What is a Doppler ultrasound?

Answerfrom Sheldon G. Sheps, M.D.

Doppler ultrasound is a noninvasive test that can be used to measure your blood flow and blood pressure

by bouncing high-frequency sound waves (ultrasound) off circulating red blood cells. A regular ultrasound

uses sound waves to produce images, but can't show blood flow.

A Doppler ultrasound may help diagnose many conditions, including:

Blood clots

Poorly functioning valves in your leg veins, which can cause blood or other fluids to pool in your legs

(venous insufficiency)

Heart valve defects and congenital heart disease

A blocked artery (arterial occlusion)

Decreased blood circulation into your legs (peripheral artery disease)

Bulging arteries (aneurysms)

Narrowing of an artery, such as those in your neck (carotid artery stenosis)

A Doppler ultrasound can estimate how fast blood flows by measuring the rate of change in its pitch

(frequency). During a Doppler ultrasound, a technician trained in ultrasound imaging (sonographer)

presses a small hand-held device (transducer), about the size of a bar of soap, against your skin over the

area of your body being examined, moving from one area to another as necessary. This test may be done

as an alternative to more-invasive procedures such as arteriography and venography, which involve

injecting dye into the blood vessels so that they show up clearly on X-ray images.

A Doppler ultrasound test may also help your doctor check for injuries to your arteries or to monitor

certain treatments to your veins and arteries.

Doppler ultrasoundDoppler ultrasound is a well established technique used to diagnose problems during pregnancy. In the same way that a speed radar measures how fast cars are travelling, Doppler ultrasound can monitor how fast blood is moving in the umbilical blood flow. Professionals can then look to see whether the blood flow is normal, indicating that the fetus is healthy, or abnormal, indicating that the fetus is under stress. The health professionals can then decide which high-risk pregnancies need assistance in delivering the baby, and which women can be left to deliver without assistance.

The aim of using Doppler is to reduce risk to the baby. However, some experts argue that it may prompt some unnecessary early interventions.The review included 18 studies which together included 10,000 women in "high risk" groups. High risk women included those who had previously lost babies during pregnancy, those carrying growth restricted babies and women with hypertension or diabetes. Women who were examined with Doppler ultrasound were compared with those who had no Doppler or with those who had cardiotocography (CTG), which monitors the baby's heartbeat. According to the results, Doppler reduced infant deaths, possibly through better timing of caesarean sections, as well as reducing the number of caesarean sections themselves, and inductions of labour. However, the researchers say the studies included were of questionable quality."A case could certainly be made for a higher quality, multi-centre trial of Doppler ultrasound than we have so far seen," said lead researcher Zarko Alfirevic, who is based at the Division of Perinatal and Reproductive Medicine at the University of Liverpool. "It is quite possible that for some so-called high risk groups fetal Doppler offers little or no benefit. Women with diabetes are one such group where fetal Doppler may, in fact, give false reassurance."It is important to point out, of course, that it is the clinical decision that follows a Doppler ultrasound examination that changes the outcome for the baby, and currently there is little agreement on what intervention should follow an abnormal Doppler finding."

New Placenta Screening For High-Risk PregnanciesScienceDaily (Apr. 2, 2007) — For the first time ever, a team of Toronto researchers are using a combination of ultrasound and blood tests to screen high-risk pregnant mothers for placental damage. By completing these non-invasive tests, most high-risk mothers can be reassured that their placenta is formed and functioning properly, so they can expect a healthy pregnancy.

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Reference

Stillbirth Placenta Maternal bond Nutrition and pregnancy

The tests are done early enough, at 16 to 23 weeks gestation, so if results are abnormal, physicians have time to improve pregnancy outcomes. "Close to 40 per cent of high-risk mothers we see in our clinic experience placental damage," says Dr. John Kingdom, Principal Investigator of the study and Maternal-Fetal Medicine Specialist at Mount Sinai Hospital. The research is among the first to look at placenta health -- a vital life line between mother and fetus through which nutrients, oxygen, antibodies and hormones pass.

