Intra-operative positioning considerations are more important for the obese patient. The supine position causes a marked increase in intra-abdominal pressure, which results in a splinting effect of abdominal contents on the diaphragm. Awake, spontaneously breathing obese patients should be in a head-up position. The Trendelenburg position can be used to engorge neck veins to facilitate central venous cannulation. Spontaneously breathing obese patients generally do not tolerate the Trendelenburg position. In mild to moderately obese patients, respiratory mechanics, lung volumes, and oxygenation all increase when changing from the supine to prone position. Due to the difficulties moving and positioning mobidly obese (MO) patients, procedures routinely performed prone are often done in the lateral decubitus position. In the lithotomy position the patient is on their back with their legs and thighs flexed at right angles. MO patients are at special risk for rhabdomyolysis (RML), a potentially fatal post-operative complication.

The primary task of the lungs is respiration. Respiration is the exchange of gases between an organism and its environment with the utilization of O2 and production of CO2. The most important physiological measure is the compliance of the intact respiratory system. The movement of O2 and CO2 in and out of the capillaries both in the lungs and in the peripheral tissues depends on gas diffusion. Positive pressure ventilation results in most of the ventilation being directed into the upper rather than the lower lung. The blood flow gradient due to gravity favors the dependent lung during one-lung ventilation (OLV). If the non-dependent lung is not ventilated any blood flow to it becomes shunt flow. This results in a larger alveolar-to-arterial oxygen tension difference with a lower PaO2 for a given oxygen concentration under identical circumstances, when compared to two-lung ventilation in the same position.

The institution of one-lung ventilation (OLV) can be problematic in approximately 20% of any case mix. A significant number will prove difficult at the lung separator insertion stage. The usual conditions of difficult intubation pertain, as well as some specific to the thoracic discipline and the pathologies encountered and exacerbated by the structure and bulk of some lung separators. The use of OLV and the adoption of the lateral decubitus position results in specific physiological changes, such as the shifts in west zones from vertical to horizontal, which are best countered by positive pressure ventilation. The open pneumothorax, weight of mediastinum, abdominal contents on adoption of the lateral decubitus position and the surgeon at work compress the dependent lung; and, all must be opposed through the narrow conduit of, often, the single lumen of a double lumen endobronchial tube (DLT).

Transrectal ultrasound (TRUS) evaluates the distal components of the ejaculatory duct system including the ampullae of the vas deferens, the seminal vesicles, ejaculatory ducts, and the prostate. Patients with complete distal ejaculatory obstruction and partial distal obstruction are ideal candidates for TRUS evaluation. The examination can be performed with the patient in the lithotomy, knee-chest, or lateral decubitus position. Lateral decubitus position is the preferred position as this provides easy access for the operator and less discomfort for the patient. On TRUS examination, the seminal vesicles appear as hypoechoic areas with fine septations. Anteroposterior diameter up to 15 mm is considered normal. Importantly, TRUS can reveal the anatomical relationship between ejaculatory channels and calcifications. It can also detect proximal dilatation of the ejaculatory tract, which indirectly implies the presence of a distal obstruction. TRUS can also be used for therapeutic aspiration and reduction in the size of obstructive cysts.