---

Assessment of Pulmonary Circulation - Seminar Summary

---

Why Is It Important?

"Assessment of pulmonary function is incomplete and unsatisfactory without the study of pulmonary circulation."

---

1. Measuring Pulmonary Blood Flow & Pressure

How?

• Done in ICU using invasive monitoring
• Tool used: Swan-Ganz catheter (flow-directed pulmonary arterial catheter)
• Measures:
  • Pulmonary arterial pressure (PAP)
  • Pulmonary capillary wedge pressure (PCW)
  • Cardiac output (CO) - by thermodilution method

Key Formula - Pulmonary Vascular Resistance (PVR):

PVR = 80 × (PAP - PCW) / CO

---

2. Disturbances in Pulmonary Circulation (PVR Increases By 4 Mechanisms)

Increased PVR --> increased pulmonary artery pressure --> decreased right ventricular output

Conditions That Increase PVR:

1. Heart diseases - e.g., mitral stenosis raises left atrial pressure
2. Lung diseases - COPD, interstitial lung disease, chest wall diseases (kyphoscoliosis), obesity hypoventilation, sleep apnea
3. Diseases of pulmonary vessels - recurrent pulmonary embolism, scleroderma

---

3. Respiratory Causes of Hypoxemia

• Non-respiratory causes: anemia, carbon monoxide poisoning, decreased inspired O₂ (high altitude)
• Respiratory dysfunction is the most common cause in adults

5 Respiratory Causes of Hypoxemia:

1. Regional hypoventilation
2. Large blood clot (occlusion of major artery)
3. Shunts
4. Generalized hypoventilation
5. Diffusion block

---

4. Alveolar-Arterial O₂ Gradient (A-aO₂ Gradient)

• Normal arterial PaO₂ = 85-95 mm Hg (Hb is ~98% saturated)
• Normal A-aO₂ gradient = 5-15 mm Hg

5 Causes of Increased A-aO₂ Gradient:

---

A) Regional Hypoventilation (most common)

• A local ventilation-perfusion (V/Q) mismatch - some lung areas are hypoventilated but still perfused
• Blood passes un-oxygenated areas → increases venous admixture
• Result: High A-aO₂ gradient, Low PO₂, Low O₂ content

---

B) Large Blood Clot

• A major artery in the lung is occluded → greater cardiac output is diverted to the remaining lung
• That remaining lung gets over-perfused relative to ventilation
• Result: Venous admixture increases, V/Q ratio falls

---

C) Shunts

• Blood bypasses the lungs entirely:
  • Right-to-left cardiac shunt (e.g., Tetralogy of Fallot)
  • Intrapulmonary shunt (e.g., foreign body or tumor obstructing a bronchus)
• Key feature: Breathing 100% O₂ does NOT correct low arterial PO₂ (because shunted blood never contacts alveolar air)
• Result: High A-aO₂ gradient, Low PO₂, Low O₂

---

D) Generalized Hypoventilation

• Occurs when overall alveolar ventilation is globally reduced (not just regional):
  • COPD (emphysema)
  • Head injury or drug overdose (morphine) - depresses respiratory center
• As ventilation falls → arterial PCO₂ rises + arterial pH falls
• KEY distinguishing feature: A-aO₂ gradient is NORMAL (unlike other causes)
• Treatment: Mechanical ventilation with room air

---

E) Diffusion Block (least common)

• Occurs when the diffusion distance across the alveolar-capillary membrane increases OR its permeability decreases
• Example: Pulmonary edema
• Features: Low PaO₂, High A-aO₂ gradient, High PaCO₂

---

5. Quick Comparison Table for Seminar

---

Memory Tips for Exams

• PVR formula: "80 times pressure drop divided by flow" = 80 × (PAP - PCW) / CO
• Shunt = the only cause where 100% O₂ fails to correct hypoxemia
• Generalized hypoventilation = only cause with normal A-aO₂ gradient
• Diffusion block = low permeability or long distance (think pulmonary edema)
• 4 mechanisms of increased PVR: Vasoconstriction, Thromboembolism, Hypertrophy, Injury (mnemonic: VTHI)

---

---

🫁 Assessment of Pulmonary Circulation - Seminar Summary

---

Why Study Pulmonary Circulation?

