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Summarise assessment of pulmonary circulation for seminar presentation with all important points easy to understand
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Here is a complete, easy-to-understand summary of Assessment of Pulmonary Circulation for your seminar presentation, based on GK Pal Physiology:
🫁 Assessment of Pulmonary Circulation
Seminar Presentation Summary
📌 Why Assess Pulmonary Circulation?
"Assessment of pulmonary function is incomplete and unsatisfactory without the study of pulmonary circulation."
Measuring pressures, vascular resistance, blood volume, and blood flow distribution helps detect:
- Venoarterial shunts
- Vascular occlusion
- Decreased pulmonary capillary volume
🩸 Normal Pulmonary Blood Flow & Pressure
| Parameter | Normal Value |
|---|---|
| Blood flow (right ventricular output) | 5 L/min |
| Mean pulmonary artery pressure | 15 mm Hg |
| Normal PVR | 50-150 dynes.s/cm⁵ |
- Pulmonary vessels are thin-walled and offer less resistance than systemic vessels
- In upright posture: pressure is lowest at apex, highest at lung bases (gravity effect)
🔬 How to Assess Pulmonary Blood Flow (Method)
Tool: Swan-Ganz (Flow-Directed Pulmonary Artery) Catheter
- Used in ICU with invasive monitoring
- Directly measures:
- PAP = Pulmonary arterial pressure
- PCW = Pulmonary capillary wedge pressure
- Cardiac output measured by thermodilution method
Formula - Pulmonary Vascular Resistance (PVR):
PVR = 80 × (PAP - PCW) / CO
Where:
- PAP = Mean pulmonary arterial pressure (mm Hg)
- PCW = Pulmonary capillary wedge pressure (mm Hg)
- CO = Cardiac output (L/min)
⚠️ Disturbances in Pulmonary Circulation
4 Mechanisms that Increase PVR:
| # | Mechanism | How it Works |
|---|---|---|
| 1 | Pulmonary Vasoconstriction | Alveolar hypoxia triggers arterial/arteriolar constriction → ↑ PVR |
| 2 | Pulmonary Thromboembolism | Intraluminal clots → ↓ luminal cross-section → ↑ PVR |
| 3 | Vascular Hypertrophy | Smooth muscle proliferation in vessel walls → ↓ lumen → ↑ PVR |
| 4 | Pulmonary Injury | Scarring or loss of alveolar walls destroys small vessels → ↓ total cross-sectional area → ↑ PVR |
Key Consequence: ↑ PVR → ↑ Pulmonary artery pressure → ↓ Right ventricular output
🏥 Conditions That Increase PVR
1. Heart Diseases
- Cardiac conditions raising left atrial pressure (e.g., mitral stenosis) → ↑ PVR
2. Lung Diseases (cause chronic pulmonary hypoxemia)
- COPD
- Interstitial lung disease
- Chest wall diseases (e.g., Kyphoscoliosis)
- Obesity hypoventilation
- Sleep apnoea syndrome
3. Diseases Affecting Pulmonary Vessels
- Recurrent pulmonary embolism
- Scleroderma (occludes small pulmonary arteries and arterioles)
😮💨 Respiratory Causes of Hypoxemia
Respiratory dysfunction is the most common cause of hypoxemia in adults.
Normal A-aO₂ Gradient
- Hb is 100% saturated in pulmonary capillaries → end-capillary PO₂ = Alveolar PO₂
- But blood in pulmonary veins returning to left atrium has lower PO₂ (due to venous admixture)
- Normal PaO₂ ≈ 95 mm Hg; Normal A-aO₂ gradient = 5-15 mm Hg
5 Respiratory Causes of Hypoxemia:
| Cause | Key Feature |
|---|---|
| 1. Regional Hypoventilation | Most common; low V/Q ratio from partially obstructed airway → venous admixture; high A-aO₂ gradient, low PO₂ |
| 2. Large Blood Clot | Occludes major artery → over-perfusion of other areas → ↑ venous admixture |
| 3. Shunts | Right-to-left cardiac or intrapulmonary shunt; 100% O₂ does NOT correct low PaO₂ because shunted blood bypasses alveoli |
| 4. Generalized Hypoventilation | Low alveolar ventilation (e.g., COPD, emphysema, morphine OD); A-aO₂ gradient is NORMAL - hypoxemia entirely from hypoventilation; correct with mechanical ventilator + room air |
| 5. Diffusion Block | Increased diffusion distance across alveolar-capillary membrane; low PaO₂, high A-aO₂ gradient, high PaCO₂; pulmonary edema is a common cause |
📊 Key Take-Home Points for Seminar
- PVR formula (Swan-Ganz catheter) is the cornerstone of pulmonary circulation assessment
- 4 mechanisms increase PVR - all reduce vascular cross-sectional area
- Hypoxia itself worsens PVR via vasoconstriction (a vicious cycle)
- A-aO₂ gradient helps distinguish type of hypoxemia - normal in generalized hypoventilation, elevated in all others
- Shunts are unique: O₂ therapy fails to correct PaO₂
- Mitral stenosis is the classic cardiac cause of ↑ PVR
🧠 Viva Points (Important to Know)
- Name the catheter used to assess pulmonary circulation → Swan-Ganz catheter
- Normal PVR → 50-150 dynes.s/cm⁵
- Normal mean PAP → 15 mm Hg
- Normal A-aO₂ gradient → 5-15 mm Hg
- Which cause of hypoxemia does NOT respond to 100% O₂? → Shunts
- In which cause of hypoxemia is A-aO₂ gradient normal? → Generalized hypoventilation
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