Defibrillation Mechanisms: The Parable of the Blind Men and the Elephant

“Defibrillation Mechanisms:
The Parable of the Blind Men
and the Elephant,”
by Ideker, Chattipakorn, and Gray.

I’ve read many scientific papers, but only one began with an eight-stanza poem about an elephant. Twenty years ago, Ray Ideker, Nipon Chattipakorn, and Rick Gray published “Defibrillation Mechanisms: The Parable of the Blind Men and the Elephant” in the Journal of Cardiovascular Electrophysiology (Volume 11, Pages 1008-1013, 2000). The opening poem by John Godfrey Saxe is reproduced below.

The purpose of the article was to review the different hypotheses that explain defibrillation of the heart. Russ Hobbie and I discuss defibrillation in Chapter 7 of Intermediate Physics for Medicine and Biology.

Ventricular fibrillation occurs when the ventricles contain many interacting reentrant wavefronts that propagate chaotically… During fibrillation the ventricles no longer contract properly, blood is no longer pumped through the body, and the patient dies in a few minutes. Implantable defibrillators are similar to pacemakers, but are slightly larger. An implanted defibrillator continually measures the [electrocardiogram]. When a signal indicating fibrillation is sensed, it delivers a much stronger shock that can eliminate the reentrant wavefronts and restore normal heart rhythm.

Ideker et al. discuss several possible mechanisms that explain how an electrical shock terminates fibrillation. This is a difficult problem, and I’ve spent much of my career trying to figure it out (I guess I’m one of the blind men).

It is possible that most of the electrical and optical mapping studies and the associated hypotheses about the mechanism of interaction of electrical stimuli with myocardium are all valid. It may be that shocks of low strength do not halt the activation fronts of fibrillation; and shocks of higher strength, depending on the circumstances, cause polarization critical points, field-recovery critical points, and/or action potential prolongation; whereas still stronger shocks slightly below the defibrillation threshold cause activation that appears focal on the epicardium either by intramural reentry, by reentry involving the Purkinje fibers, or by true focal activity, perhaps caused by delayed or early afterdepolarization… If so, then just as in the parable of the blind men and elephant, most of the reported studies and proposed defibrillation mechanisms all may be partially correct, yet all may be partially wrong because they are incomplete.

Defibrillation is a fine example of how a knowledge of physics can help solve a critical problem in medicine. Apparently a knowledge of poetry helps too.

The Virtual Museum of Medical Physics

How would you like to visit a museum dedicated solely to medical physics? Well, with COVID-19 raging, we shouldn’t visit any museums in person. But how about visiting a virtual museum? The History Committee of the American Association of Physicists in Medicine has recently opened a Virtual Museum of Medical Physics.

The Virtual Museum was launched to celebrate the 125^th anniversary of the discovery of x-rays on November 8, 1895 by Wilhelm Roentgen. Existing exhibits include those about Roentgen, Fluoroscopy, Mammography, and External Beam Radiotherapy. Exhibits under construction include Computed Tomography, Ultrasonic Imaging, Magnetic Resonance Imaging, and Nuclear Medicine. Once it’s done, the Virtual Museum will be a wonderful adjunct to Intermediate Physics for Medicine and Biology. If you want to contribute to developing an exhibit, contact the Virtual Museum.

Asimov’s Biographical Encyclopedia
of Science & Technology.

And now, the story of how Roentgen discovered x-rays, as told in Asimov’s Biographical Encyclopedia of Science & Technology.

ROENTGEN, Wilhelm Konrad 

German physicist 

Born: Lennep, Rhenish Prussia, March 27, 1845 

Died: Munich, Bavaria, February 10, 1923

...The great moment that lifted Roentgen out of mere competence and made him immortal came in the autumn of 1895 when he was head of the department of physics at the University of Wurzburg in Bavaria. He was working on cathode rays and repeating some of the experiments of Lenard and Crookes. He was particularly interested in the luminescence these rays set up in certain chemicals.

In order to observe the faint luminescence, he darkened the room and enclosed the cathode ray tube in thin black cardboard. On November 5, 1895, he set the enclosed cathode ray tube into action and a flash of light that did not come from the tube caught his eye. He looked up and quite a distance from the tube he noted that a sheet of paper coated with barium platinocyanide was glowing. It was one of the luminescent substances, but it was luminescing now even though the cathode rays, blocked off by cardboard, could not possibly be reaching it.

He turned off the tube; the coated paper darkened. He turned it on again; it glowed. He walked into the next room with the coated paper, closed the door, and pulled down the blinds. The paper continued to glow while the tube was in operation.

It seemed to Roentgen that some sort of radiation was emerging from the cathode-ray tube, a radiation that was highly penetrating and yet invisible to the eye. By experiment he found the radiation could pass through considerable thicknesses of paper and even through thin layers of metal. Since he had no idea of the nature of the radiation, he called it X rays, X being the usual mathematical symbol for the unknown. For a time, there was a tendency to call them Roentgen rays, but the inability of the non-Teutonic tongue to wrap itself about the German œ diphthong militated against that. The unit of X-ray dosage is, however, officially called the roentgen.

The SIR Model of Epidemics

In Chapter 10 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss models described by nonlinear differential equations. We provide several examples in the text and homework problems, but one topic we never address is epidemics.

The archetype mathematical description of an epidemic is the SIR model. A population is divided into three categories, corresponding to three dynamic variables:

    S: the number of susceptible people

    I: the number of infected people

    R: the number of recovered people.

