Upsala J Med Sci 93: 45-51, 1988 Ear Oximetry during Progressive Hypoxia Thorarinn Gislason,' Bo Sandhagen' and Efi Daskalopouloul Departments of 'Lung Medicine and 2Clinical Physiology, Uppsala University, Akademiska Sjukhuset, Uppsala, Sweden ABSTRACT The BIOX I11 Pulse oximeter for measuring arterial oxygen saturation (Sa02) was compared during rest and under progressively hypoxic conditions, with Sa02 values based on arterial blood samples. The measurements were performed in 16 subjects undergoing tests of ventilatory response to hypoxia, by a rebreathing method. For each individual subject, there was a linear response relationship (r=0.99), while for all 126 comparative values the regression equation was: y= 0.83 X + 14.7 (r=0.98). The observed ventilatory response was lower when the calculations were based on oximeter readings. We conclude that the oximeter has acceptable correlation between the BIOX I11 and Sa02 measurements for clinical use, especially when Sa02 is above 70%. INTRODUCTION A method for continuous non-invasive monitoring of arterial oxygen saturation (Sa02) by oximetry was described more than 50 years ago, but was not generally accepted in clinical practice until recently, because of doubts about the accuracy of the method (1, 8, 9). With the marked improvement in instrumentation, this technique, which is based on transmission of light through the vascular bed of the ear lobe ( 5 ) , is now gaining increasing attention, both for research and clinical purposes (2, 3, 4). Comparative studies between oximetry and measurements based on 45 a r t e r i a l s a m p l e s have been l i m i t e d . On t h e o t h e r hand, t h e r e i s i n c r e a s i n g i n t e r e s t i n c o n t i n u o u s m o n i t o r i n g o f b l o o d g a s changes, e s p e c i a l l y d u r i n g i n v e s t i g a t i o n s o f s l e e p - r e l a t e d b r e a t h i n g d i s t u r b a n c e s . The aim o f t h i s p r o s p e c t i v e s t u d y w a s t h e r e f o r e t o estimate t h e a c c u r a c y and r e l i a b i l i t y o f a two-wavelength o x i m e t e r f o r d e t e r m i n i n g Sa02 a s compared w i t h a r t e r i a l b l o o d samples, t a k e n a t rest a n d d u r i n g p r o g r e s s i v e h y p o x i a . MATERIAL a n d METHODS P a t i e n t s : The s t u d y comprised 1 6 c o n s e c u t i v e p a t i e n t s , who had been r e f e r r e d t o t h e Department o f Lung Medicine o f t h e U n i v e r s i t y H o s p i t a l , Uppsala, Sweden f o r i n v e s t i g a t i o n o f s l e e p r e l a t e d b r e a t h i n g d i s t u r b a n c e s . T h e i r mean a g e was 51 y e a r s ( r a n g e 38-63) ( T a b l e 1 ) . A l l w e r e h a b i t u a l s n o r e r s , and some a l s o complained of d a y t i m e s l e e p i n e s s and w e r e t h u s s u s p e c t e d o f s u f f e r i n g from t h e s l e e p apnoea syndrome ( 6 ) . They a l l had normal serum b i l i r u b i n v a l u e s a n d none had s k i n p i g m e n t a t i o n . E l e v e n o f t h e p a t i e n t s w e r e o b e s e , w i t h a body mass i n d e x ( w e i g h t / ( h e i g h t ) 2 ) 2 28.0 kg/m2 ( 7 ) , f o u r had a n a i r w a y o b s t r u c t i o n and t h r e e showed an i n c r e a s e i n r e s i d u a l volume by more t h a n 2 0 % ( T a b l e 1 ) . The h y p o x i c v e n t i l a t o r y r e s p o n s e t e s t w a s a p a r t o f t h e i r i n v