Pathophysiology: Low SVR and Volume Expansion

Show Notes

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1. Core Definition & Hemodynamic Profile

• Clinical Paradox: Congestive symptoms (pulmonary edema, JVD, peripheral edema) in the setting of a hyperdynamic, supranormal cardiac function.

• Hemodynamic Criteria:

  • Cardiac Index (CI): >4.0 L/min/m2.

  • Cardiac Output (CO): $>8\text{ L/min}$.

  • Systemic Vascular Resistance (SVR): Pathologically low (vasodilated or shunted state).

• The “Warm” Phenotype: Unlike standard HFrEF/HFpEF (often “Cold and Wet”), HOHF presents as “Warm and Wet” due to low SVR and bounding pulses.

2. Pathophysiology: The Hemodynamic Paradox

• Primary Insult: Decreased SVR (either via peripheral vasodilation or arteriovenous shunting).

• Effective Arterial Blood Volume: Paradoxically low despite high total CO.

• Neurohormonal Cascade:

  • Activation of Renin-Angiotensin-Aldosterone System (RAAS).

  • Increased Sympathetic Nervous System tone.

  • Increased Antidiuretic Hormone (ADH) secretion.

• Resultant State: Avid renal salt and water retention leading to massive plasma volume expansion.

• Cardiac Response: Chronic volume overload $\to$ eccentric remodeling $\to$ chamber dilation $\to$ eventual secondary myocardial failure/dilated cardiomyopathy.

3. Differential Diagnosis: Etiological “Buckets”

Category A: Increased Metabolic Demand (Systemic)

• Hyperthyroidism/Thyrotoxicosis:

  • Direct T3 effects: increased chronotropy/inotropy.

  • Indirect effects: metabolic byproduct accumulation causing peripheral vasodilation.

• Myeloproliferative Disorders:

  • High cell turnover and increased oxygen consumption drive compensatory CO increase.

• Sepsis (Hyperdynamic Phase):

  • Cytokine-mediated global vasodilation.

  • Note: Often transient; may transition to sepsis-induced myocardial depression.

Category B: Peripheral Vascular Effects (Shunting/Vasodilation)

• Arteriovenous Fistulas (AVF) / Malformations (AVM):

  • Most Common Cause: Iatrogenic AVF for Hemodialysis (ESRD population).

  • Bypasses high-resistance capillary beds, dumping arterial blood directly into venous circulation.

• Chronic Liver Disease (Cirrhosis):

  • Formation of “spider angiomata” and internal AV shunts.

  • Impaired clearance of endogenous vasodilators (e.g., Nitric Oxide).

• Thiamine Deficiency (Wet Beriberi):

  • Accumulation of pyruvate/lactate $\to$ systemic vasodilation.

  • Histopathology: Vacuolation, myofiber hypertrophy, and interstitial edema.

• Chronic Lung Disease:

  • Hypoxia/Hypercapnia-driven systemic vasodilation.

  • Concomitant pulmonary HTN (RV remodeling) but preserved/high LV output.

• Others: Paget’s disease of bone (extensive micro-shunting), Carcinoid syndrome, Mitochondrial diseases, Acromegaly, Erythroderma.

4. Special Focus: Hemodialysis Access-Induced HOHF

Physiologic Phases of AVF Creation:

1. Acute Phase:

   1. Immediate $\downarrow$ SVR.

   2. $\uparrow$ Stroke volume and Heart Rate (SNS-mediated).

   3. Endothelial shear stress $\to$ Nitric Oxide release $\to$ further arterial dilation.

2. Subacute Phase (Days to 2 Weeks):

   1. RAAS-driven volume expansion.

   2. $\uparrow$ Right Atrial, Pulmonary Artery, and LV End-Diastolic Pressures (LVEDP).

   3. Natriuretic peptide surge (BNP/ANP) peaks around Day 10.

3. Chronic Phase (Weeks to Months):

   1. Adaptive hypertrophy.

   2. Decompensation occurs when dilation exceeds contractility limits.

5. Point-of-Care Physical Exam & Maneuvers

• Nicoladoni-Branham Sign (Pathognomonic for Shunt-driven HOHF):

  • Maneuver: Manually compress the AVF (or inflate cuff to $>50\text{ mmHg}$ above SBP) for 30 seconds.

  • Positive Result: Reflexive bradycardia or a transient rise in systemic BP.

  • Significance: Confirms the shunt is a major contributor to the cardiac workload.

• Peripheral Pulse Assessment:

  • Water Hammer Pulses: Rapid upstroke and collapse.

  • Quincke’s Pulse: Visible capillary pulsations in the nail beds.

  • Traube’s Sign: “Pistol-shot” sounds auscultated over the femoral arteries.

• Volume Status: Rales, S3 gallop, peripheral edema (standard HF signs).

6. Diagnostic Workup (Technical Targets)

POCUS / Echocardiography:

• Left Ventricle: Hyperdynamic function; EF typically $>60\%$.

• Left Atrium: Significant dilation (Left Atrial Volume Index >34 mL/m2; Case study noted 72 mL/m2).

• IVC: Plethoric with minimal respiratory variation.

• Doppler: High flow velocities across the AV access if applicable.

Laboratory Evaluation:

• BNP/NT-proBNP: Often markedly elevated (e.g., $>70,000$ in severe cases), though mean values in literature hover around $700\text{–}800\text{ pg/mL}$.

• Hematology: CBC to evaluate for severe anemia (trigger for HOHF if $\text{Hgb}<7\text{–}8\text{ g/dL}$) or myeloproliferative markers.

• Endocrine/Metabolic: TSH (Thyrotoxicosis), Serum Thiamine (Beriberi), LFTs (Cirrhosis).

7. Management Strategy: A Stepwise Approach

Phase 1: Immediate Stabilization (Volume Offloading)

• Diuresis: Aggressive IV loop diuretics (Bumetanide/Furosemide).

• Ultrafiltration: Preferred in ESRD patients failing to respond to dialysis or with refractory congestion.

• Vasodilator Caution: Avoid aggressive Nitroglycerin or ACE-inhibitors initially.

  • Rationale: Baseline SVR is already pathologically low; further reduction may precipitate profound hypotension/circulatory collapse.

Phase 2: Targeted Therapy (Etiology Specific)

• Anemia: Transfuse to goal $\text{Hgb}>7\text{–}8\text{ g/dL}$ to reduce demand.

• Beriberi: High-dose IV Thiamine ($100\text{–}500\text{ mg}$).

• Thyrotoxicosis: Beta-blockers (Propranolol) + Antithyroid meds (PTU/Methimazole).

Phase 3: Surgical/Interventional Salvage (Refractory AVF Cases)

1. Closure of Accessory Sites: If multiple fistulas exist, close the non-dominant/unused sites.

2. Flow Reduction (Banding): Surgical narrowing of the fistula to target flow $<600\text{ mL/min}$.

3. RUDI Procedure: Revision Using Distal Inflow (moving inflow to a smaller, more distal artery).

4. Ligation: Complete closure of the AVF.

   • Note: Requires bridge to Tunneled Dialysis Catheter or AV graft (higher resistance than fistulas).

8. Key Clinical Takeaways

• The “Normal EF” Trap: Do not be reassured by an EF of $55\text{–}65\%$; in the context of pulmonary edema and high CO, this is potentially HOHF.

• Pulse Pressure: Look for a wide pulse pressure (e.g., $180/60$) as a marker of low SVR.

• ESRD Logic: If an ESRD patient is “wet” immediately after HD, the problem is likely flow (AVF), not just fluid.