Assessing Twist and Torsion

7 Assessing Twist and Torsion



Sherif F. Nagueh



Background



Left Ventricular Systole



• Left ventricular (LV) systole begins with a rapid rise in LV pressure.

• During the isovolumic contraction (IVC) period, pressure increases while LV volume is unchanged.

• During the ejection period, LV volume decreases until it reaches its lowest value at end systole.

• Invasive parameters of LV systolic function during the IVC period include rate of change in pressure over time (dP/dt).

• LV stroke volume, ejection fraction (EF), and stroke work can be used to assess LV systolic function during the ejection phase.

• LV rotation is another marker of LV systolic function during ejection.

• Apical rotation starts by an initial clockwise rotation during the IVC period, but the major rotation occurs during the ejection phase in a counterclockwise direction.

• Basal rotation starts by an initial counterclockwise rotation during the IVC period, but the major rotation occurs during the ejection phase in a clockwise direction.


Left Ventricular Diastole



• LV pressure declines rapidly during the isovolumetric relaxation (IVR) period.

• During IVR, untwisting or recoil takes place and precedes LV expansion in the longitudinal and radial directions.

• An intraventricular pressure gradient follows untwisting, which helps direct LV intracavitary flow to the LV apex.

• This is followed by LV long axis expansion and mitral annulus early diastolic recoil velocity (e’).

• When the left atrial (LA) pressure exceeds LV diastolic pressure, the mitral valve opens and transmitral early diastolic velocity (E) is recorded.

• Therefore, LV untwisting is the earliest mechanical event during diastole.


Relation of Torsion to Cardiac Fiber Orientation



• Myocardial fibers are arranged in a helical structure.

• The subendocardial fibers are arranged in a right-handed helix.

• The subepicardial fibers are arranged in a left-handed helix.

• Earlier activation of subendocardial fibers accounts for the apical clockwise rotation during IVC.

• The later contraction of the subepicardial fibers accounts for apical counterclockwise rotation during ejection. This is due to the larger radius of subepicardial fibers.


Definitions of Twist and Torsion



• Direction of rotation is determined by looking at the heart from the apex.

• Apical rotation starts with an initial clockwise movement followed by a net counterclockwise direction (positive sign) in normal hearts (Fig. 7-1).

• Basal rotation starts with an initial counterclockwise movement followed by a net clockwise direction (negative sign) in normal hearts (Fig. 7-2).

• Twist is the net difference between apical and basal rotations (algebraic difference).

• Twist is normally around 7 to 9 ± 3 to 4 degrees.

• Twist varies with age and some studies have shown that it is higher in elderly subjects (>60 years).

• Torsion is the twist in degrees divided by LV long axis (distance between basal and apical segments).

• A better account of cardiac size is possible by calculating torsion as: Twist multiplied by average value of the radii of LV apical and basal cross sections divided by length.

• The above expression can allow comparison of rotational deformation between hearts of different sizes.

image

Figure 7-1 Apical rotation in degrees of rotation (y axis) versus time in seconds (x axis). Notice the presence of an initial small clockwise rotation (deflection below baseline, arrow), followed by counterclockwise rotation (about 8 degrees, arrow). The rotation curve is shown in blue. The electrocardiogram is shown in green.


image

Figure 7-2 Basal rotation in degrees (y axis) versus time (x axis). Notice the presence of an initial small counterclockwise rotation (deflection above baseline, arrow), followed by clockwise rotation (about 3.5 degrees, arrow). Twist in this example is 8 − (−3.5) = 11.5 degrees.



Definition of Untwisting



• Untwisting or recoil occurs during the isovolumetric relaxation time (IVRT) and early LV filling period.

• It occurs in a direction opposite to that of twisting.

• Untwisting can be expressed as the amount of recoil during IVRT.

• Peak untwisting velocity is usually measured and used to reflect the direction and speed of recoil.

• Untwisting rate during IVRT only can be measured as well.

• Aside from peak untwisting velocity, the timing of peak untwisting provides additional information.

• Normally, peak untwisting precedes LV expansion and mitral inflow.


Hemodynamic Determinants of Twist/Torsion



• An increase in LV contractility leads to an increase in torsion (Figs. 7-3 and 7-4). A direct positive correlation exists between torsion and LV EF, stroke volume, and dP/dt.

• Some animal studies have shown that an increase in heart rate increases twist.

• Increased afterload with unchanged preload leads to a decrease in twist.

• In normal humans, increased preload can lead to an increase in twist.

• In patients, simultaneous changes in load and contractility often occur and limit the clinical application of twist as a pure index of LV contractility.

image

Figure 7-3 Changes in twist with alterations in left ventricular (LV) contractility from a dog with normal LV function. When contractility increased with dobutamine, twist increased from 7 to 11 degrees. When contractility was depressed with esmolol, twist decreased to 1.7 degrees.


image

Figure 7-4 Changes in LV twist from an animal model of pacing-induced heart failure. When heart failure was induced by rapid pacing for several weeks, LV twist decreased from 7 to 4.6 degrees. Pacing was then stopped and LV function improved with time. At recovery twist was back to normal at 8 degrees.



Hemodynamic Determinants of Peak Untwisting Velocity



• An increase in LV twist leads to an increase in untwisting. This positive correlation occurs in normal and diseased hearts.

• The positive association between twist and untwisting was shown in animal experiments and human studies.

• A faster decline in LV pressure during IVRT, in the absence of a change in loading conditions, leads to an increase in LV peak untwisting velocity.

• In animal studies, dobutamine infusion was accompanied by shortening of tau and an increase in peak untwisting velocity.

