Does anaemia cause faster desaturation in an anaesthetic setting?

Does Anaemia Cause Faster Desaturation in Anaesthesia?

Yes, anaemia causes faster desaturation during anaesthesia due to reduced oxygen-carrying capacity, which decreases oxygen reserves and shortens the time to critical desaturation during periods of apnoea.

Pathophysiology of Desaturation in Anaemia

• Oxygen delivery to tissues depends on three factors: blood flow, haemoglobin concentration, and oxygen extraction 1
• Anaemia reduces the oxygen-carrying capacity of blood, which directly impacts the oxygen reserves available during periods of apnoea 2
• In healthy adults with normal haemoglobin levels breathing ambient air, the delay between onset of apnoea and arterial oxygen desaturation (SpO2 <90%) is 1-2 minutes, which can be extended to 6-8 minutes with pre-oxygenation 2
• Patients with anaemia have reduced functional residual capacity (FRC) of oxygen, leading to more rapid desaturation during periods of apnoea 2

Clinical Impact of Anaemia on Desaturation

• Anaemia is an independent predictor of worse perioperative outcomes, including faster desaturation during anaesthetic procedures 3
• The risk of arterial oxygen desaturation is directly proportional to the severity of anaemia 2
• In patients with cardiovascular disease and preoperative haemoglobin of 6-9 g/dl, the odds ratio for mortality was 12.3 (2.5-62.1) compared to those with haemoglobin >12 g/dl 2
• The risk of death increases by a factor of 1.5 for every 1 g/dl decrease in haemoglobin below 7 g/dl 2

Compensatory Mechanisms in Anaemia

• Non-haemodynamic compensatory mechanisms include:
  • Increased erythropoietin production to stimulate erythropoiesis
  • Increased oxygen extraction through displacement of the haemoglobin-oxygen dissociation curve 1
• Haemodynamic compensatory mechanisms include:
  • Increased cardiac output through decreased afterload
  • Increased preload
  • Positive inotropic and chronotropic effects 1
• These compensatory mechanisms may be insufficient during anaesthesia, particularly during apnoeic periods 2

Risk Factors for Rapid Desaturation in Anaemic Patients

• Cardiovascular disease significantly increases the risk of complications from anaemia-related desaturation 2, 4
• Obesity and pregnancy (from second trimester) reduce functional residual capacity, further accelerating desaturation in anaemic patients 2
• Age greater than 55 years is a consistent predictor of desaturation during procedural sedation 2
• The presence of underlying lung disease, excessive secretions, or obesity contributes to pulmonary problems that can exacerbate desaturation 5

Management Strategies for Anaemic Patients During Anaesthesia

• Pre-oxygenation is crucial in anaemic patients to maximize oxygen reserves and delay onset of desaturation 2
• Effective pre-oxygenation is achieved when end-tidal oxygen fraction (FeO2) is greater than 90% 2
• Position optimization: semi-sitting position with head elevated at 30° allows a significant increase in FRC (average gain of 188 mL) compared to supine position 2
• Continuous monitoring with pulse oximetry is essential for early detection of desaturation 2
• Consider capnometry to detect early hypoventilation before oxygen desaturation occurs 2

Special Considerations

• In cardiac surgery, markers of adequate oxygen delivery such as lactate, cerebral near infrared spectroscopy values, and central venous oxygen saturation should be monitored 2
• For children with anaemia who are critically ill, haemodynamically stable and not bleeding, maintaining a haemoglobin >70 g/l is recommended 2
• In extreme cases of normovolemic anaemia, global oxygen transport can be maintained even at very low haemoglobin levels (as low as 3.0 g/dl) in young, healthy patients, but this requires careful monitoring of mixed venous oxygen saturation (maintained at ≥60%) 6

Prevention and Preparation

• Investigate and treat anaemia preoperatively whenever possible, especially before elective procedures 2
• Implement patient blood management strategies throughout the perioperative period 2
• For patients with severe anaemia who cannot be optimized preoperatively, consider:
  • More aggressive pre-oxygenation techniques
  • Minimizing apnoeic periods during intubation
  • Having rescue strategies readily available 2

Remember that arterial oxygen desaturation during tracheal intubation or supra-glottic device insertion manoeuvres carries significant risk of morbidity and mortality and should be prevented 2.