5.4 Advanced Hemodynamic Indices & Function

Advanced Hemodynamic Indices & Function

Myocardial Performance Index (MPI / Tei Index)

The Myocardial Performance Index (MPI), also known as the Tei Index, is an advanced Doppler-derived parameter that provides a combined assessment of both systolic and diastolic ventricular function. Unlike traditional measures such as fractional shortening, the MPI is independent of ventricular geometry and heart rate, making it particularly valuable in fetal echocardiography where ventricular shapes can be complex and heart rates are rapid.

Calculating the MPI

The MPI is calculated by measuring the time intervals of the cardiac cycle using spectral Doppler waveforms (typically obtained at the inflow and outflow tracts). The formula is: MPI = (Isovolumetric Contraction Time (ICT) + Isovolumetric Relaxation Time (IRT)) / Ejection Time (ET)

• Isovolumetric Contraction Time (ICT): The brief period between the closure of the AV valve and the opening of the semilunar valve, when pressure builds in the ventricle without a change in volume.
• Isovolumetric Relaxation Time (IRT): The brief period between the closure of the semilunar valve and the opening of the AV valve, when pressure falls in the relaxing ventricle.
• Ejection Time (ET): The duration of blood flow across the semilunar valve.

Normal Values and Clinical Application

• Normal LV MPI: Typically ranges from ~0.36 to 0.42.
• Normal RV MPI: Typically ranges from ~0.42 to 0.48.

An elevated MPI indicates global myocardial dysfunction. Prolongation of the isovolumetric times (ICT and IRT) relative to the ejection time signifies that the heart is spending a disproportionate amount of time building and releasing pressure rather than effectively pumping blood. An elevated MPI is an early marker of fetal cardiac compromise and is frequently observed in conditions such as intrauterine growth restriction (IUGR), twin-twin transfusion syndrome (TTTS), fetal anemia, and maternal diabetes, often before structural changes or overt heart failure become apparent.

Tissue Doppler Imaging (TDI)

Tissue Doppler Imaging (TDI) is an echocardiographic technique that uses Doppler principles to measure the velocity of myocardial tissue motion, rather than the velocity of blood flow. Because myocardium moves much slower but with a higher amplitude than blood, TDI requires specific machine settings: low velocity scale (low PRF) and lack of a high-pass wall filter.

TDI Waveforms and Velocities

By placing the pulsed-wave Doppler sample volume at the basal ventricular walls or the lateral tricuspid/mitral annuli, characteristic waveforms are generated that mirror the cardiac cycle:

• s' (Systolic velocity): Represents the peak myocardial velocity during ventricular systole. It is a sensitive indicator of longitudinal systolic function.
• e' (Early diastolic velocity): Represents myocardial relaxation during early diastole.
• a' (Late diastolic velocity): Represents myocardial motion during atrial contraction.

Clinical Utility of TDI

In the normal fetus, similar to the blood pool E/A ratio, the TDI e'/a' ratio is typically <1.0. TDI is highly sensitive in detecting early, subtle changes in myocardial function. A decreased s' velocity suggests impaired systolic contractility, while a reduced e' velocity or an altered e'/a' ratio points to diastolic dysfunction and impaired myocardial relaxation. TDI is increasingly used to monitor fetuses with structural heart defects or those exposed to adverse hemodynamic environments.

Speckle-Tracking Echocardiography (STE)

Speckle-tracking echocardiography (STE) is a newer, advanced modality that tracks the motion of natural acoustic markers ("speckles") within the myocardium frame-by-frame during the cardiac cycle. This allows for the calculation of myocardial strain and strain rate.

• Strain: Represents the degree of myocardial deformation (shortening or lengthening) expressed as a percentage. In the longitudinal plane, normal ventricular contraction results in a negative strain value (shortening).
• Strain Rate: Represents the speed at which this deformation occurs.

Unlike Tissue Doppler, STE is relatively angle-independent, overcoming one of the major limitations of traditional Doppler techniques. STE provides a highly detailed, quantitative assessment of global and regional myocardial mechanics. Decreased longitudinal strain is a sensitive marker of subclinical myocardial dysfunction and is being heavily researched for its prognostic value in complex fetal cardiac conditions.

Evaluating Function in High-Risk Conditions

Intrauterine Growth Restriction (IUGR)

In severe IUGR, the chronic hypoxemic environment leads to altered fetal hemodynamics. The fetus redistributes blood flow to vital organs (brain-sparing). Cardiac evaluation often reveals:

• Elevated MPI (Tei Index), indicating early global dysfunction.
• Altered venous Doppler patterns, progressing from increased pulsatility in the ductus venosus to eventual reversal of the a-wave, signaling right heart strain and impending decompensation.
• Changes in TDI parameters, reflecting impaired diastolic relaxation.

Twin-Twin Transfusion Syndrome (TTTS)

In monochorionic twin pregnancies complicated by TTTS, the recipient twin experiences severe volume and pressure overload, while the donor twin is hypovolemic.

• Recipient Twin: Echocardiography often demonstrates cardiomegaly, ventricular hypertrophy, AV valve regurgitation, and significantly impaired function characterized by an elevated MPI, reduced fractional shortening, and abnormal venous Doppler (reversed DV a-wave).
• Donor Twin: May show a structurally normal but small heart, with possible signs of decreased cardiac output and altered placental resistance profiles. Advanced modalities like MPI and TDI are crucial for staging TTTS and determining the optimal timing for interventions, such as fetoscopic laser photocoagulation.