Some science behind the scenes

Hypoxia - instruments to measure the hypoxic state


A capnometer or capnograph  measures ‘end-tidal CO2’ (the partial pressure of carbon dioxide in expired air at the end of expiration) exhaled through the nostril into a latex tube.

A narrow plastic tube is inserted into each nostril and the tubes are taped to the skin near the upper lip.  The machine then continuously monitors ‘end-tidal breath’ .  The air from the tubes is analysed by an infrared gas analyser and the signal is then sent to a computer that feeds back the information to a video-monitor.  Most machines can provide hard copy output and also provide statistical analyses of the results.  It is also possible to place a ‘goal wave’ into the machine for comparative purposes.


Pulse oximeter

Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's haemoglobin.

It is a particularly convenient noninvasive measurement method. Typically, it uses a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with a wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form; therefore, the oxy/deoxyhemoglobin ratio can be calculated from the ratio of the absorption of the red and infrared light.

The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish, (though black nail polish tends to distort readings) and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none.

From our point of view we should be able to tell whether the blood has become ‘under oxygenated’.  It is probably best to monitor both the finger and the ear.

Clinically, a pulse oximeter is useful in any setting where a patient's oxygenation is unstable, including intensive care, operating, recovery and hospital ward settings. Because of their simplicity and speed, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea. 

So from our point of view they are very useful in measuring what is happening and ensuring the levels do not go dangerously low.

Pulse oximetry is not a complete measure of respiratory sufficiency. It gives no indication of base deficit, carbon dioxide levels, blood pH, or bicarbonate concentration. The metabolism of oxygen can be readily measured by monitoring expired CO2   [which is why the capnometer is used] but saturation figures give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, the blood will carry less total oxygen, despite the hemoglobin being 100% saturated.

Erroneously low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to a limb being cold, or from vasoconstriction); incorrect sensor application; highly calloused skin; or movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform.

It is also not a complete measure of circulatory sufficiency. If there is insufficient bloodflow, tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

In June, 2009, video game company Nintendo announced an upcoming peripheral for the Wii console, dubbed the "Vitality Sensor", which consists of a pulse oximeter.


Rheoencephalography (REG), or brain blood flow biofeedback, is a biofeedback technique which measures the blood flow in the brain when a person is conscious.

An electronic device called a rheoencephalograph [from Greek rheos stream, anything flowing, from rhein to flow] is used to do the measuring. Electrodes are attached to the skin at certain points on the head.  The device then continuously measures the electrical conductivity of tissues located between the electrodes.

The brain blood flow technique is based on non-invasive methods of measuring bio-impedance. Changes in bio-impedance are generated by blood volume and blood flow and these are then registered by the device. Hypoxia does not necessarily impede the flow of blood, but if any constriction occurs then this device will be able to capture it.


Hemoencephalography or HEG biofeedback is an infrared imaging technique. As its name describes, it measures the differences in the color of light reflected back through the scalp based on the relative amount of oxygenated and unoxygenated blood in the brain. This technique and the instrument that is used to measure this is thus absolutely key in any biofeedback approach used with hypoxia techniques. 

The instrument is relatively new and research continues to determine its reliability, validity, and clinical applicability. HEG is used to treat ADHD and migraine, and for research.

In summary

Hypoxia is dangerous.  It can cause brain damage.  And if it  is suffered on a regular basis, it can cause liver damage, heart damage, kidney damage, sight impairment, and dementia.

The real advantage of using biofeedback equipment is thus principally that of safety and health.  It has the potential to be used with controlled breathing techniques for example, to make them safer. The equipment available these days can monitor oxygen levels, and CO2 levels, so the person can attain a trance like state, and lower their heart rate, without plunging their oxygen levels so low they seriously deprive their brain or other vital organs of oxygen.