Chapter 4 Notes (Hemostasis, Surgical Bleeding, and Transfusion)

Key Points

• Platelet lifespan and antiplatelet drugs
  • Platelet lifespan: $7\text{ to }10\text{ days}$.
  • Drugs that impair platelet function: aspirin, clopidogrel, prasugrel, dipyridamole, GP IIb/IIIa inhibitors.
  • Elective procedures: discontinue antiplatelet drugs for about $5\text{ to }7\text{ days}$ before procedure.
• Trauma-induced coagulopathy (TIC)
  • Laboratory evidence of trauma-related coagulopathy is present in up to about one-third of severely injured patients at admission, distinct from DIC and iatrogenic causes such as hemodilution.
  • Relationship between lab abnormalities and clinically evident bleeding remains unclear; mechanisms are multifactorial.
• Direct oral anticoagulants (DOACs)
  • DOACs lack easy monitoring methods; a new monoclonal antibody reverses dabigatran-related coagulopathy; reversal agents for direct Xa inhibitors are in trials.
• Anticoagulation and bridging in surgery
  • Reversal/bridging decisions should balance bleeding risk against thromboembolism risk; guide perioperative management and timing of reinstatement.
• Damage control resuscitation (DCR)
  • Three components: permissive hypotension, minimizing crystalloid resuscitation, balanced blood product ratios.
• Massive transfusion planning
  • Anticipate need and implement early, balanced transfusion strategies (RBCs, plasma, platelets).

Biology of Hemostasis

• Hemostasis goal and four key physiologic events
  • Goal: limit blood loss from injured vessel.
  • Four major events: vascular constriction, platelet plug formation, fibrin clot formation, fibrinolysis.
  • These events are interrelated; act as a continuum with reinforcements. See Fig. 4-1 for schematic.

Vascular Constriction

• Immediate response to vessel injury; strongest in vessels with medial smooth muscle.
• Mediators:
  • Thromboxane A2 (TXA2) via arachidonic acid in platelets; potent smooth muscle constrictor.
  • Endothelin from injured endothelium; potent vasoconstrictor.
  • Serotonin (5-HT) released during platelet aggregation; vasoconstrictor.
  • Bradykinin and fibrinopeptides also contribute to smooth muscle contraction.
• Degree of constriction correlates with injury; small arteries may remain open due to physical forces, whereas fully transected vessels may spontaneously stop bleeding.

Platelet Function

• Platelets: anucleate fragments of megakaryocytes; normal circulating count 150{,}000$–$400{,}000/\mu L; up to 30% sequestered in spleen.
• Lifespan: $7\text{ to }10\text{ days}$; removed by spleen if not consumed in clotting.
• Roles in hemostasis: form hemostatic plug; contribute to thrombin generation (Fig. 4-2).
• Early adhesion (primary hemostasis):
  • Injury exposes subendothelial collagen; von Willebrand factor (vWF) mediates adhesion by binding GP I/IX/V on platelet surface.
  • Platelets recruit others via a release reaction; adhesion/platelet aggregation begins; aggregation is initially reversible and not secretory.
• Key mediators:
  • ADP and serotonin drive platelet aggregation; TXA2 promotes aggregation and vasoconstriction.
• Biochemical details:
  • Arachidonic acid is converted by cyclooxygenase to PGG2/PGH2, then to TXA2; adjacent endothelial cells may convert arachidonic acid to prostacyclin (PGI2) which inhibits aggregation.
  • Cyclooxygenase (platelet COX) is irreversibly inhibited by aspirin; reversible by NSAIDs; COX-2 inhibitors do not affect this process.
  • A second wave (release reaction) releases ADP, Ca2+, serotonin, TXA2, and α-granule proteins; fibrinogen acts as a bridge for GP IIb/IIIa during aggregation; thrombospondin stabilizes fibrinogen-platelet interactions; PF4 and α-thromboglobulin are released.
  • Release reaction facilitates formation of enzymatically active platelet surfaces (platelet factor 3) and assembly of coagulation complexes on platelets: II→IIa (thrombin) depends on Xa, Va, and calcium; platelets may also participate in activation of factors XI and XII.
• Inhibition of second wave:
  • Aspirin/NSAIDs, cAMP elevation, and nitric oxide dampen the second wave of aggregation.
• Platelet aggregation and coagulation interface:
  • Fibrinogen bridges platelets via GP IIb/IIIa receptor; thrombospondin stabilizes platelet–platelet interactions.

