British Journal of Anaesthesia 1992; 68: 325-329
CORRESPONDENCE
D. V. THOMAS
Los Altos, California REFERENCES 1. Anderton JM. The prone position for the surgical patient: a historical review of the principles and hazards. British Journal of Anaesthesia 1991; 67: 452-463. 2. Albin MS, Ritter RR, Pruett CE, Kalff K. Venous air embolism during lumbar laminectomy in the prone position: report of three cases. Anesthesia and Analgesia 1991; 73: 346-349. Sir—Thank you for inviting me to comment on the letter from Dr Thomas. Attention is quite rightly drawn to the fact that a low central venous pressure is to be found in patients placed on the "Tarlov Seat". The gravitational effect of venous blood in abdominal viscera hanging below heart level may enhance this. The cases described were, however, at an extreme end of the spectrum of surgical and anaesthetic difficulty. All were revision procedures requiring fusion, blood loss was excessive in all and the operation times when the incident occurred were 5 h in two patients and.3.5 h in the third. I think we Should be careful not to make general recommendations for all patients placed in this position on the basis of these extreme examples. I would accept that Doppler ultrasound monitoring could reasonably be added to routine measurements of end-tidal carbon dioxide but, unless I obtain further evidence from this or read other reports of venous air embolism related to this position, I would be reluctant to add either invasive central venous pressure or direct arterial monitoring to my usual routine. The latter suffices well for single level lumbar discectomy, at which blood loss rarely exceeds 150-200 ml in an operating time of 1.5-2 h. As always, complex cases will justify more careful monitoring which could include a thermodilution catheter [1]. J. M. ANDERTON
Manchester REFERENCE 1. Backofcn JE, Schauble JF. Hemodynamic changes with prone position during general anesthesia. Anesthesia and Analgesia 1985; 64: 194.
CONSCIOUS AWARENESS Sir,—Jessop and Jones [1] stated that the origins of a voluntary act start in the unconscious. To support their claim, they quote the work of Benjamin Libet [2—4] who, in one of his experiments, asked neurosurgical patients to introspect and then, stimulating the medial lemniscus, showed that normal evoked potentials could be obtained without conscious awareness. In a later experiment [5], Libet found that readiness potentials (potentials associated with the initiation of movement) were generated before the " urge to move" was noted in subjects who were asked to introspect whilst he monitored EEG and EMG. Libet was looking at spontaneous movement and he assumed that this was related to unconscious movement. Cotterill used Libet's work to propose that conscious awareness is not necessary for the execution of a simple reaction task [6]. Some of Libet's work has been repeated by Keller and Heckhausen [7], who showed that instructing a volunteer to introspect changed the site of production of the readiness potential from the lateral premotor system in the primary auditory cortex (unconscious acts) to the supplementary motor area (voluntary spontaneous acts). This implies, however, that the subjects in Libet's study were "primed" by the instruction to introspect. Consequently, interpretation of his result is made more difficult and one may not need to invoke an unconscious initiation of acts. In fact, Keller and Heckhausen concluded that activation of the supplementary motor area and "the urge to move" occur at the same time. The implications for Libet's earlier work are similar, in that by telling the patients to introspect he may have changed what he was trying to measure. R. MUNGLANI
Cambridge REFERENCES 1. Jessop J, Jones JG. Conscious awareness during general anaesthesia—what are we attempting to monitor? British Journal of Anaesthesia 1991; 66: 635-637. 2. Libet B, Alberts WW, Wright EW, Delartrc LD, Levin G, Fenstein B. Production of threshold levels of conscious sensation by electrical stimulation of the human somatosensory cortex. Journal of Neurophysiology 1964; 27: 546-578. 3. Libet B, Alberts WW, Wright EW, Fenstein B. Responses of the human somatosensory cortex to stimuli below threshold for conscious sensation. Science 1967; 157: 1597-1600. 4. Libet B, Wright EW, Fenstein B, Pearl DK. Subjective referral of the timing for a conscious sensory experience. A functional role for the somatosensory specific projection system in man. Brain 1979; 102: 193-224. 