The desaturation response time of finger pulse oximeters during mild hypothermia*

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1 APPARATUS The desaturation response time of finger pulse oximeters during mild hypothermia* D. B. MacLeod, 1 L. I. Cortinez, 2 J. C. Keifer, 3 D. Cameron, 4 D. R. Wright, 1 W. D. White, 5 E. W. Moretti, 1 L. R. Radulescu 6 and J. Somma 1 1 Assistant Clinical Professor, 3 Associate Clinical Professor, 5 Biostatistician and 6 Research Fellow, Department of Anesthesiology, Duke University Medical Center, Durham, NC, USA 2 Assistant Clinical Professor, Hospital Clinico Universidad Catolica de Chile, Santiago, Chile 4 Specialist Registrar, Department of Anaesthesia, Royal Infirmary of Edinburgh, Scotland Summary Pulse oximeters may delay displaying the correct oxygen saturation during the onset of hypoxia. We investigated the desaturation response times of pulse oximeter sensors (forehead, ear and finger) during vasoconstriction due to mild hypothermia and vasodilation caused by glyceryl trinitrate. Ten healthy male volunteers were given three hypoxic challenges of 3 min duration under differing experimental conditions. Mild hypothermia increased the mean response time of finger oximeters from 130 to 215 s. Glyceryl trinitrate partly offset this effect by reducing the response time from 215 to 187 s. In contrast, the response times of the forehead and ear oximeters were unaffected by mild hypothermia, but the difference between head and finger oximeters was highly significant (p < ). The results suggest that the head oximeters provide a better monitoring site for pulse oximeters during mild hypothermia.... Correspondence to: David MacLeod david.macleod@duke.edu *Presented in part to the American Society of Anesthesiologists Annual Meeting, San Francisco, CA, USA, October Accepted: 4 October 2004 The use of pulse oximetry to detect hypoxaemia is now well established as a standard of care in anaesthesia [1, 2] and critical care medicine [3]. It is widely used in other clinical settings when sedatives and analgesics are administered. The accuracy of pulse oximetry in estimating the arterial oxygen saturation has previously been validated when measured under steady-state conditions in both healthy volunteers and patients [4]. However, pulse oximeters demonstrate a variable and delayed response during deliberate hypoxic challenges. Factors that affect the desaturation response time include oximeter site (e.g. ear, nose, finger and toes), poor perfusion of oximeter site (e.g. vasoconstriction, hypothermia) and vasoactive drugs [4]. The purpose of this study was to compare the desaturation response times of six pulse oximeter sensors during changes in vasoconstriction due to mild hypothermia and vasodilation induced by glyceryl trinitrate. Methods Institutional Review Board approval was granted and informed consent was obtained from all participants. Inclusion criteria were being male, aged between 21 and 40 years, no cardiac or pulmonary disease and not taking any regular medications. Specific exclusion criteria were anaemia, haemoglobinopathy, history of smoking, Raynaud s disease, current fever or septic episode at the time of the study. A full history and examination was performed and blood samples were taken at the screening for full blood count and haemoglobin electrophoresis. All subjects fasted for 6 h prior to the start of the study. Subjects were dressed in a cotton gown and placed in a semirecumbent supine position. An 18 G intravenous cannula was placed in the left forearm and flushed with 5 ml heparinised saline (50 units of heparin). Ó 2005 Blackwell Publishing Ltd 65

2 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters Anaesthesia, 2005, 60, pages A 3-lead ECG was placed and a 20 G indwelling radial artery cannula was inserted for continuous arterial blood pressure monitoring. Pulse oximeter sensors were placed on the head and fingers and connected as follows: A Nellcor Max-Fast (Nellcor, Pleasanton, CA) on right side of forehead (Nellcor N-595 monitor); Masimo LNOP (Masimo Corporation, Irvine, CA) on right earlobe (Masimo Radical monitor); Nellcor Max-A on right hand (Nellcor N-595 monitor); Masimo LNOP Adult on right hand (Masimo Radical monitor); Novametrix Y-sensor (Novametrix Medical Systems, Wallingford, CT) on right hand (Novametrix MARS p O 2 monitor); Nellcor Oxisensor II D-25 on right hand (Agilent M3 monitor, Palo Alto, CA). The assignment of sensors to fingers of the right hand was determined by random number generation. Opaque covers were