Three differential equations govern the number of people in each category.

    dS/dt = - (β/N) I S

    dI/dt = (β/N) I S – γ I

    dR/dt = γ I

where N is the total population, and β and γ are constants. Rather than analyze these equations myself, I’ll let you do it in a new homework problem.

Section 10.8

Problem 36 ½. The SIR model describes the dynamics of an epidemic. 

(a) Add the three differential equations and determine how the total number of people (S + I + R) changes with time. Does this model include people who die from the disease? 

(b) Write the equation governing the number of infected people as dI/dt = γI (r₀ – 1). Find an expression for r₀. Initially, when S = N, what does r₀ reduce to? This value of r₀ is known as the basic reproduction number. If r₀ is less than what value will the number of infected people decay, preventing an epidemic?

(c) Suppose r₀ is greater than one, so the number of infected people grows and the epidemic spreads. How low must the ratio S/N become for I to begin decreasing? Once this value of S/N is reached, the population is said to have herd immunity and the epidemic decays away.

Results from a numerical simulation of the SIR model, using S(0) = 997, I(0) = 3, R(0) = 0, β = 0.4, and γ = 0.04. By Klaus-Dieter Keller, CC0, https://commons.wikimedia.org/w/index.php?curid=77633956

The SIR model provides insight into the COVID-19 pandemic. It’s a simple model, and many researchers have modified it to be more realistic. Yet, there is value in a toy model like SIR. It lets you to gain intuition about a dynamical system without being overwhelmed by complexity. I always encourage students to first master a toy model, and only then add additional detail.

International Day of Medical Physics

Poster for the
International Day of
Medical Physics.

Tomorrow is the International Day of Medical Physics! This year’s theme is the “Medical Physicist as a Health Professional.”

The second half of Intermediate Physics for Medicine and Biology focuses on medical physics topics, such as ultrasound, radiation therapy, tomography, nuclear medicine, and magnetic resonance imaging. These concepts are central to the work of medical physicists in our hospitals. The COVID-19 pandemic reminds us of how important health care professionals are. They are truly essential workers.

Nine years ago the International Organization for Medical Physics established this annual celebration of medical physics. The IOMP represents tens of thousands of medical physicists worldwide. It’s mission is to advance medical physics practice by “disseminating scientific and technical information, fostering the educational and professional development of medical physicists, and promoting the highest quality medical services for patients.” Below is a message from the President of the IOMP, Madan Rehani.

A message from Madan Rehani, President of the International Organization for Medical Physics.
https://www.youtube.com/watch?v=yFlOi7k8IjA

How can you celebrate this special day? Tomorrow the German Cancer Research Center will host a series of live online lectures about medical physics aimed at a general audience. They will take place 3–5 PM their time, which would be 9–11 AM my time (Eastern Standard Time in the United States). You have to register to get the zoom link, but it’s free.

November 7 was chosen for the International Day of Medical Physics because it’s the birthday of Marie Curie. Below are excerpts from the Asimov’s Biographical Encyclopedia of Science & Technology entry about Curie. Enjoy!

Asimov's Biographical
Encyclopedia of
Science & Technology.

CURIE, Marie Sklodowska (kyoo-ree’) 

Polish-French chemist 

Born: Warsaw, Poland, November 7, 1867 

Died: Haute Savoie, France, July 4, 1934

Asimov begins by discussing Curie’s education.

Marie was unable to obtain any education past the high school level in repressed Poland. An older brother and sister had left for Paris in search of education and Marie worked to help meet their expenses and to save money for her own trip there, meanwhile teaching herself as best she could out of books. In 1891 her earnings had accumulated to the minimum necessary, and off she went to Paris where she entered the Sorbonne. She lived with the greatest frugality during this period (fainting with hunger in the classroom at one time), but when she graduated, it was at the top of the class….

He then describes the scientific discoveries that underlie Curie’s research

The discovery of X rays by Roentgen and of uranium radiations by A. H. Becquerel galvanized Marie Curie into activity. It was she who named the process whereby uranium gave off rays “radioactivity.” She studied the radiations given off by uranium and her reports coincided with those of Ernest Rutherford and Becquerel in showing that there were three different kinds of rays, alpha, beta, and gamma….

Later, he explains the Nobel Prize winning research Marie Curie performed with her husband Pierre.

At the physics school where the Curies worked there was an old wooden shed with a leaky roof, no floor, and very inadequate heat. The two obtained permission to work there and for four years (during which Marie Curie lost fifteen pounds) they carefully purified and repurified the tons of [uranium] ore into smaller and smaller samples of more and more intensely radioactive material… Marie’s burning determination kept the husband-and-wife team going in the face of mountainous difficulties. By 1902 they had succeeded in preparing a tenth of a gram of radium after several thousand crystallizations. Eventually, eight tons of pitchblende gave them a full gram of the [radium] salt…

Asimov ends with Curie’s final years.

Her last decades were spent in the supervision of the Paris Institute of Radium. She had made no attempt to patent any part of the extraction process of radium and it remained in the glamorous forefront of the news for nearly a generation, thanks to its ability to stave off the inroads of cancer under the proper circumstances. But in the end Marie died of leukemia (a form of cancer of the leukocyte-forming cells of the body) caused by overexposure to radioactive radiation.

Marie Curie - Scientist. https://www.youtube.com/watch?v=ZEV4KJBJvEg