How is myocardial contractility managed in clinical practice?

Management of Myocardial Contractility in Clinical Practice

Myocardial contractility is managed through assessment of cardiac function using stress echocardiography, pharmacological interventions, and optimization of preload and afterload conditions to improve patient outcomes related to morbidity, mortality, and quality of life.

Understanding Myocardial Contractility

• Contractility refers to the intrinsic ability of heart muscle to generate force and shorten, independent of changes in preload or afterload at fixed heart rates 1
• At the molecular level, contractility depends on changing concentrations of Ca²⁺ ions in myocardial cytosol, which trigger the contraction-relaxation cycle 2
• Contractility is distinct from cardiac performance, as it represents the length-independent intrinsic chemo-mechanical processes responsible for force development and velocity 1

Assessment of Contractility in Clinical Practice

Stress Echocardiography

• Exercise is the preferred stress modality for assessing contractility, as it preserves the integrity of the electro-mechanical response and provides valuable information about functional status 3
• Semi-supine bicycle exercise is technically easier than upright bicycle or treadmill exercise, especially when multiple stress parameters need assessment at peak exercise 3
• Stress echocardiography allows evaluation of:
  • Wall motion abnormalities
  • Valvular function
  • Hemodynamic responses
  • Links between symptoms and cardiovascular workload 3

Pharmacological Stress Testing

• Dobutamine stress testing is used when patients cannot exercise adequately 3
• Dobutamine acts primarily on β-1 adrenergic receptors, increasing heart rate and contractility 3
• During dobutamine infusion:
  • Left ventricular end-systolic volume decreases more than end-diastolic volume
  • Cardiac output increases due to increased heart rate and stroke volume
  • Myocardial contractility (measured as elastance) increases over four-fold in normal subjects but less than two-fold in patients with dilated cardiomyopathy 3

Advanced Assessment Methods

• Pressure-volume loops provide the most reliable index for assessing myocardial contractility in intact circulation and are relatively insensitive to changes in preload and afterload 2
• Force-frequency relationship assessment can uncover initial alterations in contractility before abnormalities appear at rest 2

Management Strategies for Altered Contractility

Pharmacological Management

Inotropic Support for Decreased Contractility

• Dobutamine is indicated for short-term inotropic support in patients with cardiac decompensation due to depressed contractility from organic heart disease or cardiac surgical procedures 4
• Milrinone acts as both a positive inotrope and vasodilator with little chronotropic activity by selectively inhibiting peak III cAMP phosphodiesterase isozyme in cardiac and vascular muscle 5
• Milrinone produces dose-related and plasma concentration-related increases in:
  • Maximum rate of increase of left ventricular pressure
  • Slope of left ventricular pressure-dimension relationship
  • Forearm blood flow in heart failure patients 5

Important Clinical Considerations

• Long-term use of cyclic-AMP-dependent inotropes (whether oral or intravenous) has not been shown to be safe or effective in controlled trials for congestive heart failure 4
• In heart failure, the ventricle may develop eccentric hypertrophy with addition of new sarcomeres to maintain preload reserve despite chamber dilation 6
• Preload reserve (heart's ability to increase stroke volume in response to increased venous return) can become exhausted in advanced heart failure 6

Special Clinical Scenarios

Hypertrophic Cardiomyopathy

• Exercise stress echocardiography is useful for evaluating symptoms and monitoring response to therapy in patients with hypertrophic cardiomyopathy 3
• Dynamic left ventricular outflow tract obstruction (>50 mmHg), abnormal blood pressure response, blunted contractile reserve, and worsened mitral regurgitation are associated with poor exercise capacity and outcomes 3

Heart Failure with Reduced Ejection Fraction

• Residual myocardial contractile reserve assessed by stress echocardiography can help distinguish ischemic from non-ischemic disease, assess outcomes, and aid clinical decision-making 3
• Both exercise and dobutamine protocols can be used to evaluate contractile reserve, with high-dose dobutamine more likely to invoke a contractile response in patients on beta-blockers 3

Congenital Heart Disease

• In patients with tetralogy of Fallot, exercise echocardiography can evaluate right and left ventricular contractile reserve 3
• Right ventricular function can be assessed by measuring right ventricular fractional area change (RVFAC), tricuspid annular plane systolic excursion (TAPSE), and tissue Doppler velocities 3

Pitfalls and Caveats

• A decrease in contractility may be masked by changes in preload or afterload, maintaining a normal ejection volume and delaying diagnosis of myocardial damage 7
• When low ejection fraction is caused by depressed contractility rather than excessive afterload, corrective surgery will be less beneficial 3
• In patients with aortic stenosis, the hypertrophied heart may have reduced coronary blood flow per gram of muscle and limited coronary vasodilator reserve, even without coronary artery disease 3
• Excessive or inappropriate hypertrophy, particularly common in elderly patients and women, can lead to high perioperative morbidity and mortality 3
• Loss of atrial contraction (as in atrial fibrillation) can cause serious clinical deterioration in patients with impaired ventricular filling 3