3.3 Venous Thromboembolism & Vascular Emergencies

3.3 Venous Thromboembolism & Vascular Emergencies

Core Clinical Rule: Whenever initiating Warfarin for acute venous thromboembolism, always co-administer therapeutic parenteral anticoagulation (LMWH or UFH) for an absolute minimum of 5 days AND until the INR is between 2.0 and 3.0 for at least 2 consecutive days. Premature cessation of heparin unmasks a paradoxical hypercoagulable state caused by rapid depletion of natural anticoagulant Protein C while factor II persists, precipitating catastrophic thrombotic extension or skin necrosis.

Venous Thromboembolism (VTE), comprising Deep Vein Thrombosis (DVT) and Pulmonary Embolism (PE), represents one of the most preventable causes of inpatient morbidity and mortality across Kenyan hospitals. In surgical, obstetric, and acute medical wards, failure to stratify venous thromboembolic risk and promptly initiate prophylactic or therapeutic anticoagulation leads to devastating clinical outcomes.

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1. Pathophysiology and Virchow's Triad

Thrombus formation within the venous circulation is governed by Virchow's Triad, described in 1856 and still foundational to vascular medicine:

                         [VIRCHOW'S TRIAD]
                                 |
       +-------------------------+-------------------------+
       |                         |                         |
[Endothelial Injury]       [Venous Stasis]        [Hypercoagulability]
       |                         |                         |
• Major orthopedic surgery • Prolonged bed rest   • Active malignancy
  (total hip/knee)         • Post-operative state • Pregnancy & puerperium
• Major pelvic/abdominal   • Paralysis / stroke     (highest in first 6 wks)
  trauma                   • Long-distance travel • Combined oral contraceptives
• Central venous catheter  • Congestive HF        • Antiphospholipid syndrome
• Direct vascular shear    • Severe obesity       • Factor V Leiden mutation

In the Kenyan clinical context, high-risk populations include women in the puerperium (especially following Cesarean Section), patients with advanced pelvic malignancies (cervical and prostate carcinomas), patients immobilized with skeletal traction for femur fractures, and patients with severe systemic infections (advanced HIV disease and sepsis).

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2. Deep Vein Thrombosis (DVT)

Clinical Presentation and Physical Signs

DVT occurs predominantly in the deep veins of the lower extremities (popliteal, femoral, and iliac veins). Symptoms and physical examination findings include:

• Unilateral limb pain and tenderness, typically worsening on weight-bearing.
• Unilateral leg swelling: A difference in calf circumference >= 3 cm measured 10 cm below the tibial tuberosity is a highly sensitive physical indicator of unilateral edema.
• Erythema, warmth, and dilated superficial collateral veins (non-varicose).
• Homans' Sign: Pain in the calf on forced passive dorsiflexion of the ankle. Clinical Caveat: Homans' sign is neither sensitive nor specific, and forceful manipulation carries a theoretical risk of dislodging the thrombus. It should not be relied upon for clinical diagnosis.

Severe Limb-Threatening Complications

• Phlegmasia Alba Dolens ('Milk Leg'): Extensive deep venous thrombosis without collateral occlusion. The leg is pale, cool, and massively swollen, but arterial pulses remain palpable.
• Phlegmasia Cerulea Dolens: Massive, near-total thrombosis of all deep and collateral venous drainage pathways of the iliofemoral system. Characterized by severe cyanosis (violaceous discoloration), excruciating pain, massive woody edema, arterial spasm, sensory motor deficits, and bullae formation. Without emergent venous decompression (catheter-directed thrombolysis or surgical thrombectomy), it rapidly progresses to venous gangrene and requires limb amputation.

