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Myocardial Mural Thickness During the Cardiac Cycle By Eric O. Feigl, M.D., and Donald L. Fry, M.D. Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 • An understanding of the relationship between forces and velocities of contraction in muscle fibers to the pressures and flows generated by the intact myocardium requires detailed information such as fiber orientation, wall curvature, sequence of contraction, and wall thickness. It was the purpose of this study to measure instantaneous myocardial thickness throughout the cardiac cycle, and, when possible, relate it to the simultaneous tangential strain occurring on the surface of the heart. Methods A transducer has been devised to measure the change in thickness of the myocardial wall during the cardiac cycle, and is illustrated in figure 1. A slender stainless steel shaft, A, is fitted with a harpoon-like toggle end piece, B. The shaft with the toggle folded flat against it can be inserted through the myocardium. When the shaft is pushed through the heart wall, the toggle piece unfolds inside the heart, forming a catch so that the shaft cannot be withdrawn. The shaft moves through a small base plate, C, which has a guiding sleeve that keeps the shaft aligned. The toggle piece is held against the inside of the heart by a light compression spring which works between the base plate and an adjustable stop, D. The to-andfro motions of the shaft with respect to the base plate are sensed by the flexure of a brass shim, E, which has one end fixed to the base plate and the other to the stop on the shaft. Two etched foil strain gauges are mounted on the brass shim and form two arms of a resistance bridge. The changes in the bridge resistance which result from motions of the shaft are sensed and suitably amplified with a standard carrier amplifier. The transducer is attached to the epicardium with shallow sutures threaded through the holes in the base plate. At the end of the experiment the instrument must be cut out of the heart muscle. From the Section of Clinical Biophysics, Cardiology Branch, National Heart Institute, U. S. Public Health Service. Received for publication November 29, 1963. Circulation Research, Volume XIV, June 1964 The transducer was calibrated following each run with a small test stand, utilizing a micrometer to move the shaft known distances. The instrument was essentially linear through the ranges encountered. A static calibration curve is shown on the left of figure 2. The dynamic amplitude vs. frequency response of the transducer is shown on the right of figure 2. Large dogs were anesthetized with chloralose (60 mgAg) and urethane (600 mg/kg) after morphine (2 mg/kg) preanesthetic. The left aspect of the heart was exposed with a sternal splitting incision and partial resection of four to six left ribs. Aortic arch pressure was recorded through a catheter inserted via the left subclavian artery. Left ventricular pressure was recorded with a cannula through an apical myocardial puncture. Pressures were measured with Statham P23d manometers. Instantaneous flow was measured at the root of the aorta just above the valves with a 400 cycles/sec gated sine wave electromagnetic flowmeter. Recording was done on a Sanborn 350 oscillograph. FIGURE 1 Myocardial wall thickness transducer. See text for description of its operation. 541 542 FEIGL, FRY The thickness gauge was inserted through the myocardium of the left ventricle midway between apex and base, either to the left or right of the anterior papillary muscle, as verified postmortem. Changes in the size of the left ventricle were estimated by continuously measuring the length of an arc of the epicardial surface. Frequently two arc lengths were recorded, one arc in the longitudinal direction, that is from base to apex, and the other at right angles to this in the transverse direction. The arc lengths were measured with electrical i 1 Static calibration of - thickness gauge i i > 20 30 Dynamic characteristics of thickness gauge 105- us 100 o y 95fDownloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 i 2 4 6 CHANGE IN THICKNESS mm 1 2 i 5 10 FREQUENCY -CPS FIGURE 2 Static calibration and dynamic amplitude vs. frequency response curves for the thickness transducer. Myocardial Mural Thickness and Strain Dog no. Body weight Minimum diastolic thickness Thickness beginning of ejection TABLE 1 Maximum systolic thickness Ejection thickness strain Ejection transverse arc strain Ejection longitudinal arc strain +.053 +.063 +.164 +.225 + .110 +.088 +.130 + .066 +.038 + .113 +.1049 -.057 -.013 -.009 + .156 + .076 +.154 + .194 +.123 +.183 +.185 +.107 +.144 +.176 + .1498 Control kg I 2 3 4 5 6 7 8 9 10 Mean 28.2 32.7 25.9 24.5 31.4 24.1 22.7 29.5 18.2 15.9 25.31 mm mm 8.97 