Coagulation (Cascade and Cell-Based Model)

• Traditional view: dual pathways (intrinsic and extrinsic) converge on a common pathway, culminating in fibrin formation (Fig. 4-3). This view is simplified; actual hemostasis involves feedback loops and platelet-endothelial interactions.
• Intrinsic pathway: activation begins with factor XII (Hageman factor) leading to XI, IX, and VIII activation; all factors are intrinsic to circulating plasma with no external surface required.
• Extrinsic pathway: tissue factor (TF) exposure on endothelium binds factor VII, activating VII to VIIa; TF-VIIa initiates coagulation at the surface.
• Common pathway: activation of factor X to Xa (in presence of VIIIa); Xa with Va converts prothrombin (II) to thrombin (IIa); thrombin then converts fibrinogen (I) to fibrin; crosslinking by factor XIII forms the clot.
• Lab correlations:
  • aPTT sensitive to intrinsic pathway abnormalities (II, IX, X, XI, XII).
  • PT/INR sensitive to extrinsic pathway abnormalities (II, VII, X). Vitamin K deficiency or warfarin use affects II, VII, IX, X.
• Cell-based model of hemostasis (initiation, amplification, propagation):
  • Initiation: TF exposure leads to VIIa; small thrombin amounts produced via prothrombinase-like complexes.
  • Amplification: platelets adhere and activate; exposure to thrombin and other stimuli further amplify coagulation on cellular surfaces.
  • Propagation: on activated platelets, tenase (VIIIa/IXa) and prothrombinase (Va/Xa) generate a thrombin burst; thrombin then converts fibrinogen to fibrin and promotes clot growth.
• Thrombin and fibrin formation details:
  • Thrombin cleaves fibrinogen to form fibrin and peptides fibrinopeptides A and B; removal of AA allows end-to-end polymerization; removal of AB peptides allows side-to-side polymerization; TAFI (thrombin-activatable fibrinolysis inhibitor) stabilizes the clot by reducing fibrinolysis.
• Redundancy and regulation:
  • Anticoagulant and regulatory pathways limit propagation to prevent excessive clotting:
  • Thrombomodulin (TM) on endothelium binds thrombin, activating protein C (APC); APC + protein S inactivates factors Va and VIIIa, reducing thrombin generation.
  • TFPI inhibits TF-VIIa complex, reducing Xa and IXa production.
  • Antithrombin III (AT-III) neutralizes procoagulant serine proteases and TF-VIIa.
  • APC/Protein S system is central to thrombin inhibition; Factor V Leiden mutation renders factor V resistant to APC cleavage, increasing venous thromboembolism risk.

Fibrinolysis

• Purpose: clot breakdown to restore blood flow after healing begins; fibrin is degraded by plasmin, derived from plasminogen.
• Plasminogen activators: tissue plasminogen activator (tPA) from endothelium; urokinase plasminogen activator (uPA) also contributes; tPA preferentially activates plasminogen bound to fibrin, promoting clot-localized lysis.
• Inhibition of fibrinolysis: plasmin is inhibited by α2-antiplasmin; TAFI removes lysine residues from fibrin, reducing plasminogen binding and slowing lysis.
• Fibrin degradation products (FDPs) and D-dimers are markers of fibrinolysis. FDPs can disrupt platelet function; D-dimer presence indicates fibrin turnover.
• Balance and clinically relevant states:
  • Hyperfibrinolysis occurs in trauma-induced coagulopathy and DIC; fibrinolysis can be excessive or overwhelmed (hyperfibrinolysis vs physiologic fibrinolysis vs fibrinolysis shutdown).
  • TAFI provides a mixed effect on clot stability by processing fibrin;
    thrombin-TM complex also affects this interplay.

Regulation and Inhibitors of Thrombin

• Endothelial anticoagulant mechanisms:
  • Thrombomodulin (TM) converts thrombin to a substrate that activates APC, dampening thrombin generation.
  • APC pathway reduces Va and VIIIa activity; Protein S acts as a cofactor.
• Antifibrinolytic and fibrinolytic balance:
  • tPA promotes fibrinolysis; Plasmin is inhibited by α2-antiplasmin; PAI-1 inhibits tPA; TFPI and AT-III help curb coagulation.

