Acquired Platelet Function Defects

Common Thrombotic Disorders Defined by CBC Platelet Parameters

The Coulter STKS mean platelet volume (MPV), platelet distribution width (PDW), mean corpuscular volume (MCV), and hematocrit (HCT) are assessed in thrombotic diseases. Complete blood counts (CBCs) were studied retrospectively in 645 consecutive hospital cases and, 850 "CBCs with blood films to the pathologist." Because the MPV and PDW are time dependant, nomograms and tables were created for the 0-25- and 26-40-min elapsed time intervals. In this study, abnormal CBC platelet parameters were found in the majority of patients with common thrombotic diseases, such as diabetes, atherosclerosis, hypertension, congestive heart failure, deep vein thrombosis (DVT), and toxemia. Thrombotic disease-related CBC categories include combined elevated MPV and PDW, elevated MPV or PDW alone, "MCV/MPV mismatch" (failure of physiologic "MCV/MPV match"), rheologic problem of elevated HCT and MPV, and mild thrombocytopenia with decreased MPV and high PDW. Anticoagulants and steroids often decrease the MPV. CBC platelet parameter abnormalities provide a new useful classification of thrombotic diseases in daily practice.

Patterns of Acquired Bleeding Disorders in a Tertiary Care Hospital

Bleeding disorders constitute a large proportion of referrals to hematology departments. Worldwide, acquired causes of bleeding are commoner than inherited ones. To identify the spectrum of these disorders, we evaluated all referrals for bleeding encountered in this tertiary care centre over a one-year period. Of the total 1342 cases, 1040 (77.5%) had underlying exclusively acquired causes, whereas inherited causes constituted 302 cases (22.5%). Amongst acquired causes, disseminated intravascular coagulation was seen in 297 (28.6%), hepatic coagulopathy in 218 (20.9%), neurosurgical causes (intracranial bleeds) in 154 (14.8%), malignancy in 89 (8.6%), and miscellaneous multiple acquired causes including those due to anticoagulant drug overdose in 282 patients (27.1%). Referrals for isolated prolonged prothrombin time or thrombocytopenia were common, but were excluded from this study because not all presented with bleeding. Prompt laboratory work-up and precise identification of acquired causes of bleeding is the key to planning appropriate patient management including transfusion support.

Effects of low temperature on shear-induced platelet aggregation and activation

BACKGROUND

Hemorrhage is a major complication of trauma and often becomes more severe in hypothermic patients. Although it has been known that platelets are activated in the cold, studies have been focused on platelet behavior at 4 degrees C, which is far below temperatures encountered in hypothermic trauma patients. In contrast, how platelets function at temperatures that are commonly found in hypothermic trauma patients (32-37 degrees C) remains largely unknown, especially when they are exposed to significant changes in fluid shear stress that could occur in trauma patients due to hemorrhage, vascular dilation/constriction, and fluid resuscitation.

METHODS

Using a cone-plate viscometer, we have examined platelet activation and aggregation in response to a wide range of fluid shear stresses at 24, 32, 35, and 37 degrees C.

RESULTS

We found that shear-induced platelet aggregation was significantly increased at 24, 32, and 35 degrees C as compared with 37 degrees C and the enhancement was observed in whole blood and platelet-rich plasma. In contrast to observation made at 4 degrees C, the increased shear-induced platelet aggregation at these temperatures was associated with minimal platelet activation as determined by the P-selectin expression on platelet surface. Blood viscosity was also increased at low temperature and the changes in viscosity correlated with levels of plasma total protein and fibrinogen.

CONCLUSION

We found that platelets are hyper-reactive to fluid shear stress at temperatures of 24, 32, and 35 degrees C as compared with at 37 degrees C. The hyperreactivity results in heightened aggregation through a platelet-activation independent mechanism. The enhanced platelet aggregation parallels with increased whole blood viscosity at these temperatures, suggesting that enhanced mechanical cross-linking may be responsible for the enhanced platelet aggregation.

Syndromes of disseminated intravascular coagulation in obstetrics, pregnancy, and gynecology. Objective criteria for diagnosis and management

This article presents current understanding of the causes, pathophysiology, clinical, and laboratory diagnosis, and management of fulminant and low-grade DIC, as they apply to obstetric, pregnant, and gynecologic patients. General medical complications leading to DIC, which may often be seen in these patients, are also discussed. Considerable attention has been given to interrelationships within the hemostasis system. Only by clearly understanding these pathophysiologic interrelationships can the obstetrician/gynecologist appreciate the divergent and wide spectrum of often confusing clinical and laboratory findings in patients with DIC. Objective clinical and laboratory criteria for diagnosis of DIC have been outlined to eliminate unnecessary confusion and the need to make empiric decisions regarding the diagnosis. Particularly in the obstetric patient, if a condition is observed that is associated with DIC, or if any suspicion of DIC arises from either clinical or laboratory findings, it is imperative to monitor the patient carefully with clinical and laboratory tools to assess any progression to a catastrophic event. In most instances of DIC in obstetric patients, the disease can be ameliorated easily at early stages. Many therapeutic decisions are straightforward, particularly in obstetric and gynecologic patients. For more serious and complicated cases of DIC in these patients, however, efficacy and choices of therapy will remain unclear until more information is published regarding response rates and survival patterns. Also, therapy must be highly individualized according to the nature of DIC, patient's age, origin of DIC, site and severity of hemorrhage or thrombosis, and hemodynamic and other clinical parameters. Finally, many syndromes that are often categorized as organ-specific disorders and are sometimes identified as independent disease entities, such as AFE syndrome, HELLP syndrome, adult shock lung syndrome, eclampsia, and many others, either share common pathophysiology with DIC or are simply a form of DIC. These entities represent the varied modes of clinical expression of DIC and illustrate the diverse clinical and anatomic manifestations of this syndrome.

