Volumetric preload measurement by thermodilution: a comparison with transoesophageal echocardiography

C. K. Hofer1,*, L. Furrer1, S. Matter-Ensner1, M. Maloigne1, R. Klaghofer2, M. Genoni3 and A. Zollinger1

1 Institute of Anaesthesiology and Intensive Care Medicine and 3 Division of Cardiac Surgery, Triemli City Hospital, Zurich, Switzerland. 2 Statistics, Department of Psychosocial Medicine University Hospital Zurich, Zurich, Switzerland

* Corresponding author: Institute of Anaesthesiology and Intensive Care Medicine, Triemli City Hospital, Birmensdorferstr. 497, 8063 Zurich, Switzerland. E-mail: christoph.hofer{at}triemli.stzh.ch

Accepted for publication February 2, 2005.


    Abstract
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Background. End-diastolic volume indices determined by transpulmonary thermodilution and pulmonary artery thermodilution may give a better estimate of left ventricular preload than pulmonary capillary wedge pressure monitoring. The aim of this study was to compare volume preload monitoring using the two different thermodilution techniques with left ventricular preload assessment by transoesophageal echocardiography (TOE).

Methods. Twenty patients undergoing elective cardiac surgery with preserved left–right ventricular function were studied after induction of anaesthesia. Conventional haemodynamic variables, global end-diastolic volume index using the pulse contour cardiac output (PiCCO) system (GEDVIPiCCO), continuous end-diastolic volume index (CEDVIPAC) measured by a modified pulmonary artery catheter (PAC), left ventricular end-diastolic area index (LVEDAI) using TOE and stroke volume indices (SVI) were recorded before and 20 and 40 min after fluid replacement therapy. Analysis of variance (Bonferroni–Dunn), Bland–Altman analysis and linear regression were performed.

Results. GEDVIPiCCO, CEDVIPAC, LVEDAI and SVIPiCCO/PAC increased significantly after fluid load (P<0.05). An increase >10% for GEDVIPiCCO and LVEDAI was observed in 85% and 90% of the patients compared with 45% for CEDVIPAC. Mean bias (2 SD) between percentage changes ({Delta}) in GEDVIPiCCO and {Delta}LVEDAI was –3.2 (17.6)% and between {Delta}CEDVIPAC and {Delta}LVEDAI –8.7 (30.0)%. The correlation coefficient (r2) for {Delta}GEDVIPiCCO vs {Delta}LVEDAI was 0.658 and for {Delta}CEDVIPAC vs {Delta}LVEDAI 0.161. The relationship between {Delta}GEDVIPiCCO and {Delta}SVIPiCCO was stronger (r2=0.576) than that between {Delta}CEDVIPAC and {Delta}SVIPAC (r2=0.267).

Conclusion. GEDVI assessed by the PiCCO system gives a better reflection of echocardiographic changes in left ventricular preload, in response to fluid replacement therapy, than CEDVI measured by a modified PAC.

Keywords: heart, cardiac output ; heart, coronary artery bypass ; heart, myocardial function


    Introduction
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Optimization of perioperative volume status for improved cardiac performance, especially in patients with a potentially limited left ventricular reserve, requires adequate preload monitoring. In contrast to the widely used cardiac filling pressures, end-diastolic volume estimates of the left ventricle are better indicators of end-diastolic left ventricular fibre length, i.e. preload according to the Frank–Starling law.1 2 Therefore, assessment of left ventricular volume by radionuclide angiography, magnetic resonance imaging and echocardiography would be the preferred techniques.3 However, these methods are either not practicable in a perioperative setting or cannot be routinely performed for logistic and economical reasons. Hence, there has been recent interest in alternative, catheter-related, volume estimates using thermodilution.

