Lung Volumes

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      • Tidal volume
        • Normal, quiet breathing involves inspiration and expiration of a tidal volume (VT).
        • Normal tidal volume is approximately 500 mL(MCQ)
        • tidal volume includes the volume of air that fills the alveoli plus the volume of air that fills the airways.(MCQ)
      • inspiratory reserve volume
        • The subject is asked to take a maximal inspiration, followed by a maximal expiration
        • The additional volume that can be inspired above tidal volume is called the inspiratory reserve volume(MCQ)
        • approximately3000 mL(MCQ)
      • expiratory reserve volume
        • The additional volume that can be expired below tidal volume is called the expira- tory reserve volume
        • approximately1200 mL.(MCQ)
      • residual volume
        • The volume of gas remaining in the lungs after a maximal forced expiration is the residual volume (RV),
        • It is approximately 1200 mL(MCQ)
        • It cannot be measured by spirometry (MCQ)

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LUNG CAPACITIES

      • inspiratory capacity (IC)
        • composed of the tidal volume plus the inspiratory reserve volume
        • approximately3500 mL (500 mL +3000 mL). (MCQ)
      • functional residual capacity (FRC
        • composed of the expiratory reserve volume (ERV) plus the residual volume
        • it is approximately 2400 mL (1200 mL +1200 mL). (MCQ)
        • FRC is the volume remaining in the lungs after a normal tidal volume is expired
        • It is the equilibrium volume of the lungs.
      • Vital capacity (VC)
        • composed of the inspiratory capacity plus the expiratory reserve volume
        • it is approximately 4700 mL (3500 mL +1200 mL)(MCQ)
      • Vital capacity
        • volume that can be expired after maximal inspiration
        • Its value increases with (MCQ)
          • body size
          • male gender,
          • physical conditioning
        • Its value decreases with age(MCQ)
      • Total lung capacity (TLC)
        • includes all of the lung volumes
        • It is the vital capacity plus the residual volume,(MCQ)
        • It is 5900 mL (4700 mL +1200 mL).(MCQ)
        • What are the lung capacities that cannot be measured by spirometry ?
          • Since residual volume cannot be measured by spirometry, lung capacities that include the residual volume also cannot be measured by spirometry
          • FRC and TLC cannot be measured (MCQ)
      • FRC
      • the volume remaining in the lungs after a normal expiration
      • it is the resting or equilibrium volume of the lungs.(MCQ)
      • There are two methods used to measure FRC (MCQ)
        • helium dilution  method
        • bodyplethysmography

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        • Dead space
          • volume of the airways and lungs that does not participate in gas exchange.
        • Anatomic Dead Space
          • The anatomic dead space is the volume of the con- ducting airways, including the nose (and/or mouth), trachea, bronchi, and bronchioles.
          • It does not include the respiratory bronchioles and alveoli.(MCQ)
          • The volume of the conducting airways is approximately 150 mL(MCQ)
          • To sample alveolar air, one must sample end-expiratory air(MCQ)
        • Physiologic Dead Space
          • Physiologic dead space includes the anatomic dead space of the conducting airways plus a functional dead space in the alveoli.
          • The functional dead space can be thought of as ventilated alveoli that do not participate in gas exchange.
          • The most important reason that alveoli do not partici- pate in gas exchange is a mismatch of ventilation and perfusion,
          • In normal persons, the physiologic dead space is nearly equal to the anatomic dead space.
          • The ratio of physiologic dead space to tidal volume provides an estimate of how much ventilation is “wasted” (either in the conducting airways or in nonperfused alveoli).
          • Partial pressure of CO2 (PCO2) of mixed expired air (PECO2)
            • Used to estimate volume of the physiologic dead space(MCQ)
            • based on three assumptions:
              • All of the CO2 in expired air comes from exchange of CO2 in functioning (ventilated and perfused) alveoli
              • there is essentially no CO2 in inspired air
              • thephysiologic dead space (non- functioning alveoli and airways) neither exchanges nor contributes any CO2.
            • If physiologic dead space is zero, then PECO2 will be equal to alveolar PCO2 (PACO2).
            • if a physiologic dead space is present, then PECO2 will be “diluted” by dead space air and PECO2 will be less than PACO2 by a dilution factor. (MCQ)
            • Therefore, by comparingPECO2 with PACO2, the dilution factor (i.e., volume of the physiologic dead space) can be measured.
          • Why is PCO2 of systemic arterial blood (PaCO2) is equal to the PCO2 of alveolar air (PACO2).
            • Alveolar air cannot be sampled directly.
            • Alveolar air normally equilibrates with pulmonary capillary blood (which becomes systemic arterial blood).
          • Volume of physiologic dead space is calculated by the following equation:(MCQ)

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            1. VD =  Physiologic dead space (mL)
            2. VT  =Tidal volume (mL)
            3. PaCO2 =  PCO2 of arterial blood (mm Hg)
            4. PECO2 =  PCO2 of mixed expired air (mm Hg)
          1. In words, the equation states that the volume of the physiologic dead space is the tidal volume (volume inspired with a single breath) multiplied by a fraction.
          2. The fraction represents the dilution of alveolar PCO2 by dead space air (which contributes no CO2).

VENTILATION RATES

          • Ventilation rate is the volume of air moved into and out of the lungs per unit time.
          • Minute ventilation
            1. total rate of air movement into and out of the lungs
            2. Minute ventilation = VT x Breaths/min(MCQ)
          • Alveolar ventilation,
            • corrects for the physiologic dead space. (MCQ)

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ALVEOLAR VENTILATION EQUATION

        • There is inverse relationshipbetween alveolar ventilation and alveolar PCO2 (PACO2).(MCQ)
        • The alveolar ventilation equation is expressed as follows:

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        1. ifCO2 production is constant, then PACO2 is determined by alveolar ventilation.
        2. For a constant level of CO2 production, there is a hyperbolic relationship between PACO2 and alveolar ventilation.
          1. Increases in alveolar ventilation cause a decrease in PACO2
          2. Decreases in alveolar ventilation cause an increase in PACO2.
        3. ifCO2 productiondoubles (e.g., during strenuous exercise), the hyperbolic relationshipbetweenPACO2 and alveolar ventilation shiftstothe right(MCQ)
          1. Under these conditions, the only way to maintainPACO2 atitsnormalvalue(approximately40mmHg) is for alveolar ventilation to double also.
          2. ifCO2 production increases from 200 mL/min to 400 mL/min, PACO2 is maintained at 40 mm Hg if, simultaneously, alveolar ventilation increases from 5 L/min to 10 L/min.
        4. FORCED EXPIRATORY VOLUMES
          1. Vital capacity
            1. volume that can be expired following a maximal inspiration(MCQ)
          2. Forced vital capacity (FVC)
            1. total volume of air that can be forcibly expired after a maximal inspiration,
          3. FEV1
            1. volume of air that can be forcibly expired in the first second is called
            2. Normally, the entire vital capacity can be forcibly expired in 3 seconds, so there is no need for “FEV4.”(MCQ)
          4. FEV1/FVC
            1. in a normal person, FEV1/FVC is approximately 0.8(MCQ)
            2. 80% of the vital capacity can be expired in the first second of forced expiration
          5. In a patient with an obstructive lung disease such as asthma(MCQ)
            1. both FVC and FEV1 are decreased
            2. FEV1 is decreased more than FVC
            3. FEV1/FVC is also decreased,
          6. In a patient with a restrictive lung disease such as fibrosis(MCQ)
            1. both FVC and FEV1 are decreased
            2. FEV1 is decreased less than FVC is.
            3. Thus, in fibrosis, FEV1/ FVC is actually increased


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