A transmission line model of the normal aorta and its branches.

A computer model of the aorta and its branches was made based on a simulation of an electrical transmission line using T elements. The model represented the aorta, with branches to the arms and legs and a branch to the head. The values for the capacitance and inductance of each T element could be specified, and a linearly increasing left ventricular pressure was used to drive the model. Transmission line equations were used to select values for the components, and an attempt was made to simulate the results of measurements of blood velocity in the aorta and peripheral arteries of normal subjects. The values obtained with the model showed a close relation to those in experimental studies. The results support the hypothesis that the arterial bed can be well represented by a "lossless" branched transmission line, with impedances matched at each branch and terminated with resistances that give a reflection coefficient of 0.5. A driving function, in which a linearly increasing left ventricular pressure provided a transient input to an aortic root of relatively low impedance, gave the best simulation of the experimental results.

Quantitative ascending aortic Doppler blood velocity in normal human subjects.

We have developed digital, Apple II microcomputer-based methods for the numerical analysis of pulsed, range-gated, ultrasonic Doppler blood velocity signals. These methods were then used to analyse Doppler data recorded every 5 ms from the ascending aorta via the suprasternal notch in normal subjects ranging in age from 3 to 62 years. Normal values for peak velocity, the integral of velocity over the time of systole, and the rate of change of velocity in early ejection are reported. It was found that, after an initial step increase, the velocity of blood flow in early ejection increased in a linear manner in more than two thirds of the individual beats analysed. The time for which the linear acceleration in the aorta was constant (circa 50 ms) was unrelated to the age or size of the subject.

Simultaneous Doppler blood velocity measurements from aorta and radial artery in normal human subjects.

We used two independent, pulsed, range-gated, ultrasonic, Doppler blood velocity meters to record blood velocities in the aorta and a peripheral artery in 32 normal subjects aged 8 to 62 years. Aortic signals were obtained from an unfocussed transducer in the suprasternal notch using a 2.25 MHz instrument. Simultaneous tracings were obtained from the radial or posterior tibial artery using an 8 MHz instrument. The audio Doppler signals were subjected to spectral analysis and mean velocity was calculated at 5 ms intervals during 11 successive heart beats at each site. The increase in mean velocity at the start of systole in the aorta followed a linear pattern for the first 45 ms of ejection in two thirds of the beats, irrespective of the age or size of the subject. A similar linear velocity increase in early systole was seen in the peripheral arterial signals after a delay due to the time taken for the flow wave to pass to the periphery. Thus the constant acceleration seen in aortic blood velocity tracings is transmitted to peripheral arteries in an attenuated and delayed but undistorted form.

Doppler blood velocity in the pulmonary artery of infants, children, and adults.

A pulsed, range gated, directional, ultrasonic, Doppler blood velocity meter was used to record signals from the pulmonary artery in 52 normal subjects ranging in age from less than 1 week to over 30 years. Signals were obtained from an unfocused transducer in the second or third left intercostal space using a 2.25 MHz instrument. The increase in mean velocity at the start of systole followed a linear pattern for at least the first 45 ms of ejection, irrespective of the age or size of the subject. The value for the acceleration at the start of systole decreased during childhood, reaching less than half that in the aorta in adults, and the duration of the linear increase in velocity increased. The pulmonary arterial signals in newborn infants closely resembled those in the aorta. The range of frequencies seen in the sample volume in the adult pulmonary arterial signals was greater than that in the aorta, whereas in infants it was indistinguishable from that in the aorta.

Spatial and temporal resolution of aortic Doppler blood velocity signals in normal human subjects.

We have used digital, microcomputer-based methods for the numerical analysis of pulsed, range-gated, ultrasonic Doppler blood velocity signals recorded with an axially aimed transducer from the ascending aorta of normal subjects. We investigated the spatial, temporal and amplitude resolution of our methods for recording and analysis, comparing aortic signals with test signals from a sine wave function generator. The spatial resolution of our system was found to be adequate in spite of the use of a relatively large sample volume. The temporal resolution was significantly influenced by the rate of change of velocity which affected the results of discrete fast Fourier transform analysis. The rate of increase in aortic blood velocity at the start of ejection was linear, following an initial jump in velocity. The amplitude resolution was affected by fluctuations in the amplitude of the Doppler signals. We conclude that our methods do not exaggerate the bluntness of the aortic velocity profile or the linearity of the increase in blood velocity at the start of left ventricular ejection.

Doppler ultrasonic investigation of Raynaud's phenomenon: effect of temperature on blood velocity.

