Positioning the parturient from supine to the left lateral tilt position (supine-to-tilt) may not effectively displace the gravid uterus, but turning from the left lateral position to the left lateral tilt position (left lateral-to-tilt) may keep the gravid uterus displaced and prevent aortocaval compression. Fifty-one full-term parturients were randomly placed in the left lateral position, supine-to-tilt and left lateral-to-tilt positions using a Crawford wedge. Femoral vein area, femoral vein velocity, femoral artery area, pulsatility index, resistance index and right arm mean arterial blood pressure and heart rate were recorded. Our results showed a lower mean (SD) femoral vein area (82.2 (14.9) vs 96.2 (16.4) mm2), a lower pulsatility index (3.83 (1.3) vs 5.8 (2.2)), a lower resistance index (0.93 (0.06) vs 0.98 (0.57)), a higher femoral artery area (33.3 (3.8) vs 30.9 (4.4) mm2) and a higher femoral vein velocity (7.9 (1.2) vs 6.1 (1.6) cm.s−1) with left lateral-to-tilt when compared with supine-to-tilt (all p < 0.001). Our results suggest that moving a full-term parturient from the full left lateral to the lateral tilt position may prevent aortocaval compression in full-term parturients more efficiently than when positioning the parturient from a supine to left lateral tilt position.
Aortocaval compression is considered one of the key factors responsible for the development of supine hypotension in pregnant women at full term. The enlarged uterus compresses the inferior vena cava and partially obstructs the lower part of the aorta; this decreases venous return to the heart and blood flow to the pelvic organs and lower extremities. It is found that without preventive measures, the incidence of hypotension may be as high as 82%, and there is evidence that this may result in fetal compromise [1–4].
To alleviate aortocaval compression, a 15° left lateral tilt position is used as routine practice, but this amount of tilt has been found in certain circumstances to be inadequate in displacing the gravid uterus, and indeed it has been shown that aortocaval compression may occur despite a lateral tilt of up to 34° [5, 6]. The reason that aortocaval compression is not relieved despite a lateral tilt of 34° is thought to be because the gravid uterus is relatively immobile; it therefore sags over the abdominal contents and may not be easily displaced by a lateral tilt. On the contrary, the displaced uterus in the full left lateral position is likely to remain displaced when the parturient is placed in the left lateral position first, then moved to the left lateral tilt position. This study was designed to compare the effect of positioning the mother from the full left lateral position to the left lateral tilt position (left lateral-to-tilt), with moving from the supine position to the left lateral tilt position (supine-to-tilt), and to investigate whether this prevented aortocaval compression more efficiently in healthy parturients at full term.
After written informed consent, 68 full term singleton parturients of ASA physical status 1-2 were recruited into the study. The study was approved by the institution’s ethics committee for human studies. Parturients with a history of hypertension, diabetes, pre-eclampsia, heart disease, placenta praevia, obesity (body mass index > 30 kg.m−2), or a haemoglobin concentration of less than 7 g.dl−1 were excluded from the study.
All the parturients were subjected to continuous colour and pulse wave Doppler studies in three different positions. Simultaneous non-invasive blood pressure (right arm) and heart rate readings were also taken. The Doppler studies were performed using 3–7 mHz linear transducer (SSA, 340 A; Toshiba, Tokyo, Japan). Femoral vessels on both sides were scanned in a transverse plane (B mode), and a regular segment of the femoral vessels was identified, just below the inguinal ligament, before the drainage of the great saphenous vein and the diameters (transverse and antero-posterior diameter) of the vessels were recorded. The femoral artery and vein sites were marked with a marker pen aligning them with the centre of the probe to ensure consistent placement of the ultrasound probe. All measurements were made below the division of the superficial and deep femoral artery, the outline of which was marked on the skin surface to ensure uniform measurements. The artery and vein were distinguished by applying external pressure using the ultrasound probe on the vessel, and venous compression was observed on the ultrasound image. Luminal diameter was measured using the internal ultrasound software. To ensure consistency of the measurement site, diameter measurements were recorded at the centre of the ultrasound probe and at the centre of the ultrasound image. Systolic and diastolic inner diameters of the femoral artery were measured using the M mode technique in the longitudinal scan.
