The use of ultrasound in the non-invasive detection of changes in the renal circulation in response to blood loss using an animal model

Doppler ultrasonographic changes in the canine kidney during normovolaemic anaemia

The haemodynamics of the canine left renal artery (LRA) and interlobar artery (ILA) were evaluated in eleven fasted, healthy, conscious beagles with severe acute (haematocrit [Hct] 16%), moderate chronic (Hct 26%) and mild chronic (Hct 34%) normovolaemic anaemia using Doppler ultrasound. Heart rate, peak systolic velocity (PSV), end diastolic velocity (EDV), time-averaged mean velocity (TAVmean), pulsatility index (PI) and resistive index (RI) were recorded. Doppler values in the dogs following the induction of anaemia states were compared with corresponding values in the same dogs prior to the induction of anaemia. Left renal artery mean PSV, mean PI and mean RI were significantly higher and the mean EDV was significantly lower in severe acute anaemia. No significant change was seen in mean values of the same parameters in moderate or mild chronic anaemia. There was no significant change in TAVmean of the LRA or mean PI and mean RI of the ILA in any grade of anaemia. Acute, severe normovolaemic anaemia significantly altered LRA Doppler parameters in resting dogs without influencing those of the ILA. Moderate or mild chronic anaemia had no effect on any renal Doppler parameter.

Resistive index: an experimental study of acute complete unilateral ureteral obstruction

RATIONALE AND OBJECTIVES

To study the effects of acute complete unilateral ureteral obstruction (UUO) and release on porcine renal resistive index (RI).

METHODS

Under general anesthesia, UUO was induced in six pigs. RI was measured bilaterally at predetermined intervals for 4 hours of UUO and 1 hour of release. Additionally, measures of renal blood flow (RBF), glomerular filtration rate (GFR), arterial blood pressure, renal vascular resistance (RVR), and ipsilateral renal intrapelvic pressure (IPP) were obtained.

RESULTS

UUO and resultant progressive IPP increase caused prompt and significant ipsilateral RI elevation, and contralateral RI decrease. Concomitantly, ipsilateral RVR increased significantly while RBF and GFR declined, both significantly. Release of obstruction saw an almost immediate normalization of ipsilateral RI, RVR and RBF while ipsilateral GFR assumed 80% of baseline value 15 minutes postobstruction. Throughout the experiment, ipsilateral RI correlated significantly with changes in IPP, GFR, RBF, and RVR with correlation coefficients of 0.844, -0.851, -0.898, and 0.836 respectively ( < 0.001).

CONCLUSIONS

UUO causes a divergent RI response that is instantly reversed upon release. IPP seems to be the principal effector of these changes in the early phases of UUO.

Short- and long-term repeatability of intrarenal resistive index in the pig

RATIONALE AND OBJECTIVES

To evaluate the short- and long-term repeatability (intraobserver variation) of the renal resistive index (RI) in a pig model.

METHODS

Under standardized ambient conditions, short- and long-term repeatabilities were assessed by measuring the RI three consecutive times at 30-minute intervals in 14 pigs and twice 4 weeks apart in 6 pigs, respectively. Repeatability was judged according to predefined criteria, which included calculation of coefficients of variation (CVs) and repeatability coefficients combined with visual assessment and estimation of bias.

RESULTS

Short-term repeatability was acceptable, having an average CV of 4.9% +/- 2.9% and of 6.1% +/- 4.7% in two comparisons, findings that were reiterated in the visual assessment. Long-term results were, however, less consistent, having an average CV of 8.1% +/- 6.1% and prominent scatter on the visual assessment graphs.

CONCLUSIONS

In this study, RI measurements showed an acceptable short-term repeatability, whereas long-term results were less consistent. The components of error contributing to the modest long-term repeatability need to be investigated in studies of larger populations.

Intraoperative transesophageal ultrasonography can measure renal blood flow

OBJECTIVE

To determine the feasibility of acquiring Doppler-derived indices of renal blood flow by transesophageal ultrasonography in the perioperative period.

DESIGN

Prospective, sequential, institutional review board-approved study.

SETTING

University teaching hospital.

PARTICIPANTS

Nine patients with normal renal function, scheduled for elective primary coronary artery bypass graft surgery.

INTERVENTIONS

Two-dimensional images of renal parenchyma and Doppler measurement of intrarenal arterial blood flow during internal mammary dissection were acquired. To effect renal blood flow changes, the renal vasodilator dopamine, 2 microg/kg/min, was infused for 20 minutes after baseline measurements were made. Renal Doppler measurements were repeated to determine whether transesophageal ultrasonography can follow these changes.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic measurements (heart rate, mean arterial blood pressure, cardiac output, and cardiac index) and Doppler velocity measurements of intrarenal arterial blood flow (peak systolic, end-diastolic, and mean velocity) were made at time 1 (T1 = baseline) and at time 2 (T2 = after 20 minutes of dopamine infusion). The derived Doppler indices, pulsatility index and resistive index, were calculated according to standard formulae. Measurements were compared by paired Student's t-test (two-tailed, p < 0.05, significant). There were no statistical differences between cardiac index (2.10 +/- 0.93 L/min/m2 v 2.21 +/- 0.92 L/min/m2, p = 0.254) and mean arterial pressure (82.3 +/- 11.2 mmHg v 83.3 +/- 14.5 mmHg, p = 0.872) between T1 and T2. Systolic renal velocity increased from 44.7 +/- 13.0 cm/s to 63.0 +/- 20.4 cm/s (p = 0.005), diastolic velocity increased from 12.7 +/- 4.0 cm/s to 22.4 +/- 7.8 cm/s (p = 0.0003), and mean velocity increased from 22.5 +/- 6.6 cm/s to 34.1 +/- 11.7 cm/s (p = 0.003) after infusion of dopamine. These results indicate an increase in renal blood flow from baseline values. The pulsatility index decreased from 1.44 +/- 0.29 to 1.21 +/- 0.24 (p = 0.0005), whereas the resistive index decreased from 0.71 +/- 0.06 to 0.64 +/- 0.06 (p = 0.0004) after dopamine. Reductions in pulsatility and resistive indices indicate a reduction in renal vascular resistance.

CONCLUSION

This study demonstrates the ability to acquire two-dimensional images of kidney and renal arterial Doppler velocities using transesophageal ultrasonography during cardiac surgery. Transesophageal renal arterial Doppler waveform analysis can follow changes in renal blood flow patterns secondary to interventional therapy.

Volumetric flow estimation in vivo and in vitro using pulsed-Doppler ultrasound

The measurement of volumetric blood flow in small vessels in vitro and in vivo poses a significant technological challenge. In this study, two pulsatile flow models were developed, one with a 3.2-mm lumen diameter and one with a 12.7-mm lumen diameter, to assess the accuracy of volumetric flow estimation of two pulsed-Doppler devices, a Crystal Biotech VF1 20-MHz system with either a cuff-mounted or a needle-mounted probe and an Advanced Technology Laboratories Ultramark 9 High Definition Imaging system with a 5-MHz linear array transducer. The VF1 volumetric flow error was measured in the 3.2-mm phantom over a variety of pulsatile and continuous waveforms. The accuracy of the VF1 was also tested in porcine femoral and renal arteries. VF1 volumetric flow error ranged from 4.8% to 54.3% in the in vivo studies. The ATL demonstrated similar volumetric flow errors in the porcine femoral artery (approximately 3.2 mm diameter), but these errors were reduced to < or = 17.4% in the 12.7-mm-diameter in vitro flow model.