Marco Bartocci

PDS represents a quite new sonographic technique, which is going to integrate more and more the standard ultrasound and Doppler examination of the newborn. This new method differs from the standard color Doppler generally based on a mean frequency shift, because displays the total integrated Doppler power in color. In fact in PDS the hue and the brightness of the color signal represent the total energy of the Doppler signal.

Conventional color Doppler is based on the mean Doppler frequency shift and therefore is a measure related to the directional component of the velocity of the blood moving through the insonated vessel. Thus, it is affected from inherent limitations, including a) the possibility that the noise overhelms the flow signal if the gain is too high or the Doppler display threshold too low, b) Doppler angle dependence, c) aliasing. PDS is calculated on the base of the integrated Power Doppler spectrum. The hue and the brightness represent the power in the Doppler signal, which is relate to the number of RBC producing the Doppler shift 8,9,10. PDS noise may be assigned to a homogeneous background (usually dark - blue), even when the gain is increased greatly over the level at which conventional color Doppler image is obscured. Finally, the mean frequency representation of CD varies with the degree of aliasing. Unlike PDS image is unaffected by aliasing because the integral of the power spectrum is the same whether the signal wraps around or not. On the other hand, it has been seen that PDS is more sensitive to tissue motion. In fact PDS signal can be hanged over by so-called flash artefacts, making it actually unsuitable in organs with large amounts of tissue motion such as heart and liver. Moreover, PDS does not allow to detect the direction and the velocity of blood flow. This last disadvantage actually compromises very little the examination in those tissue with low flow velocity 4,5.

Doppler US of the neonatal cerebral vessels has represented a corner-stone in the assessment of the brain injury in neonatal period. On the other hand, the evaluation of kidney perfusion is quite more younger field of application, although renal function is one the referral points in the every-day neonatal intensive care management. Sonographic evaluation of the kidney in the newborn can be performed both through ultrasound and pulsed Doppler and color Doppler. During prenatal, the haemodinamic alterations which take place during prolonged hypoxyemia (chronic IUGR) lead to a redistribution of the blood flow. Although at the beginning, the kidney is preserved to skin and less important organ detriment, in the end phase of the hypoxyemic insult, the blood flow in the descending aorta is decreased, as well. In spite of this the assessment of renal blood flow has not been considered as a sensitive predictor for the outcome of the baby. This may be explained by the fact that several factors may interfere with the renal resistance to flow (hypoxia, volemia, blood pressure). Because it is so complicate to identify which of these factors has the strongest influence, utilization of renal flow index for fetal surveillance appears quite difficult. Studies in healthy kidney have tried to standardize parameters for most consistent assessment of the renal vascularization. The resistive index (RI) at the level of the interlobar-arcuate arteries proved to be the best parameter to be preferred in clinical application (study on adults). In spite of these results, the assessment of newborn kidney flow and vasculature seem to be still a bit late, compared to other "noble" organs, such as brain and heart.

The evaluation of renal blood flow perfusion in perinatal life has been performed by Kempley et al. 23. Through standard Doppler technique the authors calculated blood flow velocity in renal arteries before delivery, soon after and during the first week of life. They expressed the vascularization of the kidneys by the means of the mean velocity and PI in the renal artery. Blood flow velocity in renal arteries during the first postnatal day seem to be not significantly different from the fetal values. At all ages of the study renal perfusion was lower in premature growth-retarded newborns than in the term infants. By 1 week of age renal artery blood flow increased significantly in both normal and preterm infants. Other studies by Arbeille et al. on normal and hypertensive-inducted pregnants demonstrated that renal blood flow does not represent a good indicator in the prediction of fetal growth retardation (in contrast to the highly sensitivity and specificity of the cerebroplacental ratio). The trend of the renal artery RI during normal pregnancies seem to follow a slight decrease from 26 to 30 wks GA with values spread between » 1.0-2.3. Then, after 32 wks up to 41 wks the trend is more uniform, with value between » 1.2 and 0.6. In the group of complicated pregnancies the RI modify non-homogeneously in contrast to other observations 30, where an increased renal index was observed in a population of severe intrauterine growth retarded and hypoxic fetuses. The reported studies lead us to the assumption that in severe or prolonged hypoxia, renal blood flow and may be, glomerular filtration rate, decrease, sometimes up to oliguria. On the other hand, a mild hypoxic condition lead to an increase of renal blood flow and urine excretion.

The haemodinamic assessment of the newborn kidney is not routinely performed in the neonatal intensive care units. Despite systematic Doppler measurements of blood flow velocities in the renal arteries have been reported by so long time, the first study which tries to standardize the renal Doppler examination in newborn infants, appears in 1988 17. In the described procedure the transducer is positioned in the costolateral position below costal arch in such a way to be able to insonate the renal arteries with an angle between 0° and 25° . In further studies angle of insonation was even better 19, achieving 0° -15° , and the use of the color Doppler in association to the standard ultrasound was more and more frequent. Renal artery diameter (» 0.14 cm ® » 0.28 cm) increases gradually with advancing birth weight (635 ® 4595), gestational age (25 ® 42 wks). Also the total renal blood flow (30 ® 160 ml/min) was significantly correlated to the birth weight, gestational age and cardiac output (» 160 ® 1200 ml/min) 19. Ductus arteriosus has been seen to decrease total renal blood flow stealing even 12% of the amount of the cardiac output to the kidney (i.e.: 6.6% of the CO before ductal closure ® 18.4% after ductal closure). Animal studies have demonstrated that a decrease in the relative diastolic blood velocity in the renal arteries correspond to an increase in renal vascular resistance evaluated through PI 18. Renal vascularization has been studied both in presence or not of umbilical artery catheters (UAC) such as in presence of thrombus in the descending aorta related to the catheter 18 (estimated incidence 10 ® 95%26,27). A significant difference between the presence or the absence of UAC is shown by an increase of the renal vascular resistance (PI) especially from the 7th to the 14th day after birth in premature infants. By the 21st day there was no longer significant difference in the PI, unless there was presence of thrombi 18. Studies about the usefulness of PDS in depicting interlobular vasculature in renal transplants were carried out by Martinoli et al 6. PDS appeared to be a good tool in describing renal perfusion (especially of the cortex), even if care should be taken in the diagnosis of perfusion defect since the absence of detectable flow at the interlobular level does not always correspond to cortical areas that lack perfusion in MR images.