Congenital and Platelet Deficiencies

Congenital Coagulation Factor Deficiencies (selected)

• Factor VIII deficiency: Hemophilia A.
• Factor IX deficiency: Hemophilia B (Christmas disease).
• Factor XI deficiency: Hemophilia C; more prevalent in Ashkenazi Jewish population.
• Factors II (prothrombin), V, X deficiencies: autosomal recessive; treated with FFP; prothrombin complex concentrates can treat II or X deficiencies; Factor V deficiency often requires FFP due to instability.
• Factor VII deficiency: rare autosomal recessive; bleeding risk correlates with level; treated with FFP or recombinant Factor VIIa; recombinant VIIa half-life ~2 h; FFP half-life ~4 h.
• Factor XIII deficiency: FXIII deficiency; delayed clot formation; replacement with FFP, cryoprecipitate, or FXIII concentrate; target 1–2% is usually adequate.
• Platelet disorders:
  • Glanzmann thrombasthenia (GP IIb/IIIa deficiency): autosomal recessive; defective platelet aggregation; treat with platelet transfusions.
  • Bernard-Soulier syndrome (GP Ib/IX/V deficiency): defective platelet adhesion; requires normal platelet transfusions.
  • Storage pool disease: loss of dense granules and/or α-granules; DDAVP may help in mild cases; more severe bleeding requires platelet transfusion.

Von Willebrand Disease (vWD)

• Most common congenital bleeding disorder; defect in vWF affecting platelet adhesion and carrying factor VIII.
• Types I (partial quantitative deficiency), II (qualitative defect), III (total deficiency).
• Clinical presentation: mucosal bleeding, easy bruising; menorrhagia in women.
• Treatments:
  • Type I: desmopressin (DDAVP) often effective.
  • Type II: variable response depending on defect.
  • Type III: usually unresponsive; vWF concentrates may be required.

Factor XI (Hemophilia C) and Other Factor Deficiencies

• Factor XI deficiency: often asymptomatic; bleeds after surgery/trauma; treated with FFP; anti-XI antibodies may require Factor VIIa in some contexts.
• Combined deficiencies (e.g., II/V/X): treatment with FFP; prothrombin complex concentrates can replace II/X; platelet transfusion considerations in combined defects.

Platelet Functional Defects (Inherited)

• Thrombasthenia (Glanzmann) and Bernard-Soulier syndromes are major inherited platelet disorders.
• Storage pool disease: decreased dense granules; variable bleeding; DDAVP helpful in mild cases; platelet transfusion for severe bleeding.

Acquired Hemostatic Defects

• Platelet abnormalities are common and can be quantitative or qualitative.
• Quantitative: bone marrow suppression, bleeding, sequestration (hypersplenism).
• Qualitative: platelet inhibitors, disease states (myeloproliferative disorders, liver disease).
• Acquired platelet disorders are reviewed in Table 4-1 (Etiology of acquired platelet disorders).

Acquired Platelet Disorders: Thrombocytopenias and Immune Thrombocytopenias

• Immune thrombocytopenia may be idiopathic or associated with autoimmune disease or low-grade B-cell malignancies; can be drug-induced.
• Primary (ITP) vs secondary ITP; children usually acute and viral-associated; adults usually chronic.
• Treatments include corticosteroids, IVIG, anti-D immunoglobulin; platelet transfusions usually avoided unless life-threatening bleeding.
• HIT (Heparin-Induced Thrombocytopenia): antibodies against PF4-heparin; timing typically 5–7 days after exposure; 4Ts scoring; ELISA for anti-PF4/heparin; serotonin release assay as a confirmatory test; management includes stopping heparin and starting a thrombin inhibitor (lepirudin, argatroban, or danaparoid); warfarin should be started only after full anticoagulation and platelet recovery.
• Thrombotic thrombocytopenic purpura (TTP): large von Willebrand factor multimers due to ADAMTS13 deficiency; presents with thrombocytopenia, MAHA, fever, renal/neurologic signs; treated with plasma exchange; rituximab for relapsing/refractory cases.
• Hemolytic uremic syndrome (HUS): common secondary to Shiga toxin-producing bacterial infections; renal involvement; plasmapheresis used in some cases; drug-induced HUS described with certain agents.
• Sequestration: hypersplenism reduces circulating platelets; splenectomy not routinely indicated; platelets sequestered but can be mobilized.

Acquired Coagulopathies and Fibrinolysis

Acquired Coagulation Inhibitors

• Antiphospholipid syndrome (APLS): lupus anticoagulant and anticardiolipin antibodies; can cause venous or arterial thrombosis; prolonged aPTT in vitro but increased thrombosis risk in vivo.

Anticoagulation and Bleeding Risk

• Spontaneous bleeding can occur with any anticoagulation (heparin, LMWH, warfarin, Xa inhibitors, direct thrombin inhibitors).
• Heparin bleeding risk reduced with continuous infusion; LMWH monitoring is less reliable using standard tests; anti-Xa activity assays are used when monitoring is needed.
• Warfarin reversal in major bleeding: vitamin K plus rapid reversal with plasma or PCC; four-factor PCC preferred over three-factor PCC for INR reversal; high-dose IV vitamin K provides sustained reversal.
• DOAC reversal:
  • Idarucizumab for dabigatran reversal; reversal for direct Xa inhibitors via andexanet alfa (FXa) and ciraparantag (PER977) in trials.
  • Until approved, four-factor PCCs used for Xa inhibitor reversal; hold DOACs 36–48 hours before elective procedures in patients with normal renal function.
  • When monitoring is needed for dabigatran, use ecarin clotting time or dedicated anti-Xa assays for Xa inhibitors;
  • In less urgent settings, hold DOACs before surgery as above.

