lung volumes & ventilation

• Spirometry: 

  • A Spirometer can measure most lung volumes  

  • Not all lung volumes can be measured using simple spirometry 
• Measuring RV and FRC: 

  • RV can not be expired  

  • RV and FRC can not be measured with simple spirometry  

  • Use gas dilution technique or body plethysmography 
• Why assess spirometry? 

  • Diagnosis of respiratory disease  

  • Monitoring to assess  

  – Dx progression  

  – Deterioration  

  – Drug efficiency  

  – Efficient drug delivery system 

   
• Tidal volume (VT ) – Volume of air inspired during quiet respiration  
•  Inspiratory reserve volume (IRV) – Volume air inspired from VT to maximal inspiration
• Expiratory reserve volume (ERV) – Air expelled with forced expiration  
• Residual volume (RV) – Amount air left in lungs which cannot be expelled 
• Functional residual capacity (FRC) – Lung volume at end of normal quiet expiration (ERV + RV) 
• Vital capacity (VC) – Volume air maximal inspiration to maximal expiration (IRV + TV + ERV)  
• Total lung capacity (TLC) – Volume maximal inspiration to residual volume (IRV +VT + ERV + RV 
• Factors affecting lung volumes: 

  • Body size  

  • Age  

  • Gender  

  • Muscle training  

  • Disease 
•  Tidal volume – Adequate VT necessary to maintain oxygenation and CO2 clearance  
• Inspiratory reserve volume – Required during cough & undertaking exercise 
•  Functional residual capacity – Essential to maintain distal lung patency on expiration – Critical value called “closing volume” or “closing capacity” 
• Residual volume – Can increase in lung disease due to “air trapping”  

  – When increased may alter respiratory mechanics  
• Functional Residual Capacity – Can reduce in many clinical situations 
•  Vital capacity – Critical value (1L) used to identify if patient is able to maintain spontaneous ventilation 
• Bibasalatelectasis secondary to reduced FRC post-surgery

  Associated problems: 

  • Reduced oxygenation  

  • Poor CO2 clearance  

  • Increased work of breathing  

  • Breathlessness  

  • Reduced exercise tolerance 
• Hyperinflation secondary to air trapping increasing RV 

   Associated problems: 

  • Altered respiratory mechanics  

  • Reduced tidal volume  

  • V/Q mismatch  

  • Poor gas exchange 

  • Increased work of breathing  

  • Breathlessness  

  • Pursed lip breathing  

  • Use of accessory muscles 
• Ventilation Terminology: 

  • Tidal volume (VT ) is volume of air inspired during quiet respiration  

  • Normal respiratory frequency 12 bpm 

  • Average volume 500 mls  
• Minute ventilation (V1 ) is volume of air entering lungs each minute  
• Dead Space: 

  • Not all tidal volume air is used in gas exchange  

  • Dead space is term for that part of the tidal volume NOT involved in gas exchange 

  • There are 2 types of dead space  

  – Anatomical  

  – Alveolar  

  • Together called physiological dead space 
• Anatomical Dead Space: each tidal volume 150mls remains in conducting zone up to terminal bronchioles 
• Alveolar Dead Space: 

  • Inspiratory gas reaching alveoli unable to participate in gas exchange due to insufficient blood supply  

  • In healthy subjects alveolar dead space is almost zero  

  • In disease affected by 

   – PE  

  – ventilation of non-vascular air spaces 
• Physiological Dead Space: 

  • Physiological dead space is the sum of all parts of tidal volume which does NOT participate in gaseous exchange  

  • alveolar dead space plus anatomical dead space  

  – age & sex  

  – body size & posture  

  – mechanical ventilation  

  – pulmonary disease 
• Physiological dead space = Anatomical dead space + Alveolar dead space 
• Alteration of ventilation: 

  • Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – i.e. inadequate alveolar ventilation  

   

  • Hyperventilation exists when the ratio of carbon dioxide production to alveolar ventilation decreases below normal values – i.e. excess alveolar ventilation  

   

  • Both may occur in respiratory disease 
• Hypoventilation: 

  • Localised to particular lung lobe or segment  

  – Infection  

  – sputum plug  

  – atelectasis  

   

  • Scattered throughout lung fields  

  – COPD  

  – asthma 

   

  • Generalised  

  – Pain  

  – Reduced respiratory drive or consciousness 
• How does hypoventilation affect pH?  

  • Hypoventilation --> poor CO2 excretion 

  • ↑ PACO2  

  • ↑ CO2 + H2O <-> H2CO3 <-> HCO3 + ↑ H+  

  • decrease pH 

  • respiratory acidosis 
• Hyperventilation: 

  • Anxiety / fear  

  • Metabolic disease  

  • Airway obstruction  

  • Parenchymal lung disease  

  • Altitude 
• How does hyperventilation affect pH?  

  • increase alveolar excretion of CO2 

  •  decrease PACO2  

  • CO2 + H2O <-> H2CO3 <-> HCO3 + decrease H+  

  • increas pH 

  • respiratory alkalosis 
• Alveolar ventilation: 

  • Determines O2 and CO2 levels in alveolar gas  

  • Other factors are  

  – rate of O2 consumption (VO2 )  

  – rate of CO2 production (VCO2 )  
• Factors affecting arterial O2 content: 

  • Ventilatory pattern  

  • V:Q matching 

  • Hb content of blood  

  • Hb affinity for O2  

  • Atmospheric pressure O2  

  • Specific characteristics of alveolar membrane