Home Print this page Email this page Small font sizeDefault font sizeIncrease font size
Users Online: 343

 

Home  | About Us | Editors | Search | Ahead Of Print | Current Issue | Archives | Submit Article | Instructions | Subscribe | Contacts | Login 
     


 
 
Table of Contents
ORIGINAL ARTICLE
Year : 2015  |  Volume : 5  |  Issue : 2  |  Page : 80-84

Dynamic behavior of venous collapsibility and central venous pressure during standardized crystalloid bolus: A prospective, observational, pilot study


1 Department of Research and Innovation, St Luke's University Health Network, Bethlehem, Pennsylvania, United State
2 Department of Surgery, The Ohio State University College of Medicine, Columbus, Ohio, United State
3 Department of Anesthesiology, The Ohio State University College of Medicine, Columbus, Ohio, United State
4 Department of Research and Innovation; The Research Institute, St Luke's University Health Network, Bethlehem, Pennsylvania, United State
5 Department of Emergency Medicine, The Ohio State University College of Medicine, Columbus, Ohio, United State

Date of Web Publication9-Jun-2015

Correspondence Address:
Stanislaw P Stawicki
Department of Research and Innovation, St Luke's University Health Network, 801 Ostrum Street, Bethlehem, Pennsylvania-18015
United State
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2229-5151.158392

Rights and Permissions
   Abstract 

Introduction: Measurement of intravascular volume status is an ongoing challenge for physicians in the surgical intensive care unit (SICU). Most surrogates for volume status, including central venous pressure (CVP) and pulmonary artery wedge pressure, require invasive lines associated with a number of potential complications. Sonographic assessment of the collapsibility of the inferior vena cava (IVC) has been described as a noninvasive method for determining volume status. The purpose of this study was to analyze the dynamic response in IVC collapsibility index (IVC-CI) to changes in CVP in SICU patients receiving fluid boluses for volume resuscitation.
Materials and Methods: A prospective pilot study was conducted on a sample of SICU patients who met clinical indications for intravenous (IV) fluid bolus and who had preexisting central venous access. Boluses were standardized to crystalloid administration of either 500 mL over 30 min or 1,000 mL over 60 min, as clinically indicated. Concurrent measurements of venous CI (VCI) and CVP were conducted right before initiation of IV bolus (i.e. time 0) and then at 30 and 60 min (as applicable) after bolus initiation. Patient demographics, ventilatory parameters, and vital sign assessments were recorded, with descriptive outcomes reported due to the limited sample size.
Results: Twenty patients received a total of 24 IV fluid boluses. There were five recorded 500 mL boluses given over 30 min and 19 recorded 1,000 mL boluses given over 60 min. Mean (median) CVP measured at 0, 30, and 60 minutes post-bolus were 6.04 ± 3.32 (6.5), 9.00 ± 3.41 (8.0), and 11.1 ± 3.91 (12.0) mmHg, respectively. Mean (median) IVC-CI values at 0, 30, and 60 min were 44.4 ± 25.2 (36.5), 26.5 ± 22.8 (15.6), and 25.2 ± 21.2 (14.8), respectively.
Conclusions: Observable changes in both VCI and CVP are apparent during an infusion of a standardized fluid bolus. Dynamic changes in VCI as a measurement of responsiveness to fluid bolus are inversely related to changes seen in CVP. Moreover, an IV bolus tends to produce an early response in VCI, while the CVP response is more gradual. Given the noninvasive nature of the measurement technique, VCI shows promise as a method of dynamically measuring patient response to fluid resuscitation. Further studies with larger sample sizes are warranted.

Keywords: Central venous pressure, inferior vena cava collapsibility index, intravascular volume status assessment, intravenous fluid bolus, Point-of-care ultrasound


How to cite this article:
Stawicki SP, Kent A, Patil P, Jones C, Stoltzfus JC, Vira A, Kelly N, Springer AN, Vazquez D, Evans DC, Papadimos TJ, Bahner DP. Dynamic behavior of venous collapsibility and central venous pressure during standardized crystalloid bolus: A prospective, observational, pilot study. Int J Crit Illn Inj Sci 2015;5:80-4

