Letters

Prophylaxis of venous thromboembolism

BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7089.1281b (Published 26 April 1997) Cite this as: BMJ 1997;314:1281

Article did not give enough information

  1. Thomas H S Dent, Consultant in public health medicinea,
  2. Monica A Dent, Senior registrar in public health medicinea
  1. a West Surrey Health Authority, Camberley GU16 5QE
  2. b Bristol Royal Infirmary, Bristol BS2 8W
  3. c Salisbury District Hospital, Salisbury SP2 8BJ
  4. d Royal United Hospital, Bath BA1 3NG
  5. e Centre for Molecular and Vascular Biology, University of Leuven, Campus Gasthuisberg, B-3000 Leuven, Belgium

    Editor—Systematic review is the best way of summarising the results of randomised trials. It is therefore disappointing that the Fortnightly Review on the prophylaxis of venous thromboembolism is so unsystematic.1 M Verstraete cites as sources of evidence, firstly, a set of guidelines whose method of preparation is not specified and, secondly, at least two review articles that were derived by consensus rather than by systematic appraisal of the literature. This prevents readers gauging the strength of evidence supporting his recommendations. Nor does he tell us whether he tried to find other reviews, how he selected reviews for inclusion, what he omitted from the reviews cited, how he reconciled differences between them, or whether he checked important sources of evidence such as the Cochrane Library and the Agency for Health Care Policy and Research. Three published relevant systematic reviews are not cited.2 3 4

    Were trials appraised before inclusion? This concern is underlined by the unequivocal affirmation of the effectiveness of low molecular weight heparins in spinal cord injury being based on an unrandomised trial. How did Verstraete use Cook et al 's grading system for evidence? That approach requires a trial's effect size to exceed the minimal clinically important benefit to attain grade A, and Verstraete does not specify that level.

    Perhaps because of the lack of use of meta-analyses, the review provides little quantitative information on the absolute or relative effectiveness of different methods of thromboprophylaxis. Low molecular weight heparins may be more effective than adjusted dose unfractionated heparin and other approaches, but is the difference large enough to be clinically important? Are there differences in the risks and side effects (especially rates of bleeding, a major source of surgical caution about thromboprophylaxis)? What about costs?

    A recent issue of Effective Health Care, attempting to review the literature on thromboprophylaxis in total hip replacement more systematically than Verstraete does, concluded that “many of the studies and reviews in this area are of doubtful quality or relevance to contemporary practice. On the basis of existing knowledge it is not possible to produce valid evidence-based recommendations on this issue.”5 The authors' more systematic approach may explain why their conclusions differ so markedly from Verstraete's.

    Review articles have an important role in maintaining and enhancing doctors' knowledge; all the more important then that they give readers enough information to allow assessment of the origin, quality, strength, and relevance of the evidence that they summarise. This article was not successful in that regard.

    References

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    Regional anaesthesia reduces thromboembolic morbidity

    1. T M Cook, Specialist registrar in anaesthesiab,
    2. R J H Baylis, Specialist registrar in anaesthesiac,
    3. R Marjot, Consultant in anaesthesiad
    1. a West Surrey Health Authority, Camberley GU16 5QE
    2. b Bristol Royal Infirmary, Bristol BS2 8W
    3. c Salisbury District Hospital, Salisbury SP2 8BJ
    4. d Royal United Hospital, Bath BA1 3NG
    5. e Centre for Molecular and Vascular Biology, University of Leuven, Campus Gasthuisberg, B-3000 Leuven, Belgium

      Editor—M Verstraete does not mention the role of the anaesthetic technique in the prevention of thromboembolic disease.1 Epidural analgesia can significantly reduce the incidence of deep vein thrombosis and pulmonary embolism after emergency hip surgery, elective hip replacement, and other forms of surgery. Meta-analysis has confirmed the reduction in the formation of deep vein thrombosis when regional anaesthesia is used instead of general anaesthesia.2

      How thromboembolism is reduced is unclear, but several physiological changes occur that influence Virchow's triad. Regional anaesthesia attenuates the stress response and so reduces the increase in coagulability observed postoperatively. It maintains fibrinolysis and reduces platelet adhesiveness. It also increases arterial and venous blood flow in the lower limbs. Finally, mobilisation after major surgery can occur remarkably early after regional analgesia, which may reduce the period at risk.

