Interpretation of Arterial Blood Gases

Interpretation of Arterial Blood Gases

Basic Principles

• Arterial Blood Gases (ABG) are drawn for the following reasons:

  • Determination of oxygenation

  • Determination of acid-base status

• When are arterial blood gases needed?

  • Establishing a diagnosis

  • Assessing the severity of an illness

  • Monitoring a treatment course

Pulmonary Gas Exchange

• Definition of Pulmonary Gas Exchange:

  • Refers to the transfer of O2 from the atmosphere to the bloodstream (oxygenation) and CO2 from the bloodstream to the atmosphere (CO2 elimination).

• Role of Arterial Blood Gases:

  • Help assess the effectiveness of gas exchange by providing measurements of the partial pressures of O2 and CO2 in arterial blood.

• Partial Pressure:

  • Contributions of one individual gas within a gas mixture.

  • Gases migrate from areas of higher partial pressure to lower.

  • At the alveolar-capillary membrane, air in alveoli has a higher P_{O2} and lower P_{CO2}, meaning:

  • O2 moves from alveoli to blood.

  • CO2 moves from blood to alveoli.

• Definitions:

  • P_{O2} = partial pressures of O2 in a gas mixture

  • P_{aO2} = partial pressure of O2 in arterial blood

  • P_{CO2} = partial pressures of CO2 in a gas mixture

  • P_{aCO2} = partial pressure of CO2 in arterial blood

Carbon Dioxide Elimination

• Measurement of P_{aCO2}:

  • Determined by alveolar ventilation.

  • The level of ventilation is adjusted to maintain P_{aCO2} within tight limits (reference range: P_{aCO2} = 35-45 ext{ mm Hg}).

• Increased P_{aCO2} (Hypercapnia):

  • Implies reduced alveolar ventilation.

• Regulation of Ventilation:

  • Managed by the respiratory center in the brainstem, which contains receptors that sense P_{aCO2}.

  • Adjusts the rate and depth of breathing based on abnormal P_{aCO2}.

Oxygenation

• Oxygen Transport in Blood:

  • Almost all O2 molecules in blood are bound to hemoglobin (Hb).

  • The amount of O2 is influenced by two factors:

  1. Hb concentration: Indicates the blood's capacity to carry O2.

  2. Saturation of Hb with O2 (SO2): Percentage of binding sites on Hb occupied by O2.

• Definitions:

  • SO2 = oxygen saturation in any blood

  • S_{aO2} = oxygen saturation in arterial blood (normal range 80-100 mm Hg).

• Effects of P_{aO2} on Saturation:

  • With a normal P_{aO2} (80-100 mm Hg), Hb is maximally saturated (usually S_{aO2} > 95 ext{%}).

  • Any further increase in P_{aO2} will not significantly increase arterial O2 content.

Hypoxia, Hypoxemia, and Impaired Oxygenation

• Definitions:

  • Hypoxia: Reduced O2 delivery to tissues.

  • Hypoxemia: Reduced O2 content (PaO2) in arterial blood; may result from:

  • Impaired oxygenation

  • Low hemoglobin (anemia)

  • Reduced affinity of hemoglobin for O2 (e.g., carbon monoxide).

  • Impaired Oxygenation: Hypoxemia resulting from reduced transfer of O2 from lungs to bloodstream, identified by a low P_{aO2} (<10.7 kPa; <80 mmHg).

Acid-Base Balance

• pH Measurement:

  • Measurement of acidity or alkalinity based on hydrogen (H+) ions present.

  • Normal blood pH is between 7.35-7.45.

  • H+ concentration: 35-45 nmol/L.

• Conditions based on pH Levels:

  • Acidemia: occurs if blood pH < 7.35.

  • Alkalemia: occurs if blood pH > 7.45.

• Classification of pH Disturbances:

  • Acidosis is any process that lowers blood pH.

  • Alkalosis is any process that raises blood pH.