Wells Score for DVT and Diagnostic Algorithm

+-----------------------------------------------------------------------------------+
|                         WELLS CLINICAL SCORE FOR DVT                              |
+-----------------------------------------------------------------------------------+
| Clinical Feature                                                      | Points    |
+-----------------------------------------------------------------------+-----------+
| Active cancer (treatment ongoing, within past 6 months, or palliative)| +1.0      |
| Paralysis, paresis, or recent plaster immobilization of lower limb    | +1.0      |
| Recently bedridden >= 3 days or major surgery within 12 weeks         | +1.0      |
| Localized tenderness along the distribution of the deep venous system | +1.0      |
| Entire leg swollen                                                    | +1.0      |
| Calf swelling >= 3 cm compared to the asymptomatic leg (10cm below TT)| +1.0      |
| Pitting edema confined to the symptomatic leg                         | +1.0      |
| Collateral superficial veins (non-varicose)                           | +1.0      |
| Previously documented DVT                                             | +1.0      |
| Alternative diagnosis at least as likely as DVT (e.g. cellulitis, cyst)| -2.0      |
+-----------------------------------------------------------------------------------+

                          [Suspected Lower Extremity DVT]
                                        |
                                [Calculate Wells Score]
                                        |
                   +--------------------+--------------------+
                   |                                         |
       [Wells < 2: DVT Unlikely]                 [Wells >= 2: DVT Likely]
                   |                                         |
           [Perform D-Dimer]                         [Compression Duplex]
                   |                                     [Ultrasound]
         +---------+---------+                               |
         |                   |                     +---------+---------+
     [Negative]          [Positive]                |                   |
   (< 500 mcg/L)       (>= 500 mcg/L)          [Positive]          [Negative]
         |                   |                     |                   |
    DVT Excluded    [Compression Duplex]      Initiate Full      Repeat scan in
   No treatment         [Ultrasound]          Anticoagulation    1 week or D-dimer

• Compression Duplex Ultrasonography: The non-invasive imaging modality of choice. The pathognomonic diagnostic criterion is failure of the vein lumen to completely compress under direct probe pressure in B-mode imaging, accompanied by absence of Doppler flow signals.

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3. Pulmonary Embolism (PE)

Pulmonary Embolism arises when a venous thrombus (typically originating in the iliofemoral venous system) detaches and travels through the inferior vena cava, right atrium, and right ventricle to impact within the pulmonary arterial tree.

Pathophysiology of Right Ventricular Failure and Shock

[Pulmonary Artery Thrombus Occlusion]
                |
                v
[Sudden Spike in Pulmonary Vascular Resistance (PVR)]
                |
                v
[Acute Right Ventricular (RV) Pressure Overload & Dilation]
                |
                v
[Shift of Interventricular Septum Leftward (Septal Bowing)]
                |
                v
[Impaired Left Ventricular Diastolic Filling & Reduced Stroke Volume]
                |
                v
[Systemic Hypotension, Reduced Coronary Perfusion, RV Ischemia & Shock]

Clinical Presentation and Wells Criteria for PE

• Symptoms: Sudden-onset dyspnea (most common symptom, > 80%), pleuritic chest pain (due to peripheral infarction of visceral pleura), cough, hemoptysis, and syncope (indicates transient massive RV outflow tract obstruction and heralds life-threatening instability).
• Signs: Tachypnea (RR > 20/min; most common physical sign), tachycardia (HR > 100 bpm), loud pulmonary component of the second heart sound ($P_2$), elevated JVP, and fever ($< 38.5^\circ\text{C}$).

+-----------------------------------------------------------------------------------+
|                         WELLS CLINICAL SCORE FOR PE                               |
+-----------------------------------------------------------------------------------+
| Clinical Feature                                                      | Points    |
+-----------------------------------------------------------------------+-----------+
| Clinical signs and symptoms of DVT (minimum leg swelling/pain on palpation) | +3.0 |
| An alternative diagnosis is less likely than PE                       | +3.0      |
| Heart rate > 100 beats per minute                                     | +1.5      |
| Immobilization (>= 3 days) or surgery within previous 4 weeks         | +1.5      |
| Previous documented DVT or PE                                         | +1.5      |
| Hemoptysis                                                            | +1.0      |
| Active malignancy (treated within 6 months or palliative)             | +1.0      |
+-----------------------------------------------------------------------------------+
| Wells <= 4.0: PE Unlikely -> High-Sensitivity D-Dimer (rules out PE if negative) |
| Wells > 4.0:  PE Likely   -> Immediate CT Pulmonary Angiography (CTPA)            |
+-----------------------------------------------------------------------------------+

