Indirect measurement of instantaneous arterial blood pressure in the rat KEN-ICHI YAMAKOSHI, HIDEAKI SHIMAZU, AND TATSUO TOGAWA Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Surugadai 2 chome, Kanda, Chiyoda-ku, Tokyo 101, Japan
HIDEAKI SHIMAZU, AND TATSUO Tomeasurement of instantaneous arterial blood pressure in the rat. Am. J. Physiol. 237(5): H632-H637,1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(5): H632-H637, 1979.We devised a hydraulic servo-control system for indirect blood pressure measurement in the rat’s tail, by which beat-to-beat systolic and diastolic blood pressure can be obtained. In this method the principle of “unloading vascular wall” proposed by Shirer (1962) is employed. The proposed system is composed of a transmittance photoelectric plethysmograph with an occluding cuff, a small diaphragm actuator for compressing and decompressing the segment by the hydraulic pressure, and an electromagnetic shaker driven by a volume servo circuit in accordance with the signal from the photoelectric plethysmograph. The plethysmographic signal is clamped at a proper value corresponding to the unloaded vascular volume by the instantaneous hydraulic servo control. The cuff pressure thus automatically controlled follows the intra-arterial pressure in the tail segment. The accuracy of this method was evaluated in comparison with direct measurement of blood pressure recorded simultaneously from 16 unanesthetized spontaneously hypertensive and normotensive rats. Close agreement between the simultaneous data from these two methods were observed. YAMAKOSHI, GAWA. Indirect
KEN-ICHI,
hydraulic servo control; unloaded tric plethysmograph; rat’s tail
vascular
volume;
photoelec-
METHODS of indirect measurement of arterial blood pressure in the rat have been described (1, 2, 4, 6, 7,16, 18; for review see Ref. 7). By most of these methods blood pulse or blood volume variations corresponding to the systolic end point are detected, so they can measure systolic pressure only. There is a need for an instrument capable of indirect measurement of arterial pressure at any instance in the rat. This paper describes a new method for measuring arterial pressure at any instance, including beat-to-beat systolic and diastolic blood pressure noninvasively in the rat’s tail. Using hydraulic servo-control technique, an applied counterpressure is made to equalize the intravascular pressure so as to maintain a constant arterial vascular volume, i.e., unloading of the vascular wall (11, 14; see Ref. 15). To evaluate the accuracy and reliability the values of VARIOUS
H632
10-3,
arterial pressure measured by this method were compared with those obtained simultaneously by direct measurements carried out in 16 unanesthetized spontaneously hypertensive and normotensive rats. METHODS
The instrument. Figure 1 shows a schematic diagram of the instrument. In this system the hydraulic pressure control is adapted to compress and decompress the rat’s tail by means of an occluding cuff. The occluding cuff (OC) is installed in a compression chamber (CR) which is filled with water (W) and equipped with a transmittance photoelectric plethysmograph (PL). The cuff is made of a thin-walled (0.1 mm) translucent rubber tube and formed so as not to develop any tension during the compression of the tail segment. Both flanges of the rubber cuff are firmly fixed to both ends of the chamber by a pair of annular disks (AD). According to the size of the tail segment, four different occluding cuffs having inner diameters of 8, 10, 12, and 14 mm were used. The effective width of all cuffs was 20 mm. The compression chamber is connected to a small diaphragm actuator (DA) via a lo-mm-long fluid passage of 16 mm inner diameter. The compliance of the diaphragm having an effective area of 10 cm2 was only 0.01 ml/100 mmHg. A retainer plate (RP) firmly fixed to the diaphragm is connected to the plunger (P) of an electromagnetic shaker (SHAKER, Shinken Co.). The maximum stroke and the rating of the shaker are t,2 mm and 5 kg, respectively, when drawing an input power of 50 W. The shaker is driven from the output of a power amplifier (PA). The position of the plunger is sensed by a linear displacement transducer (LT). The output of LT provides the quantitative information about the controlled volume for the adjustment of pressure on the tail segment. The compression chamber has an air vent (AV), which is also used to prime this system with water, and a side connection (SC) for a strain-gauge type pressure transducer (P23Db, Statham) to measure the hydraulic pressure in the chamber. The photoelectric plethysmograph consists of series-
0363-6135/79/0000-0000$01.25
Copyright
0 1979 the American
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INDIRECT
MEASUREMENT
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H633
FIG. 1. Schematic diagram of the servo-control system for indirect measurement of rat blood pressure. PL, photoelectric plethysmograph; LED, light-emitting diodes; PT, phototransistors; OC, thin-walled occluding rubber cuff; AD, annular disk; CR, compression chamber; SC, side connection for measuring applied cuff pressure (PC); AV, air vent; DA, diaphragm actuator; W, water; RT, retainer plate; P, plunger; LT, linear displacement transducer; AMP, amplifier; D.AMP, differential amplifier; CMP, gain and phase lead-lag compensator; PA, power amplifier; Sv, controlled volume; PG, photoelectric plethysmogram. Dimensions and electrical connections of LED and PT are shown in inset.