e s t i g a t i o n programme, which had p r e v i o u s l y b e e n a c c e p t e d by t h e E t h i c s Committee o f t h e Medical F a c u l t y o f Uppsala U n i v e r s i t y . E a r o x i m e t r y : Oxygen s a t u r a t i o n was measured by a B I O X I11 P u l s e o x i m e t e r (Ohmeda, C o l o r a d o , USA) c a l i b r a t e d a c c o r d i n g t o t h e m a n u f a c t u r e r ' s i n s t r u c t i o n s , and i t s normal r e s p o n s e mode was u s e d ( 2 ) . The ear p r o b e o f t h e o x i m e t e r was f i t t e d t o t h e ear l o b e of t h e s u b j e c t a f t e r p r o d u c t i o n o f v a s o d i l a t i o n by r u b b i n g w i t h a l c o h o l . Blood Sa02: A r t e r i a l b l o o d s a m p l e s (6-8 m l ) w e r e drawn i n t o h e p a r i n i s e d g l a s s s y r i n g e s ( 1 0 m l ) t h r o u g h a n i n d w e l l i n g c a n n u l a i n t h e r a d i a l a r t e r y and p l a c e d i n i c e - w a t e r u n t i l a n a l y s e d . The a n a l y s e s w e r e p e r f o r m e d w i t h i n 30 m i n u t e s . The s a m p l i n g p r o c e d u r e l a s t e d 5-10 s e c o n d s . Oxygen s a t u r a t i o n was measured p h o t o m e t r i c a l l y w i t h an O S M 2 oxygen s a t u r a t i o n m e t e r (Radiometer, Copenhagen, Denmark). 46 Table 1. Characteristics of the patients and results of tests. AV/ASaO2 (l/min/%Sa02 ) Case Age BMI vc RV FEV1.O based on based on NO. years kg/m2 %pred. %pred. %pred. oximetry arterial Sa02 1 39 36.0 2 48 22.8 3 38 31.0 4 54 27.1 5 57 32.6 6 41 26.9 7 52 24.5 8 61 31.4 9 63 29.0 10 49 27.8 11 38 39.1 12 39 28.4 13 60 29.0 14 58 30.4 15 43 39.8 16 59 30.5 89 84 108 106 101 112 90 57 80 103 90 102 124 93 65 83 86 122 57 93 89 65 83 92 100 119 127 71 111 103 213 91 91 82 114 117 109 119 95 53 80 113 103 106 134 96 69 80 0.39 0.43 0.12 0.23 0.37 0.74 0.64 0.73 3.78 1.15 0.55 0.85 1.59 0.70 0.53 0.45 0.41 0.62 0.12 0.24 0.46 0.90 0.63 0.89 4.40 1.20 0.65 1.00 1.63 0.85 0.57 0.48 Mean 50 30.4 93 101 98 0.83 0.94 SD 9 4.7 17 36 21 0.86 0.99 A b b r e v i a t i o n s : BMI: Body mass index; VC: v i t a l c a p a c i t y ; RV: r e s i d u a l volume; FEV1.O: Forced e x p i r a t o r y volume i n one second. Hypoxic v e n t i l a t o r y t e s t : The s u b j e c t s s a t comfortably and b r e a t h e d room a i r f o r approximately 10 m i n u t e s u n t i l a s t e a d y s t a t e v a l u e of Sa02 was a t t a i n e d . The hypoxic t e s t was performed by a method s i m i l a r t o t h a t d e s c r i b e d by Rebuck and Campbell ( 8 ) . B r i e f l y , t h e p a t i e n t s r e b r e a t h e d 8 l i t r e s of room a i r from a B e r n s t e i n s p i r o m e t e r . During t h e r e b r e a t h i n g procedure t h e c o n c e n t r a t i o n of t h e i n s p i r e d oxygen f e l l a s a consequence of oxygen consumption. The v e n t i l a t o r y response was r e g i s t e r e d a s t h e i n c r e a s e i n v e n t i l a t i o n (AV) c a l c u l a t e d f o r each 1% r e c o r d e d d e c r e a s e i n Sa02 ( ASa02 ) . 