a pulse oxymetry sensor operates with the light wavelengths 660 nm , 805 nm and 940 nm . in the case of the two wavelengths 660 nm and 940 nm the measured variable ω is ascertained according to the following equation : ## equ4 ## for the two perfusions p1 and p2 calibration curves ( see fig2 ) are ascertained for the measured variable ω . the amount of perfusion of the tissue is established from the strength of the light attenuation la at 805 nm ( isobestic point ). ## equ5 ## where : εr , εr : coefficient of extinction in the case of oxygenated and deoxygenated blood as in the present case the coefficient of extinction for oxygenated blood is identical to that of deoxygenated blood , the light attenuation al1 in the case of perfusion p1 can be allocated directly to perfusion p1 and the light attenuation al2 in the case of perfusion p2 directly to the perfusion p2 . as shown in fig2 in the case of the perfusion p1 an oxygen saturation sao 2 of approx . 0 . 6 is obtained for ω = 1 . 5 , in contrast thereto in the case of perfusion p2 the sao 2 amounts to only 0 . 38 . by ascertaining in accordance with the invention the perfusion , it is therefore possible to avoid an error of 22 % oxygen saturation . the level of perfusion can also be ascertained in this case approximately by establishing the light attenuation al at the wavelength 940 nm , since the influence of the oxygenation is low here in comparison to the perfusion , or by way of a weighting of the light attenuations over all three wavelengths . in the case of a greater level of pigmentation and pilosity it is possible to determine the perfusion in the tissue by , for example , measurements being taken in the case of light wavelengths at two isobestic points ( e . g . 560 nm and 850 nm ). the following applies for light attenuation : since the coefficient of absorption a is known for the blood and the pigmentation and the pilosity , then the measurement for the blood layer density and perfusion δ blood can be ascertained from the light attenuations . if the three light wavelengths 660 nm , 740 nm and 940 nm are used , then the following two measured variables are formed : ## equ6 ## if a value of 1 is obtained for a measurement for ω1 , then it can be assumed with a greater degree of accuracy that the oxygen saturation amounts to approximately 75 % ( as shown in fig2 ), since the number of calibration curves intersect at this point . the intersection point of the number of curves for ω2 lies likewise in the case of a value of approximately 1 , however , here the oxygen saturation value amounts to approximately 40 % ( as shown in fig3 ). owing to the fact that in order to obtain an initial signal for the oxygen saturation in the case of high saturation values ω1 is weighted more greatly than ω2 and at low saturations ω2 is weighted more greatly than ω1 , it is possible to increase the measuring accuracy using the pulse oxymetric measuring technique . note : the diagrams in fig2 and fig3 are only of qualitative importance . deviations can occur if the geometry of the sensors is different . it is difficult to calibrate a pulse oxymeter on human beings where the oxygen saturation is low , since low oxygen saturation is damaging for the tissue and a reduction in the saturation by , for example , binding an arm prevents the arterial pulsation which is necessary in order to carry out the pulse oxymetric measurement . it is possible to obviate this difficulty by injecting medical dyes in test probands . methylene blue for example absorbs the light to a maximum at approximately 660 nm . oxygenated blood absorbs light at 660 nm approximately ten times less than deoxygenated blood . thus , it is possible by injecting methylene blue to simulate the deoxygenation of the blood at 660 nm . if as a second wavelength a transmitter is used having a light wavelength of 805 nm , a wavelength at which methylene blue absorbs hardly any light and the light absorption of the haemoglobin is not dependent upon the level of oxygenation , then calibration can take place -- after taking blood samples from the tissue and carrying out a photometric analysis on said samples -- over the entire oxygen saturation range . the influence of the haemoglobin content in the tissue on the calibration process can be determined , for example , by raising and lowering body parts and the resultant change in the blood in the tissue . the resultant change in perfusion is ascertained by virtue of determining the light attenuation la through the tissue .
this is a method for measuring oxygen saturation of arterial blood in tissue using at least two different wavelengths of light directed into the tissue and measuring the amount of radiation retained by the tissue and compensating the measurements for tissue haemoglobin content and skin pigmentation and pilosity and calibrating the equipment by obtaining measurements after the injection of a medical dye into the tissue .