Myocardial Mural Thickness During the Cardiac Cycle

Transcription

1 Myocardal Mural Thckness Durng the Cardac Cycle By Erc O. Fegl, M.D., and Donald L. Fry, M.D. An understandng of the relatonshp between forces and veloctes of contracton n muscle fbers to the pressures and flows generated by the ntact myocardum requres detaled nformaton such as fber orentaton, wall curvature, sequence of contracton, and wall thckness. It was the purpose of ths study to measure nstantaneous myocardal thckness throughout the cardac cycle, and, when possble, relate t to the smultaneous tangental stran occurrng on the surface of the heart. Methods A transducer has been devsed to measure the change n thckness of the myocardal wall durng the cardac cycle, and s llustrated n fgure 1. A slender stanless steel shaft, A, s ftted wth a harpoon-lke toggle end pece, B. The shaft wth the toggle folded flat aganst t can be nserted through the myocardum. When the shaft s pushed through the heart wall, the toggle pece unfolds nsde the heart, formng a catch so that the shaft cannot be wthdrawn. The shaft moves through a small base plate, C, whch has a gudng sleeve that keeps the shaft algned. The toggle pece s held aganst the nsde of the heart by a lght compresson sprng whch works between the base plate and an adjustable stop, D. The to-andfro motons of the shaft wth respect to the base plate are sensed by the flexure of a brass shm, E, whch has one end fxed to the base plate and the other to the stop on the shaft. Two etched fol stran gauges are mounted on the brass shm and form two arms of a resstance brdge. The changes n the brdge resstance whch result from motons of the shaft are sensed and sutably amplfed wth a standard carrer amplfer. The transducer s attached to the epcardum wth shallow sutures threaded through the holes n the base plate. At the end of the experment the nstrument must be cut out of the heart muscle. The transducer was calbrated followng each run wth a small test stand, utlzng a mcrometer to move the shaft known dstances. The nstrument was essentally lnear through the ranges encountered. A statc calbraton curve s shown on the left of fgure 2. The dynamc ampltude vs. frequency response of the transducer s shown on the rght of fgure 2. Large dogs were anesthetzed wth chloralose (60 mgag) and urethane (600 mg/kg) after morphne (2 mg/kg) preanesthetc. The left aspect of the heart was exposed wth a sternal splttng ncson and partal resecton of four to sx left rbs. Aortc arch pressure was recorded through a catheter nserted va the left subclavan artery. Left ventrcular pressure was recorded wth a cannula through an apcal myocardal puncture. Pressures were measured wth Statham P23d manometers. Instantaneous flow was measured at the root of the aorta just above the valves wth a 400 cycles/sec gated sne wave electromagnetc flowmeter. Recordng was done on a Sanborn 350 oscllograph. From the Secton of Clncal Bophyscs, Cardology Branch, Natonal Heart Insttute, U. S. Publc Health Servce. Receved for publcaton November 29, FIGURE 1 Myocardal wall thckness transducer. See text for descrpton of ts operaton. 541

2 542 FEIGL, FRY The thckness gauge was nserted through the myocardum of the left ventrcle mdway between apex and base, ether to the left or rght of the anteror papllary muscle, as verfed postmortem. Changes n the sze of the left ventrcle were estmated by contnuously measurng the length of an arc of the epcardal surface. Frequently two arc lengths were recorded, one arc n the longtudnal drecton, that s from base to apex, and the other at rght angles to ths n the transverse drecton. The arc lengths were measured wth electrcal Statc calbraton of - thckness gauge 1 Dynamc characterstcs thckness gauge of > 105- us 100 o y 95f CHANGE IN THICKNESS FREQUENCY -CPS FIGURE 2 Statc calbraton and dynamc ampltude vs. frequency response curves for the thckness transducer. Myocardal Mural Thckness and Stran TABLE 1 Dog no. I Mean Mean Body weght kg Mnmum dastolc thckness Thckness begnnng of ejecton Durng Maxmum systolc thckness Control norepnephrne Ejecton thckness stran Ejecton transverse arc stran Ejecton longtudnal arc stran

3 MYOCARDIAL MURAL THICKNESS 543 LEFT VENT. PRESSURE Hg AORTIC PRESSURE Hg 9.0-, 8.0- TRANSVERSE ARC LENGTH 80- AORTIC ROOT FLOW LONGITUDINAL ARC LENGTH I sec FIGURE 3 Smultaneous records of pressures, flow, myocardal wall thckness, and two perpendcular arc lengths on the epcardal surface. Vertcal bars on the thckness record ndcate ejecton perod as judged from the flow record. calpers sewn to the surface of the heart. The calper has been descrbed n detal prevously. 1 An effort was made to place the calpers close to the pont where the thckness gauge was nserted, but ths was not always possble snce t was necessary to avod the coronary vessels on the surface of the heart. After thckness and surface stran determna- Crculalon Research, Volume XIV, June 1964 tons were made n a control state an ntravenous nfuson of norepnephrne (approxmately mg/kg per mnute) was gven to rase the arteral pressure 10 to 20 Hg and the measurements were repeated. Strans were calculated wth respect to the ntal dmenson at the begnnng of ejecton. That s, the value at the begnnng of ejecton was sub-

