434
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
Telegraph and Telephone Public Corporation, where he engaged in research first on hollowcathode discharges and then on radiation processes in gas plasmas. During 1964-1965 he worked on solid-state plasma instabilities at the Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, as a Fullbright Exchange Scholar, and during 1965-1966 he worked at the Microwave Department of the Royal Institute of Technology, Stockholm, Sweden. Since 1966 he has been working on the electromagnetic theory, 1/f fluctuations and medical engineering such as cardial potential display. He is now Professor at the Graduate School of the Tokyo Institute of Technology, Yokohama. Dr. Musha is a member of the Physical Society of Japan and the Institute of Electronics and Communication Engineers of Japan.
l
Nobuyuki Shitara was born in Tokyo in 1944. He graduated from the Department of Medicine, University of Tokyo, in 1970. He finished the post graduate neurosurgical course at the University Hospital. From 1974 to 1977 he was engaged in clinical and research work at the Neurosurgical Service in the National Cancer
Center. His research work is on cell kinetics and chemoradiotherapy of brain tumors. In 1977 he became a lecturer at the University of Tokyo.
Takeshi Kohno was born in 1939. He graduated from the Department of Medicine, University of Tokyo, in 1967. He finished the post graduate neurosurgical course at the University Hospital. Since 1975 he has been engaged in clinical and research work at the Neurosurgical Service in the National Cancer Center. His research work is on chemo and immunotherapy of brain tumor.
Kintomo Takakura was born in Tokyo on Oct. He graduated at Department of Medicine, University of Tokyo (M.D. 1958). He finished his post graduate course of Neurosurgery and received the degree of D.M.Sc. in 1963 from the University of Tokyo. He has been engaged in clinical and basic research in tumors of central nervous system. In 1963 he became a lecturer at the University of Tokyo. From 1964 to 1967, he was a research fellow under Dr. V. P. Hollander in New York and served for study of hormonal regulation on tumor development. In 1968 he became chief of the Neurosurgical Service of the National Cancer Center. His special research fields are brain tumor therapy including chemotherapy and immunotherapy.
4, 1932.
A Vaginal Photoplethysmographic Transducer H. ARMON, J. WEINMAN, SENIOR
MEMBER, IEEE, AND
D. WEINSTEIN
INTRODUCTION
Abstract-The purpose of the transducer is to monitor blood-volume changes in the vascular bed of the vagina during a menstrual cycle by monitoring the resistance of a photoconductive sensor exposed to light backscattered from the vaginal wail. The intensity of the backscattered light and therefore the resistance of the photosensor, are functions of the blood volume in the illuminated tissue. Measurements have to be performed on the same subject at repeated sessions during a menstrual cycle. The construction and calibration of the transducer ensures that at each session monitoring is performed on the same segment of the vaginal wall, illuminated by the same light intensity. The measurement proper consists in comparing the resistance of the photosensor with a known resistance by balancing a potentiometric circuit to zero and observing the zero balance on a recorder. Zero balancing is interfered with by a constantly ongoing smaller short-term vasomotor activity due to heart beat, respiration, and metabolic processes. A specially designed circuit resetting the baseline automatically to zero and measuring the degree of resetting helps to overcome this difficulty.
THE photoplethysmographic transducer described in this paper was developed as part of a larger effort initiated by the World Health Organization to detect physiological parameters undergoing periodic changes during the menstrual cycle and to investigate their temporal relationship to ovulation. The purpose behind these efforts is to explore physiological events that can predict an impending ovulation early enough to avoid unplanned pregnancies. A number of physiological processes are known to be functionally related to the event of ovulation, among them changes in the vascular bed of the reproductive tract. In domestic animals (e.g., cows and sheep) the increase of the blood supply to the uterus during estrus is a well-documented event; similarly, redness of the vulva and of the proximal part of the vagina, observed during heat, point to hyperemia in these Manuscript received December 9, 1976; revised June 6, 1977 and tissues (1). Cyclic changes in the vascular bed of the genital November 7, 1977. This work was supported by a grant from the tract were reported also in women: for instance, an increased Human Reproduction Unit of the World Health Organization. H. Armon and J. Weinman are with the Rogoff Laboratory for Bio- blood supply to the endometrium in midcycle (2). It is posmedical Engineering, Hebrew University, Hadassah Medical School, sible that also the blood supply to the vaginal walls is then inJerusalem, Israel. D. Weinstein is with the Department of Obstetrics and Gynecology, creased, resulting in a reddening of the tissue. Methods for the
Hadassah University Hospital, Jerusalem, Israel.
