Prostacyclin substitution for heparin in long-term hemodialysis

The in vitro Bleeding Time While Using a Stable Prostacyclin Analogue during Hemodialysis

A serious disadvantage of preventing clot formation by using prostacyclin (PGI2) to inhibit thrombocyte function during dialysis is that there exists no rapidly measurable monitoring parameter. The “in vitro bleeding time” fin vitro BT) is a new method for measuring primary hemostasis in vitro. Five chronic dialysis patients each underwent two dialyses: 1) with conventional full heparinization, and 2) with the stable PGI2 analogue CG 4203 and additional “low-dose” heparin. The predialytic in vitro BT is longer than normal values and values 1 h after the end of dialysis. While heparin has no significant effect on the in vitro BT, CG 4203 prolongs it concentration-dependently. Infusing CG 4203 at a rate of 25 ng/kg/min, the in vitro BT is extended beyond the measurable range of 800 μl during the first approx. 150 min of dialysis. During the next approx. 90 min it steadily decreases.

Hemodialysis with Defibrotide: Effects on Coagulation Parameters

In a crossover study conducted with eight uremic patients maintained on hemodialysis, the Authors compared the effects of heparin (100 IU/kg at the start of dialysis) and defibrotide (400 mg at the start, repeated at 2 hours of ongoing dialysis) on the parameters of blood coagulation (VIII:C, AT III, TAT, PC antigen and activity, PS, and FPA), each being assessed before dialysis and at 2, 3 and 4 hours of the ongoing procedure. Heparin-assisted dialysis resulted in a significant rise of VIII:C and AT III; with defibrotide, instead, there was evidence of thrombin activation (increased FPA and TAT). PC levels were raised with both dialysis modalities; however, PC activity and PS levels were increased only in defibrotide-assisted dialysis. There were no adverse reactions or evidence of fibrin formation. These results confirm the antithrombotic activity of defibrotide in the course of dialysis and indicate that this action is independent of thrombin neutralization.

Continuous Renal Replacement Therapy (CRRT) in the Intensive Care Unit

This review is specifically designed to address the topic of CRRT based on the needs and interests of intensivists. Some of the materials, concepts, and formulas presented in this review have been drawn from a previous chapter authored by myself and intended for individuals whose primary interest is specifically dialysis[1]. Since this previous chapter was authored in 1994, similar material presented in this review has been updated in order to present the most current information.

Toxicity of plasma from hemodialysis patients treated with heparin or prostacyclin

Hemodialysis was performed in 6 uremic patients with either a bolus dose of 5 000 IU heparin or prostacyclin at a constant infusion rate of 5 ng/kg/min. Clinical data, plasma triglycerides (TG), free fatty acids (FFA), platelet aggregation, white blood cell count and plasma toxicity were measured prior to and during both procedures. No serious side-effects were recorded. Heparin induced a fall in plasma TG, a rise in FFA and increased plasma toxicity. Prostacyclin infusion had no effect on these parameters. During both tests a marked drop in white blood cell count was found 15 min after the start of hemodialysis. During heparin dialysis no clotting was observed in the extracorporeal circuit. During prostacyclin dialysis clotting was observed at the venous line in three cases.

HEMATOLOGY: ISSUES IN THE DIALYSIS PATIENT: Anticoagulation and Continuous Renal Replacement Therapy

More than half of patients with acute renal failure in the intensive care unit require dialysis, and the majority of them have significant hemodynamic instability. Continuous renal replacement therapy (CRRT) is often the preferred dialysis modality in these patients. One requirement for CRRT is anticoagulation, which can expose patients to the risk of bleeding. However, absence of effective anticoagulation may result in clotting of the CRRT circuit and subsequently less effective treatment. While heparins are widely used for anticoagulation, because of potential side effects such as bleeding and heparin-induced thrombocytopenia, alternative anticoagulation protocols should be considered. Citrate anticoagulation, regional heparin/protamine, predilution, r-hirudin, prostacyclin, and nafamostat are among these methods.

