Methodological Aspects A Guide To The Critical Reader

The two most popular ways of obtaining automatic indirect BP records are either by use of a microphone in the cuff or by oscillometric technique [14,15]. Some monitors offer both options. While the manufacturer of the monitor is always stated in papers dealing with ambulatory BP monitoring, the technique is not necessarily described.

Mogensen CE (ed.) THE KIDNEY AND HYPERTENSION IN DIABETES MELLITUS. Copyright© 2004 by Martin Dunitz, a member of the Taylor & Francis Group, plc. All rights reserved.

No monitor is perfect and even in monitors, which have fulfilled national standards, major discrepancies between the monitors and values obtained by sphygmomanometry are observed in some of the patients. Some papers state that individually "calibration" of the monitor to each of the studied patients has been performed (by 3 to 5 simultaneous or sequential measurements). However, it is not possibly to calibrate a fully automatic monitor in strict terms (without returning to the manufacturer) and the word calibration is a misnomer in this context. The difference between auscultatory BP and the monitor can be evaluated in each patient (rather inaccurate) and this difference can either be accepted or not.

If the result of clinic BP measurement is provided it should be observed whether this is obtained by sphygmomanometry or by use of the same monitor as used for ambulatory measurements [16]. Only in the latter case are clinic and ambulatory values directly comparable.

Although more sophisticated methods exist [17,18], the diurnal variation of BP is usually reported as the night/day ratio. Obviously this must be based on individual information of the night period; otherwise the ratio is overestimated [19].

The term "non-dipper" has become a popular short term for a person who does not describe a normal reduction of BP at night. A commonly used definition of a "non-dipper" requires a relative reduction of night blood pressure less than 10 % of the day value for both systolic and diastolic BP [20]. Unfortunately no consensus exists. In addition the proportion of non-dippers also depends on the definition of night and day time which should be based on individual information of time for going to bed and rising rather than fixed periods. If individual information is not available the use of "short fixed intervals" have been proposed [21].

Patients who are hypertensive by clinic measurements but show a normal ambulatory BP are designated "white coat hypertensive" [22]. This term is well understood in literature although "isolated clinic hypertension" may be more precise [23]. The effect of the "white coat" was originally described as a transient (5 min) elevation of BP [23]. At present the "white coat effect" is usually calculated as the difference (clinic BP - day time BP) [24]. The proportion of white coat hypertensive subjects in a hypertensive population depends on:

i: The definition of hypertension (usually clinic BP > 140/90 mmHg)

ii: How carefully the hypertensive subjects are identified (if patients are labelled as hypertensive based on only one clinic BP the frequency of white coat hypertension is high, if a several clinic BP measurements -as recommended- are obtained at different occasions before diagnosis, the frequency is lower).

iii: The definition of a normal ambulatory BP (if the cut off limit for a normal ambulatory BP is defined as a day time BP < 131/86 mmHg the frequency of white coat hypertension is lower than for a cut off limit of BP < 140/90 mmHg).

iiii: The presence of other selection criteria related to clinic BP level or possible end organ signs. (The frequency of white coat hypertension is higher in a population with mild hypertension than in a population with moderate or severe hypertension even if the white coat effect is higher in the latter group [24]). If a hypertensive population are selected on the basis of criteria which may be related to elevated BP (i.e. albuminuria, retinopathy or left ventricular hypertrophy) the proportion of patients who fulfil the criteria for white coat hypertension is assumed to be low.

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