Ventilation and Perfusion

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  • Created by: LBCW0502
  • Created on: 25-03-19 17:41
What is ventilation?
Breathing air into the lungs
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What is perfusion?
Flow of blood through the pulmonary circulation
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What are the features of a healthy lung?
Ventilation is equal to perfusion (both 5 L/min at rest). Optimal efficiency oxygen absorption and CO2 removal. Blood supply (pulmonary circulation). By convention (alveolar pO2 - PAO2, arterial pO2 - PaO2)
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Outline the movement of O2 and CO2 in and out of the blood (1)
Movement in and out blood at pulmonary and systemic capillaries. O2 exchange at alveolar capillary interface. O2 transport. O2 exchange at cells. Cellular respiration (use O2, produce CO2). CO2 exchange at cells. CO2 transport
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Outline the movement of O2 and CO2 in and out of the blood (2)
CO2 exchange at alveolar capillary interface. CO2 in alveoli in lungs (released out of airways, O2 taken into airways in alveoli of lungs)
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Outline features of the lung capillary interface (1)
Consists of interstitial space (0.2-0.6 micrometres). Capillary endothelium, surfactant layer. Alveolar epithelium. Epithelial basement membrane. Capillary basement membrane. Capillary lumen. Adult lung. Terminal bronchioles, respiratory bronchioles
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Outline features of the lung capillary interface (2)
Alveolar ducts and alveolar sacs, blood vessels (branch of pulmonary artery, branch of pulmonary vein, pulmonary capillaries, alveolus)
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What are the constant factors for respiratory gases?
SA, membrane thickness and diffusion distance
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What are the variable factors for respiratory gases?
Concentration gradient (most important factor). Pressure gradient. Solubility. Temperature
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State the barriers which oxygen has to pass through in order for gas exchange to take place
O2 diffuses across alveolar epithelial cells and capillary endothelial cells -> plasma. Layers - surfactant (alveolar space), alveolar epithelium, fused basement membranes (0.1-1.5 m), (nucleus of endothelial cell), plasma and RBC (capillary lumen)
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Outline the diffusion of oxygen in the circulatory system (1)
In alveoli, pO2 is 100 mmHg. Oxygen diffuses into the arterial bloodstream (pO2 is 100 mmHg). In peripheral tissue pO2 < 40 mmHg. Movement of O2 from high conc (arterial blood) to low conc (peripheral tissue)
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Outline the diffusion of oxygen in the circulatory system (2)
pO2 in venous blood is 40 mmHg (attempt to equilibrate). pO2 in venous blood towards alveoli is 40 mmHg (movement of O2 from high conc/alveoli with pO2 of 100 mmHg to low conc/pO2 of 40 mmHg in blood). Cycle repeats
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Outline the diffusion of carbon dioxide in the circulatory system (1)
pCO2 in arterial blood towards peripheral tissue is 40 mmHg. pCO2 in peripheral tissue is >46 mmHg. Movement of CO2 from high conc/peripheral tissue to low conc/venous blood. pCO2 in venous blood is 46 mmHg (attempt to equilibrate)
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Outline the diffusion of carbon dioxide in the circulatory system (2)
pCO2 in venous blood towards alveoli is 46 mmHg. Movement of CO2 from high conc/venous blood with pCO2 of 46 mmHg to low conc/alveoli with pCO2 of 40 mmHg (CO2 breathed off)
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State the normal ranges for blood gases
PaO2 = 10-14 kPa (~90 mmHg ~ 1.75 PSI). PvO2 = 5-5.6 kPa. PaCO2 = 4.5 - 6 kPa (~ 33.6 mmHg ~ 0.65 PSI). PvCO2 = 5.5 - 6.5 kPa
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Effective gas exchange depends on which relationship? (1)
Ventilation/perfusion = V/Q ratio. Normal ventilation/perfusion are matched. Unoxygenated blood from venous system via RA and RV through pulmonary arteries to lungs. Arteries branch - alveolar capillaries
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Effective gas exchange depends on which relationship? (2)
Gas exchange - alveolar capillaries, CO2 diffuses into alveoli. O2 diffuses into capillaries
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Describe features of the ventilation and perfusion relationship
