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A-623
2003 |
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| Pulse Oximeter Response Profiles under Hypoxic and Hypothermic Conditions in Healthy Volunteers | ||||||||||||||||||||
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David MacLeod, M.D., Luis Cortinez, M.D., John Keifer, M.D., Lucian Radu, M.D., Jacques Somma, M.D. Department of Anesthesiology, Duke University Medical Center, Durham, North Carolina. |
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Introduction. Barker¹recently invited researchers to conduct laboratory studies of oximeter performance under conditions relevant to the clinical setting. It is common for patients receiving general anesthesia to become hypothermic. In addition, the volatile anesthetics produce vasodilatation. We sought to determine the performance of pulse oximeters at different sites under conditions of hypothermia in combination with peripheral vasoconstriction and vasodilatation. Method. Following IRB approval, 7 adult male volunteers provided informed consent. Pulse oximeters were placed at the following sites: Nellcor MaxFast (FH) on forehead; Masimo LNOP Ear (E) on ear; Nellcor OxiMax (DN) and Masimo LNOP Adt (DM) on fingers of ipsilateral hand. Subjects received 3 hypoxic challenges (HC) with FiO2 = 11% for 3 minutes followed by 100% O2 under 3 different conditions. HC #1 (normothermia, vasoconstriction): circulating water cooling mattress for 30 minutes; HC #2 (hypothermia, vasodilatation): 2 litres cold IV saline (4°C) and nitroglycerin (NTG) infusion at a starting rate of 50 mcg/min increased to 300 mcg/min over 30 min by 50 mcg/min increments; HC #3 (hypothermia, vasoconstriction): 15 mins after discontinuation of NTG. Desaturation (DESAT) = time for displayed SpO2 to fall below 95% after the start of the HC; resaturation (RESAT) = time for SpO2 to reach 100% after FiO2 changed to 100% at end of 3 minute period. Results. DESAT and RESAT of FH and E were significantly shorter than DN and DM (*ANOVA with Tukey correction p<0.05). DESAT of both FH and E were significantly shorter in HC #2 than HC #3 (**repeated ANOVA p<0.05). Discussion. There is significant difference in the performance of oximeters at different sites under conditions of normo- and hypothermia. The FH and E sensors demonstrated faster DESAT and RESAT than DN and DM under all 3 conditions. The lag of DN and DM was further accentuated during mild hypothermia, irrespective of presence/absence of NTG. On average FH and E responded over 2½ minutes quicker than digit sensors, independent of sensor manufacturer. This is similar to Awad's²observation that ear is immune to the vasoconstrictive effects of endogenous catecholamines. During mild hypothermia, DESAT of both FH and E were signicantly shorter in the presence of NTG. This suggests that the vasoactive responses of these sites are preserved in contrast to those of the digits. The 'central' sites provide an earlier indication of a hypoxic episode. In conclusion, we have demonstrated that the average delay of digit oximeters to detect a hypoxic episode is 2.5 minutes, with a range up to 6 minutes, when compared to FH and E sensors. The placement of digit oximeters may be inappropriate in clinical practice during mild hypothermia. References. 1. Barker, S.J. , Anesth Analg 2002;94:S17-20 2. Awad, A., Anesthiology 2000 A583 Anesthesiology 2003; 99: A623 |
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