Third-trimester arterial blood gas and acid base values in normal pregnancy at moderate altitude

Third-Trimester Arterial Blood Gas and Acid Base Values in Normal Pregnancy at Moderate Altitude G A R Y D. V. H A N K I N S , M D , S T E V E N L. C L A R K , M D , C A R O L J. H A R V E Y , R N C , M S , E D A M. U C K A N , R N C , M S N , D A V I D C O T T O N , M D , A N D J A M E S W. V A N H O O K , M D Objective: To report arterial blood gas and acid base values of normal nulliparous patients at moderate altitude for commonly used maternal positions. Methods: Ten normotensive nulliparous w o m e n between 36 and 38 weeks' gestation volunteered to undergo radial and pulmonary artery cannulation as part of a larger study. Following instrumentation, baseline assessments were made in the left lateral recumbent position after a 30-minute stabilization period. Sequential measurements were then obtained in the left lateral, right lateral, supine, knee-chest, sitting, and standing positions. Blood samples were analyzed in duplicate for oxygen content on a blood gas analyzer. Statistical analysis was performed by analysis of variance of repeated measures with significance defined at P ~ .05. Results: There was no significant difference in arterial blood gas or acid base values between any positions in this antepartum population of term healthy women. The composite mean values were as follows: pH 7.46, arterial carbon dioxide pressure (PaCO 2) 26.6 mmHg, arterial oxygen pressure 88.3 mmHg, bicarbonate 18.2 mEq/L, saturated arterial hemoglobin level 0.96. Conclusion: Arterial blood gas and acid base values are not altered by maternal position in the late third trimester of pregnancy. The PaO 2 in these w o m e n studied at moderate altitude was lower than previously reported for healthy pregnant w o m e n studied at sea level. Appropriate interpretation of arterial blood specimens of pregnant w o m e n should take into account both the pregnancy and altitude at which the w o m e n reside. (Obstet Gynecol 1996;88:347-50)

To determine the appropriate course of action in patient management, the clinician must first be able to distinguish normalcy from disease states. This is especially true in the interpretation of maternal arterial blood gas

From the University of Texas Medical Branch at Galveston, Galveston, Texas; the Universityof Utah, Salt Lake City, Utah;and Wayne State University, Detroit,Michigan.

VOL. 88, NO. 3, SEPTEMBER 1996

and acid base values, often used to assist in the management of both pulmonary and cardiac diseases that complicate pregnancy. Perusal of several standard textbooks in obstetrics ~'2 and maternal-fetal medicine 3 suggest that the normal arterial oxygen pressure (PaO2) near term is approximately 103 m m H g . Values as low as 85 m m H g , 4 attributed to maternal posture and its alterations in maternal hemodynamics superimposed upon changes in functional residual capacity and pulmonary closing volumes, have also been reported. However, perhaps a far more important variable than maternal position is the altitude at which the w o m a n lives. Sobrevilla et al 5 observed in a group of uncomplicated pregnant w o m e n near term a PaO 2 of 91 m m H g in those w o m e n living at 150 m, compared with a PaO2 of 61 m m H g in those living at 4200 m. Our purpose in this study was to determine arterial blood gas and acid base values in a group of w o m e n who lived at a moderate altitude and whose pregnancies were uncomplicated. We further sought to define the impact of maternal position changes on these values.

Materials and Methods This investigation was part of a larger study designed to assess the central hemodynamics of normal term pregnancy. 6 The study was approved b y the Hospital Institutional Review Board and by an independent pulmonologist-intensivist consultant. Volunteers were reimbursed $200 for participation in the study and were recruited from the offices of private physicians in Provo, Utah. The studies were performed at the Utah Valley Regional Medical Center in Provo in June 1988. The experimental protocol was explained to each subject, and written consent was obtained before the study.

