Laboratorium interpretation of acid base electrolites disorders
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LABORATORIUM INTERPRETATION OF ACID-BASE & ELECTROLITES DISORDERS. dr. Husnil Kadri, M.Kes Biochemistry Departement Medical Faculty Of Andalas University Padang. Arterial Blood Gases . Aids in establishing a diagnosis Helps guide treatment plan Aids in ventilator management

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LABORATORIUM INTERPRETATION OF ACID-BASE & ELECTROLITES DISORDERS

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LABORATORIUM INTERPRETATION OF ACID-BASE & ELECTROLITES DISORDERS

dr. Husnil Kadri, M.Kes

Biochemistry Departement

Medical Faculty Of Andalas University

Padang


Arterial Blood Gases

  • 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 --

    • Need for continuous BP monitoring

    • Need for multiple ABGs

  • Where to place – (with antikoagulant)

    • A. Radial

    • A. Femoral

    • A. Brachial

    • A. DorsalisPedis

    • A. Axillary


The Components

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


Arterial Blood Gases

  • Reflect oxygenation, gas exchange, and acid-base balance

  • PaO2 is the partial pressure of oxygen dissolved in arterial blood

  • SaO2 is the amount of oxygen bound to hemoglobin


Base Excess

Definition: The amount of a strong acid (like HCl) needed to bring blood to 7.40.

  • Assumes 100% oxygenation, 37oC, and pCO2 of 40.

Normal = 0

Used to calculate the metabolic component of an acid-base disturbance.


Base Excess calculations

Calculated the same way, in practice, as SID:

Buffer Base (SID) = HCO3- + A-

HCO3 calculated by pH & pCO2 (blood gas machine)

A- calculated using pH & hemoglobin (whole blood)

OR A- calculated using albumin & phos (plasma)

BE = Buffer Base – “expected buffer base”

(expected if pH = 7.4 and pCO2 = 40)


Indicators of hypoxaemia and hypoxia


Respiratory Acidosis

Respiratory Alkalosis

Metabolic Acidosis

Metabolic Alkalosis

Increased pCO2 >50

Decreased pCO2<30

Decreased HCO3 <18

Increased HCO3 >30

Is it Respiratory or Metabolic?


Compensated or Uncompensated—what does this mean?

  • Evaluate pH—is it normal? Yes

  • Next evaluate pCO2 & HCO3

    • pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis

    • pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis


Compensated vs. Uncompensated

  • Is pH normal? No

  • Acidotic vs. Alkalotic

  • Respiratory vs. Metabolic

    • pH<7.30 + pCO2>50 + normal HCO3 = uncompensated respiratory acidosis

    • pH<7.30 + HCO3<18 + normal pCO2 = uncompensated metabolic acidosis

    • pH>7.50 + pCO2<30 + normal HCO3 = uncompensated respiratory alkalosis

    • pH>7.50 + HCO3>30 + normal pCO2 = uncompensated metabolic alkalosis


Respiratory

Hypoventilation

Impaired gas exchange

Metabolic

Ketoacidosis

Diabetes

Renal Tubular Acidosis

Renal Failure

Lactic Acidosis

Decreased perfusion

Severe hypoxemia

Causes of Acidosis


Respiratory

Hyperventilation due to:

Hypoxemia

Metabolic acidosis

Neurologic

Lesions

Trauma

Infection

Metabolic

Hypokalemia

Gastric suction or vomiting

Hypochloremia

Causes of Alkalosis


Mixed Metabolic Acidosis and Chronic Respiratory Alkalosis

Examples:

  • Sepsis

  • Addition of respiratory alkalosis to metabolic acidosis further decreases HCO3- but pH may remain normal

  • Lactic acidosis plus respiratory alkalosis due to severe liver disease, pulmonary emboli, or sepsis


Mixed Metabolic Alkalosis and Chronic Respiratory Acidosis

Examples:

  • Patient with COPD receiving glucocorticoids or diuretics

  • pCO2 and HCO3- are increased by both conditions, but pH is neutralized


Mixed Alkalosis, Severe

Example:

  • Postoperative patient with severe hemorrhage stimulating hyperventilation [respiratory alkalosis] plus massive transfusion and nasogastric drainage [metabolic alkalosis]


Mixed Chronic Respiratory Acidosis and Acute Metabolic Acidosis

Examples:

  • COPD [chronic respiratory acidosis] with severe diarrhoea [metabolic acidosis]. pH is too low for pCO2 of 55 mmHg in chronic respiratory acidosis, indicating low pH due to mixed acidosis, but HCO3- effect is offset


Mixed Metabolic Acidosis and Metabolic Alkalosis

Examples:

  • Gastroenteritis with vomiting [metabolic alkalosis] and diarrhoea [metabolic acidosis due to loss of HCO3-]; surprisingly normal findings with marked volume depletion


Serum Values in Acid-Base Disturbances


Serum Values in Acid-Base Disturbances


Summary of Pure and Mixed Acid-Base Disorders

Source: Adapted from Friedman HH. Problem-oriented medical diagnosis, 3rd ed. Boston: Little, Brown. 1983


References

  • Anisman, S. Base Excess & Strong Ion Theories. ppt. 2003.

  • Klee, V. Arterial Blood Gas Analysis.ppt. 2012.

  • Perkins, J. ABG Interpretation. ppt. 2012.

  • Rashid, FA. Respiratory Mechanisms in Acid-Base Homeostasis.ppt. 2005.


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