detection of a color change in the vaginal mucosa could be,
0018-9294/78/0900-0434$00.75 C 1978 IEEE
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
therefore, of value to the efforts of predicting the fertility period because hyperemia in the genital tract is estrogen induced and a sharp estrogen peak appears in blood plasma ahead of the LH peak and therefore ahead of ovulation (3). A promising approach to the detection of color changes due to hyperemia is offered by photoplethysmography, a relatively simple technique for monitoring blood-volume changes in peripheral tissues (4). PHOTOPLETHYSMOGRAPHY OF THE VAGINAL WALLS In order to perform photoplethysmographic measurements inside the vagina, the technique was adapted to the specific conditions imposed by the anatomy of the vagina and by the long-term character (during a menstrual cycle) of the planned observations. It is especially in this latter condition-to obtain reliable results from measurements performed on the same subject at different sessions-that photoplethysmography has difficulties (5). The sources of these difficulties will be better understood by considering the basic principles of photo-
plethysmography. The photoplethysmographic transducer monitors bloodvolume changes by illuminating the investigated tissue and by recording the light backscattered from it to a photosensor; the transducer consists, therefore, in principle, of a light source and a photosensor adjacent to each other, both attached to the investigated body surface. The technique is based on the assumption that the backscattered light contains a component functionally related to blood-volume changes in the tissue. That this is indeed the case is readily demonstrated by recording photoplethysmographically the blood-volume pulse which is an increase in the tissue blood content occurring periodically with each heart contraction. The blood-volume pulse is a relatively fast event and has an easily discernible shape; slower vasomotor activity, for instance vasoconstriction or vasodilation induced by drug action, metabolic activity, temperature variations, or the expected gross blood-volume changes in the vagina, are translated by the photoplethysmographic transducer into a displacement of the recording baseline, a baseline deflection in the direction of the systolic upstroke of the blood volume pulse being interpreted as an increase in tissue blood volume (4). Due, however, to the use of a photoconductive sensor, the monitoring of baseline deflections becomes synonymous with monitoring changes in the resistance of the photoconductive cell. The measuring circuit needed for this purpose is in principle very simple and consists of a variable resistor RP in series with the photoconductive sensor RL and a battery supply (see left side of Fig. 4a). When the transducer is inserted into the vagina, the resistance of the photoconductive sensor attains a value RL depending on the amount of light backscattered from the vaginal walls. Resistance RL can be measured by adjusting the 10-turn potentiometer to a value RP where Vi = 0 and therefore RL = RP. Should, at another time, the intensity of the light backscattered from the vaginal wall become different, the resistance RL of the photosensor will be different too, and another setting of the helipot will be needed to equal the photosensor resistance and to balance V. to zero.
Experience shows that photoplethysmography normally
435
yields reliable information only as long as observations are confined to results obtained from a specific subject during a specific session. Recordings obtained from the same subject but at a different session usually cannot be reliably compared because it is difficult to decide whether the changes observed in the recordings are due to physiological causes or are artefacts. Artefacts arise primarly from difficulties in reproducing accurately: the positioning of the transducer on the surface versus the investigated sector of the vascular bed; the pressure exerted by the transducer on the vascular bed; and the intensity of the light source. When photoconductive cells are used as photosensors, a common practice in photoplethysmography because of their high sensitivity, temperature dependence and light hysteresis can also be interfering factors. Due to these circumstances, results obtained by photoplethysmography in long-term experiments are often of little clinical value. In spite of these discouraging perspectives it was possible, by taking advantage of the specific conditions prevailing in the vagina, to develop a photoplethysmographic technique which yields reliable results. Clinically useful results can be obtained even in an investigation where it is necessary to compare photoplethysmographic measurements obtained from the same subject at a relatively large number of different sessions, namely, daily during the menstrual cycle. CONSTRUCTION OF THE TRANSDUCER The transducer assembly is shown in Fig. 1. Its upper part A consists of an acrylic cylinder to which a highly polished brass cone is cemented. The acrylic cylinder houses four symmetrically distributed photoconductive cells. Part B is another acrylic cylinder cemented to the metal tube E. Tube E carries in its hollow a light guide D through which the conus cemented to A is illuminated. The four syringe needles C connect parts A and B; the needles are tightly fastened to A but allow a sliding movement of B in relation to A, thus allowing positioning of the light guide versus the conical mirror for optical uniformity and maximal intensity of the illumination. The whole transducer assembly is housed in a glass test tube into which it fits smoothly but not too tightly (Fig. 2); the finger of a surgical glove is slipped over the test tube only to demonstrate how the illumination emanating from the conical mirror is distributed spatially. The glass tube housing the transducer has an outer diameter of 26 mm and is 200 mm long. Experience shows that a tube of this diameter can be easily and without discomfort inserted in the vagina. With the tube pushed in until it touches the cervix, 5 to 10 cm of its length remain outside the vagina; care must be taken to avoid outside light striking the glass tube because the glass wall acts as a lightguide, illuminating the inside of the vagina and thus giving rise to erratic signals. In the clinic, measurements have to be performed often in sequence on a number of subjects. To cope with this situation the transducer assembly is constructed in such a way that it can be easily removed from the glass cover and a new clean glass cover used for another subject. Photoconductive cells are used as sensors because their sensitivity is high, the circuitry for detecting their response simple
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
Fig. 1. Transducer without glass housing. A: Assembly consisting of acrylic cylinder, four photoconductive cells and a polished brass conus. B: Acrylic cylinder cemented to metal tube E. C: Four syringe needles. D: Light guide. F: Acrylic ring closing the transducer housing.
