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Interpreting ABGsInterpreting ABGsInterpreting ABGsInterpreting ABGs
Suneel Kumar MDSuneel 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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Alveolar—Arterial 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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Alveolar—Arterial Gradient
A-a gradient = PAO2 – PaO2
• PAO2 = alveolar PO2 (calculated)
• PaO2 = arterial PO2 (measured)
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Alveolar—Arterial Gradient
PAO2 = PIO2 – (PaCO2/RQ)
• PAO2 = alveolar PO2
• PIO2 = PO2 in inspired gas
• PaCO2 = arterial PCO2
• RQ = respiratory quotient
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Alveolar—Arterial 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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Alveolar—Arterial Gradient
PAO2 = FiO2 (PB – PH2O) – (PaCO2/RQ)
Or
PAO2 = FiO2 (713) – (PaCO2/0.8)
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Alveolar—Arterial 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 Steps1. 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
pH7.207.307.407.507.60
[H+]6050403020
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Step 1:Acidemia or Alkalemia?
• Normal arterial pH is 7.40 ± 0.02– pH < 7.38 acidemia– pH > 7.42 alkalemia
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Step 2:Primary Disturbance
• Anything that alters HCO3 is a metabolic process
• Anything that alters PaCO2 is a respiratory process
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Step 2:Primary Disturbance
• If pH, there is either PaCO2 or HCO3
• If pH, there is either PaCO2 or HCO3
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Step 4:For Metabolic, Anion Gap?
Anion gap = Na+ - (Cl- + HCO3-)
– Normal is < 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 4:For 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 < 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- < 10 mEq/l is chloride responsive and accompanied by “contraction alkalosis” and is “saline responsive”
• Urine Cl- > 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 gap, or “gap-gap”:
Gap = Measured AG – Normal AG (12)
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Step 5: Any other process with elevated
AG?• Add gap to measured HCO3
– If normal (22-26), no other metabolic problems
– If < 22, then concomitant metabolic acidosis
– If > 26, then concomitant metabolic alkalosis
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Step 6: Adequate respiratory
compensation?Winter’s 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, Winter’s
formula does not predict the respiratory response– PaCO2 will rise > 40 mmHg, but not
exceed 50-55 mmHg– For respiratory compensation, pH will
remain > 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 10026
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
124.8
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 gap?
Gap = 24 – 12 = 12Gap + 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 PCO2 by 35 would pH by 0.28
– Chronic PCO2 by 35 would pH 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 10922
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 CO2 by 18 would pH 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 1185
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 gap?
Gap = 47-12 = 35
Gap + 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
23
7.61/24/78
Creat 6.1
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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 gap?
Gap = 52 - 12 = 40
Gap + 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 #1126
3.4
93
11
58
1.8218
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) = 22Anion gap metabolic acidosis
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Bonus Case #1• What is the gap?
Gap = 22 - 12 = 10
Gap + HCO3 = 21Non gap metabolic acidosis
• What is the osmolar gap?Calc Osm = 2x126 + 218/18 +
58/2.8Calc 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 #2142
4.3
98
10
14 108
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) = 34Anion gap metabolic acidosis
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Bonus Case #2• What is the gap?
Gap = 34 - 12 = 22
Gap + 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 = 305Osm 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) = 40Anion gap metabolic acidosis
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Bonus Case #3
• What is the gap?
Gap = 40 - 12 = 28
Gap + 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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