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On the Measurement of Intestinal Tonus
Vol. 58, No.5 Printed in U.S.A.
GASTROENTEROLOGY
Copyright © 1970 by The Williams & Wilkins Co.
ON THE MEASUREMENT OF INTESTINAL TONUS DAVID
R.
FLEISHER, M.D.
Department of Pediatrics, University of California School of Medicine, Los Angeles, California
Gut tonus has been studied previously in terms of the pressurevolume behavior of colon subjected to distension. Distension was produced by gravity flow into an intraluminal sac of fixed length from a constant hydrostatic head. This method is analyzed in terms of pressure-volume data obtained from a mechanical model. Criticism of the equation for resistance to distension, R = P/(dV/dt), is offered. This equation results in resistance curves which become progressively erratic because inherent errors in volume measurement are magnified progressively during the latter part of inflow distension under conditions of gravity-fed, variable flow. Criticism also is offered for the application of the equation for circumferential tension, T ex P yV, to pressure-volume data collected under conditions of variable flow. A viscus having low distensibility, under conditions of self-regulated inflow, may ultimately accept less volume from a fixed hydrostatic head and therefore may develop less intramural tension than a more distensible viscus. Difficulties with the volume factor are avoided when distension is produced at a constant rate of inflow. Circumferential tension is then a more reliable expression of the pressure-volume behavior of the gut. Wall tension produced by distension was examined in a latex viscus of variable wall thickness, in dog colon before and after administration of morphine, and in rabbit colon before and after sodium iodoacetate. The tension developed by both latex and smooth muscle viscera is related directly to the thickness or contractility, respectively, of the visceral walls. The tension developed by gut also is related directly to the rate at which distension is applied. In contrast, the tension developed by latex viscera is not rate-dependent. The tension differential is defined as the difference in tension, at the same volume, between rapidly and slowly distended gut. The size of the tension differential is related directly to the level of tonus. Attention is drawn to previous descriptions of similar phenomena in the pressure-volume behavior of urinary bladder. In a recent review of colon motility, Truelove l mentioned the potential value of assessing colon tone and cited the work of Lipkin et al. 2 in this regard. These workers studied the stress-strain characReceived July 14, 1969. Accepted November 14, 1969. Address requests for reprints to: Dr. David R. Fleisher, Department of Pediatrics, The Center for the Health Sciences, Los Angeles, California 90024. The author wishes to acknowledge his indebtedness to Dr. Guilio J. Barbero for training in pedi-
teristics of the human colon by recording simultaneous pressure and volume data during distension of an intraluminal sac filled by gravity from a constant hydrostatic head. Tonus was expressed in terms of the equation R = P/(dV/dt) , in which atric gastroenterology, under whose of this work was accomplished, to H. J. Pyenson for invaluable technical to Dr. M. 1. Grossman and Dr. A. J. ful criticism and suggestions.
auspices much L. Fleisher and assistance, and Brady for help-
686
FLEISHER
R signifies the resistance offered by the wall of the viscus to the distending force, P signifies the instantaneous pressure within the distending sac, and dV/dt signifies the instantaneous rate of inflow coincident with the pressure measurement. Connell') referred to this method as a potential means of assessing the effects of drugs and various physiological and clinical states on gut resistance to stretch. Hopkins4 reviewed the data of Lipkin and co-workers and demonstrated that formulating their data in terms of circumferential tension, according to the law of Laplace,5 resulted in considerably more regular graphic representations of these pressure-volume phenomena. Laplace's equation for the circumferential tension within the wall of a hollow cylinder is T = Pr, in which T signifies tension, P signifies intraluminal pressure, and r signifies the cylinder's radius. Hopkins derived the expression of relative circumferential tension from the T,aplace equation as follows: T a: P yV, in which V signifies intraluminal volume and tension is expressed in terms of arbitrary units. Hopkins did not offer an analysis as to why Lipkin's resistance curves were more erratic than his own tension curves. It is the purpose of this communication to examine critically the method of Lipkin et al. as well as Hopkins' suggestion that the law of Laplace provides a better means of expressing such pressure-volume data. Further, experimental data is presented delineating some previously unmentioned conditions that may have a marked effect on gut tonus measured in terms of intraluminal pressure and volume in a passively distended viscus.
