In the present study, we compared the effects of the VCV and PCV-VG on hemodynamic and pulmonary variables in the prone position. In VCV, the ventilator delivers a target volume with a constant flow and the airway pressure increases in a linear manner. This approach ensures minute ventilation regardless of airway compliance but cannot control airway pressure . Compared with VCV, the pressure-controlled ventilation mode (PCV) provides the tidal volume at a preset pressure with decelerating flow, and the tidal volume can be varied depending on lung compliance . PCV-VG delivers a target tidal volume with a decelerating flow and calculates lung compliance to adjust the inspiratory pressure based on the previous breath [6, 8]. It reaches the target volume with the lowest inspiratory pressure and has the benefits of both VCV and PCV .
Several studies have examined the effects of the PCV-VG compared with conventional modes (VCV or PCV). In thoracic surgery with one-lung ventilation, PCV-VG led to lower Ppeak, peak plateau pressure, and Pmean values compared with VCV [10, 11]. Similar results were obtained in a laparoscopic surgery study that showed lower Ppeak values with PCV-VG than with VCV [6, 9, 12].
Spine surgery is commonly performed with patients in the prone position. In respiratory physiology, Ppeak increases and Cdyn decreases as increased intra-thoracic pressure and abdominal pressure compromise diaphragm movement when a patient is turned to the prone position [3, 4, 13, 14]. To our knowledge, no previous study has compared VCV and PCV-VG in the prone position. However, several studies have compared PCV and VCV for patients undergoing posterior lumbar surgery and found that PCV is associated with lower Ppeak values compared with VCV [13, 14].
Pmean correlates with alveolar ventilation and improved oxygenation . Although the values for patients in the prone position were higher than for patients in the supine position in both groups, placement in the prone position did not significantly improve oxygenation. One previous study found similar results; the authors attributed them to a lack of positive end-expiratory pressure .
It is known that the CI decreases when a patient moves in the prone position. This is caused by reduced venous return and increased intra-thoracic pressure, which results in decreased arterial filling and reduced ventricular compliance . Dharmavaram et al.  compared the effects of prone positioners on hemodynamic values using transesophageal echocardiography (TEE). They found that decreases in CI in the prone position resulted from a decreased SV. This effect seemed to be caused by increased afterload rather than decreased preload because the left ventricle end-diastolic area was not significantly altered. Blood flow through the mitral valve was reduced on TEE, suggesting that reduced chest wall compliance diminished diastolic function and increased afterload. The Jackson table had less effect on cardiac function than the Wilson frame.
In the present study, the CO and CI values measured 15 min after placement in the prone position were lower compared with the same values measured in the supine position. The SV values did not significantly change as patients were positioned on the Jackson table, so it is thought that the decreased CO and CI in the prone position may have been caused by a decreased HR, rather than decreased preload or increased afterload.
It can be expected that differences in Ppeak between VCV and PCV-VG will affect pleural pressure, inducing changes in the cardiovascular response . However, we noted no significant hemodynamic change between VCV and PCV-VG. An animal study comparing the effects of VCV and PCV on anesthetized dogs showed that hemodynamic functions such as CO, CI, and SVI did not differ significantly between VCV and PCV, although the Ppeak was higher in VCV than in PCV . Balick-Weber et al.  used TEE to compare the effects of VCV and PCV on cardiac function and found that there was no echocardiographic change in right ventricular SV, left ventricular preload, or left ventricle end-systolic wall stress. They concluded that Ppeak was not associated with cardiac function and that the main determinant of right ventricle afterload was transpulmonary pressure rather than Ppeak.
The present study has several limitations. First, although the FloTrac®/Vigileo system is widely used in anesthesia, it is an uncalibrated pulse contour analysis and may be less accurate under conditions such as sepsis or reduced systemic vascular resistance . Grensemann et al.  reported that the FloTrac®/Vigileo system had a clinically unacceptable high degree of error in the prone position compared with transpulmonary thermodilution techniques and calibrated pulse contour CO analysis. We believed that the transpulmonary thermodilution technique using a pulmonary catheter would be more invasive than pulse contour analysis. As a calibrated pulse contour analysis device was not available in our center, we used an uncalibrated pulse contour analysis device as a less invasive means of measuring hemodynamic variables. Since patients in the previous study were mechanically ventilated with acute respiratory distress syndrome or acute lung injury and expected to be in a septic condition or undergoing vasopressor treatment, the effect of placement in the prone position on uncalibrated CO analysis in healthy volunteers warrants further evaluation. Second, patients with compromised cardiac or pulmonary diseases, as well as those with morbid obesity, were excluded from the study. Respiratory mechanics or hemodynamics may be affected by various lung conditions and we wanted to standardize lung function among the patients. Although there were statistically significant differences between VCV and PCV-VG for Ppeak and Cdyn, these differences were quite small and may have been more prominent for patients with pulmonary disease or morbid obesity. Additional studies are therefore needed to compare the hemodynamic and respiratory effects of VCV and PCV-VG in patients with uncompensated cardiac and pulmonary function or morbid obesity. Third, we measured Cdyn rather than static lung compliance. Static lung compliance does not depend on inspiratory flow and is influenced by the elastic properties of the lung . However, the inspiratory hold maneuver was not available in the ventilator we used, and there may have been detrimental effects on oxygenation while disconnecting the circuit to measure the plateau pressure.