Book Text

Pulmonary Considerations

During spontaneous ventilation, the rib cage and the diaphragm contribute to thoracic expansion. The abdomen also expands with inspiration to accommodate caudad motion of the diaphragm. In the supine position, abdominal expansion during inspiration is lessened and rib cage expansion is exaggerated compared with upright postures. In upright positions, the weight of the abdominal contents pulls the diaphragm caudad and helps to maintain

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**TABLE 28-3** -- Consensus Findings of the ASA Task Force on Prevention of Perioperative Peripheral Neuropathies
Preoperative Assessment
When judged appropriate, it is helpful to ascertain that patients can comfortably tolerate the anticipated operative position.
Upper Extremity Positioning
Arm abduction should be limited to 90 degrees in supine patients; patients who are positioned prone may comfortably tolerate arm abduction greater than 90 degrees.
Arms should be positioned to decrease pressure on the postcondylar groove of the humerus (i.e., ulnar groove). When arms are tucked at the side, a neutral forearm position is recommended. When arms are abducted on armboards, supination or a neutral forearm position is acceptable.
Prolonged pressure on the radial nerve in the spiral groove of the humerus should be avoided.
Extension of the elbow beyond a comfortable range may stretch the median nerve.
Lower Extremity Positioning
Lithotomy positions that stretch the hamstring muscle group beyond a comfortable range may stretch the sciatic nerve.
Prolonged pressure on the peroneal nerve at the fibular head should be avoided.
Neither extension nor flexion of the hip increases the risk of femoral neuropathy.
Protective Padding
Padded armboards may decrease the risk of upper extremity neuropathy.
The use of chest rolls in laterally positioned patients may decrease the risk of upper extremity neuropathies.
Padding at the elbow and at the fibular head may decrease the risk of upper and lower extremity neuropathies, respectively.
Equipment
Properly functioning automated blood pressure cuffs on the upper arms do not affect the risk of upper extremity neuropathies.
Shoulder braces in steep head-down positions may increase the risk of brachial plexus neuropathies.
Postoperative Assessment
A simple postoperative assessment of extremity nerve function may lead to early recognition of peripheral neuropathies.
Documentation
Charting specific positioning actions during the care of patients may result in improvements of care by (1) helping practitioners focus attention on relevant aspects of patient positioning and by (2) providing information that continuous improvement processes can lead to refinements in patient care.
From ASA Task Force on Prevention of Perioperative Peripheral Neuropathies: Practice advisory for the prevention of perioperative peripheral neuropathies. Anesthesiology 92:1168–1182, 2000.

functional residual capacity. In recumbent positions, the abdominal contents tend to displace the diaphragm cephalad, causing a decrease in functional residual capacity (by approximately 0.5 L in adults). While supine, dependent regions of the diaphragm move preferentially with breathing, such that preferential ventilation of dependent lung regions is favored. This aids in matching ventilation to perfusion because the dependent portions of the lung are also preferentially perfused. With the induction of anesthesia, the functional residual capacity further decreases in supine subjects (by approximately 0.5 L). The classic teaching is that this decrease in lung volume is caused by a cephalad shift of the diaphragm.^[3] However, later studies show that the net end-expiratory position of the diaphragm does not shift significantly during anesthesia and paralysis, although its shape does change.^[4] ^[5] ^[6] An inward motion of the rib cage and, under some conditions, increases in intrathoracic blood volume cause most of the decreases in functional residual capacity associated with anesthesia. Similar changes probably occur in other recumbent positions. These changes in thoracic configuration are associated with atelectasis in dependent lung areas, which significantly impairs gas exchange.