Discussion
This study found that overnight fasting for 8 to 10 hours has decreased SVI 5.5% which was not clinical significance. The result is similar to previous studies. Muller et al, concluded that preoperative fasting did not alter dynamic and static preload indices measured by transthoracic echocardiography.11 Kiefer et al, found no significant difference of blood volume before and after 12-hour fasting.12 By contrast, some studies has found different outcomes. Bundgaard-Nielsen et al, estimated SV by esophageal doppler and found that 70% of patients presented with intravascular volume deficit which needed about 200 mL to maximize SV.9 Jenstrup et al, used maximal venous oxygen saturation to demonstrated that preoperative volume deficit was 500 mL in fasting patients.10 There is a limitation from these two study, esophageal doppler and maximal venous oxygen saturation were measured after induction of general anesthesia. Anesthetic agents and positive pressure ventilation potentially impact hemodynamic status on blood volume, vascular tone and cardiac function. In order to avoid this potential bias, the present study was performed in volunteers without the effect of general anesthesia.
Consider SVI before and after overnight fasting, twelve volunteers (19.4%) have decreased in SVI more than 10% after overnight fasting. This could be assumed that an appropriate intravenous fluid loading could improves cardiac output in some healthy subjects. However, we did not perform subgroup analysis due to limited data and baseline cardiac contractile function was not assessed before the study.
Limitations of this study, the first we studied only in healthy population with ASA physical status classification III. We included subjects who met the inclusion criteria by interview and physical examination. So we might miss some information that affect SVI such as baseline cardiac contractility. Fasting in high-risk patients might affect hemodynamic status in different outcome. The second, we did not record intake and output of fluid. The physiologic fluid loss during fasting can be evaluated to be 0.5 mL kg-1 h-1 because of insensible perspiration and 0.5 mL kg-1 h-1 because of urine output.16 In our study, we excluded subject who possibly has excessive fluid loss such as diarrhea, nausea, vomiting, diuretic use and diagnosed diabetes mellitus. So we assumed that insensible loss and urine output were comparable in study population. The third, we measured hemodynamic parameters when volunteers were in supine position as a static status. We did not measure while passive leg raising as dynamic status. The forth, the whole body impedance cardiography is not a gold standard for hemodynamic monitoring. Pulmonary artery thermodilution is gold standard method but it is invasive, causes serious complications and the accuracy is highly dependent on operator technique. As a result, thermodilution are not always reliable. Besides recent studies found good correlation of cardiac output measured by whole body impedance cardiography and pulmonary artery thermodilution.14,15 This is the first study that determines SVI after overnight fasting in volunteers using whole body impedance cardiography which is non-invasive, safe, practical, reliable and the result is operator independent. The further research with minimized limitations should be study.
Conclusion
Overnight fasting does not clinically affect SVI measured by whole body impedance cardiography in healthy population. Some volunteers have significantly decreased in SVI after overnight fasting and appropriate intravenous fluid therapy probably improves CO.
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