Home About us Editorial board Search Ahead of print Current issue Archives Submit article Instructions Subscribe Contacts Login 

 Table of Contents  
Year : 2021  |  Volume : 31  |  Issue : 3  |  Page : 137-143

Left ventricular twist and untwist in patients undergoing elective percutaneous coronary intervention

1 Department of Cardiology, Sharm International Hospital, Sharm El Sheikh, Al Minufiyah, Egypt
2 Department of Cardiology, Menoufia Faculty of Medicine, Menoufia University, Shibin Al Kawm, Al Minufiyah, Egypt

Date of Submission06-Nov-2020
Date of Decision18-May-2021
Date of Acceptance31-May-2021
Date of Web Publication26-Oct-2021

Correspondence Address:
Ali A Elzieny
Department of Cardiology, Menoufia Faculty of Medicine, Menoufia University, Shibin Al Kawm, Al Minufiyah
Login to access the Email id

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcecho.jcecho_121_20

Rights and Permissions

Background: Left ventricular (LV) twist and untwist plays important roles in physiological adaptation and development of clinically relevant cardiac diseases. Aims: To assess LV twist and untwist in patients undergoing elective percutaneous coronary intervention (PCI) by two-dimensional (2D) speckle tracking echocardiography (STE). Subjects and Methods: Fifty patients who had stable angina pectoris and/or abnormal result from noninvasive stress tests were enrolled after undergoing elective PCI. Conventional and 2D STE were performed before elective PCI and after 3 months. Results: There was no significant systolic improvement in conventional echocardiography. However, there was a significant diastolic improvement after elective PCI as higher E, E/A, e' and lower E/e' (P < 0.034, <0.042, 0.015, and 0.033, respectively). In addition, there was a statistically significant improvement of STE-derived systolic parameters as regard higher global longitudinal strain, peak twist, and torsion (P value 0.009, 0.009, and < 0.001, respectively). Furthermore, there was significant improvement of STE-derived diastolic parameters as higher peak untwist, recoil, and lower time to peak untwist (P value 0.013, 0.001, and 0.004, respectively). Conclusions: LV and untwist parameters were improved before most of conventional echocardiographic parameters in postrevascularization of stable coronary artery disease.

Keywords: Left ventricular twist, left ventricular untwist, speckle tracking echocardiography, stable coronary artery disease

How to cite this article:
Elzieny AA, Montaser SS, Emara AM, Ahmed MK. Left ventricular twist and untwist in patients undergoing elective percutaneous coronary intervention. J Cardiovasc Echography 2021;31:137-43

How to cite this URL:
Elzieny AA, Montaser SS, Emara AM, Ahmed MK. Left ventricular twist and untwist in patients undergoing elective percutaneous coronary intervention. J Cardiovasc Echography [serial online] 2021 [cited 2022 Aug 8];31:137-43. Available from: https://www.jcecho.org/text.asp?2021/31/3/137/329303

  Introduction Top

Coronary artery disease (CAD) is considered the leading cause for left ventricular (LV) dysfunction and subsequent heart failure development.[1] Stable angina pectoris is characterized by the episodes of reversible imbalance between blood supply and metabolic demand that are usually induced by either physical or emotional stress.[2] In patients with suspected stable CAD, it is recommended to perform a resting transthoracic echocardiography for the evaluation of cardiac function and structure.[3] Although the majority of patients with stable CAD have normal systolic function that is commonly quantified by LV ejection fraction (EF), the risk of developing heart failure is not negligible, even with standard medical therapy.[4] In this respect, identification and proper early treatment of patients with stable CAD are clinically important.

Noninvasive testing can establish the likelihood of the presence of obstructive CAD. Thus, coronary angiography (CA) will rarely be necessary in patients with suspected stable CAD. Elective CA may be indicated following noninvasive risk stratification to determine the options for revascularization. On the other hand, patients who have high pretest probability and severe symptoms, or clinical signs suggesting high event risk, CA maybe the strategy to identify the lesions potentially for revascularization.[5] Speckle tracking imaging enabled a rapid validated assessment of LV function.[6],[7] LV twist and untwist mechanics have expanded our understanding of normal ventricular physiology, adaptation, and decompensated mechanisms in disease states.[8]

Percutaneous coronary intervention (PCI) is an excellent therapy for obstructive CAD. However, there is a lack of data on efficacy of PCI in patients with stable angina pectoris. Thus, the aim of this study was to assess LV twist and untwist by two-dimensional (2D) STE in patients undergoing elective PCI.

