Direct measurement of inner and outer wall thickening dynamics with epicardial echocardiography

Abstract

Simple geometric models of the left ventricle and indirect experimental measurements suggest that the inner myocardial wall contributes the largest fraction to total wall thickening. We measured transmural differences in regional wall thickening directly, using an epicardial M mode echocardiographic transducer (6 mm diameter, 5 MHz) placed on the anterior free wall of the left ventricle. Wall thickness was partitioned into inner and outer regions by inserting a waxed, 3-0 suture at different depths within the wall. The suture was used as an intramural echo target that was imaged simultaneously with the endocardium to determine inner and outer fractional contribution to total wall thickness. Data were collected in open-chest dogs at rest, during inotropic stimulation with isoproterenol, and during right heart bypass, which was used to vary cardiac output and preload. Results obtained with this method demonstrated that systolic wall thickness was nonuniform at rest and during each intervention. The fractional contributions to total wall thickening of the inner, middle, and outer thirds of the myocardial wall were estimated from the data to be 58%, 25%, and 17%, respectively. The experimental findings corresponded closely to theoretical predictions, supporting the conclusion that a gradient of thickening exists across the myocardial wall, with the inner portion of the wall contributing the largest fraction to total systolic thickening. Circulation 74, No. 1, 164-172, 1986. WHETHER OR NOT systolic wall thickening is nonuniform may be important clinically because conventional means of evaluating left ventricular function frequently assume uniformity of fiber shortening or wall thickening.' Although theoretical predictions indicate that the inner layers of the left ventricle should thicken significantly more than the outer layers,24 experimental measurements of inner and outer systolic wall thickening have not produced consistent results.7 In addition, previous measurements were indirect or required the use of methods not readily available to most investigators. Consequently, we measured differences in regional From the Thoracic Surgery Research Laboratory, Departments of Surgery (Thoracic Section) and Physiology, and Department of Medicine, The University of Michigan Medical School, Ann Arbor. Supported in part by the American Heart Association of Michigan and American Heart Association grant-in-aid 81-1161. Address for correspondence: Kim P. Gallagher, Ph.D., Thoracic Surgery Research Laboratory, R3484 Kresge I, Box 0548, University of Michigan, Ann Arbor, MI 48109. Received Nov. 29, 1985; revision accepted March 27, 1986. J. H. Myers was on sabbatical leave from Southern Illinois University, School of Medicine during these studies. K. P. Gallagher was a recipient of NIH New Investigator Award HL-30067. 164 transmural thickening with an implantable echocardiographic transducer placed on the epicardium.5 9 The myocardial wall was partitioned into inner and outer portions by passing a 3-0 suture through the wall at varying depths. The suture was used as an echo target and allowed direct measurement of the inner and outer wall contribution to total wall thickening. The method is relatively simple and enables direct measurement of transmural differences along the same axis through the wall. We applied this novel approach to test the hypothesis that systolic wall thickening is nonuniform, and to evaluate the effects of changes in preload and augmented contractility on the contribution of different myocardial layers to total systolic thickening. Methods Mongrel dogs were premedicated with morphine sulfate (1 to 2 mg/kg) 30 min before induction of anesthesia with chloralose (120 mg/kg). Additional chloralose was administered by a lowrate intravenous infusion to maintain anesthesia. After endotracheal intubation, a left thoracotomy was performed through the fifth intercostal space. The heart was suspended in a pericardial cradle to maximize exposure of the anterior free wall of the left ventricle. A Millar high-fidelity micromanometer and a Tygon CIRCULATION by gest on A ril 6, 2017 http://ciajournals.org/ D ow nladed from LABORATORY INVESTIGATION-ECHOCARDIOGRAPHY fluid-filled catheter (used to calibrate the micromanometer) were inserted into the left ventricle via the atrial appendage and mitral valve. Fluid-filled Tygon catheters were inserted into the femoral artery for measurement of aortic pressure and the femoral vein to provide access for intravenous fluids. For measurement of regional wall thickness we used a 6 mm, 5 MHz echocardiographic transducer (KB-Aerootech; Lewiston, PA) coupled to an Irex-M-mode echocardiographic recorder. The unfocused transducer was positioned on the epicardium of the anterior left ventricle (figure 1) and was oriented to obtain a continuous strong signal from the endocardium. In most studies the transducer was attached directly to the epicardial surface with cyanoacrylate adhesive (Wonder Bond, Borden, Inc.). To partition the wall into outer and inner portions, a waxed, 3-0 suture was inserted through the myocardium beneath the transducer (figure 1) with a curved surgical needle. The suture served as an intramural echo target that was imaged simultaneously with endocardial wall motion. In this manner we could track the echo target coincident with the endocardium and directly measure inner wall thickening as the distance between the suture echo and endocardial echo and outer wall thickening as the distance between the suture echo and epicardium along the same axis through the wall. The appearance of the suture echo within the wall was verified in preliminary experiments by either withdrawing the suture and recording its disappearance from the tracing or measuring its position within the wall at autopsy. Data at rest were obtained 20 min after inserting the suture into the myocardial wall. Then 2 pig of isoproterenol was injected as a bolus and wall thickness data were collected during the peak heart rate response. In this set of experiments the myocardial wall thickness data were categorized into three groups. The first group (n = 8 dogs) was obtained in experiments in which the sutures were placed within the wall at a depth of between 10% and 35% from the epicardial surface. The second group (n = 11) was collected in experiments in which sutures were inserted 35% to 65% from the epicardial surface. The third group (n = 7) was obtained in dogs in which sutures were placed 66% to 85% of the wall thickness from the epicardium. In a second group of chloralose-anesthetized dogs (n = 4), right heart bypass was instituted. Hemodynamic and wall thickness data were collected at cardiac outputs of 1, 2, 3, 4, and 5 liters/min, 2 min after each level of output was achieved. Right heart bypass was established through a sternotomy by cannulation of the venae cavae and delivery of oxygenated, temperature controlled blood into the pulmonary artery. Heart rate was controlled by atrial pacing at a fixed rate after crushing the sinoatrial node. Mean arterial blood pressure was maintained at a constant level by a combination of tourniquet adjustments around the descending thoracic aorta and varying the flow through a peripheral arteriovenous fistula. A Sarns cardiopulmonary bypass pump (Model 2000) controlled cardiac output, which was used to effect changes in end-diastolic pressure to evaluate how "preload" alterations were manifested in terms of inner and outer wall thickening. In the right heart bypass study sutures were placed only within the midmyocardial region (average depth 52 + 12%, mean + SD). To determine whether or not attaching the transducer directly to the epicardium significantly affected transmural wall motion, four animals were studied with use of a different transducer-toepicardium configuration. The transducer was attached to a hollow, lightweight rubber dome mounted on the epicardium. The dome, in turn, was attached to the edges of a skirt fashioned from a surgical glove that was glued to the epicardium approximately 1 cm from the center of the echo target. Acoustic coupling was maintained by filling the pericardium with mineral oil. The average suture depth in these experiments was 48 + 6%, which means the myocardium was divided roughly into

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@inproceedings{Hurley2005DirectMO, title={Direct measurement of inner and outer wall thickening dynamics with epicardial echocardiography}, author={Joanna Hurley}, year={2005} }