Diagnostic Investigations

1. Computed Tomography Pulmonary Angiography (CTPA): The gold-standard definitive imaging test. Demonstrates sharp intraluminal filling defects surrounded by iodinated contrast. Contraindicated in severe renal failure ($eGFR < 30\text{ mL/min}$) or severe contrast allergy.
2. Ventilation-Perfusion (V/Q) Lung Scintigraphy: Diagnostic test of choice when CTPA is contraindicated or unavailable, or in pregnancy with normal chest radiography. Demonstrates wedge-shaped areas of mismatched ventilation without perfusion.
3. 12-Lead Electrocardiogram (ECG):
   • Sinus tachycardia is the single most common ECG abnormality.
   • Classic $S_1Q_3T_3$ Pattern (McGinn-White Sign): Deep S-wave in lead I, pathological Q-wave in lead III, and inverted T-wave in lead III. Indicates acute right ventricular strain; classic for exams but present in $< 20%$ of cases.
   • T-wave inversions in right precordial leads ($V_1-V_4$) and new incomplete or complete Right Bundle Branch Block (RBBB).
4. Arterial Blood Gas (ABG): Demonstrates hypoxemia, hypocapnia (due to tachypnea-induced hyperventilation), respiratory alkalosis, and an elevated alveolar-arterial ($A-a$) oxygen gradient.

Severity Classification and Risk Stratification

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4. Anticoagulation Pharmacotherapy & Protocols

Acute Parenteral Anticoagulants

1. Low Molecular Weight Heparin (LMWH - Enoxaparin):

   • Dosing: 1 mg/kg subcutaneous 12-hourly, OR 1.5 mg/kg subcutaneous once daily.
   • Advantages: Highly predictable bioavailability and renal clearance; routine laboratory monitoring of anti-Xa activity is unnecessary in uncomplicated patients. Associated with lower incidence of Heparin-Induced Thrombocytopenia (HIT) compared to UFH.
   • Renal Impairment Modification: If $CrCl < 30\text{ mL/min}$, reduce dose to 1 mg/kg SC once daily, or transition to Unfractionated Heparin.

2. Unfractionated Heparin (UFH):

   • Dosing: Weight-based protocol: 80 units/kg IV bolus, followed by continuous infusion at 18 units/kg/hour.
   • Laboratory Monitoring: Monitor the activated Partial Thromboplastin Time (aPTT) every 6 hours, adjusting the infusion rate to maintain a therapeutic range of 1.5 to 2.5 times the control (typically 60 to 85 seconds).
   • Clinical Placement: Preferred agent in severe renal failure ($CrCl < 30\text{ mL/min}$), in patients with massive PE with shock, and in patients anticipated to require emergent surgical embolectomy due to its short half-life (60–90 minutes) and rapid reversibility.
   • Reversal Agent: Protamine Sulfate (1 mg reverses 100 units of UFH; max single dose 50 mg IV slow push).

3. Heparin-Induced Thrombocytopenia (HIT):

   • Life-threatening immune-mediated complication triggered by IgG antibodies directed against platelet factor 4 (PF4)-heparin complexes. Leads to platelet activation, consumptive thrombocytopenia (drop in platelet count by > 50% between days 5 and 10 of therapy), and catastrophic arterial and venous thrombosis ('white clot syndrome').
   • Management: Immediately discontinue all heparin products (including flushes). Switch to a non-heparin parenteral anticoagulant (e.g., Fondaparinux, Argatroban). Never initiate warfarin until platelet count has fully normalized.