"By identifying early on if there is a potential risk of complications, we can do everything possible to ensure the safety of both the mother and fetus," says Dr. Kingdom, who is also a Professor in the Department of Obstetrics and Gynaecology at the University of Toronto. "We can reassure those with normal test results that their placentas are functioning well and they can expect a healthy pregnancy and birth."If the placenta is not functioning properly it could be a potential danger to the health of the mother and fetus. Abnormalities can lead to conditions such as preeclampsia, which is maternal high blood pressure, stillbirth or the need for a pre-term delivery.The screening tests include: a maternal serum screening test used to detect Down's syndrome, which measures the hormone levels in the mother's blood; a uterine artery Doppler blood flow test, which checks the maternal blood flow in the placenta; and an ultrasound of the placental shape. Of the 212 high-risk women in the study, 19 delivered early due to poor fetal growth. None of these women had normal placental function test results. Likewise, only two of 22 stillbirths occurred in women with normal tests, and these losses were not related to abnormal placental function. This data demonstrates that the placenta screening tests can provide a good indication of which women may experience complications during pregnancy."This is an important first step in identifying placental abnormalities in early pregnancy, at a time when a number of interventions can be used to improve outcomes for those with the highest risk" says Dr. Kingdom. "This study will lead the way for future research in placenta screening and help us provide quality care for all mothers."The research will be published in the American Journal of Obstetrics and Gynecology in April 2007.

Doppler Ultrasound Findings in the Hepatic Artery Shortly After Liver Transplantation

1. Ángeles García-Criado 1 , 2. Rosa Gilabert 1 , 3. Annalisa Berzigotti 1  and4. Concepción Brú 1

+Author Affiliations1. 1Department of Radiology, Clinic Hospital of Barcelona, Villarroel 170,

08036 Barcelona, Spain. 

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Abstract

OBJECTIVE. The purpose of this article is to describe the Doppler waveform findings in the hepatic artery in the early posttransplantation period, both in the absence and presence of arterial complications.CONCLUSION. The presence of transient high-resistance Doppler waveforms in normal hepatic arteries is a common finding after grafting. Hepatic artery thrombosis and stenosis, and arterial steal syndromes can be diagnosed by Doppler in the early liver transplantation period.

Doppler sonography

 

hepatic artery

 

liver transplantation

 

sonography

ultrasound

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Introduction

Hepatic artery complications are one of the most frequent causes of morbidity and graft loss in the immediate period after liver transplantation because they can lead to liver graft ischemia [1]. The early detection of these complications is critical to treat them promptly and to reduce the liver damage. A surveillance program based on color Doppler ultrasound (CDUS) in the first days after liver transplantation has proven to be effective for the early diagnosis of hepatic artery complications, and it is now considered a standard of care [2, 3]. However, the interpretation of Doppler findings in the immediate posttransplantation phase may be difficult because the hepatic artery waveform also is commonly altered in the absence of complications [4]. Moreover, the same Doppler findings can be observed in different complications. The aim of this article is to describe the Doppler waveforms of the hepatic artery in the immediate posttransplantation period, both in patients with a normal artery and in those with arterial complications.Previous Section Next Section

Doppler Arterial Findings in the Immediate Posttransplantation Period

The normal hepatic artery shows a low-resistance waveform with continuous diastolic blood flow. The resistive index (RI) is the most commonly used Doppler parameter in hepatic artery evaluation. It allows a semiquantitative estimation of the resistance to arterial flow into the liver and its normal value, both in healthy individuals and those with transplants, and it ranges from 0.55 to 0.80 [5] (Fig. 1).In the first days after liver transplantation, almost half of patients have a transient high RI at the hepatic artery that will return to normal in a few days if there are no complications [4]. According to the degree of resistance, the high RI has been classified by García-Criado et al. [4] into four types: type 1, RI > 0.80 with continuous blood flow in the diastolic phase (Fig. 2); type 2, RI = 1, complete absence of the diastolic signal and preserved systolic velocity (Fig. 3); type 3, absence of diastolic signal and diminished systolic velocity (Fig. 4); and, in severe cases, type 4, undetectable Doppler flow. The last two types are a further progression of the