Two different techniques, transpulmonary and pulmonary artery thermodilution, are used in commercially available monitoring devices. The PiCCO system (Pulse Contour Cardiac Output system; Pulsion Medical Systems, Munich, Germany) uses integrated transpulmonary thermodilution to measure the volumetric preload parameter global end-diastolic volume index (GEDVI) and includes the total volumes of cardiac atria and ventricles as well as part of the systemic vascular blood volume. Compared with conventional pressure-derived preload assessment, volumetric preload determination by the PiCCO system has been shown to better reflect left ventricular filling.4 5 Pulmonary artery thermodilution, on the other hand, determines right ventricular end-diastolic volume index (RVEDVI). This volume index also showed a better correlation with cardiac performance than cardiac filling pressures in studies performed in critically ill patients.68 A recent modification of pulmonary artery thermodilution catheters allows the automatic and continuous determination of RVEDVI, the continuous end-diastolic volume index (CEDVI; Swan-Ganz Continuous Cardiac Output/End Diastolic Volume Thermodilution Catheter; CCOmbo CCO/SvO2/CEDV catheter 774HF75; Edwards Lifesciences, Irvine, CA, USA).

The aim of this study was to compare volumetric preload as measured by transpulmonary thermodilution (GEDVIPiCCO) and monitored by pulmonary artery thermodilution (CEDVIPAC) with left ventricular preload estimates assessed by transoesophageal echocardiography (TOE). Our hypothesis was that both volume preload parameters would comparably reflect left ventricular preload monitored by TOE.


    Patients and methods
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Patient selection
With local ethics committee approval and written informed consent, 20 patients undergoing elective off-pump coronary artery bypass grafting were enrolled. Exclusion criteria were preoperative dysrhythmias, reduced left and right ventricular function (ejection fraction <40%), valvular heart disease, intracardiac shunts, pulmonary artery hypertension, severe peripheral vascular disease and a history of oesophageal or gastrointestinal disease precluding the use of transoesophageal echocardiography.

Anaesthetic technique
After application of the routine haemodynamic monitoring (pulse oximetry, five-lead ECG and non-invasive blood pressure monitoring; CMS, Philips Medical Systems, Andover, MA, USA) a peripheral radial arterial and an i.v. line were inserted and lactated Ringer's solution 2 ml kg–1 h–1 i.v. was given continuously. Anaesthesia was induced using fentanyl 10–30 µg kg–1 i.v., lidocaine 1.5 mg kg–1 i.v. and propofol up to 2 mg kg–1 i.v., and was maintained with additional propofol (1.5–3 mg kg–1 h–1) and fentanyl (10 µg kg–1 i.v.). Muscle paralysis was achieved with pancuronium bromide (0.1 mg kg–1 i.v.). The trachea was intubated and the lungs mechanically ventilated without positive end-expiratory pressure using an inspired oxygen of 50% and tidal volume of 8 ml kg–1 to maintain end-expiratory at 4–4.5 kPa during the study period. Thus, effective applied mean tidal volumes were 610 (73) ml and peak airway pressure ranged from 14 to 24 cm H2O (mean=18 [2] cm H2O).

Haemodynamic monitoring and transoesophageal echocardiography
A 4 F thermistor-tipped arterial catheter (Pulsiocath thermodilution catheter; Pulsion Medical Systems, Munich, Germany) was inserted in the left femoral artery; its tip advanced to the abdominal aorta, and it was connected to the PiCCOplus (version 5.2.2; Pulsion Medical Systems). Cardiac output (COPiCCO), stroke volume (SVPiCCO) and global end-diastolic volume (GEDVIPiCCO) were determined using a triplicate injection of 15 ml ice-cold normal saline through an additional 7 F central venous catheter introduced in the right subclavian vein. GEDVIPiCCO is calculated from the difference of mean indicator transit time and exponential indicator down-slope time and from the cardiac index obtained from transpulmonary thermodilution. The basis of this method has been described in detail previously.9 10 The PiCCO system also displays intrathoracic blood volume index (ITBVI) as an additional volume preload variable. This variable is calculated from GEDVIPiCCO based on a fixed algorithm, established from data obtained from earlier double-indicator transpulmonary thermodilution. The bolus thermodilution measurements were made by the same observer to avoid interobserver variation.

