Cardiovascular function in pregnancy: effects of posture

British Journal of Obstetrics and Gynaecology April 2001, Vol. 108, pp. 344±352

Cardiovascular function in pregnancy: effects of posture Riccarda Del Bene a,*, Giuseppe Barletta a, Giorgio Mello b, Chiara Lazzeri c, Federico Mecacci b, Elena Parretti b, Elisabetta Martini b, Sabrina Vecchiarino c, Franco Franchi c, Giorgio La Villa c Objective To evaluate the cardiovascular response to active postural changes in pregnancy. Design Prospective study. Setting Outpatient Clinic, Fetal Maternity Unit. Participants Sixteen healthy women referred prior to pregnancy. Methods Heart rate, arterial pressure, echocardiographic end±diastolic and end±systolic left ventricular volumes (Teichholz's formula) were measured in the three months before pregnancy, at the end of the ®rst and second trimester, at mid third trimester, and six months after delivery in the supine and standing position, in thirteen women (mean age 33, range 25±38 years). Results Cardiac output (supine position) signi®cantly increased (28%): it reached its maximum at the second trimester, remained steadily elevated in the mid third trimester, and returned to baseline after delivery. Cardiac output increased during pregnancy also in the active orthostatic position, the percentage increase being greater (70%) since the standing pre±conception value was lower. The postural stress induced similar changes in heart rate, arterial pressure and left ventricular ejection fraction before, during and after pregnancy. However, the reduction in cardiac output associated with early standing attenuated signi®cantly at the second trimester and it was absent at mid third trimester (F ˆ 3.13, P ˆ 0.021). This was due to the interplay between the signi®cantly lesser increase in systemic vascular resistance, occurring since the ®rst trimester, and the signi®cantly lesser decrease in left ventricular end±diastolic volume which was observed in the mid third trimester. Conclusion These data indicate that the elevated cardiac output is adequately maintained in pregnancy during the postural challenge, due to optimisation of the responses of preload and afterload.

INTRODUCTION Bader et al. 1 ®rst described the haemodynamic pro®le of pregnant women at rest and during exercise. It is well known that cardiac output increases during gestation, but there is disagreement over the magnitude of this rise and the times at which the values are reached 2. This holds true even in longitudinal studies 3±7, the only ones that allow conclusions about the course of cardiac output in pregnancy 2. By contrast, very few studies 8±11 have investigated the cardiovascular response to postural changes in pregnancy. Only the group in Seattle 8±10 has assessed the haemodynamic effects of standing. This is somewhat surprising, since normal pregnancy does not inhibit women from spending most of the daytime life in the sitting or standing position. Furthermore, evaluation of

a

Cardiovascular Echography Section, Careggi Hospital, Italy b Dept of Internal Medicine, University of Florence School of Medicine, Italy c Fetal Maternal Unit, University of Florence School of Medicine, Italy * Correspondence: Dr G. Barletta, Via Medaglie d'Oro 43, 59100 Prato, Italy. q RCOG 2001 British Journal of Obstetrics and Gynaecology PII: S03 06-5456(00)0009 9-1

the cardiovascular response to posture may reveal the interplay between preload and afterload in physiological 12 as well as in pathological conditions 13. In order thoroughly to evaluate the adaptive response to active postural changes in pregnancy, we non±invasively analysed cardiovascular function and systemic haemodynamics at the end of the ®rst and second trimester and at mid third trimester of uneventful pregnancies, and compared them to the data obtained both before conception and after delivery. The study design comprised three echocardiographic determinations of cardiovascular function in the active standing and two in the left lateral supine positions. METHODS Sixteen women, referred to the Fetal Maternal Unit of Careggi Hospital (Florence, Italy), prospectively entered the study. These women were healthy, as assessed by history, physical examination and a complete echocardiographic study; all were non±smokers. An obstetric ultrasound scan, performed in all subjects to con®rm gestational age, ruled out multiple gestation. Seven women were primigravidae, seven had had previous normal pregnancies and two had had miscarriages. All women gave their written informed consent to participate www.bjog-elsevier.com