The probe was then turned to 90° to display a longitudinal view of the vessel and to measure velocities. With the femoral artery, the best of the waveforms were obtained over 3–4 cardiac cycles and the velocities (peak systolic velocity, mean velocity, end-diastolic velocity, pulsatility index and resistance index) were recorded by using spectral Doppler ultrasound. In the femoral vein, maximum velocity and the presence of phasicity (changes during the Valsalva maneouvre and respiration) were recorded.
Parturients were randomly positioned to the left lateral, supine-to-tilt or left lateral-to-tilt positions for recording the parameters, using a sealed envelope technique. The radiologist performing the Doppler studies was unaware whether the parturient had been positioned in a left lateral-to-tilt position or a supine-to-tilt position. An interval period of 10 min was allowed between each position. The left lateral tilt position was achieved using Crawford’s wedge (length = 55 × 30 × 10 cm) under the right pelvis .
Statistical Package for Social Sciences (SPSS version 16; Chicago, IL, USA) was used to analyse data. Continuous data were analysed with repeated measures ANOVA and post-hoc Bonferroni’s correction was applied for multiple comparisons. A p value of < 0.05 was considered significant. Data collection (combination of pulsatility and resistive index) according to a pilot study indicated that the difference in the response of matched pairs was normally distributed with SD 2.6. The true difference in the mean response of matched pairs was 1.2, and 51 pairs of subjects were required to be able to reject the null hypothesis that this response difference is zero with power 0.9. The type-1 error probability associated with this test of this null hypothesis was 0.05.
Of the 51 parturients recruited for the study, 10 were primagravida and the remaining 41 were multiparous. The mean (SD) age of the parturients included in the study was 25 (3.4) years and the gestational age was between 38 and 40 weeks. The mean (SD) weight and height were 52.8 (5.1) kg and 153.6 (4) cm, respectively.
There was no clinically significant difference in heart rate and mean arterial pressure in all three positions, even though a statistically significant difference was observed (Table 1). There was a significant increase in femoral vein area in the supine-to-tilt position, compared with the left lateral position and the left lateral-to-tilt position (Table 1, Fig. 1). Maximum femoral vein velocity decreased in the supine-to-tilt and left lateral-to-tilt positions compared with the left lateral position (Table 1, Fig. 1). Femoral artery area decreased significantly in the supine-to-tilt and left lateral-to-tilt positions compared with the left lateral position; however, the decrease was significantly greater in the supine-to-tilt position compared with the left lateral-to-tilt position (Table 1, Fig. 1). Similarly, as a result of higher resistance to forward arterial flow, the pulsatility index and resistance index were significantly higher in the supine-to-tilt position compared with the left lateral and left lateral-to-tilt positions (Table 1, Fig. 1).
Table 1. Haemodynamic and Doppler parameters in the left lateral, supine-to-tilt and left lateral-to-tilt positions in 51 parturients. Values are mean (SD).
*p value between left lateral and supine-to-tilt position.
†p value between left lateral-to-tilt and supine-to-tilt position.
‡p value between left lateral and left recumbent-to-tilt position.
Heart rate; beats.min−1
*p < 0.001 †p < 0.001 ‡p < 0.001
Mean arterial pressure; mmHg
*p < 0.001 †p < 0.001 ‡p = 0.496
Femoral vein area; mm2
*p < 0.001 †p < 0.001 ‡p = 1.000
Femoral vein velocity; cm.s−1
*p < 0.001 †p < 0.001 ‡p < 0.001
Femoral artery area; mm2
*p < 0.001 †p < 0.001 ‡p < 0.001
*p < 0.001 †p < 0.001 ‡p = 1.000
*p < 0.001 †p < 0.001 ‡p = 0.385
Gravity is often used to displace the uterus laterally. This is done either by placing a wedge under the woman’s hip or by tilting the operating table. Lateral tilt might help decrease the degree of aortocaval compression, but does not ensure its absence. This aortocaval compression reduces maternal cardiac output [1–6], an event that is often concealed, because only 10% of pregnant women will exhibit supine hypotension [8–12].
