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EXCERPTS

Efficacy of Mechanical Prophylaxis for Venous Thromboembolism in Patients With Brain Tumors

Kurtis I. Auguste, M.D.; Alfredo QuiƱones-Hinojosa, M.D.; Mitchel S. Berger, M.D.

Abstract and Introduction
Abstract

Patients with brain tumors are at considerable risk for the formation of venous thromboemboli. One method of preventing these complications is mechanical prophylaxis in which an external pneumatic compression device and graduated elastic compression stockings are used.

Evidence indicates that these devices prevent deep venous thrombosis (DVT) and pulmonary embolism (PE) by limiting venous stasis and increasing fibrinolytic activity at both the local and systemic levels.

The authors present evidence for the occurrence of both mechanisms and discuss the use of mechanical compression in the setting of surgery for brain tumors. They also present data proving the efficacy of these devices in patients who undergo craniotomy with motor mapping for resection of glioma and in whom the contralateral leg receives no prophylaxis.

Finally, they comment on the use of anticoagulation therapy both in addition to and in place of mechanical prophylaxis.

Introduction

The incidence of DVT in neurosurgical patients ranges between 2 and 50%.[1,5,10,12,19,23,27,40,47,48,51,53] The rate of DVT and PE is thus quite significant and can lead to fatal outcomes. Neurosurgical procedures entail a 1.5 to 5% risk of PE and a 9 to 50% rate of mortality from this disorder.[23]

The prevention of VTE in neurosurgery by using mechanical prophylaxis is well established. Current modalities for mechanical prophylaxis involve pneumatic compression devices, graduated elastic compression stockings, or a combination of both.

Prevention of VTE by using mechanical devices addresses tenets of the Virchow[54] triad: endothelial damage, venous stasis, and hypercoagulability.

Mechanical prophylaxis appears to prevent thromboembolic complications by limiting venous stasis and diminishing hypercoagulability.

Pneumatic Compression Devices

External pneumatic compression devices have been shown to prevent the formation of DVTs in neurosurgical patients.[47,50,51] Modern devices evacuate blood from lower-extremity vessels in an automated fashion. In brief, a microprocessor directs pressurized air (for example, at 45 mm Hg) into segmental diaphragms secured around the leg for a fixed period of time (for example, for 11 seconds).

[30] The compression is delivered in a sequential manner up the leg, producing a wavelike milking effect to evacuate leg veins. Sequential devices have been proven to be more effective than single-chamber, evenly distributed pressure in preventing DVTs.[37] The compression is set to cycle regularly (for example, every 60 seconds). Devices are available for feet, calves, and/or thighs.

Published contraindications for the use of these devices include: dermatitis

postop vein ligation

gangrene

recent skin graft

severe ischemic vascular disease

severe lower extremity edema

extreme leg deformity

suspected preexisting DVT

Pneumatic compression may exert its protective effect against thrombus formation in part by limiting venous stasis. Calnan, et al.,[11] were one of the first groups to illustrate that intermittent, rhythmic compression of the lower extremity mimics the normal pumping of calf muscles. Soon after, Sabri, et al.,[42] demonstrated a 400% increase in femoral vein pulsatility and a 250% increase in peak femoral venous blood flow when the devices were applied to the lower limbs of greyhound dogs.

Comparable results were found in the lower extremities of humans who received pneumatic compression.[41] In more modern studies investigators have shown increased blood flow velocities in the popliteal and common femoral veins by applying intermittent pneumatic compression to the foot alone.[31] Mittel man, et al.,[36] compared the use of calf and thigh compression with calf compression alone and found an enhanced effect of blood clearance with sequential compressions of the calf and thigh.

Similarly, Delis, et al.,[16] found greater outflow during foot plus calf compression when comparing it with foot compression alone. Current prophylactic devices most frequently compress at least two regions of the lower extremity, although more limited devices have retained some popularity.