and, under the conditions prevailing during measurements inside the vagina, their performance is satisfactory. Cadmium Sulphide (CdS) and not Cadmium Selenide (CdSe) photoconductive cells were chosen because the former have a lower temperature coefficient and a shorter light memory (6) and their slow speed of response (upper 3 dB point at 2 Hz) is of little importance when, as in our case, average blood-volume changes and not changes in the shape of the blood-volume pulse are monitored. The photosensor consists of four CL705LH (Clairex) cells, connected in series. Two output leads are passed through the hollows of the syringe needles C and through the tube E (Fig. 1). The flexible fiberglass lightguide D shown in Fig. I is almost 1.5 m long in order to reach the light-source assembly which, together with its regulated power supply, is located on the same mobile cart as the recording equipment. The light source is located outside the transducer to avoid heating of the photoconductive sensor by the incandescent lamp. Recent experiments showed that the long light guide can be replaced by a shorter one, not longer than the brass tube E in Fig. 1. A light source consisting of a small incandescent lamp with a lense-
Fig. 2. Transducer inside the glass housing. Finger of a surgical glove slipped over the glass tube to show distribution of light by conical mirror.
Bras
Tube
Sponge Rubber
Fig. 3. Calibrating cavity. Cut shows sponge rubber lining. One end of tube closed.
shaped glass envelope (200 mA, 2.4 V as used in flashlights) attached directly to the outside end of the light guide was found to be sufficient to supply the necessary illumination. Consequently, the transducer will become a compact unit with only two pair of leads (light supply and photosensor output) connecting it to the recording equipment. Instead of the regulated power supply used now, two NiCd batteries, with a variable resistor in series for adjusting the light intensity, will be sufficient. The intensity of the backscattered light is a reproducible function of the blood volume in the illuminated vascular bed only if the initial illumination used in repeated trials is the same. This is provided by the "calibrating cavity" shown in Fig. 3. This is a brass tube, lined with sponge rubber, into
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
437
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Fig. 4. Automating baseline resetting. a: Block diagram of the electronic circuit. Details in text. b: A vaginal photoplethysmogram illustrating the performance of the circuit. On the 12th day of the menstrual cycle RL = 61 kil and therefore Vi = 0 when also Rp is set to 61 kf2 (see channel 2). The baseline shift on channel 1 is caused by short-term blood-volume changes (symbolized by AL in 4a) in the vaginal vascular bed. When the shift exceeds the preset V(UL) = + 2 V the baseline is automatically restored to zero and channel 2 records simultaneously a step indicating that the shift was caused by an 1.6 kU increase in RI, (meaning "less light-more blood"). Similarly, the baseline is automatically restored to zero when the shift becomes more negative than the preset V(LL) = -2 V. ARL = 1.6 kn is calculated from ARL = 2RL * AVO/V f2 as explained in text. c: Photoplethysmogram made at larger paper speed to better distinguish the steeper systolic upstroke (= "more blood") and the slower diastolic downstroke of the blood volume pulse. Photoplethysmograms 4b and 4c recorded on a Hewlett Packard two channel recorder 350-1300C.
which the transducer assembly fits tightly and where it is housed until a vaginal measurement is to be performed. The conditions inside the calibrating device simulate the vagina insofar as the photosensor response is a function of the intensity of the light backscattered from the sponge rubber lining. Inside the cylinder, however, the intensity of the backscattered light is always the same fraction of the incident light intensity and, therefore, to each specific level of the incident light there corresponds always a specific photosensor response (resistance); the intensity of the incident light can be therefore easily reset
a predetermined value by adjusting the light intensity until photosensor response, corresponding to the predetermined illumination, is observed.
to a
MEASURING WITH THE TRANSDUCER The subject is in a supine position with legs spread to facilitate the insertion of the transducer. The test tube containing the transducer is inserted into the vagina until its top touches the cervix. This ensures that, whenever the measurements are repeatedly performed with the same subject resting in the
t;-!, -i -,t4| ,
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
438
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Fi.58.hooltymgam of vasooto acivt in th vacua be of th vaia wal reore by th vaia photo-+ ,x* ;:_ L transducr Blo volumechange ar exrese as chne in the resisac RL of th photcon
Fig. 5. Photoplethysmogram of vasomotor activity in the vascular bed of the vaginal wall recorded by the vaginal photoplethysmographic transducer. Blood volume changes are expressed as changes in the resistance RL of the photoconductive sensor (recorded on a potentiometric, limited upper frequency response, recorder Goerz 5 1RE).
