Hypoxemia

Background

• Hypoxemia refers to low partial pressure of oxygen (PO₂) or partial pressure of arterial oxygen (PaO₂) in the blood, which can lead to harmful physiological effects in both acute and chronic settings.
• Many conditions can disrupt normal oxygen delivery, leading to physiological effects of respiratory, cardiovascular, metabolic, neurological, and renal systems; but underlying mechanisms leading to hypoxemia include:
  • ventilation-perfusion mismatch
  • hypoventilation
  • diffusion impairment
  • right-to-left shunting of blood
  • reduced ambient oxygen content in air
• Hypoxemia may be acute or chronic.
  • Acute hypoxemia refers to a sudden drop in arterial oxygen saturation (SaO₂) < 90% concurrent with acute illness, such as, pneumonia, pulmonary embolism, or acute heart failure.
  • Chronic hypoxemia refers to routine measurements of SaO₂ < 90% even with stable illness, such as, chronic obstructive pulmonary disease and other chronic lung diseases, congenital cyanotic heart disease, and chronic neuromuscular conditions.

Evaluation

• Patients with hypoxemia may present with:
  • breathlessness
  • increased or decreased ventilation
  • tachypnea and/or tachycardia
  • nonspecific findings, such as restlessness and confusion
• For acutely ill hypoxemic patients:
  • monitor
    • respiratory rate
    • pulse rate
    • blood pressure and temperature
    • anemia
  • inspect skin and buccal mucous membranes to look for central and peripheral cyanosis
  • take detailed medical history and perform physical, if possible, to assess for acute causes of hypoxemia, such as
    • pneumonia
    • pulmonary embolism
    • chronic obstructive pulmonary disease exacerbation
    • asthma exacerbation
    • acute heart failure
    • pleural effusion
• Pulse oximetry should be used to assess oxygen saturation in patients with suspected hypoxemia.
• An arterial blood gas (ABG) assessment is the gold standard for assessing hypoxemic respiratory failure.
  • Consider ABG assessment as early as possible in patients with:
    • suspicion of alterations in pH, partial pressure of carbon dioxide (PCO)₂, and/or hemoglobin level that will affect patient outcomes
    • unexplained confusion and agitation as these may suggest hypoxemia and/or hypercapnia

Management

• Management (including target oxygen saturation) varies depending on risk of hypercapnic respiratory failure and underlying etiology.
• Emergency management may require cardiopulmonary resuscitation and emergency oxygen therapy prior to hospital arrival.
• Supplemental oxygen therapy:
  • recommended for all acutely hypoxemic patients and patients at risk of hypoxemia, such as those with major trauma and shock
  • may improve oxygenation but does not treat underlying cause of hypoxemia
  • clinical risks include worsening of hypercapnic respiratory failure, delayed recognition of clinical deterioration, and hyperoxemia with potential toxicity to lung when high fractions of inspired oxygen are used
• In critically ill patients
  • In patients without severe hypoxemia, the liberal use of supplemental oxygen to high saturation targets (> 96%) may not be beneficial and more conservative oxygen use may be warranted.
  • In patients with acute myocardial infarction or stroke, consider not administering oxygen therapy for saturations ≥ 90% on ambient air.
  • An upper threshold of 96% saturation may not apply to patients with carbon monoxide poisoning, sickle cell crisis, pneumothorax, or cluster headaches, where higher saturation goals may be indicated.
  • Oxygen administration in the most upright position possible is preferred over a supine position in fully conscious hypoxemic patients.
  • If the patient requires cardiopulmonary resuscitation, use the highest feasible inspired oxygen for ventilation (Weak recommendation).
    • After spontaneous circulation has been restored, target oxygen saturation is 94%-98% for acutely ill patients (Weak recommendation).
    • An ABG sample may help guide the ongoing oxygen therapy.
    • If blood gases suggest hypercapnic respiratory failure, reset target oxygen saturation to 89%-92% or consider mechanical ventilation (Weak recommendation).
  • If there is a clear history of treatable illness, such as sepsis, shock, major trauma, drowning, anaphylaxis, major pulmonary hemorrhage, and status epilepticus, then initiate the appropriate treatment in accordance with guidelines or standard management plans.
  • If a major head injury is present, then use early endotracheal intubation and ventilation if the patient is comatose.
  • If carbon monoxide poisoning is present, give as much oxygen as possible using a bag-valve mask, reservoir mask, or hyperbaric oxygen.
• For patients without critical illness
  • If acutely breathless and not at risk of hypercapnic respiratory failure with oxygen saturation < 85%, administer initial oxygen therapy with reservoir mask at 15 L/minute with flow rate adjusted to target 94%-98% oxygen saturation (Weak recommendation).
  • If other acute hypoxemia and no risk factors for hypercapnic respiratory failure, administer initial oxygen therapy via nasal cannulae (or a simple face mask if cannulae are not tolerated or not effective) with the flow rate adjusted to target 94%-98% oxygen saturation (Weak recommendation).
  • If oxygen therapy is ineffective using nasal cannulae or simple face mask, switch to a reservoir mask.
  • If risk for hypercapnic respiratory failure:
    • start oxygen therapy
      • target oxygen saturation 88%-92% while awaiting ABG results
      • use 24%-28% oxygen or 1-2 L/minute nasal oxygen and reduce fraction of inspired oxygen (FiO₂) if oxygen saturation as measured by pulse oximetry (SpO₂) > 92%
    • if respiratory acidosis (pH < 7.35 and PCO₂ > 6 kilopascal (kPa) [45 mm Hg])
      • treat with 24%-28% oxygen via Venturi mask to keep SpO₂ > 92%
      • consider noninvasive or invasive ventilation
    • if hypercapnic (pH ≥ 7.35 and PCO₂ > 6 kPa [45 mm Hg])
      • treat with 24%-28% oxygen via Venturi mask to keep SpO₂ > 92%
      • repeat blood gases at 30-60 minutes
      • if PO₂ ≥ 8 kPa (60 mm Hg), consider reducing FiO₂
    • if PCO₂ normal or low (≤ 6 kPa [45 mm Hg])
      • titrate oxygen to maintain target oxygen saturation 94%-98% and repeat blood gases in 30-60 minutes
      • if PCO₂ > 6 kPa (45 mm Hg) and known risk of hypercapnia, adjust oxygen to maintain target oxygen saturation 88%-92%
      • if PCO₂ > 6 kPa (45 mm Hg) and no known risk of hypercapnia, maintain target oxygen saturation 94%-98%
  • If no known risk for hypercapnic respiratory failure:
    • target oxygen saturation 94%-98%
    • if SpO₂ ≤ 94% on air or oxygen or if requiring oxygen to maintain target
      • start oxygen therapy and check blood gases
      • if PCO₂ > 6 kPa (45 mm Hg) or respiratory deterioration, consider noninvasive or invasive ventilation and treat urgently
      • if PCO₂ normal or low (≤ 6 kPa [45 mm Hg])
        • titrate oxygen to maintain target oxygen saturation 94%-98% and repeat blood gases in 30-60 minutes
        • if PCO₂ > 6 kPa (45 mm Hg) and known risk of hypercapnia, adjust oxygen to maintain target oxygen saturation 88%-92%
        • if PCO₂ > 6 kPa (45 mm Hg) and no known risk of hypercapnia, maintain target oxygen saturation 94%-98%
    • if SpO₂ within target range
      • oxygen not required
      • monitor SpO₂
      • if SpO₂ falls below target range, titrate oxygen to maintain target oxygen saturation 94%-98% and repeat blood gases in 30-60 minutes