in conventional pulse oximetry the calibration is typically developed by running a desaturation study . in these studies photoplethysmographic data are collected at a number of different oxygen saturation levels and these data are paired with simultaneously measured arterial oxygen saturation ( o 2 hb ) measurements . the o 2 hb measurements are made by running arterial blood samples through a laboratory analyzer . the photoplethysmographic data , for two wavelength pulse oximetry , are often converted into a ratio , r , which is indicative of the received photoplethysmographic signals . r is usually a ratio indicative of the absorption ( or differential absorption ) of the arterial blood at one wavelength , typically abut 660 nm , ( a 660nm ) divided by the absorption ( or differential absorption ) of the arterial blood at a second wavelength , typically about 900 nm ( a 900nm ). in this example r would be defined by equation 1 . plotting o 2 hb data as a function of the associated r values generates a calibration for these data . an example of such a calibration curve , for human use , is shown in fig1 . the equation for this curve is a calibration which provides the calculated o 2 hb for any given measured r value . typically , for a two channel photoplethysmographic device such as a pulse oximeter the calibration equation will take the form : where a , b , and c in equation 2 are calibration constants derived by fitting the calibration curve to a second order polynomial . in pulse oximetry o 2 hb is often termed s p o 2 to designate that it is the arterial oxygen saturation as measured by a pulse oximeter . by performing desaturation studies on non - human species it is possible to generate a series of calibration curves such as the ones shown in fig2 for humans , sheep , and cows . these studies are performed on a number of different test subjects over a range of oxygen saturation values and the resultant data is curve fit to give the most accurate calibration for the target species . the calibration curves may alternatively be generated mathematically by using the extinction curves for the hemoglobin of each target species . in this methodology the r value for any given o 2 hb level would be calculated as follows : r = e 660 o ⁢ ⁢ 2 ⁢ ⁢ hb · c o ⁢ ⁢ 2 ⁢ ⁢ hb · l + e 660 rhb · c rhb · l e 900 o ⁢ ⁢ 2 ⁢ ⁢ hb · c o ⁢ ⁢ 2 ⁢ ⁢ hb · l + e 900 rhb · c rhb · l equation ⁢ ⁢ 3 in equation 3 : e is the extinction of the superscripted blood analyte ( o 2 hb or rhb ) at the subscripted wavelength ( 660 nm or 900 nm ); c is the concentration of the superscripted blood analyte ; and l is the pathlength of the pulsatile absorbers in the system . because this pathlength is the same for each term , equation 3 can be reduced as shown in equation 4 . r = e 660 o ⁢ ⁢ 2 ⁢ ⁢ hb · c o ⁢ ⁢ 2 ⁢ ⁢ hb + e 660 rhb · c rhb e 900 o ⁢ ⁢ 2 ⁢ ⁢ hb · c o ⁢ ⁢ 2 ⁢ ⁢ hb + e 900 rhb · c rhb equation ⁢ ⁢ 4 note that equation 4 assumes a two component system with only two significant absorbers , o 2 hb and rhb . these equations can be expanded to include additional absorbers as this would simply add more terms , one for each absorber , into the numerator and denominator of this equation . using the extinction data supplied in the aforementioned reference by wg zijlstra et al one can then mathematically derive a set of calibrations , or calibration curves , such as those shown in fig2 . either of these two methods for deriving calibration curves can be used ; however , because the mathematical model described above is not an exact match to the actual tissue optics of the physical system , the empirical desaturation methodology always provides a more accurate calibration . the error that would be obtained by using a pulse oximeter calibrated for humans on a different species can then be calculated by measuring the vertical distance between the human calibration curve and the curve for a different species . an example of this error is shown in fig2 where , for an r value of 1 , the human calibration would provide an oxygen saturation of 90 % when the true saturation for a sheep at this r value is actually closer to 96 %. in this specific case , the error from not calibrating on the correct species would be nearly 6 %. expanding this procedure across the entire saturation range , and for a number of different species , generates the data shown in fig3 . this graph shows the errors expected , as a function of the true o 2 hb , for five different species when a pulse oximeter which is calibrated for use on humans is used on these species . installing one or more calibration curves , each derived for a specific non - human species such as the two curves shown in fig2 for cows and sheep , into a photoplethysmographic instrument such as the pulse oximeter and providing a way for the correct calibration curve to be selected for the species being monitored creates an instrument that is accurate across all species for which it is calibrated . the calibration table shown in table 1 is derived using a second order calibration equation to fit the non - human curves shown in fig2 . a , b , and c in this table are the calibration constants for the equation of the form shown in equation 2 . it is these calibration constants that are coded into the instrument software in the current embodiment of this invention . a photoplethysmographic instrument with calibrations for more than one non - human species must provide a means for selection of the desired calibration for the species to be monitored . in the current embodiment of this invention the method for selection of the correct installed calibration is illustrated in fig4 . in this figure a schematic representation of the front panel 10 of a pulse oximeter designed for use on multiple non - human species is shown . the button 20 labeled “ species ” is pressed to select the active calibration form the set of calibrations installed in the system software . in the current embodiment of this invention pressing the species button cycles the active calibration through each of the available species . the species associated with the active calibration is displayed in the “ selected species area ” 30 of the front panel display 50 . in this embodiment it is the responsibility of the operator , i . e . the “ user ”, to select the correct calibration , using the species button on the instrument , to match the species being monitored . in an alternate embodiment the active calibration would be provided by , or selected by , the specific sensor that is connected 40 into the instrument . the sensor would contain an identification element , such as a resistor of a set value for a given species , which would be read by the instrument to determine which calibration to use as the active calibration . the identification element could alternatively be a memory device in the form of an integrated circuit which contained the actual calibration . this calibration would then be read by the instrument and used as the active calibration . in this implementation the sensors would be specifically designed to be unique for a given species . the previous discussion of the invention has been presented for the purposes of illustration and description . the description is not intended to limit the invention to the form disclosed herein . variations and modifications commensurate with the above are considered to be within the scope of the present invention . the embodiment described herein is further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention as such , or in other embodiments , and with the particular modifications required by their particular application or uses of the invention . it is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art .
a method and apparatus for photoplethysmographic measurements from non - human species is disclosed . the extinction of light by hemoglobin , as a function of wavelength , varies as a function of the species of the subject being monitored . the calibration of a photoplethysmographic device should therefore also vary dependant upon the species of the subject . the photoplethysmographic device of this invention includes calibrations specifically designed to provide accurate readings on non - human subjects .