same position, the same area of the vaginal wall is illuminated and that light is backscattered to the sensor from the same part of the vascular bed. It can be also assumed that the pressure exerted on the transducer by the same segment of the vaginal wall engulfing it will remain nearly the same at each repeated trial. In order to ensure that the incident illumination is also the same, the following procedure is adopted: before a measurement inside the vagina is performed, the transducer is inserted into the calibrating cavity (Fig. 3), RP is preset to 40 ki2 on the 10-turn potentiometer dial, and the light intensity adjusted (with a resistance in series with the tungsten filament of the light source) until the voltage Vi = 0 (left side of Fig. 4a). In this way the light intensity is now set to a level where the resistance RL of the photoconductive sensor becomes RL = R = 40 kQ. Repeating this procedure before each measurement, or, rather rechecking that the condition RL = 40 kQ is maintained, ensures that the incident illumination is always the same. The choice of a set point RL = RP = 40 k92 is arbitrary but within some constraints, remembering that the four photoconductive CL705HL cells are connected in series and the resistance of each one is about 10 kM2, a value which can be comfortably obtained with the light source used by us. It is also a value which provides a convenient working point on the light intensity/resistance characteristic of the photocell where the sensitivity is sufficient but not excessive (6). It should be added that the color and the thickness of the sponge-rubber lining of the calibrated cavity was chosen by trial and error to make the intensity of the backscattered light of the same order of magnitude as the intensity of the light backscattered from the vaginal wall. Inside the vagina, the resistance of the photoconductive sensor was found to vary between 40 k2 to 80 k2, depending on the day of the menstrual cycle on which the measurement took place. The measuring procedure starts by removing the "calibrated" transducer from the calibrating cavity and introducing it into the vagina until its top touches the cervix, then measuring the resistance RL of the photoconductive sensor corresponding
to the intensity of the light backscattered from the vaginal wall by making RP = RL and thus balancing Vi to zero. In reality, however, the measuring procedure is not as simple as described here; the setting where Rp =RL is not static because RL is a changing parameter and Vi does not remain at zero but moves up and down. This is not surprising because RL is a function of the blood volume in the vaginal wall and the latter is, as in other vascular tissues, a quantity undergoing periodic variations (see Fig. 5); there are short-term variations due to the heart beat, medium-term variations caused by respiration and by various metabolic processes (7) and, hopefully, also long-term periodic variations related to the menstrual cycle-the vascular event of interest to us. The difficulties encountered could be overcome by using a time averaging process for recording the slow changes of RL. Such a procedure was not used for two reasons: one, to gather information about short-time blood-volume changes occurring in the vaginal wall-a seldom described physiological event-and, secondly, to find out whether the short-term RL changes are not of such a magnitude that they will make the measurement of RL changes, occurring during the menstrual cycle, meaningless. It is for this purpose that we used a two-channel recorder and a specially designed circuit for automatic zero-resetting of the baseline. A short description of this circuit is necessary in order to interpret properly the recordings obtained with it. AUTOMATIC BASELINE RESETTING Fig. 4a shows the block diagram of the automatic resetting circuit (8). It will be best to explain the performance of the circuit starting with the switch S in position "Operate." In this position the zero-resetting feature of the circuit is in operation, as illustrated by the recording in Fig. 4b. The position "Balance" of switch S is used only at the start of the recording to set the gain of amplifier A to a convenient value, to choose the proper upper and lower limits V(UL) and V(LL) of the window comparator and to adjust the 10-turn potentiometer, while observing the recording, until the photoplethysmogram shows during some seconds similar excursions
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
below and above the zero volt baseline. In this situation RP becomes equal to RL (Rp = RL) and V0 = A Vi = 0 everytime the photoplethysmogram crosses the baseline. The measurement is now started by throwing S from "Balance" to "Operate." It should be pointed out that in position "Balance" the input to channel 2 of the recorder is grounded and the pen therefore centered. The pen remains centered after switch S is thrown to "operate" because of the balancing RP = RL performed before. The zero resetting circuit performs as follows: the amplifier output signal V0 = A Vi is recorded on channel 1 and is simultaneously fed to the comparator; when V0 becomes larger than V(UL) or smaller than V(LL), a signal (Va) from the comparator activates the Sample and Hold Circuit which stores the peak value of (Vi) on capacitor C. At the same time, the baseline of channel 1 is reset to zero by (Vi) at the inverting input of the differential amplifier A and thus makes the two inputs to the amplifier equal. Simultaneously, the baseline of channel 2 records a step equal to voltage (Vi) stored on C. The amplitude of this step is expressed on the recording as ARL = 1.6 k92 calculated from the equation ARL = 2RL * AVO/V ohms where V = 15 volts is the supply voltage, RL = RP is the RL resistance value at balance and AV0 = 200 mV is the voltage step recorded on channel 2. This equation was derived under the assumption that changes in the light intensity due to the vasomotor activity at each measuring session are small and therefore ARL
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
Telegraph and Telephone Public Corporation, where he engaged in research first on hollowcathode discharges and then on radiation processes in gas plasmas. During 1964-1965 he worked on solid-state plasma instabilities at the Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, as a Fullbright Exchange Scholar, and during 1965-1966 he worked at the Microwave Department of the Royal Institute of Technology, Stockholm, Sweden. Since 1966 he has been working on the electromagnetic theory, 1/f fluctuations and medical engineering such as cardial potential display. He is now Professor at the Graduate School of the Tokyo Institute of Technology, Yokohama. Dr. Musha is a member of the Physical Society of Japan and the Institute of Electronics and Communication Engineers of Japan.
