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.