q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing

SVR dynes s´cm -5

2

1301 (1107-1495) * 2 1014 (907-1122) * 1 1079(967-1192) * 1 1037 (871-1203) 946 (849-1044) * 8 912 (804-1021) * 5 3 1012 (895-1129) 906 (797-1015) * 887 (781-994) * 2 1299 (1141-1456) * 6 1091 (976-1206) * 1 1031(918-1143) * 1 1319 (1115-1524) * 2 1022 (923-1121) * 1 1077(955-1198) * 1 0.007 0.081 (ns) 0.020 Effect of posture F ˆ 29.195, P , 0.0001

1

3 rd trimester

1827 (1592-2063) ³ 1 1275 (1062-1487) ² 2 1203 (1073-1333) ² 1 1639 (1446-1832) ³ 2 1820 (1591-2048) ³ 1 0.0001

2 nd trimester

6.66 (5.94-7.38) * 5.84 (4.81-6.87) 7.05 (6.11-8.00) * 6.75 (5.49-8.01) 6.84 (5.98-7.70) 6.85 (6.05-7.66) 6.43 (5.47-7.40) 7.02 (6.11-7.93) 6.76 (5.90-7.61) 5.60 (4.67-6.53) 6.42 (5.78-7.06) * 7 6.80 (5.77-7.82) * 4 6.73 (5.95-7.51) * 2 5.76 (4.72-6.87) 7.06 (6.17-7.94) * 1 0.397 (ns) 0.753 (ns) 0.998 (ns) Effect of posture F ˆ 4.422, P ˆ 0.002

1 st trimester

4.33 (3.53-5.12) 5.64 (4.54-6.74) 5.53 (4.89-6.16) 4.71 (3.92-5.50) 4.37 (3.58-5.16) 0.040

Pre-conception 3

1774 (1528-2020) ³ 1 1224 (1015-1434) ² 1 1159 (1039-1279) ² 1 1630 (1440-1820) ³ 1 1799 (1561-2037) ³ 1 , 0.0001

4.33 (3.60-5.05) ³ 5.68 (4.60-6.76) ² 3 5.62 (5.04-6.19) ² 4 4.57 (3.85-5.29) 4.28 (3.56-5.01) ³ 3 0.004

After delivery

, 0.0001 0.003 , 0.0001 , 0.0001 , 0.0001

, 0.0001 0.144 (ns) 0.016 , 0.0001 , 0.0001

P level

F ˆ 65.304 P , 0.0001

F ˆ 29.062 P , 0.0001

Effect of pregnancy

*vs pre-conception: 1 P , 0:0001; 2P ˆ 0.001; 3P ˆ 0.002; 4P ˆ 0.003; 5P ˆ 0.014; 6P ˆ 0.015; 7P ˆ 0.024; 8P ˆ 0.034. ² ˆ vs 30' standing: 1P , 0:0001; 2P ˆ 0.001; 3P ˆ 0.035; 4P ˆ 0.050. ³ ˆ vs 60' left lateral decubitus: 1P , 0.0001, 2P ˆ 0.016; 3P ˆ 0.050.

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

CO (L/min)

Stage

Table 1. Haemodynamic parameters determined before conception (within three months), at the end of the ®rst (10-12 weeks) and second (22-24 weeks) trimester, at mid third (32-34 weeks) trimester, and six months after delivery. CO ˆ cardiac output; SVR ˆ systemic vascular resistance. 95% CI is reported in brackets.

