Abstract

This is a review of the factors determining the resistance of the respiratory system and of the possibilities to measure it. Most of the work of breathing is spent for overcoming the elastic resistance of the lungs and the thoracic cage. The ability of the lung to stretch is called compliance (defined as the volume change per unit of pressure change across the lung). Another factor, non-elastic or viscous resistance depends on the resistance of the airways to air flow, the resistance of tissues as they slide over each other, and inertance. Airway resistance is the pressure difference between the alveoli and the mouth divided by air flow rate. The pressure – flow characteristics for laminar flow are well described, and resistance to flow is critically dependent upon the radius of the tube. As airway narrowing is one of the most common symptoms in persons with respiratory disease, it is crucial to have possibilities to assess changes in airway resistance.

Body plethysmography is a method for direct measurement of airway resistance. The subject seated in the body pletysmograph (box) breathes through a pneumotachograph, at the same time pressure in the alveoli and in the box are measured. Respiratory resistance can also be measured during normal breathing from an intrapleural pressure record obtained with an esophageal balloon. However, in this case, tissue viscous resistance as well as airway resistance are included. The methods used for measurement of respiratory resistance in small children are dealt with in the second part of the paper.