How to cite this URL:
Stawicki SP, Kent A, Patil P, Jones C, Stoltzfus JC, Vira A, Kelly N, Springer AN, Vazquez D, Evans DC, Papadimos TJ, Bahner DP. Dynamic behavior of venous collapsibility and central venous pressure during standardized crystalloid bolus: A prospective, observational, pilot study. Int J Crit Illn Inj Sci [serial online] 2015 [cited 2018 Jan 22];5:80-4. Available from: http://www.ijciis.org/text.asp?2015/5/2/80/158392


   Introduction Top


Hemodynamic evaluation of the critically-ill patient continues to pose a significant challenge, with lack of a reliable and reproducible "gold standard" for intravascular volume assessment. [1] Moreover, traditionally employed invasive modalities are associated with significant potential complications. [2],[3] Consequently, there is increasing interest in investigating venous collapsibility index (VCI) as a noninvasive, easily repeatable, and portable alternative to traditional invasive hemodynamic monitoring approaches. [4],[5] Although our knowledge of the characteristics of VCI in relation to CVP and other parameters affecting the dynamic behavior of the circulatory system is growing, [4],[4],[6] limited information exists regarding dynamic changes in VCI during intravenous (IV) fluid bolus administration. Most of the information available comes from studies involving observations of either venous collapsibility or vein diameters in patients with renal failure undergoing dialysis, [6],[7] pediatric surgery, tricuspid regurgitation, heart failure, [8],[9] and experimental anesthesia models. [10] The objective of this study is to describe the simultaneous behavior of VCI and CVP during standardized IV bolus infusion in the surgical intensive care unit (SICU).


   Materials and methods Top


After Institutional Review Board approval (The Ohio State University Protocol Number 2010H0247), a prospective, observational study was conducted on a convenience sample of consenting SICU patients who met clinical indications for IV fluid bolus and who had preexisting central venous access. Included in the study were patients between the ages of 18 and 89. Excluded were prisoners and patients <18 or ≥90 years of age. Two primary end-points were observed: (a) The behavior of the central venous pressure (CVP) during active IV bolus infusion; and (b) the simultaneous behavior of the VCI. Venous collapsibility was defined as the difference between the maximum and the minimum venous diameter [Figure 1] recorded on M-mode during the respiratory cycle. [4],[11]
Figure 1: Schematic representation of venous collapsibility index computations

Click here to view


IV fluid boluses were standardized to crystalloid administration of either 500 mL over 30 minutes or 1,000 mL over 60 min. Concurrent measurements of VCI (taken at the inferior vena cava (IVC) [5] as previously outlined by our group) and CVP were conducted immediately prior to initiation of IV bolus (i.e. time 0) and then at 30 and 60 min (as applicable) after the bolus was started.

Data analysis

Patient demographics (age and gender), head-of-bed elevation (degrees), ventilatory parameters (presence of mechanical ventilation and positive-end expiratory pressure (PEEP)) and systolic blood pressure assessments were recorded. Due to the purely observational nature of this pilot study with use of convenience sampling, only descriptive outcomes are reported, including mean and median CVP/VCI values. In addition, data for the initial 30 min for the two groups were combined. Graphs were prepared using Microsoft Excel™ (Microsoft Corporation, Redmond, Washington, USA).


   Results Top


Twenty patients received a total of 24 IV fluid boluses. There were 12 men and eight women. Mean age was 58.4 ± 19.0 years (median 67.5, range 20-81). Mean Acute Physiology and Chronic Health Evaluation II (APACHE II) score for the study group was 13.0 ± 4.94 (median 12, range 5-21). Mean Simplified Acute Physiology Score (SAPS II) was 35.1 ± 12.4 (median 38, range 16-54). Ventilatory support was present during 17/24 (70.8%) bolus episodes, with median PEEP of 6.5 cmH 2 O. Median head-of-bed elevation during standardized bolus administration was 25 degrees.

Overall, we recorded five episodes of a standardized 500 mL bolus given over 30 min and 19 episodes of a standardized 1,000 mL bolus given over 60 min. When comparing the dynamic response in both VCI and CVP, there was no observable difference between the 500 and the 1,000 mL bolus groups for either of the parameters. For CVP, the initial mean ± standard deviation values for the 500 and 1,000 mL groups were 6.00 ± 2.00 versus 6.05 ± 3.63, respectively. For the 30-min values, the CVP was 9.00 ± 2.55 versus 9.00 ± 3.67, respectively. For VCI, the initial values for the 500 and the 1000 mL groups were 39.9 ± 14.8 versus 45.5 ± 27.5, respectively. For the 30-min values, the VCI was 17.6 ± 12.3 versus 28.8 ± 24.6, respectively. For greater ease of presentation, both groups were combined for the composite analysis discussed below.

