New way of making artificial lungs more efficient

Artificial lungs are cumbersome beasts, but they can be a lifesaver for people with serious respiratory diseases or lung damage. Sitting outside the body, they take over the task of soaking up excess carbon dioxide from the blood and infusing it with oxygen.

The problem is that existing devices are inefficient and so have to siphon off large amounts of blood. For this they need bulky tubes, which in turn require a nasty arterial puncture to plumb them into the patient's vascular system. Now William Federspiel of the University of Pittsburgh in Pennsylvania has come up with a way of making artificial lungs more efficient, using an enzyme that helps to remove CO2 from the blood. The result, he hopes, will be a smaller device that can be used in a wider range of patients.

Artificial lungs consist of a bundle of hollow polymer fibres with the texture and diameter of a fishing line, which are bathed in blood diverted from the patient's bloodstream. Oxygen pumped through the fibres diffuses through pores in the fibre's walls and into the blood, while CO2 diffuses in the opposite direction. The oxygen-rich blood is then returned to the body (see Diagram).

One reason they are inefficient is that 90 per cent of the CO2 in blood is stored in the form of dissolved bicarbonate, which has to be broken down inside red blood cells. The freed CO2 then has to be transported in the blood to the fibres, and this requires a large volume of blood to pass over the fibres.

Federspiel has discovered that by coating the porous fibres with the enzyme carbonic anhydrase, which catalyses bicarbonate breakdown inside red blood cells, bicarbonate can be broken down into CO2 by the fibres themselves. This should cut down the rate at which blood needs to be fed to the artificial lung. Having less blood flowing over the tubes could also cut the risk of clotting and of the blood causing inflammation when it returns to the body.

To put this to the test, Federspiel flowed bicarbonate solution around both coated and uncoated fibres while oxygen was pumped through them. Sure enough, the coated fibres absorbed CO2 75 per cent faster than the uncoated ones (Biomaterials, DOI: 10.1016/ jbiomaterials.2007.03.021). His next step will be to test the equipment with real blood, and figure out how much enzyme to attach to the tubes.

The smaller units that Federspiel envisages could be used to treat a much wider range of conditions, such as temporary lung infection caused by emphysema or lung damage resulting from smoke inhalation. At the moment, people with these conditions are put on a ventilator, but the pressure this exerts on the lungs can cause further damage by stretching the tissue. “The more you can get the invasiveness down, the more applications for the device,” Federspiel says.

“This really is a good idea,” says Robert Bartlett, a thoracic surgeon at the University of Michigan, Ann Arbor. Joseph Zwischenberger at the University of Texas, Medical Branch, I Galveston, is also enthusiastic, but cautions that the technique may not work as well with real blood as it has in Federspiel's tests. “It could cause inflammation or decrease inflammation, it could cause clotting or decrease clotting, we just don't know,” he warns.