Pediatric specific anatomy and physiology includes small ventricles with low systolic mass and poor ventricular compliance . With decreased peripheral vascular resistance (SVR) . The parasympathetic nervous system is dominant. This explains why HR is delayed compared to changes in MAP and mean blood pressure, is less affected and slower to respond. As each terminal bronchiole opens into a single alveolus and initially forms clusters of clusters, rather than fully developed alveolar clusters, these clusters continue to grow until six to eight years of age to resemble adults. The diaphragm is more horizontal, which reduces its mechanical advantage. Ventilation depends mainly on the diaphragm and the rate of its movements (breathing movements). Higher compliance of the thoracic wall is also seen with increased respiratory work.

Decreased functional residual capacity (FRC) increases pulmonary shunting and leads to atelectasis. One-lung ventilation (OLV) during general anesthesia can improve the surgical field of view, which is often used in thoracic surgery. However, OLV can lead to a mismatch in the ventilation/perfusion ratio (V/Q), leading to increased shunt and pulmonary shunt fraction (Qs/Qt) within the lungs and the occurrence of hypoxia. The ratio between the rate of flow of unoxygenated blood from the pulmonary arteries to the pulmonary veins through the shunt (Qs), and the total rate of blood flow through the lungs (Qt), i.e., the ratio between the unoxygenated blood and the total blood, is known as the pulmonary shunt fraction (Qs/Qt). The FEV1/FVC ratio represents the proportion of the total vital capacity that can be exhaled in the first second of forced expiration, whereas FEV1 indicates "Force Expiratory Volume in 1 second" and FVC indicates "Forced Vital Capacity".

Hypoventilation-induced vasoconstriction (HPV) is the body's primary compensatory mechanism against pulmonary constriction, which is to divert pulmonary perfusion blood from the nonventilated area to the ventilated area, thereby reducing pulmonary constriction and improving blood hypoxia . HPV is affected by various factors, including changes in pulmonary pressure, alkalosis, vasodilators, and inhaled anesthetic gases, which have the greatest effect. The study was designed as a two-group, controlled clinical trial using propofol, which is known to have no effect on HPV.

Dexmedetomidine is widely used as an anesthetic adjuvant. It is a highly selective alpha2 adrenergic receptor agonist that directly stimulates peripheral alpha2 receptors to induce vasoconstriction and increase blood pressure, as well as act on central alpha2 receptors to suppress sympathetic excitation, which can dilate blood vessels and lower blood pressure. Alpha adrenal blockade eliminates pulmonary arrhythmias that responded to non-adrenal but not hypoxia . Due to the complex effects of dexmedetomidine on blood circulation, it is difficult to predict its effects on intrapulmonary shunt and hypoventilation-induced vasoconstriction in patients during one-lung ventilation . The hypothesis of our study predicted that dexmedetomidine would improve oxygenation Partial Pressure of Arterial Oxygen (PaO2) and reduce pulmonary shunting during OLV in children.