referring first to fig1 there is shown a graphical illustration of an “ r - wave ” portion of an electrocardiogram ( ecg ) waveform ( designated “ r ”) and the related plethysmographic waveform ( designated “ p ”). as will be appreciated by one having ordinary skill in the art , the ecg waveform comprises a complex waveform having several components that correspond to electrical heart activity . the qrs component relates to ventricular heart contraction . the r - wave portion of the qrs component is typically the steepest wave therein , having the largest amplitude and slope , and may be used for indicating the onset of cardiovascular activity . the arterial blood pulse flows mechanically and its appearance in any part of the body typically follows the r wave of the electrical heart activity ( i . e ., r - wave event , designated r 1 , r 2 , r 3 , etc . in fig2 ) by a determinable period of time that remains essentially constant for a given patient . see , e . g ., goodlin et al ., systolic time intervals in the fetus and neonate , obstetrics and gynecology , vol . 39 , no . 2 , ( february 1972 ) and u . s . pat . no . 3 , 734 , 086 . correlating the occurrence of cardiovascular activity with the detection of arterial pulses typically occurs by measuring an ecg signal , detecting the occurrence of the r - wave portion of the ecg signal , determining the time delay by which an optical pulse in the detected optical signal follows the r - wave , and using the determined time delay between an r - wave and the following optical pulse to evaluate the waveform . referring now to fig2 there is shown a graphical illustration of the arterial blood pressure ( designated “ p a ”) which is similarly follows the r - wave event by a determinable period of time . the venous blood pressure ( designated “ p v ”), in the absence of extraneous forces or events ( e . g ., restricting venous blood flow ), is typically relatively constant . referring now to fig3 there is shown a schematic illustration of one embodiment of a pulse oximeter apparatus 5 that can be employed within the scope of the invention . as discussed above , conventional pulse oximetry methods and apparatus typically employ two lights ; a first light having a discrete frequency in the range of ˜ 650 - 670 nanometers in the red range and a second light having a discrete frequency in the range of ˜ 800 - 1000 nanometers . the lights are typically directed through a finger 4 via emitters 12 , 14 and detected by a photo detector 16 . emitters 12 and 14 are driven by drive circuitry 18 , which is in turn governed by control signal circuitry 20 . detector 16 is in communication with amplifier 22 . the photo detector 16 provides an output signal s 1 that is transmitted to an amplifier 22 . the amplified signal s 1 from amplifier 22 is then transmitted to demodulator 24 , which is also synched to control signal circuitry 20 . as will be appreciated by one having skill in the art , the output signal from the demodulator 24 would be a time multiplexed signal comprising ( i ) a background signal , ( ii ) the red light range signal and ( iii ) the infrared light range signal . the demodulator 24 , which is employed in most pulse oximeter systems , removes any common mode signals present and splits the time multiplexed signal ( s 1 ) into two ( 2 ) channels , one representing the red voltage ( or optical ) signals ( designated s 3 ) and the other representing the infrared voltage ( or optical ) signals ( designated s 4 ). as illustrated in fig3 the signal from the demodulator 24 is transmitted to analog - digital converter 26 . the desired computations are performed on the output from the converter 26 by signal processor 28 and the results transmitted to display 30 . further details of the conventional pulse oximeter components , and related functions , are set forth in u . s . pat . no . 4 , 934 , 372 , which is incorporated by reference herein . referring now to fig4 the red signal s 3 includes motion artifacts or noise and the base red signal s , which comprises arterial blood ( a b ) and venous blood ( v b ) components . the infrared signal s 4 similarly includes noise and the base infrared signal s . it should be noted that if the pulse oximeter apparatus 5 is designed correctly , such that the red and the infrared light pass through substantially the same tissue , the pulses in the plethysmographic waveform in the red channel , i . e ., red voltage signal ( s ) s 3 , will be shaped identically to those in the infrared channel , i . e ., infrared voltage signal ( s ) s 4 . only their size and the slowly - varying voltage on which they sit will be different . ( see fig5 ) according to the invention , the venous blood component v b can be distinguished from the arterial blood component v a in the following manner : referring to fig6 a and 6b , in a first embodiment of the invention , the pulse oximeter 5 is operatively connected to a patient &# 39 ; s finger 4 . the patient &# 39 ; s hand 3 ( and , hence , finger 4 ) is then raised and held in a first position above the heart level ( designated h ) until substantially all of the venous ( i . e ., non - pulsating ) blood is dissipated from or flows out of the finger 4 . in a preferred embodiment , the hand 3 is held in the raised position for a first time period greater than 3 sec ., more preferably , in the range of approximately 4 . 0 to 6 . 