Trauma-related Coagulopathy (TIC) and Fibrinolysis Spectrum

• TIC is a multifactorial coagulopathy present early after trauma; mechanisms include activated protein C–mediated factor deactivation, endothelial injury, platelet dysfunction, and hyperfibrinolysis.
• Hypoperfusion and shock contribute to TIC; isolated brain injury and pulmonary contusions can cause TIC even without shock.
• Fibrinolysis spectrum:
  • Hyperfibrinolysis associated with increased mortality due to hemorrhage.
  • Fibrinolysis shutdown linked with higher mortality (often due to nonhemorrhagic causes like organ failure).
• Anticoagulant and antifibrinolytic therapy must be tailored; antifibrinolytics (e.g., tranexamic acid) reduce bleeding in trauma when used early.

Liver Disease–Related Coagulation (Coagulopathy of Liver Disease)

• The liver synthesizes most coagulation factors (Table 4-3: Vitamin K–dependent factors II, VII, IX, X; fibrinogen; V; VIII; XI; XII; XIII; AT-III; plasminogen; protein C/S).
• Liver disease causes complex coagulopathy with both bleeding risk (thrombocytopenia, prolonged PT/INR) and thrombotic risk (reduced anticoagulants C/S, antithrombin, plasminogen; elevated vWF and factor VIII).
• Thrombocytopenia in cirrhotics is often due to hypersplenism, reduced thrombopoietin production, and immune destruction; platelets may be mobilized, reducing bleeding severity.
• Conventional tests may misrepresent bleeding risk in cirrhosis; whole-blood viscoelastic tests (TEG/ROTEM) may provide better guidance.
• Treatment: FFP for coagulopathy; cryoprecipitate if fibrinogen < 200 mg/dL; cryoprecipitate also provides factor VIII; vitamin K insufficiency may occur if bile production is impaired.

Primary Fibrinolysis and Antifibrinolytics

• Antifibrinolytics such as ε-aminocaproic acid and tranexamic acid inhibit plasminogen activation and plasmin activity, stabilizing clots.
• They can be used in nontraumatic surgical settings where fibrinolysis is pathologically increased.

Trauma and Acute Coagulopathy: Trauma-Induced Coagulopathy (TIC) and Damage Control Resuscitation (DCR)

• TIC is a distinct, early coagulopathy seen after trauma; it's not merely a consequence of hemorrhagic shock and can occur with isolated organ injuries.
• DCR goals: rapid control of bleeding, minimal crystalloid use, early delivery of balanced blood products (RBC: plasma: platelets similar to whole blood), and adjuncts like TXA.
• Definitions and trials:
  • Massive transfusion traditionally defined as ≥10 units RBC in 24 hours; new measures use critical administration threshold (CAT): transfusion of ≥3 units RBC within 60 minutes, increasing risk with each additional time CAT is reached.
  • Plasma to RBC ratios (e.g., 1:1:1 vs 1:1:2) studied in PROMMTT and PROPPR trials; PROPPR found 1:1:1 improved hemorrhage control at 24 hours; mortality differences at 30 days not significant, but hemorrhagic deaths reduced at 24 hours.
  • Systematic reviews and EAST guidelines support higher plasma and platelet to RBC ratios for severe trauma.
• Endothelial glycocalyx and plasma resuscitation:
  • Plasma helps repair the EGL, reducing capillary leak and edema; crystalloid-based resuscitation can worsen edema and capillary leak.
  • Higher plasma and lower crystalloid volumes linked to better outcomes in trauma resuscitation.
• ABC score and prediction of massive transfusion:
  • ABC score uses heart rate, systolic BP, FAST results, and mechanism of injury to predict MT need; validated across centers with limitations.
• Prehospital transfusion and whole blood:
  • Early plasma/RBC administration in the field and prehospital RBC transfusion associated with improved outcomes in civilian and military settings.
  • Whole blood is resurging in trauma resuscitation; carries advantages in balancing components and may reduce the need for platelets in austere settings.