  Subjects and Methods Top

Study designs

A single center, prospective study enrolled 50 patients with stable angina pectoris at Menoufia University Hospital from March 2019 to February 2020. All patients signed an informed consent and the study was approved by the faculty ethics committee.

Inclusive criteria included

The adult patients with stable angina pectoris (Canadian Cardiovascular Society Class III or IV) and abnormal result from noninvasive stress test who underwent elective PCI.

Exclusion criteria included

The history of acute coronary syndromes; any patient that underwent angiography but was not treated with angioplasty and stent placement; history of coronary artery bypass graft surgery, systolic heart failure with reduced EF defined as clinical diagnosis of heart failure with an ejection fracture ≤40%; any cardiac arrythmias and bundle branch blocks; patient with prosthetic valve or greater than mild valvular heart disease; advanced hepatic or renal diseases; or patients with poor echocardiographic image quality.

Baseline evaluation

Each enrolled patient was subjected to full medical history and detailed physical examination. 12 leads surface electrocardiogram (ECG) was performed. CA was performed using a commercially available interventional angiography system (ArtisTM, Siemens Healthcare). The femoral approach was performed by the modified Seldinger's technique. Coronary arteries were demonstrated in various planes and on cranial and caudal angulations. Injections of contrast medium were performed manually. Assessment of coronary angiographic data was done by experienced interventional consultant cardiology and concluded with intervention procedures with stent placement.

Conventional transthoracic echocardiographic examination was performed before PCI using a commercially available ultrasound systems (General Electric Vivid S5TM and Vivid E9TM, GE Healthcare) equipped with 1.4–4.0 MHz transducer to assess LV wall thickness, internal dimensions, wall motion abnormality, systolic and diastolic function by 2D, M-Mode, Doppler wave, and tissue Doppler imaging echocardiography.

Speckle tracking imaging was acquired through LV short-axis views at the apical, mid, and basal levels, and LV apical 2-, 3-, and 4-chamber views using a high frame rate (range 80–115 fps). At each plane, three consecutive cardiac cycles were acquired at end expiratory breath holding for better image acquisitions. The studies were stored digitally on a hard disk for offline analysis. The basal short-axis view contains the mitral valve, the mid short-axis view contains the cordae tendineae, and the apical short-axis view acquired distal to the papillary muscles.

Image analysis was performed offline on a PC workstation using the analysis software (EchoPACTM workstation software, GE Healthcare) allowing semi-automated analysis of speckle-based strain. Global longitudinal strain (GLS) was calculated by averaging all the values of regional peak longitudinal strain obtained in each apical view before aortic valve closure (AVC). Global circumferential strain and global radial strain were obtained as the average of the regional values measured in the six myocardial segments of basal, middle, and apical parasternal short-axis views.

Torsion was calculated as apical rotation relative to the basal rotation. LV recoil was measured through torsion curve by determining peak torsion at AVC and torsion at time of mitral valve opening (MVO) using isovolumic relaxation time (IVRT). Subsequently, recoil was calculated as Tpeak − Tmvo/Tpeak × 100. Peak LV twist rate was determined as first positive peak after R-wave on the ECG, and peak LV untwist rate was determined as the first negative peak after AVC. Time to peak twist rate was measured as time from R-wave to peak twist rate, and time to peak untwist rate was measured as time from AVC to peak untwist rate.

Follow-up evaluation

Patients were assessed after 3 months by complete conventional echocardiography and 2D speckle tracking imaging.

Statistical analysis

Statistical analysis was performed using the SPSS software (IBM SPSS Statistics for Windows, version 22, IBM Corp., Armonk, N.Y., USA). Continuous variables were presented as means standard deviation and categorical variables as numbers or frequencies. The Chi-square test was used to compare the frequencies. One-way analysis of variance was used to compare the descriptive parameters after confirming normal distributions. Kappa test was used for the categorical data with good agreement; Pearson's correlation coefficients were used to assess the strength of relationship between continuous variables.

