Pulmonary circulation and it's clinical aspects full details for md exams 10marks answer from fishman and murray only
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This answer covers all high-yield MD exam points directly from Fishman and Murray. Quick orientation of what's inside:
| Section | Key Exam Points |
|---|---|
| Anatomy | Hilum entry, peribronchovascular sheath, capillary surface 50-70 m² |
| Normal pressures | mPAP 14 (8-20), PVR 0.9 WU, PAWP 8 mmHg |
| PVR formula | (mPAP - LAP)/CO; Swan-Ganz measurement; thermodilution |
| Recruitment & distension | Mechanism for PVR fall with exercise |
| West zones | Zone 1/2/3 + Zone 4; gravity model |
| HPV | Mechanism (redox/ROS), NO & endothelin role, fetal circulation, clinical traps |
| PH definition | mPAP >20 mmHg (updated), pre vs postcapillary criteria, DPG vs TPG |
| RV-PA coupling | Ees/Ea 1.5-2, TAPSE/sPAP ratio |
| Clinical: PE | CTPA gold standard, D-dimer, Wells/PERC, >50% obstruction for PH |
| Clinical: PAH | RHC required, vasoreactivity testing, endothelin antagonists |
| Clinical: Edema | Starling forces, HAPE, peribronchovascular cuffing |
No for 10 marks re
Here is the restructured, crisp 10-mark exam answer - every point from Fishman and Murray, nothing extra:
PULMONARY CIRCULATION AND CLINICAL ASPECTS
1. INTRODUCTION
The pulmonary circulation is a high-flow, low-pressure system evolved to optimise gas exchange. The alveolo-capillary membrane is only 0.3 μm thick. The RV is a thin-walled flow generator (not a pressure generator).
(Fishman's, Ch.13)
2. ANATOMY
(Murray & Nadel, Ch.1)
- Pulmonary artery enters each lung at the hilum alongside the bronchus in a loose peribronchovascular connective tissue sheath
- Artery branches with each airway generation down to the respiratory bronchiole - ensuring V/Q matching at every level
- Pulmonary veins lie as far from airways as possible (Miller's dictum) - peripherally in connective tissue septa
- Total capillary surface area = 50-70 m² (key to efficient gas exchange)
- Capillary volume = 60-200 mL; serves as a reservoir that can be recruited
3. NORMAL PRESSURES
(Fishman's, Table 13-1)
| Parameter | Mean | Normal Range |
|---|---|---|
| mPAP | 14 mmHg | 8-20 |
| PAP systolic/diastolic | 21/9 mmHg | 13-29 / 3-15 |
| PAWP | 8 mmHg | 2-14 |
| PVR | 0.9 Wood units | 0.2-1.8 |
| Cardiac Output | 7.3 L/min | 5.0-9.5 |
Compare: systemic mPAP ~100 mmHg for the same cardiac output - pulmonary PVR is ~1/10th systemic.
4. PULMONARY VASCULAR RESISTANCE (PVR)
PVR = (mPAP − PAWP) / CO
- Measured via Swan-Ganz catheter (triple-lumen balloon-tipped, introduced by Swan, Ganz & Forrester, 1970s)
- PAWP estimates LAP (left atrial pressure)
- CO by thermodilution (5-10 mL cold saline into RA) or direct Fick (CO = VO₂ / CaO₂ - CvO₂)
PVR is NOT fixed - it falls when pressure rises, by two mechanisms:
- Recruitment - previously closed capillaries open
- Distension - increase in caliber of open vessels
This fall in PVR during exercise limits RV work.
Effect of hematocrit: PVR rises exponentially with hematocrit - clinically important in polycythemia (COPD, altitude).
(Murray & Nadel, Ch.10; Fishman's, Ch.13)
5. DISTRIBUTION OF BLOOD FLOW - WEST ZONES
(Murray & Nadel, Ch.10)
The upright lung is ~30 cm tall; hydrostatic pressure difference = 23 mmHg - large relative to driving pressure.
| Zone | Pressure Relationship | Flow |
|---|---|---|
| Zone 1 (apex) | PA > Pa > Pv | No flow (alveolar dead space) - not present normally |
| Zone 2 (middle) | Pa > PA > Pv | Intermittent; flow ∝ Pa − PA |
| Zone 3 (base) | Pa > Pv > PA | Continuous; flow ∝ Pa − Pv |
| Zone 4 (extreme base) | Interstitial P > Pa | Reduced flow (extra-alveolar vessel compression) |
(Pa = pulmonary arterial; PA = alveolar; Pv = pulmonary venous pressure)
Non-gravitational factors: Fractal branching, regional resistance differences; at weightlessness (astronauts), distribution becomes near-uniform.
Abnormal patterns:
- Pulmonary embolism → perfusion defect with normal ventilation (V/Q mismatch)
- Left heart failure → cephalization (upper zone diversion) from raised PAWP
- COPD, fibrosis → patchy reduction in regional flow
6. HYPOXIC PULMONARY VASOCONSTRICTION (HPV)
(Murray & Nadel, Ch.10; Fishman's, Ch.13)
Definition: Local alveolar hypoxia (PO₂ < 60 mmHg) → pulmonary vasoconstriction → diverts blood to better-ventilated areas.
Unique - systemic vessels DILATE in hypoxia; pulmonary vessels CONSTRICT.
