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Hemodynamics of the rat and mouse Phelps, Robyn Lee


There are two theoretical concepts of vertebrate hemodynamics, expressed by the Windkessel and Tubular models. The "Windkessel" is a lumped parameter system in which pressure pulse wave velocity is infinite and wave reflection is impossible. Conceptually, the central arterial vessels cushion flow pulsations while the peripheral arterioles act simply as conduits. In this system, diastolic pressure falls exponentially and wave shapes and amplitudes are identical throughout the arterial tree. In the "Tubular" model, wave velocity is finite and reflected waves augment peripheral pressure and reduce peripheral flow pulsations. Diastolic pressure decline is interrupted by secondary fluctuations and wave shapes and amplitudes are altered during travel. Peripheral resistance (Rs) effects the magnitude of wave reflections while pressure pulse wave velocity (C), cardiac frequency (f₀), and arterial length (L), interact to define their effect on the heart. Simply, f₀ produces oscillating waves where incident and reflected wave summation create nodal and antinodal points in the central aorta. The correct matching of L and C/f₀ places the heart near a pressure node, thereby uncoupling it from the high terminal impedance. However, incorrect matching may promote an early, detrimental return of reflections to the heart. In most mammals, appropriate L/C/f₀ matching reduces systemic impedance, however, this may not occur in very small animals. Simultaneous measurements of aortic arch pressure and flow and abdominal aortic pressures were made under varying physiological conditions in rats and mice. These measurements were used to identify waveform alterations and to calculate systemic impedance in each species. Distal aortic wall properties were described by biomechanical tests. There were no significant differences in arterial mechanicals or anatomy in the rat as compared with other mammals. Cardiovascular indices measured in anesthetized open-chest, ventilated rats were similar to both allometrically estimated, and previously reported, values. In contrast, the abdominal aorta of the mouse was more distensible, measured mean arterial pressure lower, and C slower, than in the rat. Stroke volume was greater, f₀ slower, and Rs less than were estimated allometrically. While wave reflections were observed in the mouse their effects were not discrete. Despite a mismatch in L/C/f₀ inappropriate return of reflections to the heart was prevented by the decrease in Rs and aortic stiffness. While the hemodynamics of the rat can be described by the Tubular model, the systemic impedance of the mouse is minimized primarily through a combination of low Rs and increased central arterial distensibility.

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