INTRODUCTION:

The importance of functional right ventricular failure and resultant splanchnic venous congestion has long been under-appreciated and is difficult to assess by traditional physical examination and standard diagnostic imaging. The recent development of the venous excess ultrasound score (VExUS) and growth of point-of-care ultrasound in the last decade has made for a potentially very useful clinical tool.

The venous side of the circulation has long played second fiddle to the arterial side, who has enjoyed much of the medical fanfare both in acute and chronic disease.

Of course, while arterial blood pressure is indeed critically important, the venous side may be just as critical as its more popular counterpart, and its physiological impact on organ function is vastly more important than commonly thought.

The true perfusion pressure of an organ is in fact not mean arterial pressure (MAP) minus central venous pressure (CVP), but rather precapillary arteriolar pressure minus postcapillary venular pressure. In the former equation, the MAP is generally above 90, while in the latter, the inflow pressure can be in the 35–40 mmHg range (Fig. 1). Hence, the pressure gradient is much narrower, and the impact of raising venous pressure is much greater than commonly thought.

Note the precapillary arteriolar pressure is substantially lower than the large arterial pressures.

Portal vein pulsatility has been described about 30 years ago in the setting of heart failure although it was never incorporated in the comprehensive echocardiographic assessment.

The development of point-of-care ultrasound (POCUS) has brought together acute care clinicians and pathology with a powerful tool able to assess physiology at the bedside. The group of Denault and Beaubien-Souligny have been particularly instrumental in showing the strong link between portal vein pulsatility and acute kidney injury in the subgroup of cardiac surgical patients.

Recently, one group developed the venous excess ultrasound (VExUS) score, which incorporates hepatic venous, intrarenal venous Doppler and inferior vena cava (IVC) assessment to portal vein Doppler, and found the presence of a severe score to be very specific for the prediction of acute kidney injury following cardiac surgery, even more so than portal vein pulsatility alone.

VENOUS CONGESTION AND THE VEXUS GRADING SYSTEM

Systemic venous congestion can be evaluated and quantified by Doppler interrogation of multiple splanchnic organs. The VExUS grading system utilizes Doppler evaluation of the hepatic vein, portal vein and intrarenal venous vein. In the presence of a plethoric inferior vena cava, each of these veins are evaluated and assigned to either being normal, mild congestion, or severe congestion. VExUS grade 0 is with no sign of congestion in any organ, VExUS grade 1 is with only mild congestive findings, VExUS grade 2 is with severe findings in only one organ, and VExUS grade 3 is with severe congestive findings in at least 2 of 3 organ systems.

Hepatic vein Doppler patterns are the result of interactions between venous return-driven by the mean systemic filling pressure (Pmsf) and the right atria and right ventricle. Pmsf is generated by the elastic recoil of the sum total of the venous system. The venous system is highly compliant and acts as a large blood reservoir that can be recruited in times of hypovolemia via an increase in venous tone. Blood volume that occupies the venous compartment without distending the walls of the veins is called unstressed volume, and exerts no influence on venous return. However, any additional volume that distends the walls of the venous system will generate a pressure due to elastic recoil of the veins. The pressure resulting from this recoil, known as the Pmsf, serves as the driving force that generates venous return to the right ventricle.

Forces impeding venous return are the resistance of the venous system and the right atrial pressure that exists due to imperfect efficiency of the right ventricle. During the cardiac cycle, when right atrial pressure exceeds Pmfs, the blood flow will be away from the heart and conversely, flow will be towards the heart when Pmfs exceeds right atrial pressure.

Therefore, normal hepatic Doppler waveforms are characterized by an initial above the baseline wave associated with right atrial contraction, followed by below the baseline flow during systole (due to right atrial relaxation, movement of the tricuspid annulus toward the apex of the heart, and an increase in atrial volume capacity as the ventricles decrease their volume within the non-compliant pericardium), and finally a below the baseline flow wave during diastole as the right atrium becomes a passive conduit for blood to enter the relaxing right ventricle. These waves are known as the A, S, and D waves, respectively, and correspond to the well-known A wave, X descent, and Y descent of the jugular venous pulse. Generally, as average right atrial pressures rise, the A wave becomes more prominent and the S wave will decrease in amplitude relative to the D wave until there is a systolic reversal of flow and fusion with the A wave.

Under normal circumstances, the venous compartment is highly compliant with high capacitance, and therefore, with increasing distance from the heart, the venous pulse is dampened so that in the smaller veins, flow becomes undulating and phasic in nature. However, in states of right ventricular failure or intravascular volume overload, the venous compartment becomes congested and the limits of venous compliance are reached. Under these circumstances, the normal dampening of the venous pulse due to the compliant nature of the smaller veins is lost, and the pulsations are transmitted back into the smaller veins.

The portal vein is separated from the rest of the systemic venous circulation by the hepatic sinusoids that it empties into. The hepatic sinusoids then drain into the hepatic veins that finally drain into the IVC. Due to this distance from the heart, normal portal venous flow is undulating and phasic in nature and blood travels in a hepatopetal direction at about 20 cm/s. As venous congestion rises, the hepatic vein’s retrograde flow generated by atrial contraction is transmitted up through the hepatic sinusoids and into the portal vein where it gives rise to impedance to the portal hepatopetal flow. This causes the normal undulating flow to become progressively pulsatile and this phenomenon is compounded when there is systolic reversal of venous return to the heart. Eventually, the pulsatility becomes significant enough to cause a biphasic or back-and-forth pattern of portal flow.

Intrarenal venous Doppler waveform obtained from the interlobar or arcuate vessels is usually considered adequate when both arterial (above the baseline) and venous (below the baseline) flow signals are seen clearly for two or more cardiac cycles.

The intrarenal venous Doppler signal is normally a continuous monophasic flow below the baseline. With increasing venous congestion, the venous flow becomes pulsatile, then progresses to an interrupted biphasic flow which correlates to the S and the D waves of the hepatic vein’s flow.

Similar to the hepatic vein pattern, as venous congestion further worsens, the S wave becomes smaller and the D wave is more pronounced until the S wave disappears entirely, leaving only a monophasic D wave.

The intrarenal venous Doppler waveform is considered abnormal when there is discontinuous venous flow with either a systolic/diastolic (discontinuous biphasic) pattern or a diastolic only pattern (discontinuous monophasic).

Conclusion:

The VExUS as a window on the venous pathophysiology, might be a key to achieving precise fluid management.

The rapid evolution of knowledge about the VExUS assessment should be of interest to all first-line clinicians involved in daily decision-making about fluid balance management. Moreover, it adds to the growing body of evidence that a modern clinician should be proficient in POCUS to ensure optimal care to their patients.

Ref: Rola, P., Miralles-Aguiar, F., Argaiz, E. et al. Clinical applications of the venous excess ultrasound (VExUS) score: conceptual review and case series. Ultrasound J 13, 32 (2021). https://doi.org/10.1186/s13089-021-00232-8

Solid Organ Doppler Assessment of Venous Congestion