In what examination position is the patient lying on their back face up with the knees bent?

Variations of the Supine Position

Several variations of the supine position are frequently used. These include the lawn (or beach) chair position, frog-leg position, and Trendelenburg and reverse Trendelenburg positions. The lawn chair position (Fig. 34.4) reduces stress on the back, hips, and knees by placing the patient’s hips and knees in mild flexion. This position is often better tolerated by patients who are awake or undergoing monitored anesthesia care than the full supine position. The lawn chair position also facilitates lower extremity venous drainage because the legs are placed slightly above the level of the heart. Abdominal wall tension is also reduced becausethe xiphoid to pubic distance is decreased. Proper positioning involves positioning the patient’s hips at the break of the surgical table and avoiding venous pooling in the legs.

The frog-leg position allows procedural access to the perineum, medial thighs, genitalia, and rectum. The patient is positioned supine and then the hips and knees are flexed and the hips are externally rotated with the soles of the feet facing each other. Support of the patient’s knees to minimize stress and postoperative pain in the hips is required.

The Trendelenburg position, achieved by tilting a supine patient head down (Fig. 34.5), is linked by name to a 19th-century German surgeon, Friedrich Trendelenburg, who described its use for abdominal surgery. Walter Cannon, a Harvard physiologist, is credited with popularizing the use of Trendelenburg positioning to improve hemodynamics for patients in shock during World War I. Today the Trendelenburg position is frequently utilized to improve exposure during abdominal and laparoscopic surgery, during central line placement to prevent air embolism and distention of the central vein, and to offset hypotension by temporarily increasing venous return. A steep (30-45 degrees) head-down position is now frequently used for robotic prostate and gynecologic surgeries.

For all positions in which the head is at a different level than the heart, the effect of the hydrostatic gradient on cerebral arterial and venous pressures should be considered when estimating cerebral perfusion pressure. Careful documentation of any potential arterial pressure gradient is especially prudent.

The Trendelenburg position does produce hemodynamic and respiratory changes; however, the hemodynamic changes are not as long-lasting as often thought. Initial placement of the patient in head-down supine position will increase cardiac output approximately 9% in less than 1 minute via an autotransfusion from the lower extremities. This effect is not sustained and within approximately 10 minutes the cardiac output begins to return to baseline. Nevertheless, the Trendelenburg position is still considered an essential part of initial resuscitation efforts to treat hypotension and acute hypovolemia.19 Functional residual capacity is decreased due to gravitational pull of the diaphragm cephalad. Pulmonary compliance is increased by decreased functional residual capacity and is often further decreased in the Trendelenburg position, due to patient-positioning straps across the chest. In a spontaneously breathing patient, the work of breathing increases. In patients under general anesthesia, these pulmonary changes result in higher airway pressures. Changes to the mechanical ventilator settings can compensate for some of the respiratory changes. However, with patient body habitus and variations in positioning, the higher airway pressures, and changes to minute ventilation are too great to safely continue in the steep Trendelenburg position. Testing the position for patient tolerance after anesthetic induction and completed positioning, prior to the initiation of the surgical procedure, is recommended.

Positioning

▪

Supine position10,11:

Used during induction and emergence of anesthesia.

Supine position while awake decreases functional residual capacity (FRC).

Supine position while under anesthesia decreases FRC by another 15% to 20% due to the cephalad movement of the diaphragm

Lateral decubitus position:

Used for the majority of thoracic surgical procedures.

In adults, there is a moderate decrease in the FRC of the dependent lung owing to compression from the mediastinum and abdominal contents.

In infants and young children, there is a larger decrease in FRC because there is also compression of the dependent lung from the rib cage.

Supine Position

The supine position is a common and versatile position in cranial surgery. The flexibility of the neck and ability to rotate the torso with a shoulder roll allow a majority of brain lesions to be accessed with the patient supine. Lesions of the anterior cranial fossa are approached with the head straight up or rotated, depending on the laterality and location of the lesion. The middle and posterior fossae can be accessed by turning the head away from the side of the pathologic condition.2 Frontal craniotomy with the head straight or slightly rotated is used to access lesions of the frontal lobes, anterior interhemispheric fissure, lateral and third ventricles, and anterior skull base.

