How does Continuous Renal Replacement Therapy (CRRT) affect blood sugar and insulin levels?

How CRRT Affects Blood Sugar and Insulin

CRRT can cause significant glucose and insulin disturbances through multiple mechanisms: glucose loss in the effluent, glucose gain from dextrose-containing dialysate, altered insulin clearance, and interference with glycemic control protocols.

Mechanisms of Glycemic Disturbance During CRRT

Glucose Loss and Gain

• Glucose is freely filtered during CRRT and lost in the effluent, which can contribute to hypoglycemia, particularly in patients receiving intensive insulin therapy 1
• Dialysate or substitution fluids containing supra-physiologic glucose concentrations frequently result in excessive glucose intake and hyperglycemia and should be avoided 1
• The balance between glucose loss in effluent and glucose gain from replacement fluids creates unpredictable glycemic fluctuations that complicate insulin dosing 1

Insulin Clearance and Dosing Challenges

• Insulin requirements may increase in patients on CRRT due to the stress of critical illness, though insulin itself can be cleared by the CRRT circuit 1
• Patients receiving renal replacement therapy with bicarbonate fluids have significantly increased risk of hypoglycemia (odds ratio 14,95% CI 1.8-106) 1
• The combination of CRRT and intensive insulin therapy creates a high-risk scenario for both hyper- and hypoglycemia 1

Recommended Glycemic Management During CRRT

Target Blood Glucose Range

• Target an upper blood glucose level ≤180 mg/dL rather than intensive targets ≤110 mg/dL in critically ill patients receiving CRRT 1
• The 2024 Society of Critical Care Medicine guidelines suggest against intensive insulin therapy (targeting 4.4-6.1 mmol/L or 80-110 mg/dL) due to increased hypoglycemia risk without mortality benefit 1
• A reasonable target range is 7.8-11.1 mmol/L (140-200 mg/dL) when insulin therapy is used, as this achieves similar mortality outcomes with lower hypoglycemia risk 1

Monitoring Requirements

• Blood glucose values should be monitored every 1-2 hours until glucose values and insulin infusion rates are stable, then every 4 hours thereafter in patients receiving insulin infusions 1
• Point-of-care capillary blood glucose testing may not accurately estimate arterial blood or plasma glucose values in critically ill patients 1
• Use arterial blood rather than capillary blood for point-of-care testing if patients have arterial catheters 1

Insulin Administration Strategy

• Use continuous IV insulin infusion rather than intermittent subcutaneous insulin for acute management of hyperglycemia in critically ill patients on CRRT 1
• IV insulin allows for more rapid titration and adjustment in response to the dynamic glucose changes that occur with CRRT 1
• Implement a protocolized approach to insulin dosing, commencing when two consecutive blood glucose levels are >180 mg/dL 1

Critical Pitfalls and Safety Considerations

Hypoglycemia Risk Factors

• Patients on CRRT have multiple risk factors for severe hypoglycemia: older age, higher illness severity scores, longer ICU stay, sepsis, mechanical ventilation, and intensive insulin protocols 1
• The risk of hypoglycemia increases 5-6 fold with intensive glycemic control (relative risk 5.13-5.99) 1
• Even mild hypoglycemia (54-80 mg/dL) is associated with higher unadjusted mortality in critically ill patients 1

Fluid Composition Considerations

• Dialysate should contain physiologic concentrations of electrolytes and avoid excessive glucose 1
• Either lactate or bicarbonate can be used as buffer in most CRRT patients, but bicarbonate is preferred in patients with lactic acidosis and/or liver failure where lactate metabolism may be impaired 1
• Pre-dilution fluid administration may enhance ultrafiltration rates and reduce filter clotting, but adds complexity to glucose management 1

Monitoring Accuracy Limitations

• Glucose meters lack accuracy in critically ill patients due to concurrent anemia, hypoxia, and interfering drugs 1
• Many point-of-care devices show significant variability and bias compared to reference laboratory plasma glucose measurements 1
• The FDA allows up to 20% error in glucose meter readings, which can lead to inappropriate insulin dosing 1

Special Populations

Diabetic Patients on CRRT

• Diabetic patients have higher baseline glucose levels (170 ± 77 vs. 129 ± 35 mg/dL in non-diabetics) and require higher insulin doses 2
• BMI is an independent determinant of poor glycemic control (β = 0.73, P < 0.0001) in CRRT patients 2
• Diabetic state, septic shock, organ failure scores, and use of corticosteroids and inotropes are independent determinants of insulin requirements 2

Hemodynamically Unstable Patients

• CRRT is suggested over intermittent hemodialysis to facilitate fluid balance management in hemodynamically unstable septic patients 1
• However, these same patients are at highest risk for glycemic instability due to vasopressor use, poor perfusion, and variable insulin absorption 1