47 The p a i r s of Sa02 v a l u e s w e r e divided i n t o g r o u p s ( 9 0 - 9 9 % , 80-89% a n d s o o n ) a n d t h e i r d i f f e r e n c e s w e r e f u r t h e r a n a l y s e d . The r e s u l t s are p r e s e n t e d i n Table 2 . The v a l u e s d i s p l a y e d b y t h e o x i m e t e r were f o u n d t o be s i g n i f i c a n t l y lower t h a n t h e a r t e r i a l v a l u e s when a b o v e 9 0 % , b u t h i g h e r when below 8 0 % (Table 2 ) . Table 2 . D i f f e r e n c e s b e t w e e n o x i m e t e r r e a d i n g s a n d a r t e r i a l S a 0 2 i n d i f f e r e n t r a n g e s . n: number o f s i m u l t a n e o u s m e a s u r e m e n t s , d : mean o f t h e d i f f e r e n c e s . SEEd: s t a n d a r d e r r o r o f t h e estimate. Range n d .%Ed P 90-99% 7 0 - 1 . 3 9 1 . 1 7 < o . 0 0 1 80-89% 22 0 . 4 1 2 . 2 8 N . S . 70-79% 2 1 2 . 4 8 2 . 3 4 < o . 0 0 1 60-69% 11 2 . 8 2 2 . 6 8 < o . 0 1 The v e n t i l a t o r y r e s p o n s e t o h y p o x i a (AV/ASa02) w a s 0 . 8 3 l / m i n / l % Sa02 (k 0 . 8 6 ) when based o n o x i m e t e r r e a d i n g s , a n d 0 . 9 4 l / m i n / l % Sa02 ( f 0 . 9 9 ) when based o n a r t e r i a l v a l u e s (Table 1 ) . The v e n t i l a t - o r y r e s p o n s e s based o n a r t e r i a l a n d o x i m e t r i c v a l u e s w e r e h i g h l y c o r r e l a t e d t o e a c h o t h e r ( r = 0 . 9 9 5 ) ; b u t as a c o n s e q u e n c e of t h e r e l a t i o n s h i p s p r e s e n t e d i n Table 2 t h e v e n t i l a t o r y r e s p o n s e w a s s i g n i f i c a n t l y h i g h e r ( p < 0 . 0 5 ) when t h e c a l c u l a t i o n s w e r e based o n a r t e r i a l m e a s u r e m e n t s ( p a i r e d t t e s t ) . The d i f f e r e n c e s b e t w e e n a r t e r i a l a n d o x i m e t r i c v a l u e s w e r e n o t c o r r e l a t e d t o a g e , t o degree o f o v e r w e i g h t , o r t o t h e r e s u l t s o f t h e p u l m o n a r y t e s t s (Table 1 ) . T h e r e w a s n o e l e c t r o d e d r i f t , t h e ear p r o b e w a s w e l l t o l e r a t e d a n d n o t e c h n i c a l d i f f i c u l t i e s a r o s e d u r i n g o x i m e t r y . DISCUSSION I n t h i s s t u d y w e c h o s e t o e v a l u a t e t h e o x i m e t e r r e a d i n g s d u r i n g p r o g r - e s s i v e h y p o x i a i n o r d e r t o d e t e r m i n e t h e v a l i d i t y o f o x i m e t r y 48 An a r t e r i a l b l o o d s a m p l e w a s t a k e n a t r e s t j u s t b e f o r e t h e h y p o x i c t e s t b e g a n , a n d a sample was t h e n t a k e n e v e r y m i n u t e d u r i n g a n d o n e 2 . 5 a n d 5 m i n u t e s a f t e r t h e t e s t . When a p p r o x i m a t e l y 5 0 % of t h e a r t e r i a l s a m p l e h a d b e e n d r a w n i n t o t h e s y r i n g e , t h e Sa02 r e a d i n g as d i s p l a y e d b y t h e o x i m e t e r w a s recorded. A l l a r t e r i a l s a m p l i n g a n d r e c o r d i n g s w e r e d o n e b y t h e same p e r s o n . S t a t i s t i c a l a n a l y s e s : V a l u e s are p r e s e n t e d as mean ( f S D ) . The s t r e n g t h o f t h e c o r r e l a t i o n b e t w e e n a r t e r i a l Sa02 a n d o x i m e t e r r e a d i n g s w a s e v a l u a t e d b y l e a s t - s q u a r e l i n e a r c o r r e l a t i o n . S t a t i s t i c a l p r o b a b i l i t y w a s assessed b y S t u d e n t ' s p a i r e d t t e s t 100 90- 80 - 7 0 - BO - , RESULTS A t o t a l o f 1 2 6 p a i r s o f s i m u l t a n e o u s Sa02 r e a d i n g s w e r e o b t a i n e d , 1 6 d u r i n g s t e a d y s t a t e , 78 d u r i n g h y p o x i a a n d 32 o n e a n d t h r e e m i n u t e s a f t e r t h e t e s t ( F i g . 