This article appears in the following Europace issue: Spotlight Issue: Cardiac Imaging in EP and CRT [View the issue table of contents]
IMAGING IN CATHETER ABLATION FOR AF
Approaching regional left atrial function by tissue Doppler velocity and strain imaging
Division of Cardiology, Department of Medicine and Therapeutics, SH Ho Cardiovascular and Stroke Centre, Institute of Vascular Medicine, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin N.T., Hong Kong, Peoples' Republic of China
* Corresponding author. Tel: +86 852 26323594; fax: +86 852 26375643.E-mail address: cmyu{at}cuhk.edu.hk
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Abstract |
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 Top Abstract IntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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Atrial function is an integral part for the proper performance of the circulatory system. Assessment of its haemodynamic and mechanical characteristics by use of non-invasive echocardiography, including tissue Doppler velocity and strain imaging, may provide a better insight into atrial function and its relationship with ventricular function. From an electromechanical perspective, this review summarizes not only the various methods for evaluating regional atrial function by tissue Doppler imaging, but also the normal findings in healthy subjects and the major clinical utilities in cardiac diseases, such as atrial fibrillation, ischaemic heart disease and heart failure.
Key Words: Atrium, Myocardial function, Echocardiography
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Introduction |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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The haemodynamic function of the left atrium (LA) primarily modulates the left ventricular (LV) filling through its three components: a reservoir component during ventricular systole, a conduit component during early ventricular diastole, and a booster pump component during late ventricular diastole. The change of the LA function in different phases can be assessed non-invasively by echocardiography, using not only conventional methods such as changes in LA area and volume, but also novel techniques such as tissue Doppler imaging (TDI) and strain imaging. Tissue Doppler imaging quantifies regional tissue motion velocity whereas strain and strain rate represent the extent of local tissue deformation and its rate, respectively. These novel technologies have been validated for the assessment of both global and regional LV function and have recently been applied to the evaluation of regional LA function. From an electromechanical perspective, echocardiographic parameters that assess LA mechanical function may provide a greater understanding of atrial performance and its relationship with ventricular function.
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Assessment of left atrial and appendage function by tissue Doppler imaging |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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Both spectral pulse TDI or two-dimensional colour-coded TDI can be applied to generate a myocardial velocity curve to assess a regional LA function, by placing a small sample volume at an atrial segment of interest, usually about 2 mm for measuring velocity and preferably not more than 12 mm of length for strain and strain rate, because of its thin-walled structure. Unlike the spectral pulse TDI that has a better temporal resolution but can measure only one segment at a time, colour-coded TDI images can be processed offline and offer simultaneous multi-segment analyses of velocities and other TDI-derived parameters, such as strain and strain rate. Thus, different LA walls with their corresponding levels from the mitral annulus can be compared and assessed, in particular by offline analysis of colour TDI, such as septal and lateral walls in an apical four-chamber view, anterior and inferior walls in a two-chamber view.1
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The atrial myocardial velocity curve consists of three major deflections: ventricular systolic (Sa), early ventricular diastolic (Ea), and late ventricular diastolic (atrial contraction, Aa) waves (Figure 1A). In addition, the three components in strain and strain-rate imaging can also be readily identified (Figure 1B and C). The Aa-wave has been regarded as a direct measure of regional active atrial contraction on the longitudinal axis, which might be less load-dependent.1–3,6 The Sa and Ea waves may represent the passive expansion and emptying components of the LA function.5,7 The feasibility and reproducibility of TDI parameters, in particular the peak velocity (VAa) and peak strain rate (SRAa) of the active atrial contraction, have been demonstrated in previous studies, in which both the inter- and intra-observer variability for measuring the VAa were reported to be within 10%.1,3,5 Although the velocity data could be easily obtained in nearly all patients, strain-rate measurements were only feasible in about 95% of patients due to the relatively higher noise-to-signal ratio.4
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Left atrial appendage (LAA) is a highly contractile structure with a pattern of contractions totally different from that of the LA main body. It is more compliant and therefore plays an important role in the LA reservoir function, especially during increases in the LA pressure or volume.8
Transoesophageal echocardiography (TEE) provides essential information about LAA structure and function. In addition to the LAA size, emptying and filling velocities,8,9 a characteristic triphasic TDI profile (Figure 2) can be obtained readily at the tip, the septal, or lateral wall of the LAA by TEE. During sinus rhythm, there is an initial upward velocity in early atrial systole, i.e. in late ventricular diastole before the P-wave on ECG. This is followed by another upward velocity (after the P-wave), the late systolic wave (LSW), that has a higher amplitude and corresponds to the late emptying flow of the LAA. Lastly, the negative late diastolic wave (LDW) occurs during the LAA filling.
