Review of the MERIT-HF trial

Effect of metoprolol CR/XL in chronic heart failure

Lancet 1999;353:2001-07

Background: Beta-blockers directly reduce cardiac contractility and myocardial oxygen demand. For decades, they were avoided in patients with acute and chronic heart failure over concerns they would facilitate decompensation of the condition. The therapeutic cornerstones of treatment, prior to the modern era of clinical trials, focused on managing symptoms and quality of life with diuretics and inotropic agents like digoxin; however, new paradigms were arising that focused on addressing neurohormonal mechanisms of chronic disease that were over-activated in the failing heart.

The first major success came with inhibition of the renin angiotensin aldosterone system with angiotensin converting enzyme inhibitors whose effect on mortality for patients with mild and severe forms of chronic heart failure were demonstrated in the V-HEFT II, CONSENSUS, and SOLVD trials. Additional benefits were demonstrated with the mineralocorticoid receptor antagonist spironolactone in the RALES trial. These drug classes primarily work by reducing afterload and volume retention. Appreciating why they work for improving cardiac performance and managing symptoms in heart failure patients is straightforward when we consider the major factors that effect cardiac stroke volume - preload, afterload and contractility; however, it is also noteworthy the effects these agents have on sudden death.

How beta-blockade benefits the failing heart is less obvious (outside prevention of sudden death). Mechanistic studies in patients with chronic heart failure have consistently shown that when beta blockers are used for more than 1 month, left ventricular function improves. Beta blocker therapy appears to restore the density of beta-adrenergic receptors after they have been downregulated by the chronic overactivity of the sympathetic nervous system.

The first major placebo-controlled RCT to demonstrate a mortality benefit used the non-selective beta blocker carvedilol. The trial was small and not originally designed to test mortality and was stopped early without clearly predefined stopping rules. Furthermore, 8% of total patients selected for participation in the trial were excluded prior to randomization after a 2 week, open-label run-in phase with the study drug, which saw 2% of all patients experience worsening heart failure or death representing 24 patients (the difference in total deaths between groups was 9 when the trial was stopped). The Metoprolol CR/XL Randomised Intervention Trial in Congestive Heart Failure (MERIT-HF) was the first large scale trial designed to test the hypothesis that beta-blockade with metoprolol controlled/extended release (CR/XL) added to optimum medical therapy reduces mortality in patients with chronic systolic heart failure.

Patients: Patients were recruited from 313 sites in 13 European countries and the United States. Eligible patients were men and women between the age of 40 to 80 years with symptomatic heart failure (NYHA class II-IV) for >/= 3 months before randomization. They had to be on a diuretic and ACE inhibitor for at least 2 weeks. Other drugs, including digoxin, could also be used. Patients also had to have an EF of </=40% within 3 months before enrollment. The resting heart rate had to be >/=68 beats per minute.

Patients were excluded if: they had an MI or unstable angina within 28 days; had an indication or contraindication for treatment with beta-blocker; beta blockade within 6 weeks; heart failure due to systemic disease (i.e., amyloidosis) or alcohol abuse; scheduled or performed cardiac transplant; an ICD; procedures such as CABG or PCI planned or performed in the past 4 months; 2nd or 3rd degree AV block unless a pacemaker was present; unstable or decompensated heart failure defined by pulmonary edema or hypoperfusion or supine systolic BP <100 mmHg; any other serious disease that might complicate management and follow-up; use of calcium antagonists; use of amiodarone within 6 months before enrollment; poor compliance, defined as >25% deviation of the number of observed versus expected consumed placebo tablets during the run-in period.

Baseline characteristics: The mean age of patients was 64 years and approximately 78% were male. Slightly more than 30% of patients were above the age of 70. The average EF was 28%. The average SBP was 130 mmHg and heart rate was 82 bpm. Most patients had mild to moderate heart failure, with 41% in NYHA Class II, 56% in Class III, and only 3% in Class IV. Ischemic cardiomyopathy accounted for 65% of cases and nonischemic causes accounted for 35%. Most patients were on an ACE inhibitor or ARB (95%) and diuretic (90%). Digoxin was used in 63%.

Trial procedures: Prior to randomization, the study was preceded by a single-blind, 2-week placebo run-in period. Patients meeting eligibility were then randomized to placebo or metoprolol CR/XL. The starting dose of placebo or metoprolol CR/XL was 12.5 mg daily for patients in NYHA class III or IV and 25 mg daily for patients in NYHA class II. The dose was doubled every 2 weeks until the target dose of 200 mg daily was reached. Patients were followed every 3 months.

Endpoints: The primary outcome was all-cause mortality. It was estimated that 3,200 patients would need to be followed for 2.4 years to detect a 30% relative reduction in mortality based on annual mortality rate of 9.4% in the placebo group. This would achieve at least 80% power with a 2-sided alpha of 0.04. Patients were recruited faster then planned and so the final sample size of 3,991 patients increased the power of the study.

The study was monitored by an independent safety committee and predefined stopping rules for efficacy were based on all-cause mortality, done when 25%, 50%, and 75% of expected deaths had occurred.

Results: The trial was stopped early after the 2nd preplanned interim analysis when 50% of expected deaths had occurred. The mean duration of follow-up at the time of stopping was 1 year. The mean daily dose of metoprolol CR/XL was 159 mg once daily, with 87% receiving 100 mg or more and 64% receiving the target dose of 200 mg daily. In the placebo group, the corresponding values were 179 mg daily, 91% and 82%. The study drug was discontinued permanently in 14% of patients in the metoprolol group and 15% in the placebo group. Six months after randomization, heart rate decreased by 14 bpm in the metoprolol group compared to only 3 bpm in the placebo group. Systolic blood pressure decreased less in the metoprolol group (-2.1 vs 3.5 mmHg).

Compared to placebo, metoprolol significantly reduced all-cause mortality (7.3% vs 10.8%; RR 0.66; 95% CI 0.53—0.81). Cardiovascular mortality accounted for 91% of all deaths; with sudden death accounting for 58% and death from worsening heart failure accounting for 24% of all deaths. All 3 of these causes of death were significantly reduced by metoprolol. The relative and absolute effects on death were greatest for patients with NYHA class III heart failure.

Conclusions: In this trial of stable patients with mild to moderate chronic systolic heart failure, who were optimized on an ACEi or ARB and diuretic, metoprolol CR/XL significantly reduced all-cause mortality. Approximately 30 patients would need to be treated with metoprolol compared to placebo for 1 year to prevent 1 death.

This trial represents a significant win for beta blockade in patients with chronic systolic heart failure. While the NNT in this trial is slightly higher than in SOLVD, it is important to appreciate that follow-up time in SOLVD was more than 3x longer.

Limitations to external validity in this trial include the run-in period and stringent inclusion and exclusion criteria. Our enthusiasm is also tempered by early stopping, which has been found to be associated with false positive or exaggerated results but this concern is mitigated to some extent in this trial because the rules for early stopping were clearly defined in the protocol.