Materials and Methods Four experiments are reported. Experiment 1. The method of Lipkin et a1. was examined by determining the pressurevolume characteristics of a latex viscus subjected to inflow distension from a constant hydrostatic head. The essential feature of their method was duplicated, viz, the rate of inflow varied from one instant to the next according to the difference in pressure between the distending viscus and the fixed hydrostatic head;
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each inflow distension terminated when the pressure within the viscus rose to that of the hydrostatic head. The pressure and volume within the viscus were recorded during the course of each inflow distension by essentially the same means as Lipkin's. The viscus consisted of a large latex balloon placed within a rigid cylinder 7 cm in diameter lying on its side upon a table. The thickness of the viscus wall was altered by sheathing it with one and then two latex condoms. Four consecutive inflow distensions followed by withdrawal were recorded. Four were recorded with the sac sheathed by one condom; four more were recorded with two sheaths. Experiment 2. The internal pressure and volume were recorded from a latex viscus during distensions at constant rates of flow. Distension was produced at flow rates of 156.8 ml per min and 31.4 ml per min by a Series 900 Harvard infusion withdrawal pump (Harvard Apparatus Company, Inc. Millis, Mass.). Pressure was monitored, as in experiment 1, via an indwelling catheter connected to a Sanborn pressure transducer, model 267B (HewlettPackard Company, Inc. Palo Alto, Calif.). The viscus initially consisted of a single condom suspended vertically in a large pail of water to obviate local hydrostatic pressure artifacts within the viscus; a large pail of water was used to minimize pressure artifacts due to displacement of the surrounding water by the enlarging viscus. The thickness of the viscus wall was increased by sheathing the original condom with additional condoms of identical type so that walls of one-, two-, three-, five-, six-, seven-, and ten-condom thicknesses were tested. Two distensions were recorded at the faster rate and two at the slower rate for each wall thickness. Experiment 3. Records were made of the pressure-volume characteristics of the colon in a 20-kg dog anesthetized with pentobarbital (30 mg per kg). Distension was applied at constant rates of infusion by a Harvard pump. A large latex sac surrounding a side-vented Tygon tube 1 cm in diameter was inserted 25 to 30 cm into the dog's rectum. The unstretched volume of the sac exceeded that of the colon segment in which it was lodged. Pressure was monitored, as before, from within the sac. Distensions were produced alternately at rates of 31.4 ml per min and 156.8 ml per min. Fluid was exhausted after each distension at the same rates as inflow. An interval of at least 1 min was allowed between the completion of fluid withdrawal and the start of the next dis-
May 1970
687
MEASUREMENT OF INTESTINAL TONUS
tension. A total of 183 ml were infused and withdrawn each time. After four pairs of slow and rapid infusionwithdrawals were accomplished, morphine sulfate (0.3 mg per kg) was administered by slow intravenous push. Several minutes were allowed for the resulting acute pressure fluctuations to subside, after which three more pairs of infusion-withdrawals were recorded. Experiment 4. Records were obtained from an isolated segment of rabbit colon immersed in a physiological glucose-electrolyte solution, 6 equilibrated with 97% O2 and 3% CO 2 and maintained at 37 C. A 10-cm segment of the haustrated protion of proximal colon was excised and mounted, without tension, on a frame submerged in the constant temperature bath. A cylindrical latex sac 4 cm in length was placed within the midportion of the colon segment. Thirteen milliliters of fluid were infused and withdrawn at alternate rates of 0.63 ml per sec and 0.13 ml per sec. In all cases, 3 min were allowed between the completion of withdrawal and the start of the next distension. Mter four pairs of rapid and slow distensions were recorded, the bath was changed to one of identical composition and temperature except for the presence of 0.001 M sodium iodoacetate. 7 Mter 30 min, four more pairs of infusion-withdrawals were recorded in the previous manner.
Results Experiment 1. Pressure-volume data were recorded during inflow into a mechanical model of a distensible viscus under conditions allowing self-regulated rates of inflow. Data recorded from A, a latex sac, B, the same sac sheathed by a condom, and C, the sac sheathed by two condoms, are presented in figure 1. The data are plotted in the manner of Lipkin et al. 2 with resistance as the ordinate and duration of inflow as the abscissa. This bears comparison with figure 2 in which the same data are plotted using Hopkins' tension ordinate. When the data are plotted on the resistance ordinate, (a) the curves become progressively more erratic and imprecise and (b) the resistance of the thicker walled, less compliant viscus is constantly greater than that of the thinner walled, more compliant viscus throughout the course of distension; i.e., resistance C > B > A.
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FIG. 1. Resistance to inflow during distention of a latex viscus by gravity flow from a fixed hydrostatic head. Ordinate, resistance units (R). Abscissa, duration of inflow distension. Each point represents the mean ± SE of four consecutive distensions (SE is not depicted when < 0.5 R units). A, a latex sac; B, the sac sheathed by one condom; C, the sac sheathed by two condoms. The curves become progressively imprecise, although the relationship of wall thickness to resistance is maintained throughout the course of distension.
The curves which result using Hopkins' tension ordinate (a) are more regular and of rather constant precision from the start of inflow distension to its finish and (b) demonstrate that tension is greatest in the wall of the least compliant viscus (C) from zero to 15 sec, the relative tension of the three viscera change from 15 to 22 sec, and the tensions become reversed from 22 to 30 sec. Although during the early stage of distension tension C > B > A, in the final stages of distension tension A > B > C. It thus appears that, under conditions allowing self-regulated rates of flow, each method's advantage is accompanied by a disadvantage which seriously compromises its reliability as an expression of visceral distensibility, at least with regard to the mechanical model used. E;cperiment 2. The tension-volume behavior of a latex viscus of varying wall thickness subjected to distension at constant rates of infusion is shown in figure 3. It appears that (a) the tension in the wall
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Vol. 58, No.5
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FIG. 2. The same pressure-volume data shown in figure 1 formulated in terms of wall tension. Ordiruzte , circumferential tension (relative units). Abscissa, duration of inflow distension . Each point represents the mean ± SE of four consecutive distensions for each wall thickness. (SE is not depicted when < 2.5 units of tension.) Each curve has fairly constant precision throughout its course, although the relationship between wall thickness and tension becomes reversed during the course of distension.
of a latex viscus is directly proportional to the wall thickness and (b) no significant increase in tension occurs at a more rapid rate of distension. This sharply contrasts with the behavior of dog and rabbit colons reported below. Experiment 3. The development of tension in the distal colon of an intact dog subjected to distension at the same two rates used in experiment 2 is depicted in figure 4. It appears that (a) the administration of morphine resulted in higher tensions at comparable degrees of distension (morphine is known to cause contraction of intestinal smooth muscle in both dog and manS. 9); (b) the tensions produced by the more rapid rate of infusion were higher than those produced by the slower rate, and (c) the size of the tension differential (i.e., tension at the rapid rate minus tension at the slower rate for any given volume of distension) increased with the heightened capacity of the morphinetreated gut to develop tension when stretched.