l
Nobuyuki Shitara was born in Tokyo in 1944. He graduated from the Department of Medicine, University of Tokyo, in 1970. He finished the post graduate neurosurgical course at the University Hospital. From 1974 to 1977 he was engaged in clinical and research work at the Neurosurgical Service in the National Cancer
Center. His research work is on cell kinetics and chemoradiotherapy of brain tumors. In 1977 he became a lecturer at the University of Tokyo.
Takeshi Kohno was born in 1939. He graduated from the Department of Medicine, University of Tokyo, in 1967. He finished the post graduate neurosurgical course at the University Hospital. Since 1975 he has been engaged in clinical and research work at the Neurosurgical Service in the National Cancer Center. His research work is on chemo and immunotherapy of brain tumor.
Kintomo Takakura was born in Tokyo on Oct. He graduated at Department of Medicine, University of Tokyo (M.D. 1958). He finished his post graduate course of Neurosurgery and received the degree of D.M.Sc. in 1963 from the University of Tokyo. He has been engaged in clinical and basic research in tumors of central nervous system. In 1963 he became a lecturer at the University of Tokyo. From 1964 to 1967, he was a research fellow under Dr. V. P. Hollander in New York and served for study of hormonal regulation on tumor development. In 1968 he became chief of the Neurosurgical Service of the National Cancer Center. His special research fields are brain tumor therapy including chemotherapy and immunotherapy.
4, 1932.
A Vaginal Photoplethysmographic Transducer H. ARMON, J. WEINMAN, SENIOR
MEMBER, IEEE, AND
D. WEINSTEIN
INTRODUCTION
Abstract-The purpose of the transducer is to monitor blood-volume changes in the vascular bed of the vagina during a menstrual cycle by monitoring the resistance of a photoconductive sensor exposed to light backscattered from the vaginal wail. The intensity of the backscattered light and therefore the resistance of the photosensor, are functions of the blood volume in the illuminated tissue. Measurements have to be performed on the same subject at repeated sessions during a menstrual cycle. The construction and calibration of the transducer ensures that at each session monitoring is performed on the same segment of the vaginal wall, illuminated by the same light intensity. The measurement proper consists in comparing the resistance of the photosensor with a known resistance by balancing a potentiometric circuit to zero and observing the zero balance on a recorder. Zero balancing is interfered with by a constantly ongoing smaller short-term vasomotor activity due to heart beat, respiration, and metabolic processes. A specially designed circuit resetting the baseline automatically to zero and measuring the degree of resetting helps to overcome this difficulty.
THE photoplethysmographic transducer described in this paper was developed as part of a larger effort initiated by the World Health Organization to detect physiological parameters undergoing periodic changes during the menstrual cycle and to investigate their temporal relationship to ovulation. The purpose behind these efforts is to explore physiological events that can predict an impending ovulation early enough to avoid unplanned pregnancies. A number of physiological processes are known to be functionally related to the event of ovulation, among them changes in the vascular bed of the reproductive tract. In domestic animals (e.g., cows and sheep) the increase of the blood supply to the uterus during estrus is a well-documented event; similarly, redness of the vulva and of the proximal part of the vagina, observed during heat, point to hyperemia in these Manuscript received December 9, 1976; revised June 6, 1977 and tissues (1). Cyclic changes in the vascular bed of the genital November 7, 1977. This work was supported by a grant from the tract were reported also in women: for instance, an increased Human Reproduction Unit of the World Health Organization. H. Armon and J. Weinman are with the Rogoff Laboratory for Bio- blood supply to the endometrium in midcycle (2). It is posmedical Engineering, Hebrew University, Hadassah Medical School, sible that also the blood supply to the vaginal walls is then inJerusalem, Israel. D. Weinstein is with the Department of Obstetrics and Gynecology, creased, resulting in a reddening of the tissue. Methods for the
Hadassah University Hospital, Jerusalem, Israel.