0029-7844/96/$15.00

PII S0029-7844(96)00210-4

347

Table 1. Third-Trimester Arterial Blood Gas and Acid Base Levels Relative to Maternal Position in Ten Women Living at Moderate Altitude

pH PaCO2 (mmHg) PaO2 (mmHg) HCO 3 (mEq/L)

SaO2 (%)

Left lateral

Right lateral

Supine

Sitting

Standing

Knee-chest

7.46 -~ 0.02 26.6 2 2.7 86.2 ± 7.3 18.6 + 1.9 0.96 -+ 0.01

7.46 ± 0.06 27.4 ± 2.0 87.7 -+ 4.5 18.3 -+ 1.4 0.96 + 0.0

7.45 -+ 0.02 26.7 ± 2.1 86.7 ± 7.2 18.8 -+ 1.4 0.96 z 0.0

7.46 _+0.03 26.4 ± 2.2 91.1 ± 7.3 18.4 ± 0.8 0.96 -+ 0.0

7.46 ± 0.01 25.5 ± 2.4 88.8 ± 7.8 17.9 _+0.7 0.96 -+ 0.01

7.46 -+ 0.02 26.7 -+ 1.2 89.5 + 5.9 17.3 + 1.6 0.96 -+ 0.0

PaCO2 ~ arterial carbon dioxide pressure; PaO2 = arterial oxygen pressure; HCO3 - bicarbonate; SaO2 = saturated arterial hemoglobin level. Data are presented as mean ± standard deviation.

The s t u d y p o p u l a t i o n consisted of ten n u l l i p a r o u s w h i t e w o m e n at 3 6 - 3 8 w e e k s ' e s t i m a t e d gestational age as e s t a b l i s h e d b y clinical d a t e s a n d w i t h s o n o g r a p h i c confirmation. A l l fetuses w e r e singleton, in vertex presentation, m e t n o r m a l g r o w t h p a r a m e t e r s , h a d n o r m a l a m n i o t i c fluid v o l u m e s , a n d h a d no anomalies. M a t e r nal course w a s u n c o m p l i c a t e d ; m a t e r n a l h e m a t o c r i t s w e r e n o r m a l , a n d there w a s no alcohol, tobacco, or d r u g use b y these w o m e n . A l l w o m e n w e r e y o u n g e r than 26 y e a r s of age a n d w e r e of n o r m a l w e i g h t for their height. R a d i a l a n d p u l m o n a r y a r t e r y c a n n u l a t i o n w e r e perf o r m e d w i t h local anesthetics. The subjects lay on their left sides for 30 m i n u t e s to a l l o w a n y effects of stress f r o m i n s t r u m e n t a t i o n to d i s s i p a t e before o b t a i n i n g baseline values. Sequential m e a s u r e m e n t s in the right lateral, supine, knee-chest, sitting, a n d s t a n d i n g positions w e r e then m a d e after a m i n i m u m of 10 m i n u t e s w a s a l l o w e d for e q u i l i b r a t i o n in each of the n e w positions. All of these m e a s u r e m e n t s w e r e collected before o t h e r p o r t i o n s of this e x p e r i m e n t , such as m a t e r n a l exercise. Chest r a d i o g r a p h s w e r e o b t a i n e d on each subject after i n s t r u m e n t a t i o n to e n s u r e a p p r o p r i a t e p o sitioning of the p u l m o n a r y a r t e r y catheter a n d that pneumothoraces had not developed. E x p e r i m e n t s w e r e c o n d u c t e d in a m b i e n t c o n d i t i o n s of 22.5C d r y - b u l b t e m p e r a t u r e a n d a relative h u m i d i t y of 3 0 - 4 0 % . Barometric p r e s s u r e a v e r a g e d 650 torr. Because all subjects w e r e p e r m a n e n t residents at 1388 m elevation, the effects of m o d e r a t e a l t i t u d e w e r e conside r e d chronic. Blood gas a n a l y s e s w e r e p e r f o r m e d at 37C. N o m a ternal b o d y t e m p e r a t u r e corrections w e r e u s e d , as the w o m e n w e r e all n o r m o t h e r m i c . W e cleared the arterial lines carefully before d r a w i n g b l o o d to p r e v e n t s a m p l e dilution. Blood s a m p l e s w e r e a n a l y z e d in d u p l i c a t e u s i n g a b l o o d gas a n a l y z e r ( C o m i n g M o d e l 168; Corning, M e d f i e l d , MA). C a l i b r a t i o n s w e r e p e r f o r m e d before each subject was tested a n d r o u t i n e l y t h r o u g h o u t all e x p e r i m e n t s . The correlation coefficient for all v a l u e s w a s m o r e stringent than seen in certified clinical p a t h o l o g y laboratories. A r t e r i a l b i c a r b o n a t e w a s calculated u s i n g the H e n d e r s o n - H a s s e l b a c h equation, a n d base