  Results Top

Study population

The mean age of the studied sample was 53.3 ± 6.68 years. There were 38 males (76%) and 12 females (24%). The mean body mass index was 25.04 ± 2.3 kg/m2. Our studied sample showed that 64% were smoker, 54% were hypertensive, 58% were diabetic, and 52% had dyslipidemia [Table 1].
Table 1: Baseline criteria of studied patients

Click here to view

Conventional echocardiography data

There was a statistically significant increase in E, E/A, and e' (P value < 0.034, <0.042, and 0.015 respectively). Furthermore, E/e' showed significant decline (P value 0.033). There was no statistically significant difference in other diastolic or systolic functions at 3 months' follow-up [Table 2].
Table 2: Comparison of conventional echocardiography at baseline and 3 months' follow-up

Click here to view

Speckle tracking echocardiography-derived strain and torsional data

There was a statistically significant increase in GLS between baseline and 3 months' follow-up (−17.97 ± 5.79 vs. −20.53 ± 4.86 at P value 0.009); however, there was no statistically significance for circumferential and radial strains (P value 0.683 and 0.557, respectively). Furthermore, there was a statistically significant increase in peak untwist and recoil at follow-up as peak untwist (P value 0.013, and 0.001, respectively), while time to peak untwist was significantly lower at follow-up (P value 0.004). Other strain-derived torsional parameters of significance were apical rotation (P value < 0.001), basal rotation (P value < 0.001), torsion (P value < 0.001), and peak twist (P value 0.009) [Table 3] and [Figure 1], [Figure 2], [Figure 3].
Figure 1: Bull's eye plot of global longtudinal stain shows decreased value at baseline (a) Global longitudinal strain-18% that was increased at follow-up (b) Global longitudinal strain-24% after elective percutaneous coronary intervention in the same pateint

Click here to view
Figure 2: Torsion curve (as the net difference between apical and basal rotation) that shows lower torsion degrees at basline (a) and higher torsion degrees in the same pateint at follow-up (b) after elective percutaneous coronary intervention

Click here to view
Figure 3: Torsion rate curve shows lower peak left ventricular twist rate (as first positive peak after R wave on the electrocardiogram) and lower peak untwist rate (as the first negative peak after aortic valve closure) at baseline (a) which were improved after elective percutaneous coronary intervention in same pateint at follow up (b)

Click here to view
Table 3: Comparison of speckle tracking echocardiography parameters at baseline and 3 months follow-up

Click here to view

Correlation between recoil and peak untwist with conventional echocardiography data

Baseline recoil was inversely correlated with wall motion score index (WMSI) ratio and left ventricular end diastolic diameter (LVEDD) (P value 0.042 and 0.047, respectively). None of the other parameters showed significant correlation. However, baseline peak untwist was inversely correlated with A wave and IVRT (P value 0.038 and 0.039, respectively). Follow-up recoil showed no any significant correlation with any conventional parameters, whereas follow-up peak untwist was directly correlated with LVEDD and LVESD (P value 0.033 and 0.025, respectively) [Table 4].
Table 4: Correlation between recoil and peak untwist with conventional echocardiography parameters at baseline and follow-up

Click here to view

  Discussion Top

The main findings of this study were as follows

First: There was no significant systolic improvement by conventional echocardiography. While, there was a significant diastolic improvement after elective PCI as higher E, E/A, and lower E/e'.

Second: There was a significant improved STE-derived systolic parameters as higher GLS, peak twist ratio, and torsion after elective PCI.

Third: there was a significant improvement of STE-derived diastolic parameters as regard higher peak untwist ratio, recoil, and lower time to peak untwist after elective PCI.