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Atrial pump function and electromechanical coupling in healthy subjects |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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In healthy subjects, the right atrial (RA) free wall has been found to have the highest VAa or SRAa.1
,2 A study by Zhang and colleagues, which included 131 healthy adults aged from 22 to 81 years, measured the atrial wall VAa at mid-level. This was found to be significantly higher in the RA than in the LA free wall (9.0 ± 2.6 vs. 7.5 ± 2.4 cm/s, P < 0.001), but lowest in the inter-atrial septum (IAS) (5.6 ± 1.3 cm/s, P < 0.001 vs. either RA or LA free wall) (Figure 3).2 The difference in the atrial velocities at different sites was attributed to an atrial free-wall motion higher than that of the bounded IAS. Furthermore, the larger pectinate muscle in the RA can perhaps generate a more pronounced and sustained longitudinal movement in the relatively low pressure system of the right ventricle. In the same study, when ‘older’ (
60 years) and ‘younger’ (<60 years) age groups were compared, VAa was elevated in both the RA (9.6 ± 2.8 vs. 8.0 ± 2.1 cm/s, P < 0.01) and LA (8.1 ± 2.7 vs. 6.7 ± 1.4 cm/s, P < 0.001) free walls in the older age group. Thomas et al.10 also illustrated the effect of age on atrial pump function in their study of 92 healthy subjects, in that the older age group (50 years) had a significantly higher VAa. With ageing, there is impairment of LV myocardial relaxation and early filling, with subsequent increase in the LA pressure and volume. By the Frank–Starling law, over-stretching the atrial myocardium results in augmentation of LA contractility.10–13 The results of the direct measurement of the LA mechanical function by TDI have corroborated previous observation by Doppler echocardiography that, with ageing, there is a shift in the relative contribution of the different haemodynamic phases of the LA function in the LV filling, from a reduction of the passive to a compensatory increase in the active emptying function.14
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In addition, TDI-derived measurement of timing variables, such as the time interval from the beginning of the P-wave on ECG to the onset of the Aa-wave on the myocardial velocity curve, has been adopted as a non-invasive measure of atrial electromechanical delay.1
,2 In normal subjects, the electromechanical coupling is shortest at the RA, followed by the IAS, and then the LA lateral wall (Figure 3),1,2 and follows the same electrical activation process.15 |
Left atrial mechanical function in atrial fibrillation |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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Atrial fibrillation (AF) is the most common cardiac arrhythmia associated with an at least two-fold increase in morbidity and mortality, and occurs in 0.4% of the general population, increasing to 5% in those
65 years old.16 With the loss of atrial booster pump function, the LA–LV pressure gradient during early LV filling is enhanced by elevation of the LA pressure to maintain stroke volume.17 Thus, a reduction in both LA compliance and volume has been observed with the onset of AF that further decreases cardiac function and increases the risk of thrombo-embolism.
Using TDI-derived velocity and strain-rate parameters, it was found that the LA mechanical function was significantly decreased in AF as reflected by reduction in the Sa and/or Ea-wave in the absence of the Aa-wave during late diastole. Wang et al.5 compared 52 patients suffering from AF for less than 1 year with 27 matched normal control subjects. By placing the sample volume at the basal level of the LA wall, the velocity measured during ventricular systole (2.36 ± 1.04 vs. 3.68 ± 0.99 cm/s, P < 0.001) and early diastole (2.78 ± 1.26 vs. 3.50 ± 1.11 cm/s, P < 0.05), as well as the strain rate during ventricular systole (2.05 ± 0.96 vs. 2.83 ± 0.73 1/s, P < 0.01), were markedly reduced in the AF patients. In particular, the strain rate during ventricular early diastole was significantly lower in patients who failed the initial cardioversion or reverted to AF within 4 weeks after initial successful cardioversion when compared with those who were successfully cardioverted and remained in sinus rhythm (2.32 ± 0.95 vs. 3.17 ± 0.93 1/s, P < 0.01). Similarly, Salvo and colleagues placed the sample volume at the mid-LA walls and obtained myocardial velocity, strain and strain-rate data at both ventricular systole and early diastole in 65 patients suffering from lone AF for at least 3 months.7 All the measured myocardial properties were significantly reduced in the AF patients when compared with normal referents. Furthermore, the strain rate during ventricular systole from the individual LA wall was much lower in patients who had more than one recurrent AF episode within a 9-month follow-up period after cardioversion than those who remained in sinus rhythm. These results may reflect the decreased compliance of LA walls in patients with AF, which is in agreement with several studies demonstrating that during AF the reservoir and conduit function are impaired and the booster pump function is lost. On the other hand, a markedly impaired LAA wall contraction was also reported in AF patients using pulsed-wave TDI.18 The LSW (upward) and LDW (downward) velocities were reduced while the initial upward velocity wave during early ventricular diastole disappeared.