"Assessment of pulmonary function is incomplete and unsatisfactory without the study of pulmonary circulation."

• The pulmonary vasculature accommodates 5 L/min of right ventricular output
• Vessels are thin-walled and offer less resistance than systemic vessels
• Normal mean pulmonary artery pressure = 15 mm Hg
• In upright posture: pressure is lowest at apex, highest at lung bases

---

PART 1 - Measuring Pulmonary Blood Flow and Pressure

How Is It Done?

• Performed in ICU using invasive monitoring
• Tool: Swan-Ganz catheter (flow-directed pulmonary arterial catheter)

What Does It Measure?

• Pulmonary arterial pressure (PAP)
• Pulmonary capillary wedge pressure (PCW)
• Cardiac output (CO) - by thermodilution method

Key Formula - Pulmonary Vascular Resistance (PVR):

PVR = 80 × (PAP - PCW) / CO

Normal PVR = 50-150 dynes.s/cm³

---

PART 2 - Disturbances in Pulmonary Circulation

PVR Increases by 4 Mechanisms:

Increased PVR → Increased pulmonary artery pressure → Decreased right ventricular output

---

Conditions That Increase PVR:

• Conditions that raise left atrial pressure (e.g., mitral stenosis)

• COPD
• Interstitial lung disease
• Chest wall diseases (e.g., kyphoscoliosis)
• Obesity hypoventilation
• Sleep apnea syndrome

• Recurrent pulmonary embolism
• Scleroderma (occludes small pulmonary arteries and arterioles)

---

PART 3 - Respiratory Causes of Hypoxemia

• Non-respiratory causes: anemia, carbon monoxide poisoning, decreased inspired O₂ tension (e.g., high altitude)
• Respiratory dysfunction = most common cause of hypoxemia in adults

5 Respiratory Causes:

1. Regional hypoventilation
2. Occlusion of a major artery (large blood clot)
3. Shunts
4. Generalized hypoventilation
5. Diffusion block

---

PART 4 - Alveolar-Arterial Oxygen Gradient (A-aO₂ Gradient)

Normal Physiology:

• Hb is 100% saturated with O₂ as blood passes pulmonary capillaries
• End-capillary PO₂ = alveolar PO₂
• Blood in pulmonary veins returning to left atrium has slightly lower PO₂
• Normal PaO₂ in systemic arterial blood = 85-95 mm Hg (Hb ~98% saturated)
• Normal A-aO₂ gradient = 5-15 mm Hg

---

PART 5 - The 5 Causes Explained Simply

---

1. Regional Hypoventilation (Most Common)

• A local area of the lung is underventilated but still perfused
• Blood passes through un-oxygenated alveoli → venous admixture increases
• Only a small amount of venous admixture is normal
• Patients with abnormally low V/Q ratio:

---

2. Large Blood Clot (Occlusion of Major Artery)

• A major artery is blocked → blood is diverted to the remaining lung
• That part gets over-perfused relative to ventilation → V/Q ratio falls
• Result: venous admixture increases

---

3. Shunts

• Blood bypasses ventilated alveoli entirely:
  • Right-to-left cardiac shunt
  • Intrapulmonary shunt (foreign body or tumor obstructing airway)
• Shunted blood never contacts alveolar air
• Critical point: Breathing 100% O₂ does NOT correct the low PaO₂ (unique to shunts!)

---

4. Generalized Hypoventilation

• Overall alveolar ventilation is globally reduced (not just one area)
• Causes:
  • COPD (emphysema)
  • Head injury or drug overdose (morphine) depressing the respiratory center
• As ventilation falls → PaCO₂ rises → arterial pH falls

A-aO₂ gradient is NORMAL (unlike all other causes!)

