Preoperative fasting is considered to be refraining from taking meals for at least 6 hours and clear fluids for at least 2 hours before anesthesia. On the one hand, it is a procedure that reduces the risk of aspiration of chyme and the occurrence of Mendelson's syndrome, and on the other hand, it is one of the causes of water and carbohydrate disturbances in patients undergoing general anesthesia. Most of them experience some degree of dehydration in the preoperative period. It is associated with a worse prognosis. Due to the negative impact of fasting on homeostasis, as well as significant changes in heart rate in participants in the previous study, we decided to measure the hemodynamic parameters during fasting. The development of impedance cardiography techniques enables the measurement of these parameters by a non-invasive method.

After obtaining the participant's written consent to participate in the study, the first measurement of body weight, blood pressure using a non-invasive method and hemodynamic parameters using a non-invasive impedance cardiography method using the Niccomo ™ device (Medizinische Messtechnik GmbH) will be performed in the evening. The subject will then be asked to refrain from food for at least 10 hours and fluid for 6 hours before the next measurement. The next morning, another measurement will be made and the participant will be randomly assigned to one of two groups: subjects in the first group will be asked to refrain from eating and drinking for another 2 hours, while subjects in the second group per os will receive 400 milliliters of PreOp® (Nutricia ™ preparation intended for preoperative dietary management in surgical patients, containing carbohydrates and electrolytes). After another two hours, the third measurement of the tested parameters will be performed. The obtained results will be subjected to statistical analysis consisting in comparing the values of hemodynamic parameters between the studied groups at appropriate measurement points.

Inclusion Criteria:

• American Society of Anesthesiology (ASA) general condition assessment 1 or 2

Exclusion Criteria:

• Chronic kidney disease
• Circulatory failure
• Lung diseases
• Diseases of the heart valves
• History of hypoglycaemic episodes or any carbohydrate disturbance.
• Height less than 120 cm or greater than 230 cm
• Body weight less than 30 kg or greater than 155 kg.