4 P0
CMP
connected light-emitting diodes (LED: TLN 103, Toshiba Electric Co.) used as the light source and parallel-connected phototransistors (PT: TPS603, Toshiba) used as the photodetector, as shown in the inset. The light source (LED) is insulated from water by means of an acrylic plate. The detector is fixed directly on the root of the tail with adhesive tape. The frequency response of the photoelectric plethysmograph is flat up to more than 200 Hz. Either open- or closed-loop operation can be selected by a switch (SW), the former for the plethysmographic recording, and the latter for the blood pressure measurement. When the switch is turned to the O-position (openloop operation), the photoelectric plethysmogram can be obtained at a cuff pressure (PC) that is either set at a constant level or changed by an external electric signal REF (II). Thus the volume of the total vascular bed in the segment can be estimated in this operation (Fig. 2). In the closed-loop operation, on the other hand, the cuff pressure is controlled by the pulsating signal from the plethysmograph so that the vascular volume is clamped at a reference value REF (I). To determine the reference value for a desired volume corresponding to the unloaded vascular wall, Marey’s concept (10) was adopted, according to which the plethysmographic signal reaches the maximum amplitude when the wall of the artery is relieved of the arterial tension (see Ref. 12). The cuff pressure that allows maximum amplitude of the photoelectric pulsating signal in the open-loop operation is used as the preset value; the mean value of the photoelectric plethysmogram at this point serves as the reference value for the servo control in this experiment. This cuff pressure has been experimentally proved to be approximately equal to the intra-arterial mean pressure (8, 12, 13).
After the control loop is closed, the plethysmographic
signal is subtracted from the reference value to produce the servo control error. The error is led to the power amplifier via a gain and phase lead-lag compensator (CMP). In this operation, any variation of arterial vascular volume due to the change in intraarterial pressure is instantaneously compensated by the automatic servo regulation of the cuff pressure, which thus continuously and quantitatively follows the intra-arterial blood pressure. The gain and phase lead-lag compensator (CMP) is necessary to obtain high fidelity servo control. The use of CMP makes the frequency characteristics of the whole system flat up to about 60 Hz, a satisfactory dynamic characteristic for blood pressure measurement in the rat. Experiments in rats. Eight spontaneously hypertensive rats (SHR, Aoki-Okamoto strain) and eight normotensive rats (NR, Wistar-Kyoto), weighing 250-400 g, were anesthetized with sodium pentobarbital, 25 mg/kg ip. Femoral arterial pressure was measured directly using a Statham P23Db pressure transducer and an indwelling arterial catheter made of polyethylene tubing (ID 0.05 cm, OD 0.1 cm, length lo-15 cm; Hibiki Co.). Damping of the pressure pulse obtained by the catheter-transducer system is less than 2% as compared with that recorded via a 5-cm-long 20-gauge needle. After 10 min warming in a temperature-controlled chamber at 39°C to allow the detection of the plethysmogram in the tail, the rats were housed in the cage in an air-conditioned room. The photodetector was fixed at the root of the tail before the tail was inserted in the compression chamber through a suitable cuff. The measurements were carried out after the animal recovered from anesthesia. The experiments were performed at a room temperature of about 25°C. First, the open-loop operation was performed to deter-
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H634 mine the vascular volume of the tail segment. By this operation, a single reading of indirect systolic pressure could also be made from the plethysmographic pulsating signal corresponding to the systolic end point. After the cuff pressure was set at the preset value corresponding to the unloaded vascular volume, the closed-loop operation was started to obtain the indirect instantaneous blood pressure. Only one single setting of the preset value of the cuff pressure was necessary for a continuous measurement over a few hours. Resetting was, however, required in case of an abrupt or unexplained pressure change during the indirect recording, presumably caused by a large movement or displacement of the tail in the chamber. Two hundred sixty-two pairs of values were randomly picked from indirect and corresponding direct pressure recordings in response to bleeding, transfusion, and blowing smoke or hot air at the face of the rat. Photoelectric plethysmogram, controlled volume, cuff pressure, and intra-arterial pressure were simultaneously recorded by a multichannel recorder (San-Ei Instrument Co.).
YAMAKOSHI, mHg
SHIMAZU,
+--lOs+
AND
TOGAWA
SHR $ 31Og
INTRA-ARTERIAL PRESSURE ‘OO(Pb ) n-
CUFF PRESSURE (PC )
‘HYSMOGRAM PLET (PGJ
RESULTS
Figure 2 shows an example of simultaneous recordings of intra-arterial pressure (Pb), cuff pressure (PC), photoelectric plethysmogram (PG) and its pulsating component (PG,), and controlled volume (S,) in open-loop operation obtained from an unanesthetized SHR rat. The mean level of the photoelectric plethysmogram and the amplitude of its pulsating component changed in accordance with the gradual increase or decrease in the cuff pressure. The appearance of the pulsating signal following the gradual decrease in the cuff pressure indicates the point at which the cuff pressure represents the systolic pressure (P,,). The value of the indirect systolic pressure at this moment was nearly equal to that of the directly recorded systolic pressure. The cuff pressure corresponding to the maximum amplitude of the pulsating signal was almost equal to the arterial mean pressure. This cuff pressure and the corresponding mean level of the plethysmogram served as the preset value and the reference value for the servo control, respectively. When the control loop of the system was closed and the gain of the power amplifier was gradually increased, the cuff pressure began instantaneously to follow the arterial pressure. Conversely the superimposed pulsations disappeared and at this moment the plethysmogram was clamped at the reference value (Fig. 3). The wave form of arterial pressure measured by this indirect method was remarkably similar to that recorded simultaneously by the direct measurement. Both beat-to-beat systolic and diastolic pressure can be measured from the indirect records (Fig. 3). The accuracy of this method is dependent mainly on the pulse disappearance ratio (PDR) defined as the ratio of the amplitude of the pulsating signal during the servo control (APG,) to that before the servo control (APG), i.e., PDR = APGJAPG (see Fig. 3). Ideally, when this ratio becomes zero by increasing the total closed-loop gain of the system, the cuff pressure should follow in perfect agreement the intra-arterial pressure of the tail segment.
