Positioning the Morbidly Obese Patient for Surgery
Jay B. Brodsky, MD
SUPINE POSITION: For any patient simply changing from standing or sitting to the supine position causes an increase in venous blood return to the heart. Cardiac output, pulmonary blood flow and arterial blood pressure all increase. When supine the abdominal contents limit diaphragmatic movement reducing FRC causing a significant decline in pulmonary function. General anesthesia with muscle paralysis further reduces lung volumes. These cardio-respiratory changes are exaggerated in obesity.[1-2] In the spontaneously breathing obese patient, the increased diaphragmatic load causes a marked reduction in expiratory flow and an increase in intrinsic PEEP. The supine obese patient experiences a proportionally greater decrease in FRC, total respiratory system and pulmonary compliance, and a larger ventilation/perfusion (V/Q) mismatch than a normal weight patient, and all changes increase with increasing BMI. Spontaneously breathing extremely obese patients should never be allowed to lie completely flat. Compression of the inferior vena cava in the supine position reduces venous return to the heart. This can be avoided by tilting the operating room table or by placing a wedge under the patient, maneuvers similar to those performed during Caesarian section to reduce the pressure of the gravid uterus on the inferior vena cava.
HEAD-UP POSITIONS: Awake, spontaneously breathing obese patients should always be in a head-up position. This unloads the weight of the intra-abdominal contents from the diaphragm. Morbidly obese patients in the reverse Trendelenburg position demonstrate increases in pulmonary compliance and FRC and oxygenation compared to when they are supine. Prior to induction of general anesthesia obese patients should be positioned with their head and upper body elevated in such a way that an imaginary horizontal line can be drawn from the patient’s sternum to their ear. FIGURE 1 This “stacked”, “ramped”, or “Head Elevated Laryngoscopy Position” (HELP) improves pulmonary compliance, allows easier mask ventilation, and improves conditions for tracheal intubation. Improved laryngeal exposure increases the rate of successful tracheal intubation.
If anesthetic induction is performed with the obese patient supine, the combined effects of reduced FRC and increased metabolic rate result in rapid oxyhemoglobin desaturation. The time to arterial desaturation (“safe apnea time”) after preoxygenation is significantly longer in morbidly obese patients in a 30o reverse Trendelenburg position. Since the reverse Trendelenburg position allows extra time to secure the airway before desaturation and the HELP position improves view during laryngoscopy, our practice is to combine both for induction of anesthesia in our obese patients. If the obese patient is hemodynamically stable at the completion of surgery, the trachea should be extubated with their upper body elevated 30o-45o. Patients should then be transferred from the operating room in a semi-recumbent or tilted reverse Trendelenburg position.
HEAD-DOWN (TRENDELENBURG) POSITION: In the Trendelenburg position there is an auto-transfusion of blood from the lower extremities into the central and pulmonary circulation. The added weight of the abdominal contents pressing on the diaphragm plus the weight of the chest wall further decrease total compliance and FRC, which in turn leads to increased atelectasis and hypoxemia. Spontaneously breathing obese patients do not tolerate the Trendelenburg position and their airway should be intubated and ventilation controlled or assisted.
PRONE POSITION: For an anesthetized and paralyzed normal weight patient, placement in the prone position improves oxygenation compared to the same patient when supine. If the abdominal wall is allowed to hang freely, there is a reduction in cephalad displacement of the diaphragm and a re-opening of atelectactic lung segments. However, if the abdomen is compressed and not free to move, diaphragmatic movement will be impeded and chest wall movement restricted. In mild to moderately obese patients, respiratory mechanics, lung volumes and oxygenation all increase when changing from the supine to prone position. Less airway pressure is required to ventilate the lungs of prone morbidly obese patients compared to when those same patients were supine. Therefore, the prone position is usually well tolerated by obese patients as long as the upper chest and pelvis are adequately supported to ensure free abdominal movement. The unloading of abdominal viscera significantly reduces pressure on the diaphragm, which in turn leads to an increase in FRC.
LATERAL DECUBITUS POSITION: In normal weight patients changing from supine to the lateral decubitus position is not associated with any significant hemodynamic effects. If a kidney rest is used, cardiac output can be reduced due to a decrease in venous return and an increase in systemic vascular resistance. Our clinical experience has been that morbidly obese surgical patients tolerate the lateral position. Oxygenation with one-lung ventilation in morbidly obese patients undergoing thoracotomy in the lateral decubitus position was satisfactory in all patients. Their ability to successfully undergo one-lung ventilation may have been due to displacement of their panniculus off the abdomen, which in turn reduced intra-abdominal pressure and allowed greater diaphragmatic excursion during mechanical ventilation.
LITHOTOMY POSITION: In the lithotomy position vital capacity decreases in normal patients breathing spontaneously due to restriction of diaphragmatic movement. Venous return to the heart is increased causing an increase in cardiac output and increased pulmonary blood flow. Morbidly obese patients placed in lithotomy experience increased intra-abdominal pressure and compression of the lungs, which can further reduce chest wall compliance. For morbidly obese patients undergoing procedures in the lithotomy position, positive-pressure ventilation with an endotracheal tube is recommended to compensate for the decreased lung volume.
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