Pulmonary circulation, ventilation/ perfusion matching Flashcards
Learning outcomes
- Define the term “ventilation-perfusion ratio”
- Describe the differences in ventilation across the lung
- Explain the reasons for the differences in ventilation across the lung
- Describe the differences in perfusion across the lung
- Explain the reasons for the variation in lung perfusion
- Define the term “alveolar dead space”
- Define the term “transmural pressure”
- Describe the mechanisms for actively altering lung perfusion
- Recognise how the V/Q ratio varies across the different lung regions
- Recognise what the average V/Q ratio is for the lung
Discuss alveolar ventilation
- Importance of Pulmonary ventilation is to renew air in gas exchange areas
- Rate at which new air reaches these areas is called alveolar ventilation
- Some air that is breathed never reaches gas exchange areas but fills respiratory passages (e.g. nose, pharynx, trachea) – Dead space air (about 150ml)
• Alveolar ventilation rate
(VA) = Freq x (VT - VD)
• Alveolar ventilation is one of major factors determining O2 and CO2 concentrations in alveoli
What is the alveolar air (Gas) equation
• As consequence of gas exchange, fraction of O2 decreases and CO2 increases in alveolus
• Relationship between 2 gases is given by alveolar air equation:
P(AO2) = (P(B) – P(H20) ) x F(IO2) – (PACO2 /R)
PB = barometric pressure PH2O = water vapour pressure FIO2 = fraction of O2 in inspired air (0.21) R = Respiratory quotient (0.8)
- Alveolar air equation describes ideal case of what PAO2 should be
- If perfect transport and no venous admixture, PAO2 = PaO2
- However PaO2 affected by disease
• Difference between ideal PAO2 and PaO2 is known as Alveolar – arterial (A-a) gradient (normally less than 15 mmHg)
Discuss pulmonary ventilation
- Ventilation is not uniformly distributed in lung due to effects of gravity
- Alveoli in top of lung are more expanded than those at the bottom
- Pleural pressure is less (more negative) at apex than base of lung, with inspiration pleural pressure decreases further
- As inspiration begins alveoli in lungs are at different lung volumes
- Underinflated (smaller) alveoli at base of lung are more compliant so receive more of tidal volume
- Overinflated (expanded) alveoli at top have a lower compliance and receive less of tidal volume
Discuss the perfusion of lungs in pulmonary circulation
• Pulmonary circulation begins with RA
• Deoxygenated blood pumped via RV into
pulmonary artery
• Pulmonary artery divides into right and left main artery then enters lung tissue
• Ends in mesh like network of capillaries where rbc flow single file through alveolus
• Capillaries drain into pulmonary venules
• Finally 2 large pulmonary veins emerge from each lung to empty into LA
• Pulmonary capillary pressure is low (7mm Hg)
• Interstitial fluid pressure is approx -8 mmHg
• Pulmonary capillaries relatively leaky, so colloid osmotic pressure of pulmonary interstitial fluid is approx 14 mmHg
• Alveolar walls extremely thin and alveolar epithelium is weak and can be ruptured by a positive pressure
• Why do alveoli not fill with fluid?
– Normally pulmonary capillaries and lymphatics maintain a slight negative pressure in interstitial spaces
– Excess fluid will be sucked back into interstitial space from alveoli
Discuss the blood supply to the airways
• Lung has 2 blood supplies
– Pulmonary arteries
– Bronchial arteries
- Pulmonary arteries carry deoxygenated mixed venous blood from right ventricle to alveoli of lungs
- Pulmonary veins return oxygenated blood to left atrium
- Bronchial arteries branch from aorta and supply oxygenated blood to conducting airways
- Bronchial veins exist, but majority of blood drains into pulmonary veins - Venous admixture
Discuss pulmonary blood flow and its regulation
- Cardiac output of RV same as LV (~ 5L/min)
- Pulmonary arteries not subject to autonomic regulation to any large degree
• Regulated by PO2 and PCO2
– areas of low PO2 (hypoxia) or high PCO2 (hypercapnia)
– Arteries constrict so that blood is diverted to better oxygenated areas
– Mechanism thought to involve inhibition of K channels on smooth muscle cells
• This is a local response as remains even after section of autonomic nerves
What are the dilating agents that affect pulmonary vascular resistance
Write some notes
Discuss bronchial circulation
• Bronchial arteries arise from aortic arch, thoracic aorta or their branches
• Arteries supply oxygenated blood to smooth muscle of airways, intrapulmonary nerves and interstitial lung tissue
• Venous blood returns to heart from bronchial circulation via
– true bronchial veins
– or drains into bronchopulmonary veins where it mixes with oxygenated blood from alveoli (venous admixture)
Discuss ventilation-perfusion matching
• Ventilation and perfusion are matched when pulmonary blood flow is proportionally matched to the pulmonary ventilation - greatest efficiency for gas exchange
• Ventilation-perfusion ratio (V/Q)
– Single alveolus defined as alveolar
ventilation/capillary blood flow
– Lung defined as total alveolar ventilation/cardiac output
• If ventilation exceeds perfusion (V/Q ratio > 1)
• If perfusion exceeds ventilation (V/Q ratio < 1)
• Normal V/Q ratio 0.85 (4.2L/min / 5L/min)
Discuss ventilation-perfusion relationships
- Arterial hypoxemia – abnormal PaO2 (adult at sea level is PaO2 less than 80 mmHg)
- Hypoxia – insufficient O2 to carry out normal metabolic functions (PaO2 less than 60 mmHg)
- 2-compartment lung model is useful way to examine V/Q relationships
• 4 major causes of hypoxemia
– Anatomical shunt (perfusion that bypasses
lung)
– Physiological shunt (absent ventilation to areas being perfused)
– V/Q mismatching (low ventilation to areas being perfused)
– Hypoventilation (underventilation of lung units)
• Anatomical shunts
– Alveolar ventilation, distribution of alveolar gas and composition of alveolar gas are normal
– Distribution of CO changed as some blood now bypasses gas exchange unit
– Right-to-left shunt (as blood is deoxygenated)
– Hypoxemia cannot be abolished by giving 100% O2
– Cyanotic congenital heart diseases most common
• Shunt occurs when deoxygenated blood from RA or RV crosses septum to LA or LV
• Physiological Shunts
– If airway completely blocked alveoli supplied by that airway will receive no ventilation
– All ventilation goes to other lung units
– Perfusion will be equally distributed to both ventilated and non-ventilated lung units
– Lung unit without ventilation but with perfusion has a V/Q = 0
– Atelectasis most common cause of physiological shunt
• May be due to obstruction by mucous plug, airway oedema, foreign body or tumour
• V/Q mismatching (low V/Q)
– Most respiratory diseases produce global changes of varying extent in lungs (e.g. chronic bronchitis, asthma)
– So individual airways will have varying degrees of abnormal ventilation, but perfusion will be normally distributed
– Results in V/Q mismatching or low V/Q (V/Q < 1)
– Alveolar and end capillary gas compositions will vary according to degree of obstruction
– Supplemental O2 will correct hypoxemia as poorly ventilated units will get enriched O2
Discuss hypoventilation ventilation-perfusion relation
– Underventilation will bring less
fresh gas to alveoli
– O2 levels in alveoli will decrease, CO2 levels will increase
– If ventilation halved, arterial CO2 will double
– Patients with respiratory muscle weakness (e.g. muscular dystrophy or diaphragmatic paralysis) are at risk of hypoventilation
• Results in both hypercapnia and hypoxemia
