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Biomedical subjects

S A Altobelli

Publications and source records attributed to S A Altobelli.

18 recordsLinked to original sources
PubMed1999

Effects of gas density on experimentally obstructed ventilation during acute hypoxia.

When patients with obstructive lung disease breathe helium-oxygen mixtures, their arterial PCO2, is lowered towards normal, indicating more effective ventilation. However, there is a lack of detailed respiratory data from clinical cases, so that the mechanisms remain unclear. To study relevant variables during hypoxemia and obstruction in the absence of disease, we undertook experiments with healthy subjects breathing normoxic and hypoxic gas mixtures of differing densities (air, 13.7% O2 in N2 and 13.7% O2 in helium) through an experimental obstruction (resistive airway loading). This increased airway resistance was twice that reported from the ambient-pleural pressure differences in patients with moderately severe emphysema. Without imposed resistance the total ventilation (VE) increased 27% on both hypoxic mixtures. With normoxia, the obstruction increased tidal volume but decreased frequency so that VE and alveolar ventilation (VA) were essentially unchanged. With hypoxia, breathing pattern changed similarly, but now VE decreased while VA was maintained. Helium returned the breathing patterns toward normal. Obstruction lowered the rapid increase in VE from two or three breaths of N2, but the decrease from two or three breaths of O2 was unchanged. We detected an increase in metabolic rate with obstructed breathing that was reduced by the helium mixtures. The remarkable finding was that despite the obstruction being markedly uncomfortable because of the high resistance, we did not find any substantial disturbance in gas exchange, compared to hypoxia with no obstruction. Thus, the main mechanisms responsible for improved blood gases in patients breathing helium mixtures were outside the scope of our experiment and likely related to disease factors.

Adult↗
PubMed1999

Concentration and velocity field measurements by magnetic resonance imaging in aperiodic heterogeneous porous media.

Magnetic resonance imaging (MRI) techniques were investigated as a means to obtain concentration and velocity field measurements for the verification of a stochastic model for conservative chemical transport. MRI techniques were successfully applied to obtain one-dimensional breakthrough images and two-dimensional velocity images along the length of an aperiodic heterogeneous porous medium. Experimental moment data showed the concentration field in the experimental model to be slightly positively skewed. Velocity images showed the velocity field to be relatively uniform with no channeling or preferential flow behavior. Measured covariance functions showed evidence of negative correlation in the velocity field. The detailed spatial information provided by these imaging experiments has demonstrated that MRI is a valuable tool for obtaining experimental data for the verification of existing theoretical models.

Attention↗
PubMed1998

A programmable pre-emphasis system.

MRI systems often use magnetic field gradient and shim pulse-shaping networks (pre-emphasis) to correct for magnetic field distortions caused by eddy currents. A pre-emphasis system that uses up to 16 fixed resistor-capacitor (RC) time constants per channel with programmable amplitude coefficients is described. The magnetic fields induced by the pre-emphasis RC time constants serve as a set of basis functions for compensating eddy-current fields induced by the gradient set. The resultant time-varying magnetic field gradient accurately reflects the gradient specified by the pulse programmer. Reductions in eddy-current fields are demonstrated for actively shielded and unshielded gradient sets.

Artifacts↗
PubMed1997

NMR measurements of flow profiles in a coarse bed of packed spheres.

The velocity profiles of water flowing in a circular pipe filled with coarse, uniform, spherical beads, as well as the evolution of velocity profiles from clear flow into the bed of beads and visa versa, were measured by nuclear magnetic resonance (NMR) imaging using the phase-encoding method. Corrections for partial-volume effects were necessary in order to obtain accurate flow parameters. Phase errors due, for example, to electrical eddy currents induced by switching gradients, were minimized by the use of a static reference sample. For a tube diameter that is eight times the particle diameter, the flow reaches a steady state condition within one bead diameter into the packed bed after having a parabolic velocity profile only 1.5 tube diameters before encountering the bed. Within the packed bed, there are velocity spikes up to 16 times the average velocity in the unobstructed part of the pipe. The velocity distribution in the packed bed has a maximum that approaches the average velocity in the bed for increasing slice thickness along the flow. This distribution decreases exponentially with increasing velocity on the high-velocity side of the maximum. Even at modest flows, where velocity is steady upstream of the packed bed as well as in the bed, unsteady flow is detected downstream of the beads.

Humans↗
PubMed1986

Rapid average-flow velocity measurement by NMR.

We describe an NMR method to make a quick determination of average fluid velocity by monitoring the change in phase of the magnetization during a transient signal. We give an example utilizing the second Carr-Purcell-Meiboom-Gill echo in which we measured the velocity of water up to 54 cm/s where each velocity was determined by data taken over 2.5 ms.

Humans↗
PubMed1986

A new method for flow velocity measurement: frequency encoded NMR.

