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                                 RSPT 1207 Cardiopulmonary A&P




Ventilation
Lecture Notes
Reference & Reading: Egan’s Chapter 9
  I. Primary function of lungs is to move air in and out. VENTILATION is the process of
         moving air in and out of the lung. Differs from respiration.


  II. Mechanics of Ventilation
         a. Has two phases:
                  Inspiration
                 Expiration
         b. TIDAL VOLUME (VT)– volume of gas that moves in or out of the lungs
         c. VITAL CAPACITY (VC) – have reserves for increasing ventilation


  III. Pressure Differences During Breathing – Ventilation happens because of changes
         in pressure
         a. Pressures measured in CmH2O; related to atmospheric pressures
                  Positive pressure: > atmospheric pressure
                  Negative pressure: subatmospheric pressure
                  Mouth pressure, body surface pressure are at 0 (atmospheric
                     pressure)
                  Alveolar pressure – aka intrapulmonary pressure
                  Pleural pressure
         b. PRESSURE GRADIENT – the difference between 2 pressures
         c. Three pressures to be noted in ventilation:
                  Transrespiratory – causes flow into and out of alveoli
                  Transpulmonary – difference between alveoli & pleural space; keep
                     alveoli inflated
                  Transthoracic – difference between pleural space and body
                     surface; pressure required to expand/contract lungs & chest wall


  IV. Inspiration – Gas flow must be started from atmosphere to the airways. Negative
         pressure must be generated to create a gradient. The intrathoracic volume
         must rise so the intrathoracic pressure can drop before the airway pressure
         can drop.
         a. Diaphragm must drop and rib cage spreads apart, so that the thoracic
            cavity becomes larger. The intrathoracic pressure drops from its negative
            of approx -5 to a negative pressure of approx -10 (driving pressure 5) as
            this thoracic cavity gets larger.
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       b. The lung sticks to the thoracic cage, enlarges as the thorax enlarges.
           Pressures drop to a negative in the alveoli.
       c. Because there is a negative pressure in the airways and in the alveoli, and
           the atmosphere is still zero, there is now a driving pressure that allows
           gas flow to occur in the direction of the negative pressure.
       d. Gas moves along the airways and into the alveoli- until there is no longer
          a pressure gradient between the mouth and the alveoli.
       e. Flow stops – this point is called END-INSPIRATION.


V. Expiration – is passive. The thorax recoils, ↑ pressure in pleural space. Alveoli
       begin to deflate; positive pressure causes air to move towards mouth. The
       point in which the air has left the lung is called END-EXPIRATION.


VI. Forces opposing Inflation of the lung – several things must be overcome for
       inspiration to occur. Two categories:
                  Elastic: involves lung tissue, thorax tissue & surface tension
                  Frictional: resistance by gas flow & tissue movement
VII. Elastic forces
       a. Elastic opposition to ventilation
                  Elasticity: occurs in lungs elastic and collagen in the parenchyma
                  Negative pressures are required to stretch lung to ↑ lung volume
       b. Surface Tension Forces – lung filled with fluid is easier to fill than a lung
           without
                  Alveoli have pulmonary surfactant; which help create a gas-fluid
                interface
       c. ELASTIC RECOIL – combination of tissue elasticity & surface tension forces


VIII. Compliance – is a measurement of distensibility of the lung.
       a. It is defined as the volume of change per unit of pressure change:
                                               C = ΔV
                                                  ΔP
       b. Normally recorded in L/cmH2O or mL/cmH2O
       c. Normal compliance of normal healthy lung in 200 mL/cmH2O
       d. Emphysema vs. pulmonary fibrosis
       e. Decreased compliance requires more pressure to inflate the lung →
           increased WOB. If the pressure can’t be generated, Vt will ↓, interfering
           with gas exchange.
       f. Causes of decreased lung compliance:
                  Increased blood flow to capillary bed from CHF or renal failure
                  Disruption of capillary permeability, fluid moves into interstitial
                  space
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                Increased alveolar wall thickness
                Infections – such as interstitial pneumonia
                Increased surface tension:
                    1. Prematurity
                    2. Thermal damage
                    3. Chemical damage
                    4. Asphyxia


IX. Chest wall compliance
      a. The normal thorax will recoil to the Functional residual capacity,
          approximately 40% of the TLC
      b. When lung-chest wall relationship is disrupted, the lung collapses & the
          chest wall expands
      c. Diseases that alter that balance can affect the lung volume


X. Frictional Opposition to ventilation – occurs when respiratory system is in
      motion. Two components:
                Tissue viscous resistance
                Airway resistance


XI. Tissue viscous resistance
      a. Motion is impeded by displacement of tissue including:
                Lung
                Rib cage
                Diaphragm
                Abdominal organs
      b. Can be related to:
                Obesity
                Fibrosis
                Acites