The abscissas in figures 4 and 5 show the volume of distension on a logarithmic scale. Since the length of circumferential muscle varies with the square root of luminal volume, plotting volume logarithmically facilitates comparison of the present tension-volume data with the tensionlength curves obtained from stretched skeletal muscle by Buchtal et al. lo and Wilkie . 11 Experiment 4. The tension-volume behavior of isolated rabbit colon subjected to distension at rates of 0.63 ml per sec and 0.13 ml per sec before and after addition of iodoacetate is shown in figure 5. It appears that (a) exposing the gut to iodoacetate dimishes its capacity to develop intramural tension when stretched (this agent has been shown to decrease electrical activity and mechanical tension in visceral smooth muscle 7 ) (b) as in the intact dog, tensions produced by the more rapid rate 2000 1800 1600 14 00 -.. ~
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FIG. 3. Tension-volume curves from a latex viscus having wall thicknesses that are multiples of the original one latex. Distensions were produced at two constant rates. Curves resulting from rapid distensions are drawn as solid lines. Those resulting from slow distensions are drawn as broken lines and are shown only when they differ from the solid lines by more than 20 tension units. The development of tension in latex viscera is not affected by the rate of distension. Tension is proportional to wall thickness.
MEASUREMENT OF INTESTINAL TONUS
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FIG. 4. Tension-volume curves from a dog colon before and after administration of morphine. Solid lines, rapid distension; broken lines, slow distension. Slwded areas represent the respective tension differentials. Morphine resulted in higher tensions and larger tension differentials. The mean tensions developed during rapid distensions are significantly higher than mean tensions during slow distensions (P = 0.01 by the sign test for both pairs of curves 21 ).
of stretch are generally higher than those produced by slower stretching, and (c) the size of the tension differential diminishes as the metabolically poisoned gut loses some of its capacity to develop tension when stretched. 390
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FIG. 5. Tension-volume curves from isolated rabbit colon before and after addition of iodoacetate. Each point represents the mean ± SE of four determinations. Solid lines, rapid distension; broken lines, slow distension . Shaded areas represent the respective tension differentials. Iodoacetate (0.001 M) resulted in the gut developing less tension and smaller tension differentials.
Considerable slippage of the distension sac within the lumen of the distending gut was observed during some preliminary runs in this experiment. Great irregularities in the pressure curves accompanied these events. More regular pressure curves resulted after the gut was secured so as to minimize slippage. The poorer precision of tension-volume results recorded from the dog's colon in situ, as compared with the isolated segment of rabbit colon, is probably due to slippage of the comparatively unthethered distension sac used in the dog. This technical consideration may be significant in attempts to quantify intestinal tonus clinically. Discussion A mechanical model was used to assess the method by which resistance to stretch is measured under conditions of free inflow which allow the rate of inflow to vary constantly during the course of distension. Serious inaccuracies may result when such pressure-volume data are formulated in terms of resistance 2 or circumferential tension. 4 The source of error would appear to lie with the volume factor. The rate of inflow during the course of each inflowdistension diminishes in a somewhat exponential manner; there is very little inflow
690
FLEISHER
during the final seconds of the run. Lipkin's equation, R = P/(dV/dt), employs volume as the increment during 1 sec of inflow. Thus, dV/dt becomes a progressively smaller denominator while the numerator, P, becomes progressively larger during the course of each distension. The technical errors in volume measurement become increasingly magnified in the latter seconds of each run as the rate of inflow slows to zero. Hopkins' equation, T cc P yV, employs volume as the cumulative amount of inflow up to and including the time designated on the abscissa. Thus, technical errors in volume measurement are not magnified during the course of distension, errors in end run volume measurement are of little significance and the precision of each tension-volume curve is maintained throughout its course. However, the advantage of greater precision is largely negated by the deceptively low tension values of the less distensible viscera. The pressure within a viscus at the end of each distension equals the hydrostatic head, regardless of its resistance to inflow. This means that, under conditions of free inflow, viscera that are quite resistant to distension will ultimately accept less volume. Since T ';' P VV, such resistant viscera may develop less tension. The respective fall-offs in volume of inflow in viscera A, B, and C (fig.' 2) caused their tension curves to reverse their relative positions during the course of distension. Each degree of visceral distensibility will result in a free flow tension curve whose shape is a function of that degree of tonus. As the walls become less distensible, the initial slope of tension will be greater, the leveling off of tension rise will occur earlier, and the end inflow tensions will be lower. Lipkin and colleagues2 offered another expression for the pressure-volume behavior of gut undergoing distension, k = PlY, in which k is a function of accommodation to distension at any point during distension, P is the intraluminal pressure at that point, and V is the intraluminal volume accommodated up to that point.
