All dry powder inhalers are flow-dependent to some extent. For Turbuhaler^®, already an inspiratory flow rate above 30 L/min is enough to lift the dose from the dosing disc. Most patients achieve flow rates well above this value, a value which also can easily be achieved by children as young as 4 years of age. A large number of controlled clinical studies have documented good clinical effects of Turbuhaler^® also in situations perceived as constrained, i.e. childhood asthma, acute severe asthma and COPD including severe exacerbations.
Read more about Turbuhaler^® in situations conceived as constrained.

Some of the best scientific evidence for the full therapeutic effect of Turbuhaler^® at low inspiratory flow rates comes from the study by Pedersen et al (1990). In this study, in children aged 6 years and above, the clinical effect, measured as FEV₁, after inhaling a bronchodilator at predetermined flow rates (between 10 and 60 L/min) was investigated. Clinically effective drug delivery was achieved at flow rates as low as 13 L/min, and was maximal with a flow rate of 30 L/min (Figure), with no further increase with a flow rate of 60 L/min. The increase in FEV₁ following the addition of inhalation of 1 mg terbutaline after 5 hours shows that patients were not at the top of the dose-response curve during the inhalations from Turbuhaler^®.

Forced expiratory volume in 1 second (FEV₁) after inhalation of 0.25 mg terbutaline via Turbuhaler^® at different flow rates in children with asthma. At 5 hours terbutaline at a higher dose was inhaled via Nebuhaler^® (Pedersen et al, 1990).

In a separate part of the study it was shown that flow rates above 30 L/min could be achieved by virtually all children aged 6 years and above, and by 74% of those 4-6 years of age.

For Rhinocort^® Turbuhaler^® children as young as 6 years suffering from rhinitis are able to generate an airflow sufficient to deliver a reliable dose to the nasal mucosa. The mean delivered doses increased with flow but reached a plateau of 63-64 µg from 30 L/min, with only a small increase to 70 µg at 60 L/min (Van Zoest et al, 2000).

References

Pedersen S, et al. Influence of inspiratory flow rate upon the effect of a Turbuhaler. Arch Dis Child 1990;65:308-10.

Van Zoest JGCM, et al. Nasal peak inspiratory flow through Turbuhaler^® in children with symptomatic rhinitis and in healthy children. Pediatr Allergy Immunol 2000;11:256-9

Flow dependency is one of several factors that may compromise the delivery of inhaled drug from inhalers, but it is not always the most important one. Some of the other factors seem to be of less importance for Turbuhaler^® than for inhaled medications delivered by other dry powder devices. What is especially important is to measure the dose that actually reaches the lungs of the patients, and not rely on measurements achieved solely by in vitro studies of the devices. These other factors are further discussed in the chapter "Lung deposition from Turbuhaler^® and other inhalers" and "Turbuhaler^® or Diskus™/Accuhaler™".

As the clinical efficacy of Turbuhaler^® is unlikely to be affected by inspiratory flow there is normally no need for patients to use additional flow meters. The most important message to patients should always be to ensure a forceful and deep inhalation. However, if there is some doubt whether a patient can inhale effectively from Turbuhaler^® this can easily be checked by simple equipment that can be provided by AstraZeneca marketing companies. For accurate results it is important that AstraZeneca provided equipment is used.