348 Hankins et al

Arterial Blood Gas and Pregnancy

excess w a s d e t e r m i n e d u s i n g the S i g g a a r d - A n d e r s e n alignment nomogram. Statistical analysis w a s p e r f o r m e d b y analysis of v a r i a n c e of r e p e a t e d m e a s u r e s (StatView; A b a c u s Concepts, Berkley, CA). A l p h a w a s d e f i n e d at the <.05 level. To p e r f o r m a p o w e r calculation of the v a l u e s r e p o r t e d here, w e relied on the n o r m a t i v e d a t a from n o n p r e g n a n t i n d i v i d u a l s as r e p o r t e d b y Begin a n d Renzetti. 7 A c c e p t i n g that a n y c h a n g e a t t r i b u t a b l e to p o s i t i o n a n d rising to a level of clinical significance w o u l d equal or exceed one s t a n d a r d d e v i a t i o n of that of n o r m a t i v e data, the f o l l o w i n g v a l u e s w e r e u s e d in the p o w e r calculation: p H 0.025 units, arterial c a r b o n d i o x ide p r e s s u r e (PaCOz) 3.6 m m H g , PaO2 8.8 m m H g , b i c a r b o n a t e 1.8 m E q / L , a n d s a t u r a t e d arterial h e m o g l o b i n level 0.04. P o w e r w a s calculated u s i n g the True Epistat p r o g r a m (Richardson, TX).

Results Table 1 p r e s e n t s the values, r e p o r t e d as the m e a n -s t a n d a r d d e v i a t i o n for p H , PaO2, PaCO2, bicarbonate, a n d s a t u r a t e d arterial h e m o g l o b i n level for each of the six m a t e r n a l positions. Position d i d not significantly affect a n y of the v a l u e s u s i n g either the stringent P v a l u e of <.01 or the t r a d i t i o n a l v a l u e of <.05. P o w e r analysis at an a l p h a level of .05 s h o w e d an ability to detect significant differences b y position, as w e h a d d e f i n e d them, of 0.94 for p H , 0.96 for PaO2, 0.99 for PaCO2, 0.96 for bicarbonate, a n d 0.75% for s a t u r a t e d arterial h e m o g l o b i n level. C a l c u l a t e d base deficits also w e r e not significantly affected b y position, a n d w e r e - 3 . 3 m E q / L in the left lateral, - 3 . 5 m E q / L in the right lateral, - 3 . 8 m E q / L in the supine, - 3 . 5 m E q / L in the sitting, - 4 . 2 m E q / L in the s t a n d i n g , a n d - 5 . 0 m E q / L in the knee-chest positions. Table 2 p r e s e n t s the a l v e o l a r - a r t e r i a l o x y g e n difference a n d d e m o n s t r a t e s no significant differences acc o r d i n g to position. For this calculation, the r e s p i r a t o r y q u o t i e n t w a s a s s u m e d to be 0.8, p e r c o n v e n t i o n a l calculations.

