Anaesthesia, 1990, Volume 45, pages 1070-1071 APPARATUS
Nasal pulse oximetry overestimates oxygen saturation
J. ROSENBERG AND M. H. PEDERSEN
S-w Ten surgical patients were monitored with nasal andfinger pulse oximetry (Nellcor N-200) for five study periods with alternating mouth and m a 1 breathing and switching of cables and sensors. Nasal pulse oximetry was found to overestimate arterial oxygen saturation by 4.7(SD 1.4%) (biar,and precision).
Keywords Equipment; pulse oximeter. Oxygen; hypoxaemia.
Pulse oximetry is a commonly used technique for monitoring oxygenation in the operating theatre, recovery room and intensive care unit and was shown to give accurate estimation of arterial oxygen saturation over a wide range of values using both finger and earIprobes.'** It was suggested that nasal pulse oximetry could be of value in patients with an impaired peripheral circ~lation.~,~ Nasal probes function with the same empirical calibration algorithm as other probes and are thought to give reliable oximetry readings, but our preliminary findings suggested that oxygen saturation may be overestimated by 5-10% with this r n e t h ~ d We . ~ therefore conducted a trial on the accuracy of nasal pulse oximetry. Methods Ten patients, median age 78 (range 45-84 years), weight 68 (37-106 kg), height 170 (140-176cm), were studied. The surgical department was screened to find patients with nasal pulse oximetry oxygen saturation (Sm)below 100%. Informed consent was obtained from the patients who entered the study, seven of whom were in the postoperative period (days 2-15). Two patients had gastric surgery, three colonic resection, one cholecystectomy and one appendectomy. Three patients had not undergone surgery. Postoperative analgesia comprised intramuscular morphine on demand. Median opioid administration in the preceding 8 hours before the study was Omg morphine (range 0-10). All patients who had undergone surgery had received general anaesthesia. They were not clinically dehydrated, did not receive oxygen, and were peripherally warm with no signs of cyanosis.
Two Nellcor N-200 pulse oximeters were used (software version 2.5) with a nasal (adult nasal oxygen transducer R15) and a finger probe (adult digit oxygen transducer D 25). Oxygen saturations were simultaneously recorded on a penwriter. The study consisted of five periods of 15 minutes each. The patient was monitored in period one, with the digit and nasal probes, breathing through the nose; in period two, the nose was plugged in order to abolish nasal airflow; in period three, cables between the probes and the amplifier box were switched; in period four, cables between the amplifier box and oximeters were switched; in period five, the digit and nasal probes were replaced with new probes. The probes were attached according to instructions,6 and C-lock was used in all patients in order to minimise motion artefacts.? An arterial puncture was performed and the blood gases measured using an ABWOO, Radiometer A/S, at the beginning and end of the study. The study was in accordance with Helsinki Declaration 11, and approved by the local ethics committee. Wikoxon's, Mann-Whitney's and Spearman's tests were used for statistical analyses. Statistical significance was chosen at p < 0.05.
Results There was no difference between nasal Spo,(p > 0.16) or finger SPO,(p > 0,79) in period one compared to period two, or between nasal Spa, (p > 0.16) or finger Spo, (p > 0.12) in period four compared to period five (Fig. 1.). The median values for nasal and finger pulse oximetry are shown in Table 1. ~
~
~
~~
~~
~~~~~-
J. Rosenberg, MD, Research Fellow, Department of Surgical Gastroenterology, M.H. Pedersen, MD, Registrar, Department of Anaesthesiology, Hvidovre University Hospital, DK-2650 Hvidovre, Denmark. Accepted 15 February 1990. 0003-2409/90/12 1070+02 $03.00/0
@ 1990 The Association of Anaesthetists of Gt Britain and Ireland
1070
Nasal pulse oximetry
1071
Table 1.
