Mechanical Loadings on Pectoral Pacemaker Implants: Correlation of In-line and Transverse Force of the Pectoralis major

Abstract

Recently we presented a method for the assessment of in vivo forces on pectoral device implants motivated from technological and clinical advancements toward smaller implantable cardiac pacemakers and the altered structural demands arising from the reduced device size. Objective of this study was the investigation of the intra-species proportionality of in-line force and transverse reaction force of the Pectoralis major for the characterization of mechanical in vivo loadings on pectoral implants. Two Chacma baboons (23.9 ± 1.2 kg) received bilaterally one chronic and one acute pectoral sub-muscular instrumented pacemaker (IPM) implant. The Pectoralis major muscle was electrically stimulated and resulting in-line and transverse muscle force were measured. The correlation of in-line force and transverse force of the Pectoralis major was investigated using linear regression analyses. The proportionality of in-line and transverse force of the Pectoralis major was found to be subject-specific (R² = 0.17, p < 0.003). Including morphometric parameters, i.e., length along line of action, width over implant and stress, in the regression analysis provided a strong intra-species correlation between in-line and transverse force (R² = 0.71, p < 10⁻⁷). The novel intra-species correlation provides a tool toward the characterization of mechanical in vivo loading conditions of pectoral device implants.

Patient-specific prediction of intrinsic mechanical loadings on sub-muscular pectoral pacemaker implants based on an inter-species transfer function

“…Secondly, the effects of load rate and fibre orientation on the viscoelastic behaviour of passive muscle under compression (Van Loocke et al, 2008) can be neglected since, in our experiments, the muscle was active with increased stiffness compared to passive state and the compression acted in cross-fibre direction with lower stiffness and viscosity compared to the fibre direction. The muscle contraction was sustained for approximately 0.5 s only (de Vaal et al, 2010a) and the compression rate was similar to that for the contraction of a relaxed muscle to a maximum level of 200 s À 1 (Wilkie, 1949). This value was considerably higher than the rate reported by Van Loocke et al (2008) for which they reported that the reaction of passive muscle to compression was devoid of viscous effects for instantaneous loadings.…”

“…After conclusion of a related study (de Vaal et al, 2010a), two Chacma baboons (M b ¼23.9 7 1.2 kg) with pectoral sub-muscular implants of instrumented pacemakers (IPM) underwent imaging of the thoracic region with computed tomography (Aquilion 4, Toshiba Medical Systems, Zoetermeer, Netherlands) within two hours of euthanisation. Subsequently, the Pmajor was dissected and morphometric details were recorded as described by de Vaal et al (2010a): length along the estimated line of action L m , thickness and width at the crossbar of the buckle force transducer t m,cb and w m,cb and width over the IPM implant w m .…”

“…Subsequently, the Pmajor was dissected and morphometric details were recorded as described by de Vaal et al (2010a): length along the estimated line of action L m , thickness and width at the crossbar of the buckle force transducer t m,cb and w m,cb and width over the IPM implant w m . After excision, mass M m and volume V m of the muscle were recorded.…”

“…The availability of such data, and in particular maximum levels of mechanical loadings, is however important if not crucial for the mechanical design of implants with reduced size while ensuring reliability. We have, therefore, recently demonstrated for the first time the feasibility of a system to assess in vivo mechanical forces on implanted pacemakers and established in the non-human primate model an intra-species correlation between the force of the sternal Pmajor in line of its action and the transverse reaction force on a pectoral implant in sub-muscular position (de Vaal et al, 2010a(de Vaal et al, , 2010b.…”

“…Based on an intra-species correlation of in-line and transverse force of the Pectoralis major established previously for the Chacma baboon (de Vaal et al, 2010a), a simplified physiological model and a mechanical equivalent model were developed for a submuscular pectoral device implant considering Pectoralis major, Pectoralis minor and rib cage. By assessing the morphometric and mechanical parameters of these musculo-skeletal structures and the associated model parameters, the intra-species correlation was shown to exhibit (a) robustness for a larger intra-species subject population and (b) linear scale variance allowing application for humans under consideration of the inter-species difference of the attachment angles of Pectoralis major.…”