Transthoracic DC cardioversion is one of the most widely used and effective treatments in restoration of sinus rhythm that may ameliorate the detrimental effects of AF, as well as prevent the development of associated tachycardia-induced cardiomyopathy.19 However, it has a high recurrence rate determined by multiple factors, including patient age, origin and duration of AF, and functional class and degree of LA enlargement. Previous studies have demonstrated that severe impairment of atrial deformation during ventricular early diastole was an independent predictor of recurrent AF after adjusting for clinical and other echocardiographic parameters.5,7 Nevertheless, combining LA mechanical dysfunction with the degree of LA enlargement gave the strongest predictive value of AF recurrence.5 Recent advances in surgical or catheter ablation of AF have been emerging; however, recurrence after the procedure remains a challenge. It would be, therefore, of interest to explore in future studies whether the LA myocardial function as assessed by tissue Doppler and strain imaging could provide additional useful information, e.g. guiding pharmacological therapy and duration of anticoagulation after cardioversion in patients found to have lower atrial myocardial velocity and/or deformation.
Atrial stunning is characterized by reduced atrial mechanical function after restoration of sinus rhythm from AF, which may last several weeks with associated increased thrombo-embolic risk.20 Thomas et al. demonstrated a gradual recovery of atrial pump function after DC cardioversion, by use of strain rate, in 37 patients with chronic AF who had a restored sinus rhythm. Their LA myocardial function was evaluated at 4 h (baseline), 1- and 6-month after cardioversion. It was observed that the SRAa measured at basal LA level was significantly lower than normal controls immediately after the procedure (–0.53 ± 0.31 vs. –1.6 ± 0.73 1/s, P < 0.001), but improved over time, with maximal changes in the initial 4 weeks, in parallel to other parameters of atrial function.21 Not surprisingly, a similar temporal sequence of recovery of LAA wall motion after cardioversion could also be demonstrated by improving the strain and strain-rate parameters.22 This is consistent with previous findings that a dissociation of electrical and mechanical recovery occurs after successful cardioversion, with a delay in gradual improvement of atrial mechanical function.23,24 Intriguingly, the delayed atrial contraction or prolonged timing between aortic valve closure and peak SRAa observed soon after successful cardioversion remains unchanged over the following 6 months, despite the apparent improvement in the peak SRAa.21
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Assessment of left atrial mechanical function in other cardiac diseases |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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Ischaemic heart disease
Atrial contractile dysfunction appears early in ischaemic heart disease (IHD), irrespective of previous myocardial infarction, co-existing systolic dysfunction, or severity of diastolic dysfunction. Yu et al.1
found that the VAa measured at mid-level of the IAS and the lateral LA in the apical four-chamber view were drastically reduced in 118 patients with IHD when compared with 100 normal subjects. A poor LV-ejection fraction and the presence of a restrictive LV-filling pattern were the most important determinants of LA contractile dysfunction in IHD. Furthermore, each per cent increase in ejection fraction correlated with an increase in VAa in LA by 0.06 cm/s, while the occurrence of a restrictive filling pattern was associated with a reduction of VAa in LA by 3.17 cm/s.1 The dramatic reduction of LA VAa in the presence of a restrictive filling pattern suggested the presence of a possible LA contractile dysfunction, which has been proposed in these patients. This notion was also supported by a significant increase in the LA size and reduced active emptying fraction observed in the same cohort.1 In contrast, atrial electromechanical delay was unaffected in IHD.
