Three-Dimensional Echocardiography:
Which Role for CRT Patients?
P. NIHOYANNOPOULOS
Heart failure is a major health problem reaching epidemic proportions in the industrialised world, and is associated with substantial mortality and mor- bidity rates. It is estimated that it affects 1–2% of the population and accounts for approximately 5% of all medical admissions. Despite major advances in medical therapy during recent years, heart failure still represents a serious and steadily expanding public health and financial threat. Cardiac resynchronisation therapy (CRT) has been introduced as an adjuvant treat- ment and has since developed rapidly. However, 30% of patients fail to bene- fit from CRT when electrical asynchrony as described by the QRS complex width is used as the sole criterion for selection. This has generated an inter- est in the mechanical aspect of the ventricular dyssynchrony and its superi- ority in describing the left ventricular dysfunction [1].
Atrioventricular (AV) dyssynchrony can be assessed from conventional echocardiog raphy by evaluat ing the durat ion of mit ral inflow.
Interventricular dyssynchrony can be evaluated by assessing the extent of interventricular mechanical delay (IVMD), defined as the time difference between the left and right ventricular pre-ejection intervals. An IVMD of 40 ms or greater is considered indicative of interventricular dyssynchrony.
M-mode echocardiography may be useful for assessing intraventricular dyssynchrony from the parasternal short-axis view by measuring the septal- to-posterior wall motion delay (SPWMD). A cut-off value of 130 ms was proposed as a marker of intraventricular dyssynchrony. However, frequently the SPWMD cannot be obtained, either because the septum is akinetic fol- lowing extensive anterior infarction or because the maximal posterior
Hammersmith Hospital, Imperial College, London, United Kingdom
motion is ill-defined. In addition, it is often not possible to obtain perpen- dicular M-mode section images of the proximal left ventricle.
Tissue Doppler imaging (TDI) allows measurement of peak systolic velocity of different regions of the myocardium and timing of peak systolic velocity in relation to electrical activity (QRS complex). Based on these vari- ables, TDI can provide accurate information on electromechanical coupling, and also assess inter- and intraventricular dyssynchrony. In addition, infor- mation on diastolic function can be obtained. Yu et al. [2] used TDI to assess intraventricular dyssynchrony in 88 normal individuals, 67 patients with heart failure and a narrow QRS complex (≤ 120 ms), and 45 with a wide QRS complex (> 120 ms). In this study, 12 sample volumes were placed in the myocardium, and for each sample the time from onset of QRS complex to peak systolic velocity was measured.
Novel echocardiographic techniques such as colour-coded tissue Doppler and real-time 3D echocardiography (RT3DE) may provide more effective guidance in identifying heart failure patients who are likely to respond posi- tively to CRT. RT3DE provides a powerful tool for the qualitative and quanti- tative assessment of the left ventricle. This modality can provide information on global as well as regional left ventricular dimensions, wall thickness, and function without being limited by the inherent geometrical assumptions of conventional echocardiographic methods. The clinical use of RT3DE has been established and it has been validated against angiography, radionuclide angiography, and cardiac magnetic resonance imagin (CMRI) for the assess- ment of volumes and ventricular function. Recent technological advances have allowed the manufacturing of novel sophisticated matrix-array trans- ducers, which offer fast acquisition of high-quality datasets in real time. At the same time, the development of new dedicated semi-automatic software enables the investigator to perform more accurate measurements. This has further improved inter- and intraobserver agreement as well as the repeata- bility of this method, while strong correlation between RT3DE and CMR in volumetric measurements has been convincingly demonstrated.
Mechanical asynchrony can occur between two or more of the 16 myocar- dial regions described by the American Society of Echocardiography (ASE) from basis to apex. While tissue Doppler techniques have been extensively used to assess dyssynchrony, the limited spatial resolution using tissue Doppler, usually confined to the basal myocardial segments, together with the variable algorithms used by several manufacturers, make tissue Doppler imaging impractical for routine use in the identification of patients who might benefit from CRT.
RT3DE, on the other hand, offers ideal spatial resolution involving the entire left ventricle from base to apex and can therefore be invaluable in the assessment of mechanical asynchrony. With the additional generation of
482 P. Nihoyannopoulos
three-dimensional regional volume–time curves, accurate measurement of regional function and synchronicity in one cardiac cycle can be performed.
Regional time differences enable the identification of the most delayed regions, even when these are located in the most distal left ventricular regions and the apex. This technique could offer precise mapping of the mechanical asynchrony, which may be important in selecting heart failure patients who would benefit from CRT. Furthermore, by providing the elec- trophysiologist with regional temporal information, it could prove to be very effective in the identification of the most delayed site and thus pacing of the most delayed region.
The Dyssynchrony Index has been proposed to correlate with the ejection fraction. Moreover, the correlation between QRS width and the Dyssyn- chrony Index further empowers previous findings that mechanical rather than electrical synchrony offers a finer analysis of the anomalies amenable to resynchronisation.
References
1. Bax JJ, Ansalone G, Breithtardt OA et al (2004) Echocardiographic evaluation of cardiac resynchronization therapy: ready for routine clinical use? A critical apprai- sal. J Am Coll Cardiol: 44:1–9
2. Yu CM, Lin H, Zhang Q (2003) High prevalence of left ventricular systolic and dia- stolic asynchrony in patients with congestive heart failure and normal QRS dura- tion. Heart 89:54–60
483 Three-Dimensional Echocardiography: Which role for CRT Patients?