• Best treated by mechanical ventilation breathing room air

---

5. Diffusion Block (Least Common)

• Alveolar-capillary membrane has increased thickness or decreased permeability
• Classic example: Pulmonary edema

---

PART 6 - Master Comparison Table (Most Important for Exam!)

---

Quick Memory Tips

---

---

🫁 Pulmonary Circulation - Introduction

---

1. What Is the Pulmonary Circulation?

The pulmonary circulation is the one responsible for oxygenating blood - it carries deoxygenated blood to the alveoli and returns oxygenated blood to the left heart. Bronchial circulation accounts for only 1-2% of cardiac output and supplies the airways, connective tissue, and lung support structures.

---

2. Anatomical Path of Blood Flow

Right Ventricle
      ↓
Pulmonary Artery (only 5 cm long, then divides)
      ↓
Right + Left Pulmonary Arteries → enter each lung at the hilum
      ↓
Branch alongside bronchi at every airway generation
      ↓
Pulmonary Arterioles → Alveolar Capillaries (GAS EXCHANGE)
      ↓
Pulmonary Venules → Pulmonary Veins
      ↓
Left Atrium → Left Ventricle → Systemic Circulation

The pulmonary artery branches travel alongside airways at every generation - this close arrangement ensures ventilation and perfusion are always matched. Pulmonary veins, unlike arteries, travel away from the airways - found in the surrounding connective tissue peripherally.

---

3. Structure of Pulmonary Vessels - Why They Are Different

• High compliance allows pulmonary arteries to accommodate the full stroke volume of the right ventricle
• Low pressure protects the thin alveolar-capillary membrane from rupture, keeping it thin enough for efficient gas diffusion
• Low resistance means the right ventricle does much less work than the left ventricle

---

4. Pressures in the Pulmonary Circulation

Normal Pressure Values:

---

5. Blood Volume of the Lungs

• Total blood in lungs at any time = ~450 mL (~9% of total blood volume)
• In capillaries = ~70 mL
• Rest equally divided between arteries and veins

The Lungs Act as a Blood Reservoir!

• Under stress, up to 250 mL can be shifted from lungs to systemic circulation (e.g., when blowing hard against resistance)
• In hemorrhage, pulmonary blood can shift into systemic vessels as compensation
• In left heart failure or mitral stenosis, blood dams up in lungs - pulmonary blood volume can double, causing pulmonary edema

---

6. Blood Flow Through the Lungs

• Pulmonary blood flow = cardiac output (the entire output of the right ventricle)
• Normally ~5 L/min
• Pulmonary vessels are distensible - they widen with rising pressure and narrow with falling pressure
• This passive distension allows the lung to handle large increases in cardiac output (e.g., during exercise) without major pressure rises

---

7. Regulation of Pulmonary Blood Flow - Hypoxic Vasoconstriction

Why Does the Lung Vasoconstrict During Hypoxia?

1. O₂-sensitive K⁺ channels in smooth muscle are inhibited
2. Cell membrane depolarizes
3. Voltage-gated Ca²⁺ channels open → Ca²⁺ influx
4. Vasoconstriction occurs

• Release of vasoconstrictors (endothelin, reactive oxygen species)
• Decreased release of vasodilators (nitric oxide)

---

8. Effect of Gravity on Pulmonary Blood Flow

West's Zones of the Lung:

In normal standing humans, Zone I barely exists - most of the lung is Zone II and III. In positive pressure ventilation, Zone I expands and creates dead space.

---

9. Summary - Key Points at a Glance

---

Memory Tricks

• "Low pressure, HIGH flow" - pulmonary = the exact opposite of what you'd expect
• Pulmonary hypoxia = vasoconstriction (remember: it's the OPPOSITE of systemic)
• Bronchial = nutrition (1-2%) | Pulmonary = gas exchange (99%)
• Apex = less blood, Base = more blood (gravity wins in a low-pressure system)
• Wedge pressure ≈ Left atrial pressure (used clinically to estimate left heart filling)

---