Regarding conventional echocardiography

The findings of our study indicated a significant increase of both E-wave and E/A index and significant decrease for E/e' index after intervention. These improvements of resting LV diastolic function may be reflecting a reduction or elimination of reversible myocardial ischemia. Many trials have studied diastolic function measured by conventional echocardiography after PCI as Hashemi and colleagues[9] who studied diastolic function in 30 patients scheduled for elective PCI (before PCI, 48 hours and 3 months after PCI). They concluded that there was an improvement of both E-wave and E/A ratio which in line with our study. Furthermore, Nahid et al. conducted a study on 51 CAD patients who were scheduled for elective PCI to evaluate LV diastolic indices by conventional echocardiography before and after 3 months of PCI. Among the variables studied after intervention, DT, IVRT, e', and E/e' ratio showed an improvement.[10]

The present study showed no any significant change of LVEF. As most stable CAD patients have normal or subnormal EF, other parameters should be used to evaluate systolic function in these patients.[11] Beitnes et al. have concluded that LVEF was increased in the first 3 months, and then not-statistically significant decreased at the following 3 and 6 months which probably confirm the effect of time on cardiac indices after PCI.[12]

Regarding speckle tracking echocardiography

The present study showed significantly increase in GLS after PCI. While it showed no significant changes of either circumferential or radial strain. This can be explained by the fact that the majority of progressive myocardial diseases, including coronary ischemia, tend to cause subendocardial dysfunction. This result in a preferential involvement of longitudinal LV mechanics that can be identified even in a subclinical state. The epicardial function may remain unaffected, and circumferential strain either remains normal or shows exaggerated compensation to preserve the systolic function. Many studies have concluded that in patients with chronic stable CAD who were followed up months after PCI showed significant improvement of GLS.[13],[14],[15],[16]

Evaluation of twist and untwist could provide valuable information on either systolic or diastolic function of the left ventricle in numerous diseases.[17] Our study showed significant improvement of STE systolic parameters as apical rotation, basal rotation, torsion, and peak twist despite nonsignificant changes of conventional echocardiography parameters. As explained earlier, longitudinal strain seems to be the earliest to be affected by ischemia, as the subendocardial fibers are the first to suffer the effect of perfusion abnormalities. Thus, the impact of CAD on LVT depends on the extent and location of ischemia or/and infarct. Confined ischemia within subendocardial fiber layer increases the apical rotation. This resulting in higher dominance of the epicardial lever arm, which produces more apical rotation and twist. In contrast, transmural ischemia results in decreased apical rotation and twist.

Liszka et al. studied the effect of PCI using conventional and speckle tracking echocardiography (STE) on patients with CAD and successful PCI who were divided into Group I with myocardial infarction (MI) (either ST-elevation MI or non-ST-segment elevation MI) and Group II with stable angina pectoris. Echocardiography was performed at baseline, 30 days, and 90 days after PCI. Focusing in Group II results, it showed that there was a significant improvement in torsion, apical rotation, and basal rotation that it was compatible with our study.[18]

STE diastolic parameters were either significantly higher after PCI as peak untwist and recoil, or significantly lower after PCI as time to peak untwist. This could be explained by the fact of temporal dyssynchrony between normal and ischemic myocardium may result in impairment of LV relaxation and filling; after normalization of perfusion in ischemic zone, this regional dyssynchrony would improve. Consequently, improvement of global LV filling after PCI might result from improved regional LV function. LV diastolic filling improves gradually after PCI; thus, peak untwist value was increased and time to peak untwist was decreased.

Some studies assessed the effect of PCI on LV twisting and untwisting in patients with MI and ischemia. Cuiling et al. studied 60 patients with MI, 31 patients with myocardial ischemia, and 26 control individuals. All their patients were received CA. Twisting and untwisting were measured by STE. Two patient groups have lower parameters in torsion and untwisting rate especially MI group than control group. They concluded that torsion and untwisting rate derived by STE was decreased not only in MI group but also in patients with myocardial ischemia at rest. Furthermore, the infarction size rather than the site to influence the twisting and untwisting parameters.[19]

The present study showed significant improvement of LV recoil after PCI. The elastic LV recoil is thought to be a result of strong contraction and compression of cardiac proteins. The energy stored in these proteins (such as titin) is released during diastole which will aid myocardial relaxation and diastolic filling.[20],[21] The rapid recoil which mainly occurs during isovolumetric relaxation has a significant effect on both transmitral and LV pressures. Thus, rapid recoil during isovolumetric relaxation is crucial in the development of low LV pressure.[22] Delayed untwisting results in ineffective recoil, with little benefit to transmitral pressure gradient.[23],[24] There was a shortage in trials that studied impact of PCI on recoil for stable ischemic group.

