Pulsatile changes in arterial diameter are virtually identical to the pressure pulse, with minor differences explained in terms of nonlinear elasticity and viscosity of the arterial wall. In 1939, Hamilton and Dow defined the pressure wave contour in different arteries in terms of wave reflection between the aortic valve and peripheral sites.172 The pulse waveform recorded at any site of the arterial tree is the sum of a forward waveform and a backward-traveling one that is the "echo" of the incident wave reflected at peripheral sites. Wave reflection is an important determinant of LV load and CBF. A reflected wave occurring at systole increases systolic pressure and thereby increases ventricular afterload. In contrast, occurrence of the reflected wave at diastole is highly desirable because augmentation of pressure during diastole aids coronary perfusion.
Conventionally, the pulse is described in the time domain, where it is considered as a change in arterial pressure with time. An alternative approach that has the advantage of being quantitative is to analyze the pulse in the frequency domain. Pulse is conceived as a composite wave that can be resolved into component harmonics like a musical wave. Impedance is the measure of the opposition to flow presented by a system and can be approached quantitatively when harmonic analysis is used to relate frequency components of pressure and flow pulses.173-178 Study of impedance provides valuable insights for several issues of vascular mechanics.
Usually, there is a linear relation between pressure and flow at the same point in an artery and between pressures at different points in the arterial system. From impedance curves, it is possible to identify the factors responsible for the relation between the pulsatile pressure and flow. Furthermore, the coefficient of reflection in peripheral vessels can be calculated from the relation of resistance to the minimal and subsequent values of impedance modulus. The peripheral arterial pressure wave recorded is the summation of the incident (initial) and reflected waves. The systemic circulation has been represented by a simple asymmetric T-tube model that emphasizes the importance of wave reflection at two arteriolar reflecting sites in the upper and lower parts of the body.169 An important patient study indicates major reflection sites at the aortic level of the renal arteries and at a point distal to the terminal abdominal aorta bifurcation.170
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.