Harry Pennes, Biological Physicist

I admire scientists who straddle the divide between physics and physiology, and who are comfortable with both mathematics and medicine. In particular, I am interested in how such interdisciplinary scientists are trained. Many, like myself, are educated in physics and subsequently shift focus to biology. But more remarkable are those (such as Helmholtz and Einthoven) who begin in biology and later contribute to physics.

Which brings me to Harry Pennes. Below I reproduce his obituary published in the April 1964 issue of the American Journal of Psychiatry (Volume 120, Page 1030).

Dr. Harry H. Pennes.—Dr. Harry H. Pennes [born 1918], who had been active in clinical work and research in psychiatry and neurology died in November, 1963, at his home in New York City at the age of 45. Dr. Pennes had worked with Dr. Paul H. Hoch and Dr. James Cattell at the Psychiatric Institute of New York Columbia-Presbyterian Medical Center on new techniques of research and medical experimentation.

Dr. Pennes was born in Philadelphia and studied medicine at the University of Pennsylvania where he received a degree in 1942. In 1944 he came to New York to do research at the Neurological Institute. Soon afterward he took a two-year residency at the New York State Psychiatric Institute, and he later joined the staff as Senior Research Psychiatrist. He was also the Research Associate in Psychiatry at Columbia University. At Morris Plains, N. J., Dr. Pennes participated in intensive studies in the Columbia-Greystone Brain Research Project. He did research into chemical warfare from 1953 to 1955 at the Army Chemical Center in Maryland. Later, in Philadelphia, he was Director of Clinical Research for the Eastern Pennsylvania Psychiatric Institute for several years. He subsequently returned to New York a few years ago and resumed private practice.

Before we discuss what’s in his obituary, consider what’s not in it: physics, mathematics, or engineering. Yet, today Pennes is remembered primarily for his landmark contribution to biological physics: the bioheat equation. Russ Hobbie and I analyze this equation in Section 14.11 of Intermediate Physics for Medicine and Biology. In an article titled “Pennes’ 1948 Paper Revisited” (Journal of Applied Physiology, Volume 85, Pages 35-41, 1998), Eugene Wissler wrote:

It can be argued that one of the most influential articles ever published in the Journal of Applied Physiology is the “Analysis of tissue and arterial blood temperatures in the resting human forearm” by Harry H. Pennes, which appeared in Volume 1, No. 2, published in August, 1948. Pennes measured the radial temperature distribution in the forearm by pulling fine thermocouples through the arms of nine recumbent subjects. He also conducted an extensive survey of forearm skin temperature and measured rectal and brachial arterial temperatures. The purpose of Pennes’ study was “to evaluate the applicability of heat flow theory to the forearm in basic terms of the local rate of tissue heat production and volume flow of blood.” An important feature of Pennes’ approach is that his microscopic thermal energy balance for perfused tissue is linear, which means that the equation is amenable to analysis by various methods commonly used to solve the heat-conduction equation. Consequently, it has been adopted by many authors who have developed mathematical models of heat transfer in the human. For example, I used the Pennes equation to analyze digital cooling in 1958 and developed a whole body human thermal model in 1961. The equation proposed by Pennes is now generally known either as the bioheat equation or as the Pennes equation.

So, how did a psychiatrist make a fundamental contribution to physics? I don’t know. Indeed, I have many questions about this fascinating man.

1. Did he work together with a mathematician? No. Pennes was the sole author on the paper. There was no acknowledgment thanking a physicist friend or an engineer buddy. The evidence suggests the work was done by Pennes alone.
2. Did he merely apply an existing model? No. He was the first to include a term in the heat equation to account for convection by flowing blood. He cited a previous study by Gagge et al., but their model was far simpler than his. He didn’t just adopt an existing equation, but rather developed a new and powerful mathematical model. 
3. Was the mathematics elementary? No. He solved the heat equation in cylindrical coordinates. The solution of this partial differential equation included Bessel functions with imaginary arguments (aka modified Bessel functions). He didn’t cite a reference about these functions, but introduced them as if they were obvious.
4. Was his paper entirely theoretical? No. The paper was primarily experimental and the math appeared late in the article when interpreting the results. 
5. Were the experiments easy? No. In fact, they were a little gross. They required threading thermocouples through the arm with no anesthesia. Pennes claimed the “phlegmatic subjects occasionally reported no unusual pain.” I wonder what the nonplegmatic subjects reported?
6. Was Pennes’ undergraduate degree in physics? I don’t know.
7. Did Pennes’ interest in math arise late in his career? No. His famous 1948 paper was submitted a few weeks before his 30th birthday.
8. Did Pennes work at an institution out of the mainstream that might promote unusual or quirky career paths? No. Pennes worked at Columbia University’s College of Physicians and Surgeons, one of the oldest and most respected medical schools in the country.
9. Did Pennes pick up new skills while in the military? Probably not. He was 23 years old when the Japanese attacked Pearl Harbor, but I can’t find any evidence he served in the military during World War II. He earned his medical degree in 1942 and arrived at Columbia in 1944.  
10. Do other papers published by Pennes suggest an expertise in math? I doubt it. I haven’t read them all, but most study how drugs affect the brain. In fact, his derivation of the bioheat equation seems so out-of-place that I’ve entertained the notion there were two researchers named Harry H. Pennes at Columbia University.
11. Did Pennes’ subsequent career take advantage of his math skills? Again, I am not sure but my guess is no. The Columbia-Greystone Brain Project is famous for demonstrating that lobotomies are not an effective treatment of brain disorders. Research on chemical warfare should require expertise in toxicology. 
12. How did Pennes die? According to Wikipedia he committed suicide. What a tragic loss of a still-young scientist!

I fear my analysis of Harry Pennes provides little insight into how biologists or medical doctors can contribute to physics, mathematics, or engineering. If you know more about Pennes’ life and career, please contact me ([email protected]).

Even though Harry Pennes’ legacy is the bioheat equation, my guess is that he would’ve been shocked that we now think of him as a Biological Physicist.