Presented at the Neonatal Society 2005 Autumn Meeting.
Broughton S, Rafferty G, Milner A, Greenough A
Division of Asthma, Allergy and Lung Biology, Kings College London, London, UK
Background: The efficacy of respiratory interventions in the neonatal period given with the hope of improving respiratory outcome can be quantitatively determined by assessment of respiratory function in infancy. Unfortunately, only a few randomised trials have incorporated such assessments, as respiratory function measurements at follow up have been restricted to specialist centres, because of the expertise required to perform and interpret them. A new generation of infant plethysmographs have been produced which could make infant respiratory measurements more routinely available. One of these (Jaeger master screen baby body plethysmograph, Viasys Ltd), however, has been demonstrated to record lower lung volumes than previously published values (1) or results obtained from a helium gas dilution technique (2).
Objectives: To compare the results obtained from a new generation plethysmograph (Jaeger) and an older research plethysmograph (Hammersmith), both in vivo and in vitro and assess if any differences found in lung volume (FRCpleth) were explained by different sensitivities to the adiabatic effect.
Methods: In vivo, 11 children were measured (median postnatal age 13 (range 5-15) months) were assessed. In vitro, the systems were assessed using a variable volume lung model containing 0, 32g or 72g (completely filling the chambers of the lung model) of copper wire to minimise adiabatic effects.
In vivo Results: The airway resistance (Raw) measured by the Jaeger plethysmograph (median 3.98 (range 1.05 – 6.74) kPa/l/s) results were higher than that measured by the Hammersmith plethysmograph (2.36 (1.46-5.19) kPa/l/s) (p<0.001). In addition the within patient variability was higher with the Jaeger (median coefficient of variability 17.2 %) than with the Hammersmith plethysmograph (9.9 %) (p<0.001).
In vitro Results: The volumes measured (expressed as a percentage of the actual volume) were 60-64 %, 70-75 % and 87-89% for the Jaeger, and 73-78 %, 97-99% and 100-103% for the older plethysmograph (p<0.001) with 0, 32 and 72g of copper wire respectively. The Jaeger plethysmograph was more sensitive to the adiabatic effect (p<0.001).
Conclusion: The Jaeger plethysmograph under records FRC in vivo and in vitro, this is at least partially explained by a greater sensitivity to the adiabatic effect, it also has a wider variability in Raw measurements. These results emphasise the need to comprehensively validate all new lung function assessment systems before introducing them into clinical practice.
References
1. Hulskamp G, Hoo AF, Ljungberg H, Lum S, Pillow JJ and Stocks J. Progressive decline in plethysmographic lung volumes in infants: physiology or technology? Am J Respir Crit Care Med 2003; 168:1003-9.
2. Broughton S, Rafferty G, Milner AD and Greenough A. Progressive decline in FRCin infants, physiology or technology? Am J Respir Crit Care Med 2005; (Letter) in press.