or placebo (table I); the serum potassium concentration showed a statistically significant decrease during treatment with chlorthalidone and chlorthalidone combined with atenolol as compared with during the atenolol or placebo period (table II); and plasma renin activity rose with chlorthalidone and decreased with atenolol, though this was not statistically significant (table II).These findings have important implications. Beta-blockers alone are ineffective in black hypertensive patients, and thiazides rather than beta-blockers should be the baseline treatment of hypertension. When beta-blockers are used in black hypertensive patients they should initially be combined with a thiazide diuretic. Prevalence of urinary incontinence THELMA M THOMAS, KAY R PLYMAT, JANET BLANNIN, T W MEADE Summary and conclusionsThe prevalence of urinary incontinence was investigated by determining the number of incontinent patients under the care of various health and social service agencies in two London boroughs and by a postal survey of the 22 430 people aged 5 years and over on the practice lists of 12 general practitioners in different parts of the country. The prevalence of incontinence known to the health and social service agencies was 0 2% in women and 0 1% in men aged 15-64 and 2-5% in women and 1 3% in men aged 65 and over. The postal survey, to which 89% of the people whose correct address was known replied, showed a prevalence of urinary incontinence of 8 5% in women and 1-6% in men aged 15-64 and 116% in women and 6 9% in men aged 65 and over. Nulliparous women had a lower prevalence than those who had had one, two, or three babies, but within the parity range of one to three there were no differences in prevalence. The prevalence was appreciably increased in women who had had four or more babies. Incontinence was moderate or severe in a fifth of those who reported it in the postal survey, of whom less than a third were receiving health or social services for the condition. Incontinence is a common symptom, and many unrecognised cases appear to exist. There may be considerable scope for improving its management.

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physiologically more relevant microcirculation. A recent study, however, gives evidence of the beneficial effect of normovolaemic haemodilution on calf muscle oxygen pressures in patients measured by a direct invasive method.2The influence of blood viscosity on peripheral blood flow might be controversial. Most of the evidence on this subject, however, is for rather than against.' E ERNST

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