Maintaining Acid-Base Balance

• Key Equation:
  ext{H}_2 ext{O} + ext{CO}_2
  ightleftharpoons ext{H}_2 ext{CO}_3 + ext{H}^+ + ext{HCO}_3^{-}

• Characteristics of the Equation:

  • Dissolved CO2 becomes an acid, meaning more CO2 leads to more carbonic acid (H2CO3) produced, which dissociates to release H+.

  • Blood pH depends on the ratio of CO2 to HCO3, not absolute amounts. Changes in CO2 will not affect blood pH if balanced by changes in HCO3 that preserve the ratio.

  • CO2 is controlled by respiration; HCO3 is regulated by renal excretion, explaining how compensation can prevent changes in blood pH.

The Respiratory Buffer Response

• Lungs' Role:

  • Responsible for CO2 removal. P_{aCO2} is determined by alveolar ventilation.

  • Changes in CO2 production lead to adjustments in breathing to exhale appropriate amounts of CO2 and maintain P_{aCO2} in normal limits.

• Mechanics of CO2 Handling:

  • CO2 combines with water in blood to form carbonic acid (H2CO3). Amount of CO2 directly impacts H2CO3 levels.

• Compensation Timing:

  • Lungs' compensation starts within 1-3 minutes of an imbalance.

The Renal Buffer Response

• Kidneys' Role:

  • Responsible for excreting metabolic acids by secreting H+ ions into urine and reabsorbing HCO3− ions, helping to lower H+ concentration in blood.

• Adjustment Capability:

  • Kidneys can adjust urinary H+ and HCO3− excretion based on metabolic acid production changes, but this compensation can take hours to days.

Disturbances of Acid-Base Balance

1. Metabolic Disturbances:

   • Alter HCO3 concentration in blood:

     • Decreased serum HCO3 (base) leads to metabolic acidosis.

     • Increased serum HCO3 leads to metabolic alkalosis.

2. Respiratory Disturbances:

   • Alter pH by changing CO2 levels:

     • CO2 accumulation causes acidosis (via carbonic acid).

     • Increased respiration leads to CO2 elimination, causing respiratory alkalosis.

     • Decreased ventilation retains CO2, leading to respiratory acidosis.

Steps to Blood Gas Interpretation

• Step 1: Assess the pH to determine if blood gas is within normal range (7.35-7.45) or determine alkalotic (>7.45) or acidotic (<7.35).

• Step 2: If blood is alkalotic or acidotic, determine if it is primarily a respiratory or metabolic issue by assessing P_{aCO2} levels:

  • As pH decreases below 7.35, P_{aCO2} should rise (respiratory problem).

  • As pH increases above 7.45, P_{aCO2} should decrease.

  • If pH and P_{aCO2} move in opposite directions, problem is primarily respiratory.

• Step 3: Assess HCO_3 value:

  • As pH increases, HCO_3 should increase; as pH decreases, HCO_3 should decrease.

  • If pH and HCO_3 move in the same direction, the problem is primarily metabolic.

Compensation

• Compensatory Mechanisms:

  • When an acid-base imbalance occurs, the body attempts to compensate via lung and kidney actions, striving to restore normal pH.

  • Patient can be:

  1. Uncompensated

  2. Partially compensated

  3. Fully compensated

Compensation Scenarios

• Without Compensation (Values):

  • Respiratory Acidosis:

  • pH: ↓

  • P_{aCO2}: ↑

  • HCO_3: normal

  • Respiratory Alkalosis:

  • pH: ↑

  • P_{aCO2}: ↓

  • HCO_3: normal

  • Metabolic Acidosis:

  • pH: ↓

  • P_{aCO2}: normal

  • HCO_3: ↓

  • Metabolic Alkalosis:

  • pH: ↑

  • P_{aCO2}: normal

  • HCO_3: ↑

• Partially Compensated Status:

  • Respiratory Acid-Base:

