Access Flow in Arteriovenous Accesses by Optodilutional and Ultrasound Dilution Methods

Changes in vascular access blood flow: Etiological factors and clinical implications

METHODS

Records of 10,000 QVA measurement performed in 549 patients over 20 years were used as retrospective and anonymized data source, making ethical commission involvement unnecessary. Two approaches are used to elucidate association of QVA changes with different factors: analyses of smaller cohorts in which both the QVA and the respective factor were measured (e.g. association of QVA with cardiac output (CO)), or-in case of rare phenomena-a form of a well illustrated case reports was used (e.g. association of QVA and Kt/V).

RESULTS

Significant increase in CO after permanent VA creation (3-4-fold of the QVA value) was found. Impact of intradialytic CO changes on QVA is attenuated by relatively stable VA resistance compared to systemic resistance. Blood pressure impact is much stronger and it should therefore be noted at each QVA measurement. As reproducibility of different QVA measurement methods varies, use of the same method should be preferred. Direction of the arterial needle insertion in VA affects the QVA measured, especially in synthetic grafts, too. Also patient's own QVA variability may be quite high. All this makes KDOQI/EBPG recommended acceptable QVA drops too strict, they should be revised. In re-stenoses prone patients, measurement intervals should be shortened, too.

CONCLUSION

QVA values are significantly affected by many factors. Their knowledge appears essential for safe and effective VA surveillance and management.

Con: Vascular access surveillance in mature fistulas: is it worthwhile?

The risks and benefits of hemodialysis arteriovenous (AV) access surveillance have been debated since the introduction of AV access surveillance techniques. The debate is fuelled by the lack of robust, high-quality evidence with consistent and comparable patient-important outcomes. Additionally, there is a lack of clarity regarding the diagnostic cut points for AV access stenosis using the various surveillance techniques, questions about the appropriateness of the ‘knee-jerk’ response to intervention on a stenosis >50% regardless of the presence of clinical indicators and whether the intervention results in desired patient-important outcomes. The physiology of the AV access is complex considering the different hemodynamics within an AV fistula, which vary by time of dialysis, location, size of vessels and location of the stenosis. The current evidence suggests that the use of AV access surveillance in an AV fistula does detect more stenosis compared with clinical monitoring alone and leads to an increased number of procedures. It remains uncertain if that leads to improved patient-important outcomes such as prolonged AV fistula patency. Vascular access is an essential component of hemodialysis and further study is needed to clarify this long-standing debate. There needs to be better distinction between the strategies of vascular access surveillance, clinical monitoring with clinical indictors and preemptive intervention and their respective impacts on patient-important outcomes. Randomized controlled studies must be conducted with defined indications for intervention, reproducible methods of intervention and clinically important AV fistula and patient outcomes. The current guidelines need to be challenged and revised to permit these necessary studies to be done.

Spanish Clinical Guidelines on Vascular Access for Haemodialysis

Vascular access for haemodialysis is key in renal patients both due to its associated morbidity and mortality and due to its impact on quality of life. The process, from the creation and maintenance of vascular access to the treatment of its complications, represents a challenge when it comes to decision-making, due to the complexity of the existing disease and the diversity of the specialities involved. With a view to finding a common approach, the Spanish Multidisciplinary Group on Vascular Access (GEMAV), which includes experts from the five scientific societies involved (nephrology [S.E.N.], vascular surgery [SEACV], vascular and interventional radiology [SERAM-SERVEI], infectious diseases [SEIMC] and nephrology nursing [SEDEN]), along with the methodological support of the Cochrane Center, has updated the Guidelines on Vascular Access for Haemodialysis, published in 2005. These guidelines maintain a similar structure, in that they review the evidence without compromising the educational aspects. However, on one hand, they provide an update to methodology development following the guidelines of the GRADE system in order to translate this systematic review of evidence into recommendations that facilitate decision-making in routine clinical practice, and, on the other hand, the guidelines establish quality indicators which make it possible to monitor the quality of healthcare.

