The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

DOES A SPEAKING VALVE DECREASE INSPIRED HUMIDITY?

Heidi J. Matthew, RRT; Dean R. Hess, PhD, RRT, FAARC. Respiratory Care Department , Massachusetts General Hospital , Boston , MA .

Background.
It has been our observation that the character of secretions in patients with a tracheostomy is affected by the presence of a speaking valve. We conducted this study to determine whether the amount of humidity delivered through a tracheostomy tube is affected by the presence of a speaking valve. Methods. A Puritan-Bennett 840 ventilator was attached to one chamber of a dual-chambered test lung. A lift bar was placed between the chambers such that the ventilator triggered simulated spontaneous breathing of the second chamber. Two breathing patterns were simulated (as measured with a Novametrix NICO): 1) rate 20/min, tidal volume 0.4 L, I:E 1:2; 2) rate 28/min, tidal volume 0.3 L, I:E 1:2. A tracheostomy tube (7 mm cuffed Portex DIC) was placed through a semi-circular model simulating a patient's neck. The tracheostomy tube was inserted into a short piece of smooth bore 22 mm tube to simulate the patients' trachea. A psychrometer was connected between the distal end of the model trachea and the test lung. Unidirectional valves were used so that inhaled gas passed through the tracheostomy tube, whereas exhaled gas was diverted to atmosphere without passing through the tracheostomy tube. This simulated a patient inhaling through the tracheostomy tube and exhaling through the upper airway. For purposes of the experimental model, the cuff was inflated on the tracheostomy tube so that all of the inspired gas passed through the tracheostomy tube (i.e, no inspired gas from the upper airway). Oxygen concentration in the test lung was measured (Hudson Ventronics). The pyschrometer consisted of 2 thermal probes (Fisher Scientific Dual Thermometer), 1 dry and 1 wet (cotton wetted with water). The temperatures measured by the 2 probes were recorded after 15 min equilibrium. The psychrometric method was used to calculate RH and AH using standard formulae. The apparatus was warmed to 37 C to simulate patient conditions. Three conditions were studied: 1) tracheostomy mask placed over the tracheostomy tube without speaking valve, 2) T-piece with 6-in reservoir attached to the tracheostomy tube, 3) speaking valve (Passy-Muir PMV 2001) attached to tracheostomy tube and tracheostomy collar placed over the speaking valve. Warm humidified oxygen (Hudson Concha Therm III) was supplied to the trachesotomy collar or T-piece at a flow of 40 L/min and 40% oxygen. Psychometric data were collected in triplicate. Results. There was a significantly lower humidity when the speaking valve was used (P < 0.001; see figure). There was no difference in humidity between the 2 breathing patterns (P = 0.54). When the speaking valve was in place, the oxygen concentration measured in the test lung was 2-4% lower than with the other conditions. Conclusions. Delivered humidity may be reduced when a speaking valve is used. We speculate that this may be related to a poor fit of the tracheostomy collar over the speaking valve, resulting in dry air entrainment. This is supported by the observation of a lower oxygen concentration delivered to the lung model with the speaking valve in place. Further work is needed to assess techniques to minimize dry air entrainment with a speaking valve such as a higher gas flow or a better fit of the tracheostomy collar.

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