Vol. 58. No.5
This equation would seem to be the most valid for use with data from free inflow distension from a fixed head. It avoids the magnification of volume measurement error inherent in the resistance equation by treating the cumulative volume up to and including the second designated on the abscissa. It avoids the problem inherent to application of Hopkins' tension equation to free flow distension in that the end flow pressure is divided, rather than multiplied by end inflow volume. The smaller end-inflow volume enhances k, whereas it causes tension to be deceptively low. Despite the usefullness of this expression of gut accommodation to distension, there is one consideration which favors the use of mural tension: tension would seem to be the parameter of pressure-volume behavior most directly related to afferent sensory impulses. Hopkins 4 summarizes the work of Paintal 12 and Iggol3, 14 in this regard. It was suggested that the difficulties in applying the law of Laplace under conditions of variable rate of inflow could be avoided by making the rate of inflow constant (M. 1. Grossman, personal communication). An infusion pump was substituted for gravity-fed inflow distension. No tension differential appeared when latex viscera were stretched at different rates (fig. 15 3). Remington studied the tension-length behavior of aortic arch and ligamentum nuchae, tissues composed predominantly of elastic fibers, and found a similar lack of rate dependency in the development of tension upon stretching. A striking difference was found in both dog and rabbit colon. The tension developed by stretching these visceral smooth muscles is clearly dependent on the rate of stretch. It seems reasonable to infer that contracted gut develops more tension than relaxed gut when both are stretched to the same extent and at the same rate. More rapid distension produces increased tension in both states of gut activity. However, there is an exaggerated rise in tension in contracted gut. The stretch-tension characteristics of kitten urinary bladders were studied by Remington and Alexander. 16 Their results
May 1970
MEASUREMENT OF INTESTINAL TONUS
are of interest, since both bladder and enteric smooth muscle are of the unitary type l7 and might be expected to behave similarly in response to stretching. Their results showed that the amount of tension developed by the bladder wall was directly related to (a) the existing tonus of the bladder wall and (b) the rate at which stretch was applied. Distension of dead bladder produced much lower tensions and the rate of stretching had little or no influence on the degree of tension. Phenomena associated with stretching visceral smooth muscles have been described by several workers. 17 - 20 It is hoped that further clarification of factors affecting the measurement of tonus in intact intestine may help to elucidate the sensory and motor aspects of what is clinically referred to as gut spasm. REFERENCES 1. Truelove, S. C. 1966. Movements of the large intestine. Physiol Rev. 46: 457-512. 2. Lipkin, M., T. P. Almy, and B. M. Bell. 1962. Pressure volume characteristics of the human colon. J. Clin. Invest. 41: 1831-1839. 3. Connell, A. M. 1968. In C. F. Code [ed.], Handbook of physiology, Sect. 6, Vol. IV, Chap. 101. American Physiological Society, Washington, D. C. 4. Hopkins, A. 1966. Relation between pressure and volume in hollow viscera. Gut. 7: 521-524. 5. Laplace, P. S. 1820. Mechanique celeste. Quoted by Burton, A. C. In Ruch and Fulton, Medical physiology and biophysics, Ed. 18, p. 660. W. B. Saunders Company, Philadelphia. 6. Axelsson, J., and E. Biilbring. 1961. Metabolic factors affecting the electrical activity of intestinal smooth muscle. J. Physiol. (London) 156: 344-356. 7. Biilbring, E., and H. Liillman. 1957. The effect of metabolic inhibitors on the electrical and mechanical activity of the smooth muscle of the guinea-pig's taenia coli. J. Physiol. (London) 136: 310-323.
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8. Daniel, E. E. 1968. In C. F. Code [ed.], Handbook of physiology, Sect. 6, Vol. IV, p. 2300. American Physiological Society, Washington, D. C. 9. Daniel, E. E., W. H. Sutherland, and A. Bogoch. 1959. Effects of morphine and other drugs on motility of the terminal ileum. Gastroenterology 36: 510-523. 10. Buchthal, F., E. Kaiser, and P. Rosenfalck. 1951. The rheology of the cross striated muscle fiber. Dan. Bioi. Medd. 21: 1-318. 11. Wilkie, D. R. 1956. The mechanical properties of muscle. Brit. Med. Bull. 12: 177-182. 12. Paintal, A. S. 1954. A study of gastric stretch receptors. Their role in the peripheral mechanism of satiation of hunger and thirst. J. Physiol. (London) 126: 255-270. 13. Iggo, A. 1955. Tension receptors in the stomach and the urinary bladder. J. Physiol. (London) 128: 593-607. 14. Iggo, A. 1957. Gastrointestinal tension receptors with unmyelenated afferent fibers in the vagus of the cat. Quart. J. Exp. Physiol. 42: 130-143. 15. Remington, J. W. 1955. Hysteresis loop behavior of the aorta and other distensible tissues. Amer. J. Physiol. 180: 83-95. 16. Remington, J. W., and R. S. Alexander. 1955. Stretch behavior of the bladder as an approach to vascular distensibility. Amer. J. Physiol. 181: 240-248. 17. Burnstock, G., and C. L. Prosser. 1960. Responses of smooth muscles to quick stretch; relation of stretch to conduction. Amer. J. Physiol. 198: 921-925. 18. Biilbring, E. 1955. Correlation between membrane potential, spike discharge, and tension in smooth muscle. J. Physiol. (London) 128: 200-221. 19. Gillespie, J. S. 1962. Spontaneous mechanical and electrical activity of stretched and Ullstretched intestinal smooth muscle cells and their response to sympathetic nerve stimulation. J. Physiol. (London) 162: 54-75. 20. Tang, P. C., and T. C. Ruch. 1955. Non-neurogenic basis of bladder tonus. Amer. J. Physiol. 181: 249-257. 21. Dixon, W. J., and F. J. Massey, Jr. 1969. Introduction to statistical analysis, Ed. 3. McGraw-Hill Book Company, New York.
