What is the hangout interval in the cardiac cycle, also known as the isovolumic relaxation time (IVRT)?

Isovolumic Relaxation Time (IVRT) in the Cardiac Cycle

The isovolumic relaxation time (IVRT) is the period in the cardiac cycle that begins with aortic valve closure and ends with mitral valve opening, typically lasting 70 milliseconds or less in normal subjects, during which the left ventricle relaxes without any change in volume.

Definition and Physiological Significance

IVRT represents a critical phase in the cardiac cycle with the following characteristics:

• Timing: Occurs between the end of systole and the beginning of diastole
• Hemodynamic events:
  • Begins with aortic valve closure (when left ventricular pressure falls below aortic pressure)
  • Ends with mitral valve opening (when left ventricular pressure falls below left atrial pressure)
  • During this period, all valves are closed and ventricular volume remains constant

Measurement Methods

IVRT can be measured using several echocardiographic techniques:

• M-mode echocardiography: Time from aortic valve closure to mitral valve opening
• Doppler echocardiography: Time from end of aortic outflow to beginning of mitral inflow
• Tissue Doppler imaging: Can provide complementary information about relaxation

Normal Values and Clinical Significance

• Normal IVRT: ≤70 milliseconds in healthy subjects 1
• Prolonged IVRT:
  • Indicates impaired left ventricular relaxation
  • Common in conditions with diastolic dysfunction
  • Associated with aging, hypertension, left ventricular hypertrophy
• Shortened IVRT (≤65 milliseconds):
  • Suggests elevated left atrial pressure 1
  • Can be seen in advanced diastolic dysfunction (restrictive physiology)
  • Used as a marker of elevated filling pressures, especially in atrial fibrillation

Factors Affecting IVRT

IVRT duration is influenced by multiple factors:

1. Rate of left ventricular pressure decline: Determined by the time constant of relaxation (tau)
2. Aortic pressure at valve closure: Higher pressure requires longer time to fall
3. Left atrial pressure: Higher LA pressure shortens IVRT as mitral valve opens earlier
4. Heart rate: IVRT shortens with increasing heart rate
5. Arterial pressure: Can affect timing of aortic valve closure 2

Clinical Applications

IVRT serves as a valuable parameter in assessing diastolic function:

• Diagnostic value:

  • IVRT ≤65 msec suggests elevated left atrial pressure in patients with atrial fibrillation 1
  • Can be combined with other parameters (E/e' ratio, TR velocity) to estimate filling pressures
  • Useful when other parameters are inconclusive or unavailable

• Prognostic implications:

  • Abnormal IVRT correlates with diastolic dysfunction severity
  • Can help identify subclinical cardiac dysfunction

Special Considerations

• Atrial fibrillation: IVRT remains a valid parameter despite irregular rhythm, but requires averaging of multiple beats (5-10 consecutive cycles) 3
• Technical limitations:
  • Different measurement methods yield different values 4
  • IVRT measured from aortic closure to mitral valve opening on M-mode differs from Doppler-derived measurements
  • Results differ based on using continuous wave vs. pulsed wave Doppler 1

Interpretation in Different Cardiac Conditions

• Restrictive cardiomyopathy: IVRT <50 msec indicates advanced disease with restrictive physiology 1
• Hypertrophic cardiomyopathy: Generally prolonged IVRT due to impaired relaxation, but can be shortened in advanced disease with elevated filling pressures 5
• Heart failure with preserved ejection fraction: IVRT often prolonged in early stages, may normalize or shorten as disease progresses and filling pressures rise

Remember that IVRT should not be interpreted in isolation but as part of a comprehensive diastolic function assessment including other parameters such as E/e' ratio, left atrial volume index, and tricuspid regurgitation velocity.