We demonstrate a new rapid NMR method to measure the average velocity and the velocity distribution of a flowing fluid. We use a truncated Carr-Purcel-Meiboom-Gill pulse sequence, pi/2-T-pi-2T-pi-T-echo, in the presence of a static magnetic field gradient. The pi/2 pulse is selective and tags a narrow slice of nuclei. The second echo from the tagged nuclei is Fourier transformed after the nuclei have evolved for a time 4T to give the velocity distribution modified by the initial slice selection profile. In this way, average velocities up to 54 cm/s were measured for water flowing in a plastic tube with either a plug or a developed velocity distribution. Each measurement required about 20 ms but this could be reduced with larger field gradients. Thus, this method is fast enough to apply to such problems as measuring pulsatile flow in arteries.

Animals↗
PubMed1985

An experimental study of coronary artery fluid mechanics.

Preserved baboon and canine hearts were perfused using an in-vitro pulsatile flow system. Flow rate and pulsation frequency were controlled, and velocity profile measurements were made at several sites on the left epicardial coronary arteries of each heart. Velocity profiles were measured using a multi-channel, pulsed ultrasonic Doppler velocimeter, and the data were processed with a laboratory microcomputer system. Flow in the left main coronary artery appeared to be similar to descriptions of developing curved tube flow, but an unexplained oscillation of the velocity profiles was observed in this artery. Near the bifurcation of the main coronary artery into the anterior descending and the circumflex, the pattern of velocity profile skewing appeared to be determined by the angle through which the daughter vessels turned from the main and the overall curvature of the "plane" of bifurcation. Several diameters downstream from the bifurcation the flow appeared to be quasi-steady.

Animals↗
PubMed1983

The effects of intraventricular gradients on left ventricular ejection dynamics.

The generation of abnormal gradients between the apical cavity and the subaortic valvular region of the left ventricle in patients with hypertrophic cardiomyopathy (HCM) has traditionally been equated to a dynamic obstruction to left ventricular outflow. To examine this concept in more detail, left ventricular ejection dynamics were studied during cardiac catheterization in 30 patients with HCM and 29 patients with no evidence of cardiovascular disease. Using multisensor catheterization techniques, ascending aortic flow velocity and micromanometer left ventricular and aortic pressures were simultaneously recorded during rest (n = 47). Dynamic left ventricular emptying was also analyzed with frame-by-frame angiography (n = 46). The temporal distribution of left ventricular outflow was independently derived from both flow velocity and angiographic techniques. The HCM patients were subdivided into three groups: I, intraventricular gradients at rest (n = 9); II, intraventricular gradients only with provocation (n = 12); III, no intraventricular gradients despite provocation (n = 9). Expressed as a precentage of the available systolic ejection period (%SEP), the time required for ejection of the total stroke volume was (mean +/- 1 S.D.): Group I, 69 +/- 17% (flow), 64 +/- 6% (angio); Group II, 63 +/- 14% (flow), 65 +/- 6% (angio); Group III, 61 +/- 16% (flow), 62 +/- 4% (angio); control group, 90 +/- 5% (flow) 86 +/- 10% (angio). No significant difference was observed between any of the three HCM subgroups, but, compared with the control group, ejection was completed much earlier in systole independent of the presence or absence of intraventricular gradients. The presence of coexisting mitral regurgitation in 12 of the HCM patients did not alter these results. This study demonstrates that 'outflow obstruction', as traditionally defined by the presence of an abnormal intraventricular pressure gradient and systolic anterior motion of the mitral valve, does not impede left ventricular outflow in HCM. In a pure fluid dynamic sense, we believe that outflow obstruction does not exist in this disease entity.

Angiography↗
PubMed1981

Manipulation of ascending aortic pressure and flow wave reflections with the Valsalva maneuver: relationship to input impedance.

Dramatic changes in the shape of pulsatile ascending aortic pressure and flow wave forms occur during the Valsalva maneuver in man. To study these changes, aortic pressure and flow signals were recorded in eight patients using a multisensor catheter. Aortic input impedance was derived during the control, strain and postrelease phases of the Valsalva maneuver. During control, well-defined minima and maxima occurred in the spectral plots of impedance moduli. This pattern was accentuated during the postrelease phase. In contrast, input impedance during strain was almost equal to the characteristic impedance for all harmonics. These results imply that during the control and postrelease phases, strong reflections return to the ascending aorta, but during the strain phase, reflections are minimal, absent or more diffuse. From wave transmission theory, it also follows that pulsatile pressure and flow wave forms should be similar in shape in the absence of reflections and dissimilar in the presence of reflections. This was observed in all eight patients. By provoking changes in the arterial tree during the Valsalva maneuver, the magnitude and timing of wave reflections were significantly altered, resulting in marked changes in the shape of pulsatile aortic pressure and flow wave forms. This study demonstrates the importance of reflections in determining the shape of the arterial pulse.

Adult↗
PubMed1981

Effects of exercise on aortic input impedance and pressure wave forms in normal humans.