transient high-resistance flow, but these spectral waveforms are indistinguishable from the arterial hypoperfusion secondary to some arterial complications. Therefore, when a type 3 pattern appears in the immediate postoperative period, it is mandatory to be alert and to perform daily CDUS, suspecting a complication when the waveform does not become normal within 4 days. In these cases, CT angiography (CTA), MR angiography (MRA), or arteriography is indicated. In patients with a type 4 waveform indicating undetectable arterial CDUS flow, CDUS (if possible), CTA, or MRA is indicated to exclude hepatic artery thrombosis. If a patent artery is seen, a daily CDUS examination is mandatory until the flow becomes normal.Early and transient high RI, which has been shown to be related to an older donor and a prolonged period of ischemia, lacks clinical repercussions and long-term prognostic implications [4].Previous Section Next Section

Doppler Findings in Posttransplantation Hepatic Artery Complications

Hepatic Artery ThrombosisEarly hepatic artery thrombosis is the most serious arterial complication after liver transplantation and has an incidence of 5-7% in adults and 11% in children. Because the blood supply to the biliary tree is entirely arterial, abnormal results on liver function tests are often its first manifestation. However, hepatic artery thrombosis can be diagnosed at CDUS in the presymptomatic phase, allowing early reperfusion that obviates retransplantation [6]. Patients with hepatic artery thrombosis who are treated by revascularization before the development of clinical or laboratory alterations have a lower incidence of late biliary complications [2], which emphasizes the importance of performing close Doppler monitoring after liver transplantation.The ultrasound diagnosis of hepatic artery thrombosis is based on the absence of Doppler arterial signal at the hilus as well as in the intrahepatic arterial branches (Figs. 5A, 5B, 5C, 5D, and 5E). A high-resistance flow at the hilus (RI = 1) may be observed if the Doppler waveform is obtained in the main hepatic artery before the thrombus.Occasionally, low arterial flow may provoke a false-positive diagnosis of hepatic artery thrombosis. Contrast-enhanced ultrasound can be useful in these cases because it improves the sensitivity and accuracy of Doppler ultrasound for hepatic artery flow detection. Moreover, contrast-enhanced ultrasound helps to decrease the scanning time [7, 8] (Figs. 5A, 5B, 5C, 5D, and 5E). When contrast-enhanced ultrasound cannot be used, other noninvasive imaging techniques such as MRA or CTA can be performed after Doppler ultrasound and before arteriography.False-negative diagnoses of hepatic artery thrombosis have been described in late phases after grafting when periportal collateral arteries develop at the site of thrombosis. Unfortunately, the collateral flow is often inadequate to allow satisfactory intrahepatic biliary perfusion. To correctly establish the diagnosis, remember that the Doppler signal in the arterial collateral vessels shows a pattern with prolonged systolic acceleration time and low RI [9]. This pattern is nonspecific of hepatic artery thrombosis and can also be found in hepatic artery stenosis.Hepatic Artery StenosisHepatic artery stenosis is a frequent complication after liver transplantation, with an incidence of 4-10% [10]; in severe cases it may cause liver ischemia and graft loss. However, this complication frequently causes a subtle form of graft dysfunction, which delays the diagnosis.Hepatic artery stenosis may be suspected when an intrahepatic Doppler waveform shows a prolonged systolic acceleration time (≥ 0.08 second) and a low RI (< 0.5) [9] (Figs. 6A and 6B). In these cases, a meticulous Doppler study along the course of the main hepatic artery is mandatory because the detection of a focal peak velocity

greater than 2 m/s is diagnostic for hepatic artery stenosis [9] (Fig. 6C). When an increased focal peak systolic velocity is not detected along the course of the hepatic artery, the differential diagnosis must include hepatic artery thrombosis with the development of collateral vessels. In these cases, contrast-enhanced ultrasound examination has been advised, although it does not obviate an angiographic study to establish the diagnosis [11].Pseudoaneurysm of the Hepatic ArteryEven if pseudoaneurysm of the hepatic artery is an infrequent complication after liver transplantation, its potential for rupture and subsequent fatal hemorrhage makes early diagnosis important. The ultrasound diagnosis is based on detection of a predominantly cystic lesion at the hepatic hilus, which fills with color on CDUS and presents an arterial Doppler waveform (Figs. 7A, 7B, and 7C).Arterial Steal Syndromes

