Timing of blood pressure measurements in determining anomalies in duration of effect of an antihypertensive drug: assessment of isradipine.

Analysis of an antihypertensive drug trial, which involved measurements of blood pressure (BP) during visits to the clinic at a set time of day, showed that the initial dosage titration procedure had been inadequate in some patients. Plasma drug concentration-time curves and corresponding BP values suggested that control of BP was closely related to plasma drug concentration and that the duration of drug effect was shorter than the dosage interval of 12 h. This interpretation was supported by measurements of BP and drug concentrations taken at steady state, before and 2 h after taking the drug. Measurements of ambulatory BP revealed that some patients whose doses had been titrated at peak plasma drug concentrations had high BP at the time of trough plasma drug concentrations, whereas some of those titrated at trough times were hypotensive at peak times. Adjustment of antihypertensive therapy should entail observations of BP at times coincident with both peak and trough concentrations of the drug concerned, and can be facilitated by ambulatory BP monitoring.