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Ventilation, Transport of gases and oxygen delivery

Ventilation, Transport of gases and oxygen delivery. Dr. Megha Jain. University College of Medical Sciences & GTB Hospital, Delhi. Contents. Lung volumes Mechanics of ventilation Work of breathing Diffusion of gases Transport of gases and oxygen delivery. Lung volumes.

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Ventilation, Transport of gases and oxygen delivery

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  1. Ventilation, Transport of gases and oxygen delivery Dr. Megha Jain University College of Medical Sciences & GTB Hospital, Delhi

  2. Contents • Lung volumes • Mechanics of ventilation • Work of breathing • Diffusion of gases • Transport of gases and oxygen delivery

  3. Lung volumes • Tidal volume-Volume of air breathed in or out of the lungs, during quiet respiration. Average: 500ml in adult. • Inspiratory reserve volume- Maximal volume of air which can be inspired after normal tidal inspiration. Average: 3000 ml. • Expiratory reserve volume- Maximal volume that can be expired below normal tidal expiration. Average: 1100ml. • Residual volume- Volume of air remaining in lungs after maximal expiration. Average: 1200ml. • Total lung capacity- Volume of air contained in lungs after maximal inspiration. Average: 5800ml.

  4. Lung volumes • Vital capacity- Maximal volume of air that can be exhaled following maximal inspiration. Average: 60-70 ml/kg. • Functional residual capacity- Lung volume at the end of normal exhalation. Average: 2300ml. • Closing capacity- Volume at which the small airways begins to close in the dependent parts of the lung. Normally – well below FRC, but ↑ with age. It equals FRC in supine position( at around 44 yrs) in upright position( at around 66 yrs) Unlike FRC unaffected by posture.

  5. Lung volumes

  6. Spirometry

  7. Ventilation • Defined as mechanical movement of air into and out of the lungs. • Primary mechanism for excretion of Carbon dioxide • Cyclic activity- 2 components • Inward flow of air- Inhalation- active process • Outward flow of air- Exhalation- passive process • Minute ventilation- sum of all exhaled gas volumes in one minute. MV= RR X TV. Normal range= 5 to 10 lts/min in resting state. MV= RR X TV

  8. Ventilation • Dead space ventilation- some of the minute volume occupies space in conducting zones, does not participate in gas exchange and forms anatomic dead space Average in upright position – 150 ml or 2 ml/kg Alveolar dead space- adequately ventilated alveoli not participating in gas exchange as perfusion is absent. Physiologic dead space- sum of anatomic and alveolar dead space. • Alveolar ventilation- volume of inspired gas actually taking part in gas exchange in one minute. AV= RR X (TV – DV) = 12 X (500-150) = 4200 ml/min

  9. Ventilation • Dead space to tidal volume ratio: a numeric index of the total amount of wasted ventilation Vd/Vt = PAco2 – PEco2/PAco2 (N value=0.2 - 0.4) • It represents the primary clinical measure of efficiency of ventilation • Clinical significance: Alveolar ventilation depends on relationship b/w RR and TV. * High RR and low TV result in higher prop. of wasted ventilation per min. * Most efficient breathing pattern is slow and deep breathing.

  10. Ventilation

  11. Effectiveness of ventilation • Ventilation is effective when the body’s need for removal of CO2 is adeqately met. • Under resting metabolic conditions the body produces about 200 ml CO2 per min. • The relative balance b/w CO2 production and alveolar ventilation determines the level of CO2 in lungs and in the blood. PAco2 = V CO2/V A or total CO2 production/CO2 elimination • Normal- alveolar and arterial partial pressures of CO2 are in close equilibrium at approx 40 mmhg.

  12. Effectiveness of ventilation • In cases where alveolar ventilation is ↓ed: rate of CO2 production > rate of excretion thus PA CO2 will rise above its normal value. • Thus, ventilation that is insufficient to meet metabolic needs – hypoventilation • Very high arterial PaCO2 – depress ventilatory response (CO2 narcosis) • Alveolar hypoventilation: by definition it exists when arterial PaCO2 ↑ses above normal range of 37 to 43 mmhg(hypercarbia)

  13. Mechanics of ventilation Forces opposing lung inflation ELASTIC Lung, thorax, surface tension FRICTIONAL Airflow Tissue movement

  14. In intact thorax, Lungs & thorax recoil in opposite directions • Point at which these forces balance = resting vol of lung • AT THIS POINT Ppulm =Patm No air flows Vol. retained in lungs = FRC= 40% of TLC

  15. Elastance Physical tendency to return to original state after deformation Lung vol at any given P is slightly more during deflation than it is during inflation. ↓ HYSTERESIS ↓ (Due to surface tension)

  16. GRAVITY DEPENDENT ventilation exploited to direct ventilation towards healthy lung by changing position of patient

  17. Frictional forces opposing inflation Tissue viscous resistance(20%) Due to tissue displacement during ventilation (lungs, thorax, diaphragm) ↑ by obesity, fibrosis, ascites Airway resistance(80%) Raw = ∆P(driving P)/ ∆V(flow rate) = transrespiratory P/flow rate = 0.5-2.5 cmH2O/L/sec Flow measured by PNEUMOTACHOMETER P measured by PLETHYSMOGRAPH Affected by pattern of flow

