropriate?
• Expected PCO2 (1.5 x 12) 8 2 24 2
• It is appropriate

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Case 2

• Acute anion gap metabolic acidosis (DKA)

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Case 3

• A 71 year old male, retired machinist, is
  admitted to the ICU with a history of increasing
  dyspnea, cough, and sputum production. He has a
  120 pack-year smoking history, and quit 5 years
  previously. On exam he is moving minimal air
  despite using his accessory muscles of
  respiration. He has acral cyanosis.

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Case 3
135
93
30
7.21/75/41
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Case 3

• What is the A-a gradient?
• A-a 150 75/.8 41 15
• Acidemic or alkalemic?
• Primary respiratory or metabolic?
• Acute or chronic?
• Acute 5PCO2 by 35 would 6pH by 0.28
• Chronic 5PCO2 by 35 would 6pH by 0.105
• Somewhere in between

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Case 3

• What is the anion gap?
• AG 135 (93 30) 12

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Case 3

• Acute on chronic respiratory acidosis (COPD)

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Case 3b

• This same patient is intubated and mechanically
  ventilated. During the intubation he vomits and
  aspirates. He is ventilated with an FiO2 of 50,
  tidal volumes of 850cc, PEEP of 5, rate of 10.
  One hour later his ABG is 7.48/37/215.

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Case 3b

• What is the A-a gradient?
• A-a FiO2 (713) 37/.8 215
• A-a 310 215 95
• Why is he alkalotic with a normal PCO2?
• Chronic compensatory metabolic alkalosis and
  acute respiratory alkalosis

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Case 4

• A 23 year old female presents to the Emergency
  Room complaining of chest tightness and
  light-headedness. Other symptoms include tingling
  and numbness in her fingertips and around her
  mouth. Her medications include Xanax and birth
  control pills, but she recently ran out of both.

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Case 4
135
109
22
7.54/22/115
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Case 4

• What is the A-a gradient?
• A-a 150 22/.8 115 8
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• Acute or chronic?
• Acute 6CO2 by 18 would 5pH by 0.144
• What is the anion gap?
• AG 135 (109 22) 4

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Case 4

• Acute respiratory alkalosis (panic attack)

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Case 5

• 72 year old woman admitted from a nursing home
  with one week history of diarrhea and fever.

133
118
5
7.11/16/94
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Case 5

• What is the A-a gradient?
• A-a 150 16/.8 94 36
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 133 (118 5) 10
• Is respiratory compensation adequate?
• PCO2 (1.5 x 5) 8 2 16 2

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Case 5

• Non anion gap metabolic acidosis (diarrhea)
• Compensatory respiratory alkalosis

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Case 6

• A 27 year old pregnant alcoholic with IDDM is
  admitted one week after stopping insulin and
  beginning a drinking binge. She has experienced
  severe nausea and vomiting for several days.

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Case 6
136
70
19
7.58/21/104
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Case 6

• What is the A-a gradient?
• A-a 150 21/.8 104 20
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 136 (70 19) 47
• What is the rgap?
• rGap 47-12 35
• rGap HCO3 54

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Case 6

• Primary respiratory alkalosis (pregnancy)
• Anion gap metabolic acidosos (ketoacidosis)
• Metabolic alkalosis (vomiting)

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Case 7

• 35 year old male presents to the ER unconscious.

145
70
Creat 6.1
23
7.61/24/78
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Case 7

• What is the A-a gradient?
• A-a 150 24/.8 78 42
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 145 (70 23) 52

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Case 7

• What is the rgap?
• rGap 52 - 12 40
• rGap HCO3 63
• Metabolic alkalosis

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Case 7

• Respiratory alkalosis
• Anion gap metabolic acidosis (renal failure)
• Metabolic alkalosis

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Bonus Case 1

• 51 year old man with polysubstance abuse,
  presented to ER with 3-4 day h/o N/V and diffuse
  abdominal pain. Reports no EtOH or cocaine in 2
  weeks. He has been taking a lot of aspirin for
  pain. Denies dyspnea, but has been tachypneic
  since arrival.

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Bonus Case 1

• Afebrile, P 89, R 20, BP 142/57. Lethargic but
  arrousable, easily aggitated. Lungs clear, and
  abdomen is soft with mild tenderness in LUQ and
  LLQ.

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Bonus Case 1
126
93
58
218
11
3.4
1.8
UA 1 ketones Acetone negative Lactate 6.9 EtOH
0 Osm 272
7.46/15/107
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Bonus Case 1

• What is the A-a gradient?
• A-a 150 15/.8 107 25
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 126 (93 11) 22
• Anion gap metabolic acidosis

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Bonus Case 1

• What is the rgap?
• rGap 22 - 12 10
• rGap HCO3 21
• Non gap metabolic acidosis
• What is the osmolar gap?
• Calc Osm 2x126 218/18 58/2.8
• Calc Osm 265
• Osm gap 272 265 7

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Bonus Case 1

• Respiratory alkalosis (aspirin)
• Anion gap metabolic acidosis (aspirin)
• Non gap metabolic acidosis

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Bonus Case 2

• 20 year old college student brought to the ER by
  his fraternity brothers because they cannot wake
  him up. He had been in excellent health until
  the prior night.

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Bonus Case 2

• Afebrile, P 118, R 32, BP 120/70. Anicteric
  sclerae, pupils 8mm and poorly responsive to
  light. Fundoscopic exam with slight blurring of
  discs bilaterally and increased retinal sheen.
  Remainder of exam unremarkable.

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Bonus Case 2
142
98
14
108
10
4.3
UA negative EtOH 45 Osm 348
7.22/24/108
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Bonus Case 2

• What is the A-a gradient?
• A-a 150 24/.8 108 12
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 142 (98 10) 34
• Anion gap metabolic acidosis

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Bonus Case 2

• What is the rgap?
• rGap 34 - 12 22
• rGap HCO3 32
• Metabolic alkalosis

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Bonus Case 2

• What is the osmolar gap?
• Calc Osm 2x142 108/18 14/2.8 45/4.6
• Calc Osm 305
• Osm gap 348 - 305 43
• Is the respiratory compensation adequate?
• PCO2 (1.5 x 10) 8 2 23 2

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Bonus Case 2

• Anion gap metabolic acidosis with elevated
  osmolar gap (methanol)
• Metabolic alkalosis
• Compensatory respiratory alkalosis

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Bonus Case 3

• A 23 year old man presents with confusion. He has
  had diabetes since age 12, and has been suffering
  from an intestinal flu for the last 24 hours. He
  has not been eating much, has vague stomach pain,
  stopped taking his insulin, and has been
  vomiting. His glucose is high.

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Bonus Case 3
130
80
10
7.20/25/68
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Bonus Case 3

• What is the A-a gradient?
• A-a 150 25/.8 68 51
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
• AG 130 (80 10) 40
• Anion gap metabolic acidosis

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Bonus Case 3

• What is the rgap?
• rGap 40 - 12 28
• rGap HCO3 38
• Metabolic alkalosis
• Is the respiratory compensation adequate?
• PCO2 (1.5 x 10) 8 2 23 2

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Bonus Case 3

• Anion gap metabolic acidosis (DKA)
• Metabolic metabolic alkalosis (emesis)
• Compensatory respiratory alkalosis

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