Hypoxia and Hypoxemia

Lauren Taylor

Definitions

  • Hypoxemia: Low levels of oxygen in arterial blood, as measured by PaO2 or SpO2. Severe hypoxemia will almost always cause hypoxia. 
  • Hypoxia: Inadequate oxygen supply to either the body as a whole (general hypoxia) or to a specific region (tissue hypoxia), regardless of the level of oxygen in arterial blood.

Mechanisms of Hypoxia without Hypoxemia

  • Hypoxemic Hypoxia – low PaO2 leads to poor tissue oxygenation 
  • Anemic Hypoxia – normal PaO2 but reduced hemoglobin leads to reduced oxygen carrying capacity and thus decreased tissue oxygenation (i.e. anemia, CO poisoning, methemoglobinemia) 
  • Circulatory Hypoxia – states of inadequate perfusion (shock) leading to poor tissue oxygenation (i.e. shock, heart failure, ischemia) 
  • Histotoxic Hypoxia – inability of tissues to use oxygen (i.e. cyanide poisoning, mitochondrial dysfunction, sepsis)
  • Demand Hypoxia – oxygen requirements exceed oxygen delivery (hypermetabolic states)

Mechanisms of Hypoxia with Hypoxemia

Mechanism

Pathophysiology

Improves with O2

A-a Gradient

Examples
Low FiO2 Low fraction of inspired oxygen causes low PAO2 and thus low PaO2 Yes Normal High-Altitude
Hypoventilation Low oxygen delivery from low RR or TV to the alveoli reduces PAO2 and thus low PaO2 Yes, if ventilation (RR or TV) increase Normal Neuromuscular disease,
obesity hypoventilation syndrome,
opioid overdose
V/Q Mismatch 1. V/Q >1 (more ventilation relative to perfusion)
PAO2 is normal but inadequate perfusion limits gas exchange

2. V/Q <1 (less ventilation relative to perfusion)
blood oxygenation is impaired due to poor gas exchange 		Yes Elevated 1. Pulmonary Embolism

2. Pneumonia, ARDS, Pulmonary Edema
Right-to-left Shunt 1. Anatomic: Blood bypasses the alveolar-capillary interface and does not participate in gas exchange

2. Physiologic: Gas exchange is impaired 		1. Anatomic - no
2. Physiologic - yes 		Elevated 1. Heart defects, AV malformations

2. Diffuse parenchymal disease
Diffusion Limitation Adequate PAO2 but oxygen cannot effectively cross into the bloodstream Yes Elevated ILD

Differential diagnosis for hypoxia based on anatomical location

Anatomical Location

Differential Diagnosis
Airways COPD (chronic bronchitis), CF/bronchiectasis, bronchitis, severe asthma (hypoxia is rare with this)
Alveoli Blood (DAH), Pus (Pneumonia), Water (pulmonary edema)
Protein/Cells/Other: (ARDS, pneumonitis), atelectasis, emphysema
Interstitium/Parenchyma Interstitial lung disease
Vascular/Cardiac Pulmonary Emboli, intra/extrapulmonary shunts
Pleural Space and Chest Wall Pleural effusions*, PTX, neuromuscular weakness*, tense ascites*
*More likely to cause dyspnea, need to be severe to cause hypoxia

Evaluation

  • Perform a focused physical exam, taking particular note of signs of respiratory distress 
  • Ensure pleth has good waveform – consider switching probe to different location (earlobe, forehead) if poor circulation (vasculopathy, scleroderma) 
  • Review recent medications (ie: opioids, CNS depressants) 
  • POCUS can be helpful for evaluation of lung sliding, Kerley B lines, IVC collapsibility, as well as diaphragm paralysis

Initial Workup

  • Almost everyone: VBG vs ABG, Lactic Acid, CXR 
  • If concerned for infection: CBC, RPP, blood/sputum cultures 
  • If concerned for volume overload or PE: BNP and troponin 
  • D-Dimer rarely helpful in our patient population 
  • Shunt evaluation: TTE with bubble Study

Choosing a CT scan

  • Without contrast: best to evaluate lung parenchyma 
  • With contrast: highlights the pleura and mediastinum 
  • Angiogram: 
    • Bronchial artery protocol – for massive hemoptysis 
    • Pulmonary embolism protocol – to assess for PE 
  • High resolution (includes inspiration/expiration and prone/supine images): further characterize findings concerning for ILD
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