CARDIOVASCULAR FUNCTION IN PREGNANCY: EFFECTS OF POSTURE 345

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

HR (bpm)

100.8(91.1-110.5) ³ 4 109.7 (103.1-116.2) * 1 ³ 1 109.0 (100.4-117.6) * 3 ³ 1 81.6 (72.2-91.0) ² 7 83.8 (76.8-90.7) ² 1 87.2 (82.2-92.2) ² 1 4 1 77.6 (68.8-86.4) ² 82.7 (75.8-89.5) ² 82.9 (78.2-87.6) ² 1 93.9 (84.9-102.9) ³ 11 99.7 (92.5-106.8) ³ 4 98.0 (92.6-103.4) ³ 7 101.1 (91.3-110.8) ³ 4 110.0 (103.3-116.7) * 2 ³ 1 108.9 (100.3-117.6) * 4 ³ 1 , 0.0001 , 0.0001 , 0.0001 Effect of posture F ˆ 51.673, P , 0.0001

3 rd trimester

93.7 (86.6-100.8) ³ 2 76.9 (68.4-85.4) ² 6 73.5 (67.2-79.8) ² 2 90.6 (83.9-97.4) ³ 5 93.8 (86.5-101.2) ³ 2 , 0.0001

2 nd trimester

89.1 (83.4-94.7) ³ 9 87.1 (80.4-93.7) 87.6 (83.5-91.6) ³ 1 81.5 (75.8-87.3) 78.7 (72.8-84.7) 75.9 (70.1-81.7)² 3 78.3 (73.0-83.5) ² 8 76.7 (72.1-81.2) 72.5 (67.6-77.3) ² 1 86.6 (81.1-92.1) 85.6 (80.1-91.2) 84.1 (80.5-87.8) ³ 3 88.8 (82.9-94.7) ³ 11 87.9 (82.2-93.6) ³ 8 87.9 (83.8-92.0) ³ 1 0.014 0.008 , 0.0001 Effect of posture F ˆ 21.565, P , 0.0001

1 st trimester

93.6 (86.8-100.4) ³ 11 84.1 (76.8-91.5) 81.1 (75.4-86.8) ² 9 92.5 (85.1-99.9) 94.3 (87.2-101.4) ³ 10 0.010

Pre-conception

92.6 (85.5-99.7) ³ 2 75.2 (69.9-83.4) ² 5 72.9 (66.7-79.2) ² 2 89.3 (82.5-96.1) ³ 6 92.7 (85.3-100.1) ³ 2 , 0.0001

91.0 (84.6-97.4) 81.7 (73.9-89.6) 79.9 (73.9-85.9) 89.6 (83.0-96.1) 91.5 (84.9-98.0) 0.021

After delivery

0.002 0.119 (ns) 0.056 (ns) 0.120 (ns) 0.002

0.450 (ns) 0.369 (ns) 0.120 (ns) 0.205 (ns) 0.397 (ns)

P

F ˆ 15.342 P , 0.0001

F ˆ 5.805 P , 0.0001

Effect of pregnancy

*vs pre-conception: 1P ˆ 0:028; 2P ˆ 0.029; 3P ˆ 0.042; 4P ˆ 0.05; ² ˆ vs. 30' standing: 1P , 0:0001;2 P ˆ 0:001;3 P ˆ 0:002; 4P ˆ 0.003; 5 P ˆ 0.004; 6 P ˆ 0.007; 7 P ˆ 0.024; 8P ˆ 0.043; 9P ˆ 0.05. ³ vs 60' left lateral decubitus: 1P , 0:0001; 2P ˆ 0.001; 3P ˆ 0.002; 4P ˆ 0.003; 5P ˆ 0.005; 6P ˆ 0.009; 7P ˆ 0.010; 8P ˆ 0.037; 9P ˆ 0.043; 10P ˆ 0.044; 11P ˆ 0.05.

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

MAP (mm Hg)

Stage

Table 2. Haemodynamic parameters determined before conception (within three months), at the end of the ®rst (10-12 weeks) and second (22-24 weeks) trimester, at mid third (32-34 weeks) trimester, and six months after delivery. MAP ˆ mean arterial pressure; HR ˆ heart rate. 95% CI in brackets.

