Determining Umbilical Cord Blood Flow

Umbilical cord blood flow is among the most highly desired parameters for monitoring fetal well-being. This is because cord blood flow directly reflects placental volume flow, which is considered to be as important in the fetus as cardiac output and lung perfusion are in adults.1 Yet, presently employed noninvasive methods, such as umbilical artery Doppler waveform analyses, use surrogate flow evaluation parameters, such as systolic/diastolic ratios, which do not directly reflect placental-fetal blood flow.2,3 Volume flow estimation overcomes this by measuring true flow, and it has been shown that volume flow changes in the umbilical vein occur before umbilical artery flow indices become abnormal.4

Yet, the present volume flow measurement method has severe problems limiting its utility. These include technical difficulties in flow measurement in umbilical cords and faulty assumptions employed in the measurement. The present method using spectral Doppler is

                 Q = V × A                    (1),

where Q is volume flow, V is the mean velocity through the Doppler sample volume, and A is the cross-sectional area of the vessel of interest. This formula assumes that the 2D flow profile is cylindrically symmetric with a circular cross-section, and the line of the Doppler sampling cuts perfectly through the center of the sampled vessel. The velocity estimates require angle correction, and if the vessel is tortuous, as in umbilical cords, the sampling position placement and angle correction are hard to perform. Multiple investigators have warned that small errors in volume flow components can result in large errors in the calculation of volume flow.5-7

A new, easy-to-perform volume flow method overcomes almost all of the limitations of the standard technique. The new method is angle independent, flow profile independent, and vessel geometry independent. It works as follows:

Volume flow is defined as the total flux across any surface, S, intersecting the vessel. This is written as

Eq2

where Q is again volume flow, V is the local velocity through each area element dA, and “” is the dot product which projects the local velocity V onto the normal vector for each area element. This is known as Gauss’s theorem. The intersecting surface, known as the “C” surface, is very simple to obtain using 3D ultrasound (Figure8). In order to validate this method, we obtained an AIUM EER-funded research grant.

Fig

Figure: (A) Four-panel view of a single 3D color flow acquisition of the umbilical cord. The four views are as follows: upper-left is axial-lateral, upper-right is axial-elevational, bottom-left is elevational-lateral (ie, the c-surface), and bottom-right is a rendered 3D reconstruction. Arteries are shown in blue and the vein is shown in red. The schematic in (B) illustrates the orientation of the probe and the corresponding c-surface in the elevational-lateral imaging plane. The vessel colors in (B) match the directionality in (A). The entire umbilical cord passes through the c-surface but only the cross-sections of the umbilical arteries and umbilical vein are illustrated in (B). The two arteries are separated in power Doppler (not shown). (Printed with permission from Pinter et al. J Ultrasound Med. 2012;31(12):1927-34. © 2016 by the American Institute of Ultrasound in Medicine)

We had 2 specific aims: 1) Test the reproducibility of the volume flow measurement, and 2) evaluate the relationship of volume flow to clinical outcome in a high-risk patient population.

In the first aim, we performed studies on 35 subjects between the gestational ages of 22–37 weeks, 26 high risk and 9 normal.9 We attempted to measure umbilical cord blood flow at 3 sites in the cord in each subject, and we averaged 28.3 ± 3.3 (mean ± standard deviation) samples per site. We used a GE LOGIQ E9 ultrasound system with a 2.0–8.0 MHz bandwidth convex array transducer to acquire multiple volume 3D color and power mode data sets. Since we were measuring mean blood flow, we assessed variability using relative standard error (standard error /mean) (RSE). The average RSE for blood flow at each cord position was ±5.6% while the average RSE among the measurements in each subject was ±12.1%.

For the second aim, we compared the volume flow measurements in 5 subjects that developed preeclampsia with the 9 normal subjects. Even with these small numbers, we detected a significant difference between the mean depth-corrected, weight-normalized umbilical vein blood volume flows in the two groups (P = .035). Further, blood flow abnormalities were detected either at the same time or preceded the hypertensive disorder in 4 of the 5 subjects. This is consistent with our prior publication where blood flow changes preceded the onset of pre-eclamptic symptoms in a study subject.8

With the introduction of 2D array transducers, umbilical cord volume flow estimates can be performed in seconds and given the valuable information provided by this method, umbilical cord volume flow will hopefully become a standard component of fetal examinations.

 

References:

  1. Tchirikov M, Rybadowski C, Huneke B, Schoder V, Schroder HJ. Umbilical vein blood volume flow rate and umbilical artery pulsatility as ‘venous-arterial index’ in the prediction of neonatal compromise. Ultrasound Obstet Gynecol. 2002;20:580-5.
  2. Newnham JP, Patterson LL, James IR, Diepeveen DA, Reid SE. An evaluation of the efficacy of Doppler flow velocity waveform analysis as a screening test in pregnancy. Am J Obstet Gynecol. 1990;162:403-10.
  3. Acharya G, Wilsgaard T, Bernsten GKR, Maltau JM, Kiserud T. Doppler-derived umbilical artery absolute velocities and their relationship to fetoplacental volume blood flow: a longitudinal study. Ultrasound Obstet Gynecol. 2005;25:444-53.
  4. Rigano S, Bozzo M, Ferrazzi E, Bellotti M, Battaglia FC, Galan HL. Early and persistent reduction in umbilical vein blood flow in the growth-restricted fetus: a longitudinal study. Am J Obstet Gynecol. 2001;185:834-8.
  5. Evans DH. On the measurement of the mean velocity of blood flow over the cardiac cycle using Doppler ultrasound. Ultrasound Med Biol. 1985;11(5):735-41.
  6. Gill R. Measurement of blood flow by ultrasound: accuracy and sources of error. Ultrasound Med Biol. 1985;11:625-41.
  7. Lees C, Albaiges G, Deane C, Parra M, Nicolaides KH. Assessment of umbilical arterial and venous flow using color Doppler. Ultrasound Obstet Gynecol. 1999;14:250-5.
  8. Pinter SZ, Rubin JM, Kripfgans OD, Treadwell MC, Romero VC, Richards MS, Zhang M, Hall AL, Fowlkes JB. Three-dimensional sonographic measurement of blood volume flow in the umbilical cord. J Ultrasound Med. 2012;31(12):1927-34.
  9. Pinter SZ, Kripfgans OD, Treadwell MC, Kneitel AW, Fowlkes JB, Rubin JM. Evaluation of umbilical vein blood volume flow in preeclampsia by angle-independent 3D sonography [published online ahead of print December 15, 2017]. J Ultrasound Med. doi:10.1002/jum.14507.

