24-hour ambulatory blood pressure monitoring (ABPM) is superior to office blood pressure (BP) for assessing cardiovascular risk. On the other hand, repeated cuff inflations during ABPM can cause discomfort and reduced quality of sleep. A high frequency of measurement during ABPM might result in erroneously high BP measurement, incorrect hypertension diagnoses, unnecessary treatment or just overtreatment with antihypertensive medication and potentially serious side effects.

The purpose of this study is to investigate whether the frequency of measurements during ABPM impacts the measured BP during ABPM.

Patients will be recruited from the Clinic of Hypertension at Aarhus University Hospital, Denmark. The study will be conducted as a randomized controlled crossover study. Participants will undergo two 24-hour ABPMs: One with a low frequency of measurement, 1-time per hour during the day and 1-time per hour during the night, and one with a high frequency, 3-times/hour during the day, and 2-times/hour at night. The primary endpoint will be the difference in 24-hour mean systolic BP between the high- and low frequency measurement ABPM.

Aim:

The purpose of this study is to evaluate if 24-hour ABPM with frequent BP measurements (intervals of 20 minutes during daytime and 30 minutes during nighttime) result in erroneously elevated BP in hypertensive patients compared with non-frequent BP readings (measurement intervals of 60 minutes both day and night).

Population:

The study population will comprise all patients referred to and attending the Clinic of Hypertension, AUH, who are scheduled to undergo ABPM. At referral to the Clinic of Hypertension, AUH, patients will be contacted with an electronic or analog letter, informing them about the study and inviting them to participate. Patients who already are being treated at the Clinic of Hypertension, will be invited at an appointment in the clinic. Patients will be excluded from the study if they have atrial fibrillation, nighttime work, insomnia or sleeping disorders, and are not physically able to undergo an ABPM (e.g. too big arm circumference to fit the cuff).

Methods:

This will be a randomized controlled crossover study (figure 2). At arrival patients will be fitted with the device, a SPACELABS 90217 by a nurse. The nurse will conduct an office blood pressure measurement, in accordance with ESH guidelines to be sure the device is working, and for later comparison. Patients will have their first ABPM conducted with either frequent or non-frequent ABPM measurement protocol as described above, randomized 1:1. Patients will be instructed to remove their device after 24 hours of measurement and have an appointment to return the device between 2-7 days. At least two days between measurements ensure a sufficient washout period. The maximum of 7 days before second measurement makes sure the patient's appointment with a doctor at the clinic will not be delayed. When returning the device, patients will have the same device mounted again, measuring with the frequency not assigned at the initial randomization. Participants will be instructed to repeat the same procedures regarding the device and to, as far as possible, conduct the same daily activities on the second day of measurement as during the first day of measurement. Patients will not have any alterations in blood pressure medication between the two measuring days. After each ABPM, patients will answer a routine questionnaire from the clinic about sleep, discomfort, pain, and the degree to which the device caused interference with daily activities and/or disturbance of sleep. Since the questionnaire is standard procedure at the clinic, we will be able to compare characteristics between the group who are not interested in joining the study and patients do participate. In this way the investigators can ensure no unknown selection bias occurs. Both ABPM will be used by the clinician at the appointment in the clinic.

Sample size:

In a pilot study conducted at the Clinic of Hypertension, AUH, the investigators found a mean difference of 7 mmHg (Standard deviation = 13,76) between a 20-minute interval and a 1-hour interval between measurements, respectively. Using a sample size calculation, for crossover design with quantitative measurement, a two-sided significance level of 0.025 and a power of 0.9, and a minimal clinical difference of 10mmHg, we calculate a minimum sample size of 38 patients. It is estimated only 25% of patients with this difference will be in intermediate hypertension group as studied in the previous pilot study, which is why 152 patients wil be included.

Inclusion Criteria:

• Patients either has to be referred from primary care to the Clinic of Hypertension at Aarhus University hospital, or already being treated at the clinic. Both groups has to be scheduled for a 24-hour ambulatory blood pressure monitoring.

Exclusion Criteria:

• Below 18 years of age
• Have atrial fibrillation
• Have routine nightshift work
• Is pregnant
• Is physically inable to have the cuff applied correctly (too large arm circumference)
• Known insomnia or other sleeping disorders
• Change in blood pressure medication within 14 days before inclusion

All patients, either referred or existing patients, who has to undergo a 24-hour ambulatory blood pressure monitoring