Insulin and Blood Sugar in Heart Failure: What Patients and Clinicians Need to Know

Why Heart Failure Disrupts Blood Sugar Regulation

Heart failure creates a hostile metabolic environment that makes blood sugar management far more complicated than in otherwise healthy patients. Reduced cardiac output cuts skeletal muscle perfusion, impairs glucose uptake, and triggers neurohumoral activation, particularly of the sympathetic nervous system and the renin-angiotensin-aldosterone axis. Both responses raise circulating catecholamines and cortisol, which suppress insulin signaling at the receptor level.

A 2021 analysis published in the European Heart Journal found that skeletal muscle insulin resistance in heart failure patients correlates directly with New York Heart Association (NYHA) class, worsening at each stage from class I through class IV (1). The failing myocardium also shifts its primary fuel source away from fatty acids toward glucose, yet simultaneously becomes less efficient at extracting that glucose from the bloodstream. This metabolic mismatch means blood glucose can be elevated systemically while the heart itself is energy-starved.

Neurohormonal drugs used in heart failure, particularly loop diuretics and, to a lesser extent, beta-blockers, add another layer. Thiazide and loop diuretics cause potassium loss; hypokalemia impairs pancreatic beta-cell insulin secretion. Beta-blockers blunt the adrenergic symptoms that warn patients of hypoglycemia, a safety concern addressed directly in the 2022 AHA/ACC/HFSA Heart Failure Guideline (2).

"Patients with heart failure and concomitant diabetes represent a particularly high-risk group in whom glycemic management strategies should be individualized, with special attention to agents that may exacerbate volume overload or increase hypoglycemia risk," states the 2022 AHA/ACC/HFSA guideline writing committee (2).

How Common Is Diabetes in Heart Failure, and Does It Change Prognosis?

The overlap is substantial. Diabetes is present in approximately 40% of patients hospitalized with heart failure, and the combination carries a significantly worse prognosis than either condition alone (3). In the CHARM program (N=7,599), patients with both heart failure and diabetes had a 36% higher risk of cardiovascular death or heart failure hospitalization compared with heart failure patients without diabetes (4).

The relationship is not simply correlational. Chronic hyperglycemia stiffens the myocardium through advanced glycation end-product accumulation, drives oxidative stress, and promotes myocardial fibrosis. These changes occur even before overt systolic dysfunction appears, a condition called diabetic cardiomyopathy. By the time a patient reaches heart failure with reduced ejection fraction (HFrEF) or heart failure with preserved ejection fraction (HFpEF), significant structural damage may already exist that no glucose-lowering drug will fully reverse.

HFpEF deserves separate emphasis. More than half of heart failure patients now fall into the preserved ejection fraction category, and diabetes is present in nearly 45% of them. Insulin resistance, obesity, and systemic inflammation are considered core drivers of HFpEF pathophysiology rather than incidental findings. A fasting glucose of 126 mg/dL or an HbA1c of 6.5% or higher in a patient with exertional dyspnea and a preserved ejection fraction should prompt immediate metabolic optimization alongside standard heart failure therapy (5).

SGLT2 Inhibitors: The Evidence That Changed Practice

No drug class has shifted heart failure management more decisively over the past decade than SGLT2 inhibitors. The mechanism is partly glycosuric, partly hemodynamic, and likely also mitochondrial. Two landmark trials established the foundation.

In EMPA-REG OUTCOME (N=7,020), empagliflozin 10 mg or 25 mg daily reduced heart failure hospitalization by 35% versus placebo in patients with type 2 diabetes and established cardiovascular disease (hazard ratio 0.65 to 95% CI 0.50, 0.85, P<0.001) (6). The effect appeared within weeks, too fast for glucose lowering alone to explain.

DAPA-HF (N=4,744) then extended the story further. Dapagliflozin 10 mg daily reduced the composite of worsening heart failure or cardiovascular death by 26% in HFrEF patients, and the benefit was nearly identical in those with and without diabetes (hazard ratio 0.74 without diabetes vs. 0.75 with diabetes) (7). HbA1c dropped modestly, roughly 0.4 to 0.5 percentage points, but that clearly was not the primary driver of a 26% risk reduction.

Empagliflozin's HFpEF trial, EMPEROR-Preserved (N=5,988), showed a 21% reduction in cardiovascular death or HF hospitalization across a broad range of ejection fractions above 40%, again regardless of diabetes status (8).

The practical implication: prescribe an SGLT2 inhibitor for almost every eligible heart failure patient who has diabetes and an eGFR above 20 mL/min/1.73 m2. Watch for genital mycotic infections (incidence roughly 6 to 10% over one year) and the rarer risk of euglycemic diabetic ketoacidosis, which requires holding the drug 3 to 4 days before any elective surgery or prolonged fasting (6).