  18. Distribution of airway resistance 80% Nose, mouth, large airways TURBULENT FLOW 20% Airways < 2 mm diameter LAMINAR FLOW Branching of airway ↑ total cross sectional area with each generation ↑ area → ↓ velocity→ + laminar flow Deflation - ↑ airway diameter → ↑ resistance Wheezing heard during EXPIRATION

  19. Types of airflow LAMINAR TRANSITIONAL TURBULENT Governed by Poiseulle’s Hagon equation

  20. LAMINAR ∆P = 8ηL X flow π r4 TURBULENT ∆P = flow2 X ρ r5 ↑ Reynold’s number Re = ρ D V η η – viscosity L – length of tube r – radius ∆P – driving P ρ -density 1. Helium is less densebut more viscousthan air Advantageous in turbulent flow but not laminar flow

  21. Inferences from poiseulle’s hagen equation • ∆P = 8ηL X flow π r4 • ∆P αflow r4 • Flow α ∆P X r4 Reducing tube diameter by half requires 16 fold ↑in P to maintain same flow Small changes inbronchial calibercan markedly change flow rates. Basis for - 1. bronchodilator therapy 2. using largest practical size of artificial airway

  22. Flow – volume loops To diagnose lung pathologies as Extra / intrathoracic Variable / fixed Obstructive / restrictive

  23. FIXED AIRWAY OBSTRUCTION VARIABLE Circumferential narrowing Not affected by thoracic P INTRATHORACIC Below 6th tracheal ring Expiratory curve plateaus EXTRATHORACIC Above suprasternal notch Inspiratory curve plateaus

  24. Fixed Variable extrathoracic Variable intrathoracic

  25. Work of breathing • Done by respiratory msls to overcome elastic & frictional forces opposing inflation. W = F X S ( force X distance) = ∆P X ∆V = area under P-V curve Normal breathing – active inhalation - passive exhalation( work of exhalation recovered from potential energy stored in expanded lungs & thorax during inspiration)

  26. Area 1 = work done against elastic forces ( compliance) = 2/3 Area 2 = work done against frictional forces ( resistance work) = 1/3 Area 1+2 = total work done = 2/3 + 1/3 = 1

  27. ↑TV → ↑ elastic component of work ↑ RR ( flow) → ↑ frictional work People with diseased lungs assume a ventilatory pattern optimum for minimum work of breathing. COAD Obstructive disease Slow breathing with pursed lips (↓ frictional work) FIBROSIS Restrictive disease Rapid shallow breathing (↓elastic work)

  28. Transport of gases • Diffusion: gas movement b/w the lungs and tissue occurs via simple diffusion. • For O2 there is a stepwise downward cascade of “partial” pressure. PP of oxygen Atmospheric = 147 Alveolar = 100 Arterial = 97 Venous = 40 Tissue = 5

  29. Mechanism of diffusion • Physical process whereby gas molecules move from area of high partial pressure to low one. • Five barriers * RBC * Capillary membrane * Interstitial fluid * Alveolar membrane * Surfactant

  30. Fick’s law of diffusion • Describes bulk movement of gases through biological membranes Vgas = A X D X (P1 – P2)/T A = Cross sectional area D = Diffusion coefficient of gases T = Thickness of memb. P1 – P2 = Diff. in partial pressure • Pulmonary end capillary O2 tension (Pc’O2) depends on: # rate of O2 diffusion # pulmonary capillary blood volume # transit time

  31. Capillary transit time = pulm cap bld vol/CO = 70 ml/5000 ml per min = 0.8 seconds. • High fever, septic shock often cause ↑ed CO, limit diffusion time due to ↑ed blood flow • Maximum Pc’O2 attained after only 0.3 sec ,providing a large safety margin (like exercise where transit time ↓ due to ↑ blood flow) • For practical purposes, Pc’O2 is considered identical to PAO2.

  32. Diffusion of gases

  33. Diffusion capacity • Defined as no. of ml of a specific gas that diffuses across the ACM into the bloodstream each min for each mmhg diff in pressure gradient DLO2 = O2 uptake/ PAO2 - Pc’O2 • Carbon monoxide is preferred over O2 as test gas since its higher affinity for Hb keeps its cap pp very low, so Pc’O2 can be considered as zero DL CO = CO uptake/PA CO • Reduction in DL CO implies impaired gas transfer seen in * abnormal V/Q ratio * destruction of memb * very short capillary transit time

  34. Determinants of alveolar gas tensions • Alveolar O2 tension: * pp of O2 in air (Pi O2 = PB x Fi O2) = 760x0.21 = 159.6 mmhg * accounting for humidification for inspired gases Pi O2 = PB – PH2O x Fi O2 = 760 - 47X0.21 = 149 mmhg * accounting for residual CO2 from previous breaths final alveolar O2 tension is defined by: alveolar air equan: PAO2 = Fi O2 x (PB – 47) – (PA CO2/0.8) = 0.21 x (760 – 47) – (40/0.8) = 99 mmhg. • Arterial O2 tension: approximated by PaO2 = 102 – age/3, n range = 60 – 100 mmhg

  35. Determinants of alveolar gas tensions • Alveolar CO2 tension: PA CO2 = V CO2 x 0.863/V A = 40 mmhg • Arterial CO2 tension: readily measured, n = 38+/-4 mmhg • End tidal CO2 tension: used clinically as an estimate of PaCO2. PA CO2 – PETCO2 gradient is normally < 5 mmhg.

