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) and percentage of haemoglobin saturation with oxygen (SaO). Arterial haemoglobin saturation with oxygen was determined noninvasively by pulse co-oximetry (Masimo SpO). Pulse co-oximetry also provided arterial haemoglobin concentration (SpHb) and arterial oxygen content (SpCaO). Arterial oxygen content was calculated (CaO) in 39 samples using SaO and the value of Hüfner's constant used by Masimo Radical 7 (1.3 mL g). 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 SpO and SaO (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) but wide LoA (-39.6 to 52.6 g L); its % bias (5.2%) was still within the 7% limits recommended by the Clinical Laboratory Improvement Amendments (CLIA) for humans. When comparing SpCaO and CaO, the bias and LoA were -0.2 mL dL and -6.7 to 6.2 mL dL, respectively.

Conclusions And Clinical Relevance: Masimo SpO 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.