Venous collapsibility evaluation

Mean (median) VCI values at 0, 30, and 60 min were 44.4 ± 25.2 (36.5), 26.5 ± 22.8 (15.6), and 25.2 ± 21.2 (14.8), respectively. Graphical representation of the observed trend is depicted in [Figure 2]a.
Figure 2: Graphical depiction of composite venous collapsibility (a, left) and central venous pressure (b, right) measurements collected during standardized crystalloid bolus administration

Click here to view


CVP

Mean (median) CVP measured at 0, 30, and 60 min were 6.04 ± 3.32 (6.5), 9.00 ± 3.41 (8.0), and 11.1 ± 3.91 (12.0) mmHg, respectively. Graphical representation of the observed trend is depicted in [Figure 2]b.

Systolic blood pressure

For systolic blood pressure during IV bolus infusion, there is a nominal upward trend in the behavior or composite systolic pressure curve. Composite systolic blood pressure values for 0, 30, and 60 min were 114 ± 17.4, 120 ± 25.9, and 122 ± 27.7, respectively, as depicted in [Figure 3].
Figure 3: Observed trend in composite systolic blood pressure behavior during standardized crystalloid bolus administration

Click here to view



   Discussion Top


Accurate estimation of intravascular volume status is an essential part of management of the critically ill. Invasive procedures may lead to a variety of complications and the VCI is being increasingly recognized as a potential noninvasive replacement or a source of adjunct information. [12],[13] Nonetheless, there have been questions raised about its effectiveness. Initial evaluations of caval diameter for estimating fluid status were promising, but the static measurements obtained suffered from a number of shortcomings. [14],[15] This led to the evolution of functional hemodynamic monitoring, including calculation of the VCI, which improved predictive accuracy of sonographic volume status assessment techniques by incorporating the fractional change in central venous diameters through the respiratory cycle in addition to simple measurement of maximal or minimal IVC diameter. [4],[11],[14],[16],[17],[18] Resnick et al., studied the use of venous collapsibility looking for early ultrasound evidence of blood loss in healthy volunteers donating blood. [19] They noted that their clinical findings were confounded by frequent measurement errors, as well as difficulty acquiring sonographic data in patients with small IVC diameters and high body mass indices. [19] However, their conclusions in healthy donors are at odds with Pasquero et al., who also studied healthy blood donors who were depleted of 400-450 mL of blood volume. [12] In the latter study, the mid-hepatic long-axis view correlated well with intravascular volume changes, with the VCI being able to detect early volume responses in regard to blood loss and volume repletion after donation. [12] Sonographic changes in the IVC also occur in patients with acute decompensated heart failure undergoing diuresis. [20] Venous collapsibility changes have also been shown to correlate with IV fluid resuscitation of intraoperative surgical patients and hypotensive emergency department patients. [21],[22]

This pilot study provides important new information regarding the behavior of VCI during standardized IV fluid bolus, especially in the context of simultaneous CVP measurements. Our data further support the hypothesis that there is a dynamic relationship between fluid status administration and VCI behavior, as well as an approximate inverse relationship between VCI and CVP during fluid infusion. This is consistent with prior reports by Ferrada et al., who published studies demonstrating changes in VCI before and after IV fluid bolus and a positive correlation of IVC sonographic exam findings to clinical outcome in SICU and trauma patients. [23],[24] It is important to note that the simultaneous responses of VCI and CVP, although characterized by the previously shown inverse relationship, [4],[25] exhibit important differences. Namely, changes in venous collapsibility were characterized by a significant response during the first 30 min of infusion, with a less noticeable change during the subsequent 30 min. On the other hand, the composite response in CVP was more gradual and sustained throughout the entire 60-min interval. The gradual increase in CVP versus the more abrupt diameter changes observed in the vena cava were previously described in an animal model by Lautt and Greenway in 1976. [26] Their work demonstrated that the liver is an important buffer for changes in blood volume that occur over a short period of time. The more volume infused, the more important the liver becomes as a "volume buffer". They also indicated that as the overall venous pressure rises above the level of 6 mmHg, the capacitance vessels of the liver demonstrate the ability to accommodate and store "additional amounts" of volume. In other words, the hepatic vascular system has the capacity to sequester greater amounts of volume as a compensatory mechanism. [26]

