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Pulmonary Circulation - Applied Aspects

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1. Anatomy & Structure

• The pulmonary artery enters each lung at the hilum in a loose peribronchovascular connective tissue sheath adjacent to the main bronchus, branching with every airway generation down to the respiratory bronchiole (Murray & Nadel, p. 38).
• Pulmonary veins follow Miller's dictum: they lie as far from the airways as possible, occupying the peripheral connective tissue envelope - unlike arteries, which travel alongside bronchi (Murray & Nadel, p. 38).
• Each small muscular pulmonary artery supplies a specific volume of respiratory tissue; each vein drains portions of several such zones - an asymmetry with physiologic relevance.
• Quantitative data (humans):

• The alveolo-capillary membrane is only ~0.3 µm thick - the thinnest barrier allowing gas exchange; endothelial + epithelial basement membranes fuse at this site (unique to the lung and glomerulus) (Murray & Nadel, p. 162).

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2. Normal Pressures & Hemodynamics

• Pulmonary blood flow is a high-flow, low-pressure system - the result of evolutionary optimization for gas exchange in endothermic mammals (Fishman, p. 225).
• Key formula:
  • PVR = (mPAP - LAP) / CO (Fishman, p. 225)
  • Normal mPAP: ~14 mm Hg (upper limit of normal: 20 mm Hg)
  • Normal PVR: ~1 Wood Unit (WU) at rest
  • Normal driving pressure (mPAP - LAP): ~10 mm Hg at 5 L/min flow
• Pulmonary capillary pressure (PCP): average ~10 mm Hg (range 8-12 mm Hg) in normal volunteers (Fishman, p. 227).
• PAWP (pulmonary artery wedge pressure) = estimate of left atrial pressure; obtained by balloon occlusion of the PA. The fractal pulmonary arterial/venous tree allows stop-flow phenomenon enabling the catheter lumen to functionally extend to same-diameter pulmonary veins (Fishman, p. 226).
• Cardiac output measured by thermodilution (cooled saline into RA) or the direct Fick method: CO = VO₂ / (CaO₂ - CvO₂) (Fishman, p. 226).

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3. Pulmonary Vascular Resistance (PVR)

• PVR obeys Hagen-Poiseuille law: R = 8ηl / (πr⁴) - resistance is inversely proportional to the 4th power of radius (Murray & Nadel, p. 226).
  • A 10% change in radius → ~50% change in resistance - clinically, small changes in vessel caliber have outsized hemodynamic effects.
• PVR is not fixed - it is a function of both pressure and flow:
  • Increase in PAP or LAP → decreases PVR (by recruitment + distension of capillaries) (Murray & Nadel, p. 226).
  • This property limits right heart work during exercise.
• Two mechanisms of PVR reduction:
  1. Recruitment - opening of previously closed vessels (capillary count doubles as PAP rises from zero to ~25 cmH₂O)
  2. Distension - increased caliber of open vessels (Murray & Nadel, p. 227)
• Age and sex: Women have higher PVR than men due to smaller body size, but no sex difference after body dimension correction. PVR doubles over five decades of life due to rising mPAP and falling CO (Fishman, p. 227).
• Hematocrit and PVR: Exponential relationship - polycythemia markedly elevates PVR; effect is amplified at high hematocrit and high PAWP. A hematocrit of 70% can more than double PVR compared to normal (Fishman, p. 230).
• Pulmonary arterial compliance (PAC): PAC = SV / Pulse Pressure (systolic - diastolic PAP). PAC decreases with PAH, increasing RV afterload (Fishman, p. 230).

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4. Distribution of Blood Flow - Zones of West

• Blood flow in the upright lung is non-uniform: increases from apex to base under normal conditions.
• West's 3-Zone Model (Murray & Nadel, p. 227):

• Zone 4 (lowermost): Reduced blood flow at the lung base, especially at low lung volumes (FRC). Caused by narrow extra-alveolar vessels at the base where alveoli are less expanded due to gravitational distortion (Murray & Nadel, p. 227).
  • Zone 4 expands as lung volume decreases (FRC→RV), and is exaggerated by vasoconstrictors (e.g., serotonin) and interstitial edema (which compresses extra-alveolar vessels).