Local anticoagulation of the extracorporeal circuit with heparin and subsequent neutralization with protamine during immunoadsorption

A regimen of local anticoagulation of an immunoadsorption device was studied. The extracorporeal circuit was anticoagulated with citrate (5.5%) and a continuous infusion of heparin (2,000 U/h or 1,500 U/h), which was neutralized by a continuous infusion of protamine chloride (75% of the heparin dose) before reinfusion in 23 patients treated with low-density lipoprotein or immunoglobulin apheresis. Sufficient anticoagulation of the extracorporeal circuit was obtained (activated partial thromboplastin time [APTT] > 180 seconds; thrombin time [TT] > 120 seconds; anti-Xa activity, 1.05 +/- 0.21 U/mL) during the entire treatment of 190 minutes, whereas coagulation parameters in the patients' blood stayed within the normal range. In a control group without heparin neutralization, full systemic anticoagulation of the patients occurred (APTT, 157.8 +/- 30.6 seconds; TT, 119.8 +/- 0.4 seconds; anti-Xa activity, 0.88 +/- 0.21 U/mL). No side effects or clotting of the system were observed. Our data show that this regimen of local anticoagulation is a safe protocol for extracorporeal circulation without exposing the patients to anticoagulants.

Enhanced blood coagulation and enhanced fibrinolysis during hemodialysis with prostacyclin

In the present study the effect of unfractionated heparin (UFH) (Liquemin, 750-1000 IU/h), low molecular weight heparin (LMWH) (Fragmin, 3000-7250 IU bolus), and prostacyclin (Flolan, 5 ng/kg body weight/min) on the activation of blood coagulation and fibrinolysis, induced by polysulfone membrane dialyzers during hemodialysis, was compared. Plasma levels of thrombin-antithrombin III complex (TAT), fibrin split product D-dimer, and plasmin-plasmin inhibitor-complex (PPI) were measured in the arterial and venous line of the dialyzer at the beginning and at 10, 60, 120, and 180 minutes of hemodialysis. Five patients on chronic hemodialysis treatment were investigated in a cross over study. Clinically all three anticoagulation regimen were sufficient for hemodialysis treatment. Using UFH or LMWH TAT, PPI, and D-dimer levels were similar in the venous and the arterial line of the dialyzer. However, during prostacyclin treatment the levels of these activation markers were significantly higher in the venous line. Based on these data the dialyzer membrane can be considered as a site of activation of blood coagulation and of fibrinolysis during anticoagulation with prostacyclin in hemodialysis.

Influence of prostacyclin infusion on haemodialysis efficiency and blood cells

Dialysis efficiency, platelet and leukocyte counts, as well as malonyldialdehyde (MDA) level and lactate dehydrogenase (LDH) activity in serum were assessed in 10 patients (8 males, 2 females, aged 28-58 years) treated with repeated haemodialysis due to terminal renal failure. Patients were examined twice: during a 4-hour haemodialysis in the presence of heparin as the anticoagulant, and a week later in the course of another haemodialysis combined with infusion of heparin and prostacyclin. Statistically significant lower level of urea at the end of dialysis and significantly higher urea clearance were found during haemodialysis with prostacyclin-heparin infusion in comparison with infusion of heparin alone. As compared with the initial values obtained prior to dialysis, neutropenia and thrombocytopenia were observed during haemodialysis with heparin alone but the counts remained generally unaltered when both prostacyclin and heparin were administered. During and after haemodialysis with heparin and prostacyclin both MDA level and LDH activity were lower than in case of haemodialysis with heparin alone.

