Intraluminal pressure profiles during flexible ureterorenoscopy

The normal physiological intrarenal pressure is approximately 10 mmHg (Djurhuus 1980]) and we demonstrated intraluminal renal pelvic pressures up to 328 mmHg during ureterorenoscopy.
The threshold for pyelovenous and pyelosinous back flow has previously been shown
to be in the order of 30–45 mmHg (Thomsen 1984]). Our study confirms previous reports that this threshold is dramatically exceeded
during flexible ureterorenoscopy, which potentially may give rise to septic complications
(Wilson 1990]). The data highlights the importance of using prophylactic antibiotics in these procedures.

Despite on-going efforts miniaturizing semirigid and flexible ureteroscopes, access
difficulties due to tight conditions in the ureter are still to be sufficiently solved.
Ureteral dilatation rates of 12–25% are reported in previous series (Rehman et al.
2003]) and complications associated with the dilatation procedure have been seen in 6%
of cases. Furthermore, ureteral perforation rates of 15% have been observed (Stoller
et al. 1992]). Using a ureteral access sheath (UAS) Rehman et al. found intrarenal pressures up
to 58.9 cm H
₂
O during flexible ureteroscopy and recommended the use of UAS to prevent complications
caused by pressure elevations (Rehman et al. 2003]).

UASs are often used to minimize the hassles of repeated passages of the ureteroscope
through the ureter. Auge et al. reported that the mean intrarenal pressure measured
through a nephrostomy tube exceeded 94 mmHg when a flexible ureteroscope was present
in the renal pelvis. This pressure was reduced to 40 mmHg when a UAS was inserted
(Auge et al. 2004]). Although the intrapelvic pressure was significantly reduced using the UAS, the
pressure levels during ureterorenoscopy still exceeded the level of pyelovenous backflow.
Complications associated with the insertion of the UAS were not addressed in this
paper. The pressure levels measured in our study in patients who were not previously
drained were in the same order of magnitude. Additionally, pressure peaks close to
300 mmHg during forced irrigation and laser fragmentation were demonstrated. It is
well known from animal studies and clinical observations that a sustained high pressure
results in kidney damage (Jung et al. 2006]). Whether these short pressure peaks are harmful for the kidney is still a matter
of debate.

In the present study irrigation by gravity supplemented with on-demand flushing using
a syringe was used. Using pressure-controlled pumps and other devices for on-demand
flushing may have altered the intrarenal pressure profiles. Furthermore, pressure
bags may potentially result in sustained high pressures, and usage of such devices
is probably not recommendable.

In order to determine the optimal size of an UAS to achieve good irrigation, Ng et
al. evaluated the flow rate and the intrarenal pressure during insertion of different
access sheaths (Ng et al. 2010]). It was concluded that increased UAS diameter did not improve flow when the working
channel of the flexible ureteroscope was occupied, but the intrarenal pressure decreased
significantly, particularly when using the 16F UAS. Evidence has been put forward,
that presence of a UAS can induce transient ureteral ischaemia, promote an acute inflammatory
response and give rise to ureteral stricture (Boddy et al. 1989]). Lallas et al. found that UAS insertion caused a transient decrease in ureteral
blood flow in swine (Lallas et al. 2002]). The larger diameter of the sheath, the more pronounced the decrease in blood flow.
In a recent study by Traxer and Thomas data on a total of 359 consecutive patients
who underwent retrograde intrarenal surgery for kidney stones were prospectively collected
at 2 academic centres (Traxer and Thomas 2013]). The patients were prospectively evaluated with regard to incidence and severity
of ureteral damage due to UAS placement. UAS related ureteral wall lesions were present
in 167 patients (46.5%), 13.4% representing high-grade injuries. The authors proposed
a classification of UAS related ureteral injuries and showed that JJ pre-stenting
significantly decreased the incidence of severe UAS-related damage. However, large
prospective trials documenting the rate of stricture formation after the use of UAS
are lacking. Abrahams et al. argued against the routine use of UAS due to potential
disadvantages such as increased ureteroscopic resistance, increased risk of missing
distal ureteral pathology and the fact that the UAS does not necessarily extend all
the way to the stone, exposing the most proximal part of the urinary tract to repeated
manipulation and risk of perforation and later stricture formation (Abrahams and Stoller
2004]). On the other hand, UAS remain the only proven, data driven option to reduce intrarenal
pressures during retrograde intrarenal surgery; however their use often mandates the
use of a ureteral stent (Rapoport et al. 2007]). If a UAS is considered, it seems advisable to use the smallest possible in which
the ureteroscope fits, since this will reduce risk of ureteral damage (Traxer and
Thomas 2013]).

The type and number of different receptors in the upper urinary tract are well documented
(Park et al. 2000] ;Malin et al. 1970]; Jung et al. 2006]). ?-adrenergic stimulation causes relaxation of the ureter, while ?-adrenergic stimulation
causes contraction. Human studies in this area are quite sparse, but we have in earlier
studies demonstrated that it was possible to lower the PP during ureterorenoscopy
using topical administration of a ?-adrenergic agonist (isoproterenol) in the ureter
(Jung et al. 2008a], b]). No cardiovascular side effects were observed. The trend of extensive pressure rises
significantly decreased during isoprenalin irrigation, and the average pressure remained
below the level of intrarenal back flow. Similar results were obtained in an experimental
swine study showing that isoprenalin significantly reduced the pressure-flow relation
during semirigid ureterorenoscopy (Jakobsen et al. 2010]).

We have previously shown that intrarenal pressure correlates to pain (Pedersen et
al. 2012]). Unfortunately, postoperative pain was not monitored in our series. Reducing intrarenal
pressures during ureterorenoscopy may, however, also have important implications with
regard to pain, and should be addressed in future research.

Limitations: retrograde pressure measurement

The pelvic pressure measurements in the present study were obtained through a retrogradely
inserted pressure transducer. Endoluminal devices might themselves alter ureteral
peristalsis and hence may not optimally be applied for measurement of peristaltic
activity and intraluminal pressure. However, the baseline pelvic pressures measured
in this study were similar to pelvic pressures observed in previous studies using
antegradely inserted catheters for pressure measurement (Wilson 1990]; Auge et al. 2004]; Jakobsen et al. 2007]; Schwalb et al. 1993]). The comparable pelvic pressure levels despite the different techniques employed
serve as validation of the method applied in this study. Ethical acceptable alternatives
for pelvic pressure measurement during ureteroscopy in humans are not available.