Measurement of Nonlinearity in the Arterial System of the Dog by a New Method

One method of measurement of the nonlinearity of a vascular system is to apply a sinusoidal input perfusion flow and to examine the resultant pressures and flows at the input and elsewhere in the system for harmonic content. Unfortunately the pump may contain much higher harmonic content than will be generated by the nonlinearities in the arterial system of dogs. If two fixed flow pumps are available and can simultaneously be used in parallel to perfuse the system at two incommensurate frequencies f 1 and f 2 , then the appearance of intermodulation frequencies, such as f 1 + f 2 , f 1 - f 2 , will provide a sensitive measurement of nonlinearity. This scheme has been used, and the recorded measurements have been reduced to power spectral density form with the aid of a hybrid computer. The most important result of these studies is that the intermodulation scheme is shown to be a very sensitive method for detecting the presence of nonlinear characteristics in the arterial system of the dog, even though the system was found to be nearly linear. This scheme also proved useful in detecting changes in nonlinearity since, when injection of norepinephrine considerably increased the mean pressure, the nonlinearity was significantly less.