  • pH: ↓

  • P_{aCO2}: ↑

  • Metabolic Acidosis:

  • pH: ↓

  • HCO_3: ↓

  • Metabolic Alkalosis:

  • pH: ↑

  • HCO_3: ↑

• Fully Compensated Status:

  • Respiratory Acidosis:

  • pH: normal but < 7.40

  • P_{aCO2}: ↑

  • HCO_3: ↑

  • Respiratory Alkalosis:

  • pH: normal but > 7.40

  • P_{aCO2}: ↓

  • HCO_3: ↓

  • Metabolic Acidosis:

  • pH: normal but < 7.40

  • HCO_3: ↓

  • Metabolic Alkalosis:

  • pH: normal but > 7.40

  • HCO_3: ↑

Mixed Acid-Base Disturbances

• Occur when a primary respiratory disturbance coincides with a primary metabolic disturbance, referred to as mixed acid-base disturbances.

• Effects of Mixed Disturbances:

  • If the processes oppose each other, the resulting pH disturbance is similar to a compensated balance.

  • If the processes push pH in the same direction (e.g., metabolic acidosis and respiratory acidosis), it may lead to profound acidaemia or alkalemia.

Lactate: Basic Principles

• Biochemistry of Lactate:

  • In an adequate oxygenated state, aerobic energy extraction from cells occurs through the citric acid cycle and electron transport chain, converting pyruvate to acetyl CoA.

  • Under inadequate tissue perfusion, anaerobic metabolism occurs, resulting in pyruvate converting to lactate, producing fewer ATPs (2 compared to the 36 in aerobic metabolism).

Lactic Acidosis

• Definition:

  • Most common cause of metabolic acidosis in hospitalized patients, characterized by low HCO3 in conjunction with plasma lactate concentration > 4 mmol/L.

• Causes of Lactic Acidosis:

  • Issues with local blood supply (e.g., ischemic intestine or limb).

  • Generalized failure of tissue oxygenation (e.g., profound hypoxemia, shock, or cardiac arrest).

• Clinical Relevance:

  • Extent of lactic acidosis serves as an indicator of disease severity.

Key Values and Measures

• Concentration of Free Hydrogen Ions (H+):

  • Reference range: 35-45 nmol/L.

  • < 35 = alkalemia, > 45 = acidaemia.

• pH:

  • Normal range: 7.35-7.45.

  • < 7.35 = acidaemia, > 7.45 = alkalemia.

• Partial Pressure of O2 (Pao2):

  • Normal >10.6 kPa or >80 mmHg in arterial blood on room air.

• Partial Pressure of CO2 (Paco2):

  • Normal range: 4.7-6.0 kPa or 35-45 mmHg in arterial blood.

• O2 Saturation (So2):

  • Normal >96% on room air.

• Actual Bicarbonate (HCO3act):

  • Normal range: 22-28 mmol/L. High indicates metabolic alkalosis, low indicates metabolic acidosis.

• Standard Bicarbonate (HCO3st):

  • Normal range: 22-28 mmol/L calculated at P_{CO2} corrected to 5.3 kPa (40 mmHg).

• Base Excess (BE):

  • Normal range: -2 to +2. Positive indicates more base than normal (metabolic alkalosis), negative indicates less base (metabolic acidosis).

• Lactate:

  • Normal range: 0.4-1.5 mmol/L. Elevated levels signify tissue hypoxia.

• Hemoglobin (Hb):

  • Normal range for men: 13-18 g/dL; for women: 11.5-16 g/dL. Determines blood's capacity to carry O2.

• Normal Plasma Electrolytes:

  • Sodium (Na): 135-145 mmol/L

  • Potassium (K): 3.5-5 mmol/L

  • Chloride (Cl): 95-105 mmol/L

  • Ionized Calcium (Ca): 1.0-1.25 mmol/L

  • Glucose: 3.5-5.5 mmol/L if fasting.

Conclusion

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