placed over the finger sensors to reduce interference from incident light. All monitors were set to the manufacturers default settings for data averaging times (Table 1) and sensitivity. A forehead band was placed over the Max-Fast sensor to secure the device in accordance with manufacturer s recommendations. A BIS XP (Aspect Medical Systems, Inc., Newton, MA) sensor was placed on the forehead above the Max-Fast sensor to ensure the compatibility and ease of use with a forehead sensor. Core temperature was measured at the tympanic membrane [5] by placing a thermocouple probe (Mallinckrodt Medical, St. Louis, MO) into the left external auditory canal until an audible scratching was reported by the subject. The external auditory meatus was then occluded by packing with a cotton ball. A cooling water mattress (Cincinnati Sub-Zero Blanketrol II with Maxi-Therm Ò Blankets, Cincinnati, OH) was placed beneath the subject from the knees to shoulders. The room temperature was maintained between 21 and 25 C. The study was conducted in two sequential phases [5] (surface cooling and core cooling, followed by a recovery period). A total of three hypoxic challenges were performed, one during surface cooling and two during the core cooling phase (Fig. 1). Table 1 Settings of oximeter averaging times. Oximeter Agilent 10 Masimo 8 Nellcor 8 Novametrix 8 Averaging Time (s) Figure 1 Phases of study conduct. GTN = glyceryl trinitrate. Each hypoxic challenge was conducted in the same manner. A tight-fitting mask was placed on the subject s face whilst breathing room air. End-tidal carbon dioxide and inspired fraction of oxygen were monitored by Normocap 200 (Datex-Ohmeda, Andover, MA). Confirmation was obtained that all pulse oximeters displayed S p O 2 > 98%. The 3-way stopcock of the low resistance non-rebreathing circuit was turned to deliver an 11% O 2 in nitrogen mixture from a Douglas bag. After breathing the hypoxic gas mixture for 3 min, 100% O 2 was delivered until all oximeter monitors displayed values of 100%. The facemask was then removed and the subject 66 Ó 2005 Blackwell Publishing Ltd

3 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters was allowed to breathe room air for 5 min to permit renitrogenation. Sixteen arterial blood gases samples were taken at baseline, desaturation and resaturation phases of each hypoxic challenge. Samples were taken with heparinised (1 : 1000 units) plastic syringes, air was evacuated from the syringe and samples were placed in iced water during transport. Samples were analysed within 30 min of being drawn using an IL CO-oximeter 482 (Instrumentation Laboratory Company, Lexington, MA) to measure arterial oxygen saturation. After baseline values of vital signs and core temperature had been recorded, surface cooling started at time zero. The cooling mattress was set at 14 C. Core temperature was maintained above 36 C. The first hypoxic challenge (i.e. normothermia, vasoconstriction) was initiated after 30 min. After this, the core cooling phase started. A target core temperature between 35 and 36 C was achieved by infusing 2 litres of cold (i.e. 4 C) intravenous lactated Ringer s solution [5] at 70 ml.min )1 over 30 min and a simultaneous intravenous infusion of glyceryl trinitrate starting at a rate of 50 lg.min )1 and increasing in 50 lg increments every 5 min to a maximum of 300 lg.min )1 in order to mimic the vasodilatation encountered during the use of volatile anaesthetic agents [6]. Adequate cerebral perfusion was monitored clinically by talking to the subject. The second hypoxic challenge (i.e. hypothermia, vasodilation) commenced once the core temperature reached C and followed the same protocol. After completion, the glyceryl trinitrate infusion was stopped. The third hypoxic challenge (i.e. hypothermia, vasoconstriction) was performed 15 min after stopping the glyceryl trinitrate infusion. After the core cooling phase the subjects were rewarmed with warm blankets, clothing and warm oral fluids as tolerated. Subjects remained under observation until their core temperature was >36.5 C, vital signs had returned to baseline and they felt warm. All pulse oximeter data (except data from the Agilent) were collected using LabView (National Instruments, Austin, TX) onto a Pentium Ò 4.2 GHz dual processor. Data were transferred via serial RS232 ports at a transfer rate of 1 Hz. The displayed pulse oximeter value of the Agilent oximeter was recorded manually at 5 s intervals. Statistical analysis The primary endpoint of this study was the time required for each oximeter