detection of a color change in the vaginal mucosa could be,
0018-9294/78/0900-0434$00.75 C 1978 IEEE
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
therefore, of value to the efforts of predicting the fertility period because hyperemia in the genital tract is estrogen induced and a sharp estrogen peak appears in blood plasma ahead of the LH peak and therefore ahead of ovulation (3). A promising approach to the detection of color changes due to hyperemia is offered by photoplethysmography, a relatively simple technique for monitoring blood-volume changes in peripheral tissues (4). PHOTOPLETHYSMOGRAPHY OF THE VAGINAL WALLS In order to perform photoplethysmographic measurements inside the vagina, the technique was adapted to the specific conditions imposed by the anatomy of the vagina and by the long-term character (during a menstrual cycle) of the planned observations. It is especially in this latter condition-to obtain reliable results from measurements performed on the same subject at different sessions-that photoplethysmography has difficulties (5). The sources of these difficulties will be better understood by considering the basic principles of photo-
plethysmography. The photoplethysmographic transducer monitors bloodvolume changes by illuminating the investigated tissue and by recording the light backscattered from it to a photosensor; the transducer consists, therefore, in principle, of a light source and a photosensor adjacent to each other, both attached to the investigated body surface. The technique is based on the assumption that the backscattered light contains a component functionally related to blood-volume changes in the tissue. That this is indeed the case is readily demonstrated by recording photoplethysmographically the blood-volume pulse which is an increase in the tissue blood content occurring periodically with each heart contraction. The blood-volume pulse is a relatively fast event and has an easily discernible shape; slower vasomotor activity, for instance vasoconstriction or vasodilation induced by drug action, metabolic activity, temperature variations, or the expected gross blood-volume changes in the vagina, are translated by the photoplethysmographic transducer into a displacement of the recording baseline, a baseline deflection in the direction of the systolic upstroke of the blood volume pulse being interpreted as an increase in tissue blood volume (4). Due, however, to the use of a photoconductive sensor, the monitoring of baseline deflections becomes synonymous with monitoring changes in the resistance of the photoconductive cell. The measuring circuit needed for this purpose is in principle very simple and consists of a variable resistor RP in series with the photoconductive sensor RL and a battery supply (see left side of Fig. 4a). When the transducer is inserted into the vagina, the resistance of the photoconductive sensor attains a value RL depending on the amount of light backscattered from the vaginal walls. Resistance RL can be measured by adjusting the 10-turn potentiometer to a value RP where Vi = 0 and therefore RL = RP. Should, at another time, the intensity of the light backscattered from the vaginal wall become different, the resistance RL of the photosensor will be different too, and another setting of the helipot will be needed to equal the photosensor resistance and to balance V. to zero.
Experience shows that photoplethysmography normally
435
yields reliable information only as long as observations are confined to results obtained from a specific subject during a specific session. Recordings obtained from the same subject but at a different session usually cannot be reliably compared because it is difficult to decide whether the changes observed in the recordings are due to physiological causes or are artefacts. Artefacts arise primarly from difficulties in reproducing accurately: the positioning of the transducer on the surface versus the investigated sector of the vascular bed; the pressure exerted by the transducer on the vascular bed; and the intensity of the light source. When photoconductive cells are used as photosensors, a common practice in photoplethysmography because of their high sensitivity, temperature dependence and light hysteresis can also be interfering factors. Due to these circumstances, results obtained by photoplethysmography in long-term experiments are often of little clinical value. In spite of these discouraging perspectives it was possible, by taking advantage of the specific conditions prevailing in the vagina, to develop a photoplethysmographic technique which yields reliable results. Clinically useful results can be obtained even in an investigation where it is necessary to compare photoplethysmographic measurements obtained from the same subject at a relatively large number of different sessions, namely, daily during the menstrual cycle. CONSTRUCTION OF THE TRANSDUCER The transducer assembly is shown in Fig. 1. Its upper part A consists of an acrylic cylinder to which a highly polished brass cone is cemented. The acrylic cylinder houses four symmetrically distributed photoconductive cells. Part B is another acrylic cylinder cemented to the metal tube E. Tube E carries in its hollow a light guide D through which the conus cemented to A is illuminated. The four syringe needles C connect parts A and B; the needles are tightly fastened to A but allow a sliding movement of B in relation to A, thus allowing positioning of the light guide versus the conical mirror for optical uniformity and maximal intensity of the illumination. The whole transducer assembly is housed in a glass test tube into which it fits smoothly but not too tightly (Fig. 2); the finger of a surgical glove is slipped over the test tube only to demonstrate how the illumination emanating from the conical mirror is distributed spatially. The glass tube housing the transducer has an outer diameter of 26 mm and is 200 mm long. Experience shows that a tube of this diameter can be easily and without discomfort inserted in the vagina. With the tube pushed in until it touches the cervix, 5 to 10 cm of its length remain outside the vagina; care must be taken to avoid outside light striking the glass tube because the glass wall acts as a lightguide, illuminating the inside of the vagina and thus giving rise to erratic signals. In the clinic, measurements have to be performed often in sequence on a number of subjects. To cope with this situation the transducer assembly is constructed in such a way that it can be easily removed from the glass cover and a new clean glass cover used for another subject. Photoconductive cells are used as sensors because their sensitivity is high, the circuitry for detecting their response simple
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
Fig. 1. Transducer without glass housing. A: Assembly consisting of acrylic cylinder, four photoconductive cells and a polished brass conus. B: Acrylic cylinder cemented to metal tube E. C: Four syringe needles. D: Light guide. F: Acrylic ring closing the transducer housing.