GLP-1 Receptor Agonists in Heart Failure: Promise With Nuance

GLP-1 receptor agonists reduce cardiovascular events robustly in patients with type 2 diabetes and established atherosclerotic cardiovascular disease, but the heart failure signal is more complicated. In LEADER (N=9,340), liraglutide reduced the three-point MACE composite by 13%, yet heart failure hospitalization was not significantly reduced (HR 0.87 to 95% CI 0.73, 1.05) (9).

The SELECT trial (N=17,604) published in 2023 showed semaglutide 2.4 mg weekly reduced major adverse cardiovascular events by 20% in overweight or obese patients without diabetes but with established cardiovascular disease. Heart failure-related endpoints were not the primary driver (10).

A small but important mechanistic trial, the FIGHT study, tested liraglutide in patients with advanced HFrEF after hospitalization. Liraglutide failed to improve clinical stability and showed a trend toward worse outcomes in patients with the lowest ejection fractions (11). Most cardiologists now interpret this as a signal for caution, not contraindication, specifically in advanced systolic dysfunction.

The bottom line: GLP-1 agonists are reasonable for a heart failure patient who has type 2 diabetes plus atherosclerotic cardiovascular disease and who needs additional glucose and weight reduction. Avoid them, or use them carefully, in NYHA class III to IV HFrEF until larger dedicated trials report.

Metformin: Rehabilitated but Not for Everyone

Metformin was historically contraindicated in heart failure due to concerns about lactic acidosis, a label that persisted for decades despite thin supporting evidence. The FDA revised this contraindication in 2016, removing the blanket restriction and focusing the warning on renal impairment rather than heart failure per se (12).

Observational data consistently show lower mortality with metformin use in heart failure patients who have diabetes and preserved renal function. A 2010 meta-analysis of 9 studies (N=34,000) found metformin associated with a 20% reduction in all-cause mortality versus other oral antidiabetic agents in heart failure patients with diabetes (13).

Use it if the eGFR stays above 30 mL/min/1.73 m2 and the patient is hemodynamically stable. Hold it during acute decompensation, contrast procedures, and hospitalizations where renal function may fluctuate. The lactic acidosis risk, though real, is roughly 3 to 9 cases per 100,000 patient-years, a rate comparable with placebo in controlled trials.

Insulin Therapy: Necessary but High-Risk

Some heart failure patients with diabetes require insulin. Nothing wrong with that, provided the prescriber understands the specific risks the combination creates. Hypoglycemia is the central concern.

Severe hypoglycemia triggers massive catecholamine release. That surge increases heart rate, blood pressure, and myocardial oxygen demand simultaneously, while also prolonging the QT interval, a setup for arrhythmia in a heart already prone to conduction abnormalities. A retrospective cohort study of 4,368 hospitalized heart failure patients found that even a single documented severe hypoglycemia episode (glucose <54 mg/dL) was associated with a hazard ratio of 2.1 for 30-day cardiovascular events (P<0.001) (14).

Basal insulin (glargine U-100, detemir) is preferred over premixed or sliding-scale regimens in the inpatient setting. Target glucose of 140 to 180 mg/dL is recommended for most hospitalized heart failure patients per the American Diabetes Association Standards of Care (15). Targets below 140 mg/dL may be appropriate for surgical ICU patients, but require continuous glucose monitoring or very frequent point-of-care testing.

Fluid retention is less a concern with modern insulin analogs than it was with older formulations, but insulin still promotes renal sodium reabsorption at the tubular level. Even small doses can tip a patient in fragile fluid balance toward frank decompensation. Check daily weights, monitor for new or worsening edema, and adjust diuretic dosing proactively when initiating or uptitrating insulin in an outpatient heart failure clinic.

Drugs That Worsen Heart Failure: The Avoid List

Three drug categories deserve explicit caution or avoidance in heart failure patients who also need glucose control.

Thiazolidinediones (pioglitazone, rosiglitazone) cause sodium and water retention through PPAR-gamma-mediated renal mechanisms. The PROACTIVE trial (N=5,238) showed pioglitazone increased heart failure hospitalization by 41% versus placebo despite reducing macrovascular events (16). Both agents carry an FDA black-box warning for patients with NYHA class III or IV heart failure. Avoid them.

Saxagliptin drew attention in SAVOR-TIMI 53 (N=16,492), where it increased heart failure hospitalization by 27% versus placebo (HR 1.27 to 95% CI 1.07, 1.51, P=0.007) (17). Alogliptin showed a similar trend in EXAMINE. Other DPP-4 inhibitors (sitagliptin, linagliptin) appear neutral in heart failure trials, but saxagliptin and alogliptin should be avoided.