Nasal Spo,; % (range) Finger Spo,; YO(range)
Period 1
Period 2
Period 3
Period 4
Period 5
96.2 (89.4-97.4) 92.2 (86.4-95.0) p
Nasal pulse oximetry overestimates oxygen saturation
J. ROSENBERG AND M. H. PEDERSEN
S-w Ten surgical patients were monitored with nasal andfinger pulse oximetry (Nellcor N-200) for five study periods with alternating mouth and m a 1 breathing and switching of cables and sensors. Nasal pulse oximetry was found to overestimate arterial oxygen saturation by 4.7(SD 1.4%) (biar,and precision).
Keywords Equipment; pulse oximeter. Oxygen; hypoxaemia.
Pulse oximetry is a commonly used technique for monitoring oxygenation in the operating theatre, recovery room and intensive care unit and was shown to give accurate estimation of arterial oxygen saturation over a wide range of values using both finger and earIprobes.'** It was suggested that nasal pulse oximetry could be of value in patients with an impaired peripheral circ~lation.~,~ Nasal probes function with the same empirical calibration algorithm as other probes and are thought to give reliable oximetry readings, but our preliminary findings suggested that oxygen saturation may be overestimated by 5-10% with this r n e t h ~ d We . ~ therefore conducted a trial on the accuracy of nasal pulse oximetry. Methods Ten patients, median age 78 (range 45-84 years), weight 68 (37-106 kg), height 170 (140-176cm), were studied. The surgical department was screened to find patients with nasal pulse oximetry oxygen saturation (Sm)below 100%. Informed consent was obtained from the patients who entered the study, seven of whom were in the postoperative period (days 2-15). Two patients had gastric surgery, three colonic resection, one cholecystectomy and one appendectomy. Three patients had not undergone surgery. Postoperative analgesia comprised intramuscular morphine on demand. Median opioid administration in the preceding 8 hours before the study was Omg morphine (range 0-10). All patients who had undergone surgery had received general anaesthesia. They were not clinically dehydrated, did not receive oxygen, and were peripherally warm with no signs of cyanosis.
Two Nellcor N-200 pulse oximeters were used (software version 2.5) with a nasal (adult nasal oxygen transducer R15) and a finger probe (adult digit oxygen transducer D 25). Oxygen saturations were simultaneously recorded on a penwriter. The study consisted of five periods of 15 minutes each. The patient was monitored in period one, with the digit and nasal probes, breathing through the nose; in period two, the nose was plugged in order to abolish nasal airflow; in period three, cables between the probes and the amplifier box were switched; in period four, cables between the amplifier box and oximeters were switched; in period five, the digit and nasal probes were replaced with new probes. The probes were attached according to instructions,6 and C-lock was used in all patients in order to minimise motion artefacts.? An arterial puncture was performed and the blood gases measured using an ABWOO, Radiometer A/S, at the beginning and end of the study. The study was in accordance with Helsinki Declaration 11, and approved by the local ethics committee. Wikoxon's, Mann-Whitney's and Spearman's tests were used for statistical analyses. Statistical significance was chosen at p < 0.05.
Results There was no difference between nasal Spo,(p > 0.16) or finger SPO,(p > 0,79) in period one compared to period two, or between nasal Spa, (p > 0.16) or finger Spo, (p > 0.12) in period four compared to period five (Fig. 1.). The median values for nasal and finger pulse oximetry are shown in Table 1. ~
~
~
~~
~~
~~~~~-
J. Rosenberg, MD, Research Fellow, Department of Surgical Gastroenterology, M.H. Pedersen, MD, Registrar, Department of Anaesthesiology, Hvidovre University Hospital, DK-2650 Hvidovre, Denmark. Accepted 15 February 1990. 0003-2409/90/12 1070+02 $03.00/0
@ 1990 The Association of Anaesthetists of Gt Britain and Ireland
1070
Nasal pulse oximetry
1071
Table 1.
Nasal Spo,; % (range) Finger Spo,; YO(range)
Period 1
Period 2
Period 3
Period 4
Period 5
96.2 (89.4-97.4) 92.2 (86.4-95.0) p