Effect of heparin and prostacyclin-heparin infusion on blood coagulation in haemodialysed patients

In 10 patients (8 men and 2 women) aged 28 to 58 years (mean 44.4 years) treated by repeated haemodialysis due to end-stage renal failure, the bleeding time, whole-blood coagulation time, one-stage prothrombin time, thrombin time of plasma, activated partial thromboplastin time (APTT), fibrinogen level and euglobulin lysis time have been determined (1) during a 4-hour haemodialysis using heparin as an antithrombotic agent, and (2) one week later in the course of another haemodialysis using prostacyclin-heparin. The values for any of the above parameters with both anticoagulant treatment types did not differ. Plasma fibrinogen level after haemodialysis was significantly lower after administration of heparin alone as compared with the group treated by prostacyclin-heparin infusion. During haemodialysis performed with prostacyclin-heparin infusion, activation of the blood fibrinolytic system was manifested by a significant shortening of euglobulin lysis time, observed after 1.5 hours and after the end of haemodialysis. The above phenomenon did not occur when haemodialysis was performed with heparin alone.

[Comparison of the antithrombotic effects of heparin, enoxaparin and prostacycline in continuous hemofiltration]

Continuous anticoagulation is required during haemofiltration to prevent the deposition of fibrin and the formation of thrombus which would lead to early clotting of the haemofilter. This study aimed to compare the efficiencies of 3 different anticoagulation protocol: 150 IU.kg-1.day-1 heparin (group HEP), 1.2 mg.kg-1.day-1 enoxaparin (group ENX), and a combination of 0.8 mg.kg-1.day-1 enoxaparin with 5 ng.kg-1.min-1 prostaglandin I2 (group ENX and PGI2). A flat ANS69S (Hospal) haemofilter was used for continuous venovenous haemofiltration. Antithrombotic efficiency was assessed with a haemofilter permeability index (HPI) including the transmembraneous pressure gradient and the rate of production of ultrafiltrate. The time required for HPI to decrease to 1/3 of its initial value (HPI1/3) was used to compare the 3 protocols. Treatment tolerance was judged by monitoring the usual haemodynamic and haemostatic parameters. No adverse effects (bleeding, thrombosis, hypotension) were observed. HPI1/3 was 15.1 +/- 2.4 h, 18.3 +/- 3.1 h and 28.2 +/- 4.2 h in groups HEP, ENX and ENX and PGI2 respectively. High dose enoxaparin reached antithrombotic efficiency without increasing the risk of haemorrhage. The use of low doses of prostaglandin I2 greatly increased HPI1/3, without any deleterious haemodynamic effects. However, the high cost of prostaglandin I2 needs to be put in the balance with the increase in duration of haemofilter life. Therefore, further investigations are required to evaluate the possible synergy between heparin and prostaglandin I2, as well as the biological parameters which need to be monitored.

Role of platelet inhibitor therapy in myocardial infarction

Atherosclerotic plaque disruption is the predominant pathogenetic mechanism underlying the acute coronary syndromes. Plaque rupture leads to the exposure of collagen and vessel media, resulting in platelet and clotting activation, and occlusive thrombus formation. While drugs that interfere with platelet activation and function have been available for years, more powerful agents with novel mechanisms of action are being developed. Of the available platelet inhibitor drugs, only aspirin, sulfinpyrazone, and dipyridamole have undergone extensive clinical testing in patients with cardiovascular disease. More recently ticlopidine, a new and potent platelet inhibitor, has been successfully tested in patients with coronary and vascular disease. In acute myocardial infarction, aspirin significantly reduces cardiovascular mortality and reinfarction. Furthermore, the combination of aspirin and a thrombolytic agent produces maximal benefit. A role for heparin in the prevention of early mortality and reinfarction is emerging. This drug is effective for the prevention of left ventricular thrombosis in patients with anterior myocardial infarction. In the secondary prevention of reinfarction and cardiovascular mortality, available data support the use of a platelet inhibitor. Trials have shown that aspirin is as effective alone as in combination with dipyridamole, and is probably more effective than sulfinpyrazone. Long-term anticoagulant therapy also appears to be beneficial, but is associated with a high cost, need for extensive monitoring, and potential for hemorrhagic side effects. The role of aspirin in primary prevention is controversial. It may be indicated for patients at high risk for coronary disease in whom the benefit of therapy may outweigh the potential risk of cerebral bleeding. Coronary atherosclerotic plaque rupture, associated with thrombus formation, is fundamental to the development of acute myocardial infarction. Based on this concept, the role of antithrombotic therapy for the prevention or treatment of ischemic events in patients with coronary artery disease has stimulated enormous interest among clinicians and basic investigators. In this review we will examine: a) the pathogenesis of coronary thrombosis, b) the pharmacology of platelet-inhibitor agents, and c) their role in the management of patients with acute myocardial infarction and in primary and secondary prevention of cardiovascular disease. Platelets interact with both the coagulation and fibrinolytic systems in the pathogenesis of thrombosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Platelet inhibitor agents in cardiovascular disease: an update

