1. Correct Answer: A. The size of the kidney is normal.

Rationale: The maximal longitudinal length of the kidney is generally measured between 9 and 12 cm in an average-sized adult, and depends on age and body size. A kidney length smaller than 10 cm before 60 years of age, or smaller than 9 cm at any age, is uncommon and highly suggestive of renal atrophy. The maximal kidney length is generally reached around 18 years of age, followed by a very small reduction during adulthood. Due to loss of parenchyma, the size of the kidney tends to decrease after 50 years of age. Renal atrophy is highly suggestive of underlying chronic kidney disease. However, diabetic nephropathy can present with normal kidney size, even in the presence of significant chronic kidney disease. A hypertrophied kidney can be due to infiltrative disease, polycystic disease, pyelonephritis, or multiple myeloma.

1. Beaubien-Souligny W, Denault A, Robillard P, Desjardins G. The role of point-of-care ultrasound monitoring in cardiac surgical patients with acute kidney injury. J Cardiothorac Vasc Anesth. 2019;33(10):2781-2796.

2. O’Neill WC. Sonographic evaluation of renal failure. Am J Kidney Dis. 2000;35(6):1021-1038.

2. Correct Answer: B. Venous: away, arterial: toward

Rationale: Assessment of the kidney is accomplished with a curvilinear or a phased array probe. A large curvilinear abdominal ultrasound probe (2-5 MHz) is preferred, due to improved tissue penetration of a wider field of view, allowing for a complete view of the kidney for measurements. A phased array probe (2-10 MHz), commonly used for echocardiography, can also be used for abdominal ultrasound. Its smaller footprint compared to the curvilinear abdominal ultrasound probe makes it easier to obtain images from smaller acoustic windows, such as within an intercostal space.

In order to interrogate renal blood flow, a probe should be positioned in the posterior axillary line to obtain a longitudinal view of the kidney. Pulsed-wave Doppler is used, after the interlobar vessels are localized (usually with color Doppler, using a reduced Nyquist limit [10-25 cm/s]). It is important to adjust the sample volume, to use the low-velocity filter in order to avoid low-velocity signals, and to use a low pulse repetition frequency of 1 to 1.5 kHz to avoid aliasing. Using pulsed-wave Doppler interrogation, renal arterial flow will be displayed in the positive velocity range, directed toward the renal cortex and toward the ultrasound probe, whereas renal venous flow will be displayed in the negative velocity range, directed away from the renal cortex and away from the ultrasound probe (Figure 62.2).

1. Beaubien-Souligny W, Benkreira A, Robillard P, et al. Alterations in portal vein flow and intrarenal venous flow are associated with acute kidney injury after cardiac surgery: a prospective Observational Cohort Study. J Am Heart Assoc. 2018;7(19):e009961.

2. Beaubien-Souligny W, Denault A, Robillard P, Desjardins G. The role of point-of-care ultrasound monitoring in cardiac surgical patients with acute kidney injury. J Cardiothorac Vasc Anesth. 2019;33(10):2781-2796.

3. Correct Answer: A. (peak systolic velocity – end-diastolic velocity)/peak systolic velocity

Rationale: The RRI, or Pourcelot index, can be calculated from the maximal velocity (Vmax) occurring during systole and the minimal velocity (Vmin) occurring during diastole. An RRI between 0.50 and 0.70 is considered normal.

The RRI increases in clinical situations characterized by renal vasoconstriction or increased intracapsular pressure such as acute tubular necrosis, hepatorenal syndrome, intra-abdominal compartment syndrome, venous congestion, acute rejection in a renal transplant, or urinary obstruction. A high RRI (> 0.7) is an early marker of acute kidney injury and predicts progression toward severe stages of severe acute injury.

1. Beaubien-Souligny W, Denault A, Robillard P, Desjardins G. The role of point-of-care ultrasound monitoring in cardiac surgical patients with acute kidney injury. J Cardiothorac Vasc Anesth. 2019;33(10):2781-2796.

2. Granata A, Zanoli L, Clementi S, Fatuzzo P, Di Nicolo P, Fiorini F. Resistive intrarenal index: myth or reality? Br J Radiol. 2014;87(1038):20140004.

3. Tang WH, Kitai T. Intrarenal venous flow: a window into the congestive kidney failure phenotype of heart failure? JACC Heart Fail. 2016;4(8):683-686.

