Evaluation of pulse co-oximetry to determine haemoglobin saturation with oxygen and haemoglobin concentration in anaesthetized horses: a retrospective study

Objective: This study compared the values of variables measured by pulse co-oximetry (Masimo Radical 7; Masimo Europe Limited, UK) with those measured by a co-oximeter-enabled blood gas analyser (Siemens Rapid-point 500; Siemens Healthcare Limited, UK) in anaesthetized horses.

Study design: Retrospective study.

Animals: A total of 30 anaesthetized horses.

Methods: In total, 47 heparinized arterial blood samples were collected for blood gas analysis to determine haemoglobin concentration (tHb, g L^-1) and percentage of haemoglobin saturation with oxygen (SaO₂). Arterial haemoglobin saturation with oxygen was determined noninvasively by pulse co-oximetry (Masimo SpO₂). Pulse co-oximetry also provided arterial haemoglobin concentration (SpHb) and arterial oxygen content (SpCaO₂). Arterial oxygen content was calculated (CaO₂) in 39 samples using SaO₂ and the value of Hüfner's constant used by Masimo Radical 7 (1.3 mL g^-1). Data were compared using Bland-Altman analysis, correlation tests, accuracy root mean square (ARMS) statistics and total allowable error, where available.

Results: Low bias but wide limits of agreement (LoA) were found between Masimo SpO₂ and SaO₂ (bias = -1.4%, LoA = -4.0 to 1.3%), with an ARMS of 3%. Compared with tHb, SpHb showed low bias (6.2 g L^-1) but wide LoA (-39.6 to 52.6 g L^-1); its % bias (5.2%) was still within the 7% limits recommended by the Clinical Laboratory Improvement Amendments (CLIA) for humans. When comparing SpCaO₂ and CaO₂, the bias and LoA were -0.2 mL dL^-1 and -6.7 to 6.2 mL dL^-1, respectively.

Conclusions and clinical relevance: Masimo SpO₂ was acceptable for current ARMS standards, and SpHb measurements also met CLIA limits. The wide LoA in this study, however, suggest that the Masimo Radical 7 cannot be recommended as a substitute for direct measurements. As blood gas machines, pulse oximeters and co-oximeters use algorithms based on human haemoglobin, no true gold standard exists for horses.