K-complexes, spindles, and ERPs as impulse responses: unification via neural field theory.

To interrelate K-complexes, spindles, evoked response potentials (ERPs), and spontaneous electroencephalography (EEG) using neural field theory (NFT), physiology-based NFT of the corticothalamic system is used to model cortical excitatory and inhibitory populations and thalamic relay and reticular nuclei. The impulse response function of the model is used to predict the responses to impulses, which are compared with transient waveforms in sleep studies. Fits to empirical data then allow underlying brain physiology to be inferred and compared with other waves. Spontaneous K-complexes, spindles, and other transient waveforms can be reproduced using NFT by treating them as evoked responses to impulsive stimuli with brain parameters appropriate to spontaneous EEG in sleep stage 2. Using this approach, spontaneous K-complexes and sleep spindles can be analyzed using the same single theory as previously been used to account for waking ERPs and other EEG phenomena. As a result, NFT can explain a wide variety of transient waveforms that have only been phenomenologically classified to date. This enables noninvasive fitting to be used to infer underlying physiological parameters. This physiology-based model reproduces the time series of different transient EEG waveforms; it has previously reproduced experimental EEG spectra, and waking ERPs, and many other observations, thereby unifying transient sleep waveforms with these phenomena.

Blood oxygen equilibration time after cessation of supplemental oxygen in chronic respiratory disease.

BACKGROUND: Measurement of the arterial partial pressure of oxygen (PaO2 ) while breathing air is an informative investigation in patients with hypoxaemia due to chronic respiratory disease, but there are a lack of published data on the time needed for blood oxygen levels to equilibrate after cessation of supplemental oxygen (O2 ) in such patients. AIM: To determine the blood oxygen equilibration time after cessation of O2 and thereby provide guidance on best timing of baseline arterial blood gas analysis in this population. METHODS: Medically stable subjects with chronic respiratory disease were administered O2 at a constant concentration. Continuous pulse oximetry was recorded from before cessation of O2 to beyond the point of oxygen saturation (SpO2 ) equilibration. Data were fitted to an exponential decay model. Blood oxygen equilibration time was defined as the t90, the time taken for SpO2 to fall 90% of the difference between initial (on O2 ) and final (on air) values. RESULTS: Eighty-two (82) subjects with a mean age of 66 years were included. The largest diagnostic category was chronic obstructive pulmonary disease (37), followed by interstitial lung disease (15) and bronchiectasis (12). The median t90 was 6 min 18 s (interquartile range: 4 min 32 s-10 min 30 s). The 95th centile t90 value was 20 min. CONCLUSION: In the majority of patients with chronic respiratory disease, a time delay of 20 min between cessation of supplemental O2 and PaO2 measurement allows confidence that the result is a true baseline value.