GASTROENTEROLOGY
Copyright © 1970 by The Williams & Wilkins Co.
ON THE MEASUREMENT OF INTESTINAL TONUS DAVID
R.
FLEISHER, M.D.
Department of Pediatrics, University of California School of Medicine, Los Angeles, California
Gut tonus has been studied previously in terms of the pressurevolume behavior of colon subjected to distension. Distension was produced by gravity flow into an intraluminal sac of fixed length from a constant hydrostatic head. This method is analyzed in terms of pressure-volume data obtained from a mechanical model. Criticism of the equation for resistance to distension, R = P/(dV/dt), is offered. This equation results in resistance curves which become progressively erratic because inherent errors in volume measurement are magnified progressively during the latter part of inflow distension under conditions of gravity-fed, variable flow. Criticism also is offered for the application of the equation for circumferential tension, T ex P yV, to pressure-volume data collected under conditions of variable flow. A viscus having low distensibility, under conditions of self-regulated inflow, may ultimately accept less volume from a fixed hydrostatic head and therefore may develop less intramural tension than a more distensible viscus. Difficulties with the volume factor are avoided when distension is produced at a constant rate of inflow. Circumferential tension is then a more reliable expression of the pressure-volume behavior of the gut. Wall tension produced by distension was examined in a latex viscus of variable wall thickness, in dog colon before and after administration of morphine, and in rabbit colon before and after sodium iodoacetate. The tension developed by both latex and smooth muscle viscera is related directly to the thickness or contractility, respectively, of the visceral walls. The tension developed by gut also is related directly to the rate at which distension is applied. In contrast, the tension developed by latex viscera is not rate-dependent. The tension differential is defined as the difference in tension, at the same volume, between rapidly and slowly distended gut. The size of the tension differential is related directly to the level of tonus. Attention is drawn to previous descriptions of similar phenomena in the pressure-volume behavior of urinary bladder. In a recent review of colon motility, Truelove l mentioned the potential value of assessing colon tone and cited the work of Lipkin et al. 2 in this regard. These workers studied the stress-strain characReceived July 14, 1969. Accepted November 14, 1969. Address requests for reprints to: Dr. David R. Fleisher, Department of Pediatrics, The Center for the Health Sciences, Los Angeles, California 90024. The author wishes to acknowledge his indebtedness to Dr. Guilio J. Barbero for training in pedi-
teristics of the human colon by recording simultaneous pressure and volume data during distension of an intraluminal sac filled by gravity from a constant hydrostatic head. Tonus was expressed in terms of the equation R = P/(dV/dt) , in which atric gastroenterology, under whose of this work was accomplished, to H. J. Pyenson for invaluable technical to Dr. M. 1. Grossman and Dr. A. J. ful criticism and suggestions.
auspices much L. Fleisher and assistance, and Brady for help-
686
FLEISHER
R signifies the resistance offered by the wall of the viscus to the distending force, P signifies the instantaneous pressure within the distending sac, and dV/dt signifies the instantaneous rate of inflow coincident with the pressure measurement. Connell') referred to this method as a potential means of assessing the effects of drugs and various physiological and clinical states on gut resistance to stretch. Hopkins4 reviewed the data of Lipkin and co-workers and demonstrated that formulating their data in terms of circumferential tension, according to the law of Laplace,5 resulted in considerably more regular graphic representations of these pressure-volume phenomena. Laplace's equation for the circumferential tension within the wall of a hollow cylinder is T = Pr, in which T signifies tension, P signifies intraluminal pressure, and r signifies the cylinder's radius. Hopkins derived the expression of relative circumferential tension from the T,aplace equation as follows: T a: P yV, in which V signifies intraluminal volume and tension is expressed in terms of arbitrary units. Hopkins did not offer an analysis as to why Lipkin's resistance curves were more erratic than his own tension curves. It is the purpose of this communication to examine critically the method of Lipkin et al. as well as Hopkins' suggestion that the law of Laplace provides a better means of expressing such pressure-volume data. Further, experimental data is presented delineating some previously unmentioned conditions that may have a marked effect on gut tonus measured in terms of intraluminal pressure and volume in a passively distended viscus.
