Why is the patient experiencing hypoxia?

Understanding Why Your Patient is Hypoxic

Hypoxia occurs when tissues receive insufficient oxygen to meet metabolic demands, and this can happen through four distinct mechanisms—only one of which (hypoxemic hypoxia) responds to supplemental oxygen. 1

The Four Types of Hypoxia

Your patient's hypoxia falls into one of these categories, each requiring different management:

1. Hypoxemic Hypoxia (Low Blood Oxygen)

This is the only type that responds to supplemental oxygen. 1 The mechanisms include:

• Ventilation-perfusion (V/Q) mismatch - the most common and easily treatable cause with supplemental oxygen 2, 3
• Right-to-left shunting through cardiac defects or overwhelmed pulmonary circulation 2
• Alveolar hypoventilation with elevated PaCO2 1
• Diffusion impairment across alveolar-capillary membrane 1
• High altitude exposure with reduced inspired oxygen 1

In COPD patients specifically, V/Q mismatching is the predominant mechanism, with or without alveolar hypoventilation, and typically requires less than 3 L/min supplemental oxygen for correction. 3

2. Anemic Hypoxia (Inadequate Oxygen Carriers)

Despite normal PaO2 and SpO2, reduced hemoglobin prevents adequate oxygen delivery. 1 This includes:

• True anemia with low hemoglobin levels 1
• Functional anemia from carbon monoxide poisoning (where pulse oximetry shows falsely normal readings because it cannot differentiate carboxyhemoglobin from oxyhemoglobin) 1

Treatment requires blood transfusion or specific antidotes, not oxygen therapy alone. 1

3. Stagnant Hypoxia (Inadequate Blood Flow)

Normal blood oxygen content fails to reach tissues due to circulatory problems. 1 Clinical scenarios include:

• Low cardiac output states (heart failure, shock) 1
• Peripheral vascular disease 1
• Cold exposure causing vasoconstriction 1
• Bubble-induced circulatory obstruction at the tissue level in decompression sickness 2

Management focuses on improving cardiac output and perfusion, not increasing inspired oxygen. 1

4. Histotoxic Hypoxia (Cellular Dysfunction)

Cells cannot utilize oxygen despite adequate delivery due to disrupted cellular metabolism. 1 Primary mechanisms:

• Cyanide poisoning blocking cytochrome oxidase 1
• Sepsis-related mitochondrial dysfunction 1

Treatment requires specific antidotes and sepsis management, not oxygen supplementation. 1

Immediate Diagnostic Approach

Start by measuring SpO2 and obtaining arterial blood gas to determine if hypoxemia exists. 1 The British Thoracic Society recommends:

• Measure respiratory rate, pulse, blood pressure, temperature and assess for volume deficits and anemia using NEWS scoring 1
• Record SpO2 with the inspired oxygen device and flow rate on the observation chart 1
• Never discontinue oxygen to obtain room air measurements in patients who clearly require oxygen 1
• Obtain arterial blood gases when: SpO2 < 94% on room air, unexplained confusion/agitation, or suspected hypercapnic respiratory failure 1

Critical Clinical Pitfalls

Normal oxygen saturation does not exclude tissue hypoxia—patients with SpO2 >94% may have severe tissue hypoxia from anemic, stagnant, or histotoxic mechanisms. 1 Specific warnings:

• In carbon monoxide poisoning, pulse oximetry shows falsely normal readings and blood gas PO2 appears normal despite tissue hypoxia 1
• A sudden drop of ≥3% in SpO2 within target range should prompt full reassessment as the first sign of acute illness 1
• Cyanosis is unreliable, especially in poor lighting or with anemia/polycythemia 1

Focused History Elements

Look specifically for:

• Acute illnesses: pneumonia, pulmonary embolism, acute coronary syndrome 1
• Chronic conditions: COPD, asthma, heart failure, neuromuscular disease, chest wall deformities 1
• Cardiac disease co-existing with lung disease may present more risk than respiratory disease alone 4

Physical Examination Priorities

Perform urgent examination looking for:

• Heart failure signs: elevated JVP, peripheral edema, pulmonary crackles 1
• Pleural effusion signs: decreased breath sounds, dullness to percussion 1

Immediate Management Based on Mechanism

For critically ill patients, commence 15 L/min oxygen via reservoir mask immediately. 1 Then adjust based on the underlying mechanism:

• Target SpO2 88-92% while awaiting blood gases in patients at risk for hypercapnic respiratory failure 1
• Target SpO2 94-98% in patients NOT at risk for hypercapnic failure 1
• For anemic hypoxia: transfuse blood or administer specific antidotes 1
• For stagnant hypoxia: improve cardiac output with inotropes, fluids, or vasopressors as appropriate 1
• For histotoxic hypoxia: administer specific antidotes (hydroxocobalamin for cyanide) and treat underlying sepsis 1

Special Considerations in Cardiopulmonary Disease

Patients with cardiorespiratory disease in stable condition tolerate moderate hypoxemia well because they are effectively "acclimatized" to hypoxia—a fall in SpO2 of 10% is easily overcome by a similar percentage increase in cardiac output. 4 However, chronic hypoxemia increases mortality and morbidity for any severity of disease, and pulmonary hypertension adversely affects survival in COPD to an extent paralleling the degree of elevated mean pulmonary artery pressure. 3