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Pulmonary Circulation - Applied Aspects

For MD Exams (10 Marks) | Sources: Fishman & Murray

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1. ANATOMY AND STRUCTURE

• The pulmonary artery enters each lung at the hilum in loose peribronchovascular connective tissue, branching alongside airways down to the level of the respiratory bronchiole.
• Pulmonary veins follow Miller's dictum: they lie as far from airways as possible, occupying the peripheral connective tissue envelope of terminal respiratory units (TRUs).
• Each small muscular pulmonary artery supplies a specific volume of respiratory tissue; each vein drains portions of several such zones.
• Quantitative data (Table 1.3, Murray):
  • Arteries (>500 µm): volume 68 mL, surface area 0.4 m²
  • Arterioles (13-500 µm): volume 18 mL, surface area 1.0 m²
  • Capillaries (10 µm): volume 60-200 mL, surface area 50-70 m² (20× all other vessels combined - critical for gas exchange)
  • Venules: 13 mL / 1.2 m²; Veins: 58 mL / 0.1 m²
• The alveolo-capillary membrane is only 0.3 µm thick - a vulnerable barrier that requires complete separation of pulmonary from systemic circulation (Fishman).
• Gas exchange occurs predominantly at true capillaries; arteriolar blood flow is too rapid for significant O₂/CO₂ exchange.

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2. NORMAL HEMODYNAMIC VALUES

• Mean PAP (mPAP): ~15 mm Hg (normal); upper limit of normal ≤20 mm Hg; pulmonary hypertension defined as mPAP >20 mm Hg (updated from old threshold of 25 mm Hg).
• Pulmonary Vascular Resistance (PVR):

PVR = (mPAP - PAWP) / CO [Wood units or dynes·sec·cm⁻⁵]

• Normal PVR is far lower than systemic vascular resistance; total pressure drop across the pulmonary circulation is only ~10 mm Hg at rest (vs. ~100 mm Hg systemic).
• Normal pulmonary blood flow = cardiac output (~5 L/min).
• Total PVR (TPVR) = mPAP/CO - used when PAWP is unavailable; always larger than PVR since LAP is not negligible.
• 35-45% of total PVR resides in alveolar capillaries at FRC (unlike systemic circulation where arterioles dominate resistance) (Murray).

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3. MEASUREMENT - RIGHT HEART CATHETERIZATION

• Swan-Ganz catheter (triple-lumen, balloon-tipped, thermistor): measures RAP, RV pressure, PAP, and PAWP (pulmonary artery wedge/occluded pressure).
• PAWP estimates left atrial pressure (LAP) via stop-flow phenomenon - the fractal pulmonary vascular tree extends the fluid-filled lumen to equivalent-diameter veins on occlusion.
• Zero leveling: best reference is the intersection of three trans-thoracic planes near the hydrostatic indifference point (tricuspid valve level).
• Cardiac output measured by thermodilution (5-10 mL cooled saline into RA) or Fick method (CO = VO₂ / (CaO₂ - CvO₂)).
• Measurement errors: PAP ±8 mm Hg, CO ±1 L/min, PAWP vs. LVEDP estimates range ±12-15 mm Hg - suitable for populations but require caution in individual decision-making.

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4. PRESSURE-FLOW RELATIONSHIPS

• Pulmonary circulation is highly distensible: when PAP or LAP rises, PVR falls due to two mechanisms:
  1. Recruitment - opening of previously closed capillaries (dominant at low pressures)
  2. Distension - widening of already-open vessels (dominant at higher pressures)
• This "recruitment + distension" limits rise in mPAP during exercise and protects the right ventricle.
• Exercise: mPAP-CO slope averages 1.5 mm Hg/L/min (limits of normal: 0.5-3 mm Hg/L/min). Maximum exercise TPVR ≤3 Wood units; mPAP normally ≤30 mm Hg at CO <10 L/min (Fishman).
• Viscosity: PVR increases exponentially with hematocrit. Polycythemia markedly increases PVR, especially when PAWP is also elevated (Fishman).

