Low blood pressure may increase the risk of heart attack in patients with coronary disease, suggests a new study.

The team behind the research, from a number of international institutions including Imperial College London, recommend doctors think carefully when treating patients with coronary artery disease (also known as coronary heart disease) for high blood pressure.

A healthy blood pressure is defined as around 120/80. The first number, called systolic pressure, is the force the heart pumps blood around the body. The second number, called diastolic pressure, is the resistance to blood flow in the blood vessels. They're both measured in millimetres of mercury (mmHg). High blood pressure can put extra strain on blood vessels and organs, and is associated with a risk of diseases including heart disease and stroke.

In the study, presented at the European Society of Cardiology Congress, the researchers followed 22,672 patients with coronary artery disease, also known as heart disease, who had high blood pressure and were treated for hypertension. The patients were recruited in 45 countries between November 2009 and June 2010 and followed for up to five years.

The results suggest a blood pressure above 140 mmHg and above 80 mmHg was associated with an increased risk of heart attack and stroke.

However, the team also found that a blood pressure below a systolic blood pressure of 120 mmHg and diastolic blood pressure below 70 mmHg were both associated with an increased risk of mortality, heart attack and hospitalisation for heart failure, independent of potential confounding factors.

Professor Kim Fox, co-author of the study and head of the National Heart and Lung Institute at Imperial said: “This analysis, which involved over 22,000 patients with coronary disease and hypertension, confirms the importance of treating high blood pressure. However it also cautions against over-enthusiastic blood pressure control. Blood pressures generally considered normal, and below 120/70, may be too low in patients with coronary disease and treated hypertension"

In contrast, the results suggested that for stroke, the lower the blood pressure, the better the outcome.

The team say randomised controlled trials will be necessary to confirm the optimal blood pressure values below which harm outweighs benefit. They add this study may impact clinical practice as it strongly suggests caution when treating patients with coronary artery disease with blood pressure lowering drugs.

"Cardiovascular event rates and mortality according to achieved systolic and diastolic blood pressure in patients with stable coronary artery disease: an international cohort study" by Vidal-Petiot et al is published in The Lancet