Recoil at baseline was inversely correlated with WMSI and LVEDD. None of the other conventional echocardiographic parameters showed statistically significant correlation. Furthermore, recoil at follow-up was inversely correlated with E/A ratio without any statistically significant correlation with other mitral flow and Doppler tissue-derived diastolic parameters. The absence of correlation between recoil and both mitral flow and Doppler tissue-derived parameters can be explained by that all these parameters are load-dependent and occur after MVO. Thus, it provides a little information on process of LV relaxation that predominantly occurs prior to MVO and it evaluates only the later stages of relaxation which supported by Esch and Warburton.[25]

Study limitation

Our study had limitations. First, we enrolled a relatively small number of patients. Second, the lack of invasively derived variables, such as chamber stiffness, LV, and atrial pressures as well as pulmonary capillary wedge pressure during the study.

  Conclusions Top

Improvement of LV twist and untwist parameters was detected by 2D STE before most of conventional echocardiographic parameters in patients with stable CAD treated by elective PCI.

Ethical clearance

The study protocol was approved by Menoufia Faculty of Medicine ethics committee in November 2018, and a written informed consent was obtained from each subject before the study.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Gheorghiade M, Sopko G, De Luca L, Velazquez EJ, Parker JD, Binkley PF, et al. Navigating the crossroads of coronary artery disease and heart failure. Circulation 2006;114:1202-13.  Back to cited text no. 1
Knuuti J, Wijns W, Saraste A, Capodanno D, Barbato E, Funck-Brentano C, et al. 2019 ESC Guidelines for the diagnosis and management of chronic coronary syndromes. Eur Heart J 2020;41:407-77.  Back to cited text no. 2
Task Force M, Montalescot G, Sechtem U, Achenbach S, Andreotti F, Arden C, et al. ESC guidelines on the management of stable coronary artery disease: The Task Force on the management of stable coronary artery disease of the European Society of Cardiology. Eur Heart J 2013;34:2949-3003.  Back to cited text no. 3
Lewis EF, Solomon SD, Jablonski KA, Rice MM, Clemenza F, Hsia J, et al. Predictors of heart failure in patients with stable coronary artery disease: A PEACE study. Circ Heart Fail 2009;2:209-16.  Back to cited text no. 4
Task Force on Myocardial Revascularization of the European Society of Cardiology (ESC) and the European Association for Cardio-Thoracic Surgery (EACTS), European Association for Percutaneous Cardiovascular Interventions (EAPCI), Wijns W, Kolh P, Danchin N, Di Mario C, et al. Guidelines on myocardial revascularization. Eur Heart J 2010;31:2501-55.  Back to cited text no. 5
Amundsen BH, Helle-Valle T, Edvardsen T, Torp H, Crosby J, Lyseggen E, et al. Noninvasive myocardial strain measurement by speckle tracking echocardiography: Validation against sonomicrometry and tagged magnetic resonance imaging. J Am Coll Cardiol 2006;47:789-93.  Back to cited text no. 6
Lang RM, Badano LP, Mor-Avi V, Afilalo J, Armstrong A, Ernande L, et al. Recommendations for cardiac chamber quantification by echocardiography in adults: An update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr 2015;28:1- 39.e14.  Back to cited text no. 7
Teske AJ, De Boeck BW, Melman PG, Sieswerda GT, Doevendans PA, Cramer MJ. Echocardiographic quantification of myocardial function using tissue deformation imaging, a guide to image acquisition and analysis using tissue Doppler and speckle tracking. Cardiovasc Ultrasound 2007;5:27.  Back to cited text no. 8
Hashemi SR, Motamedi M, Khani M, Hekmat M, Gachkar L, Rezaeefar A. Evaluation of the effect of elective percutaneous coronary intervention as a treatment method on the left ventricular diastolic dysfunction in patients with coronary artery disease. J Tehran Heart Cent 2010;5:194-8.  Back to cited text no. 9
Salehi N, Saidi M, Rai A, Najafi F, Javeedannejad S, Babanejad M, et al. Effect of percutaneous coronary intervention on left ventricular diastolic function in patients with coronary artery disease. Glob J Health Sci 2016;8:270-6.  Back to cited text no. 10