The patient is placed supine on the operative table (Fig. 20-1,Video 20-2). For trauma, scalp, and calvarial lesions and for shunt procedures, the head may be placed on a foam donut or horseshoe-shaped headrest. Immobilization of the head is helpful in most other cranial procedures and is achieved in the following manner: The head attachment of the operating table is removed, and the patient's head is placed in a fixation device such as a Mayfield or Sugita head frame. Significant differences are found among the various fixation devices in terms of profile from the operative field and mechanism of retraction. Careful consideration must be given to both the planned incision and the planned craniotomy when the head is placed in fixation. With a three-pin device, the dual pins are typically placed in the coronal plane superior and inferior to the ear, and the single pin is similarly placed on the side contralateral to the incision. The temporal squama and mastoid are avoided. For bicoronal incisions, particular attention should be paid to ensure that the pins are placed sufficiently posterior to the planned incision, so that incision closure will not be compromised by excessive skin tension. The head fixation device is then secured to the operative table using the table attachment arm. Subtle differences occur at this step between surgeons who sit and those who stand while operating. It is important to move the patient on the table high enough to leave sufficient room for the seated surgeon's knees. For frontal approaches, the patient's head is slightly flexed to keep the targeted anatomy superficial within the operative field. Some degree of head tilt or rotation may be required to present the optimal perspective of the surgical target to the neurosurgeon. In contrast, for subfrontal approaches, the head is typically extended so that the orbital rims are the most superior points within the operative field, allowing the frontal lobes to fall away from the anterior cranial fossa and thereby minimizing or eliminating the need for brain retraction.

Once the patient's head is fixed, the patient is then secured to the operative table with a padded safety belt or with foam padding and tape. A pillow should be placed under the patient's knees and foam padding under the patient's heels. At this point, the operating table is manipulated. The head/back of the operative table should be raised, and the legs of the operative table should be lowered, so that the patient's hips and knees are both in slight flexion. Depending on the position of the operative table relative to the anesthesia team and scrub nurse, one or both of the patient's arms need to be padded with foam and tucked securely at the patient's sides. The ulnar grooves and bony prominences should be free from compression, and intravenous tubing should not be pressed against the skin. The hands should be free and in neutral position with thumbs facing anteriorly. It is at this point that the advantage of slight neck extension with angling of the vertex toward the ground becomes obvious. Now the surgeon can put the table in reverse Trendelenburg position, keeping the head above the heart so that there is optimal venous drainage and brain relaxation.

Myofascial syndrome

Variations of the Supine Position

Variations of the supine position are also frequently used such as the lawn-chair position, frog-leg position, and Trendelenburg positions. The lawn-chair position (Fig. 19.1D) flexes the hips and knees slightly, which reduces stress on the back, hips, and knees. This modified supine position is often better tolerated by patients who are awake or undergoing monitored anesthesia care. The legs are placed slightly above the level of the heart, which facilitates venous drainage from the lower extremities. Furthermore, the xiphoid to pubic distance is decreased, reducing tension on the abdominal musculature. Typically the back of the bed is raised, the legs below the knees are lowered to an equivalent angle, and a slight Trendelenburg tilt is used to level the hips with the shoulders.

The frog-leg position, in which the hips and knees are flexed and the hips are externally rotated with the soles of the feet facing each other, facilitates procedures to the perineum, medial thighs, genitalia, and rectum. The knees must be supported in order to minimize stress or dislocation of the hips.

Tilting a supine patient head-down with the pubic symphysis as the highest part of the trunk is called the Trendelenburg position (Fig. 19.1E). It is named after a 19th century German surgeon who first described its use for abdominal surgery. Walter Cannon, a Harvard physiologist, is credited with popularizing the use of Trendelenburg positioning to improve hemodynamics for patients in hypovolemic shock during World War I. Trendelenburg positioning is commonly used today to increase venous return during hypotension, improve exposure during abdominal and laparoscopic surgery, and prevent air emboli during central line placement.

The Trendelenburg position does produce hemodynamic and respiratory changes. Initially, placement of the patient head-down causes an autotransfusion from the legs with about a 9% from baseline increase in cardiac output in 1 minute. However, these changes are not sustained and within 10 minutes many hemodynamic variables, including cardiac output, return to baseline values. Nevertheless, Trendelenburg positioning is still part of the initial resuscitative efforts to treat hypovolemia. The abdominal contents are displaced toward the diaphragm, which decreases FRC and can also decrease pulmonary compliance necessitating higher airway pressures during mechanical ventilation. Intraocular pressure and intracranial pressure (ICP) can also increase. In patients with increased ICP and impaired cerebral autoregulation, Trendelenburg positioning should be avoided. For patients receiving general anesthesia who will be placed in the Trendelenburg position, endotracheal intubation is strongly recommended over supraglottic airways because of the risk of pulmonary aspiration of gastric contents.Prolonged head-down position can lead to swelling of the face, conjunctivae, larynx, and tongue with an increased potential for postoperative upper airway obstruction. An air leak should be verified around the endotracheal tube or the larynx visualized prior to extubation.4

Standard chest ct protocol

Supine position. Scan in suspended inspiration at total lung capacity. Scan setup:

5- × 5-mm sections from apex of the lungs to the adrenals

Six 1.25-mm high-resolution cuts throughout lung at 2.5-cm intervals

1-mm reconstructions through pulmonary nodules

Number of different combinations of pitch and section thickness

In interstitial lung disease, the six cuts are repeated in prone position. Reconstruction is done with high-resolution bone algorithm.