4. Correct Answer: B. Aortic insufficiency

Rationale: The RRI is calculated as the ratio of renal end-systolic (ES) minus end-diastolic (ED) velocities over end-systolic velocity (ES) (Figure 62.3). Hemodynamic alterations that increase arterial RRI occur with either an increase in systolic velocity or a decrease in the diastolic velocity, which therefore increases the renal pulse pressure. Thus, renal artery stenosis is associated with a decreased RRI, generally < 0.6, as a result of lower pulse pressure because of the reduced peak systolic velocity distal to the stenosis. Under normal conditions, bradycardia and aortic insufficiency should increase stroke volume, pulse pressure, and systolic velocities, and therefore increase the RRI. It is important to remember that the RRI is measured at the level of the intrarenal arteries (segmental or interlobar). Any condition associated with reduced diastolic arterial pressure can also increase the resistance index. If two completely different arterial Doppler sites (hepatic artery, splenic artery, or middle cerebral artery for instance) have similar resistance index, it is unlikely that there is a renal problem but a more global one, such as aortic insufficiency.

1. Granata A, Zanoli L, Clementi S, Fatuzzo P, Di Nicolo P, Fiorini F. Resistive intrarenal index: myth or reality? Br J Radiol. 2014;87(1038):20140004.

5. Correct Answer: B. Acute tubular necrosis

Rationale: An isolated increase in the RRI, concomitantly with normal systemic resistive indices, signifies an intrarenal etiology. Decompensated right heart failure, venous congestion, or advanced age, for example, tend to increase resistive indices in multiple different organs (liver, spleen, kidney). In acute tubular necrosis, renal flow dynamics will be altered without alteration in other organs. It is therefore important to not interpret any abnormal arterial Doppler signal in isolation.

1. Beaubien-Souligny W, Denault A, Robillard P, Desjardins G. The role of point-of-care ultrasound monitoring in cardiac surgical patients with acute kidney injury. J Cardiothorac Vasc Anesth. 2019;33(10):2781-2796.

6. Correct Answer: D. A decrease in RRI

Rationale: The RRI has been shown to decrease by increasing the mean arterial pressure with norepinephrine from 65 to 75 mm Hg in patients with septic shock. This decrease in the RRI was also associated with an increase in urine output. A decrease in the RRI might result from a proportionally larger increase in diastolic flow velocity; however, increasing mean arterial pressure from 75 to 85 mm Hg did not result in any additional decrease in the RRI. Individual optimization of mean arterial pressure with norepinephrine using end-organ perfusion indices such as the RRI might be useful to obtain optimal end-organ blood flow. This approach could be useful in patients with cardiovascular disease or long-term hypertension that might have shifted their autoregulatory curves.

1. Deruddre S, Cheisson G, Mazoit JX, Vicaut E, Benhamou D, Duranteau J. Renal arterial resistance in septic shock: effects of increasing mean arterial pressure with norepinephrine on the renal resistive index assessed with Doppler ultrasonography. Intensive Care Med. 2007;33(9):1557-1562.

7. Correct Answer: A. 2-4-1-3

Rationale: Stages of renal venous congestion can be defined by different intrarenal venous flow patterns. The absence of congestion will present as a continuous venous flow without any restriction of venous drainage. The first stage of congestion will present as a brief interruption of venous flow during atrial contraction. The second stage will present with biphasic venous flow with interruption between systole and diastole, and finally, the third stage of renal venous congestion will appear as a monophasic venous flow restricted to diastole. Congestion from elevated right atrial pressure will distend the venous system, creating a rigid liquid column between the right atrium and intrarenal venous system. Central venous oscillation will be transmitted to end-organ individual venous systems.

It is also important to mention that left renal vein phasicity may be attenuated due to its anatomic location and possible entrapment between the abdominal aorta and superior mesenteric artery (Nutcracker syndrome). Lastly, the left ovarian or left testicular veins drain into the left renal vein and may, in rare circumstances of ovarian or testicular varicosis, affect renal venous flow (Figure 62.4).

1. Beaubien-Souligny W, Denault AY. Real-time assessment of renal venous flow by transesophageal echography during cardiac surgery. A A Pract. 2019;12(1):30-32.

2. Husain-Syed F, Birk HW, Ronco C, et al. Doppler-derived renal venous stasis index in the prognosis of right heart failure. J Am Heart Assoc. 2019;8(21):e013584.

8. Correct Answer: B. (Peak maximal venous flow velocity – nadir flow velocity)/peak maximal venous flow velocity

Rationale: Renal venous flow impedance can be calculated from the maximal venous flow velocity (Vmax) and the minimal venous flow velocity (Vmin) occurring during the cardiac cycle.