and, under the conditions prevailing during measurements inside the vagina, their performance is satisfactory. Cadmium Sulphide (CdS) and not Cadmium Selenide (CdSe) photoconductive cells were chosen because the former have a lower temperature coefficient and a shorter light memory (6) and their slow speed of response (upper 3 dB point at 2 Hz) is of little importance when, as in our case, average blood-volume changes and not changes in the shape of the blood-volume pulse are monitored. The photosensor consists of four CL705LH (Clairex) cells, connected in series. Two output leads are passed through the hollows of the syringe needles C and through the tube E (Fig. 1). The flexible fiberglass lightguide D shown in Fig. I is almost 1.5 m long in order to reach the light-source assembly which, together with its regulated power supply, is located on the same mobile cart as the recording equipment. The light source is located outside the transducer to avoid heating of the photoconductive sensor by the incandescent lamp. Recent experiments showed that the long light guide can be replaced by a shorter one, not longer than the brass tube E in Fig. 1. A light source consisting of a small incandescent lamp with a lense-
Fig. 2. Transducer inside the glass housing. Finger of a surgical glove slipped over the glass tube to show distribution of light by conical mirror.
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Fig. 3. Calibrating cavity. Cut shows sponge rubber lining. One end of tube closed.
shaped glass envelope (200 mA, 2.4 V as used in flashlights) attached directly to the outside end of the light guide was found to be sufficient to supply the necessary illumination. Consequently, the transducer will become a compact unit with only two pair of leads (light supply and photosensor output) connecting it to the recording equipment. Instead of the regulated power supply used now, two NiCd batteries, with a variable resistor in series for adjusting the light intensity, will be sufficient. The intensity of the backscattered light is a reproducible function of the blood volume in the illuminated vascular bed only if the initial illumination used in repeated trials is the same. This is provided by the "calibrating cavity" shown in Fig. 3. This is a brass tube, lined with sponge rubber, into
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
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Fig. 4. Automating baseline resetting. a: Block diagram of the electronic circuit. Details in text. b: A vaginal photoplethysmogram illustrating the performance of the circuit. On the 12th day of the menstrual cycle RL = 61 kil and therefore Vi = 0 when also Rp is set to 61 kf2 (see channel 2). The baseline shift on channel 1 is caused by short-term blood-volume changes (symbolized by AL in 4a) in the vaginal vascular bed. When the shift exceeds the preset V(UL) = + 2 V the baseline is automatically restored to zero and channel 2 records simultaneously a step indicating that the shift was caused by an 1.6 kU increase in RI, (meaning "less light-more blood"). Similarly, the baseline is automatically restored to zero when the shift becomes more negative than the preset V(LL) = -2 V. ARL = 1.6 kn is calculated from ARL = 2RL * AVO/V f2 as explained in text. c: Photoplethysmogram made at larger paper speed to better distinguish the steeper systolic upstroke (= "more blood") and the slower diastolic downstroke of the blood volume pulse. Photoplethysmograms 4b and 4c recorded on a Hewlett Packard two channel recorder 350-1300C.
which the transducer assembly fits tightly and where it is housed until a vaginal measurement is to be performed. The conditions inside the calibrating device simulate the vagina insofar as the photosensor response is a function of the intensity of the light backscattered from the sponge rubber lining. Inside the cylinder, however, the intensity of the backscattered light is always the same fraction of the incident light intensity and, therefore, to each specific level of the incident light there corresponds always a specific photosensor response (resistance); the intensity of the incident light can be therefore easily reset
a predetermined value by adjusting the light intensity until photosensor response, corresponding to the predetermined illumination, is observed.
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MEASURING WITH THE TRANSDUCER The subject is in a supine position with legs spread to facilitate the insertion of the transducer. The test tube containing the transducer is inserted into the vagina until its top touches the cervix. This ensures that, whenever the measurements are repeatedly performed with the same subject resting in the
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-25, NO. 5, SEPTEMBER 1978
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Fi.58.hooltymgam of vasooto acivt in th vacua be of th vaia wal reore by th vaia photo-+ ,x* ;:_ L transducr Blo volumechange ar exrese as chne in the resisac RL of th photcon
Fig. 5. Photoplethysmogram of vasomotor activity in the vascular bed of the vaginal wall recorded by the vaginal photoplethysmographic transducer. Blood volume changes are expressed as changes in the resistance RL of the photoconductive sensor (recorded on a potentiometric, limited upper frequency response, recorder Goerz 5 1RE).
