Ghost in the machine?

Recently, I was the anaesthetist for some elective caesarean sections. All was running smoothly, with relaxing background music in dimmed ambient light. Despite this, one mother was very anxious about the impersonal medical environment. Thus when the baby was delivered, I allowed her to touch her newborn without any “foreign body” interfering. Not thinking too much, I took the saturation probe off her finger and clipped the probe onto the drip chamber of the infusion set.

To my intense surprise, not only did the screen come up with a regular waveform, resembling an electrocardiograph trace rather than an artefact, but it also displayed the “oxygen saturation” of the bloodless solution and the rate of drops infused per minute (figure⇓).

An oxygen saturation probe attached to a drip chamber produced what looked like an electrocardiogram and reassuring oxygen saturations on Ringer’s solution

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Pulse oximeters are used to determine arterial oxygen saturations by using oximetry and Beer-Lambert’s law.1 Diodes send out light of the required wavelength, usually in the red and infrared spectrum as absorbance of body tissues in this range is small. Therefore absorbance essentially results from the presence of oxygenated or deoxygenated haemoglobin. A photometer on the opposite side of the probe detects the transmitted light. The signal is converted to a DC component, representing venous blood and tissue, and an AC component, representing pulsatile flow. Only the latter component is amplified and averaged over a few cycles. Inaccuracies are the result of several factors, including bright ambient light, movement, electrical interference, venous congestion, and various pigments or molecules (such as nail polish, bilirubin, carboxyhaemoglobin, methaemoglobin, or methylene blue).

A near-infrared laser at a wavelength of 830 nm was used previously to measure lactic acid non-invasively.2 As Ringer’s solution was infused on this occasion, the lactate component (about 28 mmol/l) of the balanced crystalloid is the likely culprit, causing signal extinction and the generation of a numerical value for “oxygen saturation.” A gelatin solution was later tested, achieving a trace of inferior quality and lower readings. Physiological saline solution on the other hand produced a stronger signal, but even lower saturations. Both traces looked very much like artefacts compared with the one produced by Ringer’s lactate.

So what about the pulsatile flow—surely, there was none? Although this all happened just around Hallowe’en, surely there was no supernatural spirit in the machine? No, in fact, it was the regular intermittent light absorption, induced by the falling drops, that had fooled the sensor into recognising an AC component and then displaying a “pulse rate.”