Transfusion Medicine: Blood Components and Therapies

Typing, Crossmatching, and Transfusion Basics

• A, B, O, and Rh grouping with crossmatching; Rh-negative recipients should receive Rh-negative blood; universal donor O− RBCs and AB plasma for emergencies; low-titer type A plasma often used when AB plasma is in short supply.
• Multiplicity of blood products: RBCs, platelets, plasma; platelets do not require crossmatching; ABO compatibility is still important for plasma.
• Special considerations: cold agglutinins; crossmatching before dextran; RBC units vary by storage duration and additives.

Red Blood Cells (RBCs) and Storage

• Classic RBC storage solution now allows up to 42 days of storage; older units may contribute to inflammation and organ dysfunction in some patients.
• Storage lesion includes decreased 2,3-DPG and elevated potassium and lactate; microcirculatory impairment can occur with older units.
• Cryopreserved RBCs offer long-term storage (10 years at -80°C) and may preserve ATP/2,3-DPG better; requires ~90 minutes for thawing; practical for remote settings with future potential.
• Leukoreduced and leukocyte-reduced/washed units reduce febrile nonhemolytic transfusion reactions, alloimmunization, and CMV transmission; universal leukoreduction is common in many Western countries.

Platelets

• Indications: thrombocytopenia due to massive transfusion or production issues; qualitative platelet disorders; maintain platelet counts >100,000 per µL in certain bleeding scenarios.
• Platelet units: ~50 mL per unit with roughly 5.5×10^10 platelets; shelf life ~5 days; risk of infectious complications and alloimmunization; HLA-matched platelets used for refractory cases.

Plasma Products

• Fresh Frozen Plasma (FFP): frozen within hours; stored up to 2 years; thawing 20–30 minutes; thawed plasma (thawed FFP) usable for up to ~5 days; liquid plasma stored refrigerated.
• Freeze-dried (lyophilized) plasma (FDP): shelf-stable, reconstituted quickly; used in austere environments; not widely FDA-approved for routine use in the U.S.
• Plasma provides vitamin K–dependent factors, factor V, and other coagulation proteins; pathogen reduction and ready availability are evolving considerations.

Fibrinogen Replacement

• Fibrinogen is among the first coagulation factors to fall during major hemorrhage; low fibrinogen correlates with worse outcomes.
• Fibrinogen concentrate (lyophilized) can be reconstituted rapidly; cryoprecipitate provides fibrinogen (and factor VIII) and is used when levels are low.
• Early cryoprecipitate or fibrinogen concentrate may raise fibrinogen levels and improve coagulation in massive transfusion protocols; trials show feasibility and potential benefits.

Tranexamic Acid (TXA)

• Antifibrinolytic that inhibits plasminogen activation and plasmin activity; potent lysine-binding site blockade on plasminogen prevents binding to fibrin.
• TXA is ~10× more potent than aminocaproic acid in vitro; no significant effect on platelet counts or coagulation parameters at therapeutic levels; renal excretion; half-life ~2 hours.
• Indications include CABG, liver transplantation, orthopedic procedures, and trauma; EAST guidelines conditionally support early TXA use in severely injured patients; concerns about use in active intravascular clotting and interaction with certain PCCs/factor concentrates.
• Prehospital TXA trials are ongoing; not yet universal; use should consider fibrinolytic status and bleeding severity.

Transfusion Triggers and Triggers for Transfusion in Critical Care

• Transfusion triggers (hemoglobin/hematocrit) must be individualized; restrictive strategies (Hb 7–9 g/dL) in stable ICU patients have not shown mortality increases and may reduce RBC exposure.
• For patients with ischemic heart disease, data are mixed; guidelines generally support a restrictive approach but emphasize patient-specific factors.
• AABB and SCCM/EAST guidance suggest a minimum Hb of about $7\,g/dL$ for hemodynamically stable patients and $8\,g/dL$ for cardiac/vascular surgery patients or those with cardiovascular disease; ensure symptomatic anemia is treated and avoid transfusion for asymptomatic anemia unless necessary.

Massive Transfusion Guidelines and Ratios

• Early balanced transfusion ratios (RBC:FFP:platelets) are recommended in massive transfusion protocols, aiming for 1:1:1 when feasible.
• PROPPR trial: 1:1:1 vs 1:1:2 showed no mortality difference at 24 hours or 30 days, but 1:1:1 reduced death from hemorrhage at 24 hours and improved hemostasis; no difference in major inflammatory complications.
• Contemporary guidelines favor high and balanced ratios (≥1:1) for plasma and platelets with RBCs in severe hemorrhage.
• Initial MT guidelines: activate MTG early; deliver plasma and platelets with RBCs in 1:1:1 ratio; minimize crystalloids; stabilized laboratory monitoring (CBC, INR, fibrinogen, pH/base deficit, TEG when available).
• Table 4-6 and Table 4-7 provide stepwise recommendations for RBC, plasma, platelets, and cryoprecipitate administration during massive transfusion; details include timing, product numbers per cycle, and targets (platelet counts, fibrinogen levels).