346 R. DEL BENE ET AL.

q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

LVESV (mL)

LVEF (%)

1 st trimester

2 nd trimester

3 rd trimester

68.6 (63.9-73.3) 71.8 (68.6-75.1) 71.7 (68.5-75.0) 71.1 (66.3-76.0) 68.8 (63.9-73.7) 0.621 (ns)

21.2 (16.5-25.9) 28.3 (24.3-32.4) 30.1 (25.1-35.1) 21.8 (16.0-27.6) 21.5 (16.3-26.7) 0.013

72.6 (68.3-76.9) 75.0 (71.6-78.5) 72.4 (68.8-76.0) 73.5 (70.5-76.5) 71.0 (66.8-75.2) 71.0 (68.1-73.9) 73.0 (69.8-76.3) 71.7 (67.9-75.4) 71.8 (68.8-74.8) 73.0 (68.9-77.0) 75.5 (72.0-79.0) 74.9 (71.0-78.9) 72.4 (68.2-76.5) 75.3 (71.8-78.8) 73.3 (70.0-76.6) 0.992 (ns) 0.175 (ns) 0.440 (ns) Effect of posture F ˆ 0.003, P ns

21.7 (17.9-25.5) ³ 5 22.0 (17.4-26.6) ³ 9 23.7 (19.3-28.1) 29.1 (25.8-32.4) ² 8 33.9 (27.5-40.4) ² 6 32.3 (27.3-37.2) 33.8 (28.1-39.4) ² 7 32.0 (27.5-36.4) 30. 7 (26.2-35.2) ² 4 22.1 (18.3-25.9) ³ 9 21.7 (17.1-26.4) ³ 8 23.6 (18.6-28.6) 21.5 (17.9-25.1) ³ 4 21.7 (17.2-26.2) ³ 6 23.1 (18.7-27.5) , 0.0001 , 0.0001 0.002 Effect of posture F ˆ 27.465, P , 0.0001

79.3 (70.4-88.1) ³ 1 86.5 (75.1-97.9) * 1 ³ 1 85.6 (75.5-95.6) * 3 ³ 3 67.2 (57.8-76.6) ³ 1 100.9 (91.1-110.8) ² 1 111.0 (102.0-120.0) ² 1 115.8 (105.0-126.2) ² 2 110.9 (99.6-122.3) ² 5 113. (103.0-123.3) ² 3 105.3 (96.3-114.4) ² 1 113.4 (105.2-121.6) ² 1 118.3 (108.9-127.8) ² 1 73.5 (63.2-83.7) ³ 2 81.5 (73.3-89.7) ³ 1 86.6 (76.5-96.7) ³ 1 93.0 (80.6-105.4) * 2 67.8 (58.2-77.5) ³ 1 78.2 (69.2-87.2) ³ 1 86.1 (75.0-97.2) ³ 1 85.6 (74.7-96.4) ³ 3 , 0.0001 , 0.0001 , 0.0001 , 0.0001 Effect of posture F ˆ 72.565, P , 0.0001

Pre-conception

69.9 (65.5-74.3) 72.7 (69.7-75.6) 72.4 (70.1-74.6) 71.6 (67.3-75.9) 69.3 (64.9-73.7) 0.423 (ns)

20.2 (16.1-24.4) ³ 5 28.0 (24.6-31.4) ² 9 29.8 (26.0-33.6) ² 4 20.7 (16.2-25.2) ³ 9 20.7 (16.3-25.0) ³ 9 , 0.0001

66.9 (58.0-75.9) ³ 1 103.2 (92.4-114.0) ² 1 107.3 (97.5-117.1) ² 1 71.9 (62.6-81.1) ³ 1 66.8 (57.5-76.1) ³ 1 , 0.0001

After delivery

0.105 (ns) 0.730 (ns) 0.957 (ns) 0.387 (ns) 0.084 (ns)

0.792 (ns) 0.199 (ns) 0.683 (ns) 0.922 (ns) 0.946 (ns)

0.002 0.179 (ns) 0.223 (ns) 0.010 0.005

P

F ˆ 3.132 P ˆ 0.015

F ˆ 1.967 P ˆ 0.1 (ns)