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5. EFFECT OF LUNG VOLUME ON PVR (Murray - key exam point)

• PVR is minimal at FRC - the normal resting lung volume.
• At low lung volumes (< FRC): PVR rises due to narrowing of extra-alveolar vessels (loss of radial traction from parenchyma) and folding/distortion of capillaries.
• At high lung volumes (> FRC): PVR rises due to mechanical narrowing (stretching) of alveolar capillaries - analogous to rubber tubing narrowing when stretched laterally.
• Positive pressure ventilation raises PVR even more at high lung volumes because increased alveolar pressure compresses capillaries (reduced transmural pressure).
• Clinical implication: PEEP and mechanical ventilation with high tidal volumes increase PVR and may worsen right heart function.

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6. GRAVITY AND ZONES OF LUNG (West's Zones)

• The vertical lung height is ~24-30 cm; pulmonary artery pressure varies ~24 cm H₂O over this height.
• At mid-chest: PAP ≈ 20 cm H₂O (15 mm Hg); at lung apex: ~12 cm H₂O; at base: ~36 cm H₂O (Murray).
• Three zone model (West) based on relative magnitudes of PA pressure (Pa), Pv, and alveolar pressure (PA):

• Zone 1 does not normally exist but appears when alveolar pressure is raised (e.g., positive pressure ventilation) or when arterial pressure falls (e.g., hemorrhagic shock, hypovolemia).
• Blood volume is greater at the base due to higher luminal pressure (distension), reducing the contribution of basal vessels to total PVR.
• Swan-Ganz PAWP accurately reflects LVEDP only when catheter tip is in Zone 3 (continuous fluid column). Tip in Zone 1 or 2 gives falsely elevated readings (alveolar pressure transmitted).

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7. HYPOXIC PULMONARY VASOCONSTRICTION (HPV)

• Unique feature: while systemic vessels dilate to hypoxia, pulmonary vessels constrict - the only vasoconstrictor response to hypoxia.
• Triggered when alveolar PO₂ falls below 60 mm Hg; response proportional to degree of hypoxia.
• Site: predominantly small pulmonary arteries and arterioles (precapillary); oxygen sensor is precapillary.
• Key proof: perfusing lung with high PO₂ blood while keeping alveolar PO₂ low still produces vasoconstriction - alveolar PO₂, not arterial PO₂, is the primary determinant (Murray).
• Mechanism (Murray/Fishman - two hypotheses):
  • Redox hypothesis: hypoxia → ↓ mitochondrial ROS → reduced intracellular environment → K⁺ channel (Kv1.5) closure → membrane depolarization → voltage-gated Ca²⁺ channel opens → VSMC contraction.
  • ROS hypothesis: hypoxia → mitochondrial complex III → ↑ ROS → Ca²⁺ influx + SR Ca²⁺ release → ↑ intracellular Ca²⁺ + Rho-kinase (ROCK) activation → contraction.
• HPV is an intrinsic property of VSMCs - persists in isolated lungs and transplanted lungs without autonomic innervation.
• Modulation: Enhanced by metabolic acidosis; blunted by metabolic alkalosis and respiratory alkalosis (CO₂ effect is pH-independent). Nitric oxide (NO) inhibits HPV; NO synthase inhibitors augment it.

1. V/Q matching: diverts blood from hypoxic (underventilated) regions to well-ventilated areas - beneficial in lobar pneumonia, atelectasis, COPD.
2. Bronchodilators (e.g., salbutamol, theophylline) can abolish HPV → blood flows to underventilated areas → worsening of arterial hypoxemia (paradoxical desaturation in asthma).
3. High-altitude pulmonary edema (HAPE): global hypoxia → uneven HPV → focal high-pressure areas → capillary stress failure → edema.
4. Fetal circulation: high HPV keeps PVR elevated; only ~15% of cardiac output goes to lungs in fetal life. At birth, lungs expand + PO₂ rises → HPV released → PVR falls dramatically.
5. Chronic sustained HPV (e.g., altitude residence, COPD) leads to vascular remodeling and pulmonary hypertension.

