MedWire News: Smoking and high blood pressure are both known to raise the risk of heart disease and stroke, but researchers have now shown that the two together are particularly damaging.

Their study found that, for each 10-mmHg increase in systolic blood pressure (the top number in a reading), smokers faced an additional 15 percent increase in the risk of haemorrhagic stroke, which occurs when a blood vessel bursts inside the brain.

The team of Australian investigators explain that it is already accepted that smoking and high blood pressure substantially increase the risk of heart and blood vessel (cardiovascular) diseases.

They wanted to find out if the two risk factors have a combined, or synergistic, impact, where the effect of both risk factors interact so that they cause more harm together than simply adding the individual effect of one on top of the other.

The researchers say: "If these two risk factors have a synergistic impact on cardiovascular events, lowering blood pressure and quitting smoking will contribute more to reducing cardiovascular disease than would be expected from ignoring their interaction."

Koshi Nakamura, from the George Institute for International Health in Sydney, Australia, and colleagues studied data on the blood pressure and smoking behaviour of more than 500,000 people.

Records of heart attacks and strokes that occurred over the following 7 years were monitored alongside the participants' blood pressure and smoking behaviour.

In all, there were about 6000 strokes and about 4000 heart attacks.

Patients with high systolic blood pressure had about twice the risk of heart disease of those with low systolic blood pressure, irrespective of whether they smoked.

The researchers also found no evidence of an interaction between blood pressure and smoking on the risk of an ischemic stroke, which occurs when blood supply to the brain is blocked.

But for haemorrhagic stroke, participants with high blood pressure faced a much higher risk if they smoked.

Each 10-mmHg increase in systolic blood pressure was linked to an 81% increase in the risk of haemorrhagic stroke in smokers, compared with a 66% increase in non-smokers.

The target systolic blood pressure for most people is 140 mmHg.

"Although quitting smoking and lowering blood pressure are both crucial for prevention of cardiovascular disease, combining the two could be expected to have extra beneficial effect on preventing haemorrhagic stroke," the researchers say.