A 7.5 F pulmonary artery catheter (Swan-Ganz Continuous Cardiac Output/End Diastolic Volume Thermodilution Catheter CCOmbo CCO/SvO2/CEDV catheter 774HF75 Edwards Lifesciences) was introduced into the right internal jugular vein and attached to the Vigilance monitor for measurement of cardiac output (COPAC), stroke volume (SVPAC) and continuous end-diastolic volume (CEDVIPAC). CEDVIPAC is determined by analysis of the thermal washout curve using plateau and exponential curve analysis by analogy to the determination of right-ventricular ejection fraction and right-ventricular end-diastolic volume assessment by the fast-response thermistor-tipped pulmonary artery catheter. Details of this method have been published elsewhere.11 Central venous and pulmonary capillary wedge pressures were measured using standard transducers (CMS; Philips Medical Systems).

TOE was performed using a Philips Sonos 5500 system with an Omniplane III-TOE probe (Philips Medical Systems). The probe was positioned to obtain the transgastric midpapillary short-axis view of the left ventricle. Left ventricular end-diastolic area (LVEDA) and left ventricular end-systolic area (LVESA) were measured by manual planimetry of the area circumscribed by the leading edge of the endocardial border in this position. LVEDA was determined as the largest left ventricular cross-sectional area after the electrocardiographic T wave and LVESA as the smallest left ventricular cross-sectional area after the R wave. All TOE measurements were performed, recorded and calculated by an experienced operator blinded to the results of the haemodynamic measurements.

Experimental protocol
After induction of anaesthesia and a 15 min period of haemodynamic stabilization, haemodynamic measurements were performed before (T0) and 20 min (T1) and 40 min (T2) after a volume load. Hydroxyethyl starch solution 6% (HES 130/0.4; Voluven®; Fresenius Kabi, Stans, Switzerland) was given i.v. in a dose of 10 ml kg–1 (ideal body weight) over a period of 20 min (mean volume, 730 [60] ml). At each time point heart rate, MAP, mean pulmonary arterial pressure (MPAP), central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP) and PiCCO measurements and the COPAC readings were recorded. TOE was performed simultaneously. Surgery started after measurements at T2 were completed.

Data analysis
A sample size of >15 patients was calculated on the hypothesis of an expected 10% change in haemodynamic variables after fluid replacement (level of significance=0.05%; power=90%) according to initial observations using the different methods of preload assessment.

All haemodynamic measurements were recorded as the mean of three consecutive readings at intervals of 3 min. Ejection fraction (%) was calculated post hoc from TOE measurements: 100xLVEDA–1x(LVEDA–LVESA). All haemodynamic values were indexed to body surface area (BSA) by means of the Du Bois formula (BSA=body weight [kg]0.425xbody length[cm]0.725x71.84). Statistical analysis was performed using Statview 5.01® Software (SAS Institute, Cary, NC, USA). Analysis of variance (ANOVA) with post hoc Bonferroni–Dunn correction was done for comparison of haemodynamic data during the study period (T0–T2). Two-tailed Student's t-test was used to determine differences in preload changes and stroke volume changes between methods. Bland–Altman analysis12 was performed to compare the preload and stroke volume changes assessed by all three techniques and absolute values of cardiac output determined by the two thermodilution methods. The Pearson correlation was established for absolute values and changes between preload and stroke volume indices. Relationships between the corresponding values obtained from one method and relationships between values recorded from the different methods were calculated to exclude the possibility of mathematical coupling;13 Fisher's z transformation and a Hotelling–Williams test were used to compare correlation coefficients for statistical difference. A P-value <0.05 was considered statistically significant. Unless otherwise stated, data are presented as mean (SD).


    Results
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Four women and 16 men, ages 67.2 (8.4) yr [range 52–78 yr], body mass index=28.2 (4.1) kg m–2, ejection fraction=64.9 (9.7)% were enrolled into the study. Fluid bolus led to a significant change in all monitored haemodynamic variables, with the exception of heart rate (T1; P<0.05) (Table 1). Basic haemodynamic variables, cardiac index (CI), stroke volume index (SVI), end-diastolic volume indices assessed by both catheter systems and TOE variables increased but systemic vascular resistance decreased. Compared with T1, the haemodynamic measurements at T2 showed significant decreases in MAP, GEDVIPiCCO, SVIPiCCO and LVEDAI.


