Saturday 15 December 2012

Arterial Blood Gas (ABG) Analysis


Arterial Blood Gas (ABG) Analysis

Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism, but against hydrogen ions. - H.L. MENCKEN

A Guide to Interpretation of Arterial Blood Gases (ABGs)
 

Arterial blood gases (ABGs) are an important diagnostic tool for the evaluation of patients. They are useful for determining the ventilation status, the oxygenation status, and the acid-base balance of the patient.


 

Normal Arterial Blood Gas Values

ArterialMixed Venous
pH 7.35-7.45pH 7.34-7.37
PaC02 35-45 mmHgPvC02 44-46 mmHg
PaO2 80-100 mmHgPv02 38-42 mmHg
HC03 22-26 mEq/LiterHC03- 24-30 mEq/Liter
SaO2 > 95%Sv02  60-80%

Things to know before ABG interpretation

Things you MUST Know
• Fio2
• Patient’s temperature (some controversy here)
• Ventilator status
• PEEP
• Patient’s age
• Comorbidity

Things you SHOULD Know
• Hgb/Hct
• Drugs given
• Fraction type specific Hgb
• Chief complaint
• Diagnosis
• Respiratory rate
• Vt or MV
• I and O (fluid balance)


 

Definitions of Acid Base Disorders

Acidemia — An arterial pH below the normal range (less than 7.36).
Alkalemia — An arterial pH above the normal range (greater than 7.44).
Acidosis — A process that tends to lower the extracellular fluid pH (hydrogen ion concentration increases). This can be caused by a fall in the serum bicarbonate (HCO3) concentration and/or an elevation in PCO2.
Alkalosis — A process that tends to raise the extracellular fluid pH (hydrogen ion concentration decreases). This can be caused by an elevation in the serum HCO3 concentration and/or a fall in PCO2.
Metabolic acidosis — A disorder that causes reductions in the serum HCO3 concentration and pH.
Metabolic alkalosis — A disorder that causes elevations in the serum HCO3 concentration and pH.
Respiratory acidosis — A disorder that causes an elevation in arterial PCO2 and a reduction in pH.
Respiratory alkalosis — A disorder that causes a reduction in arterial PCO2 and an increase in pH.
Simple acid-base disorders — The presence of one of the above four disorders with the appropriate respiratory or renal compensation for that disorder.
Mixed acid-base disorders — The simultaneous presence of more than one acid-base disorder. Mixed acid-base disorders can be suspected from the patient’s history, from a lesser or greater than expected compensatory respiratory or renal response, and from analysis of the serum electrolytes and anion gap. As an example, a patient with severe vomiting would be expected to develop a metabolic alkalosis due to the loss of acidic gastric fluid. If, however, the patient developed hypovolemic shock from the fluid loss, the ensuing lactic acidosis would lower the elevated serum HCO3 possibly to below normal values, resulting in acidemia.
Clinical StateAcid-Base Disorder
Pulmonary embolusRespiratory alkalosis
HypotensionMetabolic acidosis
VomitingMetabolic alkalosis
Severe diarrheaMetabolic acidosis
CirrhosisRespiratory alkalosis
Renal failureMetabolic acidosis
SepsisRespiratory alkalosis, metabolic acidosis
PregnancyRespiratory alkalosis
Diuretic useMetabolic alkalosis
COPDRespiratory acidosis


Conditions that modify ABG Results

Delayed analysis - Iced Sample maintains values for 1-2 hours, Un-iced sample quickly becomes invalid - (PaCO2 rises 3-10 mmHg/hour, PaO2 falls at a rate related to initial value, pH falls modestly)
Excessive Heparin - Dilutional effect on results, Decreases bicarbonate and PaCO2
Large Air bubbles not expelled from sample - PaO2 rises 0-30 mmHg, PaCO2 may fall slightly
Fever or Hypothermia - Machine temperature approaches 37 C, Patient temperature shifts oxyhemoglobin curve
Hyperventilation or breath holding (due to anxiety) - May lead to erroneous lab results

Check for the consistency of ABG

While making an interpretation of an ABG always check for the consistency of the report by the modified Henderson equation 
[H+]=24(PaCO2/[HCO3])

The hydrogen ion is calculated by subtracting the two digits after the decimal point of pH from 80, e.g., if the pH is 7.24 then [H+] = 80-24 =56

