The Science Journal of the American Association for Respiratory Care

1995 OPEN FORUM Abstracts

Effects Of Changes In Inspiratory Rise Time On Imposed Work Of Breathing In A Pediatric Animal Model

Mark J. Heulitt, M.D., Shirley Holt, RRT, Sterling Wilson MS Critical Care Medicine and Respiratory Care Services, Arkansas Children's Hospital, Little Rock, AR.

INTRODUCTION: The new generation of ventilators allows clinician control of the inspiratory rise time (IRT). The IRT controls the initial flow rate of the ventilator breath and can be adjusted between 0% and 10% on the Siemens' Servo 300^{TM}. It has been shown in adults that shorter inspiratory rise times decreased work of breathing (WOB), but it has been speculated that in patients with smaller endotracheal tubes, shorter rise times may increase WOB.

Methods: We performed a prospective randomized controlled cross over study in 3 young lambs. Lambs were randomized to either IRT 1% or IRT 10% with pressure support of 5 cm H_2O. During each IRT trial, the animal was studied with and without an externally placed resistor (+R) that reduced the size of the endotracheal tube lumen. Each animal was used as its own control and studied on at least 8 separate days with both IRTs. WOB was measured with a Bicore^{TM} monitoring device as WOB of the animal (WOBp), pressure time product (PTP), peak inspiratory flow rate (PIFR), esophageal pressure (DPES), and expiratory resistance (RAWE) for each breath during the experiment. A Wilcoxon signed rank sum test was utilized for statistical analysis.


IRT 1% IRT 10%pIRT 1%+R IRT 10%+R p

(SD) (SD) (SD) (SD)

WOBp (J/L)0.23±0.540.36±0.58< 0.0010.44±0.580.57±0.59< 0.001

Pes (cm H_2O)4.2±3.2 5.9±4.4 < 0.0016.4±5.2 8.6±7.9 < 0.001

PTP (cm62.6±58 86.5±74< 0.001 87.7±82 126±94 < 0.001


PIFR (L/min) 0.36±0.120.33±0.11< 0.0010.28±0.090.27±0.07< 0.001

RAWE (cm 21.9±4.4 21.9±4.8 NS 44.6±4.1 43.3±6.8 NS


Conclusions: WOB is lower in animals supported with pressure support when IRT 1% is compared to IRT 10%. This difference is maintained when resistance is increased. We speculate that in pediatric patients with small endotracheal tubes and increased WOB, utilizing shorter IRT may reduce WOB.


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