The arterial steal syndromes have only recently been recognized as a cause of hepatic hypoperfusion after liver transplantation. These syndromes are characterized by low arterial flow toward the graft caused by a shift of flow into the splenic artery, called splenic artery steal syndrome, the most frequent; or into the gastroduodenal artery, called gastroduodenal artery steal syndrome.

Angiography is mandatory for the diagnosis. The criteria are the presence of an enlarged splenic artery (≥ 4 mm or 150% of the hepatic artery diameter) and dynamic findings in relation to hypoperfusion of the liver [12].Data about the use of Doppler ultrasound in this syndrome are scarce and include nonspecific findings such as loss of hepatic artery flow signal, decrease of hepatic artery flow velocities, or high-resistance waveform with an elevated RI in the main hepatic artery [13]. A total absence of the diastolic phase with low systolic peaks in the hepatic artery seems to be more specific [14]; however, this kind of waveform can also be found in the absence of arterial complications during the first days after liver transplantation [4]. In this latter case, no clinical or laboratory data of hypoperfusion are observed, and, most important, the waveform normalizes spontaneously some days later. In general, an arterial steal syndrome must be suspected when a high arterial resistance flow does not normalize within a few days after liver transplantation.In some arterial steal syndromes, the flow in the intrahepatic artery is scarce and slow and is not detectable on CDUS; in such situations, contrast-enhanced ultrasound can be used to confirm arterial permeability (Figs. 8A, 8B, 8C, 8D, 8E,8F, and 8G).Other ultrasound findings supporting the diagnosis are based on the accepted angiographic criteria, such as the presence of splenomegaly and of a large splenic artery with high blood flow velocity (Fig. 9A, 9B, 9C, and 9D), but no precise data have been reported.

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Fig. 1 —Color Doppler ultrasound study in 57-year-old woman 24 hours after grafting shows patent hepatic artery. Pulsed Doppler ultrasound at intrahepatic level (arrow) shows normal waveform with resistive index of 0.76.

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Fig. 2 —On second day after liver transplantation in 61-year-old woman, Doppler waveform of hepatic artery at hilus (arrow) shows high-resistance flow with presence of diastolic phase (resistive index of 0.88). This is waveform type 1 of García-Criado classification [4].

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Fig. 3 —Absence of diastolic phase with normal systolic phase in Doppler waveform of hepatic artery 24 hours after liver transplantation in 58-year-old man (resistive index = 1), waveform type 2.

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Fig. 4 —Doppler ultrasound of hepatic artery at hilus (arrow) 24 hours after liver transplantation in asymptomatic 44-year-old man shows high-resistance flow in hepatic artery without diastolic phase, as inFigure 3; but in this patient diminished systolic velocity (type 3) is also present.

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Fig. 5A —Ultrasound of liver graft in 56-year-old man with hepatic artery thrombosis. Color Doppler ultrasound shows absence of flow in hepatic artery at hilus (arrow).

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Fig. 5B —Ultrasound of liver graft in 56-year-old man with hepatic artery thrombosis. No arterial flow is detected on pulsed Doppler ultrasound.

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Fig. 5C —Ultrasound of liver graft in 56-year-old man with hepatic artery thrombosis. Contrast-enhanced ultrasound reveals no arterial perfusion in early phase at hilus level (arrow) nor at intrahepatic level.

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Fig. 5D —Ultrasound of liver graft in 56-year-old man with hepatic artery thrombosis. Later phase after contrast injection shows normal portal perfusion but no flow in hepatic artery.