The rise and fall of access blood flow surveillance in arteriovenous fistulas

Vascular access blood flow (Qa) surveillance has been described as a typical false paradigm, an example of how new tests are sometimes adopted even without good-quality evidence of their benefits. This may be true for grafts, but not necessarily for arteriovenous fistulas. We reviewed the literature on Qa surveillance in fistulas to see whether it complies with the World Health Organization's criteria for screening tests. Measuring Qa has a fairly good reproducibility. Qa shows an excellent-to-good accuracy for stenosis being the only bedside screening test that achieves a very high sensitivity while retaining a fair-to-good positive predictive value for Qa thresholds of 600 ml/minute or higher associated with a >25% drop in Qa, or findings suggesting stenosis on physical examination. The accuracy of Qa in predicting thrombosis is hard to establish because of the heterogeneity of published studies, though a Qa of 300 ml/minute seems the most reliable cutoff. Qa surveillance affords a significant 2- to 3-fold reduction in the risk of thrombosis by comparison with clinical monitoring alone when Qa criteria highly sensitive to stenosis are considered, regardless of the study design (randomized controlled trials, cohort studies with concurrent or historic controls). Using highly sensitive Qa screening criteria also halves the risk of access loss, although this effect is not statistically significant. Our analysis strongly suggests that Qa surveillance is an effective method for screening mature fistulas, though further, appropriately designed studies are needed to fully elucidate its benefits and cost effectiveness.

Computer assisted Sound Analysis of Arteriovenous Fistula in Hemodialysis Patients

The purpose of this study was to reveal the unique sound characteristics of the bruit produced by arteriovenous fistulae (AVF), using a computerized method. An electronic stethoscope (20 Hz to 20 000 Hz sensitivity) was used, connected to a portable laptop computer. Forty prevalent hemodialysis patients participated in the study. All measurements were made with patients resting in supine position, prior to the initiation of mid-week dialysis session. Standard color Doppler technique was used to estimate blood flow. Clinical examination revealed the surface where the perceived bruit was more intense, and the recording took place at a sample rate of 22 000 Hz in WAV lossless format. Fast Fourier Transform (FFT) mathematical algorithm, was used for the sound analysis. This algorithm is particularly useful in revealing the periodicity of sound data as well as in mapping its frequency behavior and its strength. Produced frequencies were divided into 40 frequency intervals, 250 Hz apart, so that the results would be easier to plot and comprehend. The mean age of the patients was 63.5 ± 14 years; the median time on dialysis was 39.6 months (mean 1 month, max. 200 months). The mean blood flow was 857.7 ± 448.3 ml/min. The mean sound frequency was approximately 5 500 Hz ± 4 000 Hz and the median, which is also expressing the major peak of sound data, was 750 Hz, varying from 250 Hz to 10 000 Hz. A possible limitation of the study is the relatively small number of participants.

Surveillance of hemodialysis vascular access

A mature, functional arteriovenous (AV) access is the lifeline for a hemodialysis (HD) patient as it provides sufficient enough blood flow for adequate dialysis. As the chronic kidney disease (CKD) and end-stage renal disease (ESRD) population is expanding, and because of the well-recognized hazardous complications of dialysis catheters, the projected placement and use of AV accesses for HD is on the rise. Although a superior access than catheters, AV accesses are not without complications. The primary complication that causes AV accesses to fail is stenosis with subsequent thrombosis. Surveying for stenosis can be performed in a variety of ways. Clinical monitoring, measuring flow, determining pressure, and measuring recirculation are all methods that show promise. In addition, stenosis can be directly visualized, through noninvasive techniques such as color duplex imaging, or through minimally invasive venography. Each method of screening has its advantages and disadvantages, and several studies exist which attempt to answer the question of which test is the most useful. Ultimately, to maintain the functionality of the access for the HD patient, a team approach becomes imperative. The collaboration and cooperation of the patient, nephrologist, dialysis nurse and technician, vascular access coordinator, interventionalist, and vascular surgeon is necessary to preserve this lifeline.

Blood Flow Measurements during Hemodialysis Vascular Access Interventions - Catheter-Based Thermodilution or Doppler Ultrasound?

Purpose

To test the clinical performance of catheter-based thermodilution and Doppler ultrasound of the feeding brachial artery for blood flow measurements during hemodialysis vascular access interventions.

