I read with interest the letter from Dr Morris (Morris. Anaesthesia 2003; 58: 1236–7) about power calculations and with even more interest the reply by Dr Pandit (Pandit. Anaesthesia 2003; 58: 1237–8). This is because Dr Morris covers many of the points that I have brooded on and lectured on over the years and Dr Pandit gives an answer to the question which I have unsuccessfully asked successive audiences of anaesthetists for the past 10 years: why demand only a 5% risk (P < 0.05) of a type-1 error (falsely claiming a real effect) and yet accept a 20% risk (80% power) of a type-2 error (claiming no effect when really one does exist). However, I cannot agree with the answer Dr Pandit quotes: that, when a new treatment costs much more, it is reasonable to take a higher risk of rejecting it even if it is good. This is because if the cost is high, then surely the important thing is to be reasonably sure (95% power) that the improvement it produces is big enough to justify the cost (a large ‘minimum clinically important difference’). The number of patients required may not be very different from that to detect a smaller difference with 80% power; but let's arrive at that number by a logical route.

11. Goudsouzian NG, Denman W, Cleveland R, Shorten G. Radiologic localization of the laryngeal mask airway in children. Anesthesiology 1992; 77: 1085-1089. 12. Brimacombe J, Berry A. Laryngeal mask airway insertion. A comparison of the standard versus neutral position in normal patients with a view to its use in cervical spine instability. Anaesthesia 1993; 48: 670-671. 13. Brain AIJ. The development of the laryngeal mask—a brief history of the invention, early clinical studies and experimental work from which the laryngeal mask evolved. European Journal of Anaesthesiology 1991; (Suppl. 4): 5-17. 14. Brain AIJ. Laryngeal mask misplacement—causes, consequences and solutions. Anaesthesia 1992; 47: 531-532. 15. Maue WM, Dickson DR. Cartilages and ligaments of the adult human larynx. Archives of Otolaryngology 1971; 94: 432^39.

We have compared the efficiency of the enclosed Magill attachment (System A) and the co-axial System D (Bain) in the Carden Ventmasta ventilator using both systems, each under five different ventilatory conditions, in each of five anaesthetized patients. Efficiency was assessed in terms of the effective alveolar ventilation as a fraction of the fresh gas flow. For System A, efficiency increased from a mean of 0.37 when the total ventilation was only 50% of the fresh gas flow, to a mean of 0.74 when ventilation was 2.3 times the fresh gas flow. The efficiency was substantially and significantly less with System D: 75% of that for System A at the smaller total ventilation (95% CI 65-85%) and 65% at the larger (95% CI 59-71%). A critical examination is made of conflicting definitions and terminology of the efficiency of breathing systems.

We have studied the effects of phonation and posture on the Mallampati classification of view of the pharyngeal structures. Differences between observers were allowed for by the experimental design and log-linear modelling. Sixty-four patients were assessed on the ward, sitting upright, with and without phonation, by each of two observers. Another 64 patients were assessed without phonation, but both upright and supine, again by both observers. Phonation (the patient saying "Ah") produced a marked, systematic improvement of view; moving to the supine posture produced a small, systematic, non-significant worsening of the view. Differences between observers were non-systematic but substantial. About 25% of patients phonated spontaneously. It is recommended that anaesthetists make their own assessments of Mallampati classification, with the patient in either of the postures but always either with or without phonation, and thereby gradually "calibrate" their assessments against the degree of difficulty encountered in intubation.

We describe the theory, construction and testing of an active model lung. This is designed so that when it is connected to a ventilator, the interaction between spontaneous ventilatory activity in the patient and the pressures and flows generated by the ventilator are reproduced faithfully. A waveform of negative pressure, pmus, equivalent to the action of the respiratory muscles, is applied indirectly to the compliance unit of a conventional model lung. It is shown that, when the compliance and resistance of a subject have been measured, the waveform of pmus can be determined. This was undertaken in three volunteers breathing under various conditions, including connection to a ventilator operating in different modes. When these conditions were reproduced with the lung model replacing, but matched to, each subject, the original waveforms of flow and pressure at the mouth were reproduced.