Tests of Hemostasis and Blood Coagulation

Conventional Coagulation Tests

• Platelet count: normal range $150{,}000–400{,}000/\mu L$; thrombocytopenia thresholds:
  • >1,000,000/µL may be associated with bleeding or thrombosis; major procedures risk when platelets <50,000/µL; minor procedures risk when <30,000/µL; spontaneous bleeding when <20,000/µL.
  • Platelet transfusion considered for ophthalmologic or neurosurgical procedures if platelets <100,000/µL (evidence limited).
• PT/INR and aPTT:
  • PT: extrinsic pathway (factors II, VII, X) and vitamin K–dependent factors; influenced by batch thromboplastin variation; INR standardizes PT across batches: $ext{INR} = \frac{PT<em>{measured}}{PT</em>{normal}}^{ISI}$, where ISI is batch-dependent.
  • aPTT: intrinsic pathway (factors I, II, V; VIII, IX, X, XII); monitored for heparin therapy (target 1.5–2.5× control; approx. 50–80 s).
• Limitations: conventional tests are plasma-based and may not reflect whole-blood coagulation status in actively bleeding patients.

Viscoelastic Assays (TEG/ROTEM)

• R (reaction time): time from start of assay to initial clot formation; reflects clotting factor activity and initial fibrin formation.
• K (clot kinetics): time to reach a defined clot strength; prolonged with hypofibrinogenemia or significant factor deficiency.
• Alpha angle (\u03b1): rate of clot formation; reflects activity of clotting factors and platelets; decreased with hypofibrinogenemia or platelet dysfunction; treated with cryoprecipitate or fibrinogen.
• MA (maximum amplitude): overall clot strength; low in platelet or fibrinogen deficiencies; treated with platelets and/or cryoprecipitate.
• G-value: overall clot strength; higher values indicate hypercoagulability; lower values indicate hypocoagulable states.
• LY30: percent clot lysis 30 minutes after MA; measures fibrinolysis activity; helps guide TXA use.
• Utility: rTEG/ROTEM can predict transfusion needs, guides TXA, and may outperform conventional tests in bleeding patients.

DIC and Other Coagulation States

• DIC: systemic activation of coagulation with thrombocytopenia, prolonged PT, low fibrinogen, elevated FDPs/D-dimers; ISTH scoring aids diagnosis and correlates with mortality, especially with infection.
• Hypocoagulable vs hypercoagulable states in liver disease and TIC require dynamic testing (TEG/ROTEM) for management.

Acquired Coagulation Defects: Anticoagulation and Reversal in the Perioperative Setting

Anticoagulants and Reversal Strategies

• Warfarin (vitamin K antagonist):
  • Reversal involves vitamin K, plasma, or prothrombin complex concentrates (PCCs); four-factor PCC preferred for rapid INR correction.
  • Vitamin K 10 mg IV for rapid onset in major bleeding (or oral in non-emergency); PCC can correct INR faster and with less volume than plasma.
  • Bridging considerations depend on thromboembolism risk; recent data suggest bridging may increase major bleeding without reducing thromboembolism significantly in elective surgeries.
• Direct Oral Anticoagulants (DOACs):
  • Dabigatran reversal with idarucizumab (approved); Factor Xa inhibitors lack widely available antidotes; andexanet alfa and ciraparantag (PER977) under development/trials.
  • When reversal agents are not available, four-factor PCCs may be used for Xa inhibitors; in non-emergent cases, hold DOACs for 36–48 hours if renal function is normal.
  • Monitoring DOACs varies: anti-Xa assays for Xa inhibitors; ecarin clotting time for dabigatran in selected centers.
• Perioperative bridging and timing:
  • Bridging strategies should weigh bleeding risk vs thromboembolism risk; recent trials show increased bleeding with bridging without reduction in thromboembolism.
  • For high thrombosis risk (mechanical valves, recent MI/stroke/PE), holding heparin 4–6 hours before procedure and restarting 12–24 hours after can be appropriate; lower risk indications may not require such strict regimens.
• Reversal in emergent settings:
  • For heparin: discontinue heparin; protamine sulfate reverses heparin effect but may have adverse reactions (hypotension, hypersensitivity).
  • For warfarin: rapid reversal with PCC/plasma; vitamin K to sustain reversal.
  • For traumatic intracranial or major bleeding, use four-factor PCC for rapid reversal; consider plasma if PCC is unavailable.

