Partial Pressure of Oxygen (pO2), Blood

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Subject: Partial Pressure of Oxygen (pO2), Blood

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Definition

  • The partial pressure of oxygen (pO2) is a measure of the tension or pressure of oxygen dissolved in the blood. The pO2 of arterial blood is primarily related to the ability of the lungs to oxygenate blood from alveolar air.

  • Normal range:

    • Arterial: >80–95 mm Hg (see Table 16.64)

    • Venous: 35–40 mm Hg

 
TABLE 16–64
Arterial pO2

Use

  • To evaluate patients with pulmonary or acid–base disturbances

  • To monitor patients with carbon monoxide poisoning, methemoglobinemia, or hemoglobin variant for O2 saturation

  • To manage patients on mechanical respirators

  • Prior to thoracic or general surgery

Interpretation

Increased In

  • Decreased ventilation

    • Airway obstruction

    • Drug overdose

    • Metabolic disorders (e.g., myxedema, hypokalemia)

    • Neurologic disorders (e.g., Guillain-Barré syndrome, multiple sclerosis)

    • Muscle disorders (e.g., muscular dystrophy, polymyositis)

    • Chest wall abnormalities (e.g., scoliosis)

  • Increased dead space in the lungs (perfusion decreased more than ventilation decreased)

    • Lung diseases (e.g., COPD, asthma, pulmonary fibrosis, mucoviscidosis)

    • Chest wall changes affecting lung parenchyma (e.g., scoliosis)

  • Increased production (e.g., sepsis, fever, seizures, excess carbohydrate loads)

Decreased In

  • Hypoventilation (e.g., chronic airflow obstruction): caused by increased alveolar CO2 that displaces O2.

  • Alveolar hypoxia (e.g., high altitude, gaseous inhalation).

  • Pulmonary diffusion abnormalities (e.g., interstitial lung disease): Supplemental oxygen usually improves pO2.

  • Right-to-left shunt: Supplemental oxygen has no effect; requires positive end-expiratory pressure.

    • Congenital anomalies of the heart and great vessels

    • Acquired (e.g., ARDS)

  • Ventilation—perfusion mismatch: Supplemental O2 usually improves pO2.

    • Airflow obstruction (e.g., COPD, asthma)

    • Interstitial inflammation (e.g., pneumonia, sarcoidosis)

    • Vascular obstruction (e.g., PE)

  • Decreased venous oxygenation (e.g., anemia).

  • Cyanosis is clearly visible at pO2 <40 mm Hg; may be seen at 50 mm Hg depending on skin pigmentation.

Limitations

  • Capillary blood is not suitable for estimation of high arterial pO2 values.

  • Values measured at 37°C must be corrected to the actual temperature of the patient.

  • Drugs causing respiratory depression, for example, barbiturates, diazepam, heron, meperidine, and midazolam, cause decrease in pO2.

 
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