Delivery to Children with Ventilators and Masks

The delivery problem is magnified in very small children, in whom endotracheal tubes are only a few millimeters in diameter, and ventilator humidity increases the chance of much change in particle size before the particle nears the patient.

Deposition in the lungs of infants from either face mask or ventilator is quite low, whether delivered from MDI/spacer or nebulizer source. Animal studies using a nebulizer to deliver isotope-labeled aerosols to rabbits indicate 1 -3% deposition at standard settings, with marked reduction when tidal volume and residence time of aerosol were reduced [84,85]. Two in vitro studies suggest that delivery may be enhanced by increasing the inspiratory time, reducing the respiratory rate, or reducing minute volume [86]. Spacer devices are now used "in-line" in some ventilator circuits. Some evidence suggests that they can improve delivery from an MDI [87], but lung delivery estimated using filter measurements can still be as low as 1.5-2%. Other studies, based on urine excretion (of sodium cromoglycate), show remarkably similar lung depositions, whether delivery to ventilated neonates is by nebulizer or by MDI [88]. Some evidence suggests that deposition may be as much as 30% greater when aerosol delivery is given by pressure-support ventilation [89]. This may be due in part to the patient contribution to the ventilation process in pressure-support mode.

The importance of particle size is well known, and it should be intuitive to anyone knowledgeable about aerosols that a 7.7-p.m particle will not reach the lung via a device such as a mask or ventilator. Cystic fibrosis patients inhaled either 3- or 7.7-p.m particles via a mask; deposition was < 1% for the larger particle and 2% for the smaller particle [90]. Many nebulized aerosols have a broad size distribution so that some of the smaller particles in the cloud will reach the lung, but the vast preponderance of the mass is in the larger particle fraction and will be lost in mask or ventilator hardware. It is not surprising that efficiency is so poor. Information such as this was not new in the 1980s, let alone in the 1990s. There is room for much improvement in this area.

Aerosols in Cystic Fibrosis

Aerosols have played a major role in the management of CF for decades, and their role continues to be important. Many airways are blocked by the inspissated, infected secretions, so delivery beyond the blockage into diseased areas is difficult.

Aerosol delivery is difficult in the best of circumstances, but it becomes especially challenging for sick infants and children, who often cannot perform necessary maneuvers to inhale most efficiently. Nasal masks are used in some of these situations, and lung delivery is very low, in the 0.3-1.6% range for infants and 2.7% for children [91]. Targeting is discussed elsewhere in this chapter and also this book (see Chaps. 3, 7). Targeting of smaller airways is often considered desirable and can be achieved in CF patients as well as others by delivering smaller particles [92]. However, size reduction is also mass reduction (at the third power), so many more particles must be delivered to achieve similar mass to small airways.

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