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Table 1 Haemodynamic variables during the study.

 
A significantly different pattern of preload index changes assessed by PAC was observed 20 and 40 min after fluid administration compared with preload changes monitored by PiCCO and TOE. Between T0 and T1, an increase >10% was observed in nine patients (45%) for CEDVIPAC compared with 17 (85%) for GEDVIPiCCO and 18 (90%) for LVEDAI (%change {Delta} GEDVIPiCCO=16.8 [6.9]%, {Delta}CEDVIPAC=11.1 [8.1]%, {Delta}LVEDAI=20.5 [10.1]%; P {Delta}GEDVIPiCCO vs {Delta}LVEDAI=0.117, P {Delta}CEDVIPAC vs {Delta}LVEDAI=0.018). Between T1 and T2, CEDVIPAC decreased by >10% in only one patient (5%), but in seven (35%) and eight (40%) patients for GEDVIPiCCO and LVEDAI, respectively ({Delta}GEDVIPiCCO=–6.2 [7.5]%, {Delta}CEDVIPAC=0.2 [10.1]%, {Delta}LVEDAI=–6.7 [7.0]%; P {Delta}GEDVIPiCCO vs {Delta}LVEDAI=0.547, P {Delta}CEDVIPAC vs {Delta}LVEDAI=0.011). Bland and Altman analysis of the preload changes induced between T0 and T1 showed lower mean bias and lower limits of agreement for LVEDAI–GEDVIPiCCO compared with LVEDAI–CEDVIPAC (Fig. 1A, B). Between T1 and T2, these differences were less pronounced (Fig. 2A, B). Comparing cardiac and stroke volume indices assessed by both thermodilution techniques during the study period, the mean bias (2 SD) was –0.04 (1.15) litre min–1 m–2 for CIPiCCO–CIPAC and –1.2 (18.2) ml m–2 for SVIPiCCO–SVIPAC. There was a good correlation between SVIPiCCO and SVIPAC and between {Delta}SVIPiCCO and {Delta}SVIPAC (r2=0.768 and 0.617, respectively [P<0.001]).



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Fig 1 Bland–Altman analysis of changes in cardiac preload indices between T0 and T1. (A) GEDVIPiCCO vs LVEDAI: mean bias (2 SD)=–3.2 (17.6)%. (B) CEDVIPAC vs LVEDAI: mean bias (2 SD)=–8.7 (30.0)%. T0, before fluid load; T1, 20 min after fluid load; GEDVIPiCCO, global end-diastolic volume index (assessed by PiCCO); LVEDAI, end-diastolic area index; CEDVIPAC, continuous end-diastolic volume index (assessed by PAC); {Delta}, change in response to fluid replacement therapy (%).

 


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Fig 2 Bland–Altman analysis of changes in cardiac preload indices between T1 and T2. (A) GEDVIPiCCO vs LVEDAI: mean bias (2 SD)=1.2 (17.9)%. (B) CEDVIPAC vs LVEDAI: mean bias (2 SD)=7.7 (24.0)%. T1, 20 min after fluid load. T2, 40 min after fluid load. GEDVIPiCCO, global end-diastolic volume index (assessed by PiCCO); LVEDAI, end-diastolic area index; CEDVIPAC, continuous end-diastolic volume index (assessed by PAC); {Delta}, change in response to fluid replacement therapy (%).

 
Linear regression analysis between the preload indices assessed by the different methods and between preload indices and stroke volume indices showed significant correlations for all volume, but not for the pressure preload indices (Table 2). Correlations for GEDVIPiCCO and LVEDAI with stroke volume indices were stronger than for CEDVIPAC. The relationship between CEDVIPAC and LVEDAI was weaker than the relationship between GEDVIPiCCO and LVEDAI (Fig. 3). Regression analysis of preload and stroke volume index changes (Table 3) as well as for volume preload index changes and changes observed by TOE (Fig. 4) showed higher correlation coefficients than for absolute values. Assessment of mathematical coupling of GEDVIPiCCO–SVIPiCCO with GEDVIPiCCO–SVIPAC and of CEDVIPAC–SVIPAC with CEDVIPAC–SVIPiCCO revealed no significant difference.


