The concept of the equal pressure point
 For air to flow through a tube, there must be a pressure difference between the two ends. In the case of forced expiration with an open glottis, this driving pressure is the difference between alveolar pressure (the sum of pleural pressure and lung elastic recoil pressure) and atmospheric pressure (assumed to be zero). Frictional resistance causes a fall in this driving pressure along the length of the conducting airways. At some point, the driving pressure may equal the surrounding peribronchial pressure; in this event, the net transmural pressure is zero. This defines the equal pressure point. Downstream (toward the mouth) from the equal pressure point, pressure outside the airway is greater than the driving pressure inside the airway. This net negative pressure tends to collapse the airway, resulting in dynamic compression. The more forcefully one expires, the more the pressure surrounding collapsible airways increases. Flow becomes effort independent. (P_pl, pleural pressure; P_L, lung elastic recoil pressure; P_alv, alveolar pressure; P_atm, atmospheric pressure.)

Classification of Causes of Respiratory Failure.

Mechanism Acute Chronic Reduced ventilation    Restrictive         Neuromuscular diseases (failure of respiratory muscles) Tetanus, botulism, poliomyelitis, polyneuritis, spinal cord injury Muscular dystrophy, myasthenia gravis     Chest wall diseases (failure of chest expansion) Pneumothorax, flail chest (trauma) Kyphoscoliosis, obesity, pleural effusion, mesothelioma, ankylosing spondylitis     Lung diseases Radiation pneumonitis Interstitial fibrosis, sarcoidosis   Obstructive Foreign bodies, epiglottitis, angioedema, diphtheria, bronchiolitis, bronchial asthma COPD,1 bronchiectasis   Abnormal perfusion  Pulmonary embolism, fat embolism Recurrent emboli, vasculitis Impaired diffusion    Interstitial diseases Shock, interstitial pneumonitis Sarcoidosis, pneumoconiosis, interstitial pneumonitis, interstitial fibrosis   Pulmonary edema Acute left ventricular failure, toxic gases, mitral stenosis Chronic left ventricular failure

1COPD = chronic obstructive pulmonary disease.