Materials and Methods Four experiments are reported. Experiment 1. The method of Lipkin et a1. was examined by determining the pressurevolume characteristics of a latex viscus subjected to inflow distension from a constant hydrostatic head. The essential feature of their method was duplicated, viz, the rate of inflow varied from one instant to the next according to the difference in pressure between the distending viscus and the fixed hydrostatic head;
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each inflow distension terminated when the pressure within the viscus rose to that of the hydrostatic head. The pressure and volume within the viscus were recorded during the course of each inflow distension by essentially the same means as Lipkin's. The viscus consisted of a large latex balloon placed within a rigid cylinder 7 cm in diameter lying on its side upon a table. The thickness of the viscus wall was altered by sheathing it with one and then two latex condoms. Four consecutive inflow distensions followed by withdrawal were recorded. Four were recorded with the sac sheathed by one condom; four more were recorded with two sheaths. Experiment 2. The internal pressure and volume were recorded from a latex viscus during distensions at constant rates of flow. Distension was produced at flow rates of 156.8 ml per min and 31.4 ml per min by a Series 900 Harvard infusion withdrawal pump (Harvard Apparatus Company, Inc. Millis, Mass.). Pressure was monitored, as in experiment 1, via an indwelling catheter connected to a Sanborn pressure transducer, model 267B (HewlettPackard Company, Inc. Palo Alto, Calif.). The viscus initially consisted of a single condom suspended vertically in a large pail of water to obviate local hydrostatic pressure artifacts within the viscus; a large pail of water was used to minimize pressure artifacts due to displacement of the surrounding water by the enlarging viscus. The thickness of the viscus wall was increased by sheathing the original condom with additional condoms of identical type so that walls of one-, two-, three-, five-, six-, seven-, and ten-condom thicknesses were tested. Two distensions were recorded at the faster rate and two at the slower rate for each wall thickness. Experiment 3. Records were made of the pressure-volume characteristics of the colon in a 20-kg dog anesthetized with pentobarbital (30 mg per kg). Distension was applied at constant rates of infusion by a Harvard pump. A large latex sac surrounding a side-vented Tygon tube 1 cm in diameter was inserted 25 to 30 cm into the dog's rectum. The unstretched volume of the sac exceeded that of the colon segment in which it was lodged. Pressure was monitored, as before, from within the sac. Distensions were produced alternately at rates of 31.4 ml per min and 156.8 ml per min. Fluid was exhausted after each distension at the same rates as inflow. An interval of at least 1 min was allowed between the completion of fluid withdrawal and the start of the next dis-
May 1970
687
MEASUREMENT OF INTESTINAL TONUS
tension. A total of 183 ml were infused and withdrawn each time. After four pairs of slow and rapid infusionwithdrawals were accomplished, morphine sulfate (0.3 mg per kg) was administered by slow intravenous push. Several minutes were allowed for the resulting acute pressure fluctuations to subside, after which three more pairs of infusion-withdrawals were recorded. Experiment 4. Records were obtained from an isolated segment of rabbit colon immersed in a physiological glucose-electrolyte solution, 6 equilibrated with 97% O2 and 3% CO 2 and maintained at 37 C. A 10-cm segment of the haustrated protion of proximal colon was excised and mounted, without tension, on a frame submerged in the constant temperature bath. A cylindrical latex sac 4 cm in length was placed within the midportion of the colon segment. Thirteen milliliters of fluid were infused and withdrawn at alternate rates of 0.63 ml per sec and 0.13 ml per sec. In all cases, 3 min were allowed between the completion of withdrawal and the start of the next distension. Mter four pairs of rapid and slow distensions were recorded, the bath was changed to one of identical composition and temperature except for the presence of 0.001 M sodium iodoacetate. 7 Mter 30 min, four more pairs of infusion-withdrawals were recorded in the previous manner.
Results Experiment 1. Pressure-volume data were recorded during inflow into a mechanical model of a distensible viscus under conditions allowing self-regulated rates of inflow. Data recorded from A, a latex sac, B, the same sac sheathed by a condom, and C, the sac sheathed by two condoms, are presented in figure 1. The data are plotted in the manner of Lipkin et al. 2 with resistance as the ordinate and duration of inflow as the abscissa. This bears comparison with figure 2 in which the same data are plotted using Hopkins' tension ordinate. When the data are plotted on the resistance ordinate, (a) the curves become progressively more erratic and imprecise and (b) the resistance of the thicker walled, less compliant viscus is constantly greater than that of the thinner walled, more compliant viscus throughout the course of distension; i.e., resistance C > B > A.
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FIG. 1. Resistance to inflow during distention of a latex viscus by gravity flow from a fixed hydrostatic head. Ordinate, resistance units (R). Abscissa, duration of inflow distension. Each point represents the mean ± SE of four consecutive distensions (SE is not depicted when < 0.5 R units). A, a latex sac; B, the sac sheathed by one condom; C, the sac sheathed by two condoms. The curves become progressively imprecise, although the relationship of wall thickness to resistance is maintained throughout the course of distension.
The curves which result using Hopkins' tension ordinate (a) are more regular and of rather constant precision from the start of inflow distension to its finish and (b) demonstrate that tension is greatest in the wall of the least compliant viscus (C) from zero to 15 sec, the relative tension of the three viscera change from 15 to 22 sec, and the tensions become reversed from 22 to 30 sec. Although during the early stage of distension tension C > B > A, in the final stages of distension tension A > B > C. It thus appears that, under conditions allowing self-regulated rates of flow, each method's advantage is accompanied by a disadvantage which seriously compromises its reliability as an expression of visceral distensibility, at least with regard to the mechanical model used. E;cperiment 2. The tension-volume behavior of a latex viscus of varying wall thickness subjected to distension at constant rates of infusion is shown in figure 3. It appears that (a) the tension in the wall
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FIG. 2. The same pressure-volume data shown in figure 1 formulated in terms of wall tension. Ordiruzte , circumferential tension (relative units). Abscissa, duration of inflow distension . Each point represents the mean ± SE of four consecutive distensions for each wall thickness. (SE is not depicted when < 2.5 units of tension.) Each curve has fairly constant precision throughout its course, although the relationship between wall thickness and tension becomes reversed during the course of distension.
of a latex viscus is directly proportional to the wall thickness and (b) no significant increase in tension occurs at a more rapid rate of distension. This sharply contrasts with the behavior of dog and rabbit colons reported below. Experiment 3. The development of tension in the distal colon of an intact dog subjected to distension at the same two rates used in experiment 2 is depicted in figure 4. It appears that (a) the administration of morphine resulted in higher tensions at comparable degrees of distension (morphine is known to cause contraction of intestinal smooth muscle in both dog and manS. 9); (b) the tensions produced by the more rapid rate of infusion were higher than those produced by the slower rate, and (c) the size of the tension differential (i.e., tension at the rapid rate minus tension at the slower rate for any given volume of distension) increased with the heightened capacity of the morphinetreated gut to develop tension when stretched.
