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Nighttime and 24-hour recordings of systolic blood pressure (BP) came out on top in their association with cardiovascular risk, according to a large longitudinal study.

Both indices had significant associations with mortality and cardiovascular events (counting cardiovascular mortality, non-fatal coronary events, heart failure, and stroke) over a median 13.8 years of follow-up:

• Nighttime systolic BP and mortality: HR 1.23 (95% CI 1.17-1.28)
• Nighttime systolic BP and cardiovascular events: HR 1.36 (95% CI 1.30-1.43)
• 24-hour systolic BP and mortality: HR 1.22 (95% CI 1.16-1.28)
• 24-hour systolic BP and cardiovascular events: HR 1.45 (95% CI 1.37-1.54)

"These associations remained significant after adjusting for conventional and automated office BP and after adjusting for the daytime BP and dipping ratio or status," reported Jan Staessen, MD, PhD, of the University of Leuven, Belgium, and IDACO collaborators in .

"These findings were also largely consistent for secondary outcomes and in sensitivity analyses performed to evaluate the influence of antihypertensive drug treatment at baseline, the use of fixed clock-time intervals vs the diary method to define day and night, and the weight of different cohorts in the overall pooled results," they stated.

However, adding 24-hour or nighttime systolic BP to a model that already has another BP index -- conventional office BP, automated office BP, or daytime ambulatory BP -- only slightly improved model performance. The area under the curve (AUC) of 0.84 increased by 0.0031 with the addition of 24-hour results and by 0.0075 with nighttime recordings for the cardiovascular endpoint.

On the other hand, incorporating other systolic BP indices to models with 24-hour or nighttime systolic BP did not improve model performance.

"Thus, 24-hour and nighttime blood pressure may be considered optimal measurements for estimating CV [cardiovascular] risk, although statistically, model improvement compared with other blood pressure indexes was small," Staessen's group concluded.

IDACO investigators followed more than 11,000 adults (median age 54.7; 49.3% women) across Europe, Asia, and South America who had ambulatory BP recordings with at least six daytime and three nighttime readings. Diastolic BP findings were consistent with those of systolic BP.

"The current population-based study confirmed previous research, indicating that ambulatory BP monitoring over and beyond measures taken in clinicians' offices improved risk stratification among patients with or those suspected of having hypertension. It strengthened the notion that nighttime BP measures carry valuable prognostic information," Staessen and colleagues said.

Such small improvements noted in the AUC challenge the idea that BP is the strongest modifiable cardiovascular risk factor, they suggested.

"Thus, an important issue in the evaluation of an additional risk prediction marker is how to interpret a small AUC increase, which many researchers believe is an imprecise metric because it increases only slightly with the introduction of an additional marker in multivariable-adjusted models, even if the marker under study carries great risk, as reflected by the odds ratio (or HR)."

The authors acknowledged that they did not use the diary approach thought to be the gold standard for differentiating between wakeful and sleeping BPs. Antihypertensive drug treatment was also only recorded at baseline.

"This is important information for patients and clinicians as they determine how to use the large amount of BP data from ABPM [ambulatory BP monitoring]," wrote Philip Greenland, MD, of the Feinberg School of Medicine at Northwestern University in Chicago, in an accompanying .

"Based on these findings, it is reasonable to consider the two most clinically relevant measurements from ABPM to be the 24-hour BP and the nighttime BP. Either could be used to justify treatment of BP that is above the treatment threshold," Greenland said.