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Title: Physiological rules for the heart, lungs and other pressure-based organs
Authors: Casha, Aaron
Camilleri, Liberato
Manche, Alexander
Gatt, Ruben
Gauci, Marilyn
Camilleri-Podesta, Marie Therese
Grima, Joseph N.
Scarci, Marco
Chetcuti, Stanley
Keywords: Biomechanics
Issue Date: 2017
Publisher: Pioneer Bioscience Publishing Company
Citation: Casha, A. R., Camilleri, L., Manche, A., Gatt, R., Gauci, M., Camilleri-Podesta, M.-T., Grima, J. N., Scarci, M., & Chetcuti, S. (2017). Physiological rules for the heart, lungs and other pressure-based organs. Journal of Thoracic Disease, 9(10).
Abstract: Background: The adherence of the heart to physical laws, such as Laplace’s Law, may act as a measure of the organ’s relative efficiency. Allometric relationships were investigated to assess the heart’s efficiency concerning end-diastolic and end-systolic volumes, cardiac pressurization energy, cardiac output and mass. Methods: Data to generate allometric relationships was obtained using a literature search, identifying heart and lung data across different mammalian and bird species. Statistical analysis was carried out using ordinary least squares (OLS) estimation. Results: Near isometric relationships exist between body mass and seven parameters indicating no “efficiency of size” with scaling of the heart, and size-matching of the heart to the lungs and whole body. Even though there was equal efficiency in pressurization energy generation, cardiac output was maximally efficient in small mammals <10 kg and birds; the human heart reached only 71% efficiency. This loss in cardiac efficiency with increasing body mass can be explained by the aortic cross-section that scales following the three-quarter allometry law, compared to end-systolic and end-diastolic volumes that scale isometrically. The heart is therefore throttled by a relatively small aorta at large body size. Conclusions: Mammalian and avian hearts operate at similar efficiencies, demonstrating a high degree of symmorphosis, however cardiac output efficiency decreases in larger animals due to a relatively negative aortic cross-section allometry. This work has a myriad of potential applications including explaining cardiac dysfunction in athletes, patient-prosthesis mismatch in aortic valve replacement and why heavy exercise is associated with a worse prognosis than mild or moderate exercise.
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