ABG Interpretation Steps

Step 1: Determine primary abnormality
  1. Determine Acidosis versus alkalosis
    1. pH <7.35: Acidosis
    2. pH >7.45: Alkalosis
  2. Determine Metabolic versus Respiratory
    1. Primary Metabolic Disorder
      1. pH changes in same direction as bicarbonate, PaCO2
      2. Metabolic Acidosis
        1. Serum pH decreased
        2. Serum bicarbonate and PaCO2 decreased
      3. Metabolic Alkalosis
        1. Serum pH increased
        2. Serum bicarbonate and PaCO2 increased
    2. Primary Respiratory Disorder
      1. pH changes in opposite direction bicarbonate, PaCO2
      2. Respiratory Acidosis
        1. Serum pH decreased
        2. Serum bicarbonate and PaCO2 increased
      3. Respiratory Alkalosis
        1. Serum pH increased
        2. Serum bicarbonate and PaCO2 decreased
Step 2: Sharpen the diagnosis
  1. Calculate the Anion Gap
    1. Helpful in Metabolic Acidosis
    2. Helpful in mixed acid-base disorders
  2. Calculate Osmolal Gap
    1. Helpful in Metabolic Acidosis
  3. Calculate Urinary Anion Gap
    1. Helpful in Non-Anion Gap Metabolic Acidosis
    2. Distinguishes renal from extra-renal cause
Step 3: Determine Compensation
  1. Metabolic Acidosis
    1. PaCO2 decreases 1.2 mmHg per 1 meq/L bicarbonate fall
  2. Metabolic Alkalosis
    1. PaCO2 increases 6 mmHg per 10 meq/L bicarbonate rise
  3. Acute Respiratory Acidosis
    1. Bicarbonate increases 1 meq/L per 10 mmHg PaCO2 rise
  4. Chronic Respiratory Acidosis
    1. Bicarbonate increases 4 meq/L per 10 mmHg PaCO2 rise
  5. Acute Respiratory Alkalosis
    1. Bicarbonate decreases 2 meq/L per 10 mmHg PaCO2 fall
  6. Chronic Respiratory Alkalosis
    1. Bicarbonate decreases 4 meq/L per 10 mmHg PaCO2 fall
Step 4: Define Associated Abnormalities
  1. Calculated PaCO2
    1. Useful in High Anion Gap Metabolic Acidosis
    2. Defines concurrent respiratory abnormalities
  2. Excess Anion Gap
    1. EAG > 30 mEq/L: Metabolic Alkalosis present
    2. EAG < 23 mEq/L: Metabolic Acidosis present


It’s not magic understanding ABG’s, it just takes a little practice!


 

ABG Examples for Practice

1. pH 7.51, pCO2 40, HCO3- 31
a. Normal
b. Uncompensated metabolic alkalosis
c. Partially compensated respiratory acidosis
d. Uncompensated respiratory alkalosis
2. pH 7.33, pCO2 29, HCO3- 16 (everything is decreased)
a. Uncompensated respiratory alkalosis
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
 3. pH 7.40, pCO2 40, HCO3- 24
a. Normal
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
4. pH 7.12, pCO2 60, HCO3- 29 (decreased pH/increased bicarb/Co2)
a. Uncompensated metabolic acidosis
b. Uncompensated respiratory acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
 5. pH 7.48, pCO2 30, HCO3- 23
a. Uncompensated metabolic alkalosis
b. Uncompensated respiratory alkalosis
c. Partially compensated respiratory alkalosis
d. Partially compensated metabolic alkalosis
6. pH 7.62, pCO2 47, HCO3- 30
a. Uncompensated metabolic alkalosis
b. Uncompensated respiratory alkalosis
c. Partially compensated respiratory alkalosis
d. Partially compensated metabolic alkalosis
7. pH 7.30, pCO2 59. HCO3- 28
a. Uncompensated metabolic acidosis
b. Uncompensated respiratory acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis

Answers and Rationale:
1. B – pH is high, Bicarb is high, Co2 is normal and not attempting to correct the problem so this metabolic alkalosis is uncompensated.
2. D – pH is low, Bicarb is low, Co2 is low and attempting to correct the problem (but has not completely helped) so this is partially compensated metabolic acidosis.
3. A – pH, Bicarb and Co2 are within normal ranges so this is a normal ABG
4. C – pH is low, Co2 is high, Bicarb is high and attempting to correct the problem (but has not completely helped) so this is partially compensated respiratory acidosis
5. B – pH is high, Co2 is low, Bicarb is normal and not attempting to correct the problem so this is an uncompensated respiratory alkalosis
6. D – pH is high, Bicarb is high, Co2 is high and attempting to correct the problem (but has not completely helped) so this is a partially compensated metabolic alkalosis
7. C – pH is low, Co2 is high, Bicarb is high and attempting to correct the problem (but has not completely helped) so this is a partially compensated respiratory acidosis.


 

References

1. All You Really Need to Know to Interpret Arterial Blood Gases, by Lawrence Martin. Lippincott Williams & Wilkins; 2nd edition (March 15, 1999)
2. The ABC of Acid-Base Chemistry, by Horace W. Davenport. University Of Chicago Press; 6th Rev edition (June 15, 1974)
3. Rose, BD, Post, TW, Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed, McGraw-Hill, New York, 2001.
4. Hansen JE. Arterial blood gases. Clin Chest Med 1989; 10:227.
5. Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980; 59(3):161-187.
6. Wrenn K. The delta (delta) gap: an approach to mixed acid-base disorders. Ann Emerg Med. 1990;19(11):1310-1313.
7. Marini, J., J. MD. Wheeler, A., P. (2009). Critical care medicine: the essentials. (4th ed.). Lippincott, Williams & Wilkins. Philadelphia
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