Are blood pressure and diabetes additive or synergistic risk factors? Outcome in 8494 subjects randomly recruited from 10 populations.

It remains unknown whether diabetes and high blood pressure (BP) are simply additive risk factors for cardiovascular outcome or whether they act synergistically and potentiate one another. We performed 24-h ambulatory BP monitoring in 8494 subjects (mean age, 54.6 years; 47.0% women; 6.9% diabetic patients) enrolled in prospective population studies in 10 countries. In multivariable-adjusted Cox regression, we assessed the additive as opposed to the synergistic effects of BP and diabetes in relation to a composite cardiovascular endpoint by testing the significance of appropriate interaction terms. During 10.6 years (median follow-up), 1066 participants had a cardiovascular complication. Diabetes mellitus as well as the 24-h ambulatory BP were independent and powerful predictors of the composite cardiovascular endpoint. However, there was no synergistic interaction between diabetes and 24-h, daytime, or nighttime, systolic or diastolic ambulatory BP (P for interaction, 0.07P0.97). The only exception was a borderline synergistic effect between diabetes and daytime diastolic BP in relation to the composite cardiovascular endpoint (P=0.04). In diabetic patients, with normotension as the reference group, the adjusted hazard ratios for the cardiovascular endpoint were 1.35 (95% confidence interval (CI), 0.87-2.11) for white-coat hypertension, 1.78 (95% CI, 1.22-2.60) for masked hypertension and 2.44 (95% CI, 1.92-3.11) for sustained hypertension. The hazard ratios for non-diabetic subjects were not different from those of diabetic patients (P-values for interaction, 0.09P0.72). In conclusion, in a large international population-based database, both diabetes mellitus and BP contributed equally to the risk of cardiovascular complications without evidence for a synergistic effect.