Obstetrics & Gynecology

Table 2. The Effects of Maternal Position on the AlveolarArterial Gradient

Position

Alveolar oxygen content PaO2 PaCO2 Alveolar-arterial (mmHg) (mmHg) (mmHg) gradient

Left lateral Right lateral Supine Sitting Standing Knee-chest

126.63 126.63 126.63 126.63 126.63 126.63

86.2 87.7 86.7 91.1 88.8 89.5

26.6 27.4 26.7 26.4 25.5 26.7

7.18 4.68 6.56 2.53 5.96 3.76

PaO2 = arterial oxygen pressure; PaCO2 = arterial carbon dioxide pressure.

Discussion This s t u d y adds to the i n f o r m a t i o n r e g a r d i n g blood gas a n d acid base status of the healthy n e a r - t e r m patient. A l t h o u g h several studies have p r o v i d e d i n f o r m a t i o n on PaO2 a n d PaCO2 values, 5"s-12 acid base data has b e e n p r o v i d e d in fewer studies (Table 3). 5"11"12The m e a n p H v a l u e of 7.46 that we f o u n d is s o m e w h a t higher than those p r e v i o u s l y reported, which r a n g e d from 7.40 to 7.44. This is explained b y a lower PaCO2 a m o n g the w o m e n n o w reported a n d is p r o b a b l y a response to increased ventilation d r i v e n b y peripheral chemoreceptors sensitive to the lower PaO2 of these w o m e n . The base deficit a n d bicarbonate levels we report are similar to p r e v i o u s reports. 5A1"12 Overall, the picture is consistent with the traditional concept of a mild respiratory alkalosis, further a u g m e n t e d b y altitude-related hypoxemia in the w o m e n n o w reported. In general, the arterial PaO2 has been reported to r a n g e from 101.8 to 85 m m H g in healthy gravidas. 5'7-n Lucius et al 4 s t u d i e d 37 w o m e n whose gestational ages r a n g e d from 8 to 42 weeks; each was s a m p l e d o n l y once a n d with the subject in the s u p i n e position. A linear

decrease in the PaCO 2 from 35.5 to 28 m m H g occurred from the tenth to the 40th week, with a c o n c u r r e n t increase in PaO2 from 85 to 92 m m H g . Arterial p H was 7.47 a n d r e m a i n e d fairly constant, a n d bicarbonate decreased from 27 to 21 m E q / L . These w o m e n lived in H a n o v e r , G e r m a n y , at a n altitude of 54 m. At the U n i v e r s i t y of D u n d e e , A n d e r s o n et al 1° f o u n d that neither PaO 2 nor PaCO 2 varied significantly b y trimester: 108 to 104 a n d 32.1 to 31.7 m m H g , respectively. Bicarbonate was also constant (range 20.6-21.2 m E q / L ) , as was base excess ( - 3 . 6 to - 2 . 8 m E q / L ) . A n d e r s e n et al 1° did note a t e n d e n c y for the PaO2 to fall with a d v a n c i n g m a t e r n a l age, from a m e a n of 114.3 2.7 m m H g at 20 years to 98.2 -+ 5.9 m m H g at 35 years of age or older. Eng et al 9 reported third-trimester blood gas data in p r e g n a n t w o m e n in Seattle, W a s h i n g t o n , w h o were 50-140% above ideal b o d y weight. They attributed the m e a n PaO2 of 85 _+ 5 m m H g to ventilat i o n - p e r f u s i o n m i s m a t c h e s from obesity, confirmed by a PaO 2 of less t h a n 550 m m H g , even w h e n b r e a t h i n g 100% oxygen. It is n o t a b l e that the w o m e n we report were at or b e l o w ideal b o d y weight as adjusted for the d u r a t i o n of their p r e g n a n c y a n d their height, u n l i k e the report b y Eng et al. 9 Sobrevilla et al 5 reported that for w o m e n living at 4200 m, the PaO 2 was 60.75 _+ 2.02 m m H g , c o m p a r e d to the 86.7 + 7.2 m m H g reported b y us for w o m e n living at 1388 m. In contrast, values from A b e r d e e n , Scotland (approximately 200 m) a n d H o u s t o n , Texas (approxim a t e l y 12 m) r a n g e d from 95 to 102 m m H g . 8'12 The variability in blood gas values has b e e n attribu t e d b y some investigators to the w o m a n ' s position w h e n the m e a s u r e m e n t was obtained. 7 For instance, A w e et al s observed a PaO2 of 101.2 - 7.0 m m H g in the sitting position in n e a r - t e r m w o m e n , falling to 94.6 + 8.8 m m H g in the s u p i n e position. They attributed the