XII. Airway Resistance – impedance to ventilation by the movement of the gas
      through the airways
      a. Accounts for 80% of frictional resistance in ventilation
      b. Ratio of driving pressure to the flow of the gas
                                       Δ P (driving pressure)
                            Raw =
                                              V (flow)


      c. When calculated it is written cmH2O/L/sec
                Driving pressure is cmH2O
                Flow is L/sec
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       d. At a given flow rate, as a tube (airway) narrows, the pressure required to
          ventilate increases
XIII. Factors that affect Raw
       a. Laminar flow – When air flow through a tube, it runs parallel to the tube
          wall. Flow increases toward center of tube
       b. Poiseuille’s law – defines laminar flow of gas through a tube with no
          branches
                                         V (flow)
                                ΔP
                                             r4
       c. Significance:
                 For gas flow to remain constant, pressure must vary
                 If gas pressure remains constant, gas flow will vary
       d. If an airway lumen is decreased by ½, Raw increases by 16, pressure
          increases
              1. Small changes in the lumen can greatly change gas flow through
                  an airway
       e. Turbulent flow – result form irregular currents
       f. Increased Raw:
                 Bronchospasm
                 Increased mucosal edema
                 Increased mucous production
       g. Results in decreased lumen, driving pressure must increase to achieve
          same Vt
                 Accessory muscles must be used
                 Air trapping may occur
XIV. Exhalation – although passive, it can be active (cough)
       a. Irritation: the tactile sensors located in the trachea and the carina note
          excessive secretions or a foreign body.
       b. Deep inspiration: the diaphragm drops down very low to create a much
          lower intrathoracic pressure than with a normal breath and the accessory
          muscles of inspiration are used to create a very low intrathoracic pressure
       c. Inspiratory hold: at least 3-5 seconds is needed for the gas to move
          down the 30 generations of airway to get into the peripheral, basal areas
          that need it most. There is no flow at this point, because the gradient is
          zero. The diaphragm now only holds that gas inside. If the glottis (hole
          between the vocal cords) was opened and the diaphragm released at this
          point—there would only be a deep breath and an exhalation.
       d. Compression phase: when the glottis is closed off completely as in a
          cough and the diaphragm is forced upward, by the accessory muscles in
          the abdomen intrapleural pressure for the first time is now a
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           positive. This pressure builds up in the alveoli and the airways. The
           airway pressure can rise to 100-200 cm
       e. Expulsion phase: The glottis releases suddenly and the high driving
           pressure of 100-200 cm H20 creates exhaled flows of as high as 300-
           500 LPM.
       f. Anything in the path of this air flow will be shirred off.


XV. Distribution of Gas - Due to the three lung zones from the base of the lung to
       the apex, there is a difference in the distribution of ventilation. Upright
       position.
       a. Zone 1: at the apex of the lung, due to gravity there is less blood flow;
           the lung is lighter and more compliant so it is easier to inflate at that
           point. The alveoli at rest are larger here with more air all the time. The
           RESIDUAL VOLUME is found here. There is less perfusion (blood flow)
           because there is some pressure in the alveoli opposing the capillary
           blood flow in this area, as well as gravity.
       b. Zone 2: equal ventilation and perfusion here
       c. Zone 3: due to gravity there is more blood at the bases of the lungs so
           that these alveoli have more blood pushing on them. They tend to be
           smaller. These alveoli in the bases will fill first, but they will not fill as
           much as the apical alveoli in Zone 1. They also will empty first. This area
           tends to be the area of atelectasis development.
XVI. Transpulmonary pressure gradient:
       a. There is more negative pressure in the apex (-10 cmH2O), zone 1.
       b. As the lungs move down, negative pressure is only -2 cmH2O
       c. On exhalation the apical alveoli are more inflated, and basal alveoli are
           closer to collapse
       d. On inspiration gas moves into alveoli in the bases first (Zone 3), then the
           middle (zone 2), then lastly into the apical alveoli
XVII. Efficiency
       a. Minute ventilation (Ve) – total volume moving in or out of the lungs per
           minute (Measured in L/min)
                    Product of rate X exhaled Vt (Ve = rate * exhaled Vt)
       b. Alveolar ventilation – efficieny of ventilation depends on gas reaching the
           alveoli (mL/min)
                    VA is a product of rate & alveolar volume (VA = f * VA)
       c. Dead space ventilation – wasted ventilation. Physiological dead space is
           the combination of
                    Anatomical dead space – volume in conducting airways that
                    doesn’t participate in gas exchange (1mL/lb of weight)
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                 Alveolar dead space – some alveoli doesn’t participate in gas
                  exchange; there is ventilation but no perfusion
       d. VD/VT – represent the efficiency of ventilation.


XVIII. Effectiveness of Ventilation
       a. Ventilation is effective when pH is maintained
       b. Balance between CO2 production and Alveolar ventilation determines pH
       c. Normal 40 mmHg CO2, pH 7.35-7.45
       d. Hypoventilation
       e. Hyperventilation
       f. Hypoventilation vs. hyperpnea

				
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