Limitations of this pilot study include its small sample size, inability to make inferential comparisons due to limited statistical power, and lack of data following post-bolus changes in CVP or VCI. Anecdotally, the authors observed that, following the completion of the bolus, both CVP and VCI appeared to partially deflect back toward the pre-bolus baseline. This observation is likely a consequence of a number of factors, including the potential "third-spacing" of the infused IV fluid [27],[28],[29] and ongoing physiologic fluid losses. [29],[30] Additionally, the volume of fluid infused in the study, while consistent with prior published literature on dynamic changes in VCI, was comparatively small to the needs of many critically-ill patients undergoing active resuscitation. [23],[31] Although the APACHE II and SAPS II scores indicated that our patient sample did not exhibit the highest acuity, this should not be perceived as a limitation for an observational pilot study in ICU patients. [32],[33]

While we have documented the various levels of PEEP, basic ventilator settings, and the angle of the head-of-bed elevation, it has to be noted that although changes in the above parameters can influence the CVP, they do not influence hepatic vascular pressure or vena cava behavior as much as one would intuitively anticipate. [4],[34],[35] In fact, increasing PEEP from 5 to 10 cm of H 2 O will increase CVP, hepatic pressures, and portal pressures by only about 1 mmHg [35] and the effect of supine versus upright patient positioning has minimal effect on IVC metrics. [34] The lack of standardization of the above parameters in our patients may be initially considered a limitation to the study. However, reports of similar physiologic responses, regardless of the above differences, in fact suggest the opposite and indirectly help corroborate our current findings.


   Conclusion Top


Observable changes in both VCI and CVP are apparent during an infusion of a standardized fluid bolus. Based on our data, the dynamic change in VCI as a measurement of responsiveness to fluid bolus is inversely related to changes seen in CVP. Moreover, an IV bolus tends to produce an early response in VCI, while the CVP response is more gradual. Given the relative simplicity of the measurement technique and its noninvasive nature, quantitative assessment of VCI shows promise as a method of dynamically measuring patient response to fluid resuscitation. Further prospective studies in larger populations are warranted to better evaluate the diagnostic value of VCI in intravascular volume assessment, and to determine which patients stand to benefit the most from this hemodynamic monitoring approach.

 
   References Top

1.
Gattinoni L, Carlesso E. Supporting hemodynamics: What should we target? What treatments should we use? Crit Care 2013;17:S4.  Back to cited text no. 1
    
2.
Evans DC, Doraiswamy VA, Prosciak MP, Silviera M, Seamon MJ, Rodriguez Funes V, et al. Complications associated with pulmonary artery catheters: A comprehensive clinical review. Scand J Surg 2009;98:199-208.  Back to cited text no. 2
    
3.
Patel C, Laboy V, Venus B, Mathru M, Wier D. Acute complications of pulmonary artery catheter insertion in critically ill patients. Crit Care Med 1986;14:195-7.  Back to cited text no. 3
[PUBMED]    
4.
Stawicki SP, Adkins EJ, Eiferman DS, Evans DC, Ali NA, Njoku C, et al. Prospective evaluation of intravascular volume status in critically ill patients: Does inferior vena cava collapsibility correlate with central venous pressure? J Trauma Acute Care Surg 2014;76:956-63.  Back to cited text no. 4
    
5.
Stawicki SP, Braslow BM, Panebianco NL, Kirkpatrick JN, Gracias VH, Hayden GE, et al. Intensivist use of hand-carried ultrasonography to measure IVC collapsibility in estimating intravascular volume status: Correlations with CVP. J Am Coll Surg 2009;209:55-61.  Back to cited text no. 5
    
6.
Haciomeroglu P, Ozkaya O, Gunal N, Baysal K. Venous collapsibility index changes in children on dialysis. Nephrology (Carlton) 2007;12:135-9.  Back to cited text no. 6
    
7.
Ando Y, Yanagiba S, Asano Y. The inferior vena cava diameter as a marker of dry weight in chronic hemodialyzed patients. Artif Organs 1995;19:1237-42.  Back to cited text no. 7
    