PULSATI LE COMPONENT OfPG (PGp1
CONTROLLED VOLUME (SV ) FIG. 2. An example of simultaneous recordings of intra-arterial pressure (Ph), cuff pressure (P,), photoelectric plethysmogram (PG), pulsatile component of the plethysmogram (PG,), and controlled volume (S,) in open-loop operation obtained from an unanesthetized spontaneously hypertensive (SHR) rat. Arrow directed upward indicates pressure value (PCs) corresponding to the systolic end point.
Figure 4 shows the relationship between PDR (in dB) and the pulse pressure ratio (PPR in percent), defined as the ratio of the amplitude of the pulse pressure in the cuff (AP,) to that in the femoral artery used as a reference for intra-arterial pressure in the tail (AP,), i.e., PPR = APJAPb. Means t SE of the measurements in all the rats are shown in this diagram. There was no statistically significant difference in the error of blood pressure measurements between NR and SHR groups in relation to the pulse disappearance ratio (P > 0.6). In order to obtain a stable measurement of pressure over a long period it was difficult in practice to attain the complete disappearance of the pulsating signal. However, less than 5% measurement error as compared with femoral arterial pulse pressure was readily attainable in both NR and SHR groups when the PDR was greater than approximately -15 dB. The responsesto such interventions asbody movement
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INDIRECT
MEASUREMENT
OF
INSTANTANEOUS
mmlig
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MOVEMENT
BLOWii&lOT AIR
mogram is clamped at the reference value. Configuration of indirectly measured arterial pressure is remarkably similar to that simultaneously recorded by direct measurement. Values at the marked points indicate systolic/diastolic pressure’ at respective times. Different stages of measurement are indicated in lower part of figure.
3. Simultaneous recording of intra-arterial pressure (Pt,), cuff pressure (P,), plethysmogram (PG), and controlled volume (S,) in closed-loop operation obtained from an unanesthetized normotensive (NR) rat. Note that with the gradual increase in the closed-loop gain of the system the cuff pressure begins to follow the arterial pressure. Conversely the superimposed pulsations disappear and the plethysFIG.
Figure 5 is a pair of scatter diagrams of the values of systolic (Fig. 5A) and diastolic (Fig. 5B) blood pressure showing the relationships between the simultaneous results obtained by the indirect and by the direct method in all unanesthetized NR and SHR rats. For these diagrams, 15-25 pairs of values were arbitrarily chosen in each rat at various levels of blood pressure. Closed and open circles in this diagram indicate the data points obtained from NR and SHR rats, respectively. The values of the indirect measurement were found to have a very close correlation with the values measured directly. The linear regression equations (solid lines) between the systolic values of indirect pressure (PCs)and direct pressure (P& and between the diastolic values of indirect pressure (Pcd) and direct pressure (Pbd) were P,, = 0.966Pb, + 2.76 and P,d = l.OlPbd - 2.30 with correlation coefficients r of 0.990 for the former and 0.975 for the latter. The P,,/Pb, and Pcd/Pbd ratios were 0.984 t 0.030 (SD) and 0.985 t 0.053, respectively. - 20
-10 PULSEDISAPPEARANCE RATIOtPDR) dB
0
4. Relationship between plethysmographic pulse disappearance ratio (PDR, dB) and pulse pressure ratio (PPR, %) obtained from unanesthetized normotensive and spontaneously hypertensive rats. Brackets represent SE. FIG.
and blowing smoke or hot air during the recording by this instrument in an unanesthetized NR rat are shown in Fig. 3. It is demonstrated that prompt responses produced by different interventions were well recorded, in close agreement with the simultaneous direct measurements.
DISCUSSION
The present study is an attempt to measure indirectly the instantaneous arterial blood pressure in the rat by using a hydraulic servo control to maintain a constant unloaded vascular volume. Most of the efforts in recent years have been made to determine more accurately only systolic blood pressure in the rat (3, 5,9, 16, 17). For this reason, the proposed indirect method seems to have the advantage of obtaining an absolute value of the arterial pressure at any instance in the rat’s tail continuously. Recently Penaz (11) has successfully applied the prin-
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YAMAKOSHI,
l
SHIMAZU,
AND
TOGAWA
NR
0 SHR
ml
NR
MO-
NOPa10.966 Pk +2.76 %=
l.Ol~-2.30
soc Y I-
100
150
200
DIRECTSYSTOLIC BLOOD PRESSURE (Pd
230
300
mHg
I 50
0’
I loo
I 150
DIRECT DIASTOLIC BLOODREWIRE(M
A
1 200 rrll~
6
FIG. 5. Comparison of direct and indirect simultaneous measurements of systolic pressures (A) and diastolic pressures (B) in unanesthetized normotensive (0) and spontaneously hypertensive (0) rats. Indirect systolic (PCs) and diastolic (Pcd) blood pressures measured by this method are plotted against corresponding direct systolic (Phs) and
diastolic (Phd) blood pressures recorded by intraarterial catheters. These data were arbitrarily collected from simultaneous recordings at various levels of blood pressure. Solid line in each diagram indicates regression line.
ciple of i ndirec t unloading for the measurement of blood pneumo-servo-control pressure in the human finger u system. But he has not reported the details and performance of the system. The response of the pneumo-servo system, however, is too slow for the measurement of rat blood pressure. The hydraulic-servo system employed by us has the requisite response, and moreover this system enables to obtain quantitatively the controlled volume (S,). Due to better frequency characteristics, this instrument can be easily used for measuring blood pressure in the human finger as well. (The details of the results obtained from the human subjects will be reported elsewhere.) The most important factor influencing the accuracy of measurement is the setting of proper reference value for the servo control. Penaz has mentioned that this value would be approxim .ately the last one-thi rd of the arterial compartment (11). In our experiments, the arterial and low pressure compartments could not be clearly differentiated, perhaps due to the presence of rigid tissue around the vascular beds in the tail. Theoretically the reference value should correspond to the unloaded vascular volume, so we adopted the mean value of the plethysmogram at the point of the maximum amplitude of the pulsating signal as the reference value. Thesetting of the proper reference value reduces the minimal measurement error, as is obvious from th .e close agreement between the indirect and direct measurements of blood pressure.
