As you're sitting there, reading these words, you're probably breathing. Down the air goes, into your lungs, via the upper respiratory tract that includes your mouth and nose; up it comes again, back out the same way, after delivering its precious oxygen payload.
This, we assume, is how most mammals breathe - but maybe it's not actually the complete picture. According to new research, rodents and pigs can also respirate through their butts.
Technically, delivery of oxygen via their rectal intestines suggests a new, enema-like means of ventilating patients under respiratory distress - if the same strange ability can be demonstrated in humans.
Intestinal respiration sounds extremely weird, but it's actually been known about for some time - in fish, anyway. In emergency low oxygen, or hypoxic, conditions, some aquatic animals such as sea cucumbers, freshwater catfish, and freshwater loaches can maximize their oxygen intake by breathing through their guts.
Naturally, this raised the fascinating question of whether other animals can do the same - including mammals. Although it seemed unlikely, a team of Japanese and American scientists led by thoracic surgeon Ryo Okabe of Kyoto University decided to try and find out, in the hopes of determining the feasibility of rectal ventilators for human patients.
"Artificial respiratory support plays a vital role in the clinical management of respiratory failure due to severe illnesses such as pneumonia or acute respiratory distress syndrome," said gastroenterologist Takanori Takebe of the Tokyo Medical and Dental University and the Cincinnati Children's Hospital Medical Center.
"Although the side effects and safety need to be thoroughly evaluated in humans, our approach may offer a new paradigm to support critically ill patients with respiratory failure."
As the team points out, standard medical options for patients with respiratory failure rely on mechanical ventilation or artificial lung systems; however, the current pandemic has resulted in a critical shortage of such devices, and a safe alternative method could provide life-saving additional support for patients in dire situations.
However, this next part is not for the squeamish.
Initially, their research subjects were mice, who were thankfully anesthetized for the next part. The researchers developed an oxygen ventilation system to be inserted anally; they induced hypoxia via tracheal intubation, and compared mice ventilated intestinally to control mice who received no ventilation.
Of the control mice, not a single one survived longer than 11 minutes. This was in marked contrast to the mice receiving intestinal oxygen, 75 percent of which survived for 50 minutes.
That's a fascinating result, but it required abrasion of the intestinal mucosa in order to achieve the most efficient oxygen delivery to the gut lumen. The mouse group that received intestinal ventilation without abrasion had a median survival time of just 18 minutes.
Gut abrasion is unlikely to be feasible for human patients - especially human patients ill enough for intestinal ventilation to be an option - so the team went looking for alternatives.
They turned to liquid perfluorochemicals, a class of chemicals in which the hydrogen atoms have been replaced with fluorine.
These have several properties that make them a good prospect for ventilation, including their high gas solubility, as well as their physical properties. In the past, patients undergoing respiratory distress have been treated using liquid perfluorochemicals by partially filling their lungs to facilitate oxygen transfer, with varying degrees of success. Perfluorochemicals have been deemed clinically safe for this purpose.
So, the team tried enriching a perfluorocarbon with oxygen, and using that to treat mice, rats and pigs.
The mice were placed in a low-oxygen chamber; those treated with the perfluorocarbon ventilation were able to walk for longer than untreated mice, and more oxygen reached their hearts. Rats were also treated to assess whether their bodies absorbed the perfluorochemical, to determine its safety.
Finally, using perfluorochemical intestinal ventilation, respiratory distress was reduced in anesthetized pigs under non-lethal hypoxic conditions. When treated, their skin grew warm and flushed, and their oxygen levels increased, without obvious side effects.
It's unclear if a similar approach would work for humans, but the team is optimistic.
"The level of arterial oxygenation provided by our ventilation system, if scaled for human application, is likely sufficient to treat patients with severe respiratory failure, potentially providing life-saving oxygenation," Takebe said.
The research has been published in Med.