Gas induces 'suspended animation'
Sewer gas can induce 'suspended animation' in mice, say US scientists, and may help to preserve organ function in critically ill patients. Hydrogen sulphide, a toxic gas that smells of rotten eggs, occurs naturally in swamps, springs and volcanoes.
But in mice, it was found to slow down heart rate and breathing and decrease body temperature, while keeping a normal blood pressure. The results were presented at the American Physiology Society conference. In the study carried out at Massachusetts General Hospital in Boston, mice were administered the gas at a concentration of 80 parts per million - a tenth of the dose which is lethal in humans.
The problem with hypothermia is it's not that easy to cool down the human body so if we can find another method to inhibit metabolism that would be very useful The researchers reported that the heart rate fell from 500 to 200 beats per minute and respiration fell from 120 to 25 breaths per minute. Core body temperature also fell from 39 to 30 degrees C. Despite the reduction in heart rate the blood pressure of the mice did not drop, which tends to happen with other techniques such as lowering body temperature.
When the researchers repeated the experiment at a higher room temperature, the heart and respiratory rate still fell significantly. The effects of the gas seemed to be reversible with the mice returning to normal two hours after the mice started to breathe normal air again. Previous research had shown the ability of hydrogen sulphide to induce a state of hibernation in mice but the effects on the cardiovascular system were unknown.
Some anaesthetics and sedatives can be used to slow down metabolism in the brain but currently the only way to protect other organs is to cool the body and induce hypothermia.
Dr Fumito Ichinose, assistant professor of anaesthesia at Harvard Medical School said that if the effects of hydrogen sulphide was confirmed in larger mammals it could be useful in helping to sustain the functionality of organs in patients undergoing cardiac surgery or in patients with severe trauma. "There was a large study published last year of patients who had cardiac arrest. They kept them in a hypothermic state for a while and the outcome was much better.
"The problem with hypothermia is it's not that easy to cool down the human body so if we can find another method to inhibit metabolism that would be very useful." Dr Ichinose added that the findings would need to be replicated in larger animals such as pigs as mice may be more susceptible to induced hibernation. He added that the safety margin was small. However, he said it was possible that a dose lower than 80 parts per million might also be effective.
But Dr Chris Pomfrett, lecturer in neurophysiology applied to anaesthesia at the University of Manchester said further studies were needed to clarify whether the reduced blood pressure and respiration were really associated with hibernation or whether the findings were a result of poisonous effects of the gas. "My big question about this work is: is it reducing brain metabolism or simply having a toxic effect on the brain stem? "Although the mice appeared normal they didn't look to see if there was any damage to the mice post-mortem. "I would also do an electroencephalogram to measure brain activity in the mice."
He added that there would be lots of problems with using hydrogen sulphide in clinical practice because it was so toxic. "This is interesting and certainly something that should be investigated but I have reservations about its use in humans," he said.