Sitting Position
The sitting position is said to offer some surgical advantages
for posterior cervical procedures and posterior fossa craniotomies. Surgeons may
weigh these advantages differently,
Figure 28-12
Lateral view of the upright shoulder position. The endotracheal
tube and head are secured to prevent movement and accidental extubation.
but most can agree that ease of surgical exposure, amount of blood pooling in the
operative field, and operative position of the surgeon are different among the sitting,
lateral, and prone positions. Access to the endotracheal tube, reduction of facial
swelling, and better cardiovascular stability are notable advantages of the sitting
position in anesthetic management. Risks of posterior fossa craniectomy in the sitting
position include venous air embolism, paradoxical air embolism to the arterial circulation,
hypotension, vascular instability resulting from brainstem
manipulation, specific cranial nerve stimulation, airway obstruction, position-related
brainstem ischemia, and macroglossia. These are not unique to the sitting position
and may occur in other positions. Management should be directed at the prevention,
early detection, and treatment of these problems.
There is much concern about the use of the sitting position, although
there is a large body of information about its safety. Several large published series
show a favorable safety record for operations using the sitting or prone positions.
[38]
[39]
[40]
A study by Black and colleagues[38]
put the problem
into perspective, and the results are consistent with those of other studies. These
authors retrospectively reviewed 579 posterior fossa craniectomies (333 sitting and
246 horizontal) performed concurrently from 1981 through 1984. Intraoperatively,
the incidence of hypotension did not differ between the two groups from induction
of anesthesia to incision or from incision to closure. About 20% of the patients
in each group became hypotensive during each of these periods; all responded to vasopressors
or fluids, or both. The incidence of venous air embolism in patients monitored by
Doppler ultrasound was significantly greater in the sitting position (45%) than in
the horizontal position (12%). Sitting patients required transfusion less often
than prone patients, and the transfused volume was lower.[38]
No prospective, randomized comparison of the sitting and prone positions has been
performed.
Several details should be considered when a patient is placed
into a sitting or semirecumbent position ( Fig.
28-13
). Strict attention to detail should be paid to the flexion of the
neck to prevent excessive flexion of the neck in sitting and other positions to prevent
spinal cord eschemia, obstruction of carotid and vertebral arteries, and embolic
Figure 28-13
The patient is in a semi-sitting position with the knees
flexed slightly. The headrest support is fastened to the upper part of the table
so that the head can be lowered without changing the relationship of the pinion head
holder to the torso. The arms must be supported (not shown) so that the weight of
the arm does not stretch the brachial plexus. The buttock area is padded. (Adapted
from Martin JT: The head-elevated positions: Anesthesiologic considerations. In
Martin JT [ed]: Positioning in Anesthesia and Surgery, 2nd ed. Philadelphia, WB
Saunders, 1987, p 81.)
or thrombotic stroke.[41]
The decision about what
constitutes excessive neck flexion is difficult; clinical experience and preanesthetic
evaluation of range of motion may be most helpful. Warner[42]
recommended avoidance of any position that would place the head and neck at the extreme
limit of a range of motion. In general, the physician should be able to place two
fingers between the chin and the sternum when positioning is complete.
In the sitting position, the arms tend to hang by the side. When
muscle relaxants are used, the downward force caused by the weight of the arms is
sufficient to stretch the brachial plexus beneath the clavicle and compress the neuromuscular
bundle to the upper extremity, causing arm paralysis or weakness. Blankets are usually
placed under the elbow and forearm to support the weight of the arm so that there
is no downward stretch on the arm, and the arm is pushed up slightly, giving the
appearance of a slight shoulder shrug.
The hips are often flexed because position tends to place the
buttocks at an angle to support the weight of the body and because it is thought
to aid venous return. The hips should be at the break in the table, with the lumbar
area against the back of the table. If the patient "slouches" lower, the cervical
operating field may be too low to clear the top of the table and the torso and thighs
may not be in a stable position on the operating table. The legs must not be outstretched,
however, because this places considerable tension on the sciatic nerve and can result
in postoperative weakness. Bending the legs at the knees removes this tension, and
placing an artificial fat pad under the buttocks and the sciatic notch of the pelvis
may reduce the chances of pressure ischemia of the sciatic nerve. Venous return
may be aided and thromboembolism may be prevented by the use of alternative
inflatable leg wraps (e.g., sequential thromboembolic disease stockings).