8.
Anter AM, Bondok RS. Peripheral venous pressure is an alternative to central venous pressure in paediatric surgery patients. Acta Anaesthesiol Scand 2004;48:1101-4.  Back to cited text no. 8
    
9.
Reeves WC, Leaman DM, Buonocore E, Babb JD, Dash H, Schwiter EJ, et al. Detection of tricuspid regurgitation and estimation of central venous-pressure by two-dimensional contrast echocardiography of the right superior hepatic vein. Am Heart J 1981;102:374-7.  Back to cited text no. 9
[PUBMED]    
10.
Nelson NC, Drost WT, Lerche P, Bonagura JD. Noninvasive estimation of central venous pressure in anesthetized dogs by measurement of hepatic venous blood flow velocity and abdominal venous diameter. Vet Radiol Ultrasound 2010;51:313-23.  Back to cited text no. 10
    
11.
Kent A, Bahner DP, Boulger CT, Eiferman DS, Adkins EJ, Evans DC, et al. Sonographic evaluation of intravascular volume status in the surgical intensive care unit: A prospective comparison of subclavian vein and inferior vena cava collapsibility index. J Surg Res 2013;184:561-6.  Back to cited text no. 11
    
12.
Pasquero P, Albani S, Sitia E, Castagno F, D′Antico S. Inferior vena cava diameters and collapsibility index changes reveal early volume depletion in a healthy donor model. Crit Ultrasound J 2012;A29.  Back to cited text no. 12
    
13.
Thanakitcharu P, Charoenwut M, Siriwiwatanakul N. Inferior vena cava diameter and collapsibility index: A practical non-invasive evaluation of intravascular fluid volume in critically-ill patients. J Med Assoc Thai 2013;96:S14-22.  Back to cited text no. 13
    
14.
Cavallaro F, Sandroni C, Antonelli M. Functional hemodynamic monitoring and dynamic indices of fluid responsiveness. Minerva Anestesiol 2008;74:123-35.  Back to cited text no. 14
    
15.
Carr BG, Dean AJ, Everett WW, Ku BS, Mark DG, Okusanya O, et al. Intensivist bedside ultrasound (INBU) for volume assessment in the intensive care unit: A pilot study. J Trauma 2007;63:495-500.  Back to cited text no. 15
    
16.
Stawicki SP, Bahner DP. Evidence tables: Inferior vena cava collapsibility index (IVCCI). OPUS 12 Scientist 2012;6:3-5.  Back to cited text no. 16
    
17.
Blair JE, Brennan JM, Goonewardena SN, Shah D, Vasaiwala S, Spencer KT. Usefulness of hand-carried ultrasound to predict elevated left ventricular filling pressure. Am J Cardiol 2009;103:246-7.  Back to cited text no. 17
    
18.
Brennan JM, Blair JE, Goonewardena S, Ronan A, Shah D, Vasaiwala S, et al. Reappraisal of the use of inferior vena cava for estimating right atrial pressure. J Am Soc Echocardiogr 2007;20:857-61.  Back to cited text no. 18
    
19.
Resnick J, Cydulka R, Platz E, Jones R. Ultrasound does not detect early blood loss in healthy volunteers donating blood. J Emerg Med 2011;41:270-5.  Back to cited text no. 19
    
20.
Tchernodrinski S, Lucas BP, Athavale A, Candotti C, Margeta B, Katz A, et al. Inferior vena cava diameter change after intravenous furosemide in patients diagnosed with acute decompensated heart failure. J Clin Ultrasound 2014;4.  Back to cited text no. 20
    
21.
Zhang X, Luan H, Zhu P, Feng J, Cui J, Zhao Z, et al. Does ultrasonographic measurement of the inferior vena cava diameter correlate with central venous pressure in the assessment of intravascular volume in patients undergoing gastrointestinal surgery? J Surg Res 2014.  Back to cited text no. 21
    
22.
Weekes AJ, Tassone HM, Babcock A, Quirke DP, Norton HJ, Jayarama K, et al. Comparison of serial qualitative and quantitative assessments of caval index and left ventricular systolic function during early fluid resuscitation of hypotensive emergency department patients. Acad Emerg Med 2011;18:912-21.  Back to cited text no. 22
    
