Ventilation_&_Perfusion.pdf

Lung Ventilation and Perfusion Dr John P Winpenny Senior Lecturer in Physiology School of Medicine University of St Andrews ([email protected]) Footer: Ventilation and Diffusion 1 Learning Outcomes • • • • • • • • • • Define the term "ventilation-perfusion ratio" Describe the differences i...

Lung Ventilation and Perfusion Dr John P Winpenny Senior Lecturer in Physiology School of Medicine University of St Andrews ([email protected]) Footer: Ventilation and Diffusion 1 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 Lung Structure 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) 4200ml/min = 12breath/min x (500ml – 150ml) VA, volume of alveolar ventilation per min Freq, frequency of respiration per min VT, tidal volume VD, dead space volume • Alveolar ventilation is one of major factors determining O2 and CO2 concentrations in alveoli 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: PAO2 = (PB – PH2O) x FIO2 – (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) 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 Effects of Gravity on Ventilation 7 Perfusion of Lungs – 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 Blood Supply to 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 Perfusion of Lungs – Pulmonary Circulation • • • • • 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 Pulmonary Blood Flow • Zone 1 – – – – • Zone 2 – – – • PPA>PA>PPV Apex to mid lung Intermittent blood flow only during the pulmonary arterial pressure peaks • Systolic Ppc > Palv • Diastolic Ppc < Palv Zone 3 – – – • PA>PPA>PPV Apex of lung under specific conditions No blood flow during all portions of the cardiac cycle Doesn’t occur normally PPA>PPV>PA Mid to lower lung Continuous blood flow during entire cardiac output • Ppc » Palv – Get distension of pulmonary capillaries Zone 4 – – – – PPA>PPV>PA Extreme base of lung Constriction of extra-alveolar vessels Peak flow decreases 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 Agents that Affect Pulmonary Vascular Resistance Dilators Constrictors  PAO2  PAO2  PACO2  PACO2  pH  pH H2 agonists H1 agonists PGI2 (prostacyclin) PGE1 Thromboxane A2 PGF2 PGE2 -adrenergic agonists -adrenergic agonists Bradykinin Serotonin Theophylline Angiotensin II Acetylcholine (Ach) Nitric oxide (NO) 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) 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) Regional Differences in Lung Location Fraction of total lung volume VA/Q PO2 (mmHg) PCO2 (mmHg) pH Q (L/min) Apex 7% 3.3 132 28 7.55 0.07 Base 13% 0.6 89 42 7.38 1.3 Overall 100% 0.84 100 40 7.40 5.0 Effect of different VA/Q regions 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) Ventilation-Perfusion Relationships • 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 Ventilation-Perfusion Relationships • 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 Ventilation-Perfusion Relationships • 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 Different Types of VA/Q Mismatching Ventilation-Perfusion Relationships • Hypoventilation – 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 References • Boron, WF & Boulpaep, EL (2017) Medical Physiology (3rd Edition) – Chapter 31 Ventilation and Perfusion of the Lungs p675-699 • Guyton & Hall (2016) Textbook of Medical Physiology (13th Edition) – • Chapter 39 Pulmonary Circulation, Pulmonary Edema, pleural fluid pp509-516 Preston RR & Wilson TE (2013) Lippincott’s Illustrated Reviews: Physiology (1st Edition) – Chapter 23 Gas Exchange p280-297 • Naish, J & Syndercombe Court, D. (2019). 3rd Edition. Medical Sciences – Chapter 13 The Respiratory System p603-642 • Ward, J, Clarke, R & Linden, R (2005). Physiology at a Glance – Chapter 26 Ventilation-perfusion matching and right to left shunts p60-61