A model to predict survival following liver retransplantation.

In the current era of critical-organ shortage, one of the most controversial questions facing transplantation teams is whether hepatic retransplantation, which has historically been associated with increased resource utilization and diminished survival, should be offered to a patient whose first allograft is failing. Retransplantation effectively denies access to orthotopic liver transplantation (OLT) to another candidate and further depletes an already-limited organ supply. The study group was comprised of 1,356 adults undergoing hepatic retransplantation in the United States between 1990 and 1996 as reported to the United Network for Organ Sharing (UNOS). We analyzed numerous donor and recipient variables and created Cox proportional-hazards models on 900 randomly chosen patients, validating the results on the remaining cohort. Five variables consistently provided significant predictive power and made up the final model: age, bilirubin, creatinine, UNOS status, and cause of graft failure. Although both hepatitis C seropositivity and donor age were significant by univariate and multivariate analyses, neither contributed independently to the estimation of prognosis when added to the final model. The final model was highly predictive of survival (whole model chi2 = 139.63). The risk scores for individual patients were calculated, and patients were assigned into low-, medium-, and high-risk groups (P <.00001). The low degree of uncertainty in the probability estimates as reflected by confidence intervals, even in our high-risk patients, underscores the applicability of our model as an adjunct to clinical judgment. We have developed and validated a model that uses five readily accessible "bedside" variables to accurately predict survival in patients undergoing liver retransplantation.

Detection and discrimination of frequency glides as a function of direction, duration, frequency span, and center frequency.

The study investigated the ability to detect and discriminate frequency glides under a variety of experimental conditions. The subjects distinguished between a comparison signal that either was level in frequency or was swept across a fixed frequency span, and a target signal that changed more in frequency than the comparison signal. Tone durations were 50 and 400 ms. Nominal center frequencies were 0.5, 2, and 6 kHz; actual center frequencies were varied randomly, or roved, over a range equal to 0.1 times the nominal center frequency. Up- and down-glides were used. The transition span of the comparison signal was either 0, 0.5, 1, or 2 times the equivalent rectangular bandwidth of the auditory filter at the nominal center frequency. Discrimination thresholds were obtained for all combinations of center frequency, direction, and span. Overall, thresholds expressed as delta Hz/ERB varied little as a function of center frequency. Glide duration had no effect on discrimination. The 50-ms down-glides were more difficult to detect than the 50-ms up-glides; otherwise, the effect of direction was not significant. With the exception of the 50-ms down-glides, detection/discrimination thresholds were similar for the 0-, 0.5-, and 1-ERB transition spans, but increased significantly for the 2-ERB span. The absence of significant variation across frequency supports a place mechanism for the detection of frequency change in gliding tones, based on the detection of changes in the excitation pattern. An excitation pattern model cannot account for the asymmetry noted for glide detection, however.

Detection and discrimination of gliding tones as a function of frequency transition and center frequency.

Two experiments were performed to investigate subjects' ability to detect and discriminate 50-ms up-glides in frequency in several different conditions. In the first experiment, the subjects indicated which of two signals increased more in pitch. The comparison, or standard signal, was a sinusoid which increased in frequency by either 0, 250, or 500 Hz. Center frequencies were 0.5, 1, 2, 4, and 6 kHz. Discrimination thresholds were obtained in both nonroved and roved conditions. In the roved condition, the actual center frequencies of the signals were varied randomly over a range equal to 0.1 times the nominal center frequency. The second experiment was the same as the first, except that the standard signals were swept over a frequency range equal to 0.5, 1, and 2 times the equivalent rectangular bandwidth (ERB) of the auditory filter at the nominal center frequency. Discrimination thresholds expressed as delta Hz/ERB varied little as a function of center frequency as long as the frequency transition of the standard was a constant proportion of ERB. In addition, discrimination thresholds did not vary significantly as a function of the frequency extent of the standard when the extent was one ERB or less, but doubled when the extent was two ERBs. The relatively small amount of variation in threshold across center frequency and the pattern of variation across different standard transitions supports a place mechanism of frequency coding for these signals based on the detection of changes in the excitation pattern.

