Physiologic Alterations

• Closing capacity is independent of body position, meaning closing volume does not change (and residual volume does not change much either). We know that functional residual capacity decreases with changes in position (especially standing/sitting to supine). If FRC decreases and RV does not change, the expiratory reserve volume must decrease.

UPRIGHT TO SUPINE

• FRC decreases by 800 mL.
• RV, TV and TLC do not change significantly. FRC decreases because abdominal contents are pulled away from the diaphragm by gravity.
  • ERV must decrease (FRC = RV + ERV) and the IRV must increase (TLC = FRC + TV + IRV). Closing volume (CV) does not change (CC = RV + CV).

TRENDELENBURG

• Total lung capacity, vital capacity, and functional residual capacity decrease significantly from the abdominal viscera compressing the diaphragm.
• Functional residual capacity (FRC) and pulmonary compliance see the greatest reduction.
• Complications:
  • Blood loss/hypovolemia/hypotension
  • Air trapped in veins of pelvis/abdomen leading to venous air embolus
  • Eye damage (retinal detachment or cerebral edema)
  • Arthralgia
  • Deep vein thrombosis
  • Injuries to fingers
  • ETT migration- endobronchial intubation
  • Atelectasis
  • Brachial plexus dysfunction
  • Aspiration
• Pulmonary edema is caused by increased left atrial pressure and is transmitted back to the pulmonary capillaries.
  • Preload ↑ (blood volume in lower extremities), lung volume ↑, and cardiac output ↓ (baroreceptor stimulation).
  • CVP and blood volume in lungs are ↑, yet pulmonary compliance, FRC, and cardiac output are ↓.
  • Baroreceptors are activated in trendelenburg because of the increased intravascular pressure, and, reflexly (baroreceptor reflex), there is peripheral vasodilation and bradycardia.
  • Pulmonary edema → ↑ venous return, ↑ pulmonary venous volume, and ↓ cardiac output.
• Abdominal contents put pressure on the diaphragm and reduces chest compliance.
• Trendelenburg causes the most decrease in FRC and pulmonary compliance.
• Shoulder braces should be placed over the acromion. Injury can occur if they are placed over the clavicle and/or neck.
• Cerebral edema can happen during prolonged lithotomy and trendelenburg.

LITHOTOMY

• FRC decreases leading to atelectasis and hypoxia. This can be made worse with head-down tilt.
• Increased venous return from raising the legs may exacerbate congestive heart failure.
• Hypotension occurs. Lowering the legs allows venous stasis and pooling to occur, which decreases venous return, stroke volume, cardiac output, and blood pressure.
• Hypotension can occur when the legs are moved abruptly or not in unison.
• Respiratory compromise:
  • The lungs and heart receive about 600 mL of blood from the lower extremities. If there is cardiac disease, this leads to pulmonary edema/CHF.
  • Vital capacity decreases.
• The decrease in FRC leads to atelectasis and hypoxia.
• The ventilatory changes are not much different than with the supine position.

PRONE

• Stroke volume decreases, cardiac output decreases, and FRC decreases.
• When healthy patients (prior to anesthesia) move from supine to prone, FRC increases.
• Diaphragm compression from the abdominal contents leads to lower chest compliance.
• BP drops when repositioned to prone. Pooling of blood in the extremities and compression of abdominal muscles leads to hypotension. Decreased preload leads to decreased stroke volume, which leads to decreased cardiac output, and finally decreased BP.
• Pressure on the abdomen causes inferior vena cava compression. This can lead to reduced cardiac output, increased blood loss from epidural veins, and clot formation in the lower extremities.

LATERAL

• Ventilation and perfusion are superior in the dependent lung when the patient is awake. Pressure on the abdominal contents is countered by the diaphragm.
• Ventilation shifts to the nondependent lung with general anesthesia. Ventilation: perfusion (V/Q) mismatching results because the lower lung remains better perfused but becomes less ventilated.
  • The diaphragm is not as effective under anesthesia; the dependent is compressed by the mediastinum and abdominal contents. This further increases the V/Q mismatch which can cause PaO₂ to decrease dramatically.
• Best position for PACU patients to prevent aspiration and post-op tonsillectomy.
• Compressed of the axillary neurovascular bundle occurs when there is no axillary roll. The chest can be lifted by an axillary roll to decrease pressure on the neurovascular bundle and avoid decreased blood flow to the arm and hand. The chest support should be periodically observed to ensure that it does not impinge on the neurovascular structures of the axilla.
• Vital capacity decreases 10%.
• Compliance of the dependent lung is decreased, but increased for the nondependent lung, so the net difference is minimal.
• Venous return and blood pressure are decreased because the inferior vena cava is compressed by the kidney rest.

SITTING

• Hypotension is the most frequent complication of the sitting position.
• Best position for epiglottitis.
• Air embolism is most likely to occur.
• Utilized in neuro because it:
  • Provides better surgical access
  • Improves venous drainage
  • Lowers ICP
  • Is easier to see the cranial motor stimulation
• For every 20 cm of difference in height of the head above the heart, there is a 15 mm Hg difference in mean arterial pressure.