Seeing top pros like Tadej Pogačar and Jonas Vingegaard engaging in the legal but controversial practice of carbon monoxide rebreathing during this month’s Tour de France has caused quite a stir.
So, what exactly is CO2 rebreathing?
The carbon monoxide rebreathing method is a technique that can be used to quantify hemoglobin mass (Hbmass) in humans. The method is based on the dilution principle where a given quantity of tracer (CO) is administered to the circulation, and where the subsequent concentration of CO in the blood allows for calculation of Hbmass. From this red blood cell volume, plasma volume and total blood volume are easily calculated.
CO re-breathing has been used to track changes in Hbmass in a multitude of scientific settings including altitude exposure, altitude training, exercise training, heat training, bed rest as well as with injections with recombinant human erythropoietin (EPO). Recently, CO re-breathing has also found its ways into clinical settings with measurements performed in patients with coronary artery disease, kidney disease and in preoperational or intensive care patients.
Absolute blood volumes were determined in animals by means of bleeding in the late 1800s. This approach was for obvious reasons not applied to humans. The use of carbon monoxide (CO) as a tracer to determine blood volume was first proposed by French scientists Grehant and Quinquaud in 1882. Soon after a usable set-up for human use was developed by eminent Oxford scientists John Haldane and Lorrain Smith and presented in the Journal of Physiology in 1900 (in which they also report on the O2 content of arterial blood).
With the goal of ensuring that all of the inhaled CO would also bind to hemoglobin and become distributed equally throughout the circulation, Haldane and Smith performed the measurements using an impressive 2 hours of re-breathing. During these early days of CO re-breathing Haldane would later report that studies were finally abandoned when the soda lime (used to eliminate CO2 in the rebreathing circuit) would make its way through the fabric of the re-breathing rubber bag, something that fortunately does not occur today.
Despite these and other technical limitations, their measurement precision was an impressive ~8%. Haldane and his Oxford colleague Claude Douglas (later famous for among others the Douglas bag method) applied their CO re-breathing method to study the effects of altitude exposure on hemoglobin mass during research expeditions to Mount Teide on Tenerife in 1910 and to Pikes Peak in Colorado in 1911. The results from the latter study suggested a 30% increase in hemoglobin mass, which however likely was a measurement artifact associated with the notorious difficulties associated with quantifying CO in blood at that time (they did not have automated blood gas analyzers back then…).
In the late 1940s, the brilliant but somewhat forgotten Swedish scientists Kjellberg, Rudhe and Sjöstrand determined blood volumes in humans using the CO re-breathing method and related this to cardiac output and exercise capacity. This was decades ahead of the later so famous “Scandinavian exercise physiology school”.
Some 50 years later the method received a facelift by Australian researchers Burke and Skinner in that they established a somewhat handier set-up.
So, what’s the controversy?
The amount of CO administrated during the test approximates the levels reached after smoking one cigarette. While cigarette smoking is certainly not recommended, the health concerns associated with smoking just one cigarette are considered to be limited. Also, it should be noted that the re-inhalation technique doesn’t provide a benefit in itself but is a tool to measure the effects achieved with the concentrations. Regarding inhaling carbon monoxide directly to achieve what some already call superaltitude, it is a novel approach that is being researched to quantify its effects. The World Anti-Doping Agency (WADA) has already taken action and is conducting its own investigations in a procedure reminiscent of the one that banned the inhalation of Xenon gas in 2014.
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