1 - Pulmonary Circulation

Question 1

Objectives: Explain bronchial and pulmonary circulation

Answer 1

• Two circulatory systems associated with the lung
  
  • Bronchial Circulation: Similar to coronary arteries, bronchial circulation is part of systemic circulation
    
    • ​Most of its venous blood drains into pulmonary vein; in absence of perfusion may act as shunt
  • Pulmonary Circulation: Driven by right ventricle through lungs; blood spends < 1sec in capillaries of lungs
    
    • Cardiac Output in R/L heart is equal; pulmonary flow = systemic flow

Question 2

Objectives: Explain the anatomy of pulmonary capillaries

Answer 2

• Pulmonary Capillaries have Low Vascular Resistenace
  
  • Vessel walls thinner
  • Vessel walls contain less smooth muscle
  • Easily distended/compressed
• Results in lower intravascular pressures vs systemic
• Anastomoses exist, but mostly relevant during disease states

Question 3

Objectives: Compare the anatomical and physical features of the pulmonary to the systemic circulation

Answer 3

• Pulmonary Circulation - Main Differences vs Systemic
  
  • Lower resistance
    
    • ​Lack of arteriole networks
  • Vessels Soft, Easily Distended/Compressed
  • Driven by Right Ventricle
  • Low Perfusing Pressures
    
    • ​(ΔP_Pulmonary = 10 mmHg)
• Avg Pulmonary Arterial Pressure = 25/8
• Systemic Circulation: Blood pumped uphill and through solid organs; requires greater force and pressure; high pressure allows redistribution under stress

Question 4

Objectives: How does gravity influence the pulmonary circulation?

Answer 4

• At Top of Lung: Working against Gravity
  
  • ​Lowest P_arterial
  • Lowest blood flow
• At Bottom of Lung: Working with Gravity
  
  • ​Highest P_arterial
  • Greatest blood flow

Question 5

Objectives: Define Zones 1, 2, 3 of the lungs

Answer 5

• Zone 1: Higher pressures (gravity) collapse capillaries
  
  • NOT perfused
  • P_A > P_a > P_V
• Zone 2: Perfused intermittently at systolic P
  
  • ​P_a > P_A > P_V
• Zone 3: Always perfused; highest blood flow and perfusion due to gravity effects
  
  • Capillaries “bulge”; radius increases, decreasing resistance
  • P_a > P_V > P_A

Question 6

Objectives: How does one measure Blood Flow?

Answer 6

• Regional Blood Flow:
  
  • Pulmonary Angiography
  • ¹³³Xe Technique
• Lung Perfusion: Microaggregates of Albumin (black/white lung screen)

Question 7

Objectives: What is PVR and what affects PVR?

Answer 7

• Pulmonary Vascular Resistance (PVR): Resistance to flow
  
  • Defined by Poiseuille’s: R=(8ηL) / (πr⁴)
  • Radius has inverse relationship to resistance
• Two Types of Control:
  
  • Active Control - Increase PVR
    
    • Alveolar Hypoxia - Pulmonary vascular contracts in low PO₂
      
      • ​This is opposite systemic
      • Beneficial in normal lungs, bad in sick lungs
      • Right Side Heart Failure
    • Altitude Sickness - Low O₂ leads to vasoconstriction
  • Passive Control - Decrease PVR
    
    • _​_Recruitment of unperfused capillaries (increase Zone 1)
    • Distension of existing pulmonary capillaries (increase radius, decrease resistance)

Question 8

Objectives: Explain pulmonary edema and explain the use of the Starling equation

Answer 8

• Pulmonary Edema: Accumulation of fluid in the lung; result is impaired gas transfer (O₂)
• Governed by Starling Law:
  
  • Q_f = K_f [(P_c - P_is) - σ(π_c - π_is)]
    
    • (P_c - P_is) = Capillary P driving fluid towards alveoli
    • (π_c - π_is) = Oncotic P driving fluid away from alveoli
    • K_f = Membrane Coefficient (fluid)
    • σ = Membrane Coefficient (protein)
• Two Type:
  
  • Hydrostatic (Cardiogenic)
  • Permeability (Non-Cardiogenic)

Question 9

How does resistance change from top to bottom of the lung?

Answer 9

Greatest at top (small radius, -gravity influence)

Least at bottom (large radius, +gravity influence)

Question 10

How does body position affect blood flow?

Answer 10

“Top” and “Bottom” of the lung (Zones 1, 2, 3) are not fixed anatomical positions

These will change with gravity’s force vector

Question 11

How do the following change lung Zone workloads?

Hemorrhage / General Anesthesia

Exerise (high Cardiac Output)

Positive Pressure Ventilator with PEEP

Answer 11

• Hemorrhage / General Anesthesia: Low systolic pressure
  
  • Increase Zone 1; reduced pressure driven resistance
• Exerise (high Cardiac Output)
  
  • Zone 1 increases greatly (most room to change)
  • Zone 2-3 increase somewhat (less room to change)
• Positive Pressure Ventilator with PEEP
  
  • Increase Zone 1; positive pressure decreases vessel collapse

Question 13

Explain how changes in lung volumes and PVR are different in Inter-Alveolar and Extra-Alveolar Pulmonary vessels

Answer 13

• Inter-Alveolar: Inhalation causes increased PVR
  
  • ​Alveoli expands, compressing internal vessel (decreasing radius, increasing resistance)
• Extra-Alveolar: Inhalation causes decreased PVR
  
  • Transmural pressure gradient and radial traction cause distension
  • Increase radius, decrease resistance

Question 14

What can be a difference in intra/extra alveolar vessels during mechanical ventilation with PEEP?

What is a downstream risk to the cardiac system?

Answer 14

• Alveolar Pressure (P_A) and Intrapleural (P_PL) are positive during inspiration
• Both intra/extra alveolar vessels are compressed (decreased radius, increased resistance)
• Result: Increase in PVR in both vessel locations
  
  • ​Right Ventricle must work harder; right side heart failure is possible

Question 15

What is the perfusion/ventilation case of Pulmonary Embolus?

Answer 15

• Poor Perfusion
• Adequate Ventilation
• Nothing is “wrong” with the air in/out of the lungs, however there is a blockage (emboli)–and no blood perfusion to alveoli; thus oxygen (perfusion limited) can not move into the blood

Question 16

Explain Hydrostatic (Cardiogenic) Edema

Cause

Result

Symptoms

Signs

Answer 16

• Cause: Increase in capillary pressure
  
  • No change to constants (K_f / σ)
  • Left Heart Failure / Mitral Stenosis
• Result: Blood returned back to pulmonary circulation due to Left Side heart defect
  
  • Increase P_C
  • Drives fluid away from capillaries
• Symptoms: Pain climbing stairs, short of breath, supine pain, sleeping in recliner, heart failure history, left side murmur
• Signs: Enlarged Cardiac Silhouette, Kerley B Lines (thickened septal lines)

Question 17

Explain Permeability (Non-Cardiogenic) Edema

Cause

Result

Symptoms

Signs

Answer 17

• Cause: Bacterial toxins, major injury, oxygen toxicity
• Result: Increase in vessel wall permeability
  
  • Increase K_f (fluid)
  • Decrease σ (protein)
  • Vascular fluid exits through more porous membrane
  • Adult Respiratory Distress Syndrome (ARDS)
• Symptoms: Breathlessness, rapid/shallow breathing, dry cough, chest pain, lung inflammation, inability to lie flat
• Signs: X-ray shows diffuse airspaces and bilateral alveolar infiltrates