Components of the inspiratory-arterial isoflurane partial pressure difference

Area of Science:

• Anesthesiology
• Respiratory Physiology
• Pharmacokinetics

Background:

• Accurate monitoring of anesthetic partial pressures is crucial for patient safety.
• Differences between end-expired and arterial anesthetic partial pressures can arise from pulmonary gas exchange abnormalities.
• Isoflurane is a commonly used inhalation anesthetic.

Purpose of the Study:

• To measure isoflurane partial pressures in various bodily compartments during anesthesia.
• To investigate if pulmonary maldistribution indices explain discrepancies between end-expired and arterial isoflurane partial pressures.
• To determine the reliability of end-expired isoflurane as a surrogate for arterial isoflurane.

Main Methods:

• Simultaneous measurement of isoflurane partial pressure in inspired gas, end-expired gas, mixed-expired gas, arterial blood, and mixed venous blood.
• Calculation of alveolar deadspace using carbon dioxide measurements.
• Measurement of shunt fraction using oxygen measurements to estimate pulmonary end-capillary blood partial pressure.

Main Results:

• Mean values: PIiso 0.69 kPa, PE'iso 0.52 kPa, PAiso 0.50 kPa, Pc'iso 0.38 kPa, Paiso 0.35 kPa, Pviso 0.22 kPa.
• Significant difference (P < 0.001) between ideal alveolar and pulmonary end-capillary isoflurane partial pressures (0.12 kPa).
• Arterial isoflurane partial pressure was consistently lower than end-expired, with differences attributed to shunt, deadspace, and incomplete equilibration.

Conclusions:

• End-expired isoflurane partial pressure may adequately reflect arterial partial pressure for isoflurane.
• Discrepancies are partly due to shunt, deadspace, and failure of isoflurane equilibration between alveolar gas and pulmonary capillary blood.
• The relationship between end-expiratory and arterial partial pressures may differ for other anesthetic agents.