Advanced heart failure and cardiac resynchronization therapy
The atria adapt to changes in ventricular filling commonly observed in congestive heart failure by adjusting the relative proportion of reservoir, conduit, and pump components in order to maintain the ventricular stroke volume. However, depression of atrial pump performance will eventually occur as the heart failure progresses, despite increased atrial preload, due to a myopathic process and/or over-distension of atrial fibres.25 Thus, in chronic heart failure, both the velocity and strain during LA contraction are attenuated, in that the former independently predicts exercise capacity.26 However, the LA mechanical function can be modified by heart failure treatment, such as cardiac resynchronization therapy (CRT), which is of proven benefit to advanced heart failure patients with prolonged QRS duration. Apart from its clinical benefits, improvement in LV systolic function and associated LV reverse remodelling have been reported.27–29 In this regard, TDI helps investigate the improvement in regional LA mechanical function from volume and pressure unloading after CRT. A landmark study by Yu et al.30 examined atrial function and remodelling in this population by a combined assessment using conventional and new echocardiographic imaging tools. It was observed that atrial remodelling was evident at 3 months after CRT, identified by a reduction in the atrial area and volume before and after atrial systole. Meanwhile, atrial contractile function was significantly improved in LA, including increases in atrial emptying fraction, peak velocity, and strain during atrial contraction (Figure 4A–D). There was also improvement of the LA peak strain during ventricular systole and early diastole, which signifies an improvement in atrial compliance. Of note, however, is that these changes were mainly observed in responders showing LV reverse remodelling (Figure 5A–D).
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s), early diastole (Mitral stenosis and mitral regurgitation
Due to inflow obstruction, the atrial booster pump contributes less to LV filling in mitral stenosis even during sinus rhythm, despite a proportional increase, with increasing severity, in the LA preload.31
The impaired atrial reservoir and pump function are associated with a reduction in LA compliance and intrinsic myocardial contractility.32 In addition, the LAA is inevitably affected in mitral stenosis, with reduced contribution to overall LA emptying fraction and increased risk of thrombus formation.33 The LSW and LDW tissue velocities recorded during sinus rhythm at the lateral wall or at the tip of the LAA were markedly reduced in subjects with mitral stenosis when compared with normal subjects.34–36 Systolic velocity was further decreased in patients with spontaneous echo contrast (SEC) in the LAA than those without.35 Interestingly, both the systolic and diastolic components of the atrial myocardial velocity curve were acutely increased after balloon mitral valvuloplasty,37,38 and SEC was diminished or even completely abolished. In addition, the mean transmitral gradient significantly correlated with LDW tissue velocity before and after the procedure.37
In contrast, the role of TDI in assessing atrial pump function in mitral regurgitation has not been fully explored. It may provide complementary information in this population, since the presence of mitral regurgitation often interferes with the interpretation of LV diastolic function as well as LA using conventional two-dimensional or Doppler parameters. Gurlertop et al.34 studied a group of patients with pure mitral regurgitation, who were compared with patients who had rheumatic mitral stenosis and with healthy subjects. It was observed that both LSW and LDW velocities of the LAA were reduced in mitral regurgitation, and were comparable to those measured in mitral stenosis. Mitral regurgitation causes concomitant atrial and ventricular volume overload. As the disease progresses, the initial augmentation of atrial shortening and expansion becomes attenuated, leading to reduction in the LA emptying fraction.39,40
Atrial septal defect and transcatheter occlusion
Atrial septal defect is the most common congenital heart disease: the secundum type can be closed by either a surgical or transcatheter approach. The atrial mechanical function can be evaluated using TDI. Rahman et al. demonstrated that peak strain rate in both RA and LA during ventricular systole and early and late diastole was comparable between patients with ASD and normal subjects. However, the peak strain rate of the late diastolic phase at mid-LA level was significantly diminished after surgical but not transcatheter closure of the ASD.41 Similar findings were also observed by Salvo et al. in ASD patients who received either surgical or transcatheter closure for more than 6 months.42 When compared with normal subjects, it was shown that the transcatheter approach preserved the LA and RA regional myocardial properties, whereas peak systolic strain rate in both atria were significantly reduced after surgical correction. Nonetheless, the baseline atrial performance was not compared between the two patient groups.
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Conclusion |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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An assessment of the atrial longitudinal contraction by TDI is feasible, and provides reproducible and clinically useful data on atrial mechanical function. Atrial stiffness as reflected by its deformation properties can be assessed using atrial strain and strain-rate imaging. Thus, the active booster pump function as well as passive reservoir and conduit functions of both atria can be evaluated by the tissue Doppler-based technique, which may be less load-dependent. It provides additional information of atrial contractile function, complementary to a conventional two-dimensional and Doppler flow echocardiography. However, with improvement in imaging technology and offline analysing capability, further studies are needed to explore the clinical utilities of TDI and its post-processing applications in a variety of cardiac diseases.
Conflict of interest: None declared.
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References |
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TopAbstractIntroductionAssessment of left atrial...Atrial pump function and...Left atrial mechanical function...Assessment of left atrial...ConclusionReferences |
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