to detect hypoxia (i.e. desaturation response time). This was defined as the time for each monitor to display a S p O 2 value = 95% after the subject started breathing 11% O 2. The delay between the fastest responding head and finger oximeters was reported for each subject at each challenge. Data were analysed using SAS Ò v8.2 (SAS Institute, Cary, NC) statistical software program. A repeated-measures analysis of variance (ANOVA) model explored whether there was a difference between the pulse oximeters desaturation response times and whether the response times varied under different physiological conditions. Post-hoc comparisons between pairs of oximeters or challenges were performed with Tukey s adjustment for multiple comparisons. As a confirmatory analysis, the non-parametric Friedman test, as described by Trivedi et al. [7], was used to compare the rank order of oximeter response. A statistician blinded to the brand and location of the pulse oximeter sensors conducted the analysis. The minimum displayed oxygen saturation values during each hypoxic challenge were compared. Accuracy and correlation between Co-oximeter arterial blood oxygen saturation and the displayed oximeter saturations of the forehead, ear and Novametrix finger oximeters (the latter taken as representative of all finger oximeters) were quantified according to Bland & Altman [8] and correlation analysis. All p-values less than 0.05 were considered statistically significant, except where adjusted for multiple comparisons. Results The 12 volunteers were American Society of Anaesthesiologists physical status I (Table 2). Two subjects were withdrawn at the discretion of the investigators (due to failure to cannulate the radial artery and hypertensive response to initial peripheral cooling). The remaining 10 subjects completed each of the three hypoxic challenges. Each oximeter displayed an oxygen saturation and plethysmograph waveform throughout all hypoxic challenges with continuous data integrity. Each subject achieved a core temperature between 35 and 36 C (an average drop of 0.8 C from baseline values) during the core cooling phase and this was maintained with and without the glyceryl trinitrate infusion. All subjects shivered vigorously during the intravenous administration of glyceryl trinitrate and cold Lactated Ringer s. The response profile of a typical subject is shown in Fig. 2. The mean and interquartile range of desaturation response times at each challenge are shown in Fig. 3. The mean desaturation response times for the forehead, ear and finger oximeters were 41 s, 60 s, 131 s (normothermia, vasoconstriction); 22 s, 33 s, 187 s (hypo- Table 2 Subject demographics. Values are means (ranges). Age; years 26 (21 32) Weight; kg 77 (61 92) Height; cm 179 ( ) Body mass index; kg.m ) ( ) Ó 2005 Blackwell Publishing Ltd 67

4 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters Anaesthesia, 2005, 60, pages Figure 2 Individual subject response profile of displayed pulse oximeter and arterial blood oxygen saturations during hypothermia and vasoconstriction. *Arterial blood gas; intermediate dashed line: forehead; long dashed line: ear; continuous line: fastest finger; short dashed line: slowest finger. thermia, vasodilation); 40 s, 60 s, 215 s (hypothermia, vasoconstriction), respectively (Table 3). The mean head finger delays were 76 s (normothermia, vasoconstriction); 141 s (hypothermia, vasodilation) and 160 s (hypothermia, vasoconstriction), respectively (Table 4). When all oximeters were considered together, the response times varied significantly, both between oximeters (p < 0.001) and between challenges (p < 0.001). Adjusted pairwise comparisons showed that each of the head oximeters responded significantly faster than any of the finger oximeters (all p < 0.001). No differences between the finger oximeters were significant. The same results were obtained when each oximeter response was tested separately at each challenge. With respect to temperature, the response times of the finger oximeters during normothermia were shorter than during both hypothermia with vasodilation (p < 0.001) and hypothermia with vasoconstriction (p < 0.001), respectively. During hypothermia the response times of the finger oximeters with glyceryl trinitrate were shorter than without glyceryl trinitrate (p = 0.011). In contrast, the response times of the head oximeters during hypothermia with glyceryl trinitrate were shorter than without glyceryl trinitrate (p < 0.001) and shorter than during normothermia (p < 0.001), respectively. The