A change in vasoconstrictive tone during the blood pressure measurements might also influence the results. In case the vasoconstrictive tone of the caudal artery is too severe to allow the detection of a Pl.ethysm.ographAC pulsating signal, the results obtained by this method cannot be interpreted satisfactorily; in fact, as observed by other investigators (1, 9), if the vasoconstrictive tone is so severe as to occlude the artery itself, it is fairly difficult to determine systolic blood pressure because of the indistinctness of the systolic end point. Therefore, the blood pressure can be indirectly measured by this method only when the plethysmogram is effectively detectable. In conclusion, the present measuring system is shown to be a useful means for accurate indirect measurement of instantaneous arterial pressure in the rat’s tail. This method will be helpful for physiological studies of blood pressure. The authors thank Ueda Electronics Works Ltd., Tokyo, for their assistance in constructing the instrument, Dr. A. R, Bukhari, Univ. of Engineering, Lahore, Pakistan, and Prof. Hiroshi Ito, Department of Physiology, Kyorin Univ. School of Medicine, for their valuable criticism and assistance in preparing the manuscript, Prof. Yukio Yamori, Dept. of Pathology, Faculty of Medicine, Shimane Univ., for his experimental expertise and the supply of the animals, and Mr. Takao Shoji for his technical assistance.
Received
26 February
1979; accepted
in final
form
21 June
1979.
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REFERENCES 1. ALEXANDER, C. S. A new simple method for indirect determination of blood pressure in the rat. Proc. Sot. Exp. BioZ. Med. 94: 368-372, 1957. 2. BEN-ZIV, G., J. WEINMAN, AND F. G. SULMAN. A photoplethysmographic method for measurement of systolic blood pressure in the rat. Arch. Int. Pharamacodyn. 149: 527-535, 1964. 3. BUNAG, R. D. Validation in awake rats of a tail-cuff method for measuring systolic pressure. J. Appl. Physiol. 34: 279-282, 1973. 4. BYROM, F. B., AND C. WILSON. A plethysmographic method for measuring systolic blood pressure in the intact rat. J. Physiol. London 93: 301-304, 1938. 5. FREGLY, M. J. Factors affecting indirect determination of systolic blood pressure of rats. J. Lab. CZin. Med. 62: 223-230, 1963. 6. FRIEDMAN, M., AND S. C. FREED. Microphonic manometer for the indirect determination of systolic blood pressure in the rat. Proc. Sot. Exp. BioZ. Med. 70: 670-672, 1942. 7. GEDDES, L. A. The Direct and Indirect Measurement of BZood Pressure. Chicago: Year Book, 1970, p. 196. 8. GEDDES, L. A., V. CHAFFEE, S. J. WHISTLER, J. D. BOURLAND, AND W. A. TACKER. Indirect mean blood pressure on the anesthetized pony. Am. J. Vet. Res. 38: 2055-2057, 1977. 9. MAISTRELLO, I., AND R. MATSCHER. Measurement of systolic blood pressure of rats: comparison of intra-arterial and cuff values. J. AppZ. PhysioZ. 26: 188-193, 1969. 10. MAREY, E. J. Pression et vitesse du sang. In: Physiologigue Experimentale, edited by G. Masson. Paris, France, 1876, vol. 2, chapt. 8,
p. 307. 11. PENAZ, J. Photoelectric measurement of blood pressure, volume and flow in the finger. Dig. Int. Conf Med. BioZ. Eng., lOth, Dresden, East Germany, 1973, p. 104. 12. POSEY, J. A., L. A. GEDDES, H. WILLIAMS, AND A. G. MOORE. The meaning of the point of maximum oscillations in cuff pressure in the indirect measurement of blood pressure. Part I. Cardiovasc. Res. Cent. BUZZ. Houston 8: 15-25, 1969. 13. RAMSEY, M., III. Noninvasive automatic determination of mean arterial pressure. Med. BioZ. Eng. Comput. 17: 11-18, 1979. 14. SHIRER, H. W. Blood pressure measuring methods. IRE Trans. Bio-Med. Electron. 9: 116-125, 1962. 15. SMITH, C. R., AND W. H. BICKLEY. The Measurement of BZood Pressure in the Human Body. Washington, DC: US Gov. Printing Office, National Aeronautics and Space Administration, SP-5006, 1964, p. l-34. 16. SHULER, R. H., H. S. KUPPERMAN, AND W. F. HAMILTON. Comparison of direct and indirect blood pressure measurements in rats. Am. J. Physiol. 141: 625-629, 1944. 17. SOBIN, S. S. Accuracy of indirect determinations of blood pressure in the rat: relation to temperature of plethysmograph and width of cuff. Am. J. Physiol. 146: 179-186, 1946. 18. WILLIAMS, J. R. JR., T. R. HARRISON, AND A. GROLLMAN. A simple method for determining the systolic blood pressure of the unanesthetized rat. J. CZin. Invest. 18: 373-376, 1939.
