Delivering warm, humidified gas to patients is important during mechanical
ventilation. Heated humidifiers are effective and popular. The humidifying
efficiency is influenced not only by performance and settings of the device
s but the settings of ventilator. We compared the efficiency of humidifying
devices with a heated wire and servo-controlled function under a variety o
f ventilator settings. A bench study was done with a TTL model lung. The st
udy took place in the laboratory of the University Hospital, Osaka, Japan.
Four devices (MR290 with MR730, MR310 with MR730; both Fisher & Paykel, Con
chaTherm IV; Hudson RCI, and HummaxII; METRAN) were tested. Hummax II has b
een developed recently, and it consists of a heated wire and polyethylene m
icroporous hollow fiber. Both wire and fiber were put inside of an inspirat
ory circuit, and water vapor is delivered throughout the circuit. The Servo
300 was connected to the TTL with a standard ventilator circuit. The venti
lator settings were as follows; minute ventilation ((V) over dot(E)) 5, 10,
and 15 L/min, a respiratory rate of 10 breaths/min, I:E ratio 1:1, 1:2, an
d 1:4, and no applied PEEP. Humidifying devices were set to maintain the te
mperature of airway opening at 32 degreesC and 37 degreesC. The greater (V)
over dot(E) the lower the humidity with all devices except Hummax II. Humm
ax II delivered 100% relative humidity at all ventilator and humidifier set
tings. When airway temperature control of the devices was set at 32 degrees
C, the ConchaTherm IV did not deliver 30 mg/L of vapor, which is the value
recommended by American National Standards at all (V) over dot(E) settings.
At 10 and 15 L/min of (V) over dot(E) settings MR310 with MR730 did not de
liver recommended vapor, either. In conclusion, airway temperature setting
of the humidifying devices influenced the humidity of inspiratory gas great
ly. Ventilatory settings also influenced the humidity of inspiratory gas. T
he Hummax II delivered sufficient water vapor under a variety of minute ven
tilation.