Adequate fluid loading is the first step in the hemodynamic optimization of surgical patients. This is especially true for cardiac patients, who, on the one hand, may have a fluid deficiency because of preoperative fasting but, in turn, may only restrictedly tolerate rapid fluid substitution depending on the underlying cardiac disease. An exact estimation of volume status is particularly difficult in these patients. The clinical approach alone is often not sufficient for the early recognition of fluid deficiency or overload to implement targeted treatment 1. Thus, volume status imbalances frequently are not recognized at all or are recognized too late, which may have drastic consequences for the hemodynamic stability of these patients 12.

Thus far, filling pressures have been used most often in the clinical routine to assess the hemodynamic status and the volume status. In a nationwide survey of internal medicine and surgical intensive care units, central venous pressure (CVP) was named in 90% of the cases as the monitoring procedure of choice for volume therapy, followed by pulmonary capillary wedge pressure (PCWP) with almost 60% 3. A more recent survey of cardiosurgical intensive care specialists showed that CVP is used for monitoring volume therapy 87% of the time, followed by mean arterial blood pressure with 84% and PCWP with 30% 4. Parameters allowing a directly measured approximation of intrathoracic fluid loading have been used in the clinical routine for some years. These parameters may contribute to more targeted and optimized volume therapy. To estimate the volume status, the global end-diastolic volume index (GEDVI) might be of more use than filling pressures 5. Dynamic parameters that assess volume reactivity, like stroke volume variation (SVV) and pulse pressure variation (PPV), are also increasingly used to monitor volume therapy 678. The aim of this study was to examine the suitability of the established parameters CVP and PCWP and the newer parameters GEDVI, PPV, and SVV to predict changes in cardiac index (CI) and stroke volume index (SVI) after sternotomy in cardiac surgery patients.

Volumetric and hemodynamic parameters were determined in 40 patients. The first set of measurements was performed directly after inducing anesthesia. Then a mean of 1,364 mL (713 mL) of autologous blood was taken from the patients, and 1,459 mL (605 mL) of HAES 6% and 1,311 mL (742 mL) of crystalloid fluid were substituted as a local standard to account for preoperative deficits and guarantee a hematocrit below 0.25 during CPB. After hemodilution and fluid therapy were concluded, the second set of measurements was performed 15 minutes after sternotomy. Patients' basic characteristics are presented in Table 1.

During the study period, there was a significant increase in heart rate and a significant decrease in CVP (Table 2). We also observed a significant decrease in systemic and pulmonary vascular resistances with a concomitant significant increase in CI and SVI (Table 2). GEDVI significantly increased and SVV and PPV significantly decreased after fluid loading (Table 2).

The changes in the CI between the two measuring points did not correlate with changes in the CVP or PCWP (Figure 1). The correlation coefficients were r = -0.280 for CVP and r = 0.017 for PCWP. In contrast, the increase in GEDVI correlated with the increase in CI (Figure 2). The correlation coefficient here was r = 0.518 (p < 0.01). Moreover, the decrease in SVV and PPV correlated with an increase in CI with correlation coefficients of r = -0.399 (p = 0.03) for SVV and r = -0.411 (p = 0.02) for PPV (Figure 3).

The best prediction of changes in SVI was observed from relative changes in SVV and PPV. Also, the change in GEDVI was predictive for an increase in SVI after fluid loading and sternotomy. None of the absolute parameters after induction of anesthesia was able to predict an increase in SVI after volume loading and chest opening (Table 3).

The most important results of this study are that the increase of CI observed after sternotomy and volume loading in our study in cardiac surgical patients did not correlate with changes in CVP and PCWP. In contrast, the changes in GEDVI, SVV, and PPV showed a significant correlation. This is of major clinical interest since the vast majority of anesthesiologists still use filling pressures as a standard monitoring tool for volume management, but as demonstrated by our results, filling pressures might not be the monitoring parameters of choice under open-chest conditions 4. Only relative changes of SVV, PPV, and GEDVI were able to predict an increase in SVI after sterotomy and volume loading.

Opening the chest causes a decrease in airway pressures and hence in the effects of the mechanical ventilation on venous return. Therefore, after sternotomy, we assumed that PPV and SVV might decrease and CI and SVI might increase alone due to the change in airway pressure. These effects might be regarded as a relative increase in 'cardiac preload' due to a facilitated venous return. This is in line with our findings and was shown previously in an experimental design 10. Fluid loading is decisively important in cardiac surgical patients to optimize cardiac output and tissue oxygenation 1112. With optimal preload, cardiac contractility increases and cardiac work is economized. However, determining left ventricular preload in the clinical routine is particularly difficult during surgery. Filling pressures like CVP and PCWP are normally used as parameters of right and left heart preload. The lack of a correlation of filling pressures as a volume parameter in our study has also been described in other studies and has several reasons: CVP and PCWP depend not only on intravascular volume and peripheral vessel tone but also on right/left ventricular compliance, pulmonary vessel resistance, and intrathoracic pressure. Sakai and colleagues 13 described a significant correlation between increasing PEEP and rising CVP (r = 0.88). Furthermore, PCWP depends on the function of the mitral valve and the contractility of the left ventricle. The therapeutic application of vasodilators and vasopressors has been shown to influence measurements 14.