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HIDEAKI SHIMAZU, AND TATSUO Tomeasurement of instantaneous arterial blood pressure in the rat. Am. J. Physiol. 237(5): H632-H637,1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(5): H632-H637, 1979.We devised a hydraulic servo-control system for indirect blood pressure measurement in the rat’s tail, by which beat-to-beat systolic and diastolic blood pressure can be obtained. In this method the principle of “unloading vascular wall” proposed by Shirer (1962) is employed. The proposed system is composed of a transmittance photoelectric plethysmograph with an occluding cuff, a small diaphragm actuator for compressing and decompressing the segment by the hydraulic pressure, and an electromagnetic shaker driven by a volume servo circuit in accordance with the signal from the photoelectric plethysmograph. The plethysmographic signal is clamped at a proper value corresponding to the unloaded vascular volume by the instantaneous hydraulic servo control. The cuff pressure thus automatically controlled follows the intra-arterial pressure in the tail segment. The accuracy of this method was evaluated in comparison with direct measurement of blood pressure recorded simultaneously from 16 unanesthetized spontaneously hypertensive and normotensive rats. Close agreement between the simultaneous data from these two methods were observed. YAMAKOSHI, GAWA. Indirect
KEN-ICHI,
hydraulic servo control; unloaded tric plethysmograph; rat’s tail
vascular
volume;
photoelec-
METHODS of indirect measurement of arterial blood pressure in the rat have been described (1, 2, 4, 6, 7,16, 18; for review see Ref. 7). By most of these methods blood pulse or blood volume variations corresponding to the systolic end point are detected, so they can measure systolic pressure only. There is a need for an instrument capable of indirect measurement of arterial pressure at any instance in the rat. This paper describes a new method for measuring arterial pressure at any instance, including beat-to-beat systolic and diastolic blood pressure noninvasively in the rat’s tail. Using hydraulic servo-control technique, an applied counterpressure is made to equalize the intravascular pressure so as to maintain a constant arterial vascular volume, i.e., unloading of the vascular wall (11, 14; see Ref. 15). To evaluate the accuracy and reliability the values of VARIOUS
H632
10-3,
arterial pressure measured by this method were compared with those obtained simultaneously by direct measurements carried out in 16 unanesthetized spontaneously hypertensive and normotensive rats. METHODS
The instrument. Figure 1 shows a schematic diagram of the instrument. In this system the hydraulic pressure control is adapted to compress and decompress the rat’s tail by means of an occluding cuff. The occluding cuff (OC) is installed in a compression chamber (CR) which is filled with water (W) and equipped with a transmittance photoelectric plethysmograph (PL). The cuff is made of a thin-walled (0.1 mm) translucent rubber tube and formed so as not to develop any tension during the compression of the tail segment. Both flanges of the rubber cuff are firmly fixed to both ends of the chamber by a pair of annular disks (AD). According to the size of the tail segment, four different occluding cuffs having inner diameters of 8, 10, 12, and 14 mm were used. The effective width of all cuffs was 20 mm. The compression chamber is connected to a small diaphragm actuator (DA) via a lo-mm-long fluid passage of 16 mm inner diameter. The compliance of the diaphragm having an effective area of 10 cm2 was only 0.01 ml/100 mmHg. A retainer plate (RP) firmly fixed to the diaphragm is connected to the plunger (P) of an electromagnetic shaker (SHAKER, Shinken Co.). The maximum stroke and the rating of the shaker are t,2 mm and 5 kg, respectively, when drawing an input power of 50 W. The shaker is driven from the output of a power amplifier (PA). The position of the plunger is sensed by a linear displacement transducer (LT). The output of LT provides the quantitative information about the controlled volume for the adjustment of pressure on the tail segment. The compression chamber has an air vent (AV), which is also used to prime this system with water, and a side connection (SC) for a strain-gauge type pressure transducer (P23Db, Statham) to measure the hydraulic pressure in the chamber. The photoelectric plethysmograph consists of series-
0363-6135/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
Society
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INDIRECT
MEASUREMENT
OF
INSTANTANEOUS
RAT
BLOOD
PRESSURE
H633
FIG. 1. Schematic diagram of the servo-control system for indirect measurement of rat blood pressure. PL, photoelectric plethysmograph; LED, light-emitting diodes; PT, phototransistors; OC, thin-walled occluding rubber cuff; AD, annular disk; CR, compression chamber; SC, side connection for measuring applied cuff pressure (PC); AV, air vent; DA, diaphragm actuator; W, water; RT, retainer plate; P, plunger; LT, linear displacement transducer; AMP, amplifier; D.AMP, differential amplifier; CMP, gain and phase lead-lag compensator; PA, power amplifier; Sv, controlled volume; PG, photoelectric plethysmogram. Dimensions and electrical connections of LED and PT are shown in inset.
4 P0
CMP
connected light-emitting diodes (LED: TLN 103, Toshiba Electric Co.) used as the light source and parallel-connected phototransistors (PT: TPS603, Toshiba) used as the photodetector, as shown in the inset. The light source (LED) is insulated from water by means of an acrylic plate. The detector is fixed directly on the root of the tail with adhesive tape. The frequency response of the photoelectric plethysmograph is flat up to more than 200 Hz. Either open- or closed-loop operation can be selected by a switch (SW), the former for the plethysmographic recording, and the latter for the blood pressure measurement. When the switch is turned to the O-position (openloop operation), the photoelectric plethysmogram can be obtained at a cuff pressure (PC) that is either set at a constant level or changed by an external electric signal REF (II). Thus the volume of the total vascular bed in the segment can be estimated in this operation (Fig. 2). In the closed-loop operation, on the other hand, the cuff pressure is controlled by the pulsating signal from the plethysmograph so that the vascular volume is clamped at a reference value REF (I). To determine the reference value for a desired volume corresponding to the unloaded vascular wall, Marey’s concept (10) was adopted, according to which the plethysmographic signal reaches the maximum amplitude when the wall of the artery is relieved of the arterial tension (see Ref. 12). The cuff pressure that allows maximum amplitude of the photoelectric pulsating signal in the open-loop operation is used as the preset value; the mean value of the photoelectric plethysmogram at this point serves as the reference value for the servo control in this experiment. This cuff pressure has been experimentally proved to be approximately equal to the intra-arterial mean pressure (8, 12, 13).
