Inspiratory flow

Mean airstream velocity diminishes as inspiratory flow moves toward the lung parenchyma because of the rapid increase in total cross-sectional area. The largest increases in area occur in the distal bronchioles and pulmonary airways, causing u to approach zero because... 

Dry powder aerosols are more complicated to sample as the commercially available devices disperse the aerosol on the patient s inspiratory flow. In order to challenge the efficiency of these devices, it is important to sample at multiple flow rates. The standard flow rate has become 60 L/min. Additional flow rates of 30 (28.3) and 90 L/min have also been employed. Each impactor must be calibrated at the different flow rates employed. In recent compendial specifications the duration of sampling (4 s) and pressure drop across the device (4 kPa) has also been suggested. This corrects for the effort on the part of a patient in a single breath. 

There have been attempts to conduct in vitro experiments in a manner that gives more meaningful data with regard to lung deposition. These methods, which are loosely based on inertial impaction, utilize inspiratory flow cycles rather than fixed flow... 

Deposition experiments were carried out in replicate hollow casts of the upper airways or a human tracheobronchial tree using 0.2, 0.15 and 0.04 ym diameter particles and cyclic inspiratory flow rates of approximately 1 8 and 34 liters per minute (L/min) (see Table I). The replicates were produced from a single solid master airway cast prepared from the lungs of 34 year old male. The airway dimensions of the cast corresponded closely with the population mean of eight adult males as reported by Nikiforov and Schlesinger (1985). The airway diameters are somewhat larger and... 

Table I. Tracheobronchial Cast Deposition Studies Cyclic Inspiratory Flow...

Gurman, J. L., M. Lippmann and R. B. Schlesinger, Particle Deposition in Replicate Casts of the Human Upper Tracheobronchial Tree Under Constant and Cyclic Inspiratory Flow. I. Experimental, Aerosol Science and Technology 3 245-252 (1984). 

Schlesinger, R. B., J.L. Gurman and M. Lippmann, Particle Deposition Within Bronchial Airways Comparisons Using Constant and Cyclic Inspiratory Flows, Ann. Occup. Hyg. 26 47-64 (1982). 

FIGURE7-1 Schematic of flow in daughter brandies of bifurcation model for steady inspiratory flow with flat profile in parent bran<. Velocity profiles in plane of bifurcation (—) and in normal plane (—) are indicated in right branch. Orientation of secondary flows and position of laminar boundary layer are shown in left branch. Redrawn wifo permisnon from Bdl. ... 

For steady inspiratory flow in hollow casts of the trachea and the first bifurcation, Dekker observed that a Reynolds number of 1,800 was... 

Dry powder inhalers are generally described as breath actuated devices, because the inspiratory airstream releases the dose from the dose system and supplies the energy for the generation of fine drug particles from the powder formulation. Because the efficiency of dose release and powder disintegration increases with increasing inspiratory flow rate for most DPIs, these devices would be better described as breath controlled devices. In Section 3.9, the effect of resistance and clinical conditions on the flow curve and relevant flow parameters for DPIs are discussed. 

Many different techniques are available for flow measurement and for recording of respiratory functions or flow parameters in particular (e.g. [115,116]). However, not all methods are appropriate for measurement of inhalation flows, either because they have low frequency responses or they influence the shape of the inspiratory flow curve by a large volume or by the inertia of the measuring instrument (e.g. rotameters). They may also interfere with the aerosol cloud from the inhalation device during drug deposition studies. 

DPIs do not utilize Freon propellants, but use either with or without lactose as vehiculum. These devices have a clinical efficacy similar to standard metered-dose inhalers, but may be easier to use in selected patients, since a minimal inspiratory flow rate is necessary to inhale from a DPI. Therefore, the DPI may be difficult for patients with an insufficient inspiratory flow rate, which occurs in children, the elderly, people with severe COPD shared with diaphragm dysfunction, and during an exacerbation. The inspiratory flow rate is associated with the internal resistance of the device used. For instance the turbuhalor has a significant internal resistance and its delivered dose is dependent from the inspiratory flow rate. 

Connolly G, Razak ARA, Hayanga A, Russell A, McKenna P, McNicholas WT. Inspiratory flow limitation during sleep in pre-eclampsia comparison with normal pregnancy and nonpregnant women. Eur Respir J 2001 18 672-676. 

Chamberlain Dyson (1956) measured deposition of unattached 212Pb in a rubber model of the trachea and main bronchi. At an inspiratory flow of 20 1 min-1, corresponding to a ventilation rate of about 0.6 m3 h-1, the activity deposited per cm2 of surface in the bronchi was 0.18% of that entering the trachea. No difference was found in the deposition of unattached 218Po (RaA) compared with unattached 212Pb (ThB). 

MDI delivery efficiency depends on the patient s inspiratory flow rate, breathing pattern and hand-mouth coordination. Increases in tidal volume and decreases in respiratory frequency enhance the peripheral deposition in the lung. Most patients need to be trained to use the MDI correctly, as up to 70% of patients fail to do so [26, 30]. 

---