SPECTRAL-ANALYSIS OF THE SYSTOLIC BLOOD-PRESSURE SIGNAL IN SECONDARY HYPERTENSION - A METHOD FOR THE IDENTIFICATION OF PHAEOCHROMCYTOMA

Objective: To seek in hypertensive patients rhythmic variations of the systolic blood pressure signal obtained by ambulatory blood pressure monitoring of any inherent cycle of intermediate value between 1 and 2 4 h. Design: Subjects (62 hypertensive, 39 normotensive) were evaluate d by 24-h ambulatory blood pressure monitoring, The hypertensive group consisted of 48 patients with essential hypertension, nine with renov ascular hypertension and five with phaeochromocytoma. The groups were matched for age and weight. Methods: The ambulatory systolic blood pre ssure recording served as the input for a filtering procedure that rej ected unacceptable values according to predetermined criteria. The who le-day systolic blood pressure series thus obtained were subjected to Fourier analysis to obtain a spectral analysis of daily systolic blood pressure fluctuations. Daily (first 12 h), nightly (second 12 h) and whole-day average systolic blood pressure values were calculated and c ompared for the various groups. Results: The average nocturnal systoli c blood pressure was found to be lower than its daily counterpart in t he normal subjects and in the patients with essential hypertension, wh ereas in the patients with renovascular hypertension or phaeochromocyt oma no such nocturnal decrease was found. The power spectrum of patien ts with phaeochromocytoma was statistically different from that of oth er aetiologies of hypertension. This was achieved due mainly to a stat istically significant difference in the power spectrum integral over t he low-frequency band (0-0.2 cycles/h) of the power spectrum of the 24 -h systolic blood pressure signal. Resection of the phaeochromocytoma normalized the power spectrum as found by analysis of the postoperativ e ambulatory blood pressure monitoring data in two patients who underw ent a repeat recording. Conclusions: The technique described enables t he discrimination of patients with phaeochromocytoma as a cause of hyp ertension from other aetiologies of hypertension. Patients with renova scular hypertension could not be distinguished from those with essenti al hypertension on the basis of their power spectrum. However, this te chnique may prove to be a valuable modality for characterizing hyperte nsive patients of different aetiologies.