Among the wide amount of drug therapies administered to the critical newborn, indomethacin (IND) has been frequently studied to better understand its side effects on others vital organs, among which also kidneys. It is well known IND side effect on renal function, which can lead to renal failure or even death 28,29. IND administration has been seen to lead to a sharp decrease in peak systolic flow velocity, i.e. an increase in resistance of the vascular bed, with a consequent decrease in renal perfusion. This effect is no longer than 1 hour after IDM treatment.

PDS could represent a very useful tool in the depiction of intrarenal vascularization. In 1994 the group of Bude, Rubin and Adler compared PDS with color Doppler sonography in the assessment of intrarenal vasculature. The study population age varied between 7 and 75 years (mean 31 y). Ten subjects were studied for 20 kidney evaluated totally. The demonstrated advantages of PDS over color Doppler sonography were several. Durick et al. Demonstrated the efficacy of PDS in the assessment of the renal blood flow changes after pharmacological manipulation (study on pigs)7. Researches by Dacher et al.1 pointed out that PDS is significantly more sensitive than conventional color Doppler sonography for acute pyelonephritis in children. The ability of PDS has also been compared to CT or DMSA in the same study: PDS could replace CT or DMSA scintigraphy in many children with urinary tract infections.

÷ Inter- observer variation in subjectivily selecting the optimum pulse repetition frequency (PRF*).

* PRF: it is a technical factor whose increasing allows to reduce flash artefacts, but, at the same time it decreases the advantage achieved with the higher gain that can be employed with PDS.

Application fields of PDS already studied:

Brain (newborn ® adults)

Kidney (animal; children ® adults)

Mulsoloskeletal (adults, neonate (femoral head vessels)

Soft-tissues (children - adults)

Liver (adults (tumours)

Heart (adults)

Bude et al. suggested future studies and fields of applications of PDS. Among these, the employ of the PDS for the study the changes of the vascularization induced by patho-physiological events or pharmacological manipulation is indicated. In this spectrum of possibilities, the application of PDS for the assessment of the renal vascularization in the perinatal period could be reasonably placed. Besides it seem not less interesting the application of PDS to the ischemic-hypoxic injury of the newborn, being able to detect so well blush areas of cerebral parenchyma.

It is by now know and still in run of further confirmation, that endothelium-derived NO is at the basis of neonatal renal vascularization (especially of the cortex). Already during prenatal life NO regulates the kidney perfusion. It has been hypothesised that NO increase renal blood flow. During perinatal life NO sintetasy activity (Ca++ dependent) increases up levels, in neonatal period, closed to those of the adult.

On the other hand, the effect of NO on the pulmonary district influencing cardiac output could regulate indirectly renal perfusion 30,31,32. So there could be the possibility of a double side effect of NO: the first direct on the endothelium of the renal arteries, mostly by endogenous NO synthesized from L-arginina by endothelial cells; by activating guanylate cyclase NO causes vasodilatation of smooth muscle cells. The second as a result of a haemodinamic circuit starting from the vasodilatation and the decreasing pressure in the lungs. The effect of administered NO on renal cortex vascularization has not been yet studied. The application of PDS in a clinical population of preterm infants who undergo to NO therapy could be helpful in understanding how renal perfusion varies during and after the therapy. On the other hand, still experimental trials are necessary to point out the different actions of NO (L-arginine levels pre and post therapy; guanilate cyclase levels.

PDS should point out:

-increased perfusion in the renal cortex (and the whole parenchyma?)

-increased of the perfusion of other organs, as well

The effects of hypoxic-ischemic insult on kidney is thought to lead to a reduction of renal perfusion ® reduction of glomerular filtrate à activation of renina-angiotensina

æ activation of endothelin 1 (ET1)

PDS should point out:

-decreased perfusion of the kidney just after anoxia

-increasing of blood perfusion during the post-hypoxic phase of active reperfusion

-correlation with PDA presence of the renal vascularization

The action of indomethacin of the kidney is quite well known and sketched above (reduction of sensitivity to Ach (proved only in the fetus); vasoconstriction of renal arteries). What still unclear is the effective direct action on the kidney and what is due to the indirect action through the closure of the ductus.

IND ® ® ® ® ® Ductus closure

æ å

renal perfusion

PDS should point out:

-impairment of renal blood flow in those VLPB which underwent to repeated maternal administration near the time of delivery

-correlation of the kidney perfusion to the IND therapy and to the haemodinamic changes after ductus closure