Why is the pulse oximeter normal in severe anaemia?
Clinical Measurement — Pulse Oximetry: Principle, Limitations and Sources of Error
The question
A woman is reviewed two hours after a major postpartum haemorrhage. She is breathing air, looks pale and is tachycardic. Her haemoglobin concentration is 58 g/L, her arterial oxygen tension is 13 kPa and the pulse oximeter reads 98% with a good trace. Which statement best explains the oximeter reading?
- A saturation of 98% confirms that oxygen delivery to the tissues is adequate
- Severe anaemia causes a falsely high reading, and the true saturation is considerably lower
- Anaemia shifts the oxyhaemoglobin dissociation curve to the left, so haemoglobin appears more saturated than it is
- The oximeter is detecting dissolved oxygen in plasma, which is unaffected by the haemoglobin concentration
- The oximeter reports the proportion of the haemoglobin present that is carrying oxygen, so it is normal even though arterial oxygen content is around 40% of normal
The answer
The oximeter reports the proportion of the haemoglobin present that is carrying oxygen, so it is normal even though arterial oxygen content is around 40% of normal
A saturation of 98% confirms that oxygen delivery to the tissues is adequate — Oxygen delivery is the product of content and cardiac output. This patient's oxygen content is severely reduced, and the tachycardia represents the compensatory mechanism sustaining delivery. The saturation figure conveys no information about either of those terms.
Severe anaemia causes a falsely high reading, and the true saturation is considerably lower — Pulse oximetry is not systematically inaccurate in isolated anaemia at normal saturations. The reading is genuine. It is the clinical inference drawn from it that is erroneous.
Anaemia shifts the oxyhaemoglobin dissociation curve to the left, so haemoglobin appears more saturated than it is — Chronic anaemia in fact raises 2,3-DPG and shifts the curve to the RIGHT, facilitating unloading. In either case, a shift acts at tissue oxygen tensions, not on the flat upper portion of the curve where arterial saturation is determined.
The oximeter is detecting dissolved oxygen in plasma, which is unaffected by the haemoglobin concentration — Pulse oximetry measures differential light absorption by oxy- and deoxyhaemoglobin. It does not detect dissolved oxygen. Dissolved oxygen contributes only approximately 1.5% of total arterial oxygen content.
The oximeter reports the proportion of the haemoglobin present that is carrying oxygen, so it is normal even though arterial oxygen content is around 40% of normal — Correct. Saturation is a ratio, not a quantity. At a normal oxygen tension, the available haemoglobin is nearly all saturated, so the reading is accurate. It simply does not indicate how much haemoglobin is present to saturate.
Explanation
A pulse oximeter shines red (approximately 660 nm) and infrared (approximately 940 nm) light through a pulsatile tissue bed and compares how much of each wavelength is absorbed. Oxyhaemoglobin and deoxyhaemoglobin have different absorption profiles at these wavelengths, and the ratio of pulsatile absorbances is converted, via a calibration curve derived from healthy volunteers, into a displayed saturation. The value displayed is functional saturation: oxyhaemoglobin expressed as a percentage of oxyhaemoglobin plus deoxyhaemoglobin.
That definition constitutes the entire answer. A percentage is a ratio and conveys nothing about the size of the pool. If the available haemoglobin is nearly all oxygenated (which it will be at a PaO2 of 13 kPa), the oximeter reads in the high nineties whether the haemoglobin is 150 g/L or 58 g/L. The number is correct in both cases. Only one of those patients has sufficient oxygen in the blood.
The quantity that matters is arterial oxygen content: CaO2 = (1.34 x [Hb] x SaO2) + (0.023 x PaO2 in kPa). At a haemoglobin of 150 and saturation of 0.99, the bound fraction is approximately 19.9 mL/dL. At 58 g/L it is approximately 7.7 mL/dL. Oxygen delivery is then content multiplied by cardiac output, which is why this patient is tachycardic: cardiac output is the only compensatory mechanism remaining once content has fallen. This also explains why anaemia is tolerated far less well by a patient who cannot mount that cardiac response.
The complementary principle is worth learning alongside this one. A two-wavelength device reading functional saturation cannot detect haemoglobin species other than oxyhaemoglobin and deoxyhaemoglobin. Carboxyhaemoglobin absorbs similarly to oxyhaemoglobin at 660 nm and is counted as saturated, so the displayed value reads falsely high. Methaemoglobin absorbs strongly at both wavelengths, driving the ratio towards 1, so the reading drifts towards the mid-eighties regardless of the true saturation. Co-oximetry (four or more wavelengths, fractional saturation) is required to distinguish these species. Other practical limitations include motion artefact, low perfusion states, venous congestion, ambient light interference, nail coverings, and the response lag from circulation time and signal averaging. That lag is why a pulse oximeter is a poor ventilation monitor and a late apnoea alarm in a preoxygenated patient.
Tips for the candidate
When a stem pairs a normal saturation with an abnormal haemoglobin, a dyshaemoglobin, or a low cardiac output, it is testing the distinction between saturation, content, and delivery. The chain is worth laying out explicitly: tension drives saturation; saturation and haemoglobin concentration determine content; content and cardiac output determine delivery. The task is to identify which link in the chain the stem has broken.
Precision about what pulse oximetry measures is essential, because distractors are written to exploit vagueness. It is a ratio of two absorbances. It reports functional, not fractional saturation. It uses two wavelengths. Every classic oximetry error follows from one of those three facts, and a candidate who can state them clearly can derive the carbon monoxide and methaemoglobin answers rather than memorising them.
Distinguishing a reading that is wrong from one that is correct but misleading is a recurrent examination theme. In anaemia, the oximeter is accurate and the error lies in reading reassurance into the number. In CO poisoning, the oximeter itself is producing an incorrect value. Questions frequently hinge on exactly that distinction, and an option beginning with "falsely high" or "falsely low" should prompt careful consideration of which situation applies.
Any option that infers adequate tissue oxygenation from a saturation figure alone should be regarded as incorrect. It is included because it represents the clinical error this topic exists to prevent. The reasoning is the same in anaemia, in shock, in CO poisoning, and in cyanide toxicity, even though the underlying mechanism differs in each case.