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Fig. 5E —Ultrasound of liver graft in 56-year-old man with hepatic artery thrombosis. Thrombosis of hepatic artery is confirmed at angiography.

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Fig. 6A —Hepatic artery stenosis in 39-year-old man. Doppler ultrasound of hepatic artery at intrahepatic level shows prolonged acceleration time of 0.153 second.

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Fig. 6B —Hepatic artery stenosis in 39-year-old man. Flow of intrahepatic artery shows diminished pulsatility and small difference between systolic and diastolic velocities that result in low resistive index of 0.40.

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Fig. 6C —Hepatic artery stenosis in 39-year-old man. Pulsed Doppler ultrasound shows elevation of blood flow velocity (2.99 m/s) at stenotic level.

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Fig. 6D —Hepatic artery stenosis in 39-year-old man. Arteriography confirmed hepatic artery stenosis.

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Fig. 7A —Pseudoaneurysm of hepatic artery in 60-year-old woman after liver transplantation. B-mode ultrasound reveals small fluid-liquid collection of 2 × 0.9 cm (arrow) at hilus.

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Fig. 7B —Pseudoaneurysm of hepatic artery in 60-year-old woman after liver transplantation. Color Doppler ultrasound shows complete filling of collection with turbulent flow (arrow). Note situation of collection above main portal vein (arrowhead), which is usual location of hepatic artery.

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Fig. 7C —Pseudoaneurysm of hepatic artery in 60-year-old woman after liver transplantation. Arteriography confirms diagnosis of pseudoaneurysm.

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Fig. 8A —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. Color Doppler ultrasound does not detect hepatic artery.

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Fig. 8B —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. No arterial flow is identified on pulsed Doppler ultrasound in usual location of hepatic artery.

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Fig. 8C —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. Contrast-enhanced ultrasound (SonoVue) shows patent hepatic artery (arrow). Note that filling of hepatic artery is delayed; also note simultaneous filling of portal vein.

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Fig. 8D —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. After contrast administration, hepatic artery flow is detected on pulsed Doppler ultrasound.

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Fig. 8E —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. Arteriography reveals sluggish flow at hepatic artery (arrow)

associated with early and intense filling of splenic artery (arrowhead), which is enlarged.

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Fig. 8F —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. Selective angiography of hepatic artery shows normal vessel (arrow).

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Fig. 8G —Splenic artery steal syndrome in 53-year-old man 4 days after liver transplantation. After surgical occlusion of splenic artery, hepatic arterial flow is normalized.

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Fig. 9A —Spleen ultrasound findings in 55-year-old woman with splenic arterial steal syndrome. B-mode ultrasound shows splenomegaly.

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Fig. 9B —Spleen ultrasound findings in 55-year-old woman with splenic arterial steal syndrome. Enlarged splenic artery in its entire course is seen on B-mode ultrasound (arrow). Figure shows splenic artery near its origin. Note location of aorta (arrowhead) and mesenteric artery (double arrowhead).

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Fig. 9C —Spleen ultrasound findings in 55-year-old woman with splenic arterial steal syndrome. Pulsed Doppler ultrasound shows high blood flow velocity in splenic artery at its origin (C) and at splenic hilus (D), with maximum velocities of 2 m/s for entire course.

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Fig. 9D —Spleen ultrasound findings in 55-year-old woman with splenic arterial steal syndrome. Pulsed Doppler ultrasound shows high blood flow velocity in splenic artery at its origin (C) and at splenic hilus (D), with maximum velocities of 2 m/s for entire course.

The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual.[75] This plateau is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.

The cardiac action potential plays an important role in coordinating the contraction of the heart.[75] The cardiac cells of the sinoatrial node provide the pacemaker potentialthat synchronizes the heart. The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles. Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers.[note 1] Conversely, anomalies in the cardiac action potential—whether due to a congenital mutation or injury—can lead to human pathologies, especially arrhythmias.[75] Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, andverapamil.[76]

Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.