Sensor: Alveolar PO₂ (not blood PO₂). Proven by perfusing lung with high-PO₂ blood under hypoxic alveolar conditions - vasoconstriction still occurs.
Site: Predominantly small pulmonary arteries and arterioles (precapillary).
Does NOT require autonomic innervation (persists in transplanted lungs).
Mechanisms - Two Hypotheses:
| Hypothesis | Mechanism |
|---|---|
| Redox hypothesis | Hypoxia → ↓ mitochondrial ROS → K⁺ channel (Kv1.5) closure → membrane depolarisation → voltage-gated Ca²⁺ entry → vasoconstriction |
| ROS hypothesis | Hypoxia → complex III generates ROS → Ca²⁺ influx + SR Ca²⁺ release → Rho kinase (ROCK) activation → myofilament sensitisation → vasoconstriction |
Modulators:
- NO (endothelium-derived): vasodilator; inhibits HPV; inhaled NO (~20 ppm) reverses HPV
- Endothelins (endothelium-derived): vasoconstrictor; augment HPV → endothelin receptor antagonists are key PAH drugs
- Metabolic acidosis: enhances HPV; metabolic alkalosis: attenuates HPV
Physiologic roles:
- Improves V/Q matching in localised lung disease
- Fetal circulation: HPV keeps PVR high → only 15% CO to fetal lungs
- Bronchodilators (β-agonists) inhibit HPV → V/Q worsens → ↓ PaO₂ (clinical trap in asthma)
7. DEFINITION OF PULMONARY HYPERTENSION
(Fishman's, Ch.13 - current expert consensus)
| Category | Criteria |
|---|---|
| Pulmonary Hypertension | mPAP > 20 mmHg (updated threshold; old: >25 mmHg) |
| Precapillary PH | mPAP >20 + PAWP ≤15 mmHg + PVR ≥3 Wood units |
| Postcapillary PH | mPAP >20 + PAWP >15 mmHg |
Diastolic Pressure Gradient (DPG) = dPAP − PAWP
- Upper limit of normal: ~5-7 mmHg
- DPG >7 mmHg in heart failure = poor survival (precapillary component added)
- DPG is superior to TPG (mPAP−PAWP) for diagnosing pulmonary vascular disease in left heart failure
Embolic obstruction threshold (Fishman's Fig 13-8):
-
50% obstruction needed to raise mPAP >25 mmHg
- Severe PH (mPAP ~50 mmHg) = ~80% obstruction
8. RV-PULMONARY ARTERIAL COUPLING
(Fishman's, Ch.13)
- Optimal coupling: Ees/Ea = 1.5-2 (RV end-systolic elastance / arterial elastance)
- EF = 1 - mPAP/Pmax
- When EF falls <35% or SV/ESV <52% → RV dilates → systemic congestion
- Noninvasive surrogate: TAPSE/sPAP ratio (echocardiography) - validated in PAH, HF, chronic lung disease
9. CLINICAL ASPECTS
A. Pulmonary Arterial Hypertension (PAH)
(Murray & Nadel, Ch.9)
- Workup: ECG, ABG, CXR, PFTs, 6-min walk test, BNP, echo
- Right heart catheterisation - mandatory to confirm diagnosis and severity
- Acute vasoreactivity test: inhaled NO / epoprostenol / adenosine → guides calcium channel blocker use
- Chronic thromboembolic PH: V/Q scan + CT pulmonary angiography
B. Pulmonary Embolism
(Fishman's, Ch.28; Murray & Nadel, Ch.9)
- Arises from lower extremity DVT; elevated troponin = high short-term mortality
- CTPA - current gold standard; high NPV; limited for subsegmental clots
- D-dimer - high NPV; poor specificity in pregnancy/surgery; negative D-dimer + low pretest probability avoids CT in ~50% outpatients
- V/Q scan (SPECT) - preferred in young women and pregnancy (lower breast dose); limited by poor specificity (PIOPED study)
- Wells, Geneva, PERC scores - pretest probability assessment (frequently underused)
- Leg Doppler US - first test if clinical DVT signs present
C. Pulmonary Edema
(Murray & Nadel, Ch.1; Fishman's, Ch.91)
- Fluid first accumulates in peribronchovascular connective tissue sheaths (seen as cuffing on CXR/CT)
- HAPE (High-Altitude Pulmonary Edema): global HPV → uneven vasoconstriction → high shear stress + vascular injury → edema
- Treatment: immediate descent, supplemental O₂, nifedipine
D. Effects of Lung/Heart Disease
- COPD → patchy HPV → if global hypoxia sustained → PH → cor pulmonale (RV hypertrophy/failure)
- Left-to-right shunts → increased flow → vascular remodelling → Eisenmenger syndrome (irreversible precapillary PH)
- Altitude: acute HPV → reversible; weeks-months exposure → structural arteriolar remodelling → RV hypertrophy
10. SUMMARY
(Fishman's Ch.13 - verbatim key point)
"The pulmonary circulation is a high-flow, low-pressure circuit with average PVR of 1 Wood unit in young adults rising to 2.5 Wood units over 4-6 decades. PVR falls with exercise. Distribution of perfusion is gravity-dependent, corrected by HPV. Optimal RV-PA coupling ratio is 1.5-2."
References: Fishman's Pulmonary Diseases & Disorders (Ch. 13, 28, 91) | Murray & Nadel's Textbook of Respiratory Medicine (Ch. 1, 9, 10)
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