Perioperative Antithrombotic Therapy Bridging (Guidelines)

• CHEST guidelines (2012) previously supported bridging for long-term anticoagulation; more recent trials question routine bridging in many elective situations.
• Decision should be individualized using thromboembolism risk vs bleeding risk; hold or bridge based on specific patient factors.

Cardio-pulmonary Bypass and Transfusion in CPB

• CPB induces platelet and coagulation factor activation, inflammatory responses, and hyperfibrinolysis; RBC/platelet dysfunction during CPB is common.
• Monitoring: activated clotting time (ACT) and thromboelastography (TEG) help guide transfusion and reversal during CPB; coagulopathy is managed with FFP, cryoprecipitate, platelets; antifibrinolytics may be used; protamine reverses heparin after CPB.
• Transfusion strategies during CPB are not universally defined; empiric FFP/cryoprecipitate transfusion is common; targeted, guided transfusion using TEG/ROTEM is increasingly used.

Local Hemostasis and Topical Agents

• Local strategies aim to prevent bleeding from a disrupted vessel and may be mechanical or adjunctive.
• Mechanical methods:
  • Direct digital pressure; tourniquets; Pringle maneuver for hepatic bleeding.
  • Simple ligature for small vessels; transfixion suture for larger pulsatile arteries.
  • Wound packing with gauze or laparotomy pads for diffuse bleeding; bone wax can control bleeding from bone surfaces.
• Thermal and electrical methods:
  • Electrocautery uses heat; proper power settings minimize tissue injury and avoid interference with monitoring equipment.
  • Direct current coagulation and argon beam can control diffuse bleeding on raw surfaces.
  • Cautions include use of grounding plates and avoidance of explosive anesthetic vapors.
• Topical hemostatic agents:
  • Absorbable agents: gelatin foam (Gelfoam), oxidized cellulose (Surgicel), microfibrillar collagen (Avitene);
  • Biologic agents: topical thrombin, fibrin sealants (FloSeal), platelet sealants (Vitagel).
  • Thrombin derivatives can cause systemic exposure in large vessels; bovine derivatives pose immunogenic risks.
  • Fibrin sealants are formed from cryoprecipitate or synthetic components; platelet sealants require autologous platelet preparation.
  • Topical agents are adjuncts; they do not replace meticulous technique.

Transfusion Medicine: Background and Product-Specific Details

Background and Historical Context

• Shift from whole blood to component therapy in the late 20th century; whole blood has seen renewed interest for resuscitation of acute hemorrhage in military and civilian settings.
• Early plasma and platelet delivery improved outcomes in trauma when using damage control resuscitation strategies.

Replacement Therapy: Typing and Crossmatching

• Serologic compatibility for ABO and Rh determined; crossmatching verifies donor-recipient compatibility.
• Emergency considerations: Rh-negative RBCs for Rh-negative females of childbearing potential; universal donor O− RBCs used for emergencies; AB plasma for universal plasma; type-specific plasma in some settings to limit TRALI risk.
• Blood warmers are used when cold antibodies exist; crossmatching is essential before transfusion of dextran or other fluids.

Whole Blood and Blood Components in Modern Practice

• Whole blood usage has increased in trauma, especially in austere environments; pilot trials compare whole blood to component therapy.
• Red blood cells: storage and age considerations; leukoreduction reduces febrile reactions and alloimmunization; cold storage fragmentation remains a hurdle for immediate use in emergencies.
• Platelets: unit volume and shelf life; transfusion thresholds; HLA-matched platelets for refractoriness.
• Plasma products: FFP, thawed plasma, liquid plasma; lyophilized/plasma alternatives for rapid use in emergencies; pathogen reduction considerations.

Fibrinogen Replacement and Cryoprecipitate

• Fibrinogen is depleted early in massive bleeding; early administration improves hemostasis and outcomes; fibrinogen concentrate avoids thawing delays and pathogen transmission risk; cryoprecipitate provides fibrinogen and Factor VIII.

Tranexamic Acid (TXA)

• Mechanism as an antifibrinolytic; reduces bleeding and transfusion needs in various surgeries and trauma.
• Cautions include avoiding use with active major bleeding with intravascular clots unless adequately monitored; interactions with PCCs and other coagulation factors require careful consideration.

