What happens to Functional Residual Capacity (FRC) in the supine position under the influence of anesthesia?

Effects of Anesthesia on Functional Residual Capacity in the Supine Position

Functional residual capacity (FRC) decreases by approximately 20% in the supine position under anesthesia due to cephalad displacement of abdominal contents, diaphragmatic dysfunction, and loss of respiratory muscle tone. 1, 2

Mechanism of FRC Reduction

The reduction in FRC during anesthesia in the supine position occurs through several mechanisms:

1. Anatomical changes:

   • Cephalad displacement of abdominal contents forcing the diaphragm upwards 1
   • Compression of dependent lung regions 1
   • Cranial shift of the diaphragm by an average of 1.9 cm 3
   • Reduction in transverse chest area by 12-20 cm² 3

2. Physiological changes:

   • Loss of inspiratory muscle tone acting on the rib cage 2
   • Decreased respiratory system compliance 1
   • Increased heterogeneity of lung expansion 1

Magnitude of FRC Reduction

• Average FRC reduction is approximately 15-20% compared to the awake state 3, 2
• The reduction is more pronounced in:
  • Obese patients 1
  • Patients with chronic obstructive pulmonary disease 2
  • Pregnant women from the second trimester 1

Clinical Consequences

The reduction in FRC under anesthesia in the supine position leads to:

1. Respiratory mechanics changes:

   • Decreased lung compliance 2
   • Increased airway resistance 2
   • Increased driving pressure (DP) 1
   • Cyclic lung overstress 1

2. Gas exchange impairment:

   • Atelectasis formation in approximately 90% of patients 1
   • Increased intrapulmonary shunt 2
   • Areas of low ventilation-to-perfusion ratio 2
   • Rapid arterial oxygen desaturation during apnea 1

Mitigating Strategies

To counteract the reduction in FRC during anesthesia in the supine position:

1. Positioning modifications:

   • Head-up or ramped position (20-30° elevation) 1
   • Prone positioning can increase FRC by approximately 350 mL compared to supine 4, 5
   • Arms raised above head can increase FRC by approximately 250 mL 5

2. Ventilation strategies:

   • Application of PEEP (5-10 cmH₂O) 6
   • Low tidal volume (6-8 mL/kg predicted body weight) with appropriate PEEP 1
   • Recruitment maneuvers to prevent atelectasis 1

Special Considerations

• Obese patients: Experience more pronounced FRC reduction and faster oxygen desaturation; benefit significantly from head-up positioning and higher PEEP levels 1
• Pregnant women: Experience FRC reduction from second trimester, aggravated by supine position; benefit from 30° head elevation 1
• Patients with neuromuscular disorders: May have baseline FRC reduction that worsens under anesthesia; require careful ventilatory management 1

Clinical Implications

The reduction in FRC during anesthesia in the supine position has important implications:

• Increased risk of perioperative pulmonary complications 1
• Shorter safe apnea time before desaturation occurs 7
• Need for effective pre-oxygenation before induction 1, 7
• Importance of individualized PEEP to optimize respiratory mechanics 1, 6

Understanding these changes is crucial for appropriate perioperative respiratory management to minimize morbidity and mortality related to anesthesia-induced respiratory dysfunction.