Facing SpO₂ and SaO₂ discrepancies in ICU patients: is the perfusion index helpful?

Peripheral oxygen saturation (SpO₂) measured by pulse oximetry is an unreliable surrogate marker for arterial oxygenation (SaO₂) in critically ill patients. We hypothesized that a higher perfusion index (PFI) would be associated with better accuracy of SpO₂ measurement. We retrospectively collected SaO₂, SpO₂, and PFI data for each arterial blood gas (ABG) analysis in a cohort of intensive care unit patients. PFI was categorised as low (PFI < 1.0), intermediate (1.0 ≤ PFI  ≤ 2.5), or high (PFI > 2.5). The correlation between SpO₂ and SaO₂ was studied using Pearson's correlation. The Bland-Altman plot was used to analyse the agreement between SpO₂ and SaO₂. Furthermore, the correlation between the (SpO₂-SaO₂) difference and PFI was assessed. The level of (dis)agreement was calculated for the three PFI categories separately. Overall, 281 patients and 1281 data points were analysed. There was a significant correlation between SaO₂ and SpO₂ (r = 0.69, p < 0.01). The Bland-Altman analysis revealed a mean difference between SaO₂ and SpO₂ of 0.2% with limits of agreement of ± 6% (SD ± 2%). The correlation between the PFI and the (SpO₂-SaO₂) difference was low; the (SpO₂-SaO₂) difference improved only marginally with higher PFI values. The accuracy of pulse oximetry for estimating arterial oxygenation was moderate and improved little with increasing PFI values. Thus, the additive value of PFI in clinical decision making is limited. Therefore, we advise performing an ABG before adjusting fraction of inspired oxygen (FiO₂) settings.