Transfusion Reactions and Safety

• Transfusion-related complications occur in about 10% of transfusions, with severe events in under 0.5%; TRALI, ABO-incompatible hemolytic reactions, and bacterial contamination of platelets are notable.
• Febrile nonhemolytic transfusion reactions are common; leukoreduction reduces risk; acetaminophen often reduces fever severity.
• TRALI risk diminished by using male or low-risk donor plasma; monitoring and rapid stopping of transfusion are critical in suspected reactions.
• Other transfusion risks include TACO (transfusion-associated circulatory overload), allergic reactions, bacterial sepsis, and hemolytic reactions (acute vs delayed).
• Transmission of infectious agents (malaria, CMV, HIV, HCV, Zika) has decreased with screening and pathogen inactivation strategies; ongoing vigilance for emerging pathogens.

Tests of Hemostasis in Transfusion Context

• Conventional tests: PT/INR, aPTT, platelet count; limitations in actively bleeding patients.
• TEG/ROTEM provide real-time, dynamic assessment of clot formation, strength, and lysis; guide transfusion decisions (e.g., TXA, platelets, cryoprecipitate).

Banked Tables and Key References (Summary Concepts)

• Table 4-3: Coagulation factors synthesized by the liver; lists vitamin K–dependent factors (II, VII, IX, X) plus fibrinogen, V, VIII, XI, XII, XIII, AT-III, plasminogen, protein C/S.
• Table 4-4: Medications that alter warfarin dosing; drug interactions influence INR and reversal needs.
• Table 4-5: Replacement therapy product characteristics (fibrinogen, prothrombin II, V, X, VII, VIII, IX, XI, XII, XIII; platelets).
• Table 4-6: Adult transfusion guideline (initial RBC transfusion steps and escalation).
• Table 4-7: Component therapy during massive transfusion (FFP, platelets, cryoprecipitate dosing; target fibrinogen).
• Table 4-8: Massive transfusion prediction models and their predictive accuracy (ABC score and other studies).
• Table 4-9: Transfusion-related complications (NHTR, TRALI, TACO, HTR, etc.).

Quick Equations and Key Numeric References (LaTeX)

• INR calculation: $ext{INR} = \left(\frac{PT<em>{\text{measured}}}{PT</em>{\text{normal}}}\right)^{ISI}$
• Platelet lifespan and count ranges remain as stated: platelets $7\text{ to }10\text{ days}$; normal count $150{,}000\text{ to }400{,}000/\mu L$.
• Transfusion ratios in MT: target 1:1:1 (RBC:FFP:platelets); alternative 1:1:2.
• Fibrinogen threshold for cryoprecipitate: if fibrinogen $\le 200\ \text{mg/dL}$, give 20 units cryoprecipitate (≈2 g fibrinogen).
• Key trauma data points: CAT defined as 3 RBCs in 60 minutes; PROMMTT findings show improved survival with higher plasma:RBC and platelet:RBC ratios; PROPPR shows 1:1:1 superior for hemorrhage control at 24 hours.
• TXA dosing and potency: TXA is 10× more potent in vitro than aminocaproic acid; half-life approximately $2\text{ hours}$; excretion via kidneys.

Connections to Prior Knowledge and Real-World Relevance

• The cell-based model of coagulation aligns with modern teaching beyond the classical cascade; emphasizes TF exposure, platelet surface reactions, and propagation on activated platelets—relevant to understanding how anticoagulants and antiplatelets alter clinically observed bleeding and clotting.
• DCR and massive transfusion protocols reflect evolving trauma care that prioritizes early, ratio-balanced product delivery and damage control principles to reduce mortality.
• Viscoelastic testing (TEG/ROTEM) provides clinically actionable hemostasis assessments beyond standard coagulation tests, guiding targeted transfusion and antifibrinolytic use.
• Reversal strategies for warfarin and DOACs illustrate the shift toward rapid, targeted reversal to control bleeding in emergencies while balancing thrombotic risks.
• The liver’s dual role in coagulation (bleeding risk via reduced synthesis; thrombotic risk via reduced anticoagulants and elevated procoagulants) underlines why liver disease presents a complex hemostatic landscape requiring nuanced management.

Practical Implications and Takeaways

• In surgical patients, assess not only platelet count but function; consider DDAVP for uremic platelet dysfunction or storage pool defects.
• For TIC, early recognition and applying DCR principles with goal-balanced transfusion improves outcomes; avoid excessive crystalloid.
• In bleeding patients, use a combination of conventional tests and viscoelastic assays where available to tailor therapy (plasma, platelets, cryoprecipitate, TXA).
• When anticoagulation needs reversal, choose rapid strategies (PCCs, vitamin K) over slower plasma-based reversal when feasible; reserve DOAC reversal agents for indicated DOACs and contexts.
• Prehospital and whole-blood strategies show promise for improving outcomes in massive bleeding; infrastructure and protocols are key for rapid delivery.