Measurement of Neonatal Residual Volume - Project Failure Report

Date: Thu, 29 Jan 1998 10:23:24 -0800 (PST)
From: Lee Hirsch <leehirsch@yahoo.com>
Subject: Project Report
To: kingpk@vuse

Measurement of Neonatal Residual Volume

By:
Tim LeCroix
Lee Hirsch
Timothy Ho

    Our group enthusiastically accepted this project because it was different than most of the other posted projects. When students enter the biomedical engineering curriculum as freshmen they think of designing prosthetics or imaging equipment. But as we progress through the curriculum we see that BME's do much more; they strive to design or reengineer any facet of the medical field which may need fixing or improving. Maybe the protocol for hospital patient admissions is inefficient or the orthopedics department needs a user-friendly database for examining case studies-Biomedical Engineers do all of these things. And it is typically the latter sorts of projects that are requested of BME's for senior projects.


    But the neonatal project was unique, and it caught our eyes. The sponsoring doctor wanted a couple of BME's to find a technique of measuring the dynamic change residual volume of neonatal lungs in order to properly adjust their respirator settings. He proposed a technique called Electrical Impedance Tomography (EIT). This made the project even more exciting since our group mates were very interested in imaging. But this is where we screwed up. We were so fascinated with the idea of coming up with either a simulation or working model of this project that we did not bother to perform a sufficient amount
of preliminary research. The only thing we knew was that we wanted this project! However, if we would have shopped around, talked with professors, and done some reading we might have saved ourselves a lot of time (a whole semester in fact).

So here were the blunders of our project:

The Physiology

    If our group would have bothered to do some simple early research on the neonate's lung physiology, then we would have seen the pitfalls of this project. The average adult lung has a volume of about 7 liters. A neonate's lung, however, is about 64 times less; this is roughly 100 ml's. The ratio of total lung capacity/residual volume does not change much throughout development; as a result, the average residual volume of a neonate would be about 12 ml's. This is a very small amount of
volume to detect-even with a spirometer. Moreover, this volume has to be distinguished from the other present volumes such as anatomical dead space. If we had this knowledge before accepting the project, then we would have seen that this volume would be extremely difficult to detect in vivo no matter what approach we took.

The EIT

This technique was recommended because it was known that the impedance across the chest of a patient is proportional to the total lung volume. Therefore, it would make sense that you could possibly extrapolate the residual volume from the impedance remaining after expiration. But we later found out that although the lung volume is proportional to impedance, the change in lung volume was not proportional to the change in impedance. In other words, the EIT may give a static representation of the lung volume, but it could not give us the dynamic measurements that we wanted.

So after blindly accepting the project, we began our research and quickly discovered a couple of other complications. First of all, there are two types of EIT's: 1) Single and 2) Multifrequency. The latter allows the image to have a full impedance spectrum at each pixel. The first type of EIT was appropriate for our purposes because we weren't as interested in an image as we were in a correlation between impedance and volume-the second type would have given us a lot of useless data. The single frequency EIT consists of a 16 electrode system that surrounds the circumference of the infant's chest. Each electrode takes a turn sending out a 20 kHz signal while the other fifteen measure the impedance relative to the signal source. After each electrode has been a signal source, background projection may be used to calculate the impedances throughout the infant's chest.

A fundamental problem with this design is that the electrodes are placed onto the chest. Therefore, there is no constant reference frame. For example, a MR scanner has a coil around the patient that emits/detects the radio frequencies. But with the electrodes placed on the chest, this reference frame moves with the patient. This is a problem due to the fact that the patient "flattens" when they lie down; as a result, the EIT would pick up less resistance between electrodes and mistakenly detect this phenomena as a decrease in volume.

The EIT uses electricity to measure volume. Unfortunately, the body happens to be an excellent conductor of electricity, but in variable amounts. Electricity will follow the path of least resistance, which, as Dr. Galloway noted, will cause the EIT current to flow in non linear paths through the chest cavity. This causes mathematical problems when calculating impedances that are not encountered with the highly linear x-rays of the CAT scanner. Blood volume is largely responsible for changing the impedance measurements of the EIT. One final bungler of the EIT measurements is the heart. The heart is, of course, a large source of electrical activity that often gives rise to considerable noise in the left lung measurements.

What Now?

After coming to the resignation that the EIT would not work, we tried to come up with another solution to the problem. We consulted with various Dr.'s, Professors, and engineers about solutions, but none of them fared fruit. Regardless, here are a couple of other solutions that we brainstormed.

Deuterated Water Vapor

This technique would have worked in a similar fashion to the He dilution method. The helium dilution method is the widely used
method for measuring residual volumes. It uses simple mass balance equations. The patient inhales a known concentration of He gas after  forced expiration. The volume and concentration of He in the exhaled gas is then measured. What ever He gas was not exhaled will be residual volume (subtracting anatomical and instrument dead space).

We proposed a deuterated water vapor method to perform this same operation in the hopes that the deuterated water could be measured with greater precision than the He gas concentration. Deuterated water has a strong absorbance in the IR region of the spectrum.   Therefore a laser/spectrophotometer setup could closely monitor the concentrations of inhaled/exhaled deuterated water vapor. But there were two problems with this solution. 1) Even if it were more precise than its predecessor (He dilution), it would still be nondynamic. 2) After talking with Dr. Harris we found that the water vapor would adhere to the lung walls (unlike a gas), which would therefore include surface area measurements as well.

Ultrasound

Next, we thought that ultrasound might provide an image of the neonatal lung. But problems arise when the acoustic wave travels past the skin and into the ribs. These bony structures have a higher acoustical impedance than the flesh, and as a result will reflect a large portion of the sound waves. Therefore, it is difficult to receive a decent image of the lungs behind the ribs. And even if we could, we are left with the hard time of discerning between different volumes within the image, such as anatomical dead space, alveolar space, and residual volume.

Sound Resonance

One other idea that we did not really pursue was the acoustic resonance approach. Imagine that the lungs were like hollow pipes; then hypothetically, if you could emit the proper frequency the sound would resonate within this volume. Of course, the lungs are in no way shaped like a cylindrical pipe. Moreover, the perimeter of the lungs is surrounded by a rough surface of surfactants, branches, and alveoli which will deflect the sound waves in an erratic matter.  Nevertheless, Dr. Galloway thought that this was a fun idea.

So here we are at the end of a whole semester with no progress on our project. Left with no other alternative, we had to abandon it. But with the benefit of hindsight, we see how easy it would have been to avoid the errors of this project. The first thing anybody should do before accepting a project is perform a little preliminary research on the subject; make sure that the project is suitable for undergraduate work instead of a Ph.D. thesis. After doing much research on the EIT, we talked with Dr. Galloway about the idea. He told us that Ph.D's have been working with the EIT for many years and have yet to produce
a reliable instrument. If we had this knowledge from the start, we would have saved ourselves a lot of time.

Another thing to avoid is narrowing your scope too soon. We painted ourselves into a corner very quickly by hastily choosing the EIT as our measurement device. We came up with the other measurement ideas after we spent considerable time on the EIT. Even though none of the other ideas worked either, the point is the same--do a lot of brainstorming at the beginning of the project. Then select the best approach; do not bet on the first horse that you find.


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