1 ) . The r a n g e o f a r t e r i a l S a 0 2 v a l u e s w a s 5 5 - 9 9 % . T h e r e w a s n o o v e r a l l b i a s , a s t h e mean Sa02 v a l u e w a s t h e same ( 8 8 . 7 % ) when m e a s u r e d w i t h t h e o x i m e t e r a s when based on t h e a r t e r i a l samples. F o r e a c h s u b j e c t t h e o x i m e t e r r e a d i n g s w e r e a l i n e a r f u n c t i o n of t h e a r t e r i a l SaOg, w i t h a mean c o r r e l a t i o n c o e f f i c e n t r o f 0 . 9 9 3 (kO.01) ( r a n g e 0 . 9 8 3 - 0 . 9 9 9 ) , w h e r e a s f o r a l l v a l u e s t h e r e g r e s s i o n e q u a t i o n w a s y= 0 . 8 3 x + 1 4 . 7 ( r = 0 . 9 7 6 ) ( F i g . 1 ) . y. r - 0 88 4, *i F i g . 1 . R e l a t i o n between 1 2 6 Sa02 v a l u e s o b t a i n e d s i m u l t a n e o u s l y by oximetry and from a r t e r i a l b l o o d samples. The l i n e a r r e g r e s s i o n e q u a t i o n ( s o l i d l i n e ) and t h e 95% c o n f i d e n c e l i m i t s (broken l i n e s ) a r e shown. 60 70 80 90 100 A r t P r l a l sao, 4- 88857 1 49 during rapid changes ot SaO2 (similar to those observed during sleep apnoea) and also to check the reliability of this method during the hypoxic ventilatory test, which is used to estimate the sensitivity of peripheral chemoreceptors (8). The high correlation between arterial and oximetric readings for each individual subject (r=0.993) suggests that the ear oximeter is very accurate in indicating changes in Sa02. The total correlation between arterial and oximeter readings is also fully acceptable for clinical purposes and even better than has been found by other authors (3). However, one must be aware of the increasing differences at low Sa02 levels ( 2 ) , which may imply that oximetry may lead to an overestimation of SaO2 in monitoring of critically ill patients and to a situation in which patients with sleep-related breathing disturbances may have more severe oxygen desaturation than is detected. The commonly found cardiac arrythmias in the sleep apnoea syndrome might also have a disturbing effect on the oximeter reading, a factor which needs to be further investigated. On the basis of calculations from the hypoxic ventilatory response tests, we agree with Chapman et a1 (2) that Sa02 values below 70% should be omitted . In summary, the Biox I11 Pulse oximeter has been found to correlate acceptabley with Sa02, is easy to apply and is well tolerated by patients. Obviously the non-invasive approach in continuous monitoring of blood gases is a rapidly expanding field which must be followed up by comparative studies to clarify the characteristics of these techniques. ACKNOWLEDGEMENTS This work was supported by grants from the Swedish National Association against Heart and Chest Diseases, Stockholm, The King Oscar I1 Jubilee Foundation, Stockholm and the Bror Hjerpstedt Foundation, Uppsala, Sweden. REFERENCES 1. Burki, N.K. & Albert, R.K.: Noninvasive monitoring of arterial 2. Chapman, K.R., D'Urzo, A. & Rebuck, A.S.: The accuracy and blood gases. Chest 83: 666-670, 1983. response characteristic of a simplified ear oximeter. Chest 3. Chapman, K.R., Lie, F.L., Watson, R.M. & Rebuck, A . S . : Range of accuracy of two wavelength oximetry. Chest 89:540-542, 1986. 83: 860-864, 1983. 50 4. 5. 6. 7. 8. 9. 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Address for reprints: Dr Thorarinn Gislason Dept. of Lung Medicine Akademiska Sjukhuset, S-751 85 Uppsala, Sweden. 51