High airway resistance is seen in
Resistance impedes airflow into (and out of) the lung. The major component of resistance is the resistance exeed by the airways (large and small), and a minor component is the sliding of the lung and the chest wall tissue elements during inspiration (and expiration). Resistance is overcome by (driving) pressure. In spontaneous breathing, Resistance (R) is calculated as driving pressure (DP) divided by the resultant gas flow (F): R = DP/F The value of airway resistance is approximately 1 cm H2O/L/sec, and is higher in obstructive lung disease (e.g., COPD, asthma); in severe asthma, it is elevated approximately tenfold. The presence of an endotracheal tube adds a resistance of 5 (or 8) cm H2O/L/min for a tube with an internal diameter of size 8 (or 7) cm. For any tube for which the airflow is laminar (smooth, streamlined), the resistance increases in direct propoion to the tube length and increases dramatically (to the fouh power) as the diameter of the tube is reduced. Two factors explain why most (approximately 80%) of the impedance to gas flow occurs in the large airways. First, as bronchi progressively branch, the resistances are arranged in parallel and the total cross-sectional area at the level of the terminal bronchioles adds up to almost tenfold that at the trachea. Second, in tubes that are large, irregular or branched, the flow is often turbulent, not laminar. When flow is laminar: F(lam) = DP/R In contrast, when the flow is turbulent: F(turb) = DP/R2 Therefore, for a given radius, far more pressure is required to achieve comparable flow where the flow is turbulent; thus, the effo required is greater, and if prolonged or severe, respiratory failure is more likely. Ref: Miller's anesthesia 8th edition Ref: Morgan & Mikhail's clinical anesthesiology 6e
Respiratory bronchiole
Intermediate bronchiole
Terminal bronchiole
Main bronchus
Fundamental concepts