Modification of the iso-shunt lines for low inspired oxygen concentrations

Area of Science:

• Physiology
• Critical Care Medicine
• Respiratory Physiology

Background:

• Arterial oxygen tension (PaO2) is a critical indicator of respiratory function.
• Existing models for predicting PaO2 based on inspired oxygen concentration (FIO2) have limitations, particularly at lower FIO2 values.

Purpose of the Study:

• To investigate the relationship between PaO2 and FIO2 in spontaneously breathing intensive therapy unit patients.
• To develop and validate an improved model for predicting PaO2 that accounts for ventilation/perfusion (V/Q) mismatch.

Main Methods:

• Studied 20 intensive therapy unit patients breathing spontaneously across a range of FIO2 (0.21-0.60).
• Calculated venous admixture using an iso-shunt diagram (model A) and developed a second model (model B) incorporating relative V/Q mismatch.
• Validated model B against published postoperative PaO2 data.

Main Results:

• A significant linear trend showed venous admixture increasing as FIO2 decreased (P < 0.001).
• Model B, incorporating V/Q mismatch, demonstrated good agreement between predicted and observed PaO2 values (mean bias -0.17 kPa).
• Model B showed excellent agreement with external postoperative data (mean bias 0.10 kPa).

Conclusions:

• Relative V/Q mismatch significantly influences PaO2 at lower FIO2 levels.
• The proposed model B extends the applicability of the iso-shunt diagram, especially for FIO2 < 0.35.
• This enhanced model has potential for broad application in anesthesia and intensive care.