• In animal studies, esmolol infusion was accompanied by prolongation of tau and a decrease in peak untwisting velocity.

• In animal studies, increased afterload was associated with reduced untwisting.

• In human studies, increased LV afterload leads to reduced and delayed untwisting.

• The impact of afterload on timing of untwisting was noted in patients with hypertrophic obstructive cardiomyopathy (HOCM).

• HOCM patients had delayed untwisting that occurred earlier after relief of dynamic obstruction.

• In animal models, an acute increase in LV end-systolic volume leads to a decrease in untwisting velocity, despite unchanged LV relaxation. Conversely, an acute decrease in LV end-systolic volume leads to an increase in peak untwisting velocity in the absence of changes in LV relaxation (Fig. 7-5).

• In normal animals where load and contractility are both altered, peak untwisting velocity tracks best the changes in LV end-systolic volume.

• In patients with systolic heart failure, LV twist, time constant of LV relaxation, and LV end-systolic volume are the hemodynamic determinants of peak untwisting velocity.

• In patients with diastolic heart failure (DHF), LV twist and LV end-systolic volume (but not the time constant of LV relaxation) are the hemodynamic determinants of peak untwisting velocity.

image

Figure 7-5 Regression plot (y versus 1/x) between LV untwisting rate and LV end-systolic volume (ESV) in animal experiments with LV volumes altered by inferior vena cava occlusion and esmolol infusion. There was no significant change in tau with changes in LV ESV.


(Data from Wang J, Khoury DS, Yue Y, et al. Left ventricular untwisting rate by speckle tracking echocardiography. Circulation. 2007;116:2580–2586.)



Relation of Peak Untwisting to Left Ventricular Filling



• The following sequence of events occurs during diastole in the normal heart:
• LV untwisting.

• Basal to apical intraventricular pressure gradient.

• LV expansion in longitudinal and radial directions.

• Mitral valve opening followed by LV filling during early diastole.

• The above normal sequence is preserved in normal hearts with exercise, which leads to earlier onset of LV untwisting and therefore adequate LV filling despite higher heart rate and shorter diastolic filling time.

• With cardiovascular disease, the above sequence is disturbed such that peak LV untwisting can be delayed and may even follow mitral valve opening. This deprives the ventricle from the beneficial effects of untwisting on LV filling, which is maintained in these hearts by increased LA pressure.

• In patients with abnormal myocardial function, delayed untwisting is associated with increased LV filling pressures.

• Patients with DHF and delayed untwisting have larger LA volumes and higher pulmonary artery pressures than patients without delayed untwisting.


Effects of Exercise on Left Ventricular Twist Mechanics



• In normal subjects, twist/torsion and untwisting velocity all generally increase with exercise.

• There are variations based on exercise type, intensity and duration, and whether measurements are obtained from elite athletes or not.

• LV untwisting occurs earlier with exercise and precedes mitral valve opening.

• Short-term high-intensity exercise is accompanied by increased torsion.

• After an ultra-long-distance triathlon, LV twist and untwisting can be reduced and delayed and accompanied by other indices of depressed LV systolic and diastolic function.


Key Points



• Apical rotation occurs in a net counter-clockwise direction when viewed from the apex (units: degrees).

• Basal rotation occurs in a net clockwise direction when viewed from the apex (units: degrees).

• Twist is the net difference between apical and basal rotations (units: degrees).

• Torsion is twist divided by LV long axis (units: degrees/cm).

• Peak untwisting rate measures the rate of recoil during diastole (units: negative degrees/s).

• Peak untwisting is the earliest event in diastole and precedes LV filling.


Key Points



• LV contractility, preload, and afterload all affect LV torsion.

• LV twist, volumes, and filling pressures all affect LV untwisting irrespective of LV EF.

• In patients with systolic, and not diastolic, heart failure, a significant inverse correlation is present between the LV time constant of LV relaxation and peak untwisting velocity.

• Peak untwisting velocity occurs before mitral valve opening and helps maintain LV filling at low filling pressures.

• Peak untwisting velocity is delayed with increased afterload, and this change renders the LV dependent on LA pressure to maintain LV filling.


Echocardiographic Approach (Table 7-1)



Acquisition



• Patients are imaged in the supine lateral position.

• Transducer frequency is 1.7 to 2 MHz; a higher frequency can be used in the absence of concerns for attenuation.

• Tissue Doppler (TD) has been used for measurement torsion and untwisting.

• TD has the advantage of high frame rate and superior temporal resolution.

• TD can be of value when imaging patients with rapid heart rates, as in children and during exercise.

• TD has the limitation of a low signal-to-noise ratio and is heavily dependent on the presence or absence of angulation between the ultrasound beam and the plane of motion.

• The above limitations of TD have limited its application for the objective of measuring torsion and untwisting.

• Speckle tracking echocardiography (STE) is the most practical technique to measure torsion at this time.

• Images are obtained from parasternal short axis views acquired using a frame rate of around 80/s.

• A basal short axis view is acquired at the mitral valve level. This is identified by noting mitral valve leaflets in the middle of the view.

Related posts:

  1. Advanced Echocardiography Approaches: 3D Transesophageal Assessment of the Mitral Valve
  2. Contrast Perfusion Echocardiography
  3. Transthoracic Three-Dimensional Echocardiography
  4. Two-Dimensional and Three-Dimensional Echocardiographic Evaluation of the Right Ventricle

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Jun 4, 2016 | Posted by in CARDIOLOGY | Comments Off on Assessing Twist and Torsion

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