This prospective cohort study clearly demonstrates the association of delirium with the increase in length of stay and mortality in a nonventilated elective and emergency SICU population. Additionally, we identified the main risk factors associated with delirium in this population.
The CAM tool was preferred, because it was already validated in Portuguese  and it is seen to be noninferior for the diagnosis of delirium compared with other tools, including the CAM-ICU in nonventilated patients .
In this study, the delirium incidence was low (9.2%), although the population studied was predominantly elderly, and aging groups are clearly at risk for the development of delirium [19, 20]. Low severity scores and short lengths of stay in our population most likely contributed to a reduced delirium incidence.
Most delirium episodes occurred in the first 3 days of admission (91%), which is similar to the findings in mechanically ventilated medical ICU patients . Interestingly, in our population, delirium occurred most frequently between the second and third day, but not on the first day, which could be expected given the chronological proximity of the surgical and anesthetic procedures. However, this timeline for development of delirium is consistent with the peak levels of inflammatory mediators in the postoperative period, such as interleukin-6 (which peaks after 24 h) and C-reactive protein (which peaks after 48 h) . It also has been demonstrated that patients with the greatest increases in inflammatory mediators are more likely to experience postoperative delirium .
There was no significant relationship between the site of surgery and the occurrence of delirium. This finding may be explained by the small sample sizes of the different types of surgery, which may prevent a more accurate comparison. However, previous studies demonstrated an association between large cardiovascular and orthopedic procedures and the development of delirium [11, 23].
We observed that the development of delirium at any time during the postoperative ICU hospital stay impairs the prognosis of SICU patients. Those who developed delirium during the SICU stay had a significantly higher mortality in both the SICU and the hospital compared to patients without delirium. This finding is similar to that described in both clinical  and surgical  patients. The occurrence of delirium increased the probability of death related to the APACHE II score. This impact was higher in terms of absolute numbers in patients with higher APACHE II scores; however, the lowest scores had the greatest proportional increase in the probability of death, as shown in Figure 3.
Among the risk factors analyzed for the occurrence of delirium, age (each year) and APS (each point) were the most important, as shown by the highest proportional relative risks. Others factors had a large variation in the CI, which may be due to the small sample size and should be interpreted with caution.
The management of delirium in the SICU includes early detection and enforcement of nonpharmacological control [24–26]. One recent study showed a benefit of using perioperative antipsychotics in preventing delirium; however, there is concern about the increase in the incidence of hypoactive delirium as a result of this practice .
Because the development of delirium has a major impact in the SICU population, risk factors and time to delirium occurrence described in the present study can help in the development of preventive actions against the development of delirium, enabling better resource allocation, providing useful family information and determining more accurate evaluation of outcomes in postoperative patients.
This study presents some limitations, including the fact that this was a single-center study. The intraoperative data were not collected systematically, but data from the literature regarding anesthesia are conflicting, and it is unclear whether the type of anesthesia affects the development of delirium . Patients with RASS −3 were excluded contributing to the lower incidence of delirium in this population. There have been changes in medical and surgical care since data collection, but recent surveys show that delirium recognition and preventive practices in SICU patients must still be improved [7, 8]. Patients were not followed up after hospital discharge to determine subsequent mortality or cognitive dysfunction.