Sylvan Fich

Measurement and Utilization of In Vivo Blood-Pressure Transfer Functions of Dog and Chicken Aortas

A method for determining the essential parameters of the aorta, namely the geometric taper, area, hoop elasticity, and effective loss factor, from in vivo pressure measurements is presented. A nonuniform hybrid model, having both geometric and elastic taper and terminated in a reflectionless impedance at the femoral bifurcation, is utilized.

Sensitivity and afterload independence of zero-load aortic flow.

The computed zero-load flow is the expected aortic flow when the aortic pressure is zero, thus eliminating the effect of afterload on ventricular ejection. Zero-load flow was computed in 15 anesthetized dogs (sodium pentobarbital 25 mg/kg, iv) by studying the response of left-ventricular pressure or aortic pressure, and aortic flow to the change in aortic input impedance induced by partial snare occlusion of the aorta. The waveform and peak value of zero-load flow were computed from a theoretical model of the left ventricle and verified by measurements of aortic flow in the first beat after transection of the aorta. To study the sensitivity, changes of zero-load flow were computed under the enhanced inotropic state produced by isoproterenol (0.1 μg/kg/min), and under the depressed contractile state induced by propranolol (0.15 mg/kg). Administration of isoproterenol resulted in an increase in the peak zero-load flow by 143.9% (p<0.001), compared with a 50.6% increase (p<0.05) in peakdp/dt. The difference of the variations was statistically significant in a pairedt test. After injection of propranolol, peak zero-load flow decreased by 32.0% (p<0.005). Afterload independence of zero-load flow was studied by computing zero-load flow before and after increasing arterial pressure by partial aortic occlusion or injection of 5 mg methoxamine. After injection of methoxamine in denervated dogs, the peak zero-load flow increased by 11.2% (N.S.), while input resistance increased by 153% (p<0.025). The peak zero-load flow decreased by 8% (N.S.) after partial aortic occlusion, while cardiac output decreased by 26.7% (p<0.001). These results may suggest that the computed peak zero-load flow is an afterload independent index of the pumping capability of the left ventricle in the intact heart.

An equivalent pressure source for the heart

Abstract A technique is described for the representation of the left ventricle as a one-port source characterized by a generator pressure and an internal impedance. The Fourier components of the pressure and impedance are determined from phasor analysis performed on recorded time-domain functions of pressure and flow under two conditions of loading. One condition is the quiescent state corresponding to normal operation of the circulatory system, and the other is obtained by use of a phase-shift balloon pump situated in the descending thoracic aorta. The use of the balloon pump for determining source parameters is novel. The Fourier components of pressure and impedance are determined for the first five harmonics of the quiescent pulse rate. Heart failure is simulated by ligation of all branches of the anterior descending coronary artery. The source parameters have been found to vary considerably between normal and failing hearts. Valve impedance and the impedance at the root of the aorta were also measured. The generator time-domain pressure waveforms were determined from the Fourier components. Effects of long duration pumping, upon the source parameters were found. The ratio of the calculated magnitudes of internal to external or load impedance indicates the possibility of considering the heart as a pressure source.

Cost function analysis applied to the determination of aorta parameters

Abstract It has been shown by Fich and Welkowitz that a model of the aorta which is tapered in area and elastic constant and is essentially reflectionless is a good representation for the calculation of pressure and flow wave propagation in the aorta. If one then measures a steady state pressure transfer function in the aorta and relates it to the steady state pressure transfer function of the model, it is possible to adjust the physical parameter values that appear in the model until the transfer functions coincide. By this means it is possible to determine the physical parameters of the aorta. Both analog and digital data techniques were developed for efficient handling of the data. Matching of the transfer functions was accomplished by finding the ‘best’ set of parameters that minimized a weighted error cost function derived from the magnitude and phase of the transfer functions.

