The sitting or beach-chair position during the surgical operation, particularly in neurosurgical interventions, is a highly controversial procedure due to the possible adverse events caused by the positioning. Lobato, Gravenstein and Kirby (2007) have mentioned that the potential hemodynamic consequence of beach-chair position is the marked decrease in the Mean Arterial Pressure or MAP as well as the Central Nervous Pressure (CVP), Pulmonary Artery Occlusion Pressure (PAOP), stroke volume, stroke volume index, cardiac output, cardiac index, and PaO2 (p.
844). According to the study of Buhre, Weyland and Buhre et al. (2000), sitting position usually triggers the decline on MAP due to the shift of intrathoracic to extrathoracic compartment; hence, causing the cardiovascular response in the form of MAP and Stroke Volume Index (SVI) decline. Anesthetized patients are the ones who commonly experience the hemodynamic decrease due to the changes in intrathoracic blood volume caused by the changes in body position, particularly in beach-chair position.
Due to the physiological variations, Lobato, Gravenstein and Kirby (2007) estimate the decline in cerebral MAP, which accounts 7 to 22mm Hg lower than the mean brachial artery pressure (p. 851). According to Lobato, Gravenstein and Kirby (2007), anesthesiologists need to be familiar in the trends of rise and fall in blood pressure, especially in the mean arterial blood pressure of sitting anesthetized patients. Such surgical positioning potentates increase in the intracranial blood pressure and cerebral fluids, and possibly causing poor neurologic outcomes as well (p. 851).
Physiological Changes Caused by Positioning Sitting position alters various physiological components of the body, particularly (1) the cerebral venous returns, (2) CVP, (3) PAOP, (4) SVI, (5) intrathoracic and extrathoracic blood volume, (6) cardiac index and lastly, (7) MAP. However, according to Buhre, Weyland and Buhre et al. (2000), the change variation among these indicators largely depend on various external and internal factors (e. g. pathological disorders, vascular resistance, aging, etc. ). Beach chair positioning affects two most important physiological mechanics, namely (1) blood and (2) fluid circulation.
The adoption of the beach-chair position for surgical cases (e. g. laminectomy, shoulder arthroplasty, etc. ) usually initiate the pooling of the blood in affected venous circulation, which further cause the reduction of venous returns thereby triggering hypotension. As supported by Lobato, Gravenstein and Kirby (2007), the changes in intrathoracic blood volume usually accompany decrease in MAP and cardiac index (p. 854-855). According to Porter, Pidgeon and Cunnigham (1999), the change in circulation is primarily due to the principle of gravitational pull, which directly affects the vascular pressure and circulatory exertion.
Three areas of the body are affected by the shifting of vascular pressure and exertion, mainly (1) brain, (2) heart, and (3) thoracic region. According to Yao, Malhotra and Fontes (2005), cerebral influence of sitting position is usually due to the elevation of the right atrium greater than the height of the central venous fluid column, which reduces the venous returns from the brain (p. 534). According to Krettek and Schemann (2005), sitting position usually requires the placement of lower extremities lower than the trunk; hence, venous downward pressure is declined causing a decrease in venous drainage returns as well (p. 123).