What causes a drop in oxygen saturation (SpO2)?

Causes of Drop in Oxygen Saturation (SpO2)

A drop in oxygen saturation (SpO2) can be caused by various respiratory, cardiovascular, and systemic conditions that impair oxygen delivery to tissues, with the most common causes being respiratory diseases like COPD, asthma, and pneumonia. 1

Respiratory Causes

• Chronic Obstructive Pulmonary Disease (COPD): Patients with COPD experience significant drops in SpO2, especially during exertion. Studies show that patients with COPD can experience a mean fall in SpO2 of 11% (from 94% to 83%) when exposed to conditions simulating altitude or during exercise 1

• Asthma: Acute asthma attacks can lead to significant drops in SpO2 due to bronchospasm and airway inflammation, accounting for approximately 9% of in-flight medical incidents 1

• Pneumonia/Respiratory Infections: These conditions impair gas exchange at the alveolar level, causing hypoxemia 1

• Pulmonary Embolism: Although many patients with minor pulmonary embolism maintain normal oxygen saturation, larger emboli can cause significant V/Q mismatch and hypoxemia 1

• Pneumothorax: Air in the pleural space prevents normal lung expansion, leading to hypoxemia in some cases 1

• Interstitial Lung Disease/Pulmonary Fibrosis: These conditions cause thickening of the alveolar-capillary membrane, impairing oxygen diffusion 1

Cardiovascular Causes

• Heart Failure: Pulmonary edema in heart failure impairs gas exchange, leading to hypoxemia. Patients with severe heart failure may experience significant drops in SpO2 1

• Cyanotic Congenital Heart Disease: Patients with this condition experience lower baseline SpO2 (mean 86%) and further drops when exposed to lower oxygen environments 1

• Pulmonary Hypertension: Impaired blood flow through the pulmonary circulation affects oxygen uptake 1

• Cardiac Arrhythmias: Can affect cardiac output and consequently oxygen delivery to tissues 1

Other Physiological Causes

• Altitude Exposure: Reduced barometric pressure at altitude decreases the partial pressure of oxygen, causing SpO2 to drop. Even healthy individuals experience a drop from 98% to 90% at simulated altitude of 8,000 feet 1

• Exercise: Physical exertion increases oxygen demand and can unmask latent hypoxemia, particularly in patients with cardiopulmonary disease 1, 2

• Hypoventilation: Inadequate breathing patterns reduce alveolar ventilation, leading to increased PaCO2 and decreased PaO2 1

• V/Q Mismatch: Ventilation-perfusion mismatching is a common cause of hypoxemia, where areas of the lung are ventilated but not perfused or vice versa 1

• Diffusion Limitation: Thickening of the alveolar-capillary membrane impairs oxygen diffusion 1

• Right-to-Left Shunt: Blood bypassing the lungs remains deoxygenated, lowering overall SpO2 1

Clinical Factors and Considerations

• Anemia: While anemia alone doesn't directly cause low SpO2 readings, it reduces oxygen content and can exacerbate hypoxemia during exercise in patients with COPD 2

• Smoking: High levels of carboxyhemoglobin in smokers can affect oxygen-carrying capacity and SpO2 readings 1

• Sleep: Normal individuals may experience transient dips in saturation to 90% or less during sleep 1

• Hyperoxia Risk: Excessive oxygen administration can lead to hyperoxia in some patients, which has been associated with adverse outcomes. Titrating oxygen based solely on SpO2 can still result in hyperoxemia in a significant proportion of patients 3

• Target Ranges: Evidence suggests that a target SpO2 range of 92-96% may be preferable to 94-98% for oxygen therapy in most adult patients without COPD 4

Monitoring Considerations

• Pulse Oximetry Limitations: Normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen. Pulse oximetry will be normal in a patient with normal PO2 but abnormal pH or PCO2 1

• SpO2/FiO2 Ratio: The time a patient spends with SpO2/FiO2 ratio ≤150 (SF-TAR) in the first 24 hours of ventilation is associated with increased mortality risk 5

• Fluctuations in SpO2: Frequent fluctuations in oxygen saturation are associated with increased risk for mortality and severe morbidities, particularly in very low birth weight infants 6