What happens to Fraction of Inspired Oxygen (FiO2), Tidal Volume (FV), Alveolar Oxygen (Fa), End-Tidal Oxygen (Ft), and Mixed Venous Oxygen (Fv) while using inhalational anesthesia in patients with shunts?

Effects of Inhalational Anesthesia on Gas Fractions in Patients with Shunts

In patients with shunts, inhalational anesthesia causes Fi (inspired fraction) to remain unchanged, FA (alveolar fraction) rises normally in ventilated units, FE (end-tidal fraction) reflects only ventilated alveoli and rises normally, while Fa (arterial fraction) rises much more slowly due to dilution by shunted blood, creating a widened FA-Fa gradient that persists throughout induction. 1, 2

Understanding the Core Physiology

The Shunt Effect on Oxygen and Anesthetic Gas Exchange

Pure shunt physiology creates a fundamental limitation: blood bypassing ventilated alveoli never contacts the anesthetic gas, so increasing FiO2 or anesthetic concentration produces only minimal increases in arterial content because the shunted blood dilutes the oxygenated/anesthetized blood from functional lung units. 1

The key principle is that oxygen therapy (and by extension, inhalational anesthetics) only works when alveolar-capillary units have functional ventilation—shunted blood receives zero anesthetic exposure regardless of inspired concentration. 1

Specific Changes to Each Gas Fraction

Fi (Fraction Inspired):

• Remains at whatever concentration you set on the vaporizer/oxygen blender
• Unaffected by shunt physiology 1

FA (Alveolar Fraction):

• Rises normally in ventilated alveolar units
• Follows typical wash-in kinetics for the specific anesthetic agent
• Critical caveat: The measured "alveolar" fraction only represents ventilated units, not the entire lung 3

FE or Ft (End-Tidal Fraction):

• Rises in parallel with FA in ventilated units
• Does not reflect shunted regions since they contribute no gas to expired breath 3
• Creates false reassurance—normal end-tidal readings despite inadequate arterial anesthesia

Fa (Arterial Fraction):

• Rises significantly more slowly than FA due to admixture of unoxygenated/unanesthetized shunted blood 1, 2
• The FA-Fa gradient widens proportionally to shunt magnitude 4, 3
• With right-to-left shunts >25% of cardiac output, induction is markedly prolonged 1, 2

Fv (Mixed Venous Fraction):

• Initially zero, rises slowly as tissues equilibrate
• In shunt patients, the mixed venous content becomes more important because shunted blood carries venous anesthetic content directly to arterial circulation 1, 4
• Lower cardiac output paradoxically speeds induction by reducing the dilutional effect of shunted blood 1

Clinical Implications by Shunt Type

Right-to-Left Shunts (Most Clinically Significant)

Induction is significantly slowed because deoxygenated, unanesthetized blood bypasses the lungs and dilutes arterial anesthetic concentration. 2

• The slowing effect is proportional to shunt magnitude—shunts >25% of cardiac output cause marked delays 1, 2
• N2O and halothane (and by extension, modern volatile agents) are both significantly affected 2
• The FA-Fa gradient remains wide throughout induction 4, 3

Left-to-Right Shunts

Minimal effect on induction speed because oxygenated blood recirculates through the lungs, getting additional anesthetic exposure. 2

Mixed Shunts (Combined L-R and R-L)

Adding a left-to-right component to an existing right-to-left shunt actually attenuates the slowing effect because some of the shunted blood gets a "second pass" through ventilated lung units. 2

Practical Management Algorithm

Step 1: Recognize Shunt Magnitude

• Calculate shunt fraction if possible (normally <5%, pathologic when >25%) 1
• Clinical clue: Hypoxemia refractory to supplemental oxygen suggests significant shunt 1

Step 2: Adjust Induction Strategy

• For significant R-L shunts: Expect prolonged induction, avoid relying on end-tidal monitoring alone 2, 3
• Consider higher initial inspired concentrations, but recognize this has limited benefit 1
• Intravenous induction may be preferable for large R-L shunts 2

Step 3: Monitor Appropriately

• Do not trust FA (end-tidal) as a surrogate for Fa (arterial) in shunt patients 3
• Use clinical signs of anesthetic depth rather than end-tidal concentration alone 2, 3
• Arterial blood gas sampling provides actual Fa but is rarely practical for anesthetic monitoring 4, 3

Step 4: Optimize Ventilation

• Maintain adequate FRC with PEEP (7-10 cmH2O) to minimize atelectatic shunt 5
• Recruitment maneuvers can reduce shunt fraction by reopening collapsed alveoli 5, 6
• Use FiO2 30-40% if lung is kept open, rather than reflexively using 100% which worsens absorption atelectasis 5, 6

Critical Pitfalls to Avoid

Assuming end-tidal equals arterial concentration: In shunt patients, the FA-Fa gradient is substantial and persistent—you may see adequate end-tidal readings while the patient remains inadequately anesthetized. 4, 3

Increasing FiO2 or anesthetic concentration excessively: Beyond a certain point, this provides minimal benefit because shunted blood never contacts alveoli regardless of inspired concentration. 1, 6

Ignoring the dynamic nature of shunt: Shunt fraction changes with FiO2—it typically decreases when FiO2 increases from 0.21 to 0.4, then paradoxically increases again at higher FiO2 due to absorption atelectasis. 6

Forgetting that mixed venous content matters more in shunt patients: As tissues saturate with anesthetic, Fv rises, and this partially compensates for the shunt effect since shunted blood now carries some anesthetic. 1, 4