Methods

Thirty patients with arteriovenous fistulas who underwent 46 interventions had access blood flow measured before and after every procedure. Two methods, catheter-based thermodilution and Doppler ultrasound, were compared to the reference method of ultrasound dilution. Catheter-based thermodilution and Doppler ultrasound were performed during the endovascular procedures while flow by ultrasound dilution was determined within three days of the procedure. The methods were compared using regression analysis and tested for systematic bias.

Results

Failure to position the thermodilutional catheter correctly was observed in 8 out of 46 (17%) pre-intervention measurements. Post-intervention measurements and ultrasound measurements were feasible in all patients. The average level of agreement was good when comparing catheter-based thermodilution to ultrasound dilution. However, blood flow by ultrasound dilution may differ by ±130 mL/min (±22%) at a flow level of 600 mL/min by thermodilution. Results from Doppler ultrasound displayed a moderate level of agreement on average when compared to ultrasound dilution. Blood flow by ultrasound dilution may differ by ±160 mL/min (±27%) at a flow level of 600 mL/min by Doppler ultrasound. No systematic bias was detected by either method.

Conclusions

On average, results from catheter-based thermodilution were more in agreement with results from the ultrasound dilution technique compared to Doppler ultrasound. However, considering the cost and the high technical failure rate of the thermodilutional system, we recommend the use of ultrasound.

Comparison of hemodialysis blood access flow rates using online measurement of conductivity dialysance and ultrasound dilution

BACKGROUND

Routine access flow (AF) surveillance is recommended by the Kidney Disease Outcomes Quality Initiative as one of several components for an arteriovenous vascular access maintenance program. A reliable, but affordable, measurement tool is needed for outpatient hemodialysis facilities.

STUDY DESIGN

Diagnostic test study.

SETTING & PARTICIPANTS

50 adult patients with 27 grafts and 23 fistulas from a single center who consented to sequential AF measurements, all performed within the first 90 minutes of the hemodialysis treatment.

INDEX TEST

AF measured by using online conductivity dialysance (OLC-AF).

REFERENCE TEST

AF measured by using ultrasound dilution (UD-AF).

RESULTS

Mean UD-AF was 1,086 +/- 629 mL/min, and mean OLC-AF was 951 +/- 575 mL/min, with a mean difference of 135 +/- 229 mL/min. OLC-AF correlated significantly with UD-AF (0.93; P < 0.0001), becoming tighter for flows less than 1,000 mL/min (0.95; P < 0.0001). The coefficient of variation for sequential measurement by using UD was 6.4%, and for OLC, 11.5%, with the difference becoming insignificant (7.6% versus 8.8%; P = 0.5) for flows less than 1,000 mL/min. The average of 2 sequential UD-AF measurements correlated tightly with that of OLC-AF (0.99; P < 0.0001).

LIMITATIONS

Tests were performed by 2 highly trained coordinators in a single clinic with a small sample size, and clinical outcomes were not evaluated.

CONCLUSION

The OLC method is a reasonable alternative to UD for flow surveillance of arteriovenous hemodialysis accesses and provides an option for widespread implementation of a vascular access surveillance program. Additional studies are needed to determine whether routine use impacts on clinical outcomes.

How to improve dialysis adequacy in patients with vascular access problems

Blood flow rate is a critical factor in the achievement of an adequate dialysis dose. The aim of this review is to evaluate the possibility of optimizing dialysis dose in terms of Kt/V in patients with reduced vascular access (VA) flow rate, considering effective blood flow (Qb eff), recirculation, access flow and hemodialyzer. In patients where the achievement of adequate blood flow rates are difficult to obtain and no surgical revision is necessary, to avoid under dialysis the increase in the treatment time should be the first choice solution. If such a solution is difficult for various reasons, a forced partial blood flow recirculation, especially in central venous catheters (CVCs) with reversed lines can be useful, on condition that the dialysis session is prolonged. The possibility of increasing the efficiency of dialysis through an increase in filter clearance has to be considered. Monitoring arterial pre-pump pressure (P asp) and optimizing ratio P asp/Qb eff during hemodialysis (HD) is one possible solution to improve blood flow rates, but it is necessary to educate and involve the staff. Recent developments in a new class of highly effective hemodialyzer due to dialysate distribution, has opened up interesting opportunities in terms of dialysis adequacy in patients with reduced VA flow rate.