Background: Hypertension is a modifiable cardiovascular risk factor. Although it is established that low-dose thiazides reduce mortality as well as cardiovascular morbidity, the dose-related effect of thiazides in decreasing blood pressure has not been subject to a rigorous systematic review. It is not known whether individual drugs within the thiazide diuretic class differ in their blood pressure-lowering effects and adverse effects.
Objectives: To determine the dose-related decrease in systolic and/or diastolic blood pressure due to thiazide diuretics compared with placebo control in the treatment of patients with primary hypertension. Secondary outcomes included the dose-related adverse events leading to patient withdrawal and adverse biochemical effects on serum potassium, uric acid, creatinine, glucose and lipids.
Search methods: We searched the Cochrane Central Register of Controlled Trials (CENTRAL 2014, Issue 1), Ovid MEDLINE (1946 to February 2014), Ovid EMBASE (1974 to February 2014) and ClinicalTrials.gov.
Selection criteria: We included double-blind, randomized controlled trials (RCTs) comparing fixed-dose thiazide diuretic monotherapy with placebo for a duration of 3 to 12 weeks in the treatment of adult patients with primary hypertension.
Data collection and analysis: Two authors independently screened articles, assessed trial eligibility, extracted data and determined risk of bias. We combined data for continuous variables using a mean difference (MD) and for dichotomous outcomes we calculated the relative risk ratio (RR) with 95% confidence interval (CI).
Main results: We included 60 randomized, double-blind trials that evaluated the dose-related trough blood pressure-lowering efficacy of six different thiazide diuretics in 11,282 participants treated for a mean duration of eight weeks. The mean age of the participants was 55 years and baseline blood pressure was 158/99 mmHg. Adequate blood pressure-lowering efficacy data were available for hydrochlorothiazide, chlorthalidone and indapamide. We judged 54 (90%) included trials to have unclear or high risk of bias, which impacted on our confidence in the results for some of our outcomes.In 33 trials with a baseline blood pressure of 155/100 mmHg, hydrochlorothiazide lowered blood pressure based on dose, with doses of 6.25 mg, 12.5 mg, 25 mg and 50 mg/day lowering blood pressure compared to placebo by 4 mmHg (95% CI 2 to 6, moderate-quality evidence)/2 mmHg (95% CI 1 to 4, moderate-quality evidence), 6 mmHg (95% CI 5 to 7, high-quality evidence)/3 mmHg (95% CI 3 to 4, high-quality evidence), 8 mmHg (95% CI 7 to 9, high-quality evidence)/3 mmHg (95% CI 3 to 4, high-quality evidence) and 11 mmHg (95% CI 6 to 15, low-quality evidence)/5 mmHg (95% CI 3 to 7, low-quality evidence), respectively.Direct comparison of doses did not show evidence of dose dependence for blood pressure-lowering for any of the other thiazides for which RCT data were available: bendrofluazide, chlorthalidone, cyclopenthiazide, metolazone or indapamide.In seven trials with a baseline blood pressure of 163/88 mmHg, chlorthalidone at doses of 12.5 mg to 75 mg/day reduced average blood pressure compared to placebo by 12.0 mmHg (95% CI 10 to 14, low-quality evidence)/4 mmHg (95% CI 3 to 5, low-quality evidence).In 10 trials with a baseline blood pressure of 161/98 mmHg, indapamide at doses of 1.0 mg to 5.0 mg/day reduced blood pressure compared to placebo by 9 mmHg (95% CI 7 to 10, low-quality evidence)/4 (95% CI 3 to 5, low-quality evidence).We judged the maximal blood pressure-lowering effect of the different thiazides to be similar. Overall, thiazides reduced average blood pressure compared to placebo by 9 mmHg (95% CI 9 to 10, high-quality evidence)/4 mmHg (95% CI 3 to 4, high-quality evidence).Thiazides as a class have a greater effect on systolic than on diastolic blood pressure, therefore thiazides lower pulse pressure by 4 mmHg to 6 mmHg, an amount that is greater than the 3 mmHg seen with angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs) and renin inhibitors, and the 2 mmHg seen with non-selective beta-blockers. This is based on an informal indirect comparison of results observed in other Cochrane reviews on ACE inhibitors, ARBs and renin inhibitors compared with placebo, which used similar inclusion/exclusion criteria to the present review.Thiazides reduced potassium, increased uric acid and increased total cholesterol and triglycerides. These effects were dose-related and were least for hydrochlorothiazide. Chlorthalidone increased serum glucose but the evidence was unclear for other thiazides. There is a high risk of bias in the metabolic data. This review does not provide a good assessment of the adverse effects of these drugs because there was a high risk of bias in the reporting of withdrawals due to adverse effects.
Authors' conclusions: This systematic review shows that hydrochlorothiazide has a dose-related blood pressure-lowering effect. The mean blood pressure-lowering effect over the dose range 6.25 mg, 12.5 mg, 25 mg and 50 mg/day is 4/2 mmHg, 6/3 mmHg, 8/3 mmHg and 11/5 mmHg, respectively. For other thiazide drugs, the lowest doses studied lowered blood pressure maximally and higher doses did not lower it more. Due to the greater effect on systolic than on diastolic blood pressure, thiazides lower pulse pressure by 4 mmHg to 6 mmHg. This exceeds the mean 3 mmHg pulse pressure reduction achieved by ACE inhibitors, ARBs and renin inhibitors, and the 2 mmHg pulse pressure reduction with non-selective beta-blockers as shown in other Cochrane reviews, which compared these antihypertensive drug classes with placebo and used similar inclusion/exclusion criteria.Thiazides did not increase withdrawals due to adverse effects in these short-term trials but there is a high risk of bias for that outcome. Thiazides reduced potassium, increased uric acid and increased total cholesterol and triglycerides.