F ˆ 14.025 P , 0.0001

Effect of pregnancy

*vs pre-conception: 1P ˆ 0.040; 2P ˆ 0.044; 3P ˆ 0.05. ² vs 30' standing; 1P , 0:0001; 2P ˆ 0.001; 3P ˆ 0.003; 4P ˆ 0.006; 5P ˆ 0.007; 6P ˆ 0.009; 7P ˆ 0.010; 8P ˆ 0.041; 9 P ˆ 0.046; ³ vs 60' left lateral decubitus: 1P , 0:0001; 2P ˆ 0.001; 3P ˆ 0.003; 4P ˆ 0.005; 5P ˆ 0.006; 6P ˆ 0.007; 8P ˆ 0.008; 9P ˆ 0.010.

30'standing 5' lateral decubitus 60' lateral decubitus 5' standing 30' standing P

LVEDV (mL)

Stage

Table 3. Left ventricular volumes and ejection fraction measured before conception (within three months), at the end of the ®rst (10-12 weeks) and second (22-24 weeks) trimester, at mid third (32-34 weeks) trimester, and six months after delivery. LVEDV ˆ left ventricular end-diastolic volume; LVESV ˆ left ventricular end-systolic volume; LVEF ˆ left ventricular ejection fraction. 95% CI in brackets.

CARDIOVASCULAR FUNCTION IN PREGNANCY: EFFECTS OF POSTURE 347

348 R. DEL BENE ET AL.

in the study, conforming to the principles outlined in the Declaration of Helsinki; the research was approved by the local ethics committee. The study was carried out before conception (within three months), at the end of the ®rst (10±12 weeks) and second (22±24 weeks) trimester, at mid third (32±34 weeks) trimester, and 6 months after delivery. Two women were excluded because they failed to keep their echocardiographic appointments and one because of abortion. Thirteen women (mean age 33, range 25±38 years) completed the study and constituted the study population; all of them had uneventful pregnancies and delivered a live infant of normal birthweight. Echocardiographic studies were performed using a Toshiba SSA 270 HG machine (Toshiba, Tokyo, Japan), equipped with a 2.5 or a 3.75 MHz transducer. The study started at 9:00am. Echocardiographic measurements were taken in ®ve consecutive steps: after at least 30 minutes of active standing (prolonged standing), after 5 and 60 minutes in left lateral decubitus (early and late decubitus), and at ®ve minute and 30 minute standing (early and late standing). Standard parasternal twodimensional long axis images were recorded and left ventricular (LV) diameters were measured in triplicate on M±mode tracings performed according to the recommendations of the American Society of Echocardiography 14. LV volumes were determined from LV diameters using the Teichholz formula 15, the ejection fraction (LVEF) was calculated as (LV end-diastolic volume minus LV end-systolic volume)/LV end-diastolic volume; and cardiac output (CO) was calculated from stroke volume and heart rate. Triplicate measurements of heart rate and arterial pressure (Sirecust 888 Siemens, Germany) were taken; mean arterial pressure (MAP) was calculated as diastolic pressure 1 1/3 pulse pressure. Systemic vascular resistance (SVR) was calculated as MAP x 80/CO. All measurements were taken by the same cardiologist to reduce inter±observer variability. The reproducibility of measurements in the prolonged orthostatic position was analysed by comparing the ®rst and last step determinations.

