Response to Nighttime Activity Influences the Evaluation of Ambulatory Blood Pressure Monitoring
Nagy and Nagy1 point out that antidepressants or hypnotics may influence sleep-time activity, especially in the hospital, and, therefore, influence dipping. Dipping can also be influenced by other factors, such as sleep-disordered breathing, age, sex, and ethnicity. We agree that sleep-disordered breathing may cause nocturia and, therefore, confound the relationship of nocturia with nondipping. Although we did not examine the relationship of antidepressants with nondipping, none of the patients were hospitalized during ambulatory blood pressure monitoring.2 Hospitalization alone is certainly not a reason for the relationship that we reported between nocturia and nondipping.
In fact, a similar relationship between nocturia and nondipping has also been reported by Perk et al.3 Perk et al3 identified 97 subjects with nocturia and calculated the absolute difference between day and night blood pressures, as well as the actual wake and actual sleep blood pressures. The actual sleep BP excluded the time that the patients were awake at night. Average systolic BP dipping was 10.1% by the day/night method and 12.0% by wake/sleep method. The prevalence of nondipping decreased from 42.2% with the day/night method to 31.2% with the wake/sleep method. Using the wake/sleep method, the prevalence of nondipping was 27.5% in those who woke up once, 33.3% in those who woke up twice, and 66.6% in those who woke up ≥3 times.
The association of nondipping with age, sex, race, sleep-disordered breathing, nocturia, and chronic kidney disease suggests that nondipping is a marker of adverse cardiovascular risk factors.4 Whether nondipping is a mediator of adverse cardiovascular and renal outcomes remains to be demonstrated.5 In trying to demonstrate the mediating relationship, adjustments for covariates should be cautious, because these covariates may lie in the causal pathway.
Sources of Funding
This work was supported by a VA Merit Review.
Nagy G, Nagy CB. Nighttime activity influences the evaluation of ambulatory blood pressure monitoring. Hypertension. 2009; 54: e139.
Agarwal R, Light RP, Bills JE, Hummel LA. Nocturia, nocturnal activity, and nondipping. Hypertension. 2009; 54: 646–651.
Perk G, Ben-Arie L, Mekler J, Bursztyn M. Dipping status may be determined by nocturnal urination. Hypertension. 2001; 37: 749–752.