In an experimental study of Buhre, Weyland and Buhre et al. (2000), controlled subjects have been infused with 14 ml kg-1 of IVF prior to the introduction of anesthesia. After the elapsed time of sitting position, the change in body position has caused a significant decline of intrathoracic blood volume (r=0. 78). According to the study of Stendel, Gramm and Schroder et al. (2000), venous pooling usually results in the filling of the left atrium leading to the reduction of stroke volume and cardiac volume output.
Despite of the increase in both systemic and pulmonary vascular resistance, the net effect usually proceeds to arterial hypotension, which eventually leads to the reduction of cardiopulmonary pressure. According to Yao, Malhotra and Fontes (2005), severe hypovolemia, cachexia, and extremes of age may actually diminish the ability of autonomic reflexes to maintain adequate cerebral and upper lung perfusion in the upright position, leasing to extremely difficult intraoperative cardiopulmonary management (p. 535).
According to Wulker, Mansat and Fu (2000), the patient’s head is fixed in the middle position to prevent any potential negative effects towards the cervical spine, cervical plexus and vascular peripheries (p. 131). In addition by Stern (2001), if the head is placed in an anti-flexed positioning and the chin comes in close contact with the thorax, such placement can potentially impede the lymphatic drainage system, which can further aggravate the increase in intracranial pressure (p. 27).
According to Maklebust and Sieggreen (2001), the physiology of sitting position and its effects on circulatory and vascular pressure are derived mainly in the principle of blockage that triggers the increase of peripheral pressure, while decreasing the exertive pressure of the circulation, thereby causing the intense decline of pressure (p. 123). b. Dangers of Inappropriate Positioning
Some of the possible dangers caused by the sitting or beach chair position include (1) potential cause of air embolism, (2) postural hypotension, (3) quadriplegia, (4) injury to the sciatic, peroneal and brachial plexus nerves, and (5) obstruction of the internal jugular vein (Lobato, Gravenstein and Kirby 2007 p. 851). Hypotension is the most common complication of sitting position and according to Walch and Boileau (1998) even on healthy individuals the adoption of beach-chair position under general anesthesia can further aggravate hypotension due to reductions on venous returns (p. 53).
Venous circulation from cerebral area is further aggravated by the inspiratory subatmospheric pressure during spontaneous ventilation, but such mechanism is being offset by the positive pressure ventilation. In addition, the obstruction on inner jugular veins can block the cerebral venous drainage, most especially during head-on-neck flexion. Meanwhile, in Abrams and Nottage’s (2003) experimental study of controlled respondents who have had shoulder arthroscopy, the findings of complications are conflictingly low (accounts to 5.
8 to 5. 9%) compared to the expected response. According to Lobato, Gravenstein and Kirby (2007), despite of the possible dangers being predisposed positioning, little is known about the adverse outcomes related to the sitting position in the context of surgical surgery, which usually is shorter in duration and less complicated than sitting neurosurgical procedures (p. 851).
In addition by Steele, Nielsen and Klein (2005), positioning the head above the heart results in a local reduction of MAP approximately 0.77 mmHg per centimeter of elevation. Such physiological effects of beach-chair position may predispose various conditions, especially for those anesthetized patients. Aside from hypotension and decline in MAP, as well as other associated circulatory pressure and exertions, other deadly conditions that may arise include (1) air embolism and (2) jugular vein obstruction. Hypotension is usually exaggerated especially if the hips are not fully flexed and if intravascular volume is depleted (to Matta, Menon and Turner 2000 p. 273).
According to Matta, Menon and Turner (2000), venous air embolism is one of the most common potential dangers of beach chair position, which occurs when the vein is held open to atmosphere and venous pressure at the operative site is sub-atmospheric (p. 273). According to Hedge and Avatgere (2000), the adverse effects of VAE are usually attributed to local endothelial reaction and mechanical impediment of blood flow. During the VAE endothelial stage, inflammatory response cascade is initiated by the release of endothelial mediators to the generation of oxygenated compositions.
In the experimental studies of Matta, Menon and Turner (2000), a precordial Doppler monitoring is used as the detection device to test the presence of VAE (Venous Air Embolism). According to the findings of the study, 25 to 50% of VAE incidence occurs among patients who are in sitting position (p. 273). As supported by Gill and Hawkins (2005), another contributing factor of VAE occurrences is the decline in end-tidal carbon dioxide concentration, which usually occurs during elective arthroscopy and upon insertion of gar or air in the distended joints.
In a more accurate study of Matta, Menon and Turner (2000) using a transoesophageal echocardiography (TOE), the documented incidence of VAE accounts to 76%. However, according to the findings in the study of to Hedge and Avatgere (2000), the assessed patients who suffered VAE conditions have not shown significant complications during the post-operative condition until recovery. Despite of VAE complications and its deadly nature, its occurrence is commonly prevented through the detection monitors (e. g. end-tidal carbon dioxide monitor).