After the control loop is closed, the plethysmographic
signal is subtracted from the reference value to produce the servo control error. The error is led to the power amplifier via a gain and phase lead-lag compensator (CMP). In this operation, any variation of arterial vascular volume due to the change in intraarterial pressure is instantaneously compensated by the automatic servo regulation of the cuff pressure, which thus continuously and quantitatively follows the intra-arterial blood pressure. The gain and phase lead-lag compensator (CMP) is necessary to obtain high fidelity servo control. The use of CMP makes the frequency characteristics of the whole system flat up to about 60 Hz, a satisfactory dynamic characteristic for blood pressure measurement in the rat. Experiments in rats. Eight spontaneously hypertensive rats (SHR, Aoki-Okamoto strain) and eight normotensive rats (NR, Wistar-Kyoto), weighing 250-400 g, were anesthetized with sodium pentobarbital, 25 mg/kg ip. Femoral arterial pressure was measured directly using a Statham P23Db pressure transducer and an indwelling arterial catheter made of polyethylene tubing (ID 0.05 cm, OD 0.1 cm, length lo-15 cm; Hibiki Co.). Damping of the pressure pulse obtained by the catheter-transducer system is less than 2% as compared with that recorded via a 5-cm-long 20-gauge needle. After 10 min warming in a temperature-controlled chamber at 39°C to allow the detection of the plethysmogram in the tail, the rats were housed in the cage in an air-conditioned room. The photodetector was fixed at the root of the tail before the tail was inserted in the compression chamber through a suitable cuff. The measurements were carried out after the animal recovered from anesthesia. The experiments were performed at a room temperature of about 25°C. First, the open-loop operation was performed to deter-
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H634 mine the vascular volume of the tail segment. By this operation, a single reading of indirect systolic pressure could also be made from the plethysmographic pulsating signal corresponding to the systolic end point. After the cuff pressure was set at the preset value corresponding to the unloaded vascular volume, the closed-loop operation was started to obtain the indirect instantaneous blood pressure. Only one single setting of the preset value of the cuff pressure was necessary for a continuous measurement over a few hours. Resetting was, however, required in case of an abrupt or unexplained pressure change during the indirect recording, presumably caused by a large movement or displacement of the tail in the chamber. Two hundred sixty-two pairs of values were randomly picked from indirect and corresponding direct pressure recordings in response to bleeding, transfusion, and blowing smoke or hot air at the face of the rat. Photoelectric plethysmogram, controlled volume, cuff pressure, and intra-arterial pressure were simultaneously recorded by a multichannel recorder (San-Ei Instrument Co.).
YAMAKOSHI, mHg
SHIMAZU,
+--lOs+
AND
TOGAWA
SHR $ 31Og
INTRA-ARTERIAL PRESSURE ‘OO(Pb ) n-
CUFF PRESSURE (PC )
‘HYSMOGRAM PLET (PGJ
RESULTS
Figure 2 shows an example of simultaneous recordings of intra-arterial pressure (Pb), cuff pressure (PC), photoelectric plethysmogram (PG) and its pulsating component (PG,), and controlled volume (S,) in open-loop operation obtained from an unanesthetized SHR rat. The mean level of the photoelectric plethysmogram and the amplitude of its pulsating component changed in accordance with the gradual increase or decrease in the cuff pressure. The appearance of the pulsating signal following the gradual decrease in the cuff pressure indicates the point at which the cuff pressure represents the systolic pressure (P,,). The value of the indirect systolic pressure at this moment was nearly equal to that of the directly recorded systolic pressure. The cuff pressure corresponding to the maximum amplitude of the pulsating signal was almost equal to the arterial mean pressure. This cuff pressure and the corresponding mean level of the plethysmogram served as the preset value and the reference value for the servo control, respectively. When the control loop of the system was closed and the gain of the power amplifier was gradually increased, the cuff pressure began instantaneously to follow the arterial pressure. Conversely the superimposed pulsations disappeared and at this moment the plethysmogram was clamped at the reference value (Fig. 3). The wave form of arterial pressure measured by this indirect method was remarkably similar to that recorded simultaneously by the direct measurement. Both beat-to-beat systolic and diastolic pressure can be measured from the indirect records (Fig. 3). The accuracy of this method is dependent mainly on the pulse disappearance ratio (PDR) defined as the ratio of the amplitude of the pulsating signal during the servo control (APG,) to that before the servo control (APG), i.e., PDR = APGJAPG (see Fig. 3). Ideally, when this ratio becomes zero by increasing the total closed-loop gain of the system, the cuff pressure should follow in perfect agreement the intra-arterial pressure of the tail segment.
PULSATI LE COMPONENT OfPG (PGp1
CONTROLLED VOLUME (SV ) FIG. 2. An example of simultaneous recordings of intra-arterial pressure (Ph), cuff pressure (P,), photoelectric plethysmogram (PG), pulsatile component of the plethysmogram (PG,), and controlled volume (S,) in open-loop operation obtained from an unanesthetized spontaneously hypertensive (SHR) rat. Arrow directed upward indicates pressure value (PCs) corresponding to the systolic end point.