Platelets interact with the coagulation and fibrinolytic systems in the maintenance of hemostasis. However, these physiologic mechanisms may become pathologic, requiring prevention and treatment. In this review, the following clinical developments are analyzed: 1) the role of platelets in thrombogenesis; 2) the pharmacology of platelet inhibitory agents; and, most important, 3) the results of recent randomized trials of platelet inhibitor agents in different cardiovascular disorders. Aspirin reduces mortality and infarction rates in unstable angina and significantly decreases vascular mortality in acute myocardial infarction. Platelet inhibitors decrease mortality and recurrent cardiovascular events in the chronic phase after myocardial infarction. They also decrease vein graft occlusion rates after coronary bypass surgery. Although platelet inhibitors are beneficial in preventing acute vessel occlusion during coronary angioplasty, they are ineffective in preventing chronic restenosis. Antiplatelet agents, combined with warfarin, reduce thromboembolic events in patients with a mechanical prosthesis. Platelet inhibitors are also effective in secondary prevention of vascular events in patients with cerebrovascular disease. Finally, the use of aspirin for primary prevention of cardiovascular disease is still evolving, particularly in individuals at high risk. In conclusion, platelet inhibitors are effective in patients with a variety of cardiovascular disorders. The best studied, most inexpensive and least toxic agent is aspirin at a daily dose of 160 to 325 mg. Studies using new platelet inhibitor agents with different mechanisms of action are currently underway.

Beneficial effect of low molecular weight heparin on the hemodialysis model in dogs

The effect of FR-860, one of low molecular weight heparins, was investigated on the hemodialysis model in dogs for comparison with that of conventional unfractionated heparin (UF-heparin). In a bolus injection model, FR-860 at 12.5-100 U/kg prolonged the dialysis time (the time until the arterial circuit pressure reaching 500 mmHg) in a dose-dependent manner and UF-heparin at 50 U/kg also prolonged it. Additionally, FR-860 (12.5-50 U/kg/hr) and UF-heparin (25 and 50 U/kg/hr) continuously inhibited the rise of the arterial circuit pressure in an infusion model. In both models, the efficacy of FR-860 was more potent than that of UF-heparin. FR-860 and UF-heparin decreased the amount of blood loss remaining in the dialyzer in the infusion model. During hemodialysis, both FR-860 and UF-heparin showed activated plasma anti-F.Xa activity, prolongation of aPTT and such of thrombin time in a dose-dependent manner in those models. However, FR-860 was higher in anti-F.Xa activity and weaker in prolongation of aPTT and thrombin time than UF-heparin. These results suggest that FR-860 is more beneficial in in efficacy and higher in safety against the bleeding risk than UF-heparin hemodialysis.

Heparin and Prostacyclin

Hemodialysis performed with prostacyclin (5 ng/kg/min) as a sub stitute for heparin was studied in 10 patients. The subjects were studied during heparin perfusion alone and during heparin perfusion together with prostacyclin. The authors inves tigated the effect of two heparin regi mens (regimen I: 2,000 U/hr and reg imen II: 500 U/hr) upon plasma anti thrombin level (IU/mL) and activated thromboplastin time (sec).

