Oxygen saturation (SpO₂) is the percentage of haemoglobin molecules in the blood that are carrying oxygen — expressed as a percentage of their maximum oxygen-carrying capacity. It is a real-time indicator of how effectively the lungs are transferring oxygen into the bloodstream.

SpO₂ is measured non-invasively by pulse oximetry, which uses two wavelengths of light (red and infrared) shone through a peripheral tissue (typically a fingertip or earlobe). Oxyhaemoglobin and deoxyhaemoglobin absorb these wavelengths differently, allowing the device to calculate the oxygen saturation ratio.

The measurement is called SpO₂ to distinguish it from SaO₂ — the arterial oxygen saturation measured directly from a blood sample (arterial blood gas). SpO₂ is a reliable proxy for SaO₂ in most clinical situations, though accuracy can be affected by poor perfusion, nail varnish, anaemia, carbon monoxide exposure, and motion artefact.

In healthy adults at sea level, normal SpO₂ is 95–100%. A reading of 94% or below is generally considered low and warrants attention; 90% is a commonly used clinical threshold below which supplemental oxygen is typically initiated.

Mild hypoxemia (90–94%) may be asymptomatic at rest but can cause exertional breathlessness, fatigue, and reduced physical capacity. It often reflects a treatable underlying condition such as pneumonia, COPD exacerbation, or heart failure.

Moderate hypoxemia (85–89%) is clinically significant and usually requires prompt evaluation. Symptoms at this level typically include dyspnoea at rest, cyanosis (bluish discolouration of lips/fingers), and impaired cognition.

Severe hypoxemia (<85%) is a medical emergency. Prolonged tissue hypoxia at this level risks end-organ damage, particularly to the brain and heart, and requires urgent clinical intervention.

Altitude adjustment is an important consideration: at high altitudes, lower atmospheric oxygen partial pressure means healthy individuals naturally have lower SpO₂ values. At 3,000 m altitude, SpO₂ of 90–92% may be acceptable in a well-acclimatised individual. The tracker offers an altitude adjustment mode to contextualise readings for travellers and mountaineers.

Pulse oximeters simultaneously measure heart rate (pulse rate) by detecting the pulsatile blood flow with each heartbeat. Normal resting pulse rate in adults is 60–100 bpm. Values below 60 suggest bradycardia; above 100 suggest tachycardia.

Pulse rate and SpO₂ often change together in cardiorespiratory compromise. As oxygen levels fall, the heart typically accelerates to compensate by delivering more oxygen per minute. Simultaneously observing both values provides a more complete clinical picture.

A very weak or absent pulse signal can cause the oximeter to under-read SpO₂ — a common issue with cold extremities, vasoconstriction, hypotension, or excessive movement. The tracker notes when a pulse signal may be unreliable.

Each reading logs SpO₂ percentage, pulse rate, measurement site, date and time, and an optional note. Common measurement sites — fingertip, earlobe, wrist, and forehead — can be recorded to account for the systematic differences between sites.

The tracker renders readings as a time-series chart with colour-coded classification bands, making trends over time immediately visible. This is particularly valuable for individuals managing chronic respiratory conditions such as COPD, asthma, pulmonary fibrosis, or post-COVID breathlessness.

Readings can be exported as a CSV file for sharing with a clinician, providing a comprehensive longitudinal record that a single in-clinic measurement cannot replicate.