Frequent Cuff Inflations May Disrupt the Accuracy of 24-hour Ambulatory Blood Pressure Monitoring

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.