Pressure Support Compensation for Inspiratory Work due to Endotracheal Tubes and Demand Continuous Positive Airway Pressure

doi:10.1378/chest.93.3.499

Chest

Volume 93, Issue 3, March 1988, Pages 499-505

https://doi.org/10.1378/chest.93.3.499 Get rights and content

We evaluated the use of pressure support to compensate for the added inspiratory work of breathing due to the resistances of endotracheal tubes and a ventilator demand-valve system for continuous positive airway pressure (CPAP). A mechanical model was used to simulate spontaneous breathing at five respiratory rates through 7-mm, 8-mm, and 9-mm endotracheal tubes with and without a ventilator demand CPAP circuit. Added work was measured as the integral of the product of airway pressure and volume during inspiration. Additional work was a function of the tube's size, and each 1-mm decrease in the tube's diameter resulted in a 67 to 100 percent increase in work. Adding the ventilator CPAP circuit further increased work and was responsible for 30 to 50 percent of the total work resulting from a tube and CPAP circuit together. Pressure support was added to a level at which net work on the airway was zero, and a relationship between mean inspiratory flow (VT/TI) and the optimal level of pressure support was established for each endotracheal tube. The inspiratory work of breathing was then measured in normal subjects breathing with and without each endotracheal tube plus the demand CPAP circuit. Work per liter of minute ventilation due to the endotracheal tube and CPAP circuit was increased from 54 to 240 percent over levels measured while breathing through an open airway. For each endotracheal tube and VT/TI, a level of pressure support (range, 2 to 20 cm H₂O) was found which eliminated added work in the spontaneously breathing subject. This level correlated well with that predicted from the data derived using the mechanical model. We conclude that when adjusting for an endotracheal tube's diameter and VT/TI, pressure support can be used to compensate for the added inspiratory work due to artifical airway resistances.

Section snippets

Mechanical Model of Lung and Computation of Inspiratory Work

A reciprocating pump (Harvard Apparatus Co) was used as a mechanical model to simulate a patient's spontaneous breathing effort (Fig 1A). The “mouth” of this model was connected to an endotracheal tube alone or to endotracheal tube and a ventilator demand CPAP circuit. Air flow (V) was measured proximal to the endotracheal tube using a screen pneumotachygraph (Electronics for Medicine, adult-size) which was calibrated to flows of 0 to 1.5 L/s. Volume (V) was integrated from the airway flow

Mechanical Model

The net added inspiratory work (Waw) increased progressively with decreasing size of the endotracheal tube and with increasing VT/TI (Fig 3). Each 1-mm decrease in the tubes diameter resulted in a 67 to 100 percent increase in work. Adding the ventilator circuit increased the work further. The proportion of added work due to the endotracheal tube alone, when compared to the added work for the tube and ventilator together, was a function of the tubes size. On average, a 9-mm tube contributed 50

DISCUSSION

This study confirms that endotracheal tubes and a ventilator demand CPAP system can markedly increase the inspiratory work of breathing. The additional work rate (WI/L) due to these added resistances increases with increased flow demand and decreased internal diameter of the tube. Pressure support can be used to compensate for this added work when the level is adjusted for the endotracheal tube's diameter and mean inspiratory flow.

The effect of the endotracheal tubes size on airway resistance

ACKNOWLEDGMENT

We thank Mr. Thomas Vagedes, Mr. Andres Obregon, Deborah Foster, R.R.T., and Robert Scott, C.R.T.T, for their eager assistance.

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Presented in part at the 52nd Annual Scientific Assembly, American College of Chest Physicians, San Francisco, Sept 23, 1986.

Manuscript received May ~; revision accepted August 26.

View full text

Chest

Pressure Support Compensation for Inspiratory Work due to Endotracheal Tubes and Demand Continuous Positive Airway Pressure

Section snippets

Mechanical Model of Lung and Computation of Inspiratory Work

Mechanical Model

DISCUSSION

ACKNOWLEDGMENT

Chest

Chest

Chest

Work of breathing through different sized endotracheal tubes.

Crit Care Med

The extra work of breathing through adult endotracheal tubes.

Anesth Analg

A comparison of intermittent mandatory ventilation systems.

Crit Care Med

Demand and continuous flow intermittent mandatory ventilation systems.

Chest

Objective indications for respiratory therapy in post-trauma and postoperative patients.

Am J Surg

Prediction of respiratory muscle fatigue by measurements of the work of breathing.

Surg Gynecol Obstet

Improved technique for estimating pleural pressure from esophageal balloons.

J Appl Physiol