Pulmonary Vascular Impedance in the Dog

Abstract

Pulmonary vascular hydraulic input impedance was measured in 13 anesthetized openchest dogs with normal sinus rhythm, and 2 dogs with surgically induced atrioventricular block, by means of electromagnetic flowmeters and strain gauge manometers of known frequency response. The linearity of the pulmonary bed was evaluated by measuring impedance while the heart rate, and hence the pulsatile input to the bed, was varied. The pulmonary bed behaved as a quasilinear system, within the limits of accuracy of the methods employed and the range of frequencies tested. The use of input impedance, or oscillatory pressure/flow ratio, to describe some characteristics of the bed is therefore justifiable, and analogies with linear models like the simple transmission line are not unreasonable. The characteristic input impedance averaged 3,094 dyne sec cm -5 kg, or about one-third the magnitude of the pulmonary vascular resistance, and was therefore a significant part of the total opposition that must be overcome in moving blood through the lungs. This impedance to pulsatile flow is not included in calculations of resistance from mean pressure and flow measurements. The pattern of the impedance spectrum suggested that reflections originating from arteries 1.0 mm and less in diameter play a large role in determining input impedance and its variations with frequency. Pulmonary vasoconstriction by 5-hydroxytryptamine (serotonin) altered the impedance pattern in a manner consistent with increased wave reflection and displacement of the dominant reflecting sites to positions nearer the main pulmonary artery. Capillary compression by increased intra-alveolar pressure also increased reflection, but did not alter the sites of reflection significantly, providing further evidence that the normal impedance pattern and its modification by serotonin are both determined by the characteristics of the arterial part of the bed.

Pulmonary vascular impedance analysis of adaptation to chronically elevated blood flow in the awake dog.

“…2) are similar in morphology in these experiments to others reported for dogs and man (McDonald, 1974;Bergel and Milnor, 1965;Milnor et al, 1966;Milnor et al 1969). The highest modulus is for Z m or impedance to mean flow.…”

“…Systolic and diastolic intervals were identified from the pulmonary flow record. Diastolic flow was assumed to be zero (Bergel and Milnor, 1965).…”

“…The computations of impedance and hydraulic power were based on Fourier analysis of the pulmonary artery pressure and flow waveforms (Bergel and Milnor, 1965;Attinger, 1963;Milnor et al, 1966). Ten harmonics were routinely calculated for all data.…”

“…These characteristics govern the amount of energy transferred to the precapillary bed and contribute to the control of regional pulmonary blood flow patterns (Maloney et al, 1968;Reuben et al, 1970). Alterations in vascular "tone" and the physiological mechanisms matching these mechanical properties to the pumping characteristics of the heart can be described only by a careful analysis of pulsatile phenomena (McDonald, 1974;Bergel and Milnor, 1965).…”

Novel Noninvasive Assessment of Pulmonary Arterial Stiffness Using Velocity Transfer Function

“…In this study, PA impedance was measured by using hybrid acquisition of pressure data from RHC and flow data from Doppler echocardiography. Although completely invasive assessment of PA impedance is feasible and was used in older studies in animals,3, 38, 39 and humans,6, 40, 41 most recent human studies on PA impedance have used the hybrid acquisition because of practicality and relative low expense 9, 12. To avoid errors attributable to hybrid acquisition of impedance, we obtained RHC and echocardiographic Doppler measurements either simultaneously or immediately next to each other, resulting in mean time difference between PA pressure and flow measurement of 0.03 hours.…”