COMPARISON OF INVASIVE AND NONINVASIVE SATURATION MONITORING IN PRESCRIBING OXYGEN DURING EXERCISE IN COPD PATIENTS

The aim of this study was to determine whether it is possible using ea r-oximetry to prescribe the correct oxygen flow rates during exercise in chronic obstructive pulmonary disease (COPD) patients on long-term oxygen therapy (LTOT). Twenty COPD patients on LTOT, with exercise des aturation breathing oxygen at resting flow rates, performed a series o f 6-min treadmill walking tests, with a progressive increase in oxygen flows until oxygen saturation measured by ear- or pulse-oximetry (Sp, O-2) was above 90%. The exercise studies were repeated the next day, s aturation being measured bath noninvasively by ear-oximetry (Sp,O-2) a nd invasively by CO-oximeter (Sa,O-2. The exercise studies continued u ntil both Sa,O-2 and Sp,O-2 were above 90%. Reproducibility and agreem ent of the results were analysed according to Bland and Altman. Sp,O-2 was significantly lower than Sa,O-1 by, on average, 0.7% (p<0.004). S p,O-2 reproducibility between the two days was good, The invasive and noninvasive oxygen flow prescriptions agreed in only 10 subjects; in s ix subjects ear-oximetry overestimated the oxygen supply (p<0.0005), w hilst in four subjects it underestimated (p<0.01). Contingency table a nalysis with coded raw data for the values of the sixth minute (that o f the deepest desaturation) showed poor agreement between CO- and puls e-oximetry (Chi-squared p<0.003). However, theoretically, if the Sp,O- 2 target had been raised to 93%, there would have been hardly any unde restimations of Sa,O-2, p=NS). We concluded that noninvasive measureme nt of oxygen saturation is not adequate for estimating arterial satura tion in chronic obstructive pulmonary disease, We suggest, as a workin g solution, that a new cut-off limit of 93% oxygen saturation measured by pulse oximetry should be used as the value below which exercise-in duced desaturation should be corrected in order to allow oxygen to be properly prescribed during activities of daily life.