Sedation modulates recognition of novel stimuli and adaptation to regular stimuli in critically ill adults

Predicting clinical physiology: a Markov chain model of heart rate recovery after spontaneous breathing trials in mechanically ventilated patients

Analysis of heart rate (HR) dynamics before, during, and after a physiologic stress has clinical importance. For example, the celerity of heart rate recovery (HRR) after a cardiac stress test (eg, treadmill exercise test) has been shown to be an independent predictor of all-cause mortality. Heart rate dynamics are modulated, in part, by the autonomic nervous system. These dynamics are commonly abstracted using metrics of heart rate variability (HRV), which are known to be sensitive to the influence of the autonomic nervous system on HR. The patient-specific modulators of HR should be reflected both in the response to stress as well as in the recovery from stress. We therefore hypothesized that the patient-specific HR response to stress could be used to predict the HRR after the stress. We devised a Markov chain model to predict the poststress HRR dynamics using the parameters (transition matrix) calculated from HR data during the stress. The model correctly predicts the exponential shape of poststress HRR. This model features a simple analytical relationship linking poststress HRR time constant (T(off)) with a standard measure of HRV, namely the correlation coefficient of the Poincaré plot (first return map) of the HR recorded during the stress. A corresponding relationship exists between the time constant (T(on)) of R-R interval decrease at the onset of stress and the correlation coefficient of the Poincaré plot of prestress R-R intervals. Consequently, the model can be used for the prediction of poststress HRR using the HRV measured during the stress. This direct relationship between the event-to-event microscopic fluctuations (HRV) during the stress and the macroscopic response (HRR) after the stress terminates can be interpreted as an instance of a fluctuation-dissipation relationship. We have thus applied the fluctuation-dissipation theorem to the analysis of heart rate dynamics. The approach is specific neither to cardiac physiology nor to transitions between mechanical and free ventilation as a specific stress. It may therefore have wider applicability to physiologic systems subject to modest stresses.

Stressful experiences in relation to depth of sedation in mechanically ventilated patients

In mechanically ventilated patients, sedatives and analgesics are commonly used to ensure comfort, but there is no documented knowledge about the impact of depth of sedation on patients' perception of discomfort. The aim of this study was, therefore, to investigate the relationship between stressful experiences and intensive care sedation, including the depth of sedation. During 18 months, 313 intubated mechanically ventilated adults admitted to two general intensive care units (ICU) for more than 24 h were included. Patients (n = 250) were interviewed on the general ward 5 days after ICU discharge using the ICU Stressful Experiences Questionnaire. Patient data including sedation scores as measured by the Motor Activity Assessment Scale (MAAS) were collected from hospital records after the interview. Of the 206 patients with memories of the intensive care, 82% remembered at least one experience as quite a bit or extremely bothersome. Multivariate analyses showed that higher proportion of MAAS score 3 (indicating more periods of wakefulness), longer ICU stay and being admitted emergent were factors associated with remembering stressful experiences of the ICU as more bothersome. The findings indicate that the depth of sedation has an impact on patients' perception of stressful experiences and that light sedation compared with heavy seems to increase the risk of perceiving experiences in the ICU as more bothersome. In reducing discomfort, depth of sedation and patient comfort should be assessed regularly, non-pharmacological interventions taken into account and the use of sedatives and analgesics adapted to the individual requirements of the patient.

Memory in relation to depth of sedation in adult mechanically ventilated intensive care patients

OBJECTIVE

To investigate the relationship between memory and intensive care sedation.

DESIGN AND SETTING

Prospective cohort study over 18[Symbol: see text]months in two general intensive care units (ICUs) in district university hospitals.

PATIENTS

313 intubated mechanically ventilated adults admitted for more than 24 h, 250 of whom completed the study.

MEASUREMENTS

Patients (n=250) were interviewed in the ward 5 days after discharge from the ICU using the ICU Memory Tool. Patient characteristics, doses of sedative and analgesic agents, and sedation scores as measured by the Motor Activity Assessment Scale (MAAS) were collected from hospital records after the interview.

RESULTS

Patients with no recall (18%) were significantly older, had higher baseline severity of illness, and experienced fewer periods of wakefulness (median proportion of MAAS score 3; 0.37 vs. 0.70) than those who had memories of the ICU (82%). Multivariate analyses showed that increasing proportion of MAAS 0-2 and older age were significantly associated with having no recall. Patients with delusional memories (34%) had significantly longer ICU stay (median 6.6 vs. 2.2 days), higher baseline severity of illness, higher proportions of MAAS scores 4-6, and more administration of midazolam than those with recall of the ICU without delusional memories.

CONCLUSIONS

Heavy sedation increases the risk of having no recall, and longer ICU stay increases the risk of delusional memories. The depth of sedation during total ICU stay as recorded with the MAAS may predict the probability of having memories of the ICU.

The effect of interruption to propofol sedation on auditory event-related potentials and electroencephalogram in intensive care patients

Introduction

In this observational pilot study we evaluated the electroencephalogram (EEG) and auditory event-related potentials (ERPs) before and after discontinuation of propofol sedation in neurologically intact intensive care patients.

Methods

Nineteen intensive care unit patients received a propofol infusion in accordance with a sedation protocol. The EEG signal and the ERPs were measured at the frontal region (Fz) and central region (Cz), both during propofol sedation and after cessation of infusion when the sedative effects had subsided. The EEG signal was subjected to power spectral estimation, and the total root mean squared power and spectral edge frequency 95% were computed. For ERPs, we used an oddball paradigm to obtain the N100 and the mismatch negativity components.

Results

Despite considerable individual variability, the root mean squared power at Cz and Fz (P = 0.004 and P = 0.005, respectively) and the amplitude of the N100 component in response to the standard stimulus at Fz (P = 0.022) increased significantly after interruption to sedation. The amplitude of the N100 component (at Cz and Fz) was the only parameter that differed between sedation levels during propofol sedation (deep versus moderate versus light sedation: P = 0.016 and P = 0.008 for Cz and Fz, respectively). None of the computed parameters correlated with duration of propofol infusion.

Conclusion

Our findings suggest that use of ERPs, especially the N100 potential, may help to differentiate between levels of sedation. Thus, they may represent a useful complement to clinical sedation scales in the monitoring of sedation status over time in a heterogeneous group of neurologically intact intensive care patients.