Direct Oral Anticoagulants (DOACs)

Modern clinical guidelines recommend DOACs as first-line therapy over Warfarin for non-cancer VTE due to fixed oral dosing, lack of routine INR monitoring, and a significantly reduced incidence of fatal intracranial hemorrhage:

• Rivaroxaban (Direct Factor Xa Inhibitor): 15 mg PO twice daily taken with food for 21 days, followed by 20 mg PO once daily taken with food. Key Advantage: No initial parenteral heparin lead-in required.
• Apixaban (Direct Factor Xa Inhibitor): 10 mg PO twice daily for 7 days, followed by 5 mg PO twice daily. Key Advantage: No initial parenteral heparin lead-in required.
• Dabigatran (Direct Thrombin Inhibitor): 150 mg PO twice daily, initiated only after at least 5 days of initial parenteral anticoagulation.

Warfarin and the Mandatory Bridging Protocol

Day 1          Day 2          Day 3          Day 4          Day 5          Day 6+
=================================================================================
[Therapeutic LMWH or UFH Infusion (Continued)] ------------------> | (Discontinue
                                                                   |  ONLY IF
[Warfarin 5 mg PO Daily Started Concurrently] -------------------> |  INR >= 2.0
                                                                   |  for 2 days)
=================================================================================
*Pharmacological Hazard: Protein C has a 6-hr half-life and plummets on Days 1-2.
 Prothrombin (Factor II) has a 60-hr half-life and remains fully active.
 Without heparin bridging, uninhibited Factor II + depleted Protein C =
 PARADOXICAL THROMBOSIS & WARFARIN SKIN NECROSIS!

• Target Therapeutic Range: Warfarin inhibits Vitamin K Epoxide Reductase ($VKORC1$). The target International Normalized Ratio (INR) for VTE is 2.0 to 3.0 (therapeutic target 2.5).
• Reversal of Warfarin Toxicity:
  • Major/Life-Threatening Bleeding: Stop warfarin, administer Vitamin K (Phytomenadione) 5–10 mg slow IV push over 20 minutes, combined with 4-Factor Prothrombin Complex Concentrate (PCC) (25–50 units/kg IV). If PCC is unavailable in resource-constrained facilities, infuse Fresh Frozen Plasma (FFP) (15 mL/kg).

Duration of Anticoagulant Therapy

• Provoked VTE (Transient/Reversible Risk Factor e.g., major surgery, fracture, estrogen therapy): 3 months of full-dose therapeutic anticoagulation.
• Unprovoked (Idiopathic) VTE or Recurrent VTE: Extended / Indefinite anticoagulation (minimum 6 months followed by ongoing risk-benefit review).
• Cancer-Associated Thrombosis (CAT): LMWH (Enoxaparin) or approved DOACs (Apixaban, Rivaroxaban) for at least 6 months, continued indefinitely while active cancer or systemic chemotherapy persists.

Emergency Management of Massive (High-Risk) PE

1. Hemodynamic Resuscitation: Place patient on high-flow oxygen. Infuse a cautious, conservative crystalloid fluid challenge (<= 500 mL normal saline). Warning: Aggressive volume resuscitation overdistends the failing right ventricle, worsens septal bowing, and precipitates immediate cardiovascular collapse. Initiate vasopressor support with Norepinephrine (0.05–0.5 mcg/kg/min) to restore systemic perfusion pressure and right coronary artery blood flow.
2. Systemic Thrombolysis (Fibrinolytic Therapy):
   • Agent & Regimen: Recombinant tissue Plasminogen Activator (Alteplase / tPA) 100 mg IV infusion over 2 hours, OR an accelerated infusion of 0.6 mg/kg over 15 minutes (max 50 mg) in cardiac arrest. Alteplase converts plasminogen to plasmin, degrading fibrin clots, reducing RV afterload, and restoring hemodynamic stability.
   • Alternative in Resource-Limited Settings: Streptokinase 250,000 IU loading dose IV over 30 minutes, followed by 100,000 IU/hour infusion for 12 to 24 hours.
3. Inferior Vena Cava (IVC) Filters: Indicated strictly when there is an absolute contraindication to therapeutic anticoagulation (e.g., active major internal bleeding, recent intracranial hemorrhage) or documented recurrent PE despite proven therapeutic anticoagulation.