Mechanical compression devices appear to exert part of their prophylactic effect through enhanced fibrinolysis.[14,25,49]

Early studies in which euglobulin clotting times were analyzed as a marker for systemic fibrinolysis activity in postoperative patients demonstrated that calf compression augments clot breakdown.[3,33,43] Weitz, et al.,[55] showed that intermittent pneumatic calf compression, by preserving the normal thrombin/plasmin ratio in blood samples obtained in patients who receive this therapy when compared with those not receiving pneumatic compression, averts the hypercoagulable state noted by Owen, et al.[39] Intermittent pneumatic compression has been shown to increase the amount of tPA release and to de crease levels of plasminogen activator inhibitor.[25]

This benefit may be short-lived, however; diminished fibrinolytic activity is seen from several minutes[25] to 18 hours[33] after discontinuation of pneumatic compression.

Various permutations of both upper- and lower-extremity compression have been tested to increase blood clearance and ultimately prevent VTEs. Nearly 30 years ago, Knight and Dawson[33] conducted a study in which they applied intermittent compression to the upper extremities of patients who had undergone surgery, and noted a reduced incidence of DVTs in the legs and increased serum fibrinolytic activity.

Tarnay, et al.,[49] also showed increased fibrinolysis in patients receiving compression in the arms, although this difference was not statistically significant. Another interesting finding of this study was the increased fibrinolysis detected in serum samples obtained in the patient’s arm after administration of pneumatic compression to the legs. This effect appeared to be proportional to the volume of tissue compressed; increased fibrinolysis was detected in patients who wore long compression boots when compared with those given shorter boots.

The results of these studies indicate that the fibrinolytic activity promoted by mechanical prophylaxis has both local and systemic effects in protecting against clot formation.

Compression Stockings

An alternative or additional means of preventing thromboembolic complications is the graduated elastic compression stocking.[2,24,26,45] These devices apply continuous circumferential pressure to the lower extremity in a graduated fashion (~18 mm Hg at the ankle to ~8 mm Hg at the thigh)[46] as a means of increasing venous clearance and preventing venous stasis.

The stockings are contoured to the lower extremity to prevent tenting and focal constriction. Similar care should be exercised in selecting patients to receive stockings as is taken in choosing patients for treatment with pneumatic devices. The contraindications listed in Table 1 can be applied for stocking use as well.

In most neurosurgical practices, compression stockings are combined with pneumatic devices as a standard of care, presumably to enhance the prophylactic benefit of each. To test the theory that the protective effect of these devices is additive, Keith, et al.,[29] used Doppler ultrasonography imaging to measure the peak venous velocities produced in superficial femoral veins by the individual and simultaneous use of graduated compression stockings and intermittent pneumatic compression boots.

Although their study was not designed to consider the effect on thrombosis formation, blood flow velocity served as a surrogate measure for the limited stasis and venous evacuation that could ultimately lead to the formation of thromboses. The results showed no synergistic effect on venous out flow when these devices were used simultaneously.

Two separate groups performed direct comparisons between compression stockings and pneumatic devices to assess the effectiveness of these modalities in reducing the risk of venous thrombosis.[9,52] Based on their results, stockings appear to be as effective in VTE prevention as pneumatic compression devices, and stockings have the added benefit of lower cost.

Patients also suffer less discomfort with stockings when compared with the pressure associated with intermittent leg compressions from pneumatic devices. In addition, the garments do not require assembly once fitted, are not subject to malfunction or trouble shooting, and facilitate patient mobility and ambulation during the postoperative recovery period.

Despite these findings, the combined approach in which both stockings and pneumatic devices are used remains popular, likely because of their relative ease of application or simply out of habit. Cost-conscious healthcare may limit their combined use in the future.

In the meantime, practitioners should not be overly concerned when discomfort leads to discontinuation of pneumatic compression; these patients are not at any proven risk as long as they continue to use their compression stockings.

Conclusions

Mechanical prophylaxis for thromboemboli is an integral part of modern neurosurgical practice. It remains an effective means of preventing DVT and PE, both intra- and postoperatively.

Compression stockings and pneumatic devices are relatively equivalent in their prevention of VTE. Their effectiveness is not diminished when one leg is not treated with compression during motor mapping.

Concomitant anticoagulation therapy during the postoperative period appears not only to be safe but also to protect patients further against thromboembolic complications

Neurosurg Focus 17(4), 2004.

Thanks to MEDSCAPE.com


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