Foreign Body Airway Obstruction
Introduction and Outcome Analysis
To develop the best treatment for choking, Patrick took the approach that it was necessary
to consider possible treatments as sequences of specific treatments where a specific treatment
includes backslaps, delivering energy to a foreign body through creation of potential energy
(what became known as the Heimlich maneuver), finger sweeps, or placing the victim upside down
(children). It would be possible to evaluate a specific treatment such as what was to become the
Heimlich maneuver or a backslap using an energy model. But to evaluate treatment sequences
of any specific treatment would require "outcome analysis." Patrick began to develop what later
would become known as "outcome analysis" about 1960. The publication of Fundamentals of
Pattern Recognition in 1972 by Prentice Hall solidified the approach.
In the early 1970's the medical community, for the most part, was not prepared to accept that
treatments could be evaluated using Statistical Pattern Recognition. Instead, the medical community
insisted that Double Blind Studies were needed, but this was not practical in the case of choking.
Patrick indicated that any outcome O could be evaluated for any treatment sequence Ti by
computing an estimate of the conditional probability of the outcome O given the treatment sequence.
Thus, one needed to estimate:
p(O|Ti), for an outcome O for each treatment sequence T1, T2,.... TN.
More precisely, one would need a set of "training samples" x to learn or estimate this outcome
probability, so that the above expression should be replaced by:
p(O|Ti, x), for an outcome O for each treatment sequence T1, T2,.... TN,
given training samples for each treatment sequence.
Using verified training samples from cases sent into talk shows, Patrick was able to estimate needed
probabilities. Dr Richard L. Day reviewed Patrick's outcome analysis and wrote that it was a "well designed
protocol" and "the arithmetic is not faulty." At the same time Dr. Day criticized the American Heart Association
data, writing "If Redding was correct when he belittled his data (Redding's American Heart Association data),
those data should not have been published." This illustrates how the journals controlled by AHA researchers could
publish flawed "peer reviewed" papers while a scientific paper on outcome analysis by Patrick was difficult to
publish.
Dr. Edward Patrick wrote the surgeon general Dr. C. Everett Koop providing Dr. Koop with the literature and research
studies so the United States Department of Health & Human Services could independently review the information.
After reviewing the information, Dr Koop called and wrote Dr Patrick to tell him that he agreed with Patrick and
would so inform the American people.
Patrick Energy Model
To model electomechanical energy conversion, the lungs/chest/diaphragm/muscles/ trachea can
be described as a system where the gas in the lungs has a volume and pressure and is connected
to an airway. In 1972 we considered a primitive model provided by Brown, Jacob and Stark where
the lung, for the purpose of gas flow, was represented as a cylinder with a plunger (or piston) where
the cylinder opened into a smaller cylinder (or tube - the airway). This motivated our construction of
the Patrick Energy Model for Foreign Body Airway Obstruction.
Energy is stored in the lungs as potential energy related to the integral of pressure as it changes
with respect to a change in volume. Through conservation of energy and electomechanical energy
conversion, this potential energy can be converted into kinetic energy of a flying object.
Electromechanical energy conversion was established as a strong discipline in Electrical Engineering
education at the Massachusetts Institute of Technology (MIT) in the 1950 - 1970 time period. For
example, it has been used by Dr. Bose of MIT who's electromehanical energy conversion work is
all around us today - Bose speakers and Bose radios.
In the case of music and speakers, the electromechanical energy conversion is from electrical
energy to acoustic energy. But in the case of foreign body airway obstruction, the Patrick Energy
Model deals with the conversion of Potential Energy, which is stored energy in a compressed gas,
to Kinetic Energy acquired by the obstructing object as it is removed.
Let P be the pressure of gas in the lungs, V the volume of this gas, dV the change in volume
as our maneuver is applied, M the mass of the obstructing object, and S the speed (velocity) obtained
by the object. Assuming no loss of energy to tissues or friction, the expression for the Patrick Energy
Model is:
Potential Energy = Kinetic Energy
∫PdV = (1/2)•M•S2
The speed at which the foreign body is expelled, given this ideal model, is:
S = [2• ∫PdV / M ]1/2
Thus the speed S at which the obstructing object flys out is higher as the mass of the object is smaller.
Pressure Alone Does Not Remove a Foreign Body Obstructing the Airway
The best method to treat foreign body airway obstruction should deliver maximum kinetic energy
to the obstructing foreign body with velocity directed out of the airway. A major cause of confusion
has been the interpretation of pressure time curves with the erroneous belief that pressure alone
removes a foreign body. Experiments have been done on humans to measure the pressure at the
airway outlet (transducer in a face mask or at the end of an endotracheal tube if the individual is
intubated) vs time. Gordon compared the pressure of airway gas produced in response to backslaps
with that produced by the Heimlich maneuver, while Heimlich had plotted the pressure vs time for
the Heimlich maneuver. Gordon presented his findings in 1975 at the National Research Council of
the National Academy of Sciences joint meeting with the National Red Cross, and subsequently he
and his colleagues distributed them elsewhere. They argued that the pressure curves curves show a
higher peak pressure for backslaps than for the Heimlich maneuver and mistakenly concluded that
backslaps therefore are superior to the Heimlich maneuver in expelling foreign bodies.
Also at the National Research Council conference in 1975, Patrick presented a paper showing
the fallacy of Gordon's conclusion that pressure alone removes a foreign body. Citing a model of the
lungs and airway utilizing electomechanical energy conversion which Patrick had developed in 1972,
Patrick explained that energy is required to remove a foreign body. If the treatment is a backslap
or the Heimlich maneuver, then energy results from pressure acting over time. He showed that energy
directed to move a foreign body is proportional to the area within the pressure-time curve (for the
segment of time concerned).
pressure acting over time