Prevention and therapy of thromboembolic complications with dextran

Dextran-70 inhibits equine platelet aggregation induced by PAF but not by other agonists

Dextrans of mean molecular weight 70 kDa (dextran-70) have had clinical use as anti-thrombotics in man. A major part of the anti-thrombotic action is mediated via inhibition of platelet function. Greatorex (1975, 1977) treated thromboembolic colic in horses with infusions of dextran-70 and reported a 90% recovery rate, but this treatment is nonetheless rarely used. We have used an in vitro method to examine the effect of dextran-70 on equine platelet suspensions, in the hope that understanding the mechanism of action of dextran-70 might lead to the development of alternative therapeutic agents. The effects of dextran-70 on equine platelets occurred immediately in vitro with an initial activation and shape change. Subsequent assessment of aggregation revealed a dose-dependent specific inhibition of platelet-activating factor (PAF)-induced aggregation, significant in rate of aggregation at dextran-70 concentrations >40 g/l (P<0.05) and in extent of aggregation at dextran-70 concentrations >50 g/l (P<0.05). Pre-incubation with 60 g/l dextran-70 significantly inhibited the rate and extent of aggregation in response to PAF (1 nmol/l) (P<0.001 and P = 0.003, respectively) but this was not dependent on the duration of pre-incubation (from 0 to 150 min). No effects were seen when the agonist was adenosine 5'-diphosphate (200 nmol/l), collagen (10 mg/l), 5-hydroxytryptamine (100 micromol/l) or U44069 (600 nmol/l) (all P>0.1). Analysis of PAF concentration-aggregation curves after pre-incubation with 60 g/l dextran-70 indicated significant noncompetitive inhibition by dextran-70 (P<0.001 for rate and extent of aggregation). The ability of dextran-70 to inhibit responses of equine platelets to PAF is probably an important component of its beneficial effect as an anti-thrombotic in colic cases.

Intravascular aggregation after acute intracranial hypertension by epidural balloon compression in cats

The authors have studied the effect of acute intracranial hypertension produced by placement of an epidural balloon (control group) in cats, on cerebral perfusion, evoked responses, and hematological parameters. These elements were measured in similarly injured animals which underwent isovolemic hemodilution with dextran 75, after relief of intracranial hypertension. Four hours after balloon deflation, perfusion was markedly impaired in 30% of the control group, and was reduced to 11% in the dextran-infused group. The suppressed N1 amplitude of somatosensory evoked responses on the compression side, the reduced platelet aggregability, and the erythrocyte-deformability by intracranial hypertension were all significantly more restored in the dextran-infused group after decompression. The percentage of platelets with volumes between 21.75 and 48.75 cu mu (normal 9.75 to 12.75 cu mu) significantly increased after decompression. Activation of platelets during intracranial hypertension leads to an increase in platelet volume from platelet aggregation, and correlates with a decrease in platelet aggregability. It was also suggested that reduction of erythrocyte deformability was not caused by erythrocyte aggregation. The authors emphasize the role of intravascular factors such as vascular obstruction by platelet aggregates, and difficulty in passage of erythrocytes through capillaries due to reduced deformability, in the disturbance of the microcirculation following acute intracranial hypertension. The protective effect of dextran 75 by inhibition of platelets as well as hemodilution is stressed.

Low molecular weight dextran and reexploration in the management of ischemic midgut-volvulus

Administration of low molecular weight dextran and reexploration in 36-48 hr, is recommended in cases of ischemic midgut volvulus. Five cases are presented utilizing this technique and many centimeters of small bowel were saved that would have been resected at time of the original exploration. One patient died as a result of sepsis due to central intravenous nutrition: the others are alive and well.

The persistence of dextran 70 in blood plasma following its infusion, during surgery, for prophylaxis against thromboembolism

An infusion of dextran (mean molecular weight 70000) in normal saline (either 1 litre or 500 ml) was given to patients undergoing hysterectomy. The infusion was started at induction of anaesthesia and continued throughout the operation and for up to 5 h thereafter. The rate of elimination of dextran was independent of the dose given. The time to eliminate half the dose was nearly two days and up to 10% was still present in the circulation after one week. The persistence of dextran in the plasma in these amounts and for this length of time may have considerable implications in the prophylaxis of postoperative deep venous thrombosis.

Dextran and the prevention of postoperative thromboembolic complications

1. The purpose of fluid administration is not only the restoration of blood volume but also the normalization of impaired nutritive flow. 2. Plasma oncotic (colloid osmotic) pressure is the only force which can draw water into the circulation. In shock the infusion of colloid solutions is able to normalize nutritive flow and peripheral resistance almost at once. 3. Five per cent solutions of pasteurized plasma protein or albumin and 6 per cent dextran 70 yield a volume expansion corresponding to the amount infused. 4. The decrease in hematocrit produced by the infusion of these three colloidal solutions is accompanied by a decrease in whole blood viscosity resulting in a rise in cardiac output as well as in nutritional tissue flow. 5. Hemodilution improves oxygen supply as long as the hematocrit does not fall below 30 per cent, although normovolemia is the critical requirement. 6. Transmission of viral hepatitis is still the greatest danger of blood transfusion. 7. The use of large amounts of Ringer's lactate is not advised, as this solution does not reduce the total number of units of blood which need to be given. Pulmonary edema may become a problem. 8. Dextrans are best suited to initial volume replacement in shock. They increase plasma volume, improve blood flow, have antithrombotic properties, and are easily available and relatively cheap. Anaphylactoid reactions are rare. 9. Every third patient undergoing general surgery and every other patient having hip surgery develops a deep venous thrombosis. Widespread prophylaxis to prevent thromboembolic complications is mandatory. 10. The antithrombotic properties of dextran are due to a reduction in platelet adhesiveness, a change in fibrin clot structure, and the increased lysability of thrombi and the improvement of blood flow. 11. In a personal controlled, prospective, randomezed trial comparing subcutaneous heparin and intravenous dextran 40, 35.8 per cent of the controls (n=95), 13.2 per cent of the 83 patients in the heparin group, and 20.5 per cent in the dextran group (n=83) developed deep venous thrombosis. The difference between dextran and heparin is not significant; however, both treatment groups show a statistically significant effect compared to the controls.

Platelet-surface interaction: effects of dextran 70 on platelet retention in extracorporeal surfaces

Dextran 70 infusion into dogs prevents platelet adhesion to extracorporeal surfaces as effectively as direct coating of the surfaces with albumin. This is demonstrated by two experimental approaches. One of these involves a detailed analysis in 14 mongrel dogs of the effects of hemoperfusion on the systemic platelet levels, hematocrits, and dextran concentrations. The second approach is in the form of 23 platelet retention tests carried out in four mongrel dogs.

Volume expansion and flow promotion in shock

Volume replacement is the most important step in treating hypovolaemic shock.

Blood is needed when the oxygen carrying capacity threatens to fall below a critical level, but has the disadvantage of transmitting virus hepatitis. Anicteric hepatitis is about four times more frequent than the icteric form. Pasteurized plasma protein solution and albumin are free from the risk of transmitting hepatitis virus, and are good volume restorers.

Dextran 70 represents the best artificial colloid with additional anti-thrombotic properties. Dextran 40 is indicated in special situations to promote flow.

There is no proof that large amounts of Ringer solution are superior for treating hypovolaemic shock in man compared with colloids and electrolytes.