What are the criteria for converting from off-pump (Off-Pump) Coronary Artery Bypass Grafting (CABG) to an on-pump procedure?

Conversion Criteria for Off-Pump to On-Pump CABG

Conversion from off-pump to on-pump CABG should occur immediately when hemodynamic instability cannot be rapidly corrected, ventricular fibrillation develops, cardiac arrest occurs, or uncontrolled bleeding is encountered—with the critical understanding that delayed conversion in the setting of severe hemodynamic compromise or cardiac arrest results in devastating neurological and myocardial complications in approximately 67% of cases. 1

Primary Indications for Immediate Conversion

Hemodynamic Instability

• Severe hemodynamic instability requiring conversion includes inability to maintain adequate perfusion pressure despite aggressive medical management 1, 2
• Target mean arterial pressure should be maintained >60 mmHg to ensure adequate coronary and renal perfusion 3, 4
• Mild-to-moderate hemodynamic instability that responds to inotropic support does not necessarily require conversion and can be managed medically 1
• Inotropic support is required more frequently during off-pump procedures (30.3% vs 15.3% in on-pump cases), which should prompt heightened vigilance 2

Electrical Complications

• Ventricular fibrillation is an absolute indication for immediate conversion 1
• Cardiac arrest requires emergent conversion to cardiopulmonary bypass 1, 5
• Intraoperative dysrhythmias occur significantly more frequently during off-pump procedures (15% vs 3.5% in on-pump cases) and should be monitored aggressively 2

Technical Surgical Issues

• Inability to complete planned revascularization due to difficult vessel access, particularly lateral wall vessels 6
• Dense adhesions that cannot be safely dissected in reoperative cases 7
• Inability to locate intramyocardial vessels 7
• Uncontrolled bleeding that cannot be managed without cardiopulmonary bypass support 5

Critical Timing Considerations

The timing of conversion is the single most important determinant of outcome—early conversion for mild-to-moderate instability results in no adverse ischemic effects, while delayed conversion after severe hemodynamic compromise or cardiac arrest leads to severe cerebral or myocardial ischemia in 67% of patients. 1

Outcomes Based on Conversion Timing

• Conversion for mild-to-moderate hemodynamic instability or difficult graft revision (n=27): no adverse ischemic complications 1
• Conversion for severe hemodynamic instability, ventricular fibrillation, or cardiac arrest (n=9): 66.6% developed severe CNS or myocardial ischemia 1
• Overall acute conversion mortality risk is 4-5 times higher than unconverted off-pump CABG 5

Conversion Rates and Risk Factors

• Expected conversion rate with experienced surgeons: 0.8-1.1% 5
• Conversion rates during learning curve: 5.1% in first 2 years, declining to 0.8% with experience 5
• Overall conversion rate across centers: 9.2% of scheduled off-pump cases or 4.6% of total CABG volume 1
• Reoperative CABG conversion rate: 5.8% 7

Specific High-Risk Scenarios Requiring Lower Threshold for Conversion

Anatomical Considerations

• Patients with extensive ascending aortic disease may benefit from off-pump approach with avoidance of aortic manipulation, but conversion threshold should be lower given stroke risk 6
• Lateral wall vessel grafting presents technical challenges that may necessitate conversion 6

Patient-Specific Factors

• Preoperative renal dysfunction (creatinine clearance <60 mL/min) patients may benefit from off-pump approach, but conversion should not be delayed if instability develops 6, 3
• Patients with preexisting hemodynamic compromise typically require on-pump strategy from the outset 6

Monitoring Parameters to Trigger Conversion Decision

Hemodynamic Thresholds

• Mean arterial pressure <60 mmHg despite maximal medical therapy 3, 4
• Inability to maintain adequate cardiac output during heart manipulation 6
• Progressive metabolic acidosis or rising lactate indicating inadequate perfusion 3

Cardiac Function

• New-onset severe ventricular dysfunction 1
• Persistent ventricular arrhythmias 1, 2
• ST-segment changes indicating ongoing ischemia that cannot be corrected 1

Post-Conversion Management

• Maintain hematocrit >19% and mean arterial pressure >60 mmHg in patients with preexisting renal dysfunction 6
• Measure biomarkers of myonecrosis (CK-MB, troponin) within first 24 hours to detect perioperative MI 6, 3
• Monitor for neurological complications given increased stroke risk with delayed conversion 1, 5

Common Pitfalls to Avoid

• Do not delay conversion in the setting of severe hemodynamic compromise hoping for medical stabilization—this results in catastrophic outcomes 1
• Do not underestimate the significance of persistent dysrhythmias, as they frequently herald hemodynamic collapse 2
• Do not attempt off-pump approach in patients with preoperative hemodynamic instability—these patients should undergo on-pump CABG from the outset 6
• Do not continue off-pump approach when incomplete revascularization is anticipated, as this is associated with worse 1-year outcomes and reduced graft patency 8

Long-Term Outcomes After Conversion

• Converted patients have 3.2-fold increased hazard of death for 3 years post-surgery compared to both unconverted off-pump and planned on-pump patients 5
• Hospital mortality odds ratio: 4.4 compared to unconverted off-pump and 4.7 compared to planned on-pump 5
• These poor outcomes are primarily driven by delayed conversion in the setting of severe instability 1, 5