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8. TRANSPULMONARY GRADIENT AND DIASTOLIC PRESSURE GRADIENT

• TPG (Transpulmonary Gradient) = mPAP - PAWP; upper limit of normal: 12-15 mm Hg (Fishman).
• DPG (Diastolic Pressure Gradient) = dPAP - PAWP; upper limit of normal: ~7 mm Hg.
• Diagnostic importance: In heart failure with raised PAWP, DPG remains stable (vascular distensibility adjusts), but TPG rises - used to diagnose a precapillary component superimposed on postcapillary (left heart) pulmonary hypertension.
• DPG >7 mm Hg = associated with worse survival in PH due to left heart disease.
• Preferred calculation of PVR uses averaged PAWP over cardiac cycle (not point reading), to avoid V-wave artifact which can artificially lower PVR.

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9. ENDOTHELIUM AND VASOACTIVE MEDIATORS

• Autonomic control is weak in normal lung (little resting tone); α- and β-adrenergic receptors both present. Sympathetic tone causes mild vasoconstriction; parasympathetic causes weak dilation.
• Vasoactive drugs are more effective at low lung volumes (less radial traction on extra-alveolar vessels) and in fetal circulation (more smooth muscle).

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10. APPLIED ASPECTS - EXAM HIGH-YIELD POINTS

A. Pulmonary Hypertension (PH)

• Defined as mPAP >20 mm Hg at rest (WHO 2022 update from prior >25 mm Hg threshold).
• 50% obstruction of pulmonary vascular bed required before mPAP exceeds 25 mm Hg at resting CO; 80% obstruction for mPAP ~50 mm Hg (massive PE model, Fishman).
• Right ventricle evolves as a "thin-walled flow generator" suited for high-flow low-pressure work; unprepared for acute pressure overload (acute cor pulmonale in massive PE).

B. Mechanical Ventilation

• PEEP raises alveolar pressure → creates/expands Zone 1 → increases dead space ventilation; also raises PVR → increases right ventricular afterload.
• PAWP readings may be overestimated if catheter tip is not in Zone 3 during positive pressure ventilation.

C. Fetal-to-Neonatal Transition

• Fetal PVR extremely high (HPV + fluid-filled alveoli + structural vascular features).
• At first breath: PO₂ rises → HPV abolished → lung expansion → mechanical opening of vessels → PVR falls 10-fold → pulmonary blood flow increases → foramen ovale and ductus arteriosus close functionally.
• Failure = Persistent Pulmonary Hypertension of the Newborn (PPHN).

D. Exercise Physiology

• PVR falls during exercise via recruitment + distension → mPAP rises only modestly despite 4-6× increase in cardiac output.
• Exercise-induced PH defined as mPAP-CO slope >3 mm Hg/L/min.
• In right heart disease, exercise intolerance is the earliest symptom as cardiac output cannot be augmented without excessive pressure rise.

E. High-Altitude Physiology

• Global alveolar hypoxia → diffuse HPV → mPAP rises.
• Chronic exposure → suppression of HPV + structural vascular remodeling → fixed pulmonary hypertension.
• HAPE: uneven HPV → focal high-pressure segments → capillary stress failure + increased permeability → non-cardiogenic pulmonary edema. Treated with descent, O₂, nifedipine (calcium channel blocker abolishes HPV).

F. COPD

• Chronic airflow obstruction → chronic alveolar hypoxia → sustained HPV → pulmonary hypertension → cor pulmonale.
• Bronchodilators may paradoxically worsen hypoxemia by relieving local bronchoconstriction while simultaneously abolishing HPV.

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• Fishman's Pulmonary Diseases and Disorders (Chapter 13: The Pulmonary Circulation - Naeije; Chapter 1: Pulmonary Vascular Anatomy)
• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 1: Pulmonary Circulation Anatomy; Chapter 10: Pulmonary Vascular Resistance, Pressure-Flow Relations, HPV; Chapter 6: Hypoxic Pulmonary Vasoconstriction)