British. Journal of Anaesthesia 1992; 68: 60-63. 2. Tuominen M, Kalso E, Rosenberg PH. Effects of posture on the spread of spinal anaesthesia with isobaric ...
329
CORRESPONDENCE hyperbaric 0.5 % bupivacaine. These studies demonstrated nicely that there are always patients who have a low block and others with an unnecessarily high spread of analgesia, irrespective of density of solution. Our conclusion was not "that individual anatomical properties are more important than baricity". We have stated: "Individual anatomical properties may play a more important role than expected in the subarachnoid spread of local anaesthetic", and "The extent of block is affected also by the baricity of the solution in relation with the position of the patient". Our conclusions were based, first, on the results in all three groups of this study; second, on our earlier studies on repeated spinal anaesthesia [5, 6]; and third, on the findings by Stienstra and van Poorten [7] and Mitchell and colleagues [8]. We are ready to accept new opinions on the prediction of spread of spinal anaesthesia, but what is needed most is greater knowledge of the anatomical-physiological determinants relative to the block in the individual patient. M. TUOMINEN M . PlTKANEN T. TATVAINEN P. H. ROSENBERG
Helsinki 1. Nicol ME, Holdcroft A. Density of intrathecal agents. British Journal of Anaesthesia 1992; 68: 60-63. 2. Tuominen M, Kalso E, Rosenberg PH. Effects of posture on the spread of spinal anaesthesia with isobaric 0.75 % or 0.5 % bupivacaine. British Journal of Anaesthesia 1982; 54: 313318. 3. Tuominen M, Taivainen T, Rosenberg PH. Spread of spinal anaesthesia with plain 0.5 % bupivacaine: influence of the vertebral interspace used for injection. British Journal of Anaesthesia 1989; 62: 358-361. 4. Taivainen T, Tuominen M, Rosenberg PH. Influence of obesity on the spread of spinal analgesia after injection of plain 0.5% bupivacaine at the 13-4 or L4-5 interspace. British Journal of Anaesthesia 1990; 64: 542-546. 5. Tuominen M, Kuulasmaa K, Taivainen T, Rosenberg PH. Individual predictability of repeated spinal anaesthesia with isobaric bupivacaine. Acta Anaesthesiologica Scandinavica 1989; 33: 13-14. 6. Taivainen T, Tuominen M, Kuulasmaa K, Rosenberg PH. A prospective study on reproducibility of the spread of spinal anaesthesia using plain 0.5% bupivacaine. Regional Anesthesia 1990; 15: 12-14. 7. Stienstra R, van Poorten JF. Plain or hyperbaric bupivacaine for spinal anesthesia. Anesthesia and Analgesia 1987; 66: 171-176. 8. Mitchell RWD, Bowler GMR, Scott DB, Edstrom HH. Effects of posture and baricity on spinal anaesthesia with 0.5% bupivacaine 5 ml. British Journal of Anaesthesia 1988; 61: 139-143.
and control patients appeared to be based on the increase in skin temperature observed. Is skin temperature related only to peripheral vasodilatation? What were the core temperature and the arterial carbon dioxide tension ? To what degree did droperidol and isoflurane contribute to this change in skin temperature? Finally, although the haemodynamic changes appeared to be similar in both uracmic and control patients, it is incorrect to conclude that propofol may be used safely for induction; morbidity and mortality were not reported. Another interpretation of the data might be that propofol is associated with equally unstable haemodynamic changes in both groups. I. H. LEWIS R. SNBYD
Ann Arbor, Michigan 1. Kirvela M, Olkkola KT, Rosenberg PH, Yli-Hankala A, Salmela K, Lindgren L. Pharmacokinetics of propofol and haemodynamic changes during induction of anaesthesia in uraemic patients. British Journal of Anaesthesia 1992; 68: 178-182. 2. Goodman NW, Hughes AO. Statistical awareness of research workers in British anaesthesia. British Journal of Anaesthesia 1992; 68: 321-324. 3. Frieman JA, Chalmers TC, Smith H, Kuebler RR. The importance of beta, the type II error and sample size in the design and interpretation of the randomized control trial. New England Journal of Medicine 1978; 299: 690-695.
BLEEDING TIME Sir,—I am sorry to leam that O'Kelly, Lawes and Luntley [1] have not found the bleeding time to be a useful clinical tool. They suggest that the Editorial in the Lancet [2] states that the bleeding time is of little relevance in individual patients before regional anaesthesia. However, the Editorial states "used judiciously, the bleeding time deserves to remain as part of the assessment of individual patients with histories suggestive of bleeding disorders". The problems of aspirin in obstetric patients are as follows: aspirin causes platelet dysfunction; platelet dysfunction prolongs the bleeding time; the incidence of venous puncture is about 18% during siting of an extradural catheter; if the bleeding time is prolonged will there be a significantly increased volume of blood in the extradural space?; what volume of blood is required to produce an extradural haematoma which will cause neurological deficit? Until we know the answers to the last two of these points, I am still of the opinion that a bleeding time should be performed on all patients receiving aspirin, before an extradural is sited. Armed with the result, anaesthetists are better able to deliberate the advantages and disadvantages of extradural block; for example if subarachnoid block might be more appropriate. They can also choose a technique designed to reduce the incidence of vessel puncture. R. MACDONALD
Leeds PROPOFOL IN URAEMIC PATIENTS Sir,—In the recent article by Dr Kirvela and colleagues [1], the authors concluded that, in terms of pharmacokinetics and haemodynamic changes, propofol may be used safely for induction of general anaesthesia in uraemic patients. We believe that these conclusions are open to question, for the following reasons. The mean elimination half-lives of propofol were stated to be similar in both uraemic and control patients. However, the omission of statements regarding the level of significance which was sought, the probability of a type II error (beta) or the calculation of sample size make this result and the other pharmacokinetic data difficult to assess. The recent commentary by Goodman and Hughes [2] and an earlier article by Freiman and colleagues [3] both emphasized the importance of statistical power in the design and interpretation of studies resulting in a "negative" outcome. The haemodynamic data appear to have been collected using a non-invasive oscillotonometric method. There were no statements regarding the accuracy of the apparatus used or the fact that peaks and troughs in arterial pressure may be missed by a technique limited to intermittent measurement. The conclusion that propofol caused marked peripheral vasodilatation in both uraemic
1. O'Kelly SW, Lawes EG, Luntley JB. Bleeding time: is it a useful clinical tool? British Journal of Anaesthesia 1992; 68: 313-315. 2. The bleeding time. Lancet 1991; 337: 1447-1448.
Sir,—Drs O'Kelly, Lawes and Luntley recently attempted to assess the use of the skin bleeding time in clinical practice, and suggested that it was an unreliable test [1]. I would suggest that their paper contained serious errors of methodology which render any conclusions invalid. In the methods section, the authors state that the estimation of bleeding time was "as described in the Simplate II test package", and that "a standard cut" was made using this spring loaded device. They do not mention that the Simplate II template device has two blades and makes two cuts (hence the "II"), and that the mean of the two measured bleeding times is recorded. There is, indeed, another apparatus, "Simplate", from the same manufacturers (General Diagnostics, Organon Teknika), which only makes one cut. Are the authors able to tell us exactly which template device they used for their study?
