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10th August, 2011

Airway Resistance

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Doctor DarwinAccording to the Hagen-Poiseuille law if the airway tube will reduct twice - to maintain the same flow a gradient of the pressure creating this flow should be increased in 16 (!) times.
According to the Hagen-Poiseuille law if the airway tube will reduct twice - to maintain the same flow a gradient of the pressure creating this flow should be increased in 16 (!) times.

The abbreviation for the term "airway resistance" is Raw. It is measured in cmH2O/L/sec or mbar/ml/sek. Normal range for adult healthy person is 0,6-2,4 cmН2О/L/sec.

The physical sense of this parameter represents a gradient of pressure (forcing pressure) in current system to provide a flow of 1 litre per second. It isn't a big problem for modern ALV devices to calculate airway resistance - it's enough to divide pressure rate on the flow rate.

Raw = (PIP–Pplateau)/V`

where Raw = airway-resistance,

PIP = maximal inspiratory pressure

Pplateau = pressure of the inspiratory plateau

V` = flow

The respiratory mechanics considers resistance of airways to an air. Airway resistance depends on:

(length) - (-[s2]) (q:airways) ||
(diameter) - (s2: > [q,w,e]) - of (w:endotracheal tube) ||
(passableness) - (-[s2]) (e:breathing circuit)

Airway resistance grows during all chronic and acute obstructive diseases of the lungs leading to airway diameter reduction, as well as during mucus or sputum accumulation or breathing circuit tubes bend. According to the Hagen-Poiseuille law if the airway tube will reduct twice - to maintain the same flow a gradient of the pressure creating this flow should be increased in 16 (!) times. It is also important to remember that airway resistance of all system is defined by the region of maximum resistance (the "bottle-neck"). So, the removal of a foreign body from airways, elimination of a trachea stenosis, for example, will contribute normalization of ventilation conditions. Some useful tips about resistance:

  • ALV device can measure airway resistance only in forced ventilated at relaxed patient.
  • Airway resistance represents the obstructive problems (connected with airways passableness).
  • The greater is a flow , the greater is airway resistance.