Table 3. Studies of Arterial Blood Gas and Acid Base Values in Pregnant Women Altitude (m)

PaO 2

Investigator

152-304 Templetonl2/ University of Aberdeen 11.5 AweS/Baylor, Houston

Position

Semirecumbent101.8± 1.0 with 15° LL tilt Sitting 101.2 ± 7.0 Supine Not stated

50

Engg/Seattle

44.8

Blechner ll/University Supine of Florida SobrevillaS/Lima, Supine Peru SobrevillaS/Cerrode Supine Pasco, Peru

150 4200

(mmHg)

94.6 ± 8.8 85 -+ 5

PaCO2 (mmHg)

pH

BD (mEq/L)

30.4 -+-0.6

7.43 ± 0.006

3.1 + 0.2

7.44 _+0.04

28.7

7.42

EGA (wk)

N

38

20

36-40

23

36-40 23 Third 10 trimester 34-44 23

-4.2 ± 2.6

22.0 ± 2.1

91 + 2.09 32.26+ 1.06 7.397± 0.12

-4.14 _+0.66

19.46± 0.63

Term

12

60.75 ± 2.02 24.54+ 1.48 7.432 ± 0.11

-6.33 = 0.68

15.88z 0.76

Term

12

Arterial Blood Gas and Pregnancy

349

96.6

29.7 ± 2.8

HCO3 (rnEq/L)

BD = base deficit; EGA = estimated gestational age; LL = left lateral; all other abbreviations as in Table 1.

VOL. 88, NO. 3, SEPTEMBER 1996

Hankins et al

difference to a n a b n o r m a l a l v e o l a r - a r t e r i a l o x y g e n grad i e n t i n t h e s u p i n e p o s i t i o n . I m p o r t a n t l y , the m e a n a l v e o l a r - a r t e r i a l g r a d i e n t r e p o r t e d in the s i t t i n g p o s i t i o n w a s 14.3, c o m p a r e d w i t h 20 i n t h e s u p i n e p o s i t i o n . I n contrast, w e f o u n d a n a l v e o l a r - a r t e r i a l g r a d i e n t of o n l y 2.53 s i t t i n g a n d 6.56 s u p i n e . T h e n o r m a l a l v e o l a r arterial g r a d i e n t for h e a l t h y subjects is c o n s i d e r e d to b e less t h a n 15. A n e l e v a t i o n of 1388 m ( b a r o m e t r i c p r e s s u r e 650 torr) is c o n s i d e r e d to b e m i l d to m o d e r a t e altitude. M o o r e et a113 u s e d a r t e r i a l i z e d v e n o u s b l o o d f r o m w o m e n at 1520 feet a n d r e p o r t e d at-rest v a l u e s s i m i l a r to those w e f o u n d . T h u s , it is a p p a r e n t that a p p r o p r i a t e i n t e r p r e t a t i o n of b l o o d gas v a l u e s for p r e g n a n t w o m e n r e q u i r e s the e s t a b l i s h m e n t of n o r m a t i v e d a t a a p p r o p r i a t e to the e l e v a t i o n w h e r e the w o m e n are r e c e i v i n g their care. I n the c u r r e n t i n v e s t i g a t i o n , w e d i d n o t f i n d a n y s i g n i f i c a n t effects of m a t e r n a l p o s i t i o n o n a n y b l o o d gas or acid b a s e p a r a m e t e r s . This r e p o r t r e p r e s e n t s o n l y a s u b s e t of d a t a collected at the t i m e of a m u c h l a r g e r study. 6 As a consequence, no power analysis was done b e f o r e the s t u d y . N e v e r t h e l e s s , it is u n l i k e l y that w e would have retained additional study participants, bec a u s e the s t u d y i n v o l v e d c o n s i d e r a b l e i n c o n v e n i e n c e a n d d i s c o m f o r t to o t h e r w i s e h e a l t h y v o l u n t e e r s . E v e n so, p o s t - s t u d y p o w e r a n a l y s i s , u s i n g criteria for d e f i n i n g a difference that w e b e l i e v e d w o u l d b e clinically i m p o r t a n t to u s as i n t e n s i v i s t s , s h o w e d that the d a t a has g o o d reliability.