Sulfonylureas carry class effects of hypoglycemia and, in some analyses, excess cardiovascular mortality, though trial data are conflicting. In a patient with heart failure, the hypoglycemia risk alone usually tips the risk-benefit analysis against them, particularly in older adults or those with renal impairment. If a sulfonylurea is used, glipizide is preferred over glyburide because of its shorter duration of action and lower hypoglycemia burden.

Glycemic Targets in Heart Failure: Relaxed, but Not Relaxed Entirely

A practical glycemic target framework for heart failure patients stratified by functional class and comorbidity:

NYHA Class I to II, age <70, eGFR above 45, no frailty: HbA1c target 7.0 to 7.5%. SGLT2 inhibitor as backbone. Metformin if tolerated. Avoid saxagliptin and thiazolidinediones.

NYHA Class II to III, age 65 to 80, eGFR 30 to 45, mild frailty: HbA1c target 7.5 to 8.0%. SGLT2 inhibitor if eGFR allows. Metformin with caution. Basal insulin if needed. Strict hypoglycemia avoidance takes priority over tight HbA1c.

NYHA Class III to IV, age above 75, eGFR <30, significant frailty or recurrent hospitalizations: HbA1c target 8.0 to 8.5%, or simply targeting symptomatic hyperglycemia avoidance rather than a numeric HbA1c goal. Basal insulin at low doses if oral agents are insufficient. Avoid adding any agent that increases volume retention or hypoglycemia risk.

The 2023 ADA Standards of Care explicitly state: "Less stringent A1C goals (such as <8% or <8.5%) may be appropriate for patients with limited life expectancy, long duration of diabetes, advanced complications, or extensive comorbid conditions" (15).

Monitoring Blood Sugar in Acute Decompensated Heart Failure

Hospitalization for decompensated heart failure creates unique glucose management challenges. Stress hyperglycemia appears in patients without prior diabetes diagnosis, driven by elevated cortisol and catecholamines. Diuresis alters drug pharmacokinetics. Oral intake may be poor.

Point-of-care glucose checks every 4 to 6 hours are standard in the ICU or step-down unit. Continuous glucose monitoring (CGM) devices are increasingly used but can show lag times of 10 to 15 minutes during rapid glucose changes, a clinically meaningful delay in a patient on insulin infusion. Capillary glucose may also be unreliable when peripheral perfusion is poor, a common finding in low-output states. Arterial blood gas glucose (measured on co-oximetry) is more accurate in those situations.

Discharge planning must include a clear medication reconciliation step. Glucose-lowering drugs held at admission, SGLT2 inhibitors, metformin, and sometimes oral sulfonylureas, need a deliberate restart decision, not an automatic refill. An outpatient follow-up within 7 to 14 days of discharge to reassess renal function, fluid status, and HbA1c is recommended by the AHA Get With The Guidelines program (2).

Special Populations Within Heart Failure

Older adults (age 75 and above): Hypoglycemia awareness diminishes with age. The threshold for liberalizing HbA1c targets is lower, and the threshold for using complex insulin regimens is higher. Prioritize once-daily basal insulin over multiple-injection regimens if insulin is needed (15).

Renal disease: SGLT2 inhibitors retain cardiovascular and renal benefits at eGFR as low as 20 mL/min/1.73 m2 per the CREDENCE and DAPA-CKD trials, though their glucose-lowering effect is blunted below eGFR 45. Metformin must be stopped at eGFR <30. Dose adjustments for renally cleared agents (certain insulins, DPP-4 inhibitors) are necessary (18).

Pregnancy: Preexisting heart failure with diabetes in pregnancy is rare but serious. Metformin is generally continued through the first trimester and often through the second; insulin remains the primary glucose-lowering agent in pregnancy because it does not cross the placenta. SGLT2 inhibitors and GLP-1 agonists are contraindicated in pregnancy per FDA labeling (19).

Children: Heart failure in pediatric patients with diabetes is most commonly seen in the context of congenital heart disease, cardiomyopathy, or type 1 diabetes. Insulin is the primary therapy. SGLT2 inhibitors are not FDA-approved in children under 18 for heart failure indications.

Why the Drug Choice Matters as Much as the Number

A blood glucose of 180 mg/dL reached by holding insulin to prevent hypoglycemia is very different from a blood glucose of 180 mg/dL caused by an inadequate SGLT2 inhibitor dose. One decision protects the patient; the other misses an opportunity to prevent 25 to 35% of future heart failure hospitalizations. The glucose number on the monitor tells you where the patient is. The drug producing that number determines where they are going.

Initiate dapagliflozin 10 mg or empagliflozin 10 mg in every eligible patient with heart failure and diabetes (or even without diabetes in HFrEF) once eGFR is confirmed above 20 mL/min/1.73 m2, then build the remaining regimen around it.