Cirrhosis does not affect the pharmacokinetics and pharmacodynamics of clopidogrel

Clopidogrel, a new platelet ADP receptor antagonist used for the prevention of vascular ischemic events, is converted to an active metabolite via the cytochrome P450 system. Patients with cirrhosis may not metabolize drugs normally and may, in addition, have a number of defects in the coagulation system. To assess the effect of cirrhosis on the pharmacokinetics and pharmacodynamics of clopidogrel, the authors performed an open-label, parallel-group study of 12 patients with Child-Pugh Class A or B cirrhosis and 12 matched controls. All 24 subjects received clopidogrel 75 mg PO QD for 10 days. Pharmacokinetics of clopidogrel and the major metabolite SR 26334 were analyzed on Days 1 and 10; pharmacodynamics were assessed by the inhibition of ADP-induced platelet aggregation and by bleeding time prolongation factor. Pharmacokinetic analysis of clopidogrel was limited due to low plasma concentrations arising from rapid hydrolysis to SR 26334. The Cmax at SS for clopidogrel was higher in cirrhotics than in normals. However, exposures to the metabolite SR 26334, as measured by AUC(tau), were comparable. At Day 10, there was not a statistically significant difference in mean inhibition of platelet aggregation (49.2% +/- 38.6% in cirrhotics vs. 66.7% +/- 7.5% in normals) or in bleeding time prolongation factor (1.64 +/- 0.49 in cirrhotics vs. 1.54 +/- 0.87 in normals) between groups. No significant adverse events, including bleeding events, were reported. In conclusion, there were no significant differences in the pharmacokinetics and pharmacodynamics of clopidogrel in this group of subjects with cirrhosis and matched normals. Therefore, no dosage adjustment of clopidogrel is required in patients with Child-Pugh Class A or B cirrhosis.

Platelet function defects associated with hemorrhage or thrombosis

Platelet dysfunction, especially acquired forms, is a common cause of hemorrhage, especially when associated with trauma or surgery. Although the hereditary platelet function defects are generally quite rare, hereditary storage pool disease is common enough to be suspected in an individual, usually a child, with characteristic historical and clinical findings. The acquired platelet function defects, especially those resulting from drugs, are common and should promptly be suspected in patients developing easy and spontaneous bruising, mild-to-moderate mucosal membrane hemorrhage, or unexplained bleeding associated with trauma or surgery. The template bleeding time is generally useful as a screening test of platelet function, but a normal template bleeding time, in the face of a suggestive history, suggestive clinical findings, or in a patient frankly bleeding, is not reliable, and platelet aggregation or lumiaggregation should be done in appropriate clinical situations. Also, prolongation of the template bleeding time is an unreliable predictor of clinical bleeding propensity. The mainstay of therapy for almost all these defects, if bleeding is significant, is the liberal infusion of appropriate numbers of platelet concentrates. The acquired platelet function defects should also be managed by attempts to treat or control the underlying disease, if possible, and offending drugs or potentially offending drugs should immediately be stopped.

Extracardiac thrombotic, embolic, and hemorrhagic causes of sudden death

The integrity of the vascular tree is essential to sustain life. However, blood vessels are the most vulnerable structure in the cardiovascular system, and disruption of the vasculature by occlusion (thrombosis/thromboembolism) or leakage (hemorrhage) can lead to sudden death. This review will concentrate on the extracardiac manifestations of two opposite, but intimately related, pathophysiologic processes that disturb vascular integrity: namely, thrombosis and hemorrhage. Thrombosis is a pathological process that leads to activation of the hemostatic system at an abnormal time and location. We are only just beginning to appreciate the myriad causes of thrombosis, some of which include vascular factors such as stasis and endothelial injury, as well as blood factors such as excessive coagulation, decreased coagulation inhibitors, or decreased fibrinolysis. Hemorrhage is a frequent cause of sudden death and may be attributable to blood vessel disease/rupture or to abnormalities within the blood itself that impair hemostasis. In this review, the pathophysiologic mechanisms of thrombosis/thromboembolism and hemorrhage are delineated and multiple system-specific disease entities are discussed.