We have used spectral analysis of signals from a pulsed, range-gated Doppler ultrasonic instrument to make quantitative measurements of arterial blood flow velocity in the hands of normal subjects and persons with Raynaud's phenomenon. We measured the peak velocity during the cardiac cycle and the time integral of the velocity signal over the cardiac cycle. This latter parameter gives a sensitive indication of the degree of vasoconstriction in response to cold. Our preliminary results, based on findings in 13 subjects, suggest that Doppler ultrasound can differentiate persons with Raynaud's phenomenon from normal subjects.

Comparison of methods of recording and analysis of Doppler blood velocity signals in normal subjects.

Doppler blood velocity signals were recorded from the aorta and pulmonary artery in normal adults, children and premature infants, using three different pulsed, range-gated instruments. The tracings analyzed by three independent methods of spectral analysis with axially aimed transducers showed velocity patterns with a narrow range of frequencies (blunt velocity profile) with a constant acceleration of blood flow in early systole. A similar velocity pattern was seen in the premature infant's aorta in which the transducer beam was larger than the vessel insonated. We conclude that the normal velocity pattern in the central circulation is close to blunt, and that tracings obtained with transducers that insonate varying proportions of the vessel give similar signals.

A model of the systemic arterial bed showing ventricular-systemic arterial coupling.

A multielement model of the systemic arterial bed was implemented on a microcomputer and was used to simulate the transient response of the arterial circulation to a specific driving function: a linearly increasing left ventricular pressure. The elements (compliances and inductances) that simulate the aorta and its branches are determined by the characteristic impedances and time delays in the arterial system. The simulations of pressure and blood velocity signals found in a previous version of the model were improved by making the delays in the system nonuniform, with slow central and rapid peripheral wave speeds. This reduced the impedance into which the heart has to pump and increased the distal impedance, raising the peripheral diastolic pressures. When the reflection coefficient was decreased as the heart rate was increased, normal pressure and velocity signals could be maintained over a range of heart rates up to 150 min-1. Extrapolating this relationship to a heart rate of 200 min-1 indicated that the reflection coefficient could be zero at that heart rate, giving optimal power transmission through the arterial bed.

A model of the pulmonary arterial bed in adults and infants.

A microcomputer model based on an equivalent model of the systemic arterial circulation was used to simulate the pulmonary arterial circulation. The elements chosen to represent the physical properties of the pulmonary arterial bed were determined from the characteristic impedances and tme delays in the system. A terminal resistance that provided a reflection coefficient of 0.56 at a heart rate of 75 min-1 was chosen. The model simulated normal values for pressure and blood velocity in the main pulmonary artery and pulmonary capillary bed of adults and newborn infants. The results indicate that the relatively simple physical principles used to explain systemic hemodynamics are equally applicable to the pulmonary arterial circulation.

Skin gas tensions and resistances measured by mass spectrometry in adults.

We used a dual-sampling chamber 2.5 cm in diameter, controlled by an Apple II microcomputer, to measure gas tensions in the skin of adults by mass spectrometry. The tensions of O2, CO2, N2, and water vapor and the resistances to the flows of gases through the skin were calculated from the voltages for the different gases passing through two separate membranes of different permeability. We compared the skin O2 and CO2 tensions with arterial values in 13 patients with lung disease during a standardized exercise test. The skin values followed changes in the arterial values. The regression line relating skin to arterial O2 tension had a slope of 1.05 and that for CO2 a slope of 1.51. We conclude that mass spectrometry is capable of providing measurements of gas tension from the heated skin that are similar to those found using electrochemical methods in adults.

Dynamics of expiration and gas trapping in rabbits during mechanical ventilation at rapid rates.

Mechanical ventilation at rapid rates may cause gas trapping by decreasing the time available for expiration. The volume of gas trapped within the lungs depends not only on the expiratory time, but also on the rate of emptying of the lungs. Because newborn infants are frequently ventilated at rapid rates, we examined the factors determining the rate of lung emptying, and thereby the development of gas trapping, at ventilatory rates of 40 to 120 breath/min in anesthetized and paralyzed rabbits. We found that flow and volume were related nonlinearly during the first segment of expiration, and linearly during the second segment. Only the second segment could therefore be described with a single time constant. The expiratory resistance of the respiratory system and endotracheal tube was up to 4.5 times greater than the inspiratory resistance, a finding explained by the higher transmural airway pressures during inspiration and by the dynamic effects of the increase in cross-sectional airway area at the junction of endotracheal tube and trachea. This high expiratory resistance lengthened the time constant of the second segment of expiration which, combined with the delay caused by the slow opening of the exhalation valve during the first segment, promoted gas trapping when the expiratory time was shortened.