Biering-Sørensen T, Hoffmann S, Mogelvang R, Zeeberg Iversen A, Galatius S, Fritz-Hansen T, et al. Myocardial strain analysis by 2-dimensional speckle tracking echocardiography improves diagnostics of coronary artery stenosis in stable angina pectoris. Circ Cardiovasc Imaging 2014;7:58-65.  Back to cited text no. 11
Beitnes JO, Gjesdal O, Lunde K, Solheim S, Edvardsen T, Arnesen H, et al. Left ventricular systolic and diastolic function improve after acute myocardial infarction treated with acute percutaneous coronary intervention, but are not influenced by intracoronary injection of autologous mononuclear bone marrow cells: A 3 year serial echocardiographic sub-study of the randomized-controlled ASTAMI study. Eur J Echocardiogr 2011;12:98-106.  Back to cited text no. 12
Sang Jin HA, Kim SH, Kim JB, Kim SJ, Kim W, Kim W. Improvement in regional myocardial function assessed by strain rate imaging in patients who had coronary artery disease but normal wall motion undergoing percutaneous coronary intervention. Circulation 2012;126:A17807.  Back to cited text no. 13
Jin-Oh C, Woo PS, Jeong Hoon Y, Won CS, Jin CS, Yeub LS, et al. Longitudinal 2-dimensional strain of left ventricle at rest predicts high risk coronary artery disease without regional wall motion abnormality. Circulation 2007;116:329-30.  Back to cited text no. 14
Nozari Y, Oskouei NJ, Khazaeipour Z. Effect of elective percutaneous coronary intervention on left ventricular function in patients with coronary artery disease. Acta Med Iran 2012;50:26-30.  Back to cited text no. 15
Hossain M, Fazlur A, Siddique A, Krishna S, Meskat C, Hoque H, et al. Study of changes in various echocardiographic parameters in patients with chronic stable angina undergoing percutaneous coronary intervention. Univ Heart J 2013;9:2.  Back to cited text no. 16
Badano LP, Muraru D. Twist mechanics of the left ventricle. Circ Cardiovasc Imaging 2019;12:e009085.  Back to cited text no. 17
Liszka J, Haberka M, Tabor Z, Finik M, Gąsior Z. Two-dimensional speckle-tracking echocardiography assessment of left ventricular remodeling in patients after myocardial infarction and primary reperfusion. Arch Med Sci 2014;10:1091-100.  Back to cited text no. 18
Cuiling L, Hon L, Rui F, Fengjuan Y, Kun L. Assessment of left ventricular twisting and untwisting in patients with myocardial infarction and ischaemia. Heart 2010;96 Suppl 3:A214.  Back to cited text no. 19
Fukuda N, Sasaki D, Ishiwata S, Kurihara S. Length dependence of tension generation in rat skinned cardiac muscle: Role of titin in the Frank-Starling mechanism of the heart. Circulation 2001;104:1639-45.  Back to cited text no. 20
Helmes M, Lim CC, Liao R, Bharti A, Cui L, Sawyer DB. Titin determines the Frank-Starling relation in early diastole. J Gen Physiol 2003;121:97-110.  Back to cited text no. 21
Notomi Y, Martin-Miklovic MG, Oryszak SJ, Shiota T, Deserranno D, Popovic ZB, et al. Enhanced ventricular untwisting during exercise: A mechanistic manifestation of elastic recoil described by Doppler tissue imaging. Circulation 2006;113:2524-33.  Back to cited text no. 22
Fuchs E, Müller MF, Oswald H, Thöny H, Mohacsi P, Hess OM. Cardiac rotation and relaxation in patients with chronic heart failure. Eur J Heart Fail 2004;6:715-22.  Back to cited text no. 23
Notomi Y, Srinath G, Shiota T, Martin-Miklovic MG, Beachler L, Howell K, et al. Maturational and adaptive modulation of left ventricular torsional biomechanics: Doppler tissue imaging observation from infancy to adulthood. Circulation 2006;113:2534-41.  Back to cited text no. 24
Esch BT, Warburton DE. Left ventricular torsion and recoil: Implications for exercise performance and cardiovascular disease. J Appl Physiol (1985) 2009;106:362-9.  Back to cited text no. 25


  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3], [Table 4]


Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

  In this article
Subjects and Methods
Article Figures
Article Tables

 Article Access Statistics
    PDF Downloaded98    
    Comments [Add]    

Recommend this journal