Arterial Blood Gas Analysis

What is an Arterial Blood Gas (ABG)? The components pH / PaCO2 / PaO2 / HCO3 / O2sat / BE Desired ranges pH – 7.35 - 7.45 PaCO2 – 35 - 45 mmHg PaO2 – 80 - 100 mmHg HCO3 – 21 - 27 O2sat – 95 - 100% Base excess – +/-2 mEq/L

Why Order an ABG? Aids in establishing a diagnosis Helps guide treatment plan Aids in ventilator management Improvement in acid/base management allows for optimal function of medications Acid/base status may alter electrolyte levels critical to patient status/care

Logistics When to order an arterial line Where to place – The options Need for continuous BP monitoring Need for multiple ABGs Where to place – The options Radial Femoral Brachial Dorsalis Pedis Axillary

Acid Base Balance The body produces acids daily 15,000 mmol CO2 50 - 100 mEq Nonvolatile acids The lungs and kidneys attempt to maintain balance

Acid Base Balance Assessment of status via bicarbonate-carbon dioxide buffer system CO2 + H2O <--> H2CO3 <--> HCO3- + H+ ph = 6.10 + log ([HCO3] / [0.03 x PCO2])

The Terms Acids Bases Acidemia Acidosis Alkalemia Alkalosis Respiratory CO2 Metabolic HCO3 Bases Alkalemia Alkalosis Respiratory CO2 Metabolic HCO3

Respiratory Acidosis ph, CO2, Ventilation Causes CNS depression Pleural disease COPD/ARDS Musculoskeletal disorders Compensation for metabolic alkalosis

Respiratory Acidosis Acute vs. chronic Acute – Little kidney involvement. Buffering via titration, via Hb for example. pH by 0.08 for 10mmHg  in CO2 Chronic – Renal compensation via synthesis and retention of HCO3 (Cl to balance charges  hypochloremia) pH by 0.03 for 10mmHg in CO2

Respiratory Alkalosis  pH,  CO2,  Ventilation  CO2   HCO3 ( Cl to balance charges  hyperchloremia) Causes Intracerebral hemorrhage Salicylate and progesterone drug usage Anxiety   lung compliance Cirrhosis of the liver Sepsis

Respiratory Alkalosis Acute vs. chronic Acute – HCO3 by 2 mEq/L for every 10mmHg  in PCO2 Chronic – Ratio increases to 4 mEq/L of HCO3 for every 10mmHg  in PCO2 Decreased bicarb reabsorption and decreased ammonium excretion to normalize pH

Metabolic Acidosis  pH,  HCO3 12 - 24 hours for complete activation of respiratory compensation  PCO2 by 1.2mmHg for every 1 mEq/L  HCO3 The degree of compensation is assessed via the Winter’s Formula  PCO2 = 1.5(HCO3) +8  2

The Causes Metabolic gap acidosis Non-gap metabolic acidosis M – Methanol U – Uremia D – DKA P – Paraldehyde I – INH L – Lactic acidosis E – Ehylene glycol S – Salicylate Non-gap metabolic acidosis Hyperalimentation Acetazolamide RTA (Calculate urine anion gap) Diarrhea Pancreatic fistula

Metabolic Alkalosis  pH,  HCO3  PCO2 by 0.7 for every 1mEq/L  in HCO3 Causes Vomiting Diuretics Chronic diarrhea Hypokalemia Renal failure

Mixed Acid-Base Disorders Patients may have two or more acid-base disorders at one time Delta gap Delta HCO3 = HCO3 + Change in anion gap >24 = metabolic alkalosis

The Steps Start with the pH Note the PCO2 Calculate anion gap Determine compensation

Sample Problem #1 An ill-appearing alcoholic male presents with nausea and vomiting ABG – 7.4 / 41 / 85 / 22 Na – 137 / K- 3.8 / Cl- 90 / HCO3- 22

Sample Problem #1 Anion gap = 137 - (90 + 22) = 25  Anion gap metabolic acidosis Winter’s Formula = 1.5(22) + 8  2 = 39  2  Compensated Delta gap = 25 - 10 = 15 15 + 22 = 37  Metabolic alkalosis

Sample Problem #2 A 22-year-old female presents for attempted overdose. She has taken an unknown amount of Midol containing aspirin, cinnamedrine, and caffeine. On the exam she is experiencing respiratory distress.

Sample Problem #2 ABG – 7.47 / 19 / 123 / 14 Na – 145 / K- 3.6 / Cl- 109 / HCO3- 17 ASA level – 38.2 mg/dL

Sample Problem #2 Anion gap = 145 - (109 + 17) = 19  Anion gap metabolic acidosis Winter’s Formula = 1.5 (17) + 8  2 = 34  2  Uncompensated Delta gap = 19 - 10 = 9 9 + 17 = 26  No metabolic alkalosis

Sample Problem #3 47-year-old-male experienced crush injury at construction site ABG – 7.3 / 32 / 96 / 15 Na – 135 / K-5 / Cl- 98 / HCO3- 15 / BUN- 38 / Cr- 1.7 CK – 42, 346

Sample Problem #3 Anion gap = 135 - (98 + 15) = 22  Anion gap metabolic acidosis Winter’s Formula = 1.5 (15) + 8  2 = 30  2  Compensated Delta gap = 22 - 10 = 12 12 + 15 = 27  Mild metabolic alkalosis

Sample Problem #4 1-month-old male presents with projectile emesis x 2 days ABG – 7.49 / 40 / 98 / 30 Na – 140 / K- 2.9 / Cl- 92 / HCO3- 32

Sample Problem #4 Metabolic alkalosis, hypochloremic Winter’s Formula = 1.5 (30) + 8  2 = 53  2  Uncompensated