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Table 2 Correlation coefficients (r2) between absolute values of cardiac preload and stroke volume indices.

 


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Fig 3 Correlation between cardiac preload indices. (A) GEDVIPiCCO vs LVEDAI. (B) CEDVIPAC vs LVEDAI. GEDVIPiCCO, global end-diastolic volume index (assessed by PiCCO); LVEDAI, left ventricular end-diastolic area index; CEDVIPAC, continuous end-diastolic volume index (assessed by PAC). Analysis of pooled data (T0–T2).

 

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Table 3 Correlation coefficients (r2) between change in cardiac preload and stroke volume indices. {Delta}, change in response to fluid replacement therapy (%).

 


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Fig 4 Correlation between changes of cardiac preload indices. (A) GEDVIPiCCO vs LVEDAI. (B) CEDVIPAC vs LVEDAI. GEDVIPiCCO, global end-diastolic volume index (assessed by PiCCO); LVEDAI, end-diastolic area index; CEDVIPAC, continuous end-diastolic volume index (assessed by PAC); {Delta}, changes in response to fluid replacement therapy (%). Analysis of pooled data (changes between T0 and T1 and changes between T1 and T2).

 

    Discussion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
These results, obtained in patients with preserved left ventricular function, indicate that global end-diastolic volume index assessed by the PiCCO system (GEDVIPiCCO) gives a better reflection of echocardiographic changes of left ventricular end-diastolic area index (LVEDAI) in response to fluid replacement therapy than continuous end-diastolic volume index measured with a modified pulmonary artery catheter (CEDVIPAC). Furthermore, the relationship of absolute values and changes of GEDVIPiCCO with stroke volume index (SVI) was stronger than for the respective values of CEDVIPAC with SVI. For both thermodilution techniques, mathematical coupling appeared to be unlikely.

As with previously published results, conventional pressure preload parameters did not adequately reflect left ventricular filling,1 2 48 indicating superiority of volumetric monitoring of cardiovascular volume status over conventional preload pressure monitoring. In clinical practice, when logistic and financial considerations limit the use of echocardiography and other imaging technologies, thermodilution-based volume assessment must be regarded as the preferred method. However, to our knowledge, a comparison of the different commercially available volumetric preload assessment techniques has not been performed.

Transpulmonary thermodilution integrated in the PiCCO system does not require pulmonary artery catheter placement and thus avoids the related risks.14 Based on the injection site (usually central venous access) and the detection site (thermistor in the distal descending aorta) the measured volume includes the total volumes of the heart and the aortic blood volume (GEDVI). In most studies published during the last decade, GEDVI and the closely related intrathoracic blood volume index (ITBVI), which includes the central blood volumes of GEDVI and the pulmonary blood volume, were both assessed by a double-indicator (iced water and indocyanine green injection) dilution technique using the COLD system (Pulsion Medical Systems). These studies were performed in a variety of clinical settings (critically ill,15 sepsis,16 cardiac surgery,17 neurosurgery18). Results indicate that these volume preload indices are closely correlated to volume status and to changes in cardiac output in response to changes in circulating blood volume. Moreover, this method of volumetric preload assessment has been shown to be a measure of cardiac preload equivalent to preload assessment by TOE.19 However, the results raised concerns of mathematical coupling which can occur if two variables calculated from the same measurement are compared, allowing correlations between the variables to be artificially improved.20 This issue has been addressed in studies by changing cardiac output using dobutamine21 or ß-antagonists.22 In our study, independent changes in cardiac output and volume preload indices mean that correlations between measured volumes and cardiac output were unlikely to be attributed primarily to mathematical coupling.

