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Automated, real-time calibration of the respiratory inductance plethysmograph and its application in newborn infants

Respiratory inductive plethysmography (RIP) is widely used in infants, children and adults. The technique is well accepted as it provides important qualitative information on the pattern of breathing, although its ability to record volume accurately was questioned due to calibration uncertainties. Existing calibration methods require two-position calibration, or patient cooperation in performing various breathing manoeuvres, or prolonged calibration paradigms. The disadvantages from calibration difficulties are even more pronounced in infants. We present a computer system that is capable of performing a single-posture, real-time RIP calibration during natural breathing and is suitable for use in newborns. The calibration algorithm is based on interactive, point-by-point calculations of maximal correlation between airflow at the mouth, V̇ao, and summed differentiated RIP signals. The quantities are calculated interactively at every sample point, and the process continues until stable results are reached and convergence criteria met. A graphic user interface was developed to assist in the rapid implementation and ease of use. Validation schemes were evaluated in 33 newborn infants against actual V̇ao. Calibration factors were obtained within 21 ± 11 s with a mean correlation coefficient of 0.97 ± 0.03. All RIP-derived values were similar to actual airflow signals, with error values ranging from 0.4 ± 3.0% for respiratory rate to 1.8 ± 7.3% for tidal volume. Calibration was found to be stable and reliable for up to 3.5 h and in changing sleep states. It is concluded that the new single-posture real-time RIP calibration system is safe and simple to use, and also quick, accurate and stable. The system was found to be suitable for use in newborns during natural breathing while asleep.