References 1. Cunningham FG, MacDonald PC, Gant NF, Leveno KJ, Gilstrap LC III. Maternal adaptations to pregnancy. In: Williams obstetrics. 19th ed. East Norwalk, Connecticut: Appleton & Lange, 1993:209. 2. Gabbe SG, Niebyl JR, Simpson JL. Obstetrics, normal and problem pregnancies. 2nd ed. New York: Churchill Livingstone, 1991. 3. De Swiet M. Pulmonary disorders. In: Creasy RK, Resnik R, eds. Maternal-fetal medicine: Principles and practice. 2nd ed. Philadelphia: WB Saunders, 1989:875. 4. Lucius H, Gahlenbeck H, Kleine HO, Fabel H, Bartels H. Respira-

350 H a n k i n s et al

Arterial Blood Gas and Pregnancy

5.

6.

7.

8.

9. 10.

11.

12.

13.

tory function, buffer system, and electrolyte concentrations of blood during human pregnancy. Respir Physiol 1970;9:311-7. Sobrevilla LA, Cassinelli MT, Carcelen A, Lamage JM. Human fetal and maternal oxygen tension and acid-base status during delivery at high altitude. Am J Obstet Gynecol 1971;111:1111-8. Clark SL, Cotton DB, Lee W, Bishop C, Hill T, Southwick J, et al. Central hemodynamic assessment of normal term pregnancy. Am J Obstet Gynecol 1989;161:1439-42. Begin R, Renzetti AD. Alveolar-arterial oxygen pressure gradient. I. Comparison between an assumed and actual respiratory quotient in stable chronic pulmonary disease. II. Relationship to aging and closing volume in normal subjects. Respir Care 1977;22:491-9. Awe RJ, Nicotra MB, Newsom TD, Viles R. Arterial oxygenation and alveolar-arterial gradients in term pregnancy. Obstet Gynecol 1979;53:182-6. Eng M, Butler J, Bonica JJ. Respiratory function in pregnant obese women. Am J Obstet Gynecol 1975;123:241-5. Andersen GJ, James GB, Mathers NP, Smith EL, Walker J. The maternal oxygen tension and acid-base status during pregnancy. J Obstet Gynaecol Br Commw 1969;76:16-9. Blechner JN, Cotter JR, Stenger VG, Hinkley CM, Prystowsky H. Oxygen, carbon dioxide, and hydrogen ion concentrations in arterial blood during pregnancy. Am J Obstet Gynecol 1968;100: 1-6. Ternpleton A, Kelman GR. Maternal blood-gases, (PaO2-PaO2), physiological shunt, and VD/VT in normal pregnancy. Br J Anaesth 1976;48:1001-4. Moore LG, McCullough RE, Weil JV. Increased HVR in pregnancy: Relationship to hormonal and metabolic changes. J App1 Physiol 1987;62:158-63.

Address reprint requests to:

Gary D. V. Hankins, MD Department of Obstetrics and Gynecology University of Texas Medical Branch at Galveston 301 University Boulevard Galveston, TX 77555-0587

Received January 16, 1996. Received in revised form May 1, 1996. Accepted May 8, 1996. Copyright © 1996 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc.

Obstetrics & Gynecology