Metabolic Demand Under General Anesthesia
General anesthesia significantly reduces the body's metabolic demand by approximately 25-30% compared to awake resting state. Energy expenditure under general anesthesia is about one-quarter lower than preoperative awake resting energy expenditure in patients undergoing surgery. 1
Physiological Changes During General Anesthesia
Oxygen Consumption and Energy Expenditure
- Median energy expenditure decreases from approximately 953 kcal/day/m² when awake to 680 kcal/day/m² under general anesthesia 1
- This represents a 27% reduction in metabolic demand
- Both oxygen consumption (VO₂) and carbon dioxide production (VCO₂) decrease significantly during anesthesia 2
- Oxygen delivery (DO₂) also decreases during anesthesia induction 2
Metabolic Alterations
- General anesthesia reduces the body's resting metabolic rate
- This reduction is primarily due to:
- Decreased neuronal activity
- Reduced muscle tone and activity
- Controlled ventilation
- Direct effects of anesthetic agents on cellular metabolism
Clinical Implications
Hemodynamic Management
- The reduced metabolic demand under anesthesia means lower oxygen requirements
- However, this creates a potential oxygen supply-demand imbalance that requires careful monitoring 2
- Intraoperative oxygen consumption may be inadequate to meet even these reduced metabolic requirements in some cases 3
Thermoregulation
- Reduced metabolic rate contributes to hypothermia under anesthesia
- Temperature monitoring and management are essential to prevent complications
Special Considerations for Mitochondrial Disorders
- Patients with mitochondrial disease require special attention as they have impaired ATP generation via oxidative phosphorylation 4
- These patients may be more vulnerable to metabolic decompensation during anesthesia
- Despite theoretical concerns, adverse events after general anesthesia in patients with mitochondrial disorders are rare 4
Obesity Considerations
- Obese patients have increased resting metabolic rate and oxygen demand when awake 4
- Under anesthesia, they experience:
- Reduced functional residual capacity
- Significant atelectasis and shunting in dependent lung regions
- Increased work of breathing
- Higher minute oxygen demand 4
- This combination means arterial oxygen levels decrease rapidly following cessation of breathing in obese patients 4
Postoperative Metabolic Changes
- Following surgery, there is a compensatory increase in cardiac output and oxygen delivery to support increased oxygen consumption 3
- This represents the body's response to intraoperative oxidative and metabolic deficits
- Additional energy requirements for wound healing contribute to increased postoperative metabolism 3
Common Pitfalls and Caveats
- Don't assume "normal" hemodynamic parameters are adequate during anesthesia
- Titration of therapy should support the necessary physiologic compensations, especially postoperatively
- Patients with underlying metabolic disorders require special consideration and monitoring during anesthesia 5
- The reduced metabolic state during anesthesia may mask underlying metabolic problems that become apparent during emergence
Understanding the reduced metabolic demand under general anesthesia is crucial for appropriate hemodynamic management and ensuring adequate oxygen delivery despite lower requirements.