Understanding Hypoxemia vs. Hypoxia

This chapter explores the critical differences between hypoxemia and hypoxia, underscoring their clinical relevance. It emphasizes a systematic approach to assessing patients with low oxygen saturation and identifies key causes, particularly in urgent care settings.

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🧭 REBEL Rundown

📌 Key Points

- 🫁 Hypoxemia = low blood oxygen
- 🧠 Hypoxia = low tissue oxygen
- 🔍 5 causes of hypoxemia, but most hospital cases are either:
  - 🚫 Shunt = doesn’t improve with oxygen therapy
  - 💨 Dead space = causes tachypnea but is easier to oxygenate
- 💡 Always start with maximizing oxygen delivery (💊💨),
  but recognize quickly when positive pressure (🫁➡️) is needed
- ️ V/Q mismatch
- 🩸 Shunt (refractory to oxygen therapy)

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📝 Introduction

In this episode, we break down a practical bedside approach to hypoxemia. We clarify the difference between hypoxemia (low oxygen in the blood) and hypoxia (low oxygen at the tissue level), and walk through the major causes of hypoxemia that you need to recognize quickly at the bedside.

🔑 Key Concepts

Hypoxemia vs. Hypoxia: Know the Difference

Hypoxemia = Low oxygen in the blood.
- Measured indirectly by SpO₂ (pulse oximeter) or directly by PaO₂ (arterial oxygen tension) or SaO₂ (oxygen saturation).
Hypoxia = Low oxygen at the tissue level.
- Can happen with or without hypoxemia.

Four Types of Hypoxia

Hypoxemic Hypoxia: Blood oxygen is low, so tissues get less oxygen. (e.g., severe pneumonia)
Anemic Hypoxia: Low hemoglobin levels mean less oxygen-carrying capacity, even if oxygen levels are normal. (e.g., hemorrhage, hemolysis)
Ischemic Hypoxia: Blood flow to tissues is blocked or reduced. (e.g., MI, stroke, severe shock)
Histotoxic Hypoxia: Oxygen delivery is normal, but tissues can’t use it. (e.g., carbon monoxide or cyanide poisoning)

Five Major Causes of Hypoxemia

Hypopnea/Apnea (Decreased Respiratory Drive)
- Inadequate breaths (or no breaths) means lower oxygen intake.
- Seen in cardiac arrest, drug overdose, severe brain injury.
- Easy to recognize as patients are encephalopathic or apneic.
High Altitude
- Lower barometric pressure = less available oxygen, despite 21% FiO₂.
- Rarely relevant inside hospitals, but important to know.
Diffusion Defect
- Impaired oxygen transfer across alveoli, often due to chronic lung disease.
- Examples: interstitial lung disease, idiopathic pulmonary fibrosis.
- Patients are usually known to have underlying disease.
V/Q Mismatch (Dead Space Disease)
- Problem with perfusion relative to ventilation
- Common examples:
  - Pulmonary embolism (classic dead space).
- Other causes include:
  - Severe emphysema: Alveolar walls are destroyed, so air reaches areas with no capillary blood flow.
  - Pulmonary hypertension: High pressure damages and narrows vessels, reducing blood flow to ventilated alveoli.
  - Low-flow states (shock): Poor systemic perfusion limits blood reaching alveoli, creating ventilated but unperfused areas.
  - Excessive PEEP on ventilation: Overdistended alveoli compress nearby capillaries, blocking blood flow despite good ventilation.
- Key concept: Easy to oxygenate, but tachypneic due to perfusion/ventilation mismatch.
- Key point: Patients often oxygenate “ok” at rest but are tachypneic
Shunt (Most Common and Most Concerning)
- “Crap in the alveoli” blocks oxygen diffusion:
  - Pneumonia (pus)
  - Pulmonary edema (water)
  - Atelectasis (collapse)
  - Pulmonary hemorrhage (blood)
- Blood moves from right to left without being oxygenated.
- Refractory hypoxemia despite oxygen therapy = shunt physiology.
- Key Move: High FiO₂ (non-rebreather mask) → if still hypoxemic, they need positive pressure (NIV or intubation).

🛌 Practical Bedside Approach

Give as much FiO₂ as possible (non-rebreather mask).
Watch SpO₂ response:
- If it improves → V/Q mismatch or dead space more likely.
- If it doesn’t improve → think shunt physiology.
If refractory hypoxemia persists → Start positive pressure ventilation (HFNC, CPAP, BiPAP, or intubation depending on the situation).

🚨 Clinical Bottom Line

Mastering the basics of hypoxemia helps you recognize dangerous physiology early — before your patient crashes. Keep in mind the four types of hypoxia and the five major causes of hypoxemia.

Post Peer Reviewed By: Marco Propersi, DO (Twitter/X: @Marco_propersi), and Mark Ramzy, DO (X: @MRamzyDO)