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
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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.
BRITISH JOURNAL OF ANAESTHESIA
326 that this apparent paradox had to be addressed. We stated that both Libet and Cotterill had proposed that conscious awareness is not necessary for the execution of a simple reaction task. In contrast, we proposed that, in this specific situation (which lies between a conditioned reflex and a voluntary movement), a motor programme might be set up by biasing a set of synapses so that the stimulus automatically triggers the required response. We state that "consciousness is undoubtedly necessary for this programme to be set up and kept in a state of readiness". The later work of Keller and Heckhausen to which Dr Munglani refers is concerned with the initiation of voluntary movements. Whilst this is fascinating in its own right, it is some way from the point of our editorial, which was concerned with conscious awareness. We are sure that Dr Munglani will agree that Libet's work is not familiar to most anaesthetists; nevertheless its wider dissemination will help us to make progress in the difficult problem of understanding and detecting conscious awareness under anaesthesia. J. JESSOP
Doncaster J. G. JONES
Cambridge
BLOOD CONSERVATION Sir,—Dr Turner's editorial on blood conservation [1] dealt with the role of autotransfusion and we wish to comment further. Our experience with the Haemocell 350 system now exceeds 40 patients who have received more than 60 units of salvaged blood. Haemolysis always occurs, but the degree depends on how carefully the surgeon collects salvaged blood, and there is a fast learning curve; it is not clinically significant and does not affect a patient with reasonable renal function. These systems are designed to save red blood cells that would otherwise be discarded, so any cells saved are a bonus. The majority of cells have a perfectly normal survival time. There has been no case of postoperative coagulopathy. Our research indicates that red blood cells passed through a cell washer improve their 2,3-DPG concentrations. Air embolism, loss of platelets and fibrinogen, and microemboli are similar risks to any i.v. transfusions of stored blood. We have been using the system routinely for elective aortic aneurysm, where it makes a beneficial impact on transfusion requirements and gives confidence in using the system in emergency situations. D. W. RYAN D. BELL
Newcastle upon Tyne EUROPEAN STANDARDISATION COMMITTEE ON ANAESTHETIC EQUIPMENT Sir,—As members of the United Kingdom delegation to the European Standardisation Committee on Anaesthetic Equipment CEN/TC215 and its Working Groups, we would like to point out that reference to a private draft committee document made by Dr Zbinden and his colleagues in their article on fresh gas utilization of eight circle systems [1] has not been authorized by the Technical Committee. The test procedure included in the CEN Committee's working document was that proposed by Dr Zbinden's group. It is possible that the proposed fresh gas utilization test will be deleted or changed substantially before publication of the Standard. Some members of the CEN Working Group have expressed doubts about the value of this test in clinical anaesthesia because, as the authors state, the results can be modified greatly by changes in ventilation volume, flow pattern, compliance of the breathing system, leakage, etc. The design and arrangement of components of a circle system to achieve 100% fresh gas utilization would not improve patient safety if the exhaust valve was badly sited, the system resistance high or the leakage excessive. We therefore request that the reference to CEN/TC215/WG1 N36 is not interpreted as providing published international support for this method of apparatus evaluation. R. GREENBAUM
Member of CEN/TC215 B. R. SUGG
Member of CEN/TC215 B. HAYES
Chairman of CEN/TC21S REFERENCE 1. Zbinden AM, Fcigcnwinter, P, Hutmacher M. Fresh gas utilization of eight circle systems. British Journal of Anaesthesia 1991; 67: 492^199.
INCORRECT STATEMENT Sir,—In my article entitled " Indications for Use of Bicarbonate in Patients with Metabolic Acidosis" [1], I stated that THAM is an experimental drug. It is, in fact, approved for use in humans and has been for several years. A. I. ARIEFF
San Francisco REFERENCE 1. Arieff AI. Indications for use of bicarbonate in patients with metabolic acidosis. British Journal of Anaesthesia 1992; 67: 165-177.
REFERENCE 1. Turner DABT. Blood conservation. British Journal of Anaesthesia 1991; 66: 281-284.
MANAGEMENT OF HYPONATRAEMIA Sir,—We read with interest Professor Swales' review on the management of hyponatraemia [1]. However, we should like to draw attention to a situation he has not discussed, namely hyponatraemia caused by natriuresis initiated by intracranial disease. Although relatively rare, this condition is important, as its management differs from that of other causes of hyponatraemia. The term "cerebral salt wasting" was used first in the early 1950s to explain hyponatraemia in patients after subarachnoid haemorrhage (SAH). The cause was thought to be renal sodium loss [2]. However, after the description of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) in 1957, it was assumed widely that water retention was the cause of the (dilutional) hyponatraemia and the term fell into disuse [3]. It was reintroduced in 1981 by Nelson and colleagues, who investigated 12 patients with intracranial pathology who fulfilled established laboratory criteria of SIADH [4]. In 10 of these patients there was a decrease in red cell mass, plasma volume and total blood volume—results which could not be explained by SIADH, as water excess should lead to an expanded circulation in this condition. Natriuresis is a better explanation and cerebral salt wasting a preferable term, as the aetiology is unknown. Cerebral salt wasting has since been reported in many other intracranial pathologies, including disease of the pituitary and its surrounds, head injury and infections including meningitis. Further studies have given support to this concept and atrial natriuretic peptide has been implicated as a possible cause [5-7]. This condition is of more than academic interest, as the treatment of cerebral salt wasting (replace salt and restore circulating volume) is the opposite of fluid restriction used to manage SIADH. Hyponatraemia is common after subarachnoid haemorrhage and recent work indicates that 67 % of these cases are a result of natriuresis and not SIADH [8]. Fluid restriction in any intracranial pathology is potentially dangerous and in SAH increased mortality and morbidity are well documented [9]. Clinicians involved in treating such patients therefore should be aware of this distinction, as inappropriate fluid restriction is hazardous. K. A. SEEX P. R. ELDRIDGE
Liverpool REFERENCES 1. Swales JD. Management of hyponatraemia. British Journal of Anaesthesia 1991; 67: 146-153. 2. Cort JH. Cerebral salt wasting. Lancet 1954; 1: 752-754.