Calibration of Respiratory Inductive Plethysmography on Lambs and Piglets Receiving Controlled Mechanical Ventilation

Abstract

Respiratory inductive plethysmography (RIP) is a method of assessing breathing pattern noninvasively, without the use of airway connections. We undertook this study to investigate the accuracy of the single-position graphic (SPG) calibration technique with the respiratory inductive plethysmograph in small animals receiving controlled mechanical ventilation. Materials and Methods: We employed the SPG calibration technique for gain-factor calculation in 5 lambs and 5 piglets receiving controlled mechanical ventilation; each animal was studied three times. The SPG technique utilizes selection of two breaths (from a 20-s run of breaths) with different rib-cage and abdomen cross-sectional areas, as indicated by the RC and AB bands, respectively, of the plethysmograph; these ratios of rib cage and abdomen areas to a volume measurement made simultaneously by pneumotachography (RC/PNT and AB/PNT, respectively) are used for gain-factor calculation. Validation of gain factors was performed by comparing the volumes of 10 breaths measured by RIP with volumes measured simultaneously by integrated PNT. Results: On the initial ventilator settings, 87% (130) of the 150 lamb breaths used for validation were within 10% of the integrated PNT volume, and 81% (121) of the 150 piglet validation breaths were within 10% of integrated PNT. Following a significant change in ventilator-delivered frequency and tidal volume, new gain-factor calculation was required in 6 of the 15 lamb studies (40%) and 9 of the 15 piglet studies (60%) because of significantly altered RC amplitude, AB amplitude, or both. On the second ventilator settings, validation runs, including those on animals requiring new gain factors, revealed 91% (137) of the 150 lamb breaths and 94% (141) of the 150 piglet breaths to be within 10% of the integrated PNT standard. Total study time ranged between 30 and 90 minutes. Conclusions: The results of this study suggest that the SPG calibration technique provides an accurate method of calibration of RIP in small, sedated animals receiving controlled mechanical ventilation. Accuracy of gains cannot be assumed following ventilator setting changes.