      Regional anaesthesia with local anaesthetics produces these effects. Whether regional analgesia with opioids produces the same benefit is unclear. Also unclear is whether regional anaesthesia (peroperatively) alone prevents postoperative formation of thrombus or whether blockade of longer duration is necessary (postoperative regional analgesia).

      The recent national confidential inquiry into perioperative deaths found that a fifth of the orthopaedic patients who died and underwent postmortem examination died of pulmonary embolus.3 The report commented on the lack of thromboembolic prophylaxis but did not draw strong conclusions.

      Verstraete includes major gynaecological surgery in patients aged over 40 with at least one risk factor (one of which is pregnancy) in a medium risk group requiring prophylaxis. The working party of the Royal College of Obstetricians and Gynaecologists includes age over 35 and emergency caesarean section in labour as risk factors requiring prophylaxis with heparin.4 Fatal pulmonary embolus was the commonest cause of direct maternal death in 1991-3,5 and 13 of the 17 deaths occurred after caesarean section. We recently completed a survey of anaesthetists in 243 hospitals in Britain, in which we examined the practice of thromboembolic prophylaxis in obstetric surgery. We received 176 (72%) complete replies (table 1).

      Table 1

      Responses of anaesthetists to questions about routine use of thromboembolic prophylaxis in obstetric surgery (176 complete replies were received)

      View this table:

      Both the national confidential inquiry into perioperative deaths and our survey identify a lack of use of thromboembolic prophylaxis. Regional anaesthesia for operations on the lower limbs and pelvis seems likely to reduce thromboembolic morbidity when other methods are not used. Whether regional anaesthesia and analgesia have a role when mechanical or anticoagulant prophylaxis is used is uncertain. While levels of prophylaxis are low, however, the use of regional techniques has much to commend it.

      References

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      Author's reply

      1. M Verstraete, Professore
      1. a West Surrey Health Authority, Camberley GU16 5QE
      2. b Bristol Royal Infirmary, Bristol BS2 8W
      3. c Salisbury District Hospital, Salisbury SP2 8BJ
      4. d Royal United Hospital, Bath BA1 3NG
      5. e Centre for Molecular and Vascular Biology, University of Leuven, Campus Gasthuisberg, B-3000 Leuven, Belgium

        Editor—Thomas H S Dent and Monica A Dent criticise the fact that my review was based largely on European and American guidelines and consensus reports, including the recent one by the Scottish Intercollegiate Guidelines Network (my reference 4). The brief that I received from the BMJ was to put the latter into perspective. I indicated in my review that I used the definition of evidence and the grading described by Cook et al (my reference 9) and mentioned for each class of thrombotic risk the grade of evidence, which, as I also mentioned, accords with the grades recently published by Clagett et al (my reference 2).

        I concede that, because of limitations on space, I could not mention all reviews (systematic or not), meta-analyses, and opinions that have been published on the subject. The most provocative, controversial, and anti-educational conclusion quoted by Dent and Dent and published in Effective Health Care—that “there is no valid evidence-based recommendation on the use of thromboprophylaxis in total hip replacement”—is quite opposite to the evidence based recommendation of reputed critical experts.1 On the basis of level I data, their grade A recommendation for routine use in hip replacement surgery is postoperative prophylaxis with twice daily, fixed dose, unmonitored subcutaneous low molecular weight heparin; low intensity (international normalised ratio 2.0-3.0) oral anticoagulants (started preoperatively or immediately after operation); or adjusted dose unfractionated heparin (started preoperatively). These are the most effective anticoagulant based prophylactic regimens for this indication. In total knee replacement surgery the same group of experts recommends postoperative subcutaneous unmonitored low molecular weight heparin (twice daily at a fixed dose) as the most effective anticoagulant based prophylactic regimen (grade A recommendation on level I data).1

        I agree with T M Cook and colleagues that not mentioning the impact of the anaesthetic technique on the incidence of deep venous thrombosis and pulmonary embolism is a shortcoming in my review. Their letter compensates for my omission but also indicates that many uncertainties still obscure this issue. The same authors rightly indicate that emergency caesarean section in patients aged over 35 belongs to the category of medium thrombotic risk, which erroneously was not mentioned in box 2 of my review.

        References

        1. 1.
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