Methodologic issues associated with secretion weight as a dependent variable in research using closed-system suction catheters

Closed-system suction catheters (CSSC) were designed to eliminate the need to disconnect the patient from the ventilator during endotracheal suctioning (ETS). During data collection on an NIH-funded study, it was noted that moisture accumulated on the inside of the CSSC and sleeve when attached to the patient for 30 minutes. Because CSSC are not disconnected, they present unique methodologic problems related to measurement of secretions as a dependent variable in clinical research.

To describe a valid, reliable, and practical method for weighing secretions obtained during ETS using a CSSC; and to determine the change in weight of a CSSC after its attachment for 30 minutes to a mechanically ventilated patient.

After being weighed, a CSSC and sputum trap were attached to the endotracheal tube of a mechanically ventilated adult and remained attached for 43 minutes (30 minutes to allow positive end expiratory pressure and oxygenation levels to return to normal and 13 minutes to mimic the time frame for the ETS procedure used in another study). No ETS occurred. The CSSC and sputum trap were then removed and reweighed.

A convenience sample consisted of 50 adults who were critically ill and mechanically ventilated. Independent variables included tidal volume, pressure support, body temperature, and respiratory rate. The dependent variable was wet weight of the CSSC, determined by subtracting the preprotocol catheter weight from the postprotocol catheter weight. The mean wet weight for all catheters was 0.5142 +/- 0.1215 grams. In a subset of 37 patients, two wet weights (74 paired observations) were determined. The mean wet weight for these catheters was 0.54014 +/- 0.1404 grams. The paired wet weights were statistically different (t = 2.433; df = 36; p = 0.02). Pearson correlation coefficients and beta coefficients were computed. While tidal volume and pressure support were highly correlated (r = 0.678; p = 0.011), there were no other statistically significant associations.

The amount of secretions is a common dependent variable in ETS research. During the time that CSSC are attached to the patient and ventilator, moisture from either the ventilator's humidification system or the patient accumulates in the CSSC. This wet weight is not actually part of the secretions retrieved during ETS and should not be considered in the actual weight of secretions. Further study on the determinants of wet weight is warranted.