The abscissas in figures 4 and 5 show the volume of distension on a logarithmic scale. Since the length of circumferential muscle varies with the square root of luminal volume, plotting volume logarithmically facilitates comparison of the present tension-volume data with the tensionlength curves obtained from stretched skeletal muscle by Buchtal et al. lo and Wilkie . 11 Experiment 4. The tension-volume behavior of isolated rabbit colon subjected to distension at rates of 0.63 ml per sec and 0.13 ml per sec before and after addition of iodoacetate is shown in figure 5. It appears that (a) exposing the gut to iodoacetate dimishes its capacity to develop intramural tension when stretched (this agent has been shown to decrease electrical activity and mechanical tension in visceral smooth muscle 7 ) (b) as in the intact dog, tensions produced by the more rapid rate 2000 1800 1600 14 00 -.. ~
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FIG. 3. Tension-volume curves from a latex viscus having wall thicknesses that are multiples of the original one latex. Distensions were produced at two constant rates. Curves resulting from rapid distensions are drawn as solid lines. Those resulting from slow distensions are drawn as broken lines and are shown only when they differ from the solid lines by more than 20 tension units. The development of tension in latex viscera is not affected by the rate of distension. Tension is proportional to wall thickness.
MEASUREMENT OF INTESTINAL TONUS
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FIG. 4. Tension-volume curves from a dog colon before and after administration of morphine. Solid lines, rapid distension; broken lines, slow distension. Slwded areas represent the respective tension differentials. Morphine resulted in higher tensions and larger tension differentials. The mean tensions developed during rapid distensions are significantly higher than mean tensions during slow distensions (P = 0.01 by the sign test for both pairs of curves 21 ).
of stretch are generally higher than those produced by slower stretching, and (c) the size of the tension differential diminishes as the metabolically poisoned gut loses some of its capacity to develop tension when stretched. 390
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FIG. 5. Tension-volume curves from isolated rabbit colon before and after addition of iodoacetate. Each point represents the mean ± SE of four determinations. Solid lines, rapid distension; broken lines, slow distension . Shaded areas represent the respective tension differentials. Iodoacetate (0.001 M) resulted in the gut developing less tension and smaller tension differentials.
Considerable slippage of the distension sac within the lumen of the distending gut was observed during some preliminary runs in this experiment. Great irregularities in the pressure curves accompanied these events. More regular pressure curves resulted after the gut was secured so as to minimize slippage. The poorer precision of tension-volume results recorded from the dog's colon in situ, as compared with the isolated segment of rabbit colon, is probably due to slippage of the comparatively unthethered distension sac used in the dog. This technical consideration may be significant in attempts to quantify intestinal tonus clinically. Discussion A mechanical model was used to assess the method by which resistance to stretch is measured under conditions of free inflow which allow the rate of inflow to vary constantly during the course of distension. Serious inaccuracies may result when such pressure-volume data are formulated in terms of resistance 2 or circumferential tension. 4 The source of error would appear to lie with the volume factor. The rate of inflow during the course of each inflowdistension diminishes in a somewhat exponential manner; there is very little inflow
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during the final seconds of the run. Lipkin's equation, R = P/(dV/dt), employs volume as the increment during 1 sec of inflow. Thus, dV/dt becomes a progressively smaller denominator while the numerator, P, becomes progressively larger during the course of each distension. The technical errors in volume measurement become increasingly magnified in the latter seconds of each run as the rate of inflow slows to zero. Hopkins' equation, T cc P yV, employs volume as the cumulative amount of inflow up to and including the time designated on the abscissa. Thus, technical errors in volume measurement are not magnified during the course of distension, errors in end run volume measurement are of little significance and the precision of each tension-volume curve is maintained throughout its course. However, the advantage of greater precision is largely negated by the deceptively low tension values of the less distensible viscera. The pressure within a viscus at the end of each distension equals the hydrostatic head, regardless of its resistance to inflow. This means that, under conditions of free inflow, viscera that are quite resistant to distension will ultimately accept less volume. Since T ';' P VV, such resistant viscera may develop less tension. The respective fall-offs in volume of inflow in viscera A, B, and C (fig.' 2) caused their tension curves to reverse their relative positions during the course of distension. Each degree of visceral distensibility will result in a free flow tension curve whose shape is a function of that degree of tonus. As the walls become less distensible, the initial slope of tension will be greater, the leveling off of tension rise will occur earlier, and the end inflow tensions will be lower. Lipkin and colleagues2 offered another expression for the pressure-volume behavior of gut undergoing distension, k = PlY, in which k is a function of accommodation to distension at any point during distension, P is the intraluminal pressure at that point, and V is the intraluminal volume accommodated up to that point.
