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Plethysmograph and interrupter resistance measurements in prematurely born young children
  1. M R Thomas1,
  2. G F Rafferty1,
  3. R Blowes1,
  4. J L Peacock3,
  5. N Marlow4,
  6. S Calvert2,
  7. A Milner1,
  8. A Greenough1
  1. 1Department of Child Health, Guy’s, King’s and St Thomas’ Medical School, King’s College London, UK
  2. 2Department of Child Health, St George’s Hospital Medical School, London, UK
  3. 3School of Health Sciences and Social Care, Brunel University, West London, UK
  4. 4Academic Division of Child Health, School of Human Development, University of Nottingham, Nottingham, UK
  1. Correspondence to:
    Professor Greenough
    Department of Child Health, 4th Floor Golden Jubilee Wing, King’s College Hospital, Bessemer Road, London SE5 9RS, UK; anne.greenough{at}


Background: Airways obstruction in premature infants is often assessed by plethysmography, which requires sedation. The interrupter (Rint) technique does not require sedation, but has rarely been examined in children under 2 years of age.

Objective: To compare Rint results with plethysmographic measurements of airway resistance (Raw) in prematurely born, young children.

Design: Prospective study.

Setting: Infant and Paediatric Lung Function Laboratories.

Patients: Thirty children with a median gestational age of 25–29 weeks and median postnatal age of 13 months.

Interventions and main outcome measures: The infants were sedated, airway resistance was measured by total body plethysmography (Raw), and Rint measurements were made using a MicroRint device. Further Raw and Rint measurements were made after salbutamol administration if the children remained asleep.

Results: Baseline measurements of Raw and Rint were obtained from 30 and 26 respectively of the children. Mean baseline Rint values were higher than mean baseline Raw results (3.45 v 2.84 kPa/l/s, p  =  0.006). Limits of agreement for the mean difference between Rint and Raw were −1.52 to 2.74 kPa/l/s. Ten infants received salbutamol, after which the mean Rint result was 3.6 kPa/l/s and mean Raw was 3.1 kPa/l/s (limits of agreement −0.28 to 1.44 kPa/l/s).

Conclusion: The poor agreement between Rint and Raw results suggests that Rint measurements cannot substitute for plethysmographic measurements in sedated prematurely born infants.

  • Raw, airways resistance measured by total body plethysmography
  • Rint, interrupter resistance
  • airways resistance
  • interrupter technique
  • premature
  • plethysmography

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  • Published Online First 20 October 2005

  • Competing interests: none declared