Significance of Left Ventricular Power Waveform on Changes in Stroke Work During Pulsatile Left Ventricular Bypass

Abstract The object of this paper is to introduce the “peakedness” of the left ventricular power waveform as an index of the effectiveness of the pulsatile ventricular bypass (LVBP). The “peakedness” is determined by the position of the peak and the rate of ascent and decline of the instantaneous left ventricular power waveform. In eight open-chested dogs with coronary artery ligation, it has been found that the “peakedness” of the LV power waveform during assistance is important in determining the efficacy of LVBP. When the LV power waveform, the instantaneous product of LV pressure and aortic flow, has a broad width about the peak with an early rise to peak (low value of “peakedness” factor), the chance of improvement in stroke work after bypass is greater than when the waveform is sharply peaked with a delayed rise (high value of “peakedness”). Improvement in stroke work (SW) occurred when the percentage bypass flow was between 45 and 60%. Among the pumping runs with improved SW and CO, there was no consistent relationship between the percentage changes in SW and CO and the percentage bypass flow. The data suggest that “peakedness” may be of importance, because it evaluated the effect of left ventricular bypass on the performance of the native ventricle during postischemic periods.

Relation between computed zero-load aortic flow and cardiac muscle mechanics☆

Abstract Computed zero-load flow, qs(t), is the expected aortic flow at zero aortic input impedance, thus eliminating the effect of afterload on ventricular ejection. It is computed from an equivalent model of the ventricle using the Fourier components of left ventricular (LV) pressure and aortic flow, qa(t), of two consecutive beats with different input impedance produced by partial aortic occlusion. Actual qs(t) may be measured during the first beat after transection of the aorta. In animal experiments computed qs(t) was similar to the waveform and magnitude of actual qs(t) and was sensitive to inotropic interventions. A relationship between zero-load flow and parameters of cardiac muscle mechanics is derived from the Thevenin equivalent model of the pumping function of the ventricle and the three dimensional force-velocity-length diagram. The present analysis indicates that aortic flow can be expressed as the difference of afterload-independent zero-load flow and externally-controlled afterload-dependent internal flow.

Dynamic optimization of in-series cardiac assistance by means of intra-aortic balloon pumping.

Analytical techniques are developed which permit objective control of assist device driving systems. In addition to being objective, the techniques described in this paper are optimal in the sense of minimizing a performance index which consists of a term involving left ventricular power and a term involving deviations of aorta hemodynamic parameters from normal values. Comparisons are included of off-line computations and measurements on dogs with experimentally induced myocardial infarctions undergoing intra-aortic balloon pumping.

The Design of a Piezoelectric Heart Assist Device

A piezoelectric heart assist device was designed, and preliminary tests were performed in vitro and in vivo. The device has the advantages of simple construction, low power consumption (approximately one watt), electrical rather than pneumatic drive, and noiseless operation. The device consists of piezoelectric bender elements forming two cantilevers. A unique feature of the device is that two tungsten alloy masses, 0.44 kg each, were added to the free ends of the cantilevers to reduce the resonant frequency to 2.5 Hz. The driving voltage was a 320 V peak-to-peak square wave synchronized with a paced heartbeat.

Frequency analysis of time-varying elastance model of the left ventricle

Zadehs transfer function method for linear time-variable systems is used to apply frequency-domain analysis to a periodically time-varying elastance model of the left ventricle. Left ventricular pressure computed from the system function of the time-varying elastance and the phasors of aortic flow shows a typical waveform of the measured ventricular pressure.

A Catheter Flow Probe for Measurement of Left Ventricular Source Parameters

A balloon-tip electromagnetic velocity flow probe was used to measure source pressure Pg and zero-load aortic flow Qs in seven mongrel dogs before and after coronary artery ligation, where Qs is the expected aortic flow at zero aortic input impedance and Pg is the expected left ventricular pressure for the open-circuit condition of the equivalent model of the left ventricle. The source parameters Pg and Qs were measured by two types of flow probes (cuff-type and catheter-tip flow probe). The source parameters measured by the cuff-type flow probe are designated by Pg and Qs, and Pgv and Vs are source parameters measured by the catheter probe. The correlation coefficient between the mean values of Pg and Pgv was 0.83 in 47 measurements, and correlation between Q5 and Vs was 0.64. After coronary artery ligation, Qs, Pg, Pgj,, and Vs decreased by 35.9, 19.87, 16.67, and 2.6 percent, respectively. A change of 10 to 30 percent in aortic input resistance could be produced by balloon inflation in two consecutive heartbeats. This paper presents a new application of a catheter flow probe for diagnosis of cardiac pump performance during catheterization procedures.