Increased low amplitude delta waves may be related to septic encephalopathy. In the study by Cooper et al, septic encephalopathy was thought less likely due to negative blood culture results. However, in the study by Gabor et al, five of seven patients had positive blood culture findings, but SWS was only 2.7% of TST. Many of our patients had positive culture results (all sources), but only one patient had a positive blood culture result; no correlation existed with mDelta activity. It is well supported in the literature that 50% of patients admitted to the ICU with pneumonia do not have positive culture results, particularly if they have received antibiotics. Therefore, systemic inflammatory response to infection despite negative blood culture results cannot be eliminated. Cytokines released, specifically interleukin (IL)-1 and tumor necrosis factor (TNF), lead to an increase in delta activity. there
IL-1 and TNF act in the thermoregulatory area of the hypothalamus and induce a pyogenic and som-nogenic effect. Delta activity occurs in response to an increase in cerebral or body temperature. Although no correlation of temperature (fever) with mDelta was found, the majority of patients were admitted with fever, pneumonia, ARDS, and likely sepsis syndrome. We did not draw IL-1 or TNF levels, which may have been helpful in correlating delta activity. Delta activity in normal subjects is strongly related to prior wakefulness and sleep deprivation (homeostatic need for sleep). It is unknown how much sleep is required during illness and whether increased delta activity may be due to homeostatic need for restorative sleep.
Serial EEG analysis throughout illness would be helpful to reevaluate delta amplitude and recovery.
It is well documented that SWS declines with aging. However, numerous studies using frequency and period amplitude analysis indicate that the amount of delta activity is actually preserved but that wave amplitude decreases significantly with age. Although it was not statistically significant, there was a 5% increase in delta activity in the CS/NMBA group using the mDelta amplitude criteria compared with standard criteria of Rechtschaffen and Kales (Table 3). This difference may reflect the effect of age in the CS/NMBA group. Scoring with traditional Rechtschaffen and Kales criteria may underestimate SWS in middle-aged and older populations. Some researchers have suggested that traditional amplitude criteria be abandoned and frequency scoring be implemented.