by the t test with the Bonferroni correction. Repeatability of measurements was assessed by Bland and Altman correlation coef®cient between difference and average of each couple of values 16. When P value is # 0.05, the exact P value is reported. The statistical analysis was performed by means of SPSS 9.0 (SPSS Inc, Chicago Illinois, USA). RESULTS Systemic haemodynamics, LV volumes and LVEF obtained before, during and after pregnancy in prolonged active standing, early and late left lateral decubitus and early and late active standing are reported in Tables 1±3. During pregnancy CO in late left lateral decubitus signi®cantly increased, reaching its maximum at the second trimester (128%, 95% con®dence interval from 115% to 140%); it remained steadily elevated in the mid third trimester. The apparently greater increase in CO (170%, 95% con®dence interval 145% to 196%) in prolonged active standing resulted from the lower CO values observed in the same posture before pregnancy. SVR in late left lateral decubitus signi®cantly decreased in the second trimester (P ˆ 0.0001) as compared with non-pregnant states, and it decreased further in the third trimester. During prolonged active standing, SVR exhibited a small increase in the mid third trimester (Fig. 1). A different behaviour of SVR in the different postures was observed during pregnancy (two±way analysis of variance pregnancy and posture F ˆ 2.599, P ˆ 0.001, P ˆ test power 0.992).

Statistical analysis Data are reported as mean with 95% con®dence interval.Variations in haemodynamic parameters were evaluated by using repeated measures two±way analysis of variance (period of pregnancy and postural stage), followed by ANOVA and t tests with the Bonferroni correction as appropriate. The power of univariate and multivariate F tests based on ®xed±effect assumptions was assessed by computing alpha level ˆ 0.05. To analyse better the effects of pregnancy on the cardiovascular response to active standing (postural stress), differences in haemodynamic parameters between 60 minute left lateral decubitus and 5 minute standing were calculated, and then analysed by ANOVA followed

Fig. 1. Systemic vascular resistance behaviour before conception (within three months), at the end of the ®rst (10±12 weeks) and second (22±24 weeks) trimester, at mid third (32±34 weeks) trimester, and 6 months after delivery in left lateral decubitus (closed squares) and in response to the postural stress (30 min active standing, open squares). Each bar represents 2SE.

q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

CARDIOVASCULAR FUNCTION IN PREGNANCY: EFFECTS OF POSTURE 349

In prolonged active standing, but not in late left lateral decubitus, pregnancy was associated with a signi®cant increase in LVEDV and HR, while no signi®cant changes in LVESV or in LVEF or MAP were observed. Postural stress (i.e. moving to the standing posture from left lateral decubitus) modi®ed LVEDV, LVESV, HR, CO, SVR and MAP. However some differences in response were observed: in the non±pregnant states, standing was associated with decrements in LVEDV, LVESV and CO, and increments in HR, SVR and MAP. These effects of postural stress were attenuated or absent in pregnancy (Fig. 2). In particular, changes in SVR were signi®cantly less after the ®rst trimester, with a continuous trend towards third trimester. Changes in CO were signi®cantly less in the second trimester and were lacking in the third. This was associated with a signi®cantly smaller decrease in LVEDV. The effects of posture on HR, MAP and LVEF did not differ between non±pregnant states and pregnancy. No appreciable differences were found between the early and late determinations taken in the supine position, as well as among the three determinations taken in the standing posture (short term and prolonged). The two prolonged active standing periods were compared in order to assess the repeatability of measurements: they did not appear to differ and their difference showed a uniform distribution on Bland±Altman scatter plots (Fig. 3). Mean paired differences, their 95% con®dence interval from the mean and the coef®cients of Bland±Altman correlations (none signi®cant) are reported in Table 4. DISCUSSION According to Schrier 17, pregnancy is a condition of primary peripheral arterial vasodilation, which initiates a series of haemodynamic and hormonal events resulting in high cardiac output and plasma volume expansion. The hyperdynamic circulation of pregnancy, with high CO and low SVR has been investigated by a number of studies, that were usually performed in the supine position 3±7,18±20. By contrast, maternal haemodynamics in the orthostatic posture has been little described 8±10. The hyperdynamic circulation of pregnancy develops in early gestation 21, and the increase goes on to the end of the second trimester. The behaviour of CO in late pregnancy is still a matter of disagreement, since invasive and non±invasive studies have found CO either to fall 3±20 or to reach a plateau 4,5,18, or to increase further 6,7,19. Our longitudinal echocardiographic evaluation of cardiovascular function, starting before conception and ending after delivery, joins in this contention: supine CO progressively increased to the second trimester, and remained unchanged thereafter. Cardiac output was high also in the orthostatic position during the whole q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

Fig. 2. Postural stress effects on cardiac output, systemic vascular resistance and left ventricular end±diastolic volume. Each graph shows the mean (each bar represents 2SE) value of variations of the corresponding parameter in the transition from the supine position to the active standing posture before, during and after pregnancy.

