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DVT is a common problem in orthopedics,[242] and pulmonary embolism is a major cause of postoperative mortality.[243] [244] [245] Rates are low after upper extremity surgery, spinal surgery,[246] and knee arthroscopy (3%)[247] but more common after total-hip replacement (30% to 50%),[248] [249] [250] total-knee replacement (40% to 60%),[191] [251] [252] and trauma to the lower extremities (20% to 50%). Proximal DVT (i.e., popliteal femoral or iliac vessels) is more common after total-hip replacement (10% to 20%)[249] [250] [253] and is more likely to cause pulmonary embolism.
In the presence of surgical injury, emboli form because of venous stasis. During total-knee replacement, there is absolute stasis with the tourniquet inflated, and on release of the tourniquet, acute increases in markers of coagulation can be detected in blood[254] ( Fig. 61-8 ). Concurrently, thromboemboli can be detected in the right side of the heart on echocardiography.[255] During total-hip replacement, obstruction of the femoral vein occurs during surgery on the femur. [84] [256] [257] [258] After obstruction of the vein is relieved when the hip is relocated, the level of fibrinopeptide A, a marker of thrombosis, increases ( Fig. 61-9 ), and echogenic material can be seen on echocardiography.[74] Efforts to reduce DVT during surgery should be directed during these phases of venous occlusion. Maneuvers include reducing the duration of surgery (surgeon's responsibility), [253] augmenting lower extremity blood flow during surgery to reduce venous stasis,[259] and administering anticoagulants during this phase of surgery.[84] If 15 to 20 units/kg of unfractionated heparin is administered before surgery on the femur,
Figure 61-8
Fibrinopeptide A in patients receiving general or epidural
anesthesia. Determinations were made before the induction of anesthesia (sample
1), 20 minutes after the start of the surgical procedure (sample
2), 45 seconds after deflation of the tourniquet at the end of surgery
(sample 3), and 1 hour postoperatively (sample
4). Values are given as the mean ± SEM. Notice the increase in
fibrinopeptide A after deflation of the tourniquet (sample 3),
representing thrombosis in the leg while the tourniquet was inflated. (Adapted
from Sharrock NE, Go G, Williams-Russo P, et al: Comparison of extradural and general
anaesthesia on the fibrinolytic response to total knee arthroplasty. Br J Anaesth
79:29–34, 1997.)
Figure 61-9
Blood levels of fibrinopeptide A (FPA) in patients receiving
saline (n = 25), 10 units/kg of intravenous unfractionated heparin (n = 25), or 20
units/kg of intravenous unfractionated heparin (n = 25) during hybrid total-hip replacement.
Five blood samples were taken at specific times during surgery: sample
1, baseline, which was before the epidural injection; sample
2, immediately after insertion of the acetabulum; sample
3, 5 minutes after insertion of the femoral prosthesis; sample
4, 2 minutes after hip relocation; and sample 5,
30 minutes after surgery. Notice the increase in FPA in the saline group and the
significant suppression in the groups receiving 10 units/kg or 20 units/kg during
insertion of the femoral component. Values are given as the mean ± SEM.
Asterisk, P = .0001
compared with 10 and 20 units/kg. (Adapted from Sharrock NE, Go G, Sulco
TP, et al: Dose response of intravenous heparin on markers of thrombosis during
primary total hip replacement. Anesthesiology 90:981–987, 1999.)
Intravenous unfractionated heparin has also been studied in total-knee arthroplasty, with overall pulmonary embolus rates as low as 0.096% reported.[261] Epidural or spinal anesthesia reduces DVT rates after total-knee replacement by 20% (from 50% to 40%)[191] and after total-hip replacement by approximately 40%.[262] [263] DVT rates during total-hip replacement performed with epidural anesthesia can be reduced to 10% with the concurrent use of low-dose epinephrine infusions.[253] [264] The mechanism of action of epinephrine is unknown,[265] but it does augment lower extremity blood flow during epidural anesthesia, thereby minimizing venous stasis.[259]
Postoperative epidural analgesia does not appear to provide additional benefit in reducing DVT rates.[251] Epidural infusion of 0.1% bupivacaine does not increase femoral venous blood flow, whereas simple flexion-extension exercises of the foot do increase extremity blood flow.[266] The benefit of epidural analgesia may be that it facilitates early ambulation, which is beneficial in the prophylaxis of DVT.
There is controversy about using epidural anesthesia and heparin. [193] The effects of heparin and epidural anesthesia on DVT prophylaxis are additive.[263] [267] DVT rates of 33% are observed using LMWH with general anesthesia, whereas rates of 19% occur with LMWH and epidural anesthesia.[263] Perioperative anticoagulants are necessary if general anesthesia is used, but the question is whether LMWH is useful (or even dangerous) when given in conjunction with conduction anesthesia.[193] The risk of spinal hematoma, although rare, must be considered when performing neuraxial blockade on patients receiving anticoagulation therapy. The American Society of Regional Anesthesia and Pain Medicine (ASRA) consensus on neuraxial anesthesia and anticoagulation recommends delaying placement of epidural or spinal 10 to 12 hours after the last dose of LMWH. For postoperative thromboprophylaxis treatment, they recommend the first dose to be administered no earlier than 24 hours postoperatively, indwelling catheters be removed before initiation of therapy, and LMWH administered no sooner than 2 hours after catheter removal.[268]
Warfarin is frequently administered to orthopedic patients for thromboprophylaxis. Warfarin should be discontinued 4 to 5 days before surgery, and the international normalized ratio (INR) should be checked before neuraxial anesthesia. It has been suggested that neuraxial catheters should be removed when the INR is less than 1.5. This number is derived from studies correlating hemostasis with clotting factor activity levels rather than actual clinical outcome data. In one study, 844 patients who received epidural analgesia and warfarin for DVT prophylaxis were evaluated. The epidural catheters were discontinued on postoperative day 2 to 4, with a mean INR of 1.47 ± .37. There was no evidence of epidural hematomas.[269] The practice had been to withdraw catheters without checking the patients' INR values.
Postoperative modalities to reduce DVT that can safely be used in conjunction with epidural anesthesia include pneumatic compression boots,[270] foot pumps,[271] [272] foot exercises,[266] early ambulation, aspirin, and low-dose warfarin started the day after surgery. Anticoagulation is not generally recommended after knee arthroscopy or spinal surgery. In high-risk cases, vena cava filters may be placed preoperatively.
The role of the anesthesiologist in the prevention of DVT will change with recognition of the role of anesthesia in the prevention of thrombosis and a realization that the thrombi form in the operating room rather than after surgery. Because they form during "our watch," we have an opportunity and a responsibility to prevent thrombi from forming.
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