  36. Compliance • Compliance = Distensibility of lung • Elastance = resisting deformation • Compliance = 1/ elastance = ∆V/ ∆P = 0.2L/cm H2O (lung) = 0.2L/cm H2O (Thorax) = 0.1L/cm H2O (lung+ thorax) Affected by Obesity Kyphoscoliosis Ankylosing spondylitis Fibrosis Emphysema

  37. Steep curve + Lt shift = ↑compliance (loss of elastic tissue) Flat curve + Rt shift = ↓compliance (↑ connective tissue)

  38. Compliance Static compliance: measured when air flow is absent, reflects elastic resistance of lung & chest wall. =Corrected tidal vol./(plateau pressure – PEEP) n value: 40 to 60 ml/cm H2O. Dynamic compliance: measured when air flow is present, reflects airway + elastic resistance, = Corrected tidal vol./(peak airway pressure – PEEP) n value: 30 to 40 ml/cm H2O. LOW Compliace: Lung expansion difficult. HIGH Compliance: Incomplete CO2 elimination.

  39. Compliance is reduced in DYNAMIC STATIC Atelectasis ARDS Tension Pneumothorax Obesity Retained secretions Bronchospasm Kinking of ET tube Airway obstruction

  40. Transport of oxygen • 2 forms: 1. Small amount dissolved in plasma 2. Chemically combined with Hb in RBC • Dissolved oxygen: henry’s law Gas conc = S x PP in soln * S - gas solubility coefficient for given soln at a given temp Dissolved O2 = 0.003 x 100 = 0.3ml/dl

  41. Transport of oxygen • Chemically combined with Hb: accounts for max blood oxygen • Hemoglobin - conjugated protein: four polypeptide (globin) chain, each combined with a porphyrin complex called heme. • each heme complex has a central ferrous ion to which O2 binds converting Hb into oxygenated state. • 1 gram of normal Hb carries 1.34 ml of O2, if Hb is 15 g/dl , O2 carrying capacity of blood = 1.34 ml x 15 g/dl = 20.1 ml/dl

  42. Transport of oxygen • O2 content: dissolved + combined with Hb • O2 content = (0.003 x PO2) + (Hb x 1.34 x SaO2) = (0.003 x 100) + (15 x 1.34 x 0.975) = 19.5 ml/dl (arterial) • O2 content = (0.003 x 40) + (15 x 1.31 x 0.75) = 14.8 ml/dl (venous)

  43. Transport of oxygen • Total oxygen delivery to tissues: = oxygen content x CO = 20 ml/dl x 50 dl blood/min = 1000 ml/min. • O2 Flux: amount of O2 leaving the left ventricle per min in the arterial blood. • Fick equation describes O2 consumption (VO2) = CO x diff b/w arterial and venous oxygen content = 250 ml/min. • Extraction ratio for O2= (Ca O2 - Cv O2)/ Ca O2 = 5/20 = 25%

  44. Oxygen stores • Normally in adults = 1500 ml * O2 remaining in lungs * bound to Hb * dissolved in body fluids • O2 contained within lungs at FRC – most imp source of oxygen. • Apnea in pt breathing room air = FiO2 x FRC = 0.21 x 2300 = 480 ml depleted in 90 sec • Preoxygenation with 100% oxygen for 4-5 min leaves 2300 ml of oxygen – delays hypoxemia following apnea

  45. HbO2 Dissociation Curve • Relates SpO2 to the PO2 • Sigmoid shaped (comb of 1st heme Hb molecule with O2↑ affinity of other heme molecules) SHIFTING AFFINITY

  46. Measure of Hb affinity for O2 • quantified by P50. • P50 - PO2 at which Hb is 50% saturted. • P50 = 26 mmhg at PCO2 40 mmhg, pH 7.4, temp. 37°C. ↑ P50 (facilitates O2 release) ↓ Hb affinity, Rt shift of ODC

  47. Factors affecting O2 loading and unloading • Blood pH • Body temp • Organic phosphates in RBC • Variations in structure of Hb

  48. Shift of curve to right • Fall in blood pH due to a. ↑ CO2 b. Presence of any acid in blood • ↑ temp • Inhalational anesthetics: Isoflurane shifts P50 to right by 2.6 mmhg. • ↑ conc of 2,3- DPG • By product of glycolysis (accumulates in anaerobic met.) • Competes with O2 for binding sites on Hb • ↓ in: acidosis, blood stored in acid citrate dextrose sol in blood bank • ↑ in: high altitude, chronic anemia, exercise

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