 

How do you determine umbilical cord blood flow? What problems have you encountered using the traditional method? Comment below or let us know on Twitter: @AIUM_Ultrasound.

Jonathan Rubin, MD, PhD, FAIUM, is Professor Emeritus of Radiology at University of Michigan.

 

Vascular Access for Fiona

Life as a vascular access nurse can be very challenging and diverse in a pediatric hospital. A typical day is fast-paced and includes neonatal, pediatric, and adult patients. Veins may be small, tortuous and often found in unusual locations, eg an extremity or scalp vein. For many patients, imaging tools such as ultrasound are essential for successful placement of IVs, midline catheters, and PICCs. The Vascular Access Team sees patients in both the inpatient and outpatient settings. While many of our procedures are routine, a phone call in February 2017 forever changed the way we view our specialty of vascular access.

The caller on the phone was Amy from the Cincinnati Zoo Marketing Department. She described an urgent clinical situation with Fiona, a 3-week-old premature hippo who was dehydrated and needed IV access. The Zoo staff was desperate as Fiona was not taking any bottles and her IVs were only lasting 8–12 hours. Amy had previous experience with the Vascular Access Team when her daughter had surgery at our institution. She referred to our team as the “Vein Whisperer.” Amy wanted to know if we would be able to use the same tools we used on her daughter to gain IV access with Fiona.pic 8

Fiona was already a star in the eyes of the Cincinnati community. Fiona was born on January 24, 2017, the first premature hippo on record to survive. Fiona was small, around 30 pounds, and was being cared for by a specialized team of experts at the Zoo. Her day-to-day progress was being reported on social media and the local news.

My answer to Amy was, “Of course we can help Fiona!” In my mind, I was thinking of all the things we would need to bring to the Zoo. Supplies included an ultrasound machine, probe cover, ultrasound gel, skin antisepsis, varying sizes and lengths of IV and midline catheters, dressings, etc. I kept thinking…this is a premature hippo, what will we need to insert and maintain the catheter? I asked my colleague Blake to accompany me to the Zoo. Blake is an experienced vascular access nurse and is always up for a challenge! We gathered all our supplies and began our journey to the Zoo.

We arrived in the Hippo Cove area of the Cincinnati Zoo. We met two of the veterinarians who updated us on her condition. Fiona was dehydrated, on oxygen, and extremely weak. They described her condition as critical. We put on special scrubs and removed our shoes. As we were led into the small room where Fiona was, the room temperature was very warm as an effort to maintain Fiona’s body temperature. Fiona was on the floor, laying on a blanket.

Fiona was surrounded by 2–3 Hippo team specialists. Amid their worried looks, they quickly reviewed Fiona’s history, IV access issues, and her inability to take a bottle. Fiona was receiving nutrition through an intermittent naso-gastric tube.

Time was of the essence; we began setting up the 2D ultrasound machine and the necessary supplies. Initially, I scanned her head to assess for any scalp veins, there were no visible veins identified. Blake began scanning her hind leg; she was able to locate a viable vein, about 0.2 cm below the skin. The vein easily compressed and had a straight pathway. Based on her assessment and fluid requirements, we decided to use a 3Fr 8cm midline catheter.

The vein was accessed under ultrasound guidance, using a transverse approach. The midline catheter initially threaded with ease but we were unable to advance it fully. Fluids were connected to the catheter but it only lasted 20 minutes before leaking. The midline catheter was discontinued. Another vein was visualized under ultrasound guidance on the hind leg; the midline catheter was trimmed to 7 cm and threaded with ease. The midline catheter flushed and aspirated with ease.

pic1

 

Due to Fiona’s occasional activity of standing up, we really wanted a secure catheter. The midline catheter was sutured to her skin and a dressing was applied. We discussed the care and maintenance with the veterinary staff, and the decision was made to infuse continuous fluids through her midline catheter to maintain patency.

Over the next 2 days, Fiona gradually began to regain her strength. She began slowly taking her bottles and standing up. Fiona received 5 liters of fluids over 6 days through her midline catheter. The catheter was discontinued on day 6.

Fast forward and now Fiona has celebrated her 1st birthday. She did so with the Hippo team that provided the delicate care that she needed. The Vascular Access Team is so proud to have been part of her care. On that cold February day, we were able to use our 20+ years of experience and knowledge to provide the right catheter under imaging to provide her with the lifesaving fluids she needed.

 

Have you preformed ultrasound in an unusual situation? Tell us your story by commenting below or letting us know on Twitter: @AIUM_Ultrasound.

Darcy Doellman MSN, RN, CRNI, VA-BC, is Clinical Manager of the Vascular Access Team at Cincinnati Children’s Hospital.