interpreting abgs suneel kumar md. arterial blood gases written in following manner: ph/paco 2 /pao...

Interpreting ABGsInterpreting ABGsInterpreting ABGsInterpreting ABGs

Suneel Kumar MDSuneel Kumar MD

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

Oxygenation• Hypoxia: reduced oxygen

pressure in the alveolus (i.e. PAO2)

• Hypoxemia: reduced oxygen pressure in arterial blood (i.e. PaO2)

Hypoxia with Low PaO2

• Alveolar diffusion impairment

• Decreased alveolar PO2

– Decreased FiO2

– Hypoventilation– High altitude

• R L shunt• V/Q mismatch

Hypoxia with Normal PaO2

• Alterations in hemoglobin– Anemic hypoxia– Carbon monoxide poisoning– Methemoglobinemia

• Histotoxic hypoxia– Cyanide

• Hypoperfusion hypoxia or stagnant hypoxia

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

Alveolar—Arterial Gradient

A-a gradient = PAO2 – PaO2

• PAO2 = alveolar PO2 (calculated)

• PaO2 = arterial PO2 (measured)

Alveolar—Arterial Gradient

PAO2 = PIO2 – (PaCO2/RQ)

• PAO2 = alveolar PO2

• PIO2 = PO2 in inspired gas

• PaCO2 = arterial PCO2

• RQ = respiratory quotient

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)

Alveolar—Arterial Gradient

PAO2 = FiO2 (PB – PH2O) – (PaCO2/RQ)

Or

PAO2 = FiO2 (713) – (PaCO2/0.8)

Alveolar—Arterial Gradient

• For room air:PAO2 = 150 – (PaCO2/0.8)

• And assume a normal PaCO2 (40):

PAO2 = 100

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.

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?

Henderson-Hasselbach Equation

pH = pK + log [HCO3/PaCO2] x K(K = dissociation constant of CO2)

Or

[H+] = 24 x PaCO2/HCO3

Henderson-Hasselbach Equation

pH7.207.307.407.507.60

[H+]6050403020

Step 1:Acidemia or Alkalemia?

• Normal arterial pH is 7.40 ± 0.02– pH < 7.38 acidemia– pH > 7.42 alkalemia

Step 2:Primary Disturbance

• Anything that alters HCO3 is a metabolic process

• Anything that alters PaCO2 is a respiratory process

Step 2:Primary Disturbance

• If pH, there is either PaCO2 or HCO3

• If pH, there is either PaCO2 or HCO3

Step 4:For Metabolic, Anion Gap?

Anion gap = Na+ - (Cl- + HCO3-)

– Normal is < 12

Increased Anion Gap• Ingestion of drugs or toxins

– Ethanol– Methanol– Ethylene glycol– Paraldehyde– Toluene– Ammonium chloride– Salicylates

Increased Anion Gap• Ketoacidosis

– DKA– Alcoholic– Starvation

• Lactic acidosis• Renal failure

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

Nongap Metabolic Acidosis

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

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

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

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

Step 5: Any other process with elevated

AG?• Calculate gap, or “gap-gap”:

Gap = Measured AG – Normal AG (12)

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

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

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

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

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).

Case 1

138 10026

7.08/80/37

Case 1• What is the anion gap?

AG = 138 – (100 + 26)AG = 12

Case 1

• Acute respiratory acidosis

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.

Case 2

135 99

124.8

7.23/25/113

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

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

Case 2

• Acute anion gap metabolic acidosis (DKA)

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.

Case 3

135 93

30

7.21/75/41

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

Case 3• What is the anion gap?

AG = 135 – (93 + 30) = 12

Case 3

• Acute on chronic respiratory acidosis (COPD)

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.

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

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.

Case 4

135 10922

7.54/22/115

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

Case 4

• Acute respiratory alkalosis (panic attack)

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

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

Case 5

• Non anion gap metabolic acidosis (diarrhea)

• Compensatory respiratory alkalosis

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.

Case 6

136 70

19

7.58/21/104

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

Case 6

• Primary respiratory alkalosis (pregnancy)

• Anion gap metabolic acidosos (ketoacidosis)

• Metabolic alkalosis (vomiting)

Case 7• 35 year old male presents to the

ER unconscious.

145 70

23

7.61/24/78

Creat 6.1

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

Case 7

• What is the gap?

Gap = 52 - 12 = 40

Gap + HCO3 = 63–Metabolic alkalosis

Case 7

• Respiratory alkalosis• Anion gap metabolic acidosis

(renal failure)• Metabolic alkalosis

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.

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.

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

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

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

Bonus Case #1

• Respiratory alkalosis (aspirin)• Anion gap metabolic acidosis

(aspirin)• Non gap metabolic acidosis

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.

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.

Bonus Case #2142

4.3

98

10

14 108

UA negative

EtOH 45

Osm 348 7.22/24/108

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

Bonus Case #2• What is the gap?

Gap = 34 - 12 = 22

Gap + HCO3 = 32

Metabolic alkalosis

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

Bonus Case #2

• Anion gap metabolic acidosis with elevated osmolar gap (methanol)

• Metabolic alkalosis• Compensatory respiratory

alkalosis

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.

Bonus Case #3

130 80

10

7.20/25/68

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

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

Bonus Case #3

• Anion gap metabolic acidosis (DKA)

• Metabolic metabolic alkalosis (emesis)

• Compensatory respiratory alkalosis