Normally, during good health, air can flow in and out of the lungs with little difficulty. During periods of illness though, the airways become inflamed or constricted (e.g. bronchospasm in asthma), and it is noticeably more difficult to breathe. This resistance to the free movement of air results in a reduction of the speed of air travelling in and out of the lungs a fact already familiar to many asthmatics that rely on the peak expiratory flow meter to provide a simple indicator of how the disease is affecting them.
How airflow is affected by an increase in resistance.
Placing an inhaler device in the mouth, or near it, also affects how easily air can be inhaled. An inhaler that allows air to travel with little difficulty will have little effect of the speed of inhalation, and will be found to have a low resistance.
But those inhalers that prevent air from moving easily will have a large effect on the speed of inhalation, and as would be expected, have a high resistance.
The above observations are supported with results from laboratory and clinical research - those inhalers with a high resistance will reduce the speed of inhalation far more than those with low resistance.
It is not possible to differentiate high resistance inhalers from low resistance ones by just looking. Both of the inhalers pictured below are in common use worldwide, but the way in which they work, and hence their internal resistance, is very different!
Commonly used inhalers - pMDI (left), a low resistance inhaler and Turbohaler (right), a high resistance inhaler.
The route air follows when travelling through an inhaler will determine its internal resistance - airflow through a large open tube, in a straight line from one end of the device to the other, will be associated with free movement of air, and hence a very low resistance.
Airflow in a low resistance inhaler
However, if the air that the patient is inhaling has to follow a frequently changing route, with complicated turns and narrow passages to travel along, then the relative difficulty of doing this will result in a difficulty in moving air through, and the resistance consequently will be higher.
Airflow in a high resistance inhaler
Summary
It can be seen that the freedom of air to move in and out of the lungs is directly affected by the resistance encountered. When breathing through an inhaler, the internal resistance of the device will be additional to any airway resistance being experienced through illness or bronchoconstriction.
By putting additional effort into breathing in, the speed of inhalation can be increased when a resistance is present - but the higher the resistance experienced, the greater the effort needed to overcome it.
As individuals, everyone will have their own, personal, maximum inspiratory flow for a resistance they encounter. It will depend on the strength of their respiratory muscles, and the effort that is comfortable for each individual when inhaling, and will be reduced if fatigue or exhaustion is present.
Whilst the equipment needed to measure the internal resistance of an inhaler is likely to be found only in testing laboratories and dedicated research centres, it is possible to use knowledge of each inhaler's resistance to improve how inhaled medication is used by professionals and patients alike.
"....inspiratory flow rate is determined by the resistance in the patient's airways, the respiratory muscle function and the resistance of the device. When airway obstruction and thus airway resistance increase, the inspiratory flow rate generated by the patient decreases."
Dekhuijzen P.N.R., Professor of Pulmonology, University Medical Centre, Nijmegen, The Netherlands. 2001
