Liver Note

Blood Flow Measurements in Orthotopic Liver Surgery

Intraoperative Blood Flow Measurements

Courtesy of J. Michael Henderson, M.D., F.A.C.S., The Cleveland Clinic Foundation, Cleveland, Ohio

Introduction

Abnormal hepatic hemodynamics and physiology in the transplanted liver pose continuing challenges for the surgeon. A practical method for measuring two of these hemodynamic parameters, portal venous and hepatic arterial flows, is by intraoperative flow measurements. Transit-time ultrasound technology is well suited to measure these flows. Flowprobes are easily applied and do not have to be applied tightly to vessels; they simply encompasss the vessel.

Why Measure Flow?

Liver transplantation offers a unique opportunity for intraoperative blood flow measurement. Since the blood supply to the liver originates from two sources, the portal vein and the hepatic artery, a red color indicating perfusion of the liver following transplant does not guarantee that the two conduits, the portal vein and the hepatic artery are both functional. Transit-time ultrasound flowmetry assures this functionality. Other reasons include:

Donor/Recipient Graft Mismatch - LDLT
Troisis, et al² stress that in living donor liver transplant (LDLT), graft size mismatch is a major concern because hyperperfusion to the newly grafted liver may lead to graft dysfunction and poor survival. They advocate flow measurement during liver transplant surgery in order to determine if there is a donor/recipient graft mismatch and to decide whether measures should be taken to remedy the mismatch.
Check the Quality of the Hepatic Artery Anastomosis and Its Functionality
Although the hepatic artery only supplies 20% of the blood to the liver, its blood is oxygenated and it is the sole contributor to the biliary tree in the liver. For successful transplantation, it is critical for the hepatic artery to remain functional after transplant. Although microsurgical techniques have reduced the incidence of hepatic artery thrombosis (HAT), intraoperative blood flow measurement of the hepatic artery offers a functionality test of the hepatic artery before closure.
Check for Kinking of the Portal Vein
A technical problem that Starzl cites in his Atlas of Transplant Anatomy¹ is the importance of the a snug fitting portal vein over the liver. It is crucial to correctly size the portal vein for anastomosis to the recipient portal to prevent kinking of the portal when the liver is retracted into place in the abdominal cavity. As in CABG surgery, intraoperative blood flow measurement would detect any such kinking of the vein so that it could be immediately corrected.

Surgical Approach

Measurement of portal venous and hepatic arterial flows can be easily done at the completion of orthotopic liver transplantation using Transonic flowprobes. Following completion of the vascular anastomoses, the new liver is reperfused, and hemostasis achieved. Prior to biliary reconstruction, the flowprobes are placed on the reconstructed portal vein and hepatic artery.

The probes are chosen to comfortably encompass - but not constrict - the vessels, and are placed such that extraneous tissue is excluded. The field is then immersed in saline which serves as a good acoustic contact with the vessels. Readings stabilize rapidly, usually within 1-2 minutes, and in stable patients fluctuate less than ± 10% when left in situ for 10-15 minutes. If there is wider fluctuation, this usually indicates improper positioning of the flowprobes with poor alignment or extraneous tissue, and can normally be corrected by repositioning.

Discussion

Combined portal venous and hepatic artery flow are usually between 15 - 25% of cardiac output. Of clinical importance is hepatic artery patency and flow, as survival of the graft depends on this. Flowprobes provide a volumetric measure of hepatic artery flow, and when this is low can be used to determine if there is a fixed anatomic limitation to flow or a physiologic limitation.

For example, in a patient with a cardiac output of 10 L/min, portal flow of 2000 ml/min and hepatic artery flow of 75 ml/min, reduction of portal flow to 1000 ml/min resulted in a hepatic artery flow increase to 125 ml/min. Thus, the low basal hepatic artery flow results from a high physiologic resistance rather than a fixed, potentially surgically correctable low inflow. This kind of data can be collected on a strip chart recorder for permanent record.

The information obtained with these transit-time ultrasound flowprobes is often at variance with "clinical impression." Accurate information on volumetric flow at the time of operation can either be reassuring, or may indicate an unexpected problem which can be fixed at this time. In a procedure such as liver transplant, where the stakes are high, this technology can be a useful adjunct in operative decisions.

Hepatic Hemodynamics: Transplanted Liver (34 patients)1

Mean ± SD (ml/min)

Range (ml/min)

Total Liver Blood Flow

2,091 ± 932

570 - 4,540

Portal Vein Blood Flow

1,808 ± 929

300 - 4,500

Hepatic Artery Blood Flow

268 ± 157

30 - 675

Recommended Flowprobes

Conduit

Recommended probe size (mm)

Hepatic artery

4 - 6

Portal vein

10-12

References

Stieber, AC, Makowka, L., Starzl, T.E., “Orthotopic Liver Transplantation,” in Starzl, T.E., Shapiro, R., and Simmons R.L. (eds), Atlas of Organ Transplantation, Gower Medical
Publishing, New York, 1991.
Troisis, R., Hemptinne, B., Clinical Relevance of Adapting Portal Vein Flow in Living Donor Liver Transplantation in Adult
Patients,” Liver Transplantation 2004 9(9) S36-41.
Henderson, J. M., Gilmore, G.T., Mackay, G.J., Galloway, J.R., Dobson, T., Kutner, M.H., "Hemodynamics During Liver Transplantation: The Interaction between Cardiac Output and Portal Venous and Hepatic Arterial Flows," Hepatology 16: 715-718, 1992.
Henderson, J. M., Mackay, G.J., Kutner, M.H., Noe, B., "Volumetric and Functional Liver Blood Flow Are Both Increased in the Human Transplanted Liver," Journal of Hepatology,17: 204-207, 1993.