Functional assessment of the circulation of the single kidney

A non-invasive magnetic resonance imaging approach for assessment of real-time microcirculation dynamics

We present a novel, non-invasive magnetic resonance imaging (MRI) technique to assess real-time dynamic vasomodulation of the microvascular bed. Unlike existing perfusion imaging techniques, our method is sensitive only to blood volume and not flow velocity. Using graded gas challenges and a long-life, blood-pool T₁-reducing agent gadofosveset, we can sensitively assess microvascular volume response in the liver, kidney cortex, and paraspinal muscle to vasoactive stimuli (i.e. hypercapnia, hypoxia, and hypercapnic hypoxia). Healthy adult rats were imaged on a 3 Tesla scanner and cycled through 10-minute gas intervals to elicit vasoconstriction followed by vasodilatation. Quantitative T₁ relaxation time mapping was performed dynamically; heart rate and blood oxygen saturation were continuously monitored. Laser Doppler perfusion measurements confirmed MRI findings: dynamic changes in T₁ corresponded with perfusion changes to graded gas challenges. Our new technique uncovered differential microvascular response to gas stimuli in different organs: for example, mild hypercapnia vasodilates the kidney cortex but constricts muscle vasculature. Finally, we present a gas challenge protocol that produces a consistent vasoactive response and can be used to assess vasomodulatory capacity. Our imaging approach to monitor real-time vasomodulation may be extended to other imaging modalities and is valuable for investigating diseases where microvascular health is compromised.

320-row CT renal perfusion imaging in patients with aortic dissection: A preliminary study

OBJECTIVE

To investigate the clinical value of renal perfusion imaging in patients with aortic dissection (AD) using 320-row computed tomography (CT), and to determine the relationship between renal CT perfusion imaging and various factors of aortic dissection.

METHODS

Forty-three patients with AD who underwent 320-row CT renal perfusion before operation were prospectively enrolled in this study. Diagnosis of AD was confirmed by transthoracic echocardiography. Blood flow (BF) of bilateral renal perfusion was measured and analyzed. CT perfusion imaging signs of AD in relation to the type of AD, number of entry tears and the false lumen thrombus were observed and compared.

RESULTS

The BF values of patients with type A AD were significantly lower than those of patients with type B AD (P = 0.004). No significant difference was found in the BF between different numbers of intimal tears (P = 0.288), but BF values were significantly higher in cases with a false lumen without thrombus and renal arteries arising from the true lumen than in those with thrombus (P = 0.036). The BF values measured between the true lumen, false lumen and overriding groups were different (P = 0.02), with the true lumen group having the highest. Also, the difference in BF values between true lumen and false lumen groups was statistically significant (P = 0.016), while no statistical significance was found in the other two groups (P > 0.05). The larger the size of intimal entry tears, the greater the BF values (P = 0.044).

CONCLUSIONS

This study shows a direct correlation between renal CT perfusion changes and AD, with the size, number of intimal tears, different types of AD, different renal artery origins and false lumen thrombosis, significantly affecting the perfusion values.

Assessment of renal hemodynamics and function in pigs with 64-section multidetector CT: comparison with electron-beam CT

PURPOSE

To prospectively evaluate the feasibility of obtaining reliable measurements of renal hemodynamics and function by using 64-section multidetector CT.

MATERIALS AND METHODS

This study was approved by the Institutional Animal Care and Use Committee. Eight pigs (two with induced unilateral renal artery stenosis) were studied with both electron-beam CT and 64-section multidetector CT at 1-week intervals in randomized order. Both kidneys were scanned repeatedly, without table movement, for about 3 minutes after intravenous (IV) administration of a bolus of contrast medium and again during vasodilator challenge (acetylcholine). Images were reconstructed on each CT console but were analyzed on the same independent workstation. Attenuation changes in the kidneys were plotted as function of time, and time-attenuation curves (TACs) were subsequently analyzed to determine regional perfusion and volume, glomerular filtration rate (GFR), and renal blood flow (RBF). Statistical analysis utilized Student t test, analysis of variance (ANOVA), linear regression, and Bland-Altman analysis.

RESULTS

TACs obtained with multidetector CT were qualitatively similar to those obtained with electron-beam CT, as were the quantitative values of renal perfusion and function. RBF correlated significantly between the two techniques (RBF(MD) = 0.96 . RBF(EB) mL/min; R = 0.77, P < .01). GFR(MD) was also similar to GFR(EB) (77.6 +/- 8.3 vs 79.8 +/- 8.8 mL/min, p > .05). Bland-Altman plots showed good agreement between the two techniques. Both techniques similarly detected the differences between stenotic and contralateral kidneys.

CONCLUSION

The clinical multidetector CT scanner provides reliable measurements of single-kidney hemodynamics and function, which are similar to those obtained with previously validated electron-beam CT.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

Correlation between noninvasive and endovascular Doppler in patients with atherosclerotic renal artery stenosis: A pilot study

Considerable interest exists in using endovascular techniques for assessing the renal circulation. The vasomotor response of the renal circulation in patients with hypertension and severe atherosclerotic renal artery stenosis was evaluated. Sixteen patients (5 men and 11 women) with a mean age of 73.2+/-7.1 years (range, 59-82 years) underwent noninvasive renal duplex ultrasonography and endovascular flow wire evaluation. The flow wire was positioned in the main renal artery distal to the stenosis and used to examine the reactivity of the renal circulation in response to intra-arterial papaverine before and after percutaneous revascularization. Resistive index was computed as (MPV - EDV)/MPV, where MPV is the maximum peak velocity and EDV is the end-diastolic velocity. Renal flow reserve was defined as the ratio of peak hyperemic response to baseline flow. Intervention was technically successful in 18/18 arteries. Heart rate (69+/-11 vs. 65+/-10 beats/min; P=NS) and systolic blood pressure (168+/-19 vs. 163+/-24 mm Hg; P=NS) remained constant following injection of intrarenal papaverine, while diastolic blood pressure decreased (77+/-13 vs. 65+/-11 mm Hg; P<0.006). Papaverine injection resulted in significant increases in average peak velocity (APV; 148% of baseline; P=0.0003), MPV (146% of baseline; P=0.001), and EDV (161% of baseline; P=0.0026), but had no effect on the endovascular resistive index (0.81+/-0.12 vs. 0.84+/-0.08; P=NS). Renal artery stenting produced no effect on APV or EDV, but a small increase in MPV (123% of baseline; P<0.05). In contrast, renal stenting resulted in a significant increase in the endovascular resistive index (0.77+/-012 vs. 0.84+/-0.08; P=0.003), pulsatility index (1.84+/-0.61 vs. 2.23+/-0.70; P<0.014), and renal flow reserve (1.49+/-0.44 vs. 1.86+/-0.73; P<0.0071). A correlation was found between the noninvasive and endovascular measures diastolic/systolic ratio (r=0.73; P<0.0009) and resistive index (r=0.63; P<0.0053). Patients with atherosclerotic renal artery stenosis have preserved vasomotor response to papaverine, as well as marked increases in renal flow and resistive index following successful renal artery stenting. Endovascular flow wire measurements are safe, easy to perform, and correlate well with noninvasive Doppler measurements.