Describe Pulse Oximetry in detail.

Principle

The pulse oximeter continuously and non-invasively measures the arterial oxygen saturation (SpO₂) and the pulse rate. It works by spectrophotometry: oxygenated and deoxygenated haemoglobin absorb red and infrared light differently, so by shining two wavelengths through a pulsatile tissue bed (finger, ear) and analysing the pulsatile (arterial) component, the device calculates the percentage saturation.

Value

Pulse oximetry is one of the most important safety monitors: it gives an early warning of hypoxaemia (before cyanosis is visible), is non-invasive and continuous, and also displays a plethysmographic pulse waveform and heart rate. It has greatly improved anaesthetic safety.

Limitations & Sources of Error

SpO₂ can be inaccurate when there is poor peripheral perfusion (hypotension, cold, vasoconstriction), motion or shivering, nail varnish/dyes, and bright ambient light. Crucially, it is unreliable in abnormal haemoglobins: carboxyhaemoglobin (reads falsely high) and methaemoglobin (reads toward 85%). It reflects oxygenation, not ventilation — a patient on oxygen can retain CO₂ with a normal SpO₂ — and it is a late sign of hypoventilation (especially with preoxygenation).

Oxygen Dissociation Curve & Interpretation

Interpreting the SpO₂ requires an understanding of the oxygen dissociation curve: because the curve is flat at the top, saturation stays near 100% across a wide range of high arterial oxygen tensions, so a normal reading does not exclude a falling oxygen reserve, and a patient breathing added oxygen may maintain a normal SpO₂ for some time even while ventilation deteriorates. Once saturation begins to fall it does so steeply, because the patient is now on the steep part of the curve, which is why a dropping SpO₂ is a relatively late and then rapidly-worsening sign. This is the physiological reason capnography is needed alongside oximetry to detect hypoventilation early.

Probe Placement & Practical Use

In everyday use the reliability of the pulse oximeter depends on obtaining a good pulsatile signal, which is checked by looking at the plethysmographic waveform on the monitor: a clear, regular trace confirms the reading, whereas a poor or erratic trace warns that the number may be unreliable. The probe is placed on a well-perfused, warm site — a finger, toe or ear lobe — kept still and shielded from very bright light, and moved if perfusion is poor; nail varnish is removed where it interferes with the light path through the digit. These simple measures, together with an awareness of the device’s limitations, allow the monitor to fulfil its role as an early and reliable warning of falling oxygenation before cyanosis becomes visible.

Measures saturation, not oxygen delivery or ventilation.

• Pulse oximeter: non-invasive continuous SpO₂ via differential red/infrared absorption of oxy/deoxy-Hb (pulsatile signal).
• Early warning of hypoxaemia (before cyanosis); shows plethysmograph & pulse.
• Errors: poor perfusion, motion, nail varnish, ambient light; unreliable in carboxy-/methaemoglobin.
• Measures oxygenation not ventilation; a late sign of hypoventilation — pair with capnography.