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The change of alertness influences F-wave amplitude and persitence in humans
- Jan H. Baars, Benno Rehberg (2 June 2006)
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Jan H. Baars Department of Anaesthesiology, Campus Mitte, Charité-Universitätsmedizin Berlin-, Benno Rehberg
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Dear Editor, we would like to thank Drs. Kakinohana and Sugahara for their well conducted study that points out to what extend the level of consciousness influences F-waves during propofol sedation in humans [1]. At first glance, their results seem to contradict the results of our study on F- wave during propofol sedation [2], where we have shown that the F-wave amplitude shows a gradual attenuation but not a sudden decrease around the change of consciousness. How can this difference be explained? Kakinohana et al. measured the F-waves on the abductor pollicis whereas we measured the F-wave of the abductor hallucis. It cannot be excluded that the motoneuron pool of the upper limb is subject to stronger supraspinal control than that of the lower limb. Another fundamental difference between both studies is the used study design. Kakinohana et al. intentionally changed the patients’ level of consciousness by a mild physical stimulus and a verbal command whereas we tried to keep it constant (during consciousness) by regularly giving verbal commands asking the patients whether they could still hear us. Similar to Kakinohana and Sugahara, we have also observed in a pilot study an abrupt increase of F-wave amplitude upon arousal after propofol sedation. This has been avoided in our study by regular verbal commands during decreasing propofol concentrations (and close to the loss of consciousness) which allowed a more gradual wake up which is also reflected by the BIS values (Fig.6 of reference [2]). The fact that in the study of Kakinohana and Sugahara the BIS value was instantly increased above 90 after the mild wake up of tugging the earlobe indicates that propofol has probably induced “natural” sleep in the patients. Taking together the results of Kakinohana’s study and those of our study one can conclude that propofol has two different effects on F-waves: First, it induces “sleep” and secondary to that it also reduces spinal excitability, presumably by supraspinal, descending inhibitory input. As demonstrated by Kakinoana and Sugahara this effect is reversible as long as the patient can be woken up. It would have been interesting to see the time course of the F-waves and the BIS after the arousal since parallel time courses would be a further argument for supraspinal effects. Second, our results have shown a concentration-dependent decrease of F- wave amplitude and persistence by propofol under conditions that tried to exclude additional effects caused by induced sleep as far as possible. This effect could be caused by a direct spinal action of propofol or another effect on higher centres of motor control. It appears rational to assume that both effects interact synergistically especially at propofol concentrations above those causing the loss of consciousness. To give further evidence that changes of alertness measured by the BIS also influences the F-wave amplitude, we registered F-waves and the BIS (Version 3.30) in 4 awake volunteers not familiar with the aims of the study. We employed the same protocol for F-wave stimulation and recording as Kakinohana and Sugahara. During the "eyes open" measurement the volunteers sat still in an armchair with their eyes open reading or talking to the experimenter. The arm on which F-waves were recorded was relaxed and loosely fixed on an arm-rest. During the "eyes open" measurement the volunteers had their eyes closed and were listening to a relaxing tape (natural sounds from the rain forest). When the BIS values were below 92 the recording commenced. Under both conditions 125 F-waves values were collected in each volunteer. The average values (SE) during the "eyes closed" state were 86.8µV(0.56), 103(5.0),0.64 (0.009)and 96.8 (0.21),227µV (8.4), 0.85 (0.008) during the "eyes open" state for BIS, F- amplitude, F-persistence, respectively. This corresponds to an average increase of 11.5%, 120%, 32% for BIS, F-amplitude, F-persistence, respectively. These results are in accord with Bathien and Morin [3] who have shown in awake subjects that the H-reflex as another parameter of motoneuron excitability increases during alertness, at least when the level of attention is high. Reference List 1. Kakinohana M, Sugahara K: Level of consciousness affects the excitability of spinal motor neurones during propofol sedation in humans. Br.J Anaesth. 2006; 96: 742-6 2. Baars JH, Tas S, Herold KF, Hadzidiakos DA, Rehberg B: The suppression of spinal F-waves by propofol does not predict immobility to painful stimuli in humans. Br.J Anaesth. 2006; 96: 118-26 3. Bathien N, Morin C: Comparing variations of spinal reflexes during intensive and selective attention (author's transl). Physiol Behav. 1972; 9: 533-8 Conflict of Interest:None declared |
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