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Biomedical subjects

J R Ligas

Publications and source records attributed to J R Ligas.

11 recordsLinked to original sources
PubMed1997

Experiences as preceptor to acute-care nurse practitioner students: one physician's view.

The role of the physician preceptor is determined by the needs of the individual student and the curriculum preceding the practicum. The preceptor must be prepared to listen to the student, determine the student's level of understanding, and begin at that point. The didactic curriculum must provide the students with an in-depth understanding of pathophysiology. The students must spend more time addressing real patient problems, whether at the bedside or in problem-based learning sessions.

Attitude of Health Personnel↗
PubMed1993

Enhancement of urine output and glomerular filtration in acutely oliguric patients using low-dose norepinephrine.

A prospective clinical trial was initiated to test the hypothesis that low-dose norepinephrine enhances urine output and renal function in oliguric surgical patients. Norepinephrine (0.05 or 0.1 micrograms/kg/min) was infused into nine oliguric (< or = 0.5 ml/kg/hr), volume-replete, hemodynamically stable patients. There was an average increase of urine output of 13 ml/hr (48% over baseline) and an increase of 31.7 ml/min (47% over baseline) in creatinine clearance. Both these results were statistically significant. Mean arterial pressure was the only hemodynamic parameter that changed significantly, rising an average of 12 mm Hg. We conclude that low-dose norepinephrine infusion may enhance renal function and urine output in acutely oliguric surgical patients who have been appropriately fluid resuscitated.

Acute Disease↗
PubMed1989

The unsteady form of the Bernoulli equation for estimating pressure drop in the airways.

Previous investigators have used the steady Bernoulli equation to correct the underestimation of alveolar pressure by airway pressure observed during unsteady flow conditions. Using a simple idealized geometry, we demonstrate a method of including the unsteady term of the Bernoulli equation. Our approach was to determine the axial velocity distribution from the wave equation and employ this solution with the unsteady Bernoulli equation. In addition, we estimate the importance of inertia and viscosity and show that these effects can be quantified by the Womersley number.

Air Pressure↗
PubMed1986

Sepsis due to triple lumen central venous catheters.

We prospectively studied 96 consecutive catheterizations in 65 patients who required triple lumen central venous catheters. The incidence of catheter associated bacteremia was 3.1 per cent. Staphylococcus epidermidis was the organism involved in all instances.

Catheters, Indwelling↗
PubMed1985

Parameter estimation and sensitivity analysis of a nonlinearly elastic static lung model.

A model for the static pressure-volume behavior of the lung parenchyma based on a pseudo-elastic strain energy function was tested. Values of the model parameters and their variances were estimated by an optimal least-squares fit of the model-predicted pressures to the corresponding data from excised, saline-filled dog lungs. Although the model fit data from twelve lungs very well, the coefficients of variation for parameter values differed greatly. To analyze the sensitivity of the model output to its parameters, we examined an approximate Hessian, H, of the least-squares objective function. Based on the determinant and condition number of H, we were able to set formal criteria for choosing the most reliable estimates of parameter values and their variances. This in turn allowed us to specify a normal range of parameter values for these dog lungs. Thus the model not only describes static pressure-volume data, but also uses the data to estimate parameters from a fundamental constitutive equation. The optimal parameter estimation and sensitivity analysis developed here can be widely applied to other physiologic systems.

Animals↗
PubMed1984

Static mechanics of excised whole lung: theoretical framework and experimental studies.

A theoretical framework is presented in which to view models of static pulmonary mechanics. To test common simplifying assumptions of these models, we performed a set of experiments using normal lungs excised from dogs. Transpulmonary pressure (Ptp) and lung volume (VL) were measured for air-filled lungs in air and saline-filled lungs in saline during stepwise-static deflations at different vascular volumes and temperatures. Simultaneously, we measured displacements between points on the lung surface. Changes in vascular volume shift the location but not the shape of the Ptp-VL relationship. As long as the vascular pressure is in the normal range, changes in the volume (and weight) of the perfusate do not significantly stiffen the parenchyma. Furthermore, Ptp-VL data obtained between 16 degrees C and 40 degrees C were superimposable, indicating that parenchymal mechanical properties evaluated at room temperature are valid at body temperature. Finally, the common assumptions of uniform deflation, homogeneity, and isotropy of bulk lung tissue appear consistent with the relationship between surface displacement and volume changes.

Animals↗
PubMed1984

Static mechanics of excised whole lung: pleural mechanics.

Continuum analyses of lung mechanics require that the boundary condition of stress transmitted to the outermost alveoli be known. Depending upon the exact geometry of the pleural-parenchymal coupling, this stress could possibly be influenced by the pleural mechanical properties. The relation between pleural tension and extension ratio was obtained from tissue specimens from mongrel dog lungs. Using the worst-case geometry, this relationship was compared with the equivalent relation between pressure and volume ratio for the whole lung of the same mongrel dogs. The results of this comparison and a suitable mathematical analysis indicate that the pleura transmits applied pressure differences to the underlying alveolar walls essentially without modification.

Animals↗
PubMed1984

A non-linearly elastic, finite deformation analysis applicable to the static mechanics of excised lungs.

Continuum mechanical analyses of lung behavior require a constitutive relationship for the parenchyma. For a homogeneous, isotropic, elastic material this relationship is dependent upon the strain invariants. Furthermore, the sparse data available indicate that the relationship should be exponential in form. A suitable equation has previously appeared in the literature. In this paper it is developed for application to whole lung experiments.

Biomechanical Phenomena↗
PubMed1977

Comparison of measures of forced expiration.

Theoretical relationships among a number of parameters were derived for idealized timed vital capacity (TVC) and maximal expiratory flow-volume (MEFV) curves to determine a minimal set of independent parameters. Normal pediatric subjects and those with cystic fibrosis were studied to verify these relationships experimentally. The average flow over the middle half (FEF25-75%) of the forced vital capacity (FVC) and flows at various exhaled percentages of the FVC (FEF50%, FEF75%), as well as moments of the TVC and MEFV curves were computed. From the TVC moments, a mean transit time (MTT) and an index of dispersion (ID) were also calculated. The minimum information needed to detect pulmonary mechanical changes associated with obstructive lung disease requires at least two reproducible measures: one related to the mean slope (e.g., FEF25-75%/FVC or MTT) and the other to the shape (e.g;, ID) of the effort-independent portion of the MEFV curve.

Adolescent↗
PubMedDate not supplied

Continuous blood density measurement for hemodynamic monitoring: an analysis of its accuracy and sensitivity.

Continuous measurement of arterial blood density after bolus injection of fluids of different density into the right atrium has been used to measure cardiac output and mean transit time through the central circulation. The transit time distribution for density, however, differs from that for plasma-phase tracers such as indocyanine green. This difference may yield important information about red cell transit times through the microcirculation. We analyzed the potential of the density technique to resolve small changes in transit time distributions. Rayleigh's Method was used to calculate the relationship between density distribution within the U-tube and frequency of oscillation. Fourier integral transformation of a functional representation of indocyanine green dye curves provided an estimate of amplitude versus frequency for likely input density signals. We found that the ability of the densitometer to accurately follow blood density changes depends upon physiologic parameters associated with the experimental animal and upon the physical characteristics of the densitometer itself. Even for small animals, such as a rabbit, the densitometer theoretically has the ability to accurately follow rapid density changes over time.

Animals↗