REPORTS OF ORIGINAL INVESTIGATION • Ultrasound Evaluation of Inferior Vena Cava Compression in Tilted and Supine term Parturients

BACKGROUND

One of the most common surgeries worldwide – cesarean delivery (CD) – accounts for 31.7% of births in the United States and 29.4% of births in Canada in 2019. Maternal hypotension is a frequent adverse event during CD under spinal anesthesia (SA) and can be detrimental to the fetus and mother. The incidence of hypotension can be decreased by positioning, fluids, and pharmacologic interventions.

IVC compression by the gravid uterus is a major contributor to parturient’s hypotension when placed supine and may cause maternal hypotension and fetal compromise. Maternal hypotension may be exacerbated by neuraxial anesthesia-induced sympathetic blockade.

Large variations of IVC diameter using ultrasound during expiration and inspiration have been shown to be related to hypotension in non-pregnant patients. The inferior vena cava collapsibility index (IVCCI) is a measure of the difference between the maximal and minimal diameter of the inferior vena cava (IVC) with respiration and may reflect the impact of IVC compression by the gravid uterus on the perceived preload.

This study was conducted to explore the practicality of point-of-care IVCCI measurements in parturients in the supine and the 15° tilted positions. In addition, the study was intended to investigate the effect of a spinal anesthetic with a phenylephrine infusion on IVCCI variations with different patient positions. The authors looked at the clinical correlation between the pre-spinal IVCCI measurements and phenylephrine requirements. They hypothesized that the IVCCI as measured by ultrasound will be lessened in the 15° tilt position compared with the supine position in third trimester parturients undergoing CD before and after SA with a phenylephrine infusion.

METHODS

This was a prospective exploratory study of parturients of American Society of Anesthesiologists Physical Status II with singleton fetuses at 37–42-week gestation scheduled for elective CD under SA.

Exclusion criteria included any cardiac pathology such as valvulopathy, cardiomyopathy, or congenital disease, active labor, emergency CD, morbid obesity (body mass index [BMI] > 40 kg·m^-2), fetal abnormalities or prematurity, multiple gestation, inability to cooperate, contraindication or patient’s refusal for SA, pre-eclampsia, and pathologies requiring strong hemodynamic support or anti-hypertensive medications.

Standard monitors were placed on the patients after arrival to the operating theater.  Ultrasound images of the IVC were obtained using a cardiac transducer with an intercostal IVC view with M-mode recording. Each image was analyzed to identify the smallest IVC diameter (during inspiration) and the largest IVC diameter (during expiration). Each patient was placed for two minutes in each position before obtaining ultrasound images. The 15° tilted position was achieved by verifying the tilt of the operating table with a digital inclinometer (iPhone Measure application).

Before SA, the images in supine (position #1) and in the 15° tilted position (position #2) were taken and saved. The anesthesia technique was standardized. SA was performed in the sitting position at the lumbar level (L2–L5) with a 25-G Whitacre needle. The solution injected consisted of 10.5 mg of 0.75% bupivacaine, 15 µg of fentanyl, and 150 µg of morphine. The patient was then immediately placed in supine with a phenylephrine infusion at 0.5 µg･kg^-1･min^-1 which was begun after injection of the spinal anesthetic to avoid hypotension. Hypotension was defined as a decrease of the mean arterial blood pressure below 80% of baseline values based on two consecutive blood pressure values. A 1.5 µg･kg^-1 phenylephrine bolus (lean body weight) was administered if the patient presented hypotension or a nausea or vomiting episode. If the heart rate decreased under 50 beats･min^-1 while the arterial pressure remained within 20% of baseline values, a 0.2 mg iv glycopyrrolate bolus was administered. If the heart rate decreased under 50 beats･min^-1 while the mean arterial pressure decreased under 80% of baseline values, a 5 mg iv ephedrine bolus was administered. If the mean arterial blood pressure increased above 120% of baseline values for two consecutive blood pressure values, the phenylephrine infusion was diminished by 25%.

Ten minutes following intrathecal injection, with confirmation of a sensory block at T4 determined with ice, ultrasound images of the IVC after SA were obtained in the supine (#3) and tilted positions (#4) as described above.

The mean value of the three minimum and maximum IVC diameters was used to calculate the IVCCI for each of the four positions. The IVCCI was calculated as follows: IVCCI (%) = 100 x (maximal IVC diameter – minimal IVC diameter) / maximal IVC diameter.

The primary outcome of the study was to compare the supine position with a 15° tilt position using IVCCI measurements by ultrasound for term parturients scheduled for CD prior to SA.

Secondary outcomes included comparing the IVCCI measurements after SA in the same two positions. In addition, the authors studied the relationship between the pre-spinal IVCCI measurements (supine and the difference between supine and tilt) and the phenylephrine requirements during the surgery.

RESULTS

Out of 32 patients who were screened, 20 completed the study. The mean age of patients was 34 (5) yr, the mean gestational age was 39.1 (0.9) weeks, and the mean BMI was 32.1 (5.0) kg·m^-2. Position was a significant source of variation (7.1%; P = 0.005), contrary to the SA (0.7%; P =0.36) or the interaction between the position and the SA (0.1%; P = 0.70).

Post-hoc analysis to correct for multiple comparisons showed that the mean (SD) IVCCI (%) before SA was higher in the supine 19.5 (8.0) than in the tilted 15.0 (6.4) position (mean difference, 4.5; P = 0.04). After SA, there was no significant difference between IVCCI (%) in the supine 17.8 (8.3) and tilted 14.2 (6.9) position (mean difference, 3.5; P =0.13).

The mean (SD) phenylephrine dose administered from the time of SA until the end of the operation was 2.1 (0.7) mg. There was no correlation between the pre-spinal supine IVCCI and the quantity of phenylephrine used during the surgery, nor between the pre-spinal IVCCI supine vs tilt difference and the quantity of phenylephrine used during the surgery.

CONCLUSIONS

The authors concluded that in pregnant women before SA, the IVCCI measured using bedside ultrasound imaging was significantly lower in the 15° tilt position than in the supine position. This effect, however, was not significant after SA with a phenylephrine infusion.

They demonstrated that ultrasound imaging of the IVC is easy to acquire and can detect IVCCI changes in the supine versus tilted position in term parturients.

They commented on the fact that Phenylephrine may influence the IVCCI as previously seen in previous studies evaluating cardiac index and systolic ejection volume have shown that the effect of phenylephrine may vary with preload. Specifically, a phenylephrine bolus in preload independent patients likely increase venous return through venous and splanchnic vasoconstriction.

Therefore, they suggest that the phenylephrine infusion initiated right after SA might have negated the changes in IVCCI that existed prior to SA between positions (supine vs tilted) by increasing venous return in those participants who received a preload bolus of colloid.