An acoustical implementation of the Esophageal Detector Device
August 6, 1996
The EDD works on the principle that due to the rigid structure of the trachea air can flow freely into an endotracheal tube placed there while air flow is impeded when the endotracheal tube is inserted into the esophagus because of its soft structure which allows it to collapse around the end of the tube. The suction used to detect this difference in impedance is generally supplied by a syringe or a collapsible rubber bulb. SCOTI, U.S. patent #5,331,967, works on the same principle but, instead of measuring the energy required to suck air into the tube once, does it about 600 times per second. This device has a speaker connected to the end of the endotracheal tube so that it propagates a sound wave down the tube. Each time the unit is used it self configures and selects the exact frequency which gives the greatest sensitivity between an open endotracheal tube and a closed one. Then it locks on this frequency and continuously determines the impedance to air movement at the end of the endotracheal tube just as the EDD does. To repeat a measurement with the EDD one has to manually recompress the rubber bulb while SCOTI automatically repeats its measurement as the cone of the speaker moves back and forth. The cone of the speaker acts as a kind of automatic rubber bulb which is electrically controlled and monitored. From a physics standpoint what this device is doing is exactly the same as the EDD, it just repeats the test about 600 times per second. Because a person can not perceive 600 changes per second the output is modified to update only 15 times per second which still feels instant to the user.
SCOTI continually monitors the acoustical impedance of the end of the tube and displays this with a number from "00" to "99". To assist the operator it also displays a red light indicating esophagus if the number is 15 or less and a green light indicating trachea if the number is 20 or greater. There is also an audible alarm which indicates the color of the light. This device is automatically configured each time it is used to adjust itself to the exact parameters of the actual endotracheal tube to be used. If, for any reason, it fails to configure adequately it turns itself off to prevent any chance of misinterpretation. Between the speaker and the endotracheal tube is a disposable bacterial filter and a 70 cm corrugated tube which allows for unencumbered movement of the endotracheal tube during intubation and protects against cross contamination between patients. This disposable circuit is provided assembled and has only one style and will be supplied by the manufacturer of SCOTI.
The EDD and SCOTI can both be viewed as blocked tube
detectors. Their operation can be tested in the lab by simply testing them
with the end of the endotracheal tube left open and with the end of the
endotracheal tube blocked with two fingers. It is highly advisable to check
the placement of the endotracheal tube before forcing air into the
endotracheal tube as is required to use any CO2 monitoring device,
such as capnography, or even to listen for breath sounds. Forcing air through
an endotracheal tube which may indeed be in the esophagus heavily increases
the chance of regurgitation and subsequent aspiration of gastric contents.
This accepted method of using capnography as the primary means to verify
tube placement actually does something that creates a new risk.
2) Dr. D. Murray, Dr. Michael E. Ward and Dr. J.W. Sear of the Nuffield Department of Anaesthetics in Oxford, England wrote a report 11/10/94 of a clinical trial that was done at The John Radcliff Hospital with 90 patients. These patients were all intubated in the esophagus and also in the trachea with the results recorded. The results indicated a confidence level greater than 98% for both the esophagus and the trachea. This report was published in Anaesthesia.
3) Dr. B. Martin and Dr. E. Wheatley wrote an abstract and a paper "Clinical Evaluation of the ĎSonar Confirmation Of Tracheal Intubationí Device" 9/28/95 which describes a clinical trial that was done at St. Bartholomewís Hospital in London, England with 125 patients. In this study 51 patients were intubated in the esophagus and also in the trachea with the results recorded. A further 74 patients were intubated in the trachea only. The results indicated a confidence level greater than 98% for both the esophagus and the trachea. This paper was presented by Martin at the 11th World Congress of Anaesthesiologists in Sydney, Australia in April, 1996 and the abstract is included in their publication of abstracts.
SCOTI, while basically an automatic version of the
EDD, gives an added benefit of indicating tube position while the intubation
is taking place where the EDD can only be used to confirm tube placement
after the intubation. In fact, while using SCOTI the operator never even
fully inserts the endotracheal tube into the esophagus since as soon as
it starts to enter the esophagus the alarm starts to beep and the light
turns red signaling the operator to reposition the tube. While it may seem
obvious to compare the times required to use and interpret the results
for capnography, colormetric CO2 monitors, EDD and SCOTI this
isnít really the relevant time. The important issue is the cycle time between
intubation attempts when there has been an esophageal intubation. This
would be the time from when the device indicated that there was an esophageal
intubation until the patient had been intubated again and the device had
given its result a second time. For capnography, for instance, this would
include the times needed to disconnect the airway circuit, reinsert the
laryngoscope, reintubate, reconnect the airway circuit and interpret the
results of the capnograph. This time could be substantial for a patient
who isnít breathing. For SCOTI this time is reduced to less than 1 second
since this device indicates tube position while the tube is being inserted
and the laryngoscope would not even be withdrawn until the tube was correctly
placed. In the event of a difficult intubation or with an inexperienced
operator there could be several of these cycles before the patient was
successfully intubated. If a test was done using all of these methods,
SCOTI would always give its result first. In the case of emergency intubations
time is even more critical and the experience levels are often lower. If
there is a cardiac arrest then intidal CO2 monitoring techniques
do not work and time is really critical.
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