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5. Hypoxic Pulmonary Vasoconstriction (HPV)

• HPV is the most clinically important vasomotor response of the pulmonary circulation - the opposite of the systemic response to hypoxia (Fishman, p. 233).
• Mechanism (Fishman, p. 234):
  • Low PO₂ → inhibits smooth muscle cell voltage-gated K⁺ channels → membrane depolarization → Ca²⁺ influx → vasoconstriction
  • Possible O₂ sensors: mitochondria, NADPH oxidases
  • Candidate mediators: reactive oxygen species, redox couples, AMP-activated kinases
  • Profound hypoxia reverses HPV via activation of ATP-dependent K⁺ channels
• Purpose of HPV:
  • Diverts blood from poorly ventilated/hypoxic lung regions to better-aerated zones - optimizes V/Q matching
  • Limits perfusion of the fetal lung (maintaining high PVR in utero)
  • Preventing HPV (e.g., with vasodilators in ARDS) → blood flows to edematous/dependent lung → marked drop in PaO₂
• Clinical significance:
  • Whole-lung HPV at high altitude → fixed pulmonary hypertension (mild-borderline in humans)
  • Chronic HPV + vascular remodeling → cor pulmonale in COPD/ILD

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6. Endothelium and Vasoactive Mediators

• The endothelium has segment-specific phenotypes: artery, capillary, and vein differ in origin, morphology, Weibel-Palade body content (absent in capillaries), NO production (higher precapillary), and barrier tightness (Murray & Nadel, p. 162).
• Vasodilator mediators:
  • Nitric oxide (NO)
  • Prostacyclin (PGI₂)
  • Endothelium-derived hyperpolarizing factor (EDHF)
• Vasoconstrictor:
  • Endothelin-1 (major endothelium-derived constrictor)
• Applied: Imbalance toward endothelin/reduced NO/PGI₂ is central to pulmonary arterial hypertension (PAH) pathogenesis, forming the basis for:
  • Prostacyclin analogs (epoprostenol, treprostinil)
  • PDE-5 inhibitors (sildenafil, tadalafil) - enhance NO signaling
  • Endothelin receptor antagonists (bosentan, ambrisentan) (Fishman, p. 234)

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7. Pulmonary Circulation During Exercise

• Exercise increases CO up to 6-fold and VO₂ up to 20-fold above rest (Fishman, p. 225).
• With exercise, PAP, PAWP, and CO all rise; PVR falls due to recruitment and distension.
• Pressure-flow slope is the best measure of pulmonary vascular reserve during exercise (Fishman, p. 234).
• Abnormal exercise response (mPAP >30 mm Hg at CO of 10 L/min) suggests early pulmonary vascular disease before resting PAP is elevated.
• Autonomic control: Sympathetic activation stiffens proximal pulmonary arterial tree but does not change PVR significantly; parasympathetic has minimal role (Fishman, p. 234).
• Strenuous exercise can cause stress failure of pulmonary capillaries (barrier disruption → alveolar hemorrhage) - mechanism for exercise-induced hemoptysis in horses and elite athletes.

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8. Right Ventricle and Ventriculo-Arterial Coupling

• The RV is a thin-walled flow generator - evolutionarily reshaped for the low-pressure pulmonary circuit (Fishman, p. 225).
• Coupling is assessed by the Ees/Ea ratio (end-systolic elastance / arterial elastance):
  • Optimal coupling: Ees/Ea ≈ 1.5
  • In PAH: Ees rises adaptively (homeometric adaptation) but Ees/Ea is decreased → insufficient adaptation → impending RV failure (Fishman, p. 232)
• PA compliance (PAC) and PVR are inversely related - their product (RC time constant) is relatively fixed in pulmonary vascular diseases; loss of compliance increases RV pulsatile load even when mean PVR appears controlled.
• A decrease in PAC is an independent predictor of mortality in PAH independent of PVR.

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9. Pulmonary Edema - Applied Aspects

• Pulmonary edema results when the Starling forces favor net fluid movement out of capillaries exceeding lymphatic drainage capacity.
• Key factors:
  • Hydrostatic (cardiogenic): elevated PAWP > 18 mm Hg → interstitial edema; >25 mm Hg → alveolar flooding
  • Permeability (non-cardiogenic, ARDS): endothelial barrier breakdown via inflammatory mediators
• Interstitial edema compresses extra-alveolar vessels, increasing PVR at the lung base - Zone 4 expands (Murray & Nadel, p. 227).
• Weibel-Palade bodies in artery/vein endothelium (absent in capillaries) store vWF and P-selectin - relevant in inflammation-driven permeability (Murray & Nadel, p. 162).

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10. Pulmonary Hypertension - Applied Summary

• Definition (ESC/Fishman): mPAP > 20 mm Hg at rest on RHC
• Old threshold was 25 mm Hg; revised down to 20 mm Hg based on population data.
• ~50% obstruction of pulmonary vascular bed required before mPAP rises above 25 mm Hg at normal CO; severe PH (mPAP ~50 mm Hg) corresponds to ~80% obstruction (Fishman, Fig. 13-8).
• Diagnostic: RHC with PAWP ≤15 (precapillary/Group 1,3,4,5) vs. PAWP >15 (postcapillary/Group 2 = LHD).
• Key groups: PAH (Group 1), LHD (Group 2), Lung disease/hypoxia (Group 3), CTEPH (Group 4), Miscellaneous (Group 5).
• CTEPH: Chronic thromboembolic PH - treatable by pulmonary endarterectomy; HPV-mediated reversible component may coexist.

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