350 R. DEL BENE ET AL.

Fig. 3. Repeatability of measurements of cardiac output, systemic vascular resistance and left ventricular end±diastolic volume obtained in the standing position (from top to bottom): the strict correlation between the data of the ®rst (stage 1) and the last (stage 5) prolonged standing periods is demonstrated in the plots on the left hand side, while the Bland±Altman scatter plots of the mean vs. the difference of the determinations made in the ®rst (stage 1) and the last (stage 5) prolonged standing periods are displayed on the right.

course of pregnancy, or at least until the mid third trimester. This observation sharply contrasts with the results by Easterling et al. 8 who found a signi®cant fall in cardiac output in response to active orthostatic stress at the end of pregnancy. Such a difference may depend on

the timing of cardiovascular and haemodynamic monitoring, probably the end of pregnancy in the study of Easterling et al. 8, a period when the descent of the fetus may hamper more consistently the venous return 22,23 (documented by a reduction in LVEDV in the postural stress). q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

CARDIOVASCULAR FUNCTION IN PREGNANCY: EFFECTS OF POSTURE 351 Table 4. Repeatability of measurements between the two prolonged standing periods. Paired differences Parameter

Heart rate Mean arterial pressure Cardiac output Systemic vascular resistance End-diastolic volume End-systolic volume LV ejection fraction

Mean

0.15 0.41 0.000 7.9 -0.19 -0.09 1.1

On the other hand, it is not conceivable to suppose that the different results are a matter of methodological problems (i.e. estimation of ventricular volumes by means of the Teichholz M±mode method vs Doppler± derived stroke volume technique). Certainly, the Doppler method has been validated in the pregnant woman 24, while the same is not true for the Teichholz method. Nevertheless, both methods have been demonstrated equally valid in estimates of stroke volume in symmetrically contracting ventricles over a wide range of LV sizes, and in epidemiological as well as clinical investigations 25; and the reliability of echocardiographic measurements was demonstrated in several studies 26. Furthermore, repeatability of measurements in the standing posture was con®rmed in our study. In our study, pregnancy modi®ed the haemodynamic response to the postural challenge, in that the physiological reduction in cardiac output associated with active standing was signi®cantly attenuated after the second trimester and was absent at mid third trimester. This went along with a signi®cantly lesser increase in SVR. The reduction in end±diastolic volume (preload) during the orthostatic stress was signi®cantly diminished in late pregnancy, probably because of blood volume expansion 27 and because of reductions in the distensibility and viscoelastic properties of the lower limb veins 28 which improve venous return during standing. Therefore, our data are in keeping with the statement that the elevated preload improves tolerance of the pregnant woman to the orthostatic stress 11. This seems consistent with previous data by Nisell et al. 11, who studied the sympathoadrenal and cardiovascular responses to tilting at the end of pregnancy and after delivery, and found a reduced reactivity of arterial plasma norepinephrine and forearm vasoconstriction to passive tilting. Adaptation of the maternal cardiovascular system to the standing posture was immediate and long lasting, since no differences were found between determinations made after ®ve and 30 minutes of standing. In conclusion, the high cardiac output of pregnancy is maintained in the q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352

95% CI Difference

Bland Altman

Lower

Upper

r

-0.12 0.11 -0.054 -7.6 -0.72 -0.39 -2.9

0.43 0.72 0.053 23.4 0.34 0.22 5.1

0.134 0.002 0.037 -0.004 0.099 0.080 -0.004

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Accepted 30 August 2000

q RCOG 2001 Br J Obstet Gynaecol 108, pp. 344±352