19.VQ mismatch.pdf

Ventilation and Perfusion 7 September 2023 Prof. A. Kariyawasam Y1S2 Dept. of Physiology 2021/ 2022 batch  Ventilation Perfusion Ratio (V / Q) Adequate exchange of gases requires well matched ventilation and perfusion in each of the alveolar un...

Ventilation and Perfusion 7 September 2023 Prof. A. Kariyawasam Y1S2 Dept. of Physiology 2021/ 2022 batch  Ventilation Perfusion Ratio (V / Q) Adequate exchange of gases requires well matched ventilation and perfusion in each of the alveolar units 1. Explain the term ventilation- perfusion ratio (V/Q ratio) 2. State the V/Q ratio in a normal lung 3. Explain how ventilation, perfusion and V/Q are changed in the healthy upright lung 4. Explain V/Q mismatch giving examples 5. Explain the changes in PO2 and CO2 in alveolar and arterial blood when there is a mismatch  Ventilation / Perfusion Ratio (V/Q ratio) Ventilation – alveolar ventilation (VA) Perfusion- pulmonary blood flow (Q) In adults, typically Alveolar Ventilation (VA) = 4 L/min Pulmonary blood flow ( perfusion; Q) = 5 L /min Thus, ventilation perfusion ratio VA / Q = 0.8 * In certain situations some areas of the lung –well ventilated but have almost no blood flow Some areas –rich blood flow but little or no ventilation Ventilation perfusion inequalities / mismatch Either case gas exchange is impaired Severe respiratory distress * At normal ratio, O2 = PAO2 = 100 mmHg 100 mmHg and PACO2 = 40 mmHg. CO2 = 40 mmHg VENTILATION/PERFUSION RATIOS IN NORMAL LUNGS (upright position) * In the upright lung gravity causes - a pleural pressure gradient alters distribution of ventilation - a hydrostatic pressure gradient alters distribution of blood flow Difference in ventilation Intrapleural Pressure and transpulmonary pressure at the apex and the base Vertical gradient in pleural pressures from the apex to the base Alveoli at the apex are exposed to a greater distending pressure (PA-Ppl = 0 – (-10) = 10 cm H2O) than those at the base At the base (PA-Ppl= 0 – (-2.5)= 2.5 cm H2O).  Difference in ventilation Apex of the lung * Intrapleural pressure less * Intrapleural negative pressure more negative * Smaller transpulmonary * Greater pressure gradient transpulmonary pressure gradient * Alveoli smaller, more * Alveoli larger, compliant less compliant * Ventilation is greater * Ventilation is less Base of the lung Difference in blood flow  Apex Difference in blood flow of the lung Lower intravascular pressures, Less recruitment, distension, Greater intravascular Higher resistance, pressures, Less blood flow More recruitment, distension, Lower resistance, Base Greater blood flow  Apex of the Difference in ventilation blood flow lung * Intrapleural pressure less * Intrapleural pressure negative more negative * Smaller transpulmonary * Greater transpulmonary pressure gradient pressure gradient * Alveoli smaller, more * Alveoli larger, less compliant compliant * Ventilation is greater * Ventilation is less Greater intravascular Lower intravascular pressures, pressures, More recruitment, Less recruitment, distension, distension, Lower resistance, Higher resistance, Greater blood flow Less blood flow Base Apex Less Ventilation Less blood flow Base Greater Ventilation Greater blood flow  Base Apex Ventilation Blood flow VA/Q ratio APEX Lowest Lowest Greatest (V >>Q) BASE Greatest Greatest Lowest (Q>>V) Regional Variations in Alveolar Gas Tensions Apex of the Apex: High V A/Q > 1. lung Therefore, high PAO2 & low PACO2 Middle of lung: average PAO2 & average PACO2 due to ideal VA/Q =1 Base: Low V A/Q < 1. Therefore, Base low PAO2 & high PACO2 Regional Variations in Alveolar Gas Tensions Apex: High V A/Q > 1. Therefore, high PAO2 & low PACO2 Middle of lung: average PAO2 & average PACO2 due to ideal V A/Q = 1 Base: Low V A/Q < 1. Therefore, low PAO2 & high PACO2 Apex: High V A/Q > 1. Therefore, high PAO2 & low PACO2 Base: Low V A/Q < 1. Therefore, low PAO2 & high PACO2 Mycobaterium tuberculosis (organism causing tuberculosis) favours lung regions where O2 levels are high and typically establishes at the apices  V/Q mismatch The V /Q can range from zero (no ventilation) to infinity (no lung perfusion) in lung diseases No perfusion – No ventilation- shunt alveolar dead space  Normal V / Q Inspired Mixed Venous Alveolus Gas Blood & Capillary PO2 150 40 100 PCO2 0 45 40 (in mmHg) O2 = 100 CO2 = 40  Extremes of V/Q Inequality 1. Shunt – Perfusion of lung units without ventilation Unoxygenated blood enters the systemic circulation, V/Q = 0; wasted blood flow A fraction of the venous blood passing through the pulmonary capillaries does not become oxygenated. This fraction is called shunted blood. Causing the PO2 of arterial blood to be less- hypoxaemia  V/Q = 0 (No ventilation = Shunt) Inspired Gas Mixed Venous Alveolus & Blood Capillary PO2 150 40 40 PCO2 0 45 45 Normal Shunt  2. Alveolar Dead Space V A/Q =∞ (no perfusion). – Ventilation of lung units without perfusion Gas enters and leaves lung units without contacting blood Wasted ventilation V/Q is infinity  V’/Q’ = Infinity (No perfusion) Inspired Gas Mixed Venous Alveolus Blood PO2 150 40 150 PCO2 0 45 0 Alveolar Normal dead space VA/Q ratios affect alveolar and end capillary blood gas composition (PO2 –PCO2-VA/Q diagram) VA/Q= 0 PO2= 40 VA/Q= 1 PCO2 = 45 PO2= 100 PCO2 = 40 VA/Q= ∞ PO2= 150 PCO2 = 0  Local control of VA/Q To maintain a normal ventilation perfusion ratio , changes in the PAgas and tissues affect -contractile activity of bronchiolar smooth muscle and - the smooth muscle in the arterioles 1. Bronchioles - alter the local distribution of ventilation 2. Small pulmonary vessels - alter the local distribution of blood flow  Area in which blood flow (perfusion) is greater than airflow (ventilation) Q > V Helps Helps balance balance Decreased O2 in alveoli Increased CO2 in area Increased contraction of local Relaxation of airway pulmonary arteriolar smooth muscle smooth muscle Constriction of local Dilatation of local airways blood vessels Decreased airway resistance Increased vascular resistance Increased air flow Decreased blood flow Areas where ventilation is greater than blood flow (perfusion) V> Q Constriction of local airways Dilatation of local pulmonary blood vessels  Ventilation Perfusion ratio (V/Q) 4/ 5 = 0.8 Apex V and Q , V>>Q , V /Q High V/Q mismatch Local control of VA/Q mismatch - Base 1. Bronchioles – constriction/dilatation V and Q , Q>>V 2. Small pulmonary vessels – constriction/ V/Q low dilatation