Figure 4 shows the relationship between PDR (in dB) and the pulse pressure ratio (PPR in percent), defined as the ratio of the amplitude of the pulse pressure in the cuff (AP,) to that in the femoral artery used as a reference for intra-arterial pressure in the tail (AP,), i.e., PPR = APJAPb. Means t SE of the measurements in all the rats are shown in this diagram. There was no statistically significant difference in the error of blood pressure measurements between NR and SHR groups in relation to the pulse disappearance ratio (P > 0.6). In order to obtain a stable measurement of pressure over a long period it was difficult in practice to attain the complete disappearance of the pulsating signal. However, less than 5% measurement error as compared with femoral arterial pulse pressure was readily attainable in both NR and SHR groups when the PDR was greater than approximately -15 dB. The responsesto such interventions asbody movement
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INDIRECT
MEASUREMENT
OF
INSTANTANEOUS
mmlig
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I
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H635
PRESSURE
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NR $ 380g 1
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.
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CUFFPRESSURE 100
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T
CONTROllED uoL VOLUME
STARTOf MEASUREMENT
t
BLOWING SMOKE
MOVEMENT
BLOWii&lOT AIR
mogram is clamped at the reference value. Configuration of indirectly measured arterial pressure is remarkably similar to that simultaneously recorded by direct measurement. Values at the marked points indicate systolic/diastolic pressure’ at respective times. Different stages of measurement are indicated in lower part of figure.
3. Simultaneous recording of intra-arterial pressure (Pt,), cuff pressure (P,), plethysmogram (PG), and controlled volume (S,) in closed-loop operation obtained from an unanesthetized normotensive (NR) rat. Note that with the gradual increase in the closed-loop gain of the system the cuff pressure begins to follow the arterial pressure. Conversely the superimposed pulsations disappear and the plethysFIG.
Figure 5 is a pair of scatter diagrams of the values of systolic (Fig. 5A) and diastolic (Fig. 5B) blood pressure showing the relationships between the simultaneous results obtained by the indirect and by the direct method in all unanesthetized NR and SHR rats. For these diagrams, 15-25 pairs of values were arbitrarily chosen in each rat at various levels of blood pressure. Closed and open circles in this diagram indicate the data points obtained from NR and SHR rats, respectively. The values of the indirect measurement were found to have a very close correlation with the values measured directly. The linear regression equations (solid lines) between the systolic values of indirect pressure (PCs)and direct pressure (P& and between the diastolic values of indirect pressure (Pcd) and direct pressure (Pbd) were P,, = 0.966Pb, + 2.76 and P,d = l.OlPbd - 2.30 with correlation coefficients r of 0.990 for the former and 0.975 for the latter. The P,,/Pb, and Pcd/Pbd ratios were 0.984 t 0.030 (SD) and 0.985 t 0.053, respectively. - 20
-10 PULSEDISAPPEARANCE RATIOtPDR) dB
0
4. Relationship between plethysmographic pulse disappearance ratio (PDR, dB) and pulse pressure ratio (PPR, %) obtained from unanesthetized normotensive and spontaneously hypertensive rats. Brackets represent SE. FIG.
and blowing smoke or hot air during the recording by this instrument in an unanesthetized NR rat are shown in Fig. 3. It is demonstrated that prompt responses produced by different interventions were well recorded, in close agreement with the simultaneous direct measurements.
DISCUSSION
The present study is an attempt to measure indirectly the instantaneous arterial blood pressure in the rat by using a hydraulic servo control to maintain a constant unloaded vascular volume. Most of the efforts in recent years have been made to determine more accurately only systolic blood pressure in the rat (3, 5,9, 16, 17). For this reason, the proposed indirect method seems to have the advantage of obtaining an absolute value of the arterial pressure at any instance in the rat’s tail continuously. Recently Penaz (11) has successfully applied the prin-
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YAMAKOSHI,
l
SHIMAZU,
AND
TOGAWA
NR
0 SHR
ml
NR
MO-
NOPa10.966 Pk +2.76 %=
l.Ol~-2.30
soc Y I-
100
150
200
DIRECTSYSTOLIC BLOOD PRESSURE (Pd
230
300
mHg
I 50
0’
I loo
I 150
DIRECT DIASTOLIC BLOODREWIRE(M
A
1 200 rrll~
6
FIG. 5. Comparison of direct and indirect simultaneous measurements of systolic pressures (A) and diastolic pressures (B) in unanesthetized normotensive (0) and spontaneously hypertensive (0) rats. Indirect systolic (PCs) and diastolic (Pcd) blood pressures measured by this method are plotted against corresponding direct systolic (Phs) and
diastolic (Phd) blood pressures recorded by intraarterial catheters. These data were arbitrarily collected from simultaneous recordings at various levels of blood pressure. Solid line in each diagram indicates regression line.
ciple of i ndirec t unloading for the measurement of blood pneumo-servo-control pressure in the human finger u system. But he has not reported the details and performance of the system. The response of the pneumo-servo system, however, is too slow for the measurement of rat blood pressure. The hydraulic-servo system employed by us has the requisite response, and moreover this system enables to obtain quantitatively the controlled volume (S,). Due to better frequency characteristics, this instrument can be easily used for measuring blood pressure in the human finger as well. (The details of the results obtained from the human subjects will be reported elsewhere.) The most important factor influencing the accuracy of measurement is the setting of proper reference value for the servo control. Penaz has mentioned that this value would be approxim .ately the last one-thi rd of the arterial compartment (11). In our experiments, the arterial and low pressure compartments could not be clearly differentiated, perhaps due to the presence of rigid tissue around the vascular beds in the tail. Theoretically the reference value should correspond to the unloaded vascular volume, so we adopted the mean value of the plethysmogram at the point of the maximum amplitude of the pulsating signal as the reference value. Thesetting of the proper reference value reduces the minimal measurement error, as is obvious from th .e close agreement between the indirect and direct measurements of blood pressure.