9.40 7.98 7.24 8.05 7.90 8.47 8.35 7.50 9.92 8.378 9.41 10.72 8.78 7.87 8.88 8.44 8.89 10.60 8.62 11.02 9.323 mm 9.91 11.40 10.22 9.64 9.86 9.18 10.04 11.30 8.95 12.26 10.276 -.038 -.055 -.085 -.036 -.080 -.007 -.008 -.035 -.072 -.044 -.0376 -.0460 -.066 -.046 + .020 During norepinephrine 1 2 3 4 5 6 7 8 9 10 Mean 28.2 32.7 25.9 24.5 31.4 24.1 22.7 29.5 18.2 15.9 25.31 8.72 9.19 7.98 7.97 8.54 7.40 8.23 8.29 6.14 9.79 8.225 9.22 11.77 9.34 10.78 9.58 8.32 9.50 11.43 7.82 11.48 9.924 10.66 12.66 10.78 12.87 10.76 9.84 11.26 12.65 8.95 13.50 11.393 -.017 -.076 -.127 +.028 -.104 -.021 -.020 -.031 -.071 -.123 -.0533 -.0468 Circulation Research, Volume XIV, June 1964 MYOCARDIAL MURAL THICKNESS 543 LEFT VENT. PRESSURE mmHg 150 — 0 AORTIC PRESSURE mmHg 80AORTIC ROOT FLOW Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 10.0- 9.0-, 8.0TRANSVERSE ARC LENGTH mm 16.25LONGITUDINAL 21.0 ARC LENGTH mm 20.520.0I sec • FIGURE 3 Simultaneous records of pressures, flow, myocardial wall thickness, and two perpendicular arc lengths on the epicardial surface. Vertical bars on the thickness record indicate ejection period as judged from the flow record. calipers sewn to the surface of the heart. The caliper has been described in detail previously.1 An effort was made to place the calipers close to the point where the thickness gauge was inserted, but this was not always possible since it was necessary to avoid the coronary vessels on the surface of the heart. After thickness and surface strain determinaCirculalion Research, Volume XIV, June 1964 tions were made in a control state an intravenous infusion of norepinephrine (approximately 0.001 mg/kg per minute) was given to raise the arterial pressure 10 to 20 mm Hg and the measurements were repeated. Strains were calculated with respect to the initial dimension at the beginning of ejection. That is, the value at the beginning of ejection was sub- 544 FEIGL, FRY tracted from the value at the end of ejection and the resulting difference divided by the value at the beginning of ejection. Thus a positive strain indicates an increase in size during ejection, a negative strain a decrease. The ejection period was determined from the aortic root flow record. Results Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 A representative record of mural thickness is shown in figure 3. The change in thickness of the heart wall during the cardiac cycle has a characteristic pattern. A sharp increase in thickness at the beginning of systole during the "isovolumic" phase was regularly observed. With the onset of ejection a somewhat slower increase in thickness was observed. Thickness became fairly constant in the later portion of ejection and often remained so for a brief period after the aortic valves had closed. The wall became thinner during diastole, usually thinning in a smooth manner. Successful determinations were made in ten animals. The data are summarized in table 1. The table lists the dogs with their weights and is divided into two similar parts: control and norepinephrine. The minimum diastolic and maximum systolic wall thicknesses during a cardiac cycle are given in millimeters. Also the strains during the ejection period are given for thickness, as well as longitudinal and transverse arcs. Under control conditions the average increase in wall thickness from diastole to the beginning of ejection in ten animals was + 0.113. The average control thickness strain during ejection was -f- 0.105. The average of seven transverse surface strains during the same ejection period was — 0.038. The average of six longitudinal strains during ejection was — 0.046. With a norepinephrine infusion the change in thickness during ejection changed to an average strain of + 0.150 in the same animals. The average transverse arc strain was — 0.053 in seven animals with norepinephrine. The average longitudinal arc strain in six animals was — 0.047 with norepinephrine. Discussion The toggle piece in the transducer used was held against the endocardium by a light com- pression spring. The turgor of the myocardium is less during diastole than systole which means that the toggle could indent the muscle more during diastole than during systole when it became stiffer. This effect would tend to exaggerate the change in thickness observed between diastole and systole. It was not possible to determine the magnitude of this error, but it was estimated to be small since special care was used to keep the spring tension as light as possible. The change in turgor of the myocardium during the ejection phase of systole is probably not very great so that the measurements made in this period and the strains calculated would be less subject to this type of error. Since muscle is predominately composed of water it would be expected that little