The Solubility of Halothane in Blood and Tissue Homogenates. By Larson CP, Eger EI, Severinghaus JW. Anesthesiology 1962; 23:349–55.
Measured samples of human and bovine blood, human hemoglobin, and tissue homogenates from human fat and both human and bovine liver, kidney, muscle, whole brain, and separated gray and white cortex were added to stoppered 2,000-ml Erlenmeyer flasks. To each flask, 0.1 ml of liquid halothane was added under negative pressure using a calibrated micropipette. After the flask was agitated for 2 to 4 h to achieve equilibrium between the gas and blood or tissue contents, a calibrated infrared halothane analyzer was used to measure the concentration of halothane vapor. Calculated partition coefficients ranged from 0.7 for water to 2.3 for blood and from 3.5 for human or bovine kidney to 6 for human whole brain or liver and 8 for human muscle. Human peritoneal fat had a value of 138. The human blood–gas partition coefficient of 2.3 as determined by this equilibration method was well below the previously published value of 3.6.
At the end of my first year of anesthesia residency at the University of California, San Francisco (UCSF), I decided to take a break from clinical training and accept a research fellowship in the Cardiovascular Research Institute (CVRI), also at UCSF. The CVRI was established in 1958, a year before I came to UCSF, with a mission to undertake innovative research in heart and vascular diseases and their origins. By the time I became a fellow in the institute in 1960, it had attracted a number of creative, young scientists whose areas of research interest included respiratory diseases in adults and infants. Since I was certain that I wanted to pursue an academic career when I finished my clinical training, the opportunity to work with distinguished CVRI scientists for a year was a no-brainer. At the start, I worked in the laboratories of John W. Severinghaus, M.D., and Julius H. Comroe, M.D., and later expanded into the laboratories of Jay Nadel, M.D., and others working on respiratory physiology and function. The fellowship proved to be an exciting, educationally rewarding experience that served as a touchstone for the remainder of my academic career (figs. 1 and 2).
Shortly after starting the fellowship, John suggested to me that he thought the blood–gas partition coefficient for halothane (3.6) as published by British scientists Drs. William A. M. Duncan and James Raventos1 was too high. At the time, Duncan and Raventos were working at Imperial Chemical Industries, a chemical powerhouse and the bluest of British blue-chip companies. Duncan and Raventos were the principal developers of halothane as an anesthetic agent.
John thought that I should check their accepted halothane blood–gas value using equilibration technology. It was well known that it was critical to have accurate partition coefficients for blood and tissues to be able to model and predict reliable values for the uptake and distribution of inhaled anesthetics. Anesthetic solubility in blood and tissues was thus understood to be a primary factor that differentiated one agent from another.2 The result of our work was the 1962 publication in Anesthesiology titled “The Solubility of Halothane in Blood and Tissue Homogenates.”3
In the past, the solubility of anesthetics was determined using gas extraction devices like the Van Slyke–Neill manometric apparatus. The trouble with these techniques was that not all of the anesthetic dissolved in the liquid or tissue could be extracted. This led to higher-than-actual values for the blood–gas or tissue-gas coefficients. In contrast, we found that equilibration of halothane between water, blood, or tissue samples and the overlying gas phase was complete within 2 to 4 h of agitation, so accurate blood–gas values could be obtained.
The human blood was obtained from expired blood bank supplies or from used and discarded cardiopulmonary bypass units. Human tissue was obtained from discarded specimens in the pathology laboratory. Bovine blood and tissues were obtained from the Swift and Co. stockyards in South San Francisco. Workers at the plant were initially perplexed and later amused when I arrived in my Volkswagen “Bug” with buckets to carry the blood and tissues back to the lab on Parnassus Heights. Driving the hills of San Francisco with all this gore in the back would be catastrophic if I should stall the car, hit or be hit by something, or be stopped by the police. Any collision or interruption would require quite an explanation.
Once in the laboratory, I made the measurements over and over again to make certain of their accuracy and reproducibility. This was a tedious but necessary part of the study. Using the values obtained, partition coefficients were calculated for all of the different human and bovine tissues, as well as for hemoglobin, lecithin, olive oil, water, and saline (table 1). Once again, John was right: the accepted blood–gas coefficient was too high. Our other coauthor, Edmond “Ted” I. Eger II, M.D., was an excellent mathematician and was extremely helpful in completing the statistical analyses. Ted had a strong vested interest in this research study since having the correct blood–gas and tissue-gas values was critical for his pioneering work in modeling the uptake and distribution of halothane in human beings.4
After publication of the work in Anesthesiology, I was invited to attend a 2-day symposium on the Uptake and Distribution of Anesthetic Agents at the Barbazon–Plaza Hotel in New York City from April 23 to 24, 1962. The symposium was cosponsored by the Committee on Anesthesia of the National Research Council and the Section on Anesthesiology and Resuscitation of the New York Academy of Medicine. The audience was in the hundreds. All of the heavyweights in the field of uptake and distribution of anesthetics were in attendance. Much to my trepidation, I was the opening speaker at the conference.
In spite of my apprehension, I survived the presentation without a hitch, and a question and comment period followed. Two experts in the field gave brief remarks on their studies of solubility of anesthetics in other tissues. Then came the highlight. The next man who came to the podium was none other than Dr. Duncan, who with Dr. Raventos had published the halothane blood–gas value of 3.6. To my surprise, Dr. Duncan apologized for publishing the wrong value. He and Dr. Raventos had derived it using a now-disproved extraction technique. He reported that he had repeated his analysis of the halothane water-gas partition coefficient using an equilibration technique and had found essentially the same value (0.70) as the one I had reported (0.74) in my halothane solubility study.3 This was a satisfying conclusion to all of the hours spent transporting blood and tissues from the stockyard to the laboratory, agitating and equilibrating the samples, calibrating the halothane analyzer, measuring and remeasuring halothane vapor concentrations, and finally calculating the various partition coefficients. The proceedings from this symposium were published in full in 1963.5
The story of arriving at a correct halothane blood–gas partition coefficient illustrates why scientific accuracy matters. Halothane’s high blood solubility relative to air resulted in a slower rise in alveolar and brain halothane concentration than would occur with less soluble anesthetics like nitrous oxide, ethylene, and cyclopropane (table 2). Our new blood–gas value allowed us to make accurate and clinically relevant predictions about the rate of rise of halothane in the human body during general anesthesia. All subsequent calculations of uptake and distribution of halothane relied on this measure.
Using the equilibration method enabled investigators to determine accurately not only the solubilities of older anesthetics such as diethyl ether and chloroform but also, more importantly, the solubilities of modern agents like enflurane, isoflurane, sevoflurane, and desflurane. The uptake and distribution profiles for these anesthetics were made with greater confidence than would otherwise have been the case.