A change in vasoconstrictive tone during the blood pressure measurements might also influence the results. In case the vasoconstrictive tone of the caudal artery is too severe to allow the detection of a Pl.ethysm.ographAC pulsating signal, the results obtained by this method cannot be interpreted satisfactorily; in fact, as observed by other investigators (1, 9), if the vasoconstrictive tone is so severe as to occlude the artery itself, it is fairly difficult to determine systolic blood pressure because of the indistinctness of the systolic end point. Therefore, the blood pressure can be indirectly measured by this method only when the plethysmogram is effectively detectable. In conclusion, the present measuring system is shown to be a useful means for accurate indirect measurement of instantaneous arterial pressure in the rat’s tail. This method will be helpful for physiological studies of blood pressure. The authors thank Ueda Electronics Works Ltd., Tokyo, for their assistance in constructing the instrument, Dr. A. R, Bukhari, Univ. of Engineering, Lahore, Pakistan, and Prof. Hiroshi Ito, Department of Physiology, Kyorin Univ. School of Medicine, for their valuable criticism and assistance in preparing the manuscript, Prof. Yukio Yamori, Dept. of Pathology, Faculty of Medicine, Shimane Univ., for his experimental expertise and the supply of the animals, and Mr. Takao Shoji for his technical assistance.
Received
26 February
1979; accepted
in final
form
21 June
1979.
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INDIRECT
MEASUREMENT
OF
INSTANTANEOUS
RAT
BLOOD
PRESSURE
H637
REFERENCES 1. ALEXANDER, C. S. A new simple method for indirect determination of blood pressure in the rat. Proc. Sot. Exp. BioZ. Med. 94: 368-372, 1957. 2. BEN-ZIV, G., J. WEINMAN, AND F. G. SULMAN. A photoplethysmographic method for measurement of systolic blood pressure in the rat. Arch. Int. Pharamacodyn. 149: 527-535, 1964. 3. BUNAG, R. D. Validation in awake rats of a tail-cuff method for measuring systolic pressure. J. Appl. Physiol. 34: 279-282, 1973. 4. BYROM, F. B., AND C. WILSON. A plethysmographic method for measuring systolic blood pressure in the intact rat. J. Physiol. London 93: 301-304, 1938. 5. FREGLY, M. J. Factors affecting indirect determination of systolic blood pressure of rats. J. Lab. CZin. Med. 62: 223-230, 1963. 6. FRIEDMAN, M., AND S. C. FREED. Microphonic manometer for the indirect determination of systolic blood pressure in the rat. Proc. Sot. Exp. BioZ. Med. 70: 670-672, 1942. 7. GEDDES, L. A. The Direct and Indirect Measurement of BZood Pressure. Chicago: Year Book, 1970, p. 196. 8. GEDDES, L. A., V. CHAFFEE, S. J. WHISTLER, J. D. BOURLAND, AND W. A. TACKER. Indirect mean blood pressure on the anesthetized pony. Am. J. Vet. Res. 38: 2055-2057, 1977. 9. MAISTRELLO, I., AND R. MATSCHER. Measurement of systolic blood pressure of rats: comparison of intra-arterial and cuff values. J. AppZ. PhysioZ. 26: 188-193, 1969. 10. MAREY, E. J. Pression et vitesse du sang. In: Physiologigue Experimentale, edited by G. Masson. Paris, France, 1876, vol. 2, chapt. 8,
p. 307. 11. PENAZ, J. Photoelectric measurement of blood pressure, volume and flow in the finger. Dig. Int. Conf Med. BioZ. Eng., lOth, Dresden, East Germany, 1973, p. 104. 12. POSEY, J. A., L. A. GEDDES, H. WILLIAMS, AND A. G. MOORE. The meaning of the point of maximum oscillations in cuff pressure in the indirect measurement of blood pressure. Part I. Cardiovasc. Res. Cent. BUZZ. Houston 8: 15-25, 1969. 13. RAMSEY, M., III. Noninvasive automatic determination of mean arterial pressure. Med. BioZ. Eng. Comput. 17: 11-18, 1979. 14. SHIRER, H. W. Blood pressure measuring methods. IRE Trans. Bio-Med. Electron. 9: 116-125, 1962. 15. SMITH, C. R., AND W. H. BICKLEY. The Measurement of BZood Pressure in the Human Body. Washington, DC: US Gov. Printing Office, National Aeronautics and Space Administration, SP-5006, 1964, p. l-34. 16. SHULER, R. H., H. S. KUPPERMAN, AND W. F. HAMILTON. Comparison of direct and indirect blood pressure measurements in rats. Am. J. Physiol. 141: 625-629, 1944. 17. SOBIN, S. S. Accuracy of indirect determinations of blood pressure in the rat: relation to temperature of plethysmograph and width of cuff. Am. J. Physiol. 146: 179-186, 1946. 18. WILLIAMS, J. R. JR., T. R. HARRISON, AND A. GROLLMAN. A simple method for determining the systolic blood pressure of the unanesthetized rat. J. CZin. Invest. 18: 373-376, 1939.
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