Ventilation and perfusion are normally matched. Gas exchange (ventilation) ~ pulmonary arterial blood flow (perfusion). Mismatched V/Q leads to impairment of O2 and CO2 transfer. pO2 measured (ration ventilation to blood flow). V/Q ratio
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What does the ventilation/perfusion relationship also apply to?
Carbon dioxide, nitrogen and any other gas
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How is V/Q measured?
Alveolar (A) - arterial (a) pO2 difference. Normal PAO2 - PaO2 gradient = 10 (increasing 5-6 per decade over 50 years)
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What does a low V/Q ratio indicate?
Inadequate ventilation (shunt). Pulmonary circulation is normal but there is a reduction in the amount of O2 available to alveoli (not enough for normal diffusion). Some blood not oxygenated (e.g. pneumonia)
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What does a high V/Q ratio indicate?
Inadequate perfusion (dead-space ventilation). Ventilation is normal but there is a reduction in alveolar perfusion (or absent) due to narrowed capillary. E.g. perfusion defect = pulmonary embolism or disorder that reduces CO
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How does the pulmonary circulation differ to systemic circulation? (1)
Pulmonary circulation is in series to systemic circulation. Pulmonary blood flow is more pulsatile in capillaries. Low pressure system (RV only 15 mmHg pressure to drive CO to lungs). In systemic circulation the pressure is 100 mmHg
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How does the pulmonary circulation differ to systemic circulation? (2)
Resistance to blood flow pulmonary vessels is low and more evenly distributed. Control resistance in vessels only weakly influenced by para/sympathetic innervation
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What is the major influence for pulmonary circulation?
Hypoxia (low O2 levels). Blood from areas of hypoxia is diverted to areas with good O2 supply
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Describe features of shunts (1)
Ideally all blood flowing through the lungs will be oxygenated but not so. Part of blood serving bronchial airways (form aorta) feeds into pulmonary vein and contributed deoxygenated blood (shunt)
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Describe features of shunts (2)
In heart part of coronary venous, blood drains directly into the LV via the venae cordia minimae (Thebesian veins). Addition of right-sided (deoxygenated blood) to left sided (oxygenated blood). Right-left shunts (equivalent to 2% of CO, OK-normal)
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Abnormal right to left shunt can occur in which disease conditions? (1)
Collapsed lung (atelectasis) - obstruction in bronchi - collapse of tissues, distal to obstruction (no gas exchange in this area, perfusion wasted). Pneumonia - infection -> ++ inflammatory cells (PMN, lymphocytes/macrophages)
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Abnormal right to left shunt can occur in which disease conditions? (2)
Oedema formation/fibrin deposits. No gas exchange in infected area/perfusion wasted. Both - mixing de-oxygenated blood with oxygenated blood from healthy areas -> lowering pO2
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What other conditions cause shunts? (1)
Fallot's tetralogy (hole in the heart condition, congenital heart disease). Stenosis (narrowing of vascular lumen) at the start of pulmonary artery occurs. RV wall thickens (hypertrophy)
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What other conditions cause shunts? (2)
Some deoxygenated blood flows into aorta from RV (right to left shunt) -> cyanosis (blue babies)
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What other conditions cause shunts? (3)
Hole (septal defect) present between RV and LV immediately adjacent to an overriding aorta (connected to RV and LV)
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What other conditions cause shunts? (4)
Not all hole in heart conditions lead to right to left shunt. Many which connect atria or ventricle do not give cyanosis because higher pressure in left results in left to right shunt (not resulting in reduced arterial pO2/not cyanotic, less painful)
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What other conditions cause shunts? (5)
Atrial septal defect (detected in adult life). Ventricular septa defect (1/500 live births, can give pulmonary hypertension). Many will self correct or may require surgery to close septal defect
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What are the normal values for ventilation and perfusion?