Our findings show: (1) prostacy clin can substitute for heparin anti coagulaton in hemodialysis; (2) the concomitant administration of pro stacyclin enhances the anticoagulant effect of heparin, based on the mea surement of the activated partial thromboplastin time; (3) the anti thrombin activity is increased by both treatments but more so with prostacyclin; and (4) platelet activa tion plays a role in limiting heparin anticoagulation, a conclusion partly supported by the finding that acti vated partial thromboplastin time is somewhat more prolonged by hepa rin when measured in platelet-poor rather than in platelet-rich plasma in the presence of prostacyclin. Physio pathologic implications of these pre liminary findings are discussed.

Safety of regional citrate hemodialysis in acute renal failure

Regional citrate anticoagulation is an alternative to heparin anticoagulation for hemodialysis of patients at increased risk of bleeding. We report the successful use of this technique in 326 dialyses in 49 high bleeding risk patients with acute renal failure. Systemic anticoagulation did not occur as a result of any dialysis procedure, and in no instance was bleeding observed. Dialysis was effective, as judged by removal of creatinine. The safety of this procedure is demonstrated by the lack of bleeding complications and the small incidence of electrolyte and acid-base abnormalities. In addition we document the absence of citrate intoxication by serial measurements of serum citrate levels. Regional citrate anticoagulation is a safe and effective method of performing hemodialysis in patients with acute renal failure at increased risk of bleeding.

Epoprostenol (PGI2, prostacyclin) during high-risk hemodialysis: preventing further bleeding complications

The frequency of hemodialysis-associated hemorrhage was studied prospectively in two successive, parallel, heparin-controlled studies using epoprostenol (PGI2; average dose, 4.1 ng/kg.min) as the sole antithrombotic agent. Sixty-three patients with active or recently active bleeding underwent 163 hemodialysis treatments in each of which prospective bleeding risk was assessed. PGI2 was associated with up to 50% overall reduction in the frequency of bleeding, particularly in the highest risk circumstances. PGI2 also allowed successful completion of the full, prospectively prescribed hemodialysis time in the most treatments (82% versus 93% with heparin). Furthermore, the efficiency of hemodialysis using PGI2, as indicated by the reduction in concentration of blood urea nitrogen and serum creatinine, was equal to that using heparin, even though there was a tendency toward modest reduction in residual volume of the hollow fiber dialyzer and slightly more frequent early termination of treatment from dialyzer clotting with PGI2. No severe vasodilatory side effects of PGI2 were observed during these studies. Hypotension was equally frequent during hemodialysis with heparin as with PGI2. The current results suggest that PGI2 should be considered as a substitute for heparin during high-risk hemodialysis because PGI2 may reduce the incidence of dialysis-associated bleeding without severe adverse side effects.

Prostacyclin and iloprost do not affect action of standard dose heparin on haemostatic function during haemodialysis

The actions of prostacyclin and iloprost, as supplements to heparin therapy, were examined by measuring indices of blood coagulation and platelet activation during the first two hours of haemodialysis in six patients with stable chronic renal failure. Patients received either prostacyclin 5 ng kg-1 min-1, iloprost 2 ng kg-1 min-1 or placebo at random on three separate occasions as a supplement to standard therapy with heparin 50 I.U. kg-1 loading dose and 30 I.U. kg-1 hr-1 infusion. Neither prostacyclin nor iloprost significantly affected dialysis induced changes in whole blood platelet count and plasma concentrations of beta thromboglobulin, (beta TG) platelet factor 4 (PF4), fibrinogen and fibrinopeptide A. Thus platelet count still fell by 12 +/- 8% within 15 min of the start of dialysis, slowly returning to the initial count over the study period and plasma PF4 increased nearly 10 fold over the same time. In addition, serial measurements of KCCT failed to indicate any sparing effect of the prostanoids on this dose of heparin.