Peak velocity of saccadic eye movements was studied in six healthy volunteers who received either 0.9% sodium chloride (as a control) or a stepwise rising propofol infusion regime of 0.24, 0.6 and 1.5 mg kg-1 h-1, which produced arterialized venous concentrations equivalent to 1%, 6% and 13% of the estimated EC50 for propofol. The infusion lasted for 75 min (25 min at each infusion rate) and was followed by a 2-h post-infusion recovery period. Peak saccadic velocity was highly significantly depressed during the 15-25-min period after the start of the second and third propofol infusions compared with saline. The relationship between arterialized venous blood concentration and peak saccadic velocity was similar during recovery when concentrations were falling to that during increasing blood propofol concentrations. Peak saccadic velocity decreased linearly with increasing log10 propofol concentration in the range of 25-800 ng ml-1 (14% of control per decade). Measured arterialized venous-blood propofol concentrations were variable, and systematically greater than computer predictions at the two higher infusion rates. In only half the subjects were the subjective assessments significantly correlated with log10 propofol concentrations or with percentage reduction in peak saccadic velocity.

A control system was used to bring the tension of anaesthetic in the brain to any value specified (in MAC units) by the anaesthetist and then maintain it constant until a new value was specified. The control was applied to a volatile agent but allowance was automatically made for the anaesthetic effect of any nitrous oxide concomitantly administered by the anaesthetist. The inspired concentration required to achieve the desired brain tension was calculated from a model of the patient and set automatically on the vaporizer. The quantification of the model was matched to the patient on the basis mainly of body mass and periodic non-invasive measurements of alveolar ventilation and cardiac output. In order to adapt the model to the patient an arterial blood sample was taken every 30 min to obtain the arterial tension of halothane for use as feedback. The system has been tested on eight Alsatian dogs. After omitting results affected by avoidable errors, the SD of the measured-to-computed arterial tension ratio was less than 10%.

Equine Veterinary JournalVolume 16, Issue 3 p. 155-157 Mathematical modelling of the horse W. W. Mapleson, W. W. Mapleson University of Wales, Welsh National School of Medicine CardiffSearch for more papers by this authorG. E.Staddon, G. E.Staddon General Hospital, BristolSearch for more papers by this authorB. M. Q. Weaver, B. M. Q. Weaver Department of Veterinary Surgery, University of BristolSearch for more papers by this author W. W. Mapleson, W. W. Mapleson University of Wales, Welsh National School of Medicine CardiffSearch for more papers by this authorG. E.Staddon, G. E.Staddon General Hospital, BristolSearch for more papers by this authorB. M. Q. Weaver, B. M. Q. Weaver Department of Veterinary Surgery, University of BristolSearch for more papers by this author First published: May 1984 https://doi.org/10.1111/j.2042-3306.1984.tb01889.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume16, Issue3May 1984Pages 155-157 RelatedInformation

Mapleson's (1973) physiological, circulation-time model of the distribution of inhaled anaesthetics has been elaborated to be suitable for modelling agents in which hepatic metabolism and renal excretion are important factors; as well as the obvious improvement of providing separate compartments for liver and kidney, the arterial and portal supplies to the liver are separately represented, as is the portal blood pool. The separate portal pool also leads to a more realistic total circulation time for the majority of the cardiac output. The quantification for a "standard man" is fully documented and makes use of the latest (1975) report of the International Commission on Radiological Protection, Reference Man, and includes data on the water, fat and protein composition of each tissue compartment. Suggestions are included on adapting the quantification to non-standard men and to other species.

A technique is described for analysing small discrete samples of gas (about 100 μl) by injecting them into a stream of carrier gas being continuously sampled by a ‘respiratory’ mass spectrometer. This involves interrupting the normal respiratory monitoring use of the mass spectrometer for only 20s per sample. The theory for calculating the composition is given for the case when the carrier gas is totally different from the discrete sample, and for the case when it is air and the discrete sample may contain air. Allowance is made for difference of viscosity between sample and carrier and for different response times to different components of the sample. The method was developed for the analysis of gas bubbles equilibrated with blood. When tested on a mixture of 1·2% halothane and 5% CO2 in 50/50 N2O/O2, with air as the carrier, the standard deviation between repeat determinations was about 0·5% of the actual concentration of each component.

If a small bubble gas is equilibrated with a volume of blood the partial pressures of the components of the bubble after equilibration approximate to the original tensions in the blood. A method is described in which the bubble is analysed by a respiratory mass spectrometer connected to a mini-computer. The normal respiratory monitoring function of the mass spectrometer need be interrupted for only 20 s for each sample and the complete equilibration and analysis procedure takes only 3 min. Theoretical studies, experimental tests and records of experimental usage show that, in a subject anaesthetized with halothane in nitrous and oxygen, tension can be measured with an overall SD of 5% for carbon dioxide and halothane and, certain reservations, an SD of about 3% for nitrous oxide.

