(G-269) Mechanism of the Developed Pressure – Preload Relationship in the Isolated Perfused Heart

Introduction:: The presence of a critical transition preload at which the developed pressure peaks and decreases with further increase in the preload is a well-established feature in isolated heart preparation using a Langendorff setup. This behavior mirrors that observed in isometric tension test conducted on skinned cardiac muscle fiber, where there exists an optimal range of sarcomere length (2.2 – 2.4 μm) [1] at which the developed tension peaks. While these two behaviors occurring at different spatial scales are similar, it is not clear whether they are causally related (i.e., the critical transition preload is associated with the optimal range of sarcomere length) because of the difficulty in measuring sarcomere length in the Langendorff set up. The cellular to organ connection is also not straightforward because of many confounding factors such as perfusion and myocardial-coronary interactions in the Langendorff set up. To address this issue, we integrate computational modeling with Langendorff experiment to test the hypothesis that the critical transition preload associated with peak developed pressure at the organ level is associated with the optimal sarcomere length found in the skinned cardiac muscle fiber test.




Materials and Methods:: Approval for conducting experiments using male Sprague-Dawley rat weighting 300-350 g was granted by the Animal Ethics Committee of Michigan State University. The heart was isolated from the rat after heparin injection and isoflurane anesthesia. The isolated heart was immediately perfused via the Langendorff constant flow mode with oxygenated Krebs-Henseleit buffer. To measure the left ventricular (LV) pressure during the isovolumic contraction, a balloon was inserted into the LV chamber. Pacing wires were attached to the right atrium and the heart was paced at 4Hz. The LV underwent isovolumic contraction in each beat over a range of preload that was imposed by changing the balloon volume (Fig. 1a). The LV chamber pressure waveform was recorded for different preload and used to construct the end-diastolic pressure volume relationship (EDPVR) and end-systolic pressure volume relationship (ESPVR). A finite element model (FE) of the LV based on an idealized ellipsoidal geometry with a realistic transmural variation of myofiber orientation (Fig. 1b) [2] was developed to simulate isovolumic contraction measured in the experiment. In the FE model, the functional relationship between the LV pressure and volume is obtained by minimizing a Lagrangian function consisting of myocardial tissue strain energy function and terms associated with the enforcement of constraints. Mechanical behavior of the LV is described using an active stress formulation, in which the passive and active mechanical behaviors are described using a Fung-type strain energy function and a modified time-varying elastance model, respectively. The computer model is implemented using the open-source library FE FEniCS.


Results, Conclusions, and Discussions:: Results: A representative pressure waveform measured during isovolumic contraction is shown in Fig. 1c. Developed pressure (DP) in the LV chamber (defined as the difference between ESPVR and EDPVR) increased with an increase in cavity chamber volume (preload) until the increase is 120 μL Beyond this transition the DP decreased with increasing chamber volume (Fig. 1d). At the transition, LV EDP and ESP are 7 and 89 mmHg, respectively, which are both within the in vivo heart’s operating ranges. Finite element model predictions of pressure waveform, ESPVR, EDPVR and the variation of DP with preload agree with the measurements (Fig. 1c-1d). Largest difference between model prediction and measurements in the pressure waveform is found during the isovolumic relaxation phase. Average peak myofiber strain and sarcomere length in the LV wall (over the entire isovolumic contraction cycle) at the transition volume are estimated (in the FE model) to be 15% and 2.12 μm, respectively.

Sarcomere length in the LV wall associated with peak DP (2.12 μm) in the Langendorff isolated heart preparation is comparable and slightly lower than the reported optimal sarcomere length of 2.2 – 2.4 μm [1], which is associated with the peak tension under isometric tension in rat skinned fiber test. This result suggests that DP-preload relationship is controlled by the length-tension relationship of the myocytes.

The measured EDPVR, ESPVR and DP are consistent with previous observations in Langendorff isolated heart preparation. Besides the DP rises with increasing chamber volume or preload (i.e., Frank-Starling mechanism) before reaching its peak value, followed by a decrease with preload. This behavior mirrors isometric tension test on skinned fiber. The mechanism of this length-tension relationship rests on the degree of overlap between the thick and thin filaments at different sarcomere lengths during cross-bridge cycling. However, it is challenging to directly connect them because of the difficulty in measuring myofiber strain and sarcomere length in the deep myocardial wall in the Langendorff preparation. To circumvent this issue, we developed an LV FE model to simulate isovolumic contraction and estimate these two quantities.


Acknowledgements (Optional): : This work was supported by the National Institute of Health (R01 HL160997) and American Heart Association Postdoctoral Fellowship (AHA835298).

References (Optional): : [1] Weiwad, W., J. Mol. Cell. Cardiol., 2000.

[2] Fan L. et al., Comput. Biol. Med., 141, 105050, 2022.