Objective The dramatic spread of SARS-CoV-2 infections calls for reliable, inexpensive tools to quickly identify patients with a poor prognosis. In this study, acute respiratory distress syndrome (ARDS) was assessed within 72 h after admission of each of 153 consecutive, SARS-CoV-2 infected, adult patients to either of two hospitals in Tenerife, Spain, using suitable routine laboratory tests for lymphocyte counts, as well as ferritin, lactate dehydrogenase (LDH), and C-reactive protein levels. Results were correlated with the patients’ respiratory function, defined through their pulse oximetric saturation/fraction of inspired oxygen (SpO2/FiO2) ratio. Results Within 72 h from admission, criteria matched ARDS (SpO2/FiO2 < 235) in 13.1% of cases. We found a significant, negative correlation between SpO2/FiO2 ratios and d-dimer, ferritin, and LDH levels (− 0.31, − 0.32, and − 0.41; p = 0.004, 0.004, and < 0.0001, respectively). In patients with ARDS, the mean LDH was 373 U/L (CI95%: 300.6–445.3), but only 298 U/L (CI95%: 274.7–323.1) when they did not develop the syndrome (p = 0.015). None of the additionally evaluated biomarkers correlated with the SpO2/FiO2 ratios. Serum LDH levels in patients hospitalised for COVID-19 correlate with ARDS, as defined by their SpO2/FiO2 ratio, and might help to predict said complication.

Under intermittent food schedules animals develop temporally organized behaviors throughout interfood intervals, with behaviors early in the intervals (interim) normally occurring in excess. Schedule-induced drinking (a prototype of interim, adjunctive behavior) is related to food deprivation and food frequency. This study investigated the interactions that resulted from combining different food-deprivation levels (70%, 80% or 90% free-feeding weights) with different food-occurrence frequencies (15-, 30- or 60-s interfood intervals) in a within-subjects design. Increases in food deprivation and food frequency generally led to increased licking, with greater differences due to food deprivation as interfood intervals became shorter. Distributions of licking were modestly shifted to later in the interfood interval as interfood intervals lengthened, a result that was most marked under 90% food deprivation, which also resulted in flatter distributions. It would therefore appear that food deprivation modulates the licking rate and the distribution of licking in different ways. Effects of food deprivation and food frequency are adequately explained by a theory of adjunctive behavior based on delayed food reinforcement, in contrast to alternative hypotheses.

Three food-deprived rats (80% of their free-feeding weights) developed schedule-induced drinking after being exposed to a multiple fixed-time schedule (FT 60-s FT 18-s) of food-pellet presentation. A 3-s signaled delay was then initiated by each lick, and the rate of licking was reduced to a much greater extent in the FT 18-s component in two rats. With these rats, a 9-s lickdependent signaled delay then occurred in the FT 60-s component only, and a reduction was observed in licks per minute similar to that observed previously with the 3-s delay in the richer component. With the third rat the delays which were effective in reducing licking were 6 and 18 s in the FT 18-s and FT 60-s components, respectively. Measures of the percentage of interfood intervals with at least one lick produced less pronounced effects. These results suggest that the ratio between delay length and interfood interval length is critical for lick-dependent delays to be effective in punishing schedule-induced drinking. Falk (1961 ) found that food-deprived rats exposed to a variableinterval schedule (VI 1-min) of food reinforcement drank excessive amounts of water concurrently with their performance of the operant task. This behavior is unusual because the rats were not deprived of water and no contingency was arranged between their drinking and the delivery of food. Falk (1971) suggested that this schedule-induced polydipsia was the prototype of a behavioral class named "adjunctive behavior;' different from operant behavior. The amount of schedule-induced behavior is related to the parameters that define the food reinforcer, such as its magnitude or quality, the rate at which food is presented, or the animal's level of food deprivation (see reviews by Pell6n, 1992;Reid & Staddon, 1990;Wetherington, 1982).

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