In a group of seven lambs, pressure-volume relationships were obtained for the initial artificial expansion of the lungs in the intact chest and subsequent re-expansion afer the lungs had been excised and collapsed. In a second group, of 12 severely asphyxiated lambs, the lungs were expanded in steps and circulatory resuscitation was achieved at pressures ranging from 1.75 to 3.5 kPa, geometric mean 2.3 kPa, with estimated corresponding volumes of 3--24 ml kg-1, geometric mean 5.8 ml kg-1. In a third group, of 24 severely asphyxiated lambs (mean pH 6.84), 22 were successfully resuscitated in the same manner and nine of these survived in apparent good health for more than 6 months. Any resuscitation regime for human neonates should take account of the finding that resuscitation occurred at pressures less than those necessary for full expansion and that, after the first sign of an increase in heart rate, there was a delay of 17--45 s before full circulatory improvement and of 3 min before full improvement in PaO2.

AnaesthesiaVolume 34, Issue 2 p. 163-172 Free Access From Clover to computer Towards programmed anaesthesia?* W.W. MAPLESON, W.W. MAPLESON W. W. Mapleson, DSc, FInst P, Professo: of the Physics of Anaesthesia, Department of Anaesthetics, Welsh National School of Medicine, Heath Park, Cardiff, CF4 4XN.Search for more papers by this author W.W. MAPLESON, W.W. MAPLESON W. W. Mapleson, DSc, FInst P, Professo: of the Physics of Anaesthesia, Department of Anaesthetics, Welsh National School of Medicine, Heath Park, Cardiff, CF4 4XN.Search for more papers by this author First published: February 1979 https://doi.org/10.1111/j.1365-2044.1979.tb06272.xCitations: 2 * This paper is an edited version of the 19th Joseph Clover Lecture, delivered to the Faculty of Anaesthetists of the Royal College of Surgeons of England on 15th March 1978. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References 1 Saidman, L.J. & Eger, E.I. (1964) Effect of nitrous oxide and of narcotic premedication on the alveolar concentration of halothane required for anesthesia. Anesthesiology, 25, 302. 2 Chilcoat, R.T. (1975) Servo loops in the control of anaesthetic parameters. MSc thesis, University of Wales. 3 Traer, J.R. (1862) Surgical, medical and obstetrical instruments in the international exhibition of 1862. Medical Times and Gazette, 2, 148. 4 Clover, J.T. (1873) Description of a new double current inhaler for administering ether. British Medical Journal, 1, 282. 5 Clover, J.T. (1874) Remarks on the production of sleep during surgical operations. British Medical Journal, 1, 200. 6 Clover, J.T. (1876) On an apparatus for administering nitrous oxide gas and ether, singly or combined. British Medical Journal, 2, 74. 7 Mapleson, W.W. (1963) Theory of the uptake and distribution of methoxyflurane in man. In: Proceedings of the Symposium on Methoxyflurane Held at the Royal Society of Medicine, 1963, p. 7. Abbott Laboratories Ltd, Queenborough , Kent . 8 Cowles, A.L., Borgstedt, H.H. & Gillies, A.J. (1972) Digital computer prediction of the optimal anaesthetic inspired concentration. British Journal of Anaesthesia, 44, 420. 9 Bellville, J.W. & Attura, G.M. (1957) Servo control of general anesthesia. Science, 126, 827. 10 Coles, J.R., Brown, W.A. & Lampard, D.G. (1973) Computer control of respiration and anaesthesia. Medical and Biological Engineering. 11, 262. 11 Lowe, H.J. & Mostert, J.W. (1974) Quantitative closed circuit anaesthesia. Anaesthesia, 29, 110. 12 Salamonsen, R.F. & Smith, K. (1976) An approach to programmed anaesthesia. Anaesthesia, 31, 1043. 13 Allott, P.R., Steward, A. & Mapleson, W.W. (1976) Pharmacokinetics of halothane in the dog. Comparison of theory and measurement in individuals. British Journal of Anaesthesia, 48, 219. 14 Mapleson, W.W., Allott, P.R. & Steward, A. (1974) A non-feedback technique for programmed anaesthesia. British Journal of Anaesthesia. 46, 805. 15 Mapleson, W.W. (1973) Circulation-time models of the uptake of inhaled anaesthetics and data for quantifying them. British Journal of Anaesthesia, 45, 319. Citing Literature Volume34, Issue2February 1979Pages 163-172 ReferencesRelatedInformation