Effect of Patient Position on Cardiovascular and Pulmonary Function

Abstract

The intraabdominal exposure required to perform laparoscopic surgery is most commonly provided by a carbon dioxide (CO 2) pneumoperitoneum that ele-vates the abdominal wall, suppresses the viscera, and creates a working space in the abdominal cavity which permits the safe introduction of trocars and in-struments. The CO 2 pneumoperitoneum has a number of cardiopulmonary and physiologic effects. CO 2 , which is highly soluble, is readily absorbed through the visceral and parietal peritoneum and almost always results in hypercarbia and a respiratory acidosis. In addition, the increased intraabdominal pressure (IAP) pushes the diaphragms cephalad, which makes it more difficult to ventilate the patient. To compensate for these deleterious changes the minute ventilation is increased, by increasing either the tidal volume or the respiratory rate. Elevated peak inspiratory pressures are usually required to effect these changes. The increased IAP may also have an impact on cardiovascular function. During laparoscopic procedures, the body position of the patient is often changed to expose the organ of interest and provide the best operative field. In general, more radical body positions are used in minimally invasive surgery than in open cases because retraction of the intestine and other mobile viscera is more difficult in minimally invasive cases. Although laparotomy pads and other packs are used in conjunction with retractors to create an operative field during an open procedure, gravity is the principal means by which retraction is accomplished during a laparoscopic case. The body positioning options include Trendelenburg (head down), reverse Trendelenburg (head up), lateral " airplaning " of the table (left or right side of patient up or down), or a combination of these. Radical patient positioning, by itself, will have an impact on cardiopulmonary function. The effect, which varies depending on the position, may either exacerbate or alleviate the pneumoperitoneum effects. Although for short laparoscopic pro-cedures the cardiopulmonary changes are not problematic, during advanced procedures and in patients with considerable cardiopulmonary disease clinically significant cardiorespiratory changes may result. In this chapter the cardiovas-cular and pulmonary changes associated with various patient positions during laparoscopic procedures are discussed. A brief overview of the cardiovascular and the pulmonary changes with the patient in the supine position is given, fol-lowed by discussion of the cardiopulmonary impact of the specific positions. A. Supine Position Cardiovascular Changes Although not all reports agree about the impact of pneumoperitoneum on the cardiac output during laparoscopic procedures, the majority have noted increases in systemic vascular resistance (SVR) and mean arterial pressure (MAP). As mentioned, intraperitoneal insufflation with CO 2 causes hypercarbia and acidosis: hypercarbia stimulates the sympathetic nervous system, which may lead to an increase in blood pressure, heart rate, and vascular tone [1]. Increased IAP also compresses the inferior vena cava, which compromises venous return from the lower extremities. As a result cardiac preload will decrease. Afterload is increased as a result of the elevated increased SVR that is caused by com-pression of the abdominal vessels and increased sympathetic activity. Reduced preload may cause a reduction of cardiac output and a compen-satory increase in heart rate. Therefore, if preload is markedly reduced, it may be critical to expand the intravascular volume. The effect of increased IAP on venous return is dependent on the intravascular volume status and central venous pressure (CVP). At low or normal right atrial pressure, venous return is reduced with increased IAP by compression of the inferior vena cava (IVC). In subjects with a high right atrial pressure, the IVC remains patent despite the IAP and, in fact, venous return is augmented. Several investigators have demonstrated that femoral vein blood flow decreases with increased IAP, which implies that instal-lation of pneumoperitoneum increases pooling of blood in the peripheral circu-lation [2] (Table 42.1). 42. Patient Position on Cardiovascular and Pulmonary Function 411