same position, the same area of the vaginal wall is illuminated and that light is backscattered to the sensor from the same part of the vascular bed. It can be also assumed that the pressure exerted on the transducer by the same segment of the vaginal wall engulfing it will remain nearly the same at each repeated trial. In order to ensure that the incident illumination is also the same, the following procedure is adopted: before a measurement inside the vagina is performed, the transducer is inserted into the calibrating cavity (Fig. 3), RP is preset to 40 ki2 on the 10-turn potentiometer dial, and the light intensity adjusted (with a resistance in series with the tungsten filament of the light source) until the voltage Vi = 0 (left side of Fig. 4a). In this way the light intensity is now set to a level where the resistance RL of the photoconductive sensor becomes RL = R = 40 kQ. Repeating this procedure before each measurement, or, rather rechecking that the condition RL = 40 kQ is maintained, ensures that the incident illumination is always the same. The choice of a set point RL = RP = 40 k92 is arbitrary but within some constraints, remembering that the four photoconductive CL705HL cells are connected in series and the resistance of each one is about 10 kM2, a value which can be comfortably obtained with the light source used by us. It is also a value which provides a convenient working point on the light intensity/resistance characteristic of the photocell where the sensitivity is sufficient but not excessive (6). It should be added that the color and the thickness of the sponge-rubber lining of the calibrated cavity was chosen by trial and error to make the intensity of the backscattered light of the same order of magnitude as the intensity of the light backscattered from the vaginal wall. Inside the vagina, the resistance of the photoconductive sensor was found to vary between 40 k2 to 80 k2, depending on the day of the menstrual cycle on which the measurement took place. The measuring procedure starts by removing the "calibrated" transducer from the calibrating cavity and introducing it into the vagina until its top touches the cervix, then measuring the resistance RL of the photoconductive sensor corresponding
to the intensity of the light backscattered from the vaginal wall by making RP = RL and thus balancing Vi to zero. In reality, however, the measuring procedure is not as simple as described here; the setting where Rp =RL is not static because RL is a changing parameter and Vi does not remain at zero but moves up and down. This is not surprising because RL is a function of the blood volume in the vaginal wall and the latter is, as in other vascular tissues, a quantity undergoing periodic variations (see Fig. 5); there are short-term variations due to the heart beat, medium-term variations caused by respiration and by various metabolic processes (7) and, hopefully, also long-term periodic variations related to the menstrual cycle-the vascular event of interest to us. The difficulties encountered could be overcome by using a time averaging process for recording the slow changes of RL. Such a procedure was not used for two reasons: one, to gather information about short-time blood-volume changes occurring in the vaginal wall-a seldom described physiological event-and, secondly, to find out whether the short-term RL changes are not of such a magnitude that they will make the measurement of RL changes, occurring during the menstrual cycle, meaningless. It is for this purpose that we used a two-channel recorder and a specially designed circuit for automatic zero-resetting of the baseline. A short description of this circuit is necessary in order to interpret properly the recordings obtained with it. AUTOMATIC BASELINE RESETTING Fig. 4a shows the block diagram of the automatic resetting circuit (8). It will be best to explain the performance of the circuit starting with the switch S in position "Operate." In this position the zero-resetting feature of the circuit is in operation, as illustrated by the recording in Fig. 4b. The position "Balance" of switch S is used only at the start of the recording to set the gain of amplifier A to a convenient value, to choose the proper upper and lower limits V(UL) and V(LL) of the window comparator and to adjust the 10-turn potentiometer, while observing the recording, until the photoplethysmogram shows during some seconds similar excursions
ARMON et al.: VAGINAL PHOTOPLETHYSMOGRAPHIC TRANSDUCER
below and above the zero volt baseline. In this situation RP becomes equal to RL (Rp = RL) and V0 = A Vi = 0 everytime the photoplethysmogram crosses the baseline. The measurement is now started by throwing S from "Balance" to "Operate." It should be pointed out that in position "Balance" the input to channel 2 of the recorder is grounded and the pen therefore centered. The pen remains centered after switch S is thrown to "operate" because of the balancing RP = RL performed before. The zero resetting circuit performs as follows: the amplifier output signal V0 = A Vi is recorded on channel 1 and is simultaneously fed to the comparator; when V0 becomes larger than V(UL) or smaller than V(LL), a signal (Va) from the comparator activates the Sample and Hold Circuit which stores the peak value of (Vi) on capacitor C. At the same time, the baseline of channel 1 is reset to zero by (Vi) at the inverting input of the differential amplifier A and thus makes the two inputs to the amplifier equal. Simultaneously, the baseline of channel 2 records a step equal to voltage (Vi) stored on C. The amplitude of this step is expressed on the recording as ARL = 1.6 k92 calculated from the equation ARL = 2RL * AVO/V ohms where V = 15 volts is the supply voltage, RL = RP is the RL resistance value at balance and AV0 = 200 mV is the voltage step recorded on channel 2. This equation was derived under the assumption that changes in the light intensity due to the vasomotor activity at each measuring session are small and therefore ARL