Recently, the time-consuming and expensive double-indicator technique (COLD system) has been replaced by a single-indicator technique (PiCCO system). Using the PiCCO system, GEDVI is measured and ITBVI is calculated from GEDVI based on a fixed algorithm established with data from the double-indicator technique. Adequate accuracy and precision between end-diastolic volume assessment by the COLD system and the PiCCO system has been demonstrated.23 Furthermore, the superiority of the PiCCO system as a left ventricular preload monitoring compared with conventional pressure preload assessment was confirmed4 5 and the influence of mathematical coupling was again found to be negligible.10

In contrast to the global end-diastolic volume assessed by the PiCCO system, continuous end-diastolic volume index (CEDVIPAC) is measured using a pulmonary artery catheter and the continuous cardiac output measurement technique; thus, end-diastolic volume of the right heart is determined. Earlier versions of a modified pulmonary artery catheter (mounted with fast reacting thermistors) assessed right ventricular end-diastolic volume (RVEDVI) by the iced water bolus method. RVEDVI has been validated against radionuclide angiography, contrast ventriculography and echocardiography of the right heart.11 24 Several studies on RVEDVI, used as left ventricular preload substitute in critically ill patients, showed a superior relationship between this preload variable and cardiac output compared with standard pressure measurement8 25 and mathematical coupling was also not a factor.2527 However, difficulties in correct catheter placement prevented wider clinical use of this technique. The modified pulmonary artery catheter (CCOMBO-EDV) gives access to continuous volumetric preload assessment of the right heart.

To our knowledge, the present data on CEDVIPAC represent the first clinical experience with this technique. CEDVIPAC reflected left ventricular preload better than the conventional cardiac filling pressures and the results are comparable with previous clinical investigations of RVEDVI as volume preload index. However, a poorer relationship between CEDVIPAC and echocardiographic preload assessment and poorer performance in comparison with GEDVIPiCCO or stroke volume indices highlight major limitations in using right-heart catheterization for volumetric left ventricular preload assessment. Right ventricular function differs considerably from left ventricular function. The major determinant of left ventricular function is myocardial wall tension, whereas for the right it is ventricular afterload, which is primarily controlled by pulmonary vascular resistance and indirectly by left ventricular function and various pulmonary factors.28 Based on clinical experience, excluding the right ventricle from the circulation, the right heart may act as a conductance vessel and therefore the influence of right ventricular end-diastolic volume on cardiac performance may be limited.29 Furthermore, CEDVIPAC readings may be influenced by interventricular dependence, right ventricular dysfunction and increased right ventricular afterload. Therefore, the relationship between right ventricular preload assessment and cardiac output readings may be weak. However, our findings do not preclude a valid assessment of right heart end-diastolic volumes. In addition, delayed reactivity to rapid changes of intravascular volume by the pulmonary artery catheter compared with the PiCCO system could explain different findings for CEDVIPAC and GEDVIPiCCO. However, stroke volume changes in this study assessed with both the PiCCO system and the pulmonary artery catheter were comparable.

Certain limitations of the clinical utility of CEDVIPAC monitoring have to be considered. CEDVIPAC was assessed here as a substitute for left ventricular preload only and has not been validated as right ventricular preload parameter against radionuclide angiography or magnetic resonance imaging. However, valid echocardiographic monitoring of right heart volume based on anatomical structures is questionable due to lack of suitable mathematical models. Moreover, CEDVIPAC has not been tested in patients with clinical left- or right-heart failure and its value in this context is unknown. The limitations of transoesophageal echocardiography as the gold standard for monitoring left ventricular preload have to be emphasized. Quantitative assessment of left ventricular end-diastolic area by transoesophageal echocardiography may not necessarily reflect volume status due to myocardial wall motion abnormalities in patients undergoing cardiac surgery, and may be altered by dislocation of the probe from the midpapillary level.30

In conclusion, the present study, comparing two thermodilution-based volumetric preload assessment tools with echocardiographic preload monitoring, indicates that GEDVI assessed by the PiCCO system better reflects left ventricular preload than CEDVI measured by a modified pulmonary artery catheter.


    Acknowledgments
 
This study was supported by a grant of the Swiss Society of Anaesthesiology (SGAR). Material support was in part provided by Edwards Lifesciences, Switzerland for the CCombo-V CEDV pulmonary artery catheter (Edwards Lifesciences, Irvine, CA, USA). None of the authors is related to or has any financial interests in the manufacturers of the products studied.


    References
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
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Articles by Hofer, C. K.
Articles by Zollinger, A.
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Articles by Hofer, C. K.
Articles by Zollinger, A.