The role of frequency resolution and temporal resolution in the detection of frequency modulation.

The experiment investigated subjects' ability to detect short-duration changes in frequency. In an adaptive, 2AFC task, three normal-hearing subjects were asked to distinguish a sinusoidal signal that increased in frequency in a series of discrete steps from a standard that was identical except that its frequency increased essentially continuously. The signals were 60 ms in duration with center frequencies of 0.25, 0.5, 1, 2, 3, 4, and 6 kHz. The smallest frequency increase between steps (FI) at which the stepped signal could be distinguished from the standard was determined as a function of the number of steps in the signal. As the number of steps increased and the step duration decreased, the FI at first decreased and then reached a roughly asymptotic level. Eventually, however, at a certain number of steps, the FI increased rapidly. The data were analyzed using a model of auditory temporal resolution that included a bank of bandpass filters, a nonlinearity, a temporal integrator, and a decision device. The analysis yielded ERDs that ranged from 3.8 to 5.0 ms and did not change systematically with frequency. Detector efficiency varied considerably, being greatest at 0.5 and 1 kHz, and declining at higher and lower center frequencies.

Temporal resolution in normal-hearing and hearing-impaired listeners using frequency-modulated stimuli.

This study compares the temporal resolution of frequency-modulated sinusoids by normal-hearing and hearing-impaired subjects in a discrimination task. One signal increased linearly by 200 Hz in 50 msec. The other was identical except that its trajectory followed a series of discrete steps. Center frequencies were 500, 1000, 2000, and 4000 Hz. As the number of steps was increased, the duration of the individual steps decreased, and the subjects' discrimination performance monotonically decreased to chance. It was hypothesized that the listeners could not temporally resolve the trajectory of the step signals at short step durations. At equal sensation levels, and at equal sound pressure levels, temporal resolution was significantly reduced for the impaired subjects. The difference between groups was smaller in the equal sound pressure level condition. Performance was much poorer at 4000 Hz than at the other test frequencies in all conditions because of poorer frequency discrimination at that frequency.

Validity of the 24-hr. dietary recall and seven-day record for group comparisons.

The internal validity of a 24-hr. dietary recall and a seven-day dietary record was investigated among a group of non-institutionalized elderly subjects who were participating in a congregate meals program. Internal validity was assessed by comparing reported intake with unobtrusively obtained data on actual intake. Validity results suggest that the recall is prone to over-reporting low intakes and under-reporting high intakes. This pattern has been referred to as the "flat-slope syndrome." Records collected during the first few dyas were less prone to this syndrome; however, validity declined by the fifth, sixth, and seventh record days. Also, as the record progressed to the seventh day, the demographic nature of the sample became biased due to drop-outs and decreased usability of the records.

Validity of the 24-hr. recall. Analysis of data obtained from elderly subjects.

Tests of the validity of the 24-hr. dietary recall were done by comparing actual with recalled intakes for eight nutrients and the MAR (mean adequacy ratio) for a sample of seventy-six subjects age sixty years or older. Validity was tested by using paired-t tests and regression analysis. In the paired-t test, no significant difference was found between the mean recalled and the mean actual intake of nutrients, with the exception of calories. Using regression analysis, results indicated that for three of the eight nutrients considered (calories, protein, and vitamin A), small intakes tend to be over-reported and large intakes under-reported (p less than .05). Thus, for these three nutrients, the recall seems to be statistically conservative for group comparisons; it would seldom, if ever, indicate a difference in intake where no difference exists. But, it could yield a false negative, i.e., an indication of no significant difference, when, in fact, a difference does exist. Clearly, more research is needed, both to replicate this study and to develop techniques with greater internal validity for comparing the dietary intakes of groups.