True about Hyaline membrane disease:
B i.e. FRC below the closing volume - Closing volume (CV) or critical CV is minimal volume of gas necessary to prevent small airway collapse. In young adults it is 10% of vital capacity (ie = 480 ML) and becomes equal to - 40% of VC (ie = FRC) at the age of 65 years. In RDS (hyaline membrane disease), the FRC decreases below closing volume (FRC < CV) indicating collapse of some alveolar segments during tidal breathing. - Volume of air in lung at the end of (1) force ful inspiration is TLC; (2) forceful expiration is residual volume (RV) and ; (3) normal tidal expiration is FRC. RV is 1200mL and FRC is 2200 mL. Therefore normally closing volume is more close to RV than to FRC. Closing Volume (CV) and Closing Capacity (CC) CV is volume below which the terminal airway connected to alveoli gets closed and hence the gas cannot come in or out of alveoli. In young subjects, the closing volume is about 10% of vital capacity (V C). It increases with age and becomes equal to about 40% of VC (i.e. = FRC) at the age of 65 years. Relatively small amounts early disease in the small airways apparently increases the closing volume. Single Breath N2 Washout Test to measure CV From mid inspiration, the subject takes a deep breath (vital capacity single breath) of pure (100%) 02 and then exhales steadily. The changes in N2 concentration of expired air (at lips) is continuously measured during subsequent full exhalation. 4 phases of N2 conc. can be recognized one after another. Phase I or initial gas exhaled is the gas that filled the pure dead space. It contains no N2 because inhaled gas is 100% 02. Phase II: Mixture of dead space & alveolar gas is exhaled (with increasing N2 conc). The volume of dead space is volume of gas expired from peak inspiration to mid poion of phase II. 3. Phase III: Pure alveolar gas is exhaled (with constant N2 conc). Phase III terminates at (ie transition from phase III to IV) is closing volume (CV). The CV is the lung volume above RV (residual volume) at which the airways in lower dependent pas of lungs begin to close off because of the lesser transmural pressure in these areas. In most normal persons phase III also has a slight positive slope indicating a gradual increase in the propoion of expired gas coming from relatively N2 rich upper poions of lung. Phase IV: At the end of expiration, an abrupt increase in N2 conc. is seen signaling closure of airways at the base of lung. This is caused by preferential emptying of apex, which has relatively high concentration of Nz. The gas in upper lung is richer in N2 than lower dependent poions because the upper lung alveoli are more distended at the sta of inspiration and therefore the N2 in them is less diluted with 100% 02 inspired from mid inspiration (because during a VC breath of 100% 02, apex expands less and therefore previously occupied N2 there is less diluted with 02). Functional Residual Capacity (FRC) or Relaxation Volume It is volume of air remaining in lungs after normal (tidal) exhalation. and is determined by the balance b/w the elastic recoil pressure generated by lung parenchyma to become smaller (inward recoil) and the pressure generated by chest wall to become larger (outward recoil). At FRC, transmural pressure across respiratory system (chest wall + lung) is zero (0). It is positive at greater & negative at smaller volumes. - FRC decreases when the chest wall muscles are weak, (eg paralysis) or the elastic recoil of lung (i.e. tendency of alveoli to collapse) is more eg hyaline membrane diseaseQ (i.e. lung elastic recoil > chest wall muscle force). Disease characterized by low FRC include pulmonary edema, pneumonitis & RDS/HMD. In hyaline membrane disease, FRC usually decreases below closing volumeQ. When closing capacity exceeds FRC (FRC < CC), some alveolar segments are collapsed during a poion of tidal breathing. This decreases V/P ratio and may cause hypoxia and hypercarbia. And when the CC > FRC + TV. (i.e. C C exceeds FRC plus tidal volume), lung segments will be collapsed during inspiration and expiration of tidal breathing. This fuher exaggerate V/P mismatch and lit complete atelectasis. So respiratory therapy is designated to increase the lung volume towards normal FRC. - Positive end expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) raise FRC to a level above CV/CC in diseases associated with alveolar collapse; So the terminal airways remain open throughout the respiratory cycle. In presence of airway obstruction, FRC increases b/o premature airway closure, which traps air in the lung.
FRC above the closing volume
FRC below the closing volume
FRC is equal to closing volume
FRC doesn't depend on closing volume