5. Libet B, Gleason CA, Wright EW, Pearl DK. Time of conscious intention to act in relation to onset of cerebral activity (readiness potential). Brain 1983; 106: 623-642. 6. Cotterill R. No Ghost in the Machine. London: Heinemann, 1989. 7. Keller I, Heckhausen H. Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control. Electroencephalography and Clinical Neurophysiology 1990; 76: 351-361. Sir,—Thank you for the opportunity to reply to Dr Munglani. Our editorial focused on the hypothesis that changes in early cortical waves in the auditory evoked potential could be used as an indicator of conscious awareness. These waves, with latencies of about 50 ms, provide evidence of one aspect of cortical function, but not necessarily of cognition, and we quoted Libet and colleagues who showed, in conscious man, that these waves may occur in response to a stimulus without that stimulus reaching consciousness. Furthermore, these workers provide evidence that conscious awareness of a sensory stimulus occurred only after a delay of about 500 ms. However, as it is well known that the reaction time for a simple task may be less than 200 ms, we felt
Downloaded from http://bja.oxfordjournals.org/ at University of Lethbridge on September 9, 2015
VENOUS AIR EMBOLISM AND THE PRONE POSITION Sir,—I enjoyed reading Dr Anderton's very full review of the prone position for the surgical patient [1]. He mentioned that, whenever veins are opened at a surgical procedure above heart level, there is a "theoretical" risk of air embolism (my quotation marks), and that "the actual risk of venous air embolism seems to be very small indeed." He then quoted "... one report of a fatality in the literature, but in this patient posterior fossa surgery was being performed for a large arterio-venous malformation ". I regret to point out that, since Dr Anderton wrote his review, a very recent report [2] gives details of three cases of major air embolism. All three patients were placed in the "abdomen free" or kneeling position, and two of the three died. There has been a recent revival of interest in this "kneeling, free abdomen" position in the U.S.A. and a corresponding increase in the number of manufacturers offering the relevant equipment. As a result, some of my surgeons have been trying it and have been enthusiastic. This recent report of two deaths is, to say the least, food for thought (although I must comment that the operating times and the blood loss were far greater than I am used to seeing in my hospital). You can be sure that all my orthopaedic and ncurosurgical colleagues have been given copies of the article in question, and I thought that your readers should be made aware of it too.
CORRESPONDENCE
D. V. THOMAS
Los Altos, California REFERENCES 1. Anderton JM. The prone position for the surgical patient: a historical review of the principles and hazards. British Journal of Anaesthesia 1991; 67: 452-463. 2. Albin MS, Ritter RR, Pruett CE, Kalff K. Venous air embolism during lumbar laminectomy in the prone position: report of three cases. Anesthesia and Analgesia 1991; 73: 346-349. Sir—Thank you for inviting me to comment on the letter from Dr Thomas. Attention is quite rightly drawn to the fact that a low central venous pressure is to be found in patients placed on the "Tarlov Seat". The gravitational effect of venous blood in abdominal viscera hanging below heart level may enhance this. The cases described were, however, at an extreme end of the spectrum of surgical and anaesthetic difficulty. All were revision procedures requiring fusion, blood loss was excessive in all and the operation times when the incident occurred were 5 h in two patients and.3.5 h in the third. I think we Should be careful not to make general recommendations for all patients placed in this position on the basis of these extreme examples. I would accept that Doppler ultrasound monitoring could reasonably be added to routine measurements of end-tidal carbon dioxide but, unless I obtain further evidence from this or read other reports of venous air embolism related to this position, I would be reluctant to add either invasive central venous pressure or direct arterial monitoring to my usual routine. The latter suffices well for single level lumbar discectomy, at which blood loss rarely exceeds 150-200 ml in an operating time of 1.5-2 h. As always, complex cases will justify more careful monitoring which could include a thermodilution catheter [1]. J. M. ANDERTON
Manchester REFERENCE 1. Backofcn JE, Schauble JF. Hemodynamic changes with prone position during general anesthesia. Anesthesia and Analgesia 1985; 64: 194.