volume change would accompany contraction. The volume change that skeletal muscle undergoes during contraction has been shown to be extremely small, less than one hundredth of one per cent.2 It is unlikely that the changes in thickness observed were influenced by changes in volume of the cardiac muscle. If muscle tissue is incompressible, then the sum of infinitesimal strains in three mutually perpendicular directions at a point will be zero. However, when strains are estimated in a curved object such as the heart and finite dimensions and finite strains are measured, this is only approximately true. For a given stroke volume, the endocardial surface must undergo a greater strain than the epicardial surface. Since the change in thickness represents the strain integrated across the entire wall, the strain estimated from the change in thickness will be greater than the sum of the tangential strains on the epicardial surface. Although the data manifest considerable scatter, inspection of table 1 shows that the sum of the transverse and longitudinal epicardial strains with ejection was on the average somewhat smaller than the mean radial strain computed from change in thickness. These comparisons lend support to the values observed and indicate that the thickness measurements were probably of the correct order of magnitude. Circulation Research, Volume XIV, June 1964 MYOCARDIAL MURAL THICKNESS Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 It was not possible with this transducer to measure thickness changes of the ventricular wall where the ventricle has a short radius of curvature, as at the apex or close to the aortic ring. It can only be assumed that these measurements, made midway between apex and base, are representative of the left ventricle generally. The average of ten dogs showed an increase in wall thickness of over 10% between diastole and the beginning of ejection and a further increase of 10% during ejection. These values increased by about half again when norepinephrine was given. When tension (force per unit area) is to be calculated for the heart wall the changes in thickness during the cardiac cycle should be kept in mind. Moreover, myocardial force gauges which are sutured to the outer surface of the heart register forces which may be altered in a complex manner by changes in thickness. 545 ness increased an additional 10% during the ejection period of systole. These average values increased to 20% and 15% respectively with the administration of norepinephrine (approximately 0.001 mg/kg per minute). Transverse and longitudinal epicardial arc strains were compared with thickness strain of the myocardium. Assuming the myocardium is incompressible, reasonable agreement was found in the three strains, which lends support to the measurements that were made. It is concluded that changes in the thickness of the myocardial wall during the cardiac cycle may be important in some considerations of the heart's performance. Acknowledgment We thank Mr. Raymond P. Kelly for expert help in designing and fabricating the transducer used in this study. We also thank Mr. Joseph M. Pearce for his careful technical assistance. Summary References The instantaneous and continuous thickness of the left ventricular wall was measured in ten dogs with a specially designed transducer. The thickness change during the "isovolumic" phase of systole was 11%. The average thick- 1. MALLOS, A. J.: An electrical caliper for continuous measurement of relative displacement. J. Appl. Physiol. 17: 131, 1962. Circulation Research, Volume XIV, June 1964 2. ABBOTT, B. C , AND BASKIN, R. J.: Volume changes in frog muscle during contraction. J. Physiol. 161: 379, 1962. Myocardial Mural Thickness During the Cardiac Cycle ERIC O. FEIGL and DONALD L. FRY Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017 Circ Res. 1964;14:541-545 doi: 10.1161/01.RES.14.6.541 Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1964 American Heart Association, Inc. All rights reserved. Print ISSN: 0009-7330. Online ISSN: 1524-4571 The online version of this article, along with updated information and services, is located on the World Wide Web at: http://circres.ahajournals.org/content/14/6/541 Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the Editorial Office. Once the online version of the published article for which permission is being requested is located, click Request Permissions in the middle column of the Web page under Services. Further information about this process is available in the Permissions and Rights Question and Answer document. Reprints: Information about reprints can be found online at: http://www.lww.com/reprints Subscriptions: Information about subscribing to Circulation Research is online at: http://circres.ahajournals.org//subscriptions/