The effects of supine bicycle exercise on the input impedance of the ascending aorta were studied in thirteen male subjects undergoing cardiac catheterization, but in whom no cardiovascular disease was found. Ascending aortic flow velocity and pressure were recorded simultaneously from a multisensor catheter with an electromagnetic flow velocity probe and a pressure sensor mounted at the same location. A second pressure sensor at the catheter tip provided left ventricular pressure. Fick cardiac outputs were used to scale the velocity signal to instantaneous volumetric flow. Input impedance was calculated from 10 harmonics of aortic pressure and flow. For each subject, impedance moduli and phases from a minimum of 15 beats during rest and exercise were averaged. Peripheral resistance decreased from a resting value of 1142 +/- 51 (+/- SE) dynes sec/cm5 to 712 +/- 39 dynes sec/cm5 during exercise. Characteristic impedance remained unchanged with a resting value of 47 +/- 4 dynes sec/cm5 and an exercise value of 45 +/- 4 dynes sec/cm5. These results were associated with an increase in aortic pressure (96 +/- 2 to 111 +/- 2 mm Hg) and pulse wave velocity implying a decrease in aortic compliance. An increase in aortic cross-sectional area apparently offsets the effects of these changes in compliance and pulse wave velocity to result in an unchanged characteristic impedance. The increase in pulse wave velocity caused wave reflections to occur earlier during exercise, but the general characteristics of the pressure wave shapes remained unchanged.

Adult↗
PubMed1980

Dynamics of left ventricular ejection in obstructive and nonobstructive hypertrophic cardiomyopathy.

The purpose of this study was to examine the dynamics of left ventricular ejection in patients with obstructive and nonobstructive hypertrophic cardiomyopathy (HCM). 30 patients with HCM and 29 patients with no evidence of cardiovascular disease were studied during cardiac catheterization. Using a single multisensor catheter, electromagnetically derived ascending aortic flow velocity and high fidelity left ventricular and aortic pressures were recorded during rest (n = 47) and provocative maneuvers (n = 23). Dynamic ventricular emptying during rest was also analyzed with frame-by-frame angiography (n = 46). Left ventricular outflow was independently derived from both flow velocity and angiographic techniques. The HCM patients were subdivided into three groups: (I) intraventricular gradients at rest (n = 9), (II) intraventricular gradients only with provocation (n = 12), and (III) no intraventricular gradients despite provocation (n = 9). During rest, the percentage of the total systolic ejection period during which forward aortic flow existed was as follows (mean +/- 1 SD): group I, 69 +/- 17% (flow), 64 +/- 6% (angio); group II, 63 +/- 14% (flow), 65 +/- 6% (angio); group III, 61 +/- 16% (flow), 62 +/- 4% (angio); control group, 90 +/- 5% (flow), 86 +/- 9% (angio). No significant difference was observed between any of the HCM subgroups, but compared with the control group, ejection was completed much earlier in systole independent of the presence or absence of intraventricular gradients. These results suggest that "outflow obstruction," as traditionally defined by the presence of an abnormal intraventricular pressure gradient and systolic anterior motion of the mitral valve, does not impede left ventricular outflow in HCM.

Blood Flow Velocity↗
PubMedDate not supplied

A comparison of digital and analog methods of Doppler spectral analysis for quantifying flow.

Ultrasonic methods can be used for calculating flow when the mean Doppler frequency is representative of spatial average velocity. We have examined the capabilities of two commercially available methods of Doppler spectral analysis for providing measurements of spatial average velocity and flow. In a steady state flow model, Doppler audio spectra were recorded using a 5-MHz duplex scanner. Fast Fourier transform (FFT) spectral analysis was used to determine mean (M), mode (MO), and maximum (MAX) frequencies. An analog method (offset zero crossing detector = ZC) was used to determine root mean square (RMS) frequencies. The results of comparing Doppler flow estimates (QM, QMO, QMAX and QRMS) with direct flow measurements (n = 10; range = 128-1098 ml/min) were (1) QM = 0.67Q + 23 ml/min (SEE = 36 ml/min); (2) QMO = 0.96Q + 152 ml/min (SEE = 32 ml/min); (3) QMAX = 1.19Q + 171 ml/min (SEE = 23 ml/min); and (4) QRMS = 0.93Q + 76ml/min (SEE = 92 ml/min). Estimates of flow using M and RMS frequencies were adversely affected by experimental conditions likely to result in turbulence. We conclude that application of commercially available FFT determined M frequencies could result in significant errors in calculations of spatial average velocity and flow. Alternatively, FFT determined MO frequencies and ZC determined RMS frequencies resulted in accurate estimates of flow in this model. This study demonstrates the importance of evaluating the capabilities of commercially available methods of Doppler spectral analysis when using ultrasound for determining velocity and flow.

Blood Flow Velocity↗
PubMedDate not supplied

NMR imaging experiments for the verification of stochastic transport theory.

Nuclear magnetic resonance (NMR) imaging techniques were investigated as a means to obtain velocity field measurements for the verification of a stochastic transport theory. A modified stimulated-echo pulse sequence was used to make velocity images in an aperiodic heterogeneous porous medium. Construction of the velocity covariance from measurements of the fluctuating velocity has demonstrated that nuclear magnetic resonance imaging is a valuable tool for obtaining experimental data for the examination of existing stochastic transport theory.

Computer Simulation↗