Vol. 58. No.5
This equation would seem to be the most valid for use with data from free inflow distension from a fixed head. It avoids the magnification of volume measurement error inherent in the resistance equation by treating the cumulative volume up to and including the second designated on the abscissa. It avoids the problem inherent to application of Hopkins' tension equation to free flow distension in that the end flow pressure is divided, rather than multiplied by end inflow volume. The smaller end-inflow volume enhances k, whereas it causes tension to be deceptively low. Despite the usefullness of this expression of gut accommodation to distension, there is one consideration which favors the use of mural tension: tension would seem to be the parameter of pressure-volume behavior most directly related to afferent sensory impulses. Hopkins 4 summarizes the work of Paintal 12 and Iggol3, 14 in this regard. It was suggested that the difficulties in applying the law of Laplace under conditions of variable rate of inflow could be avoided by making the rate of inflow constant (M. 1. Grossman, personal communication). An infusion pump was substituted for gravity-fed inflow distension. No tension differential appeared when latex viscera were stretched at different rates (fig. 15 3). Remington studied the tension-length behavior of aortic arch and ligamentum nuchae, tissues composed predominantly of elastic fibers, and found a similar lack of rate dependency in the development of tension upon stretching. A striking difference was found in both dog and rabbit colon. The tension developed by stretching these visceral smooth muscles is clearly dependent on the rate of stretch. It seems reasonable to infer that contracted gut develops more tension than relaxed gut when both are stretched to the same extent and at the same rate. More rapid distension produces increased tension in both states of gut activity. However, there is an exaggerated rise in tension in contracted gut. The stretch-tension characteristics of kitten urinary bladders were studied by Remington and Alexander. 16 Their results
May 1970
MEASUREMENT OF INTESTINAL TONUS
are of interest, since both bladder and enteric smooth muscle are of the unitary type l7 and might be expected to behave similarly in response to stretching. Their results showed that the amount of tension developed by the bladder wall was directly related to (a) the existing tonus of the bladder wall and (b) the rate at which stretch was applied. Distension of dead bladder produced much lower tensions and the rate of stretching had little or no influence on the degree of tension. Phenomena associated with stretching visceral smooth muscles have been described by several workers. 17 - 20 It is hoped that further clarification of factors affecting the measurement of tonus in intact intestine may help to elucidate the sensory and motor aspects of what is clinically referred to as gut spasm. REFERENCES 1. Truelove, S. C. 1966. Movements of the large intestine. Physiol Rev. 46: 457-512. 2. Lipkin, M., T. P. Almy, and B. M. Bell. 1962. Pressure volume characteristics of the human colon. J. Clin. Invest. 41: 1831-1839. 3. Connell, A. M. 1968. In C. F. Code [ed.], Handbook of physiology, Sect. 6, Vol. IV, Chap. 101. American Physiological Society, Washington, D. C. 4. Hopkins, A. 1966. Relation between pressure and volume in hollow viscera. Gut. 7: 521-524. 5. Laplace, P. S. 1820. Mechanique celeste. Quoted by Burton, A. C. In Ruch and Fulton, Medical physiology and biophysics, Ed. 18, p. 660. W. B. Saunders Company, Philadelphia. 6. Axelsson, J., and E. Biilbring. 1961. Metabolic factors affecting the electrical activity of intestinal smooth muscle. J. Physiol. (London) 156: 344-356. 7. Biilbring, E., and H. Liillman. 1957. The effect of metabolic inhibitors on the electrical and mechanical activity of the smooth muscle of the guinea-pig's taenia coli. J. Physiol. (London) 136: 310-323.
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8. Daniel, E. E. 1968. In C. F. Code [ed.], Handbook of physiology, Sect. 6, Vol. IV, p. 2300. American Physiological Society, Washington, D. C. 9. Daniel, E. E., W. H. Sutherland, and A. Bogoch. 1959. Effects of morphine and other drugs on motility of the terminal ileum. Gastroenterology 36: 510-523. 10. Buchthal, F., E. Kaiser, and P. Rosenfalck. 1951. The rheology of the cross striated muscle fiber. Dan. Bioi. Medd. 21: 1-318. 11. Wilkie, D. R. 1956. The mechanical properties of muscle. Brit. Med. Bull. 12: 177-182. 12. Paintal, A. S. 1954. A study of gastric stretch receptors. Their role in the peripheral mechanism of satiation of hunger and thirst. J. Physiol. (London) 126: 255-270. 13. Iggo, A. 1955. Tension receptors in the stomach and the urinary bladder. J. Physiol. (London) 128: 593-607. 14. Iggo, A. 1957. Gastrointestinal tension receptors with unmyelenated afferent fibers in the vagus of the cat. Quart. J. Exp. Physiol. 42: 130-143. 15. Remington, J. W. 1955. Hysteresis loop behavior of the aorta and other distensible tissues. Amer. J. Physiol. 180: 83-95. 16. Remington, J. W., and R. S. Alexander. 1955. Stretch behavior of the bladder as an approach to vascular distensibility. Amer. J. Physiol. 181: 240-248. 17. Burnstock, G., and C. L. Prosser. 1960. Responses of smooth muscles to quick stretch; relation of stretch to conduction. Amer. J. Physiol. 198: 921-925. 18. Biilbring, E. 1955. Correlation between membrane potential, spike discharge, and tension in smooth muscle. J. Physiol. (London) 128: 200-221. 19. Gillespie, J. S. 1962. Spontaneous mechanical and electrical activity of stretched and Ullstretched intestinal smooth muscle cells and their response to sympathetic nerve stimulation. J. Physiol. (London) 162: 54-75. 20. Tang, P. C., and T. C. Ruch. 1955. Non-neurogenic basis of bladder tonus. Amer. J. Physiol. 181: 249-257. 21. Dixon, W. J., and F. J. Massey, Jr. 1969. Introduction to statistical analysis, Ed. 3. McGraw-Hill Book Company, New York.