CONSCIOUS AWARENESS Sir,—Jessop and Jones [1] stated that the origins of a voluntary act start in the unconscious. To support their claim, they quote the work of Benjamin Libet [2—4] who, in one of his experiments, asked neurosurgical patients to introspect and then, stimulating the medial lemniscus, showed that normal evoked potentials could be obtained without conscious awareness. In a later experiment [5], Libet found that readiness potentials (potentials associated with the initiation of movement) were generated before the " urge to move" was noted in subjects who were asked to introspect whilst he monitored EEG and EMG. Libet was looking at spontaneous movement and he assumed that this was related to unconscious movement. Cotterill used Libet's work to propose that conscious awareness is not necessary for the execution of a simple reaction task [6]. Some of Libet's work has been repeated by Keller and Heckhausen [7], who showed that instructing a volunteer to introspect changed the site of production of the readiness potential from the lateral premotor system in the primary auditory cortex (unconscious acts) to the supplementary motor area (voluntary spontaneous acts). This implies, however, that the subjects in Libet's study were "primed" by the instruction to introspect. Consequently, interpretation of his result is made more difficult and one may not need to invoke an unconscious initiation of acts. In fact, Keller and Heckhausen concluded that activation of the supplementary motor area and "the urge to move" occur at the same time. The implications for Libet's earlier work are similar, in that by telling the patients to introspect he may have changed what he was trying to measure. R. MUNGLANI
Cambridge REFERENCES 1. Jessop J, Jones JG. Conscious awareness during general anaesthesia—what are we attempting to monitor? British Journal of Anaesthesia 1991; 66: 635-637. 2. Libet B, Alberts WW, Wright EW, Delartrc LD, Levin G, Fenstein B. Production of threshold levels of conscious sensation by electrical stimulation of the human somatosensory cortex. Journal of Neurophysiology 1964; 27: 546-578. 3. Libet B, Alberts WW, Wright EW, Fenstein B. Responses of the human somatosensory cortex to stimuli below threshold for conscious sensation. Science 1967; 157: 1597-1600. 4. Libet B, Wright EW, Fenstein B, Pearl DK. Subjective referral of the timing for a conscious sensory experience. A functional role for the somatosensory specific projection system in man. Brain 1979; 102: 193-224. 5. Libet B, Gleason CA, Wright EW, Pearl DK. Time of conscious intention to act in relation to onset of cerebral activity (readiness potential). Brain 1983; 106: 623-642. 6. Cotterill R. No Ghost in the Machine. London: Heinemann, 1989. 7. Keller I, Heckhausen H. Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control. Electroencephalography and Clinical Neurophysiology 1990; 76: 351-361. Sir,—Thank you for the opportunity to reply to Dr Munglani. Our editorial focused on the hypothesis that changes in early cortical waves in the auditory evoked potential could be used as an indicator of conscious awareness. These waves, with latencies of about 50 ms, provide evidence of one aspect of cortical function, but not necessarily of cognition, and we quoted Libet and colleagues who showed, in conscious man, that these waves may occur in response to a stimulus without that stimulus reaching consciousness. Furthermore, these workers provide evidence that conscious awareness of a sensory stimulus occurred only after a delay of about 500 ms. However, as it is well known that the reaction time for a simple task may be less than 200 ms, we felt
Downloaded from http://bja.oxfordjournals.org/ at University of Lethbridge on September 9, 2015
VENOUS AIR EMBOLISM AND THE PRONE POSITION Sir,—I enjoyed reading Dr Anderton's very full review of the prone position for the surgical patient [1]. He mentioned that, whenever veins are opened at a surgical procedure above heart level, there is a "theoretical" risk of air embolism (my quotation marks), and that "the actual risk of venous air embolism seems to be very small indeed." He then quoted "... one report of a fatality in the literature, but in this patient posterior fossa surgery was being performed for a large arterio-venous malformation ". I regret to point out that, since Dr Anderton wrote his review, a very recent report [2] gives details of three cases of major air embolism. All three patients were placed in the "abdomen free" or kneeling position, and two of the three died. There has been a recent revival of interest in this "kneeling, free abdomen" position in the U.S.A. and a corresponding increase in the number of manufacturers offering the relevant equipment. As a result, some of my surgeons have been trying it and have been enthusiastic. This recent report of two deaths is, to say the least, food for thought (although I must comment that the operating times and the blood loss were far greater than I am used to seeing in my hospital). You can be sure that all my orthopaedic and ncurosurgical colleagues have been given copies of the article in question, and I thought that your readers should be made aware of it too.
