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Ventilation perfusion matching and dead space Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Rules before lectures Define dead space and list the types. Differentiate physiological dead space and anatomical dead space. Define pulmonary ventilation an...

Ventilation perfusion matching and dead space Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Rules before lectures Define dead space and list the types. Differentiate physiological dead space and anatomical dead space. Define pulmonary ventilation and alveolar ventilation. Describe ventilation/ perfusion ratio- (V/Q) ratio- and explain regional variation of the V/Q across the lungs (apex vs. base). Describe the V/Q mismatching in case of impaired perfusion and in case of impaired ventilation. Dead Space Definition: The volume of air in the respiratory passages that does not undergo gases exchange with the blood. Dead space= 150ml What is the relationship of dead space and conducting zone??? Dead Space Types of dead space 1-Anatomical dead space It is Volume of air present in conducting zone (from nose to terminal bronchiole) where gaseous exchange does not take part. 2- Alveolar dead space Presents in non-functional alveoli-of poor blood supply 3- Physiological dead space Anatomical dead space + alveolar dead space Normally , there is no alveolar dead space because all alveoli are functioning So, Anatomical dead space = Physiological dead space Dead Space Factor affecting the volume of dead space 1. Sympathetic stimulation → bronchodilation → increase the volume of dead space. 2. Parasympathetic stimulation → bronchoconstriction→ decrease the volume of dead space. Importance of dead space 1. Warm , moist, and filter the inspired air. 2. It causes the difference in composition between the alveolar air and expired air. 3. Regulates the resistance to air flow. 4. It is responsible for the difference between pulmonary ventilation and alveolar ventilation Pulmonary ventilation Definition: It is the volume of air breathed in or out in one minute. Pulmonary ventilation = Tidal volume x Respiratory rate = 500 ml x 12 = 6L /min during rest Pulmonary ventilation= Minute ventilation= Minute respiratory volume Alveolar ventilation Definition: It is the volume of air reaching the alveoli per minute. Alveolar ventilation = Tidal volume - Dead space x Respiratory rate = 500 ml - 150 x 12 = 350 x 12 = 4.2 L /min during rest. Factor affecting alveolar ventilation 1. Rate and depth of breathing: shallow rapid breathing→ decrease alveolar ventilation 2. Volume of the dead space: increased dead space → decrease alveolar ventilation Which is important pulmonary ventilation or alveolar ventilation?? Alveolar ventilation. It is an indicator of gas exchange In case of hypoxia (decrease O2), the alveolar ventilation is decrease while pulmonary ventilation is normal as in 1- Shallow rapid breathing 2- Increased alveolar dead space Normal breathing Shallow rapid breathing Tidal volume 500 200 Respiratory rate 12 30 Pulmonary ventilation 6 L/min 6L/min Alveolar ventilation 4.2 L/min 1.5 L/min Pulmonary Perfusion Pulmonary perfusion is the flow of blood through pulmonary capillaries. Inside the lung there are 3 zone (In standing position) 1. Zone 1: No blood flow because alveolar pressure > pulmonary capillary pressure. 2. Zone 2: Intermittent blood flow, The blood flow occurs only in systolic blood pressure. 3. Zone 3: Continuous blood flow, The blood flow occurs during the cardiac cycle (during systolic blood pressure and diastolic blood pressure) Normally, Lung contains only Zone 2 and 3 At Apex of the lung: little blood flow At base of the lung: five times increase in the blood flow. Zone 1 Zone 2 Zone 3 Zone 1 blood flow occurs only under abnormal conditions. 1- Increased intra-alveolar pressure (Alveolar inflammation and alveolar edema). 2- Marked decrease in blood flow as in sever blood loss or right-side heart failure Ventilation Perfusion Ratio (VA/Q) Alveolar ventilation VA = 4 L/min. Pulmonary perfusion Q = cardiac output = 5 L/min. So, Ventilation Perfusion Ratio(VA/Q) = 4/5= 0.8. However, the ventilation and the perfusion are not equal throughout the lung in standing position. In the Figure: Ventilation is decrease from Base (Bottom) to the apex (Top) of the lung. Perfusion is Markedly decrease from base to the apex of the lung Ventilation Perfusion Ratio (VA/Q) At the apex of the lung, there is much more ventilation than perfusion. At the base of the lung, there is much more perfusion than ventilation. Ventilation Perfusion Ratio (VA/Q) At the apex of the lung: Ventilation > perfusion. VA/Q = 3 3 Average VA/Q = 0.8 At the base of the lung: Perfusion > ventilation. VA/Q = 0.6 0.6 Causes of Pathological Ventilation Perfusion Ratio (V A/Q) High V/Q ratio: Pulmonary thrombi or emboli→ less perfusion → decreased gas exchange. Emphysema → high ventilation→ decreased gas exchange. Low V/Q ratio: Bronchial asthma →less ventilation → decreased gas exchange. For health lung , the body needs adequate ventilation and perfusion in order to maintain adequate gas exchange Ventilation Perfusion scan Lung scintigraphy, or ventilation-perfusion (V/Q) scan, is one of the commonly performed studies in nuclear medicine. A V/Q scan consists of two portions, the V or ventilation portion and the Q or perfusion portion.(Derenoncourt et al., 2021) Uses: 1. pulmonary embolism. 2. Emphysema Review Questions Which of the following scenarios would be most likely to cause a ventilation/perfusion (V̇/Q̇) ratio of 0.5? A. B. C. D. E. Destruction of pulmonary capillaries by emphysema Partial mucus plugging of a bronchiole Pulmonary embolus Severe left heart failure Severe right heart failure https://exchange.scholarrx.com/brick/pulmonary-ventilation-andperfusion Which of the following correctly compares physiologic values at the base of the lung to those at the apex? A. B. C. D. E. Perfusion and V̇/Q̇ ratio are both higher at the base Perfusion and V̇/Q̇ ratio are both lower at the base Perfusion is higher, and V̇/Q̇ ratio is lower at the base Ventilation and V̇/Q̇ ratio are both higher at the base Ventilation and V̇/Q̇ ratio are both lower at the base https://exchange.scholarrx.com/brick/pulmonary-ventilation-andperfusion Summary Define dead space and list the types. Differentiate physiological dead space and anatomical dead space. Define pulmonary ventilation and alveolar ventilation. Describe ventilation/ perfusion ratio- (V/Q) ratio- and explain regional variation of the V/Q across the lungs (apex vs. base). Describe the V/Q mismatching in case of impaired perfusion and in case of impaired ventilation. Learning Resources  Hall JE, Hall ME. Guyton and Hall textbook of medical physiology e-Book. 14ed, Elsevier Health Sciences; 2021. Chapter 40, 511-520 https://www-clinicalkeycom.gmulibrary.com/#!/content/book/3-s2.0-B9780323597128000400  https://exchange.scholarrx.com/brick/ventilationperfusion-basics  https://exchange.scholarrx.com/brick/pulmonary-ventilation-and-perfusion  Derenoncourt PR, Felder GJ, Royal HD, Bhalla S, Lang JA, Matesan MC, Itani M. Ventilation-Perfusion Scan: A Primer for Practicing Radiologists. RadioGraphics. 2021 Oct 22.  Power-point presentation in the Moodle. www.gmu.ac.ae COLLEGE OF MEDICINE Thank you