difference between hypothermia with vasoconstriction and normothermia with vasoconstriction was not significant (p = 0.920). Repeated-measures analysis of variance of the response times of the two head oximeters showed a shorter response time by the forehead oximeter (p = 0.002), which was consistent across all challenges. The forehead oximeter had a faster response time than the ear oximeter Figure 3 Box and whisker plots showing the response times of the oximeters during hypoxic challenges. H Nell : Nellcor forehead; H Mas : Masimo ear; F Nell : Nellcor finger; F Mas : Masimo finger; F Nova : Novametrix finger; F Agil : Agilent finger. by 17 s overall. The Friedman test of rank order found a strong difference between all oximeters (p < at each challenge). Overall, the forehead oximeter was 68 Ó 2005 Blackwell Publishing Ltd

5 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters Table 3 Desaturation response times for head and finger oximeters during each hypoxic challenge. Values are mean (standard deviation) and [range] in s. Head oximeters Finger oximeters Challenge Nellcor forehead Masimo ear Both Nellcor Masimo Novametrix Agilent All Normothermic 41 (14) [17 65] Hypothermic 22 (11) glyceryl trinitrate on [2 35] Hypothermic 40 (11) glyceryl trinitrate off [20 56] 60 (16) [40 90] 33 (14) [10 55] 60 (36) [29 144] 50 (18) [17 90] 27 (13) [2 55] 50 (28) [20 144] 127 (41) [72 180] 175 (61) [76 282] 211 (63) [104 )318] 125 (41) [83 189] 188 (86) [71 372] 215 (72) [98 331] 127 (45) [61 197] 187 (86) [84 341] 221 (74) [ (48) [90 225] 199 (86) [85 335] 211 (68) [120 )325] 130 (43) [61 225] 187 (77) [71 372] 215 (67) [98 332] Table 4 Mean (standard deviation) [range] delays between head and finger oximeter response times (seconds) of individual subjects during each hypoxic challenge. Challenge Mean SD Range Normothermic Hypothermic glyceryl trinitrate on Hypothermic glyceryl trinitrate off ranked first before the ear oximeter in 28 of the 30 challenges. The minimum saturation displayed during the hypoxic challenges showed no difference between the forehead, ear and finger sensors. Correlation coefficients of forehead, ear and Novametrix finger oximeters with arterial blood gases were 0.90, 0.87 and )0.01, respectively. The bias (and precision) for forehead, ear and Novametrix finger oximeters was 0.79 (1.91), )0.61 (2.04) and )1.3 (7.17), respectively (Fig. 4). Arterial blood pressure remained within ±10% of baseline values. The placement of the forehead oximeter and BIS probes was satisfactory in all subjects. In addition, no artifacts attributable to interference or inconsistent data were noted during the study. Discussion This study demonstrated that mild hypothermia significantly prolongs the response times of finger oximeters. The hypothermic response time was 215 s compared to 130 s during normothermia. Compared to the finger oximeters, the head oximeters consistently were significantly faster under all study conditions (60 s or less). We believe that the peripheral vasoconstriction induced by mild hypothermia significantly impaired the performance of the finger oximeters. The addition of glyceryl trinitrate decreased the response times from 215 to 187 s, thereby supporting the proposed mechanism. In contrast, the response times of the forehead and ear oximeters (40 s and 60 s, respectively) were similar during hypothermic vasoconstriction. The response times for both oximeters were further shortened by glyceryl trinitrate (22 s and 33 s, respectively). Differences in the desaturation response times between oximeter sites under normothermic conditions has been demonstrated in earlier studies. Severinghaus et al. [9] noted response times of s for the ear and s for the finger oximeters. Similarly, Trivedi et al. [7] showed that the ear and forehead sensors performed consistently better than the finger sensors with mean desaturation response times of 38 s, 42 s and 57 s, respectively. An individual s finger oximeter response time was always greater than the head oximeter response time. For individual subjects and experimental conditions, the four finger oximeters yielded similar response times, suggesting that each of the finger oximeters performed equally well. The delay between the fastest head and fastest finger response times within each single subject is perhaps more clinically relevant than the absolute response times as it represents the relative response times of the oximeters for that individual subject. In the study by Trivedi et al. [7] they showed a maximum difference in response times