Some physical properties of the pulmonary arterial bed deduced from pulsatile arterial flow and pressure

Use of input impedance to determine changes in the resistance of arterial vessels at different levels in feline femoral bed

In many studies, the functional state of vessels of different caliber was determined by fitting the lumped parameters of a mathematical model of the bed in order to fit the vascular input impedance (Z in) data. However, reliability of the results obtained in such a way remains uncertain. In this study, we employed a mathematical model with seven lumped parameters and Z in experimental data to analyze the distribution of resistance across the arterial bed of the hind limb in anesthetized cats, to test reliability of this distribution and to describe the process of ascending arterial dilation followed occlusion of iliac artery. The vascular bed was divided into three segments: large arteries, medium-sized arterial vessels and precapillary resistance vessels together with venous part of the bed. Based on the data of Z in measured in a wide frequency range (from 0 to 150 Hz) we showed that pharmacologically induced constriction and dilation of the arterial microvessels were reflected in the model by the changes in the resistance of distal precapillary vessels only, whereas the local constriction or dilation of femoral and iliac arteries as well as artificial stenosis of the femoral artery resulted exclusively in the changes of the resistance describing the state of large arteries. Using the input impedance method we could demonstrate and quantitatively describe the process of ascending arterial dilation during the post-occlusion (reactive) hyperemia. All these results prove that the model of vascular bed with seven lumped elements used in combination with input hydraulic impedance data can be an effective tool permitted to quantitatively analyze the functional state of arterial vessels of different caliber and to describe the changes in resistance of arterial vessels during vascular reactions.

Input impedance of the pulmonary arterial system in normal man. Effects of respiration and comparison to systemic impedance

Input impedance of the pulmonary arterial system was determined in 10 subjects undergoing elective cardiac catheterization. No cardiovascular or pulmonary disease was found in these patients. In five of the subjects, systemic arterial impedance was also obtained, so that both systems could be compared. Pulmonary and systemic peripheral resistances were 79 +/- 9 dynes sec/cm5 (mean +/- SEM) and 1016 +/- 50 dynes sec/cm5, respectively. Characteristic impedance of the pulmonary circulation was lower than the characteristic impedance of the systemic circulation: 20 +/- 1 dynes sec/cm5 vs. 47 +/- 9 dynes sec/cm5, respectively. Pulmonary pressure and flow spectra for both systems are also presented. The amplitudes of the harmonics of pressure and flow are smaller for the pulmonary circulation, which is consistent with the lower pressures and more rounded waveforms of the normal pulmonary circulation. In all 10 subjects, input impedance of the pulmonary system was examined during both the inspiratory and expiratory phases of respiration. There was no difference between inspiration and expiration in either pulmonary vascular resistance (77 +/- 10 dynes sec/cm5 vs. 80 +/- 9 dynes sec/cm5, respectively), characteristic impedance (20 +/- 1 dynes sec/cm5 vs. 20 +/- 1 dynes sec/cm5) or in the overall impedance spectrum. Quiet respiration, thus, has no effect on the pulmonary arterial load, and changes in pressure and flow must result from alterations in right ventricular performance.

Impedance matching between ventricle and load

Impedance matching in the cardiovascular system is discussed in light of two models of ventricle and load: a Thevenin equivalent consisting of a hydromotive pressure source and an internal, source resistance and compliance in parallel; and a time-varying compliance filled from a constant pressure source and ejecting into a load of three components, a central resistor, a compliance, and a peripheral resistance. According to the Thevenin analog, the energy source and the load are matched when the load resistance is T/t times the internal source resistance (T is total cycle length, t is systolic time interval). Both from this model and from the variable compliance model it appears that optimum matching between source and load depends on the compliance of the Windkessel, as low compliance shifts the matching load resistance to a low value. Animal experiments (isolated cat hearts) indicated that both left and right ventricles at normal loads work close to their maxima of output hydraulic power, and, according to experiments in the right ventricle, maximum power output is related to load resistance and compliance as predicted by the above models. From an experimentally determined relationship among instantaneous ventricular pressure and volume (right ventricle of isolated cat hearts), an optimum load impedance was calculated on the basis of the assumption that the ratio between stroke work and static, potential energy developed in the ventricular cavity is maximum. The optimum load impedance found by this procedure closely resembles the normal input impedance of the cat lung vessel bed.

Blood volume and extravascular water content in the rat lung during acute alveolar hypoxia

Values for pulmonary blood volume and extravascular lung water (estimated as wet weight of lung tissue) were arrived at in intact, anesthetized rats by labelling of blood constituents with isotopes and rapidly freezing the whole animals in liquid nitrogen. On ventilating the animals with 10% O2 in N2/N2O, a reduction in lung blood content could be demonstrated. The degree of reduction depended on the type of anesthesia and ventilation used. In some animals the volume reduction was so marked that both arterial, venous and capillary blood compartments have most probably been involved. The water content of the lung tissue was also rapidly and markedly reduced during hypoxia. Increased plasma osmolarity in mixed venous blood could partly be responsible for this tissue water reduction.

Improved left ventricular performance by the transmission of pulse waves through the pulmonary vascular bed

The influence on left ventricular performance of pulsewaves transmitted through the pulmonary vascular bed was studied in a cat lung preparation with the right heart bypassed by a pulsatile blood pump. The pump worked at a frequency slightly different from the intrinsic heart rate; transmitted pulse waves were thereby forced to arrive the left atrium at different phases of the left heart cycle. Slow fluctuations of left atrial pressure, left ventricular systolic pressure and left ventricular dP/dt were observed. Left ventricular systolic pressure and left ventricular dP/dt were maximum when the transmitted pulsewaves arrived left atrium just prior to the atrial contraction. The observed variation in left ventricular systolic pressure was found to be directly dependent on the magnitude of pulsatile hydraulic power transmitted to the left atrium.