Diagnosing internal bleeding is difficult because of the way the body reacts to it. "You'd think it'd be easy to spot, but it's far from it," says John Heyworth, president of the British Association for Accident and Emergency Medicine and a consultant at Southampton General Hospital. "We desperately need a good way to do this." A drop in blood pressure is one sign, but by the time that happens it's often too late.
The body compensates for blood loss by constricting certain blood vessels in order to channel blood only to the major organs and maintain blood pressure. But this simply puts off the inevitable. "You only see a drop in blood pressure once they've lost 25 to 30 per cent of their blood," says Heyworth. "The blood pressure then drops through the floor, and by that time, they're very sick."A steadily increasing pulse rate is another sign of internal bleeding, but it's unreliable, he says, since pain and shock can also increase it.
Now there's a device that should offer a quick and easy way to spot internal bleeding. It works by measuring the concentration of oxygen in the blood in the veins and arteries of the eye.
Blood pumped into the eye through the ophthalmic artery is at least 95 per cent oxygenated-a level that remains steady even if the person is suffering a haemorrhage. But the oxygen content of blood leaving the eye is very sensitive to the amount of blood in circulation, says Chris Gregory, head of biomedical devices at Sarnoff, the technology development company based in Princeton, New Jersey, where the gadget was invented. The eye tissue will always try to consume a constant amount of oxygen. So if there's less blood available, the tissues will have to strip more oxygen from the blood as it circulates. The result is an instant drop in blood oxygen level in the veins of the eye.
To measure blood oxygen levels, the device bounces four different colours of low-powered laser light off the patient's retina. Reflections from blue and green laser light are used to map the blood vessels on the retina and distinguish between veins and arteries, which differ slightly in colour.
The other two lasers produce red and near-infrared light. Some of this light is absorbed by blood in the vessels rather than being reflected back off the retina. Because oxygenated blood is a brighter red than deoxygenated blood, it absorbs light differently. The device uses a microcomputer to work out the ratio of oxygen in the blood in the different vessels. Any drop in blood oxygen level in the veins will be immediately apparent, indicating internal bleeding.
Gregory says his "retinal oximeter" could be just what staff in the emergency room need. He is working on miniaturising the device, which is still too large to be carried by hand, and arranging clinical trials.
Heyworth says the device could make a huge difference. Once you know a person has internal bleeding, you can X-ray them to find out where the haemorrhage is, he says. At the same time, the person can be given a transfusion to top up their blood level or intravenous saline to bulk up their blood. These techniques are often successful, says Heyworth, but staff need to know when they are needed.
Author: Ian Sample
New Scientist issue: 8 June 2002
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