of 29 s between the fastest head and slowest finger oximeters. Bebout et al. [10] found that the difference in response times between forehead and finger oximeters in normothermic subjects exposed to cold was 74 s. We found a difference of 76 s during normothermia and vasoconstriction, which was further increased during hypothermia. Our study suggests that the increased response times of finger oximeters during mild hypothermia is of far greater magnitude than previously reported. The vasoactivity of the blood vessels at the oximeter sensor site affects pulse oximeter performance [11 13]. Fingers have the greatest degree of vasoactivity with intense vasoconstriction. In contrast, the vessels supplying the forehead and ear do not vasoconstrict as much. Ó 2005 Blackwell Publishing Ltd 69

6 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters Anaesthesia, 2005, 60, pages When compared to the head oximeters, the desaturation response curves of the finger oximeters were similar both in duration and the minimum displayed S p O 2 value but demonstrated a temporal right shift of the curve (Fig. 2). The displayed S p O 2 of both forehead and ear oximeters showed good agreement with the measured radial arterial oxygen saturation, whereas the finger oximeters showed poor agreement. A key function of pulse oximeters is to identify the onset of hypoxia and this may be affected by two factors: mild hypothermia (core body temperature C) and the presence of vasoactive drugs. First, mild hypothermia frequently occurs in the operating room, as a result of both environmental factors and the action of general anaesthetics upon thermoregulation [6]. Second, both volatile and intravenous anaesthetic agents produce peripheral vasodilation. In addition, many drugs administered clinically have significant effects upon vascular tone, e.g. inotropes. It is possible that the response times seen in the healthy volunteers may underestimate the monitoring delay that might occur in different patient populations, e.g. peripheral vascular disease, but we believe these effects are important. In conclusion, this study demonstrated that mild hypothermia significantly prolonged the desaturation response time of finger pulse oximeters. This effect was reduced partly by the presence of glyceryl trinitrate. Acknowledgements We wish to thank Drs Richard Moon, Mark Newman and Kerri Robertson, Duke University Medical Center, for their invaluable support. This project was funded by Nellcor Tyco Healthcare, Pleasanton, CA. References Figure 4 Bland-Altman plots of arterial blood oxygen saturations and forehead, ear and finger pulse oximeter saturations. 1 American Society of Anesthesiologists. American Society of Anesthesiologists Standards for Basic Anesthetic Monitoring, sgstoc.htm. 2 Pedersen T, Moller AM, Pedersen BD. Pulse oximetry for perioperative monitoring. systematic review of randomized, controlled trials. Anesthesia and Analgesia 2003; 96: Wahr JA, Tremper KK. Noninvasive oxygen monitoring techniques. Critical Care Clinics 1995; 11: Severinghaus JW, Kelleher JF. Recent developments in pulse oximetry. Anesthesiology 1992; 76: Frank SM, Raja SN, Bulcao CF, Goldstein DS. Relative contribution of core and cutaneous temperatures to thermal comfort and autonomic responses in humans. Journal of Applied Physiology 1999; 86: Ó 2005 Blackwell Publishing Ltd

7 D. B. MacLeod et al. Æ Desaturation response times of pulse oximeters 6 Sessler DI. Temperature monitoring. In: Miller RD, ed. Anesthesia, 5th edn. Philadelphia: Churchill Livingstone, 2000: Trivedi NS, Ghouri AF, Lai E, Shah NK, Barker SJ. Pulse oximeter performance during desaturation and resaturation: a comparison of seven models. Journal of Clinical Anesthesia 1997; 9: Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; i: Severinghaus JW, Naifeh KH, Koh SO. Errors in 14 pulse oximeters during profound hypoxia. Journal of Clinical Monitoring 1989; 5: Bebout DE, Mannheimer PD, Jopling MW. Site-dependent lag times in saturation during low perfusion. Anesthesia and Analgesia 2002; 94: S Hertzman AB, Roth LW. The absence of vasoconstrictor reflexes in the forehead circulation: effects of cold. American Journal of Physiology 1942; 136: Evans ML, Geddes LA. An assessment of blood vessel vasoactivity using photoplethysmography. Medical Instrumentation 1988; 22: Awad AA, Ghobashy MA, Ouda W, Stout RG, Silverman DG, Shelley KH. Different responses of ear and finger pulse oximeter wave form to cold pressor test. Anesthesia and Analgesia 2001; 92: Ó 2005 Blackwell Publishing Ltd 71