Extracorporeal enzymatic heparin removal: use in a sheep dialysis model

Extracorporeal medical devices such as the hemodialyzer rely on systemic heparinization to prevent thrombus formation. Heparin, however, can lead to serious hemorrhagic complications. A blood filter containing immobilized heparinase, a heparin specific enzyme, was used to degrade heparin into small fragments which have significantly less anticoagulant activity than the parent compound. The heparinase filter was tested in the extracorporeal circuit during the hemodialysis of adult sheep. At a blood flow of 200 ml/min, the clearance of heparin varied from 50 to 70 ml/min (N = 16) depending on the amount of immobilized heparinase in the filter. Hemolysis was insignificant as measured by the animals' red cell counts, hematocrit, total hemoglobin and a plasma-free hemoglobin value of 89 +/- 33 mg/dl (N = 16) (less than 1% of the total hemoglobin). The white cell counts dropped to 47 +/- 7% (N = 16) of the initial value at 20 minutes and rebounded to 72 +/- 10% (N = 16) after one hour. The platelet counts decreased to 55 +/- 8% (N = 16) of the initial value after one hour. No change in heparin clearance was observed when reactors were used repeatedly in adult sheep over a 10 week period. The red cell counts, white cell counts, platelet counts, total hemoglobin and hematocrit did not change after 10 weeks of exposure to the device. These results suggest that with further study, heparinase may be useful in removing heparin used to anticoagulate blood in extracorporeal circuits.

Plasma Exchange with Prostaglandin I2 and ACD Solution: Comparative Effects

Discontinuous plasma exchange with prostaglandin I2 (5ng/kg/min) and low dosage heparin (5-6 IU/kg/min) (treatment I), and ACD solution alone (treatment II) was studied. During both treatments the activated partial thromboplastin time remained within the normal range. After treatment I platelet count was not decreased but in vitro platelet aggregation was reduced (p < 0.001). After treatment II platelet count was reduced and in vitro platelet aggregation unchanged.

Prostaglandin I2 at this dosage caused no cardiovascular complications. The physiopathological implications of these differences are discussed.

Regional anticoagulation: hemodialysis with hypertonic trisodium citrate

We have developed a simplified method for performing regional citrate anticoagulation during hemodialysis. High ultrafiltration rates and specialized equipment were obviated by the use of a 1.6-mol/L trisodium citrate solution and a standard calcium-containing dialysate. Thirty-six dialyses were performed with this technique on 14 stable and 22 high bleeding risk patients. There was no significant decline in plasma-ionized calcium during citrate dialysis, ie, 3.85 +/- 0.34 mg/dL (mean +/- SE) predialysis, to 3.31 +/- 0.26 postdialysis; furthermore, no patient developed neuromuscular symptoms or evidence of cardiovascular instability from hypocalcemia. Serum sodium rose with this procedure, but not to hypernatremic levels. This method of citrate dialysis is safe and effective during continuous blood flow (double-needle) hemodialysis, and is no more difficult to perform than conventional heparin dialysis. Single-needle (reciprocating blood flow) hemodialysis was successfully performed by the additional use of a calcium-free dialysate and separate calcium chloride infusion (10% calcium chloride), but risks the production of unexpected hypercalcemia.

Ginger: inhibition of thromboxane synthetase and stimulation of prostacyclin: relevance for medicine and psychiatry

Ginger, the common spice, has recently been found to act as a potent inhibitor of thromboxane synthetase, raising levels of prostacyclin, without a concomitant rise in PGE2 or PGF2 alpha. Indications for use of ginger to replace either thromboxane inhibitors having serious side effects or prostacyclin are given.

