Blood pressure variability and ambulatory monitoring.

The inherent variability of blood pressure means that the accuracy of a small number of clinic readings in estimating the true blood pressure is limited. In many patients, a clinic visit provokes an increase in blood pressure such that they may be misclassified as being hypertensive (white coat hypertension). This applies to about 20% of hypertensives. Ambulatory monitoring has the potential to overcome these limitations. The white coat effect may still occur in patients taking antihypertensive medications, although in some cases medication may lower the clinic pressure without affecting the ambulatory pressure. The diurnal rhythm of blood pressure is another major source of variability and is influenced by both extrinsic and intrinsic factors. Its pathologic significance is unclear. Antihypertensive medication generally lowers the set point of blood pressure but has relatively little effect on variability.