Pulmonary Embolism: A sudden drop in SpO2 can be indicative of a pulmonary embolism, which is a blockage of an artery in the lungs that can significantly impair oxygenation of the blood. This condition is life-threatening and requires immediate medical attention.

Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Patients with COPD can experience a drop in SpO2 during exacerbations, which can be triggered by infections, air pollution, or other factors.
Pneumonia: Infection of the lungs can lead to inflammation and consolidation, impairing gas exchange and causing a drop in SpO2.
Asthma Exacerbation: Severe asthma attacks can cause significant airway obstruction, leading to decreased oxygenation.
Heart Failure: Decreased cardiac output can lead to pulmonary congestion, reducing oxygenation of the blood.

Cardiac Arrest: Although less common, cardiac arrest can cause a sudden drop in SpO2 due to the cessation of cardiac output and subsequent lack of oxygen delivery to tissues.
Tension Pneumothorax: A life-threatening condition where air enters the pleural space and cannot escape, causing the lung to collapse and severely impairing breathing and oxygenation.
Anaphylaxis: A severe allergic reaction that can cause airway obstruction and significant impairment of oxygenation.

High-Altitude Pulmonary Edema: Occurs in individuals who ascend to high altitudes too quickly, leading to fluid accumulation in the lungs and impaired oxygenation.
Cystic Fibrosis Exacerbation: Patients with cystic fibrosis can experience a drop in SpO2 during exacerbations due to increased airway obstruction and infection.
Goodpasture Syndrome: A rare autoimmune disease that can cause pulmonary hemorrhage, leading to a drop in SpO2.
Mitral Stenosis: A valvular heart disease that can lead to pulmonary congestion and decreased oxygenation, particularly during periods of increased cardiac demand.