Other studies evaluating volumetric parameters in different surgical areas generally agree with our results. Thus, it was shown that ITBVI and GEDVI correlate better with cardiac preload than filling pressures 15161718. Moreover, no significant correlation was found between the percentage of changes in CI and the SVI, CVP, and PCWP changes over a 24-hour period in patients after uncomplicated coronary artery bypass surgery 19. Lichtwarck-Aschoff and colleagues 18 showed for the first time that volumetric parameters like ITBV were superior to filling pressures (CVP and PCWP) for assessing the preload. This was also confirmed in other studies with different patient populations 202122. In burn patients, optimized volume and fluid loading are of particular clinical interest since greater volume shifts are seen in these patients over a short time and empirical volume therapy frequently underestimates the real fluid requirement. In these patients, significant correlations were found between ITBVI, CI, and oxygen supply parameters 23, whereas CVP and PCWP failed to predict changes in CI dependent on volume loading. Furthermore, Reuter and colleagues 7 showed that there was a significant decrease in CI and GEDVI after taking autologous blood for hemodilution following sternotomy. Particularly with mechanical ventilation and changes in the intrathoracic pressure, volumetric parameters like ITBVI and GEDVI are clearly superior to filling pressures (CVP and PCWP) for estimating the cardiac preload 24. This is especially relevant for cardiac surgery patients since intrathoracic pressure markedly changes after sternotomy, which may decisively affect the usability of changes in the filling pressures.

Positive intrathoracic pressure following mechanical ventilation induces a reduction in biventricular preload. This is reflected by variations in the SV. These variations during a defined interval have proven to be useful parameters of cardiac preload 25. SVV and PPV are parameters derived from changes in SV dependent on mechanical ventilation and have been found useful for monitoring volume reactivity volume therapy in the perioperative phase of cardiac patients 26. The decrease in SVV and PPV with a concomitant increase in CI following volume loading observed in our study agrees with previous studies. Wiesenack and colleagues 2728 described a good correlation between changes in SVI and SVV base values after volume loading. In systemic vascular resistance changes, however, the measurement algorithm required recalibration since an increase in the SVI and CI cannot always be reliably determined by pulse contour analysis 272829. Furthermore, a significant correlation between SVV and tidal volumes was described before and after fluid loading in a controlled study 30. In regard to this, tidal volumes were kept constant throughout the study period. Thus, it seems important to observe absolute values and relative changes of SVV and PPV during fluid loading to assess volume loading and to exclude volume overload. Only a decrease in SVV and PPV with a concomitant increase in SVI or CI indicates a volume deficit requiring therapy. This is in line with our finding that only relative changes of PPV, SVV, and GEDVI were predictive of volume responsiveness in our study.

In summary, our results suggest that filling pressures CVP and PCWP are not suitable parameters for monitoring volume therapy during surgery in cardiac patients. Direct volume measurement, such as GEDVI determination or the dynamic volume-reactive parameters SVV and PPV, is clearly superior to the pressure parameters CVP and PCWP, especially under pressure conditions in the thorax changed by sternotomy. Only relative changes in SVV, PPV, and GEDVI were predictive for volume response in this study.

• We observed no correlation between the change of cardiac index (CI) and the changes in the central venous pressure or pulmonary capillary wedge pressure before and after sternotomy and volume loading in patients undergoing coronary artery bypass graft surgery.

• In contrast to this, our study could establish a good correlation between the increase in global end-diastolic volume index (GEDVI) and the increase in CI prior to and after sternotomy and volume loading.

• Moreover, the decrease in stroke volume variation (SVV) and pulse pressure variation (PPV) correlated with an increase in CI.

• Only relative changes in GEDVI, SVV, and PPV predicted a positive volume response.

CI = cardiac index; CPB = cardiopulmonary bypass; CVP = central venous pressure; DSt = downslope time; GEDVI = global end-diastolic volume index; HAES = hydroxyaethyl starch; ITBVI = intrathoracic blood volume index; MTt = mean transit time; PCWP = pulmonary capillary wedge pressure; PPV = pulse pressure variation; SV = stroke volume; SVI = stroke volume index; SVV = stroke volume variation.

The authors declare that they have no competing interests.

MS and CvH prepared the manuscript, carried out the measurements, conceived the study, and performed the statistical analysis. KB, AF, MC, and MK helped with the recruitment of the patients and the drafting of the manuscript. HG participated in the study design and helped with the recruitment of patients. CDS drafted the manuscript and helped with the study design and coordination. All authors read and approved the final manuscript.