Air Detector - Fresenius Medical Care 4008 E Technical Manual

1.2.4

Air detector

The penetration of air into the patient's extracorporeal blood circuit may cause an air embolism.
In order to catch limited amounts of air and to separate accompanying air bubbles, the venous
blood line is expanded (venous drip chamber). A major task of the air detector is to monitor the
filling level in the venous drip chamber.
Ultrasonic air detector
The protection system against air infusion uses the method of ultrasonic transmission. Ultrasonic
converters are attached on either side of the venous bubble catcher. At periodic intervals of
approx. 90 ms, a transmitting resonator generates attenuated ultrasonic vibrations at a natural
resonance of approx. 90 kHz, which are absorbed by a receiving resonator. The amplitude of the
signal received is dependent upon the medium between the converters. Its value is at its
minimum with the bubble catcher empty (air) and at its maximum with bubble-free fluids. The
amplitude decreases with increasing air content (foam). The signal path is fail-safe up to and
including the receiving resonator, i.e. the failure of any component always leads to a smaller
amplitude and, thus, to an alarm. Starting at the receiving resonator, the signal voltage is always
sent onto two independent receiver paths. As soon as the signal is too weak, one of these
receiver paths causes the blood pump to stop and the other the venous line clamp to close.
The and
As long as the
module opens, and the blood level rises. The blood pump runs at reduced speed (180 ml/min). As
long as the
detector module opens, the ventilation pump is running, and the blood level sinks. The blood
pump runs at the preselected speed.
Optical detector
The optical detector serves to detect if there is blood or saline solution or air in the venous return
line downstream of the bubble catcher. In the hemodialysis machine, the hemodialysis phase is
defined by presence of a dark medium and the preparation phase by presence of a clear medium.
Venous pressure measurement
The venous pressure measuring equipment comprises a piezo-resistive pressure sensor provid-
ed on the P.C.B. with following operational amplifier. The pressure-proportional output voltage is
supplied onto the logic P.C.B. in the monitor. There, the pressure is indicated on a quasi-analog
LED scale, and the transmembrane pressure is computed by determinig the difference between
the dialysate pressure and the venous pressure.
1-82
Fresenius Medical Care 4008 4/08.03 (TM)
keys are used to both raise and lower the blood level in the venous bubble catcher.
key is pressed, the venous line clamp closes. The vent valve in the air detector
key is pressed, the venous line clamp remains open. The vent valve in the air