0 sec . during this first period of time , the red and infrared optical ( or oximetry ) signals are obtained . the patient &# 39 ; s hand 3 is then lowered to a second position below the heart level ( h ) for a substantially equal time period ( see fig6 b ). during this second period of time , the red and infrared optical ( or oximetry ) signals are also obtained . referring now to fig7 there is shown a graphical illustration of the venous blood flow during the above described test cycle . as illustrated in fig7 when the finger 4 is in the second position , i . e ., below the heart level ( h ), venous blood flows into the finger 4 , designated v b ( in ) . when the finger is raised to the second position , venus blood dissipates from the finger 4 , designated v b ( out ) . after the oximetry signals are acquired , the ratios of the low frequency ( i . e ., 0 . 08 to 0 . 12 hz ) and high frequency ( i . e ., 0 . 8 - 1 . 2 hz ) signals of s 3 ′ ( red ) and s 4 ′ ( infrared ) are then computed and compared ( see fig8 ). according to the invention , the ratio of the extended time period ( e . g ., 10 sec ) reflects the venous blood component ( v b ). indeed , as will be appreciated by one having ordinary skill in the art , the ratio of the extended time period can only be attributed to the venous blood component ( v b ), since a period of 10 sec . could not be attributed to the heart rate . the following is an illustrative example : an oximeter sensor arrangement is coupled to a finger on patients a &# 39 ; s left hand . the hand is raised over patient a &# 39 ; s head and held for 4 sec . the hand is then lowered to patient &# 39 ; s a &# 39 ; s side and held for 4 sec . referring now to fig9 there is shown a representative modulation of red r and infrared i light for patient a when both the venous concentration and arterial concentration are varying with time . as illustrated in fig9 the arterial blood concentration is varying at approximately 1 cycle / sec . and the venous blood concentration is varying at approximately 1 cycle / 10 sec . as will be appreciated by one having ordinary skill in the art , the 1 cycle / sec . variation is due to the periodic changes in blood pressure attendant with each heart beat . the 1 cycle / 10 sec . variation corresponds to the raising and lowering of the hand 3 above and below the heart level h within a 10 . 0 sec . period . as illustrated in fig9 the 1 cycle / sec . modulation has different amplitudes for red r and infrared i signals ( i . e ., “ saw - tooth ” shaped waveform ). the 1 cycle / 10 sec . modulation has the same amplitude for the red r and infrared i signals . according to the invention , the same ratio variations at low frequencies , designated generally f 1 , is indicative of a venous blood saturation of approximately 81 %. the 2 : 1 amplitude variations ( of the infrared to red signals ) at 1 cycle / sec . is indicative of an arterial blood saturation of approximately 97 %. the following data can thus be acquired from fig9 : according to the invention , the arterial and venous blood components of the noted oximetry signals can be determined as follows : v a = ln r max / r min / ln i max / i min = 0 . 52 eq . 1 v b = ln r ′ max / r ′ min / ln i ′ max / i ′ min = 1 . 0 eq . 2 the arterial blood component , v a , would thus correspond to a saturation level of approx . 97 %. the venous blood component , v b , would thus correspond to a saturation level of approx . 80 %. in an additional embodiment of the invention , the red ( r ) and infrared ( i ) signals are achieved by sequentially blocking and releasing the blood flow to the tissue ( e . g ., finger ) coupled to the pulse oximeter . various means may be employed to block and release the blood flow , such as a cuff disposed on the finger . ( see , e . g ., u . s . pat . no . 4 , 883 , 055 ) as will be appreciated by one having ordinary skill in the art , the noted method can be employed with virtually all pulse oximeter methods and apparatus to enhance the accuracy of the output data ( i . e ., plethysmographic waveform ). the method can similarly be employed in the method and apparatus described in co - pending application ser . no . 09 / 815 , 827 , filed mar . 23 , 2001 , entitled “ method and apparatus for determining physiological characteristics .” the method of the invention is also particularly advantageous when employed in any device where venous saturation could interfere with the accuracy of the output data . without departing from the spirit and scope of this invention , one of ordinary skill can make various changes and modifications to the invention to adapt it to various usages and conditions . as such , these changes and modifications are properly , equitably , and intended to be , within the full range of equivalence of the following claims .
a method for determining the blood constituents of a patient comprising coupling an oximeter sensor arrangement to a tissue region of the patient ; passing first and second lights through the patient &# 39 ; s tissue region for a first period of time while the venous blood in the tissue region has a first volume and for a second period of time while the venous blood in the tissue region has a second volume , the first light being substantially in a red light range and the second light being substantially in an infrared light range ; detecting a red light signal and an infrared light signal , the red and infrared signals having at least first and second frequencies ; computing a first ratio of the red and infrared signals at the first frequency ; computing a second ratio of the red and infrared signals at the second frequency ; comparing the first and second ratios to determine a first blood constituent .