4 544 FEIGL, FRY tracted from the value at the end of ejecton and the resultng dfference dvded by the value at the begnnng of ejecton. Thus a postve stran ndcates an ncrease n sze durng ejecton, a negatve stran a decrease. The ejecton perod was determned from the aortc root flow record. Results A representatve record of mural thckness s shown n fgure 3. The change n thckness of the heart wall durng the cardac cycle has a characterstc pattern. A sharp ncrease n thckness at the begnnng of systole durng the "sovolumc" phase was regularly observed. Wth the onset of ejecton a somewhat slower ncrease n thckness was observed. Thckness became farly constant n the later porton of ejecton and often remaned so for a bref perod after the aortc valves had closed. The wall became thnner durng dastole, usually thnnng n a smooth manner. Successful determnatons were made n ten anmals. The data are suarzed n table 1. The table lsts the dogs wth ther weghts and s dvded nto two smlar parts: control and norepnephrne. The mnmum dastolc and maxmum systolc wall thcknesses durng a cardac cycle are gven n mllmeters. Also the strans durng the ejecton perod are gven for thckness, as well as longtudnal and transverse arcs. Under control condtons the average ncrease n wall thckness from dastole to the begnnng of ejecton n ten anmals was The average control thckness stran durng ejecton was -f The average of seven transverse surface strans durng the same ejecton perod was The average of sx longtudnal strans durng ejecton was Wth a norepnephrne nfuson the change n thckness durng ejecton changed to an average stran of n the same anmals. The average transverse arc stran was n seven anmals wth norepnephrne. The average longtudnal arc stran n sx anmals was wth norepnephrne. Dscusson The toggle pece n the transducer used was held aganst the endocardum by a lght compresson sprng. The turgor of the myocardum s less durng dastole than systole whch means that the toggle could ndent the muscle more durng dastole than durng systole when t became stffer. Ths effect would tend to exaggerate the change n thckness observed between dastole and systole. It was not possble to determne the magntude of ths error, but t was estmated to be small snce specal care was used to keep the sprng tenson as lght as possble. The change n turgor of the myocardum durng the ejecton phase of systole s probably not very great so that the measurements made n ths perod and the strans calculated would be less subject to ths type of error. Snce muscle s predomnately composed of water t would be expected that lttle volume change would accompany contracton. The volume change that skeletal muscle undergoes durng contracton has been shown to be extremely small, less than one hundredth of one per cent. 2 It s unlkely that the changes n thckness observed were nfluenced by changes n volume of the cardac muscle. If muscle tssue s ncompressble, then the sum of nfntesmal strans n three mutually perpendcular drectons at a pont wll be zero. However, when strans are estmated n a curved object such as the heart and fnte dmensons and fnte strans are measured, ths s only approxmately true. For a gven stroke volume, the endocardal surface must undergo a greater stran than the epcardal surface. Snce the change n thckness represents the stran ntegrated across the entre wall, the stran estmated from the change n thckness wll be greater than the sum of the tangental strans on the epcardal surface. Although the data manfest consderable scatter, nspecton of table 1 shows that the sum of the transverse and longtudnal epcardal strans wth ejecton was on the average somewhat smaller than the mean radal stran computed from change n thckness. These comparsons lend support to the values observed and ndcate that the thckness measurements were probably of the correct order of magntude.

5 MYOCARDIAL MURAL THICKNESS 545 It was not possble wth ths transducer to measure thckness changes of the ventrcular wall where the ventrcle has a short radus of curvature, as at the apex or close to the aortc rng. It can only be assumed that these measurements, made mdway between apex and base, are representatve of the left ventrcle generally. The average of ten dogs showed an ncrease n wall thckness of over 10% between dastole and the begnnng of ejecton and a further ncrease of 10% durng ejecton. These values ncreased by about half agan when norepnephrne was gven. When tenson (force per unt area) s to be calculated for the heart wall the changes n thckness durng the cardac cycle should be kept n mnd. Moreover, myocardal force gauges whch are sutured to the outer surface of the heart regster forces whch may be altered n a complex manner by changes n thckness. Suary The nstantaneous and contnuous thckness of the left ventrcular wall was measured n ten dogs wth a specally desgned transducer. The thckness change durng the "sovolumc" phase of systole was 11%. The average thckness ncreased an addtonal 10% durng the ejecton perod of systole. These average values ncreased to 20% and 15% respectvely wth the admnstraton of norepnephrne (approxmately mg/kg per mnute). Transverse and longtudnal epcardal arc strans were compared wth thckness stran of the myocardum. Assumng the myocardum s ncompressble, reasonable agreement was found n the three strans, whch lends support to the measurements that were made. It s concluded that changes n the thckness of the myocardal wall durng the cardac cycle may be mportant n some consderatons of the heart's performance. Acknowledgment We thank Mr. Raymond P. Kelly for expert help n desgnng and fabrcatng the transducer used n ths study. We also thank Mr. Joseph M. Pearce for hs careful techncal assstance. References 1. MALLOS, A. J.: An electrcal calper for contnuous measurement of relatve dsplacement. J. Appl. Physol. 17: 131, ABBOTT, B. C, AND BASKIN, R. J.: Volume changes n frog muscle durng contracton. J. Physol. 161: 379, 1962.