Effects of connecting tubes and swivels on the breathing pattern and work of breathing during pressure support ventilation: a laboratory and simulation study

Introduction

The influence of the resistance of endotracheal tubes (ETT) on the additional work of breathing (WOB) has been extensively evaluated. However, not only the ETT resistance causes additional WOB, but also the connecting tubes and swivels (CT+S) that are placed between the Y-piece and ETT. This study was performed to test the hypothesis that CT+S significantly increase the resistance of the breathing circuit with the consequence of increased WOB during PSV.

Methods

Four different types of CT+S (Rüsch, Medisil, Medisize, Mallinckrodt) were ventilated using a sinusoidal flow pattern, and the flow-dependent pressure drop across the CT+S was recorded. Flow dependency of the pressure drop was determined by fitting Rohrer's equation (ΔP = K1*V' + K2*V'²). The resulting coefficients K1 and K2 were used in a mathematical simulation of PSV ventilation to calculate the influence of CT+S on breathing pattern, minute ventilation (VE) and WOB in simulated normal, obstructive and restrictive patients.

Results

The resistance of the different models of CT+S widely varied. The CT+S type used in our ICU had a resistance of 3.1 mbar/l/s (at 1 l/s), comparable with a #9 ETT. Dependent on the patient's disease and muscle strength (Pmus) and on the ventilatory demand, the use of CT+S reduced minute ventilation by up to 13% (Fig. 1). If the additional WOB is accomplished by the patient, he/she has to increase the Pmus by up to 45% (Fig. 2). If the pressure support is increased instead, an additional pressure of up to 37% has to be applied (Fig. 3). In some simulated patients with pulmonary obstructive disease, this additional pressure support caused missed efforts and additional dynamic hyperinflation. Interestingly, in simulated pulmonary restriction, a paradox effect of the additional resistance on tidal volume during PSV was observed: the reduction of peak inspiratory flow led to a delayed cycling of inspiration. As the cycling criterion during PSV (flow drop to 25% of peak inspiratory flow) results in a very short inspiratory time in these patients, a delayed cycling led to an increased inspiratory time and hence to an increased tidal volume.

Figure 1

(abstract P36)

Figure 2

(abstract P36)

Figure 3

(abstract P36)

Conclusion

The resistance of CT+S adds significant load to the respiratory system. CT+S consisting of low resistive parts should therefore be preferred. In restrictive patients, PSV without variable cycling may be an inappropriate mode.