Use of IV contrast:

Evaluation of vascular structures, AVM, aortic dissection

Evaluation of mediastinal tumors, enlarged lymph nodes

Hilar masses

Neck masses

Body position

Supine position with both legs flexed 90 degrees at hip/knee or with one knee flexed and the other leg extended along the mat. Hold the strap between the hands and bend the right knee and place the right foot in the strap. Keeping the pelvis neutral and stable and the hip neutral in rotation, extend the right knee to align the right leg with the axis of the right hip (Fig. 6.24A).

Adjust the hands on the strap so that the arms extend until the elbows are still slightly flexed and the scapulae are able to draw down the back and stabilize on the ribcage. Allow the weight of the right leg to drop the femoral head down into the hip socket.

Supine Position

In the supine position, first reported by Andrews and Carson3 in 1985, the patient lies supine with the shoulder over the edge of the operating table. The shoulder is abducted to 90 degrees and is in neutral rotation with 90 degrees of elbow flexion. The elbow is suspended by a traction device that is attached to either the hand or forearm (Fig. 37-1).

There are several advantages of the supine position in elbow arthroscopy.19,21 The supine position allows the anesthesiologist an easier access to the patient's airway, and there is more flexibility to the choice of anesthesia. The conceptualization of the intra-articular anatomy is facilitated with the elbow in the supine position with the elbow joint in a more familiar anatomic orientation. Conversion of an arthroscopic procedure to an open procedure is readily facilitated in the supine position when required.

Disadvantages of the supine position include the requirement for a traction device with the assistance of a second person to stabilize the arm during arthroscopy. Manipulation of the elbow is more difficult with the overhead traction device. Finally, this position provides poor access to the posterior compartment of the elbow joint.

Supine Position

The supine position allows the most flexibility in the choice of anesthesia. It facilitates the use of regional block anesthesia for elbow arthroscopy, because patients often do not tolerate the prone or lateral decubitus position if awake. The supine position also allows safe conversion from regional to general anesthesia if necessary, without repositioning or redraping.11 The supine position facilitates airway maintenance in general anesthesia.

For the surgeon, the supine position presents the patient's elbow joint in a more familiar anatomic orientation, with the anterior compartment facing up and the posterior compartment down. This allows easier conceptualization of the intra-articular anatomy. Although the reversed anatomic orientation as seen in the prone or lateral decubitus position can become familiar with experience, the complexity of the elbow joint anatomy presents abundant inherent challenges to arthroscopy, and the standard anatomic reference provided by the supine position may initially facilitate the procedure.

The significant disadvantage of supine positioning stems mostly from the limited posterior access and arm instability. Additionally, the suspension device is costly to acquire and time-consuming to set up. A second device or a scrubbed assistant is also needed to stabilize the arm during arthroscopy because the arm is suspended on a traction rope and would swing freely without a stabilizing force. Manipulation of the elbow during arthroscopy can be limited with the arm in traction. In addition, posterior compartment access and visualization may be compromised because elbow extension is more difficult to achieve and sustain; with the instruments and operative field above the hands of the surgeon, extensive procedures in the posterior compartment can lead to the surgeon's fatigue and difficulty in maintaining the instruments' position.

4.1 Baseline Measures

Supine position can reduce functional residual capacity compared to upright position due to gravity-dependent effects on the thoracic cavity volume (e.g., Gisolf et al., 2004). We did not perform any additional respiratory function tests (e.g., FVC, FEV1.0/FVC), as the aim of the study was to compare central respiratory chemoreflex responses to increases in CO2 with tilt. However, we did quantify resting respiratory variables during tilt (VT, fR, VE, and PETCO2). Subjects were hyperventilating in all five positions during baseline, as evidenced by the high ventilation (~ 16 L/min) and resulting hypocapnia (< 30 Torr PETCO2; see Table 1). This was likely due to (a) Calgary being ~ 1000 m above sea level (e.g., Steinback and Poulin, 2007), (b) the use of a nose clip and mouthpiece being somewhat unnatural, (c) the subject being instrumented on a tilt table (i.e., discomfort, stress), and (d) the gas sample port in this study was distal to the flow head, increasing the dead space and thus mixing the end-tidal measure with residual room air from the apparatus dead space. Recent data in our lab (n = 19) sampling PETCO2 proximal to the mouth in supine position reveal more representative values of ~ 34 Torr (unpublished values). Given that these values at rest are not representative of accepted values, these may not be reliable measures with which to make conclusions about the effects of steady-state tilt on baseline respiratory variables. Regardless, there are no differences between baseline respiratory variables in any position, which is consistent with some studies (e.g., Baydur et al., 1987; Segizbaeva et al., 2011).

Although these baseline measures may not be representative for the methodological reasons we stated above, this is inconsequential to the experimental findings during rebreathing. During rebreathing, the entire circuit is equilibrated, as evidenced by the fact that the end-expiratory and end-inspiratory fluctuations in the CO2 signal are almost eliminated as CO2 accumulates and respiratory responses are mounted (see Fig. 2).