23.
Ferrada P, Anand RJ, Whelan J, Aboutanos MA, Duane T, Malhotra A, et al. Qualitative assessment of the inferior vena cava: Useful tool for the evaluation of fluid status in critically ill patients. Am Surg 2012;78:468-70.  Back to cited text no. 23
    
24.
Ferrada P, Vanguri P, Anand RJ, Whelan J, Duane T, Wolfe L, et al. Flat inferior vena cava: Indicator of poor prognosis in trauma and acute care surgery patients. Am Surg 2012;78:1396-8.  Back to cited text no. 24
    
25.
Stawicki SP, Braslow B, Gracias VH. Exploring measurement biases associated with esophageal Doppler monitoring in critically ill patients in intensive care unit. Ann Thorac Med 2007;2:148-53.  Back to cited text no. 25
[PUBMED]  Medknow Journal  
26.
Lautt WW, Greenway CV. Hepatic venous compliance and role of liver as a blood reservoir. Am J Physiol 1976;231:292-5.  Back to cited text no. 26
[PUBMED]    
27.
Drobin D. A single-model solution for volume kinetic analysis of isotonic fluid infusions. Acta Anaesthesiol Scand 2006;50:1074-80.  Back to cited text no. 27
    
28.
Brauer KI, Svensen C, Hahn RG, Traber LD, Prough DS. Volume kinetic analysis of the distribution of 0.9% saline in conscious versus isoflurane-anesthetized sheep. Anesthesiology 2002;96:442-9.  Back to cited text no. 28
    
29.
Brandstrup B. Fluid therapy for the surgical patient. Best Pract Res Clin Anaesthesiol 2006;20:265-83.  Back to cited text no. 29
    
30.
Streat SJ, Beddoe AH, Hill GL. Measurement of total body water in intensive care patients with fluid overload. Metabolism 1985;34:688-94.  Back to cited text no. 30
[PUBMED]    
31.
Barbier C, Loubieres Y, Schmit C, Hayon J, Ricome JL, Jardin F, et al. Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients. Intensive Care Med 2004;30:1740-6.  Back to cited text no. 31
    
32.
Wong DT, Knaus WA. Predicting outcome in critical care: The current status of the APACHE prognostic scoring system. Can J Anaesth 1991;38:374-83.  Back to cited text no. 32
    
33.
Sakr Y, Marques J, Mortsch S, Gonsalves MD, Hekmat K, Kabisch B, et al. Is the SAPS II score valid in surgical intensive care unit patients? J Eval Clin Pract 2012;18:231-7.  Back to cited text no. 33
    
34.
Panebianco NL, Shofer F, Cheng A, Fischer J, Cody K, Dean AJ. The effect of supine versus upright patient positioning on inferior vena cava metrics. Am J Emerg Med 2014;32:1326-9.  Back to cited text no. 34
    
35.
Sand L, Rizell M, Houltz E, Karlsen K, Wiklund J, Odenstedt Hergès H, et al. Effect of patient position and PEEP on hepatic, portal and central venous pressures during liver resection. Acta Anaesthesiol Scand 2011;55:1106-12.  Back to cited text no. 35
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]


This article has been cited by
1 Prevention of arterial hypotension after spinal anaesthesia using vena cava ultrasound to guide fluid management
S. Ceruti,L. Anselmi,B. Minotti,D. Franceschini,J. Aguirre,A. Borgeat,A. Saporito
British Journal of Anaesthesia. 2018; 120(1): 101
[Pubmed] | [DOI]
2 Assessment of hemodynamic response to fluid resuscitation of patients with intra-abdominal sepsis in low- and middle-income countries
Egide Abahuje,Robert Munyaneza,Robert Riviello,Faustin Ntirenganya
Journal of Surgical Research. 2017; 218: 162
[Pubmed] | [DOI]
3 Advancing clinician-performed sonography in the twenty-first century: building on the rich legacy of the twentieth century pioneers
R. Jeanmonod,S. P. Stawicki,D. P. Bahner,M. Zago
European Journal of Trauma and Emergency Surgery. 2016;
[Pubmed] | [DOI]



 

Top
 
  Search
 
    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
    Abstract
   Introduction
    Materials and me...
   Results
   Discussion
   Conclusion
    References
    Article Figures

 Article Access Statistics
    Viewed1357    
    Printed22    
    Emailed0    
    PDF Downloaded119    
    Comments [Add]    
    Cited by others 3    

Recommend this journal