Our study has demonstrated that aortocaval compression may be prevented by using the left lateral-to-tilt position and not by the supine-to-tilt position. The degree of tilt required to alleviate aortocaval compression varies from parturient to parturient, and a 15° tilt from supine may not be sufficient to achieve this. The gravid uterus is relatively immobile and is not easily displaced by a tilt that is comfortable to the patient or to the operating surgeon . Aortic compression may still be present even despite a tilt of 34° , but at a 30° tilt, the subject may slide off the inclined plane . On the contrary, the displaced uterus in the left lateral position is likely to remain displaced when the patient is turned back into left lateral-to-tilt position, as demonstrated by the current study. This was apparent from the mean arterial pressure measurements, which were significantly lower when the parturients were moved from the supine-to-tilt position, than when they were moved from the left lateral-to-tilt position. Perhaps consequent to the fall in cardiac output, there was a compensatory higher heart rate in the supine-to-tilt position, compared with the left lateral-to-tilt position. Doppler parameters also suggested that the supine-to-tilt position predisposed the parturients to significant aortocaval compression, compared with the left lateral-to-tilt position. Femoral vein area and femoral vein velocity measured in the left lateral-to-tilt position and the lateral position, respectively, demonstrated absence of inferior vena caval obstruction, and did not reveal any difference between the two positions. However, left lateral-to-tilt did not relieve the aortic compression completely and the partial aortic obstruction observed in the left lateral-to-tilt position was significantly less than that seen in the supine-to-tilt position. Kinsella et al.  demonstrated that there is a 40% reduction in blood flow in the supine position, and neither right nor left pelvic tilt was associated with a significant change in leg blood flow or maternal heart rate compared with the supine position, which is consistent with our results.
Several investigators have used Doppler ultrasound for blood flow examination in the femoral veins during pregnancy [14–16], and its reliability was demonstrated by Peiffer et al.  to detect the state of uteroplacental and fetal circulation. Pulsatility index (S−D/M), resistive index (S-D/S) and systolic diastolic flow ratio (S/D) are commonly used indices that are derived from blood flow velocity waveforms where S = highest systolic velocity, D = end-diastolic velocity and M = mean velocity over one cardiac cycle. Decreased uteroplacental flow is depicted by an increase in ratio because of increased downstream impedance to flow. These Doppler indices associate well with changes in downstream impedance, if central haemodynamics remain stable. However, peripheral resistance is pertinent only to steady non-pulsatile flow conditions. Doppler indices have high intra-observer reproducibility and have been used in many studies related to uterine and fetal haemodynamics. It has also been demonstrated that vascular impedance spectra derived from blood flow velocity with electromagnetic flow measurements are similar at rest and vasodilatation [18, 19]. Demorais and Johnston  have shown that analysis of Doppler recordings is a simple, non-invasive method to evaluate the flow in the femoral artery and is potentially useful for determining if a haemodynamically significant lesion is present in the aorto-iliac segment. Pulsatility index in particular has a close correlation with peripheral arterial resistance [20, 21]. A rise in pulsatility index occurs if the proximal obstruction is < 85% and a drop is seen if the obstruction is > 85% . None of the parturients in the left lateral-to-tilt or supine-to-tilt positions exhibited a drop in pulsatility index compared with the left lateral position. However, pulsatility index was seen to increase in the supine-to-tilt position. In an experimental model, resistance index also changed in a similar way to pulsatility index, with alterations in vascular resistance . In our study, because of partial proximal aortic obstruction, the pulsatility index increase was accompanied by an increase in the resistance index subsequent to a compensatory increase in peripheral vascular resistance, probably as a result of decreased lower limb blood flow . Earlier studies have documented that a decrease in peripheral vascular resistance due to epidural anaesthesia is associated with an increase in peak systolic and end-diastolic flow velocities and a reversal of post-systolic flow velocity to a positive forward flow [22, 24].
To conclude, positioning a full-term parturient from the left lateral position to the lateral tilt position is as efficient as keeping the patient in the full left lateral position for preventing aortocaval compression.
No external funding or competing interests declared.