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Interpreting ABGs Suneel Kumar MD

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Arterial Blood Gases Written in following manner: pH/PaCO 2 /PaO 2 /HCO 3 –pH = arterial blood pH –PaCO 2 = arterial pressure of CO 2 –PaO 2 = arterial pressure of O 2 –HCO 3 = serum bicarbonate concentration

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Oxygenation Hypoxia: reduced oxygen pressure in the alveolus (i.e. P A O 2 ) Hypoxemia: reduced oxygen pressure in arterial blood (i.e. P a O 2 )

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Hypoxia with Low PaO 2 Alveolar diffusion impairment Decreased alveolar PO 2 –Decreased FiO 2 –Hypoventilation –High altitude R L shunt V/Q mismatch

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Hypoxia with Normal PaO 2 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 FiO 2, from loss of hypoxic vasoconstriction in the lungs

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Alveolar—Arterial Gradient A-a gradient = P A O 2 – P a O 2 P A O 2 = alveolar PO 2 (calculated) P a O 2 = arterial PO 2 (measured)

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Alveolar—Arterial Gradient P A O 2 = P I O 2 – (P a CO 2 /RQ) P A O 2 = alveolar PO 2 P I O 2 = PO2 in inspired gas P a CO 2 = arterial PCO 2 RQ = respiratory quotient

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Alveolar—Arterial Gradient P I O 2 = FiO 2 (P B – P H2O ) P B = barometric pressure (760 mmHg) P H2O = partial pressure of water vapor (47 mmHg) RQ = V CO 2 /V O 2 RQ defines the exchange of O 2 and CO 2 across the alveolar-capillary interface (0.8)

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Alveolar—Arterial Gradient P A O 2 = Fi O 2 (P B – P H2O ) – (P a CO 2 /RQ) Or P A O 2 = FiO 2 (713) – (P a CO 2 /0.8)

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Alveolar—Arterial Gradient For room air: P A O 2 = 150 – (P a CO 2 /0.8) And assume a normal P a CO 2 (40): P A O 2 = 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 [HCO 3 /PaCO 2 ] x K (K = dissociation constant of CO 2 ) Or [H + ] = 24 x PaCO 2 /HCO 3

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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 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 HCO 3 is a metabolic process Anything that alters PaCO 2 is a respiratory process

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Step 2: Primary Disturbance If pH, there is either PaCO 2 or HCO 3 If pH, there is either PaCO 2 or HCO 3

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Step 4: For Metabolic, Anion Gap? Anion gap = Na + - (Cl - + HCO 3 - ) –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 HCO 3 –Diarrhea –Pancreatic fistulas Renal loss of HCO 3 –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 NH 4 + excretion –Negative gap indicates normal NH 4 + 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 HCO 3 –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 PaCO 2 = (1.5 x HCO 3 ) + 8 ± 2 –If measured PaCO 2 is higher, then concomitant respiratory acidosis –If measured PaCO 2 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 –PaCO 2 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 PaCO 2 or HCO 3 PaCO 2 and HCO 3 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 138100 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 13599 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 = 12 Gap + HCO 3 = 12 + 12 = 24 –No other metabolic abnormalities Is the respiratory compensation appropriate? Expected PC O 2 = (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 13593 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 PCO 2 by 35 would pH by 0.28 –Chronic PCO 2 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 = [FiO 2 (713) – 37/.8] – 215 A-a = 310 – 215 = 95 Why is he alkalotic with a normal PCO 2 ? –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 135109 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 CO 2 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. 133118 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? PCO 2 = (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 13670 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 + HCO 3 = 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. 14570 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 + HCO 3 = 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 3.4 93 11 58 1.8 218 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 gap? Gap = 22 - 12 = 10 Gap + HCO 3 = 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 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) = 34 Anion gap metabolic acidosis

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Bonus Case #2 What is the gap? Gap = 34 - 12 = 22 Gap + HCO 3 = 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? PCO 2 = (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 13080 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 gap? Gap = 40 - 12 = 28 Gap + HCO 3 = 38 Metabolic alkalosis Is the respiratory compensation adequate? PCO 2 = (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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