The cardiovascular and platelet actions of 9 beta-methyl carbacyclin (ciprostene), a chemically stable analogue of prostacyclin, in the dog and monkey

9 beta-Methyl carbacyclin (9 beta Me; ciprostene) is a synthetic, chemically stable analogue of prostacyclin (PGI2; epoprostenol). The platelet anti-aggregating and cardiovascular effects of 9 beta Me have been compared to PGI2 in anaesthetized monkeys and dogs. In addition, their haemodynamic effects have been compared in open-chest anaesthetized dogs and conscious dogs. Intravenous infusion of 9 beta Me and PGI2 to the anaesthetized monkey resulted in a dose-dependent hypotension, tachycardia and inhibition of ex vivo ADP-induced platelet aggregation. 9 beta Me was 72 times less active than PGI2 both as a hypotensive and anti-aggregating agent. Intravenous infusion of 9 beta Me and PGI2 to the anaesthetized beagle dog resulted in a qualitatively similar haemodynamic profile. Thus both substances induced a dose-dependent hypotension accompanied initially by a slightly increased heart rate, a dose-dependent increase in cardiac output, stroke volume and an increased peak LV dP/dt. At the higher doses studied, the initial increases in the parameters measured were succeeded by dose-dependent falls. 9 beta Me was 76 times less active than PGI2 as a hypotensive agent. In the anaesthetized greyhound, a dose-dependent anti-aggregating and hypotensive effect was seen with either drug, with 9 beta Me being 23 and 40 times less active than PGI2, respectively. Intravenous infusion of 9 beta Me and PGI2 to the conscious beagle dog induced a dose-dependent hypotension and a variable effect on heart rate. 9 beta Me was 33 times less active than PGI2 as an hypotensive agent. The duration of the hypotensive response induced by 9PMe was not significantly different from that induced by PGI2 in either monkey or beagle dog.

A chemically stable analogue, 9 beta-methyl carbacyclin, with similar effects to epoprostenol (prostacyclin, PGI2) in man

The effects of 9 beta-methyl carbacyclin, a chemically stable analogue of epoprostenol (prostacyclin, PGI2) were studied, in comparison with epoprostenol, both in vitro and in vivo in man. In vitro 9 beta-methyl carbacyclin and epoprostenol inhibited platelet aggregation induced by ADP, collagen, the endoperoxide analogue U46619 and arachidonic acid. The potency of 9 beta-methyl carbacyclin relative to epoprostenol was comparable in ADP and collagen-aggregated platelet rich plasma (PRP), 9 beta-methyl carbacyclin being 0.01 times as active as epoprostenol. The anti-aggregatory potencies of the two compounds were comparable in PRP and whole blood. The phosphodiesterase inhibitor isobutyl methyl xanthine enhanced the anti-aggregatory activity of both compounds in vitro. 9 beta-methyl carbacyclin and epoprostenol elevated platelet cyclic AMP, 9 beta-methyl carbacyclin being 0.04 times as active as epoprostenol. In a placebo controlled trial both drugs produces significant headache and facial flushing when compared with placebo. Nasal stuffiness, abdominal discomfort and nausea were reported on all three treatments. Both drugs caused significant and comparable increase in heart rate and decrease in pre-ejection (PEP) and PEP/left ventricular ejection time (LVET) ratio compared with placebo. Systolic and diastolic blood pressure, LVET and QS2 index were unchanged. Platelet aggregation responses to ADP were significantly inhibited by all three doses of both drugs compared with placebo. Bleeding time was significantly longer during epoprostenol infusion than either placebo or 9 beta-methyl carbacyclin infusion. Neither drug had significant effect, compared with placebo, on kaolin activated clotting time in PPP, PRP or in PRP in the presence of heparin, prothrombin time, partial thromboplastin time, thrombin clotting time, fibrinogen, fibrinogen degradation products or euglobulin clot lysis time. The pharmacodynamic effects and duration of action of 9 beta-methyl carbacyclin and of epoprostenol are similar; 9 beta-methyl carbacyclin is approximately 100 times less potent than epoprostenol in man.