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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 seminar summary of Assessment of Pulmonary Circulation from GK Pal Physiology:
Assessment of Pulmonary Circulation
Seminar Presentation Summary
1. WHY ASSESS PULMONARY CIRCULATION?
Assessment of pulmonary function is incomplete without studying pulmonary circulation.
It helps detect:
- Venoarterial (right-to-left) shunts
- Vascular occlusion
- Decreased pulmonary capillary volume
2. NORMAL VALUES TO REMEMBER
| Parameter | Normal Value |
|---|---|
| Pulmonary vasculature blood flow | 5 L/min (= entire RV output) |
| Mean pulmonary artery pressure (PAP) | 15 mm Hg |
| Pulmonary Vascular Resistance (PVR) | 50-150 dynes.s/cm⁵ |
Key point: Pulmonary vessels are thin-walled and offer LESS resistance than systemic vessels. In upright posture, pressure is lowest at the apex and highest at the lung bases (gravity effect).
3. HOW IS PULMONARY CIRCULATION ASSESSED?
Tool: Swan-Ganz (Pulmonary Arterial) Catheter
- A flow-directed catheter inserted into the pulmonary artery
- Directly measures:
- Pulmonary Arterial Pressure (PAP)
- Pulmonary Capillary Wedge Pressure (PCW) - reflects left atrial pressure
- Cardiac Output (CO) is measured separately by the thermodilution method
- Usually done in Intensive Care Units (ICU)
4. THE PVR FORMULA (Must Know!)
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)
- Normal PVR = 50-150 dynes.s/cm⁵
Think of it as: How much resistance does blood face while flowing through the lungs?
5. DISTURBANCES IN PULMONARY CIRCULATION
PVR increases by 4 mechanisms:
| # | Mechanism | How it increases PVR |
|---|---|---|
| 1 | Pulmonary Vasoconstriction | Arteries narrow in response to alveolar hypoxia |
| 2 | Pulmonary Thromboembolism | Blood clots block lumens, reducing cross-sectional area |
| 3 | Vascular Hypertrophy | Smooth muscle overgrowth thickens vessel walls, narrows lumen |
| 4 | Pulmonary Injury | Scarring or loss of alveolar wall destroys small vessels |
Chain of events: Increased PVR → Increased Pulmonary Arterial Pressure → Decreased Right Ventricular Output
6. CONDITIONS THAT INCREASE PVR
A. Heart Diseases
- Mitral stenosis - elevated left atrial pressure backs up into pulmonary circuit
B. Lung Diseases (cause chronic hypoxemia → vasoconstriction)
- COPD
- Interstitial lung disease
- Chest wall diseases (e.g., Kyphoscoliosis)
- Obesity hypoventilation
- Sleep apnea syndrome
C. Pulmonary Vascular Diseases
- Recurrent pulmonary embolism
- Scleroderma (occludes small pulmonary arteries and arterioles)
7. RESPIRATORY CAUSES OF HYPOXEMIA
Respiratory dysfunction is the most common cause of hypoxemia in adults.
Non-respiratory causes (for contrast): Anemia, CO poisoning, high altitude
Normal A-aO₂ Gradient
- Normally Hb is 100% saturated in pulmonary capillaries
- Arterial PaO₂ ≈ 95 mm Hg; Hb ≈ 98% saturated
- Alveolar PO₂ ≈ 100-102 mm Hg; Arterial PO₂ ≈ 85-95 mm Hg
- Normal A-a gradient = 5-15 mm Hg
5 Respiratory Causes of Hypoxemia
| Cause | Key Feature |
|---|---|
| 1. Regional Hypoventilation | Most common physiological cause; V/Q mismatch from partially obstructed airway → venous admixture; High A-a gradient, low PO₂ |
| 2. Large Blood Clot (Pulmonary Embolism) | Blocks major artery → blood overperfuses other areas → V/Q mismatch → venous admixture |
| 3. Shunts | Right-to-left cardiac shunt or intrapulmonary shunt; High A-a gradient, low PO₂; Breathing 100% O₂ does NOT correct it (shunted blood bypasses lungs) |
| 4. Generalized Hypoventilation | Low overall alveolar ventilation (COPD, drug overdose/morphine, head injury); A-a gradient is NORMAL (the only cause where A-a gradient stays normal!); Best corrected by mechanical ventilation |
| 5. Diffusion Block | Alveolar-capillary membrane thickened or impermeable (e.g., pulmonary edema); Low PaO₂, High A-a gradient, high PaCO₂ |
8. QUICK CLINICAL COMPARISON TABLE
| Feature | Regional Hypoventilation | Shunt | Generalized Hypoventilation | Diffusion Block |
|---|---|---|---|---|
| A-a gradient | High | High | Normal | High |
| PO₂ with 100% O₂ | Improves | Does NOT improve | Improves | Improves |
| PaCO₂ | Normal/low | Low | High | High |
9. KEY EXAM POINTS (Important to Know)
- Swan-Ganz catheter is the tool for measuring pulmonary pressures
- PVR formula: 80 × (PAP - PCW) / CO; normal = 50-150 dynes.s/cm⁵
- 4 mechanisms increase PVR: vasoconstriction, thromboembolism, hypertrophy, injury
- Alveolar hypoxia is the most potent cause of pulmonary vasoconstriction
- Generalized hypoventilation is the ONLY respiratory cause with a NORMAL A-a gradient
- Shunts are the ONLY cause where 100% O₂ breathing does NOT correct hypoxemia
- Regional hypoventilation = most common physiological cause of hypoxemia
- FEV₁ is the most sensitive non-invasive test of lung function
- Blood gas analysis is the most sensitive measure of diffusion capacity
Source: GK Pal Physiology, Section 10: Respiratory System, Chapter 114 - Pulmonary Function Tests (pp. 970-972)
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