Tan SK Patrick, FRCA;
Alan KH Teh, FRCP
Incidence
The risk of venous thromboembolic disease(VTE) following knee and hip
arthroplasty in the western population is well described. The level of risk in the
western population varies according to patient factors, surgical factors and
rehabilitation regimens. Genetic predispositions to thrombosis are listed in
Table 1. On the other hand, the risk of postoperative thromboembolic
phenomena were believed to be low in Asia, based on several papers
published in the 1980s. The thrombophilic factors, factor V Leiden mutation
and prothrombin variant 20210A, are almost nonexistent in Asian populations.
Factor V Leiden mutation is a point mutation of Factor V that confers
resistance to breakdown by activated protein C and thus predisposes to
thrombosis. It is commonest in the Greek population(7%) and occurs variably
between 3 and 5% in Europe and "European" South America. The prothrombin
variant 20210A is commonest in Southern Europe(3%) and increases the risk
of thrombosis by a factor of 2 or 3.
The other thrombogenic states
< Table 1> are, however, present in Asian populations. Activated protein C
binds to protein S on cell surfaces where it proteolytically inactivates
coagulation factors Va and VIIIa. The odds ratios for developing DVT increased
to 10.6 and 6.7, respectively, for patients with protein C and protein S
deficiency. Other conditions did not seem to affect the level of risk.
Recent research has revealed that the incidence of thromboembolic disease
in Asian patients is higher than previously thought, though still lower than in
Caucasians. The incidence of DVT has been found to be between 30 and 60%
in unprotected Asians undergoing hip and knee replacements, whereas the
incidence in a similar Caucasian population lies between 50 and 75%. The risk
is highest in knee replacements(and simultaneous knee replacement in
particular), followed by hip fracture surgery and total hip replacement(THR). In
the majority, the thromboses are found in the distal calf veins. Patients
undergoing knee replacement were thought to be at higher risk because of
the use of tourniquets to provide a bloodless field. However, the evidence is
equivocal.
Diagnosis
The clinical diagnosis of DVT has long been known to be unreliable. Classical
signs of DVT such as Homan`s sign, leg swelling and oedema are difficult to
elicit after surgery to the lower limbs. Other investigative modalities include
serum D dimer assay, compression ultrasound and venography. Venography
remains the gold standard but is an invasive and painful procedure.
Compression ultrasound scan, in expert hands, has a sensitivity of 83% and a
specificity of 98%. Serum D dimer assays following surgery show variations
with the type of surgery and the time after surgery the specimen was
collected. Whereas the D dimer assay is elevated in patients with DVT on the
3rd postoperative day, a specific cutoff level could not be determined to allow
specific diagnoses for an individual to be made.
Screening
Clinical suspicion based on the risk factors listed in Table 1 is probably the
most effective way of screening patients. In Asian patients, activated protein C
resistance is unlikely to be useful. Protein C and protein S deficiency are more
likely to be useful markers. However, mass application of these tests may not
be cost-effective. Postoperative screening with ultrasound to detect DVT is
also possible, but is not widely practised for various reasons. The technique
works best with a well trained and experienced investigator.
Prophylaxis
Several treatments are available for the prophylaxis of VTE. These can be
divided into mechanical and pharmaceutical methods. Mechanical prophylaxis
with various pneumatic sequential compression boots or intermittent
compression foot pumps(IPC) are known to be very effective. However, these
can be difficult to apply to the limb undergoing surgery. Postoperative
mechanical methods include various foot pumps and graduated compression
elastic stockings(ES).
Pharmaceutical methods include aspirin, warfarinization, low-dose
unfractionated heparin(LDUH), low-molecular-weight heparins(LMWH) and
newer synthetic, highly selective, indirect inhibitors of activated factor X. These
methods can be complementary. The recommendations of the Sixth American
College of Chest Physicians consensus conference on antithrombotic therapy
with reference to major orthopaedic surgery are listed in Table 2. These
recommendations are presented with reference to the strength of evidence
(see Appendix). The benefits of prophylaxis are also evident in Asian patients
undergoing joint replacement.
The current trend is towards prescribing preoperative LMWH and maintaining
the prophylaxis for at least 7 days for lower limb surgery. This strategy is more
cost-efficient than warfarinization or LDUH. However, the current generation of
LMWH are derived from animals(mainly porcine) and this has encountered
some religious and cultural reservations in Asia.
Neuraxial blockade(spinal or epidural anaesthesia) independently reduces the
morbidity of DVT, PE, transfusion requirements, pneumonia and respiratory
depression significantly. It is now the preferred mode of anaesthesia in the
elderly orthopaedic population. There have been some worries that LMWH
administered within 12 hours of insertion or withdrawal of an epidural catheter
could result in the rare complication of epidural haematoma. This complication
is more commonly reported in the United States and may be related to
dosages of particular LMWH drugs used. However, not all anaesthetists agree
on this. In the absence of epidemiological studies confirming the link between
LMWH and epidural haematoma, the general feeling is that the benefits of
neuraxial blockade far outweighs this particular risk. However, the general
recommendation is that LMWH should be administered outside the 12-hour
period. This complicates patient management in an era of rapid turnover and
same day admissions.
■ 기사 요지
메디메디아아시아 발간 `MEDICAL PROGRESS` 3월호에 게재된 글로, 아시아인에게서 나
타나는 무릎·둔부 관절성형술 후 심부정맥혈전증(DVT)의 특징이 주내용이다.
최근 연구에서는 아시아인의 혈전색전증 유병률이 백인에 비해 여전히 낮지만, 예상 보다
높게 나타나고 있으며 그 특징 또한 백인과 구별되는 것으로 보고되고 있다. 서구인들에게
는 `Facgtor V Leiden 돌연변이`와 `prothrombin variant 20210A`가 주요 예지인자지만,
아시아인의 경우는 protein C와 protein S 결핍시 DVT 발생 가능성이 높다. 진단법으로는
혈청D-이합체분석(Serum D-dimer Assay)·정맥조영술·compression ultrasound 등이
있다. 예방을 위한 약물요법에는 아스피린·와파린·저용량 비분획 헤파린(LDUH)·저분자량헤파
린(LMWH) 등이 포함된다.
정리·이상돈 기자
sdlee@kimsonline.co.kr