Why does mitral regurgitation (MR) worsen when coming off cardiopulmonary bypass (CPB)?

Why Mitral Regurgitation Worsens When Coming Off Cardiopulmonary Bypass

Mitral regurgitation (MR) worsens when coming off cardiopulmonary bypass primarily because the elimination of the low-resistance regurgitant pathway suddenly increases left ventricular afterload, unmasking pre-existing LV dysfunction that was previously concealed by favorable loading conditions. 1

Pathophysiological Mechanisms

The worsening of MR during weaning from cardiopulmonary bypass (CPB) occurs due to several key mechanisms:

1. Sudden Afterload Increase:

   • In chronic MR, the LV adapts to volume overload by developing eccentric hypertrophy with increased end-diastolic volume
   • The LV operates under favorable loading conditions with low afterload due to the "pop-off valve" effect of regurgitation into the low-pressure left atrium
   • After valve repair/replacement, the LV must suddenly eject its entire stroke volume into the higher-resistance systemic circulation 1

2. Unmasking of LV Dysfunction:

   • Chronic MR leads to compensatory mechanisms that mask underlying LV dysfunction
   • Even mildly reduced LVEF in MR (e.g., <60%) may indicate significant dysfunction
   • This latent contractile dysfunction becomes apparent when the regurgitant pathway is eliminated 1

3. Reduced Preload:

   • Elimination of the regurgitant volume reduces preload
   • The previously volume-overloaded LV now faces both increased afterload and reduced preload simultaneously 1

Complicating Factors

Several additional factors can exacerbate difficulties in weaning from CPB:

• Pulmonary Hypertension: Often develops from chronic MR and may persist after valve surgery 1
• Right Ventricular Dysfunction: Can result from long-standing pulmonary hypertension 1
• Potential Surgical Complications:
  • Left circumflex coronary artery injury during valve surgery
  • Paravalvular leaks
  • Air embolism to coronary arteries
  • Prosthetic valve dysfunction 1

Management Strategies

To address worsening MR when coming off bypass:

1. Diagnostic Assessment:

   • Transesophageal echocardiography is crucial to:
     • Confirm adequate valve repair/replacement
     • Assess ventricular function
     • Rule out technical complications
     • Evaluate for residual air 1

2. Pharmacological Support:

   • Inotropic Agents: Dobutamine or milrinone to support contractility
   • Vasodilators: Nitroprusside or nitroglycerin to reduce afterload 1

3. Mechanical Support for refractory cases:

   • Intra-aortic balloon pump
   • Temporary mechanical circulatory support 1

Prevention Strategies

To minimize difficulties when weaning from CPB:

1. Earlier Intervention: Consider surgery when LVEF approaches but hasn't fallen below 60% 1

2. Surgical Technique:

   • Valve repair is preferred over replacement when feasible
   • Preservation of subvalvular apparatus during replacement
   • Meticulous de-airing to prevent air embolism 1

Common Pitfalls

• Underestimation of LV Dysfunction: LVEF often overestimates true LV performance in MR due to favorable loading conditions 1
• Inadequate Preload Management: Overlooking the need for volume optimization
• Overlooking Right Heart Failure: Focusing solely on left ventricular function
• Residual MR: Incomplete repair can exacerbate difficulties in weaning 1

Understanding these mechanisms is crucial for anticipating and managing the hemodynamic challenges that occur when weaning patients with MR from cardiopulmonary bypass.