Normal healthy lungs will have alveolar ventilation of ~5 L/min (VA = 5 L/min). Perfusion (Q) also ~ 5 L/min (Q = 5 L/min). V/Q = 1, normal gas exchange (if ratio is significantly different from 1 -> indication of reduction in oxygenated blood)
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What is the effect of a 20% right to left shunt in lung on arterial O2 and CO2 content? (1)
In 80% O2 content = 20 mL/dL, CO2 content = 48 mL/dL. In 20% O2 content = 15 mL/dL and CO2 content = 52 mL/dL
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What is the effect of a 20% right to left shunt in lung on arterial O2 and CO2 content? (2)
Content = weighted average of 2 streams. Arterial O2 content = 80/100 x 20 + 20/100 x 15 = 19 mL/dL. Arterial CO2 content = 80/100 x 48 + 20/100 x 52 = 48.8 mL/dL. Effect of pO2 and pCO2 can be calculated from contents using dissociation curves
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What is the effect of a 20% right to left shunt in lung on arterial O2 and CO2 content? (3)
[content vs P, but you cannot average pressures)
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What is the effect of a 20% right to left shunt in lung on arterial O2 and CO2 content? (4)
CO2 dissociation curve - moderate rise [0.8 mL] in CO2 content causes a small rise in pCO2 [5.3-5.5 kPa], shunt gives a differential effect. O2 dissociation curve - moderate fall (1 mL) in O2 content causes a large fall in pO2 [12.9-9.0]
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What is the effect of a 20% right to left shunt in lung on arterial O2 and CO2 content? (5)
CO2 dissociation curve is almost a straight line but the O2 dissociation curve is a sigmoid shape
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What effect doo shunts have on the respiratory system if it is otherwise OK?
Low pO2 and raised pCO2. Stimulate chemoreceptors. Increase ventilation. Ventilated areas lose more CO2 but pick up little extra O2 due to areas already being saturated. Shunted blood still unaffected. Final blood gases - low PaO2, normal/low PaCO2
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Describe features of V/Q mismatching (inequalities) (1)
Many diseases don't have a region with no gas exchange (no matching). But many regions with a variety of V/Q ratios. Some regions may be under-perfused relation ventilation (areas behave like alveolar dead space)
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Describe features of V/Q mismatching (inequalities) (2)
Some regions may be under-ventilated (disease state) in relation to perfusion - behave like right to left shunts
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What is the effect of mixing mismatched areas? (1)
In artery (small flow with high pO2, normal O2 content, low pCO2, low CO2 content, V/Q = 4/1). Vein (large flow with low pO2, low O2 content, high pCO2, high CO2 content, V/Q = 5/15)
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What is the effect of mixing mismatched areas? (2)
Combined to give low O2 content, high CO2 content, low pO2, slightly high pCO2. Peripheral/central chemoreceptors, increase ventilation/HR/stretch receptors. Outcome - low O2 content, normal/low CO2 content, low pO2, normal/low pCO2
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Describe features of high V/Q areas
High V/Q areas don't compensate for low V/Q areas in their effects on arterial blood gases (especially O2 content). High V/Q areas don't have high O2 content (once blood is saturated), raising local pO2 further does little to increase O2 content
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Describe features of low V/Q areas
In chronic lung diseases where areas of low V/Q may persist, hyperventilation may lead to exhaustion and CO2 content and pCO2 may rise [pCO2 of 6.3 kPa in COPD]
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Describe the effect of ventilation and perfusion on the normal upright lung (1)
Gravity affects pleural pressure which is less negative at the base than the apex (leads to alveoli at apex > expanded at FRC, less able to expand further than alveoli at base (more ventilation at base). Perfusion gravity effects even greater
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Describe the effect of ventilation and perfusion on the normal upright lung (2)
Hydrostatic pressure varies with height. At base > apex, distend vessels at base, increase flow at base. Pulmonary - low pressure system/pulsatile, in diastole pressure, vessels at apex may < alveolar pressure (collapse for part of cardiac cycle)
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What is the effect of gravity on perfusion in the lungs?
Gravity affects perfusion much more than ventilation with a mismatch at the apex of the lung. Usually inconsequential in young but can be important in old people (reduction in arterial pO2) - graph (expressed as rate/unit volume vs lung height)
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Describe features of hypoxic vasoconstriction of pulmonary vessels (1)
Correction is mismatch. Hypoxic vasoconstriction pulmonary vessels helps reduce blood flow to poorly ventilated areas, diverting it to well ventilated areas. Improves ventilation - perfusion matching and arterial oxygenation
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Describe features of hypoxic vasoconstriction of pulmonary vessels (2)
Not helpful for global hypoxia (altitude, hypoxic lung disease) where constriction leads to lower arterial pO2. In systemic circulation hypoxia causes dilation
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What are the 3 methods of measurement for assessment for V-Q mismatching? (1)
Alveolar - arterial pO2 gradient (provides an estimate of V /Q match, alv PO2 ~ 13.3, art. PO2 ~ 12.5, a gradient of 0.6-1.0 kPa normal, >2 kPa – V /Q mismatch)
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What are the 3 methods of measurement for assessment for V-Q mismatching? (2)
Measurement of alveolar dead space (using Bohr equation, normal value ~ 150 mL, >150 indicates mismatch). Isotope ventilation and perfusion scans of the lungs, radio-isotopes (gamma emitters) either inhaled/perfused images - gamma camera
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What are the 3 methods of measurement for assessment for V-Q mismatching? (2)
Provides a visualisation of airways or pulmonary circulation (particularly good for detecting emboli)
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Describe features of ventilation perfusion scans (1)
Perfusion detected by IV infusion of Technetium [TC 99m] labelled macroaggregated albumin. Remain lodged in narrow capillaries for a few hours. Where they don't go suggests emboli in vessels [6 hr gamma emitter]
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Describe features of ventilation perfusion scans (2)
Ventilation detected by inhalation of a gas labelled with Xenon [Xe 133] and analysis with gamma camera (beta and gamma emitter)
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What are the causes of low alveolar pO2? (1)
Inspired air - abnormally low oxygen (altitude). Alveolar ventilation is inadequate (decreased lung compliance e.g. emphysema, increased airway resistance e.g. asthma, overdose of drugs). (respiration maintains arterial CO2/O2 levels at constants)
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What are the causes of low alveolar pO2? (2)
Low inspired pO2 due to altitude (low PaO2 and normal/low PaCO2). Hypoventilation (e.g. head injury, anaesthetics, drugs, chronic lung disease, low PaO2/high PaCO2, if neural control is compromised or adaptation has occurred)
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What are the causes of low alveolar pO2? (3)
Right to left shunts and ventilation perfusion mismatching (e.g. due to certain congenital heart diseases, lung collapse, lobar pneumonia, asthma, chronic obstructive airway disease). Primary effect - low PaO2/slightly raised PaCO2
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What are the causes of low alveolar pO2? (4)
With ventilatory response - low PaO2/normal or low PaCO2
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Outline pathological conditions which reduce alveolar ventilation and gas exchange (1)
Emphysema (pO2 normal or low in alveoli, pO2 low in capillary). Fibrotic lung disease (pO2 normal or low in alveoli, pO2 low in capillary). Pulmonary oedema (pO2 normal in alveoli, increased diffusion distance, pO2 low in capillary)
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Outline pathological conditions which reduce alveolar ventilation and gas exchange (2)
Asthma (pO2 in alveoli low due to bronchioles constricted, pO2 low in capillary)
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What is perfusion?

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Flow of blood through the pulmonary circulation

Card 3

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Preview of the front of card 3

Card 4

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Outline the movement of O2 and CO2 in and out of the blood (1)

Back

Preview of the front of card 4

Card 5

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Outline the movement of O2 and CO2 in and out of the blood (2)

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