NKF K/DOQI GUIDELINES 2000
GUIDELINES FOR VASCULAR ACCESS
II. Monitoring, Surveillance, and Diagnostic Testing
Definition of Terms
As they are used in relation to dialysis vascular access, the following terms will apply:
MonitoringThis term refers to the examination and evaluation of the vascular access by means of physical examination to detect physical signs that would suggest the presence of pathology.
SurveillanceThis term refers to periodic evaluation of the vascular access by means of tests, that may involve special instrumentation, for which an abnormal test result suggests the presence of pathology.
Diagnostic TestingThis term refers to testing that is prompted by some abnormality or other medical indication and which is undertaken to diagnose the presence of pathology.
Monitoring Dialysis AV Grafts
of an access graft should be performed weekly and should include, but not be
limited to, inspection and palpation for pulse and thrill at the arterial, mid,
and venous sections of the graft. (Opinion)
Fig III-1. Arterial and venous pressure monitoring.
The Work Group recommends
an organized monitoring approach with regular assessment of clinical parameters
of the AV access and dialysis adequacy. Data from the clinical assessment and
dialysis adequacy measurements should be collected and maintained for each patients
access and made available to all staff. The data should be tabulated and tracked
within each dialysis center as part of a Quality Assurance/Continuous Quality
Improvement (QA/CQI) program. (Opinion)
Table III-3. Access Flow Protocol Surveillance
Access flow measured by ultrasound dilution, conductance dilution, thermal dilution, doppler or other technique should be performed monthly. The assessment of flow should be performed during the first 1.5 hours of the treatment to eliminate error caused by decreases in cardiac output related to ultrafiltration. The mean value of 3 separate determinations performed at a single treatment should be considered the access flow.
AV Graft and AV Fistula
Access Flow less than 600 mL/min, the patient should be referred for fistulogram.
Access Flow less than 1,000 mL/min that has decreased by more than 25% over 4 months should be referred for fistulogram.
Surveillance of AV Grafts
Prospective surveillance of AV grafts for hemodynamically significant stenosis, when combined with correction, improves patency and decreases the incidence of thrombosis. (Evidence) Techniques, not mutually exclusive that can be used in surveillance for stenosis in AV grafts include, in order of decreasing preference:
A. Intra-access flow (Protocol provided in Table III-3) (Evidence)
C. Dynamic venous pressures (Protocol provided in Table III-5) (Evidence)
Other studies or information that can be useful in detecting AV graft stenosis include:
D. Measurement of access recirculation using urea concentrations (see Guideline 12: Recirculation Methodology, Limits, Evaluation, and Follow-up) (Evidence)
E. Measurement of recirculation using dilution techniques (nonurea-based) (Evidence)
F. Unexplained decreases in the measured amount of hemodialysis delivered (URR, Kt/V) (Evidence)
G. Physical findings of persistent swelling of the arm, clotting of the graft, prolonged bleeding after needle withdrawal, or altered characteristics of pulse or thrill in a graft (Evidence/Opinion)
H. Elevated negative arterial pre-pump pressures that prevent increasing to acceptable blood flow104,105 (Evidence/Opinion)
I. Doppler ultrasound (Evidence/Opinion)
Diagnostic Testing in AV Grafts
Persistent abnormalities in any of these parameters should prompt referral for venography. (Evidence)
Table III-4. Static Intra-Access Pressure (IAP) Surveillance Protocol
1. Establish a baseline when the access has matured and shortly after the access is first used. Trend analysis is more useful than any single measurement.
2. Assure that the zero setting on the pressure transducers of the dialysis delivery system being used has been calibrated to be accurate within ±5 mm Hg. If uncertain check the calibration (Step 8).
3. Measure the mean arterial blood pressure (MAP) in the arm contralateral to the access.
4. Enter the appropriate output or display screen where venous and arterial pressures can be visualized (this varies for each dialysis delivery system). If a gauge is used to display pressures, the pressure can be read from the gauge.
5. Stop the blood pump and cross clamp the venous line just proximal to the venous drip chamber with a hemostat (this avoids having to stop ultrafiltration for the brief period needed for the measurement). On the arterial line, no hemostat is needed since the occlusive roller pump serves as a clamp.
6. Wait 30 seconds until the venous pressure is stable, then record the arterial and venous intra-access pressure (IAP) values. The arterial segment pressure can only be obtained if a pre-pump drip chamber is available and the dialysis system is capable of measuring absolute pressures greater than 40 mm Hg.
7. Unclamp the venous return line and restore the blood pump to its previous value.
8. If uncertain about the accuracy of the zero value on the pressure transducers, clamp the tubing from the drip chamber(s) to the pressure transducer protector(s). Pull off the pressure protector(s) from their nipples and record the zero value(s), P0 (these are usually close to zero, but may deviate by 10 mm Hg or more below or above zero). Replace the pressure transducer(s) protector(s) and unclamp the line(s).
9. Determine the offset pressure(s), Poffset, between the access and the drip chamber(s) either by direct measurement (A) or using a formula (B) based on the difference in height between the top of the drip chamber and the top of the arm rest of the dialysis chair (Δ).
A. Measure the height from the venous or arterial needle to the top of the blood in the venous drip chamber in cm. The offset in Hg = height (cm) x 0.76. For practical purposes the same value can be used for both if the drip chambers are at the same height.
B. Use the formula, offset in mm Hg = 3.6 + 0.35 x Δ. The same value can be used for both if the drip chambers are the same height. If the drip chambers are not at equal heights, the arterial and venous height offsets must be determined individually. In a given patient with a given access the height offsets need to be measured only once and then used until the access location is altered by construction of a new access.
10. Calculate the normalized arterial and venous segment static intra-access pressure ratio(s), PIA.
Arterial PIA = (arterial IAP + arterial Poffset - arterial P0)/MAP
Venous PIA = (venous IAP + venous Poffset - venous P0)/MAP
NOTE. If the P is less than zero, algebraically subtracting a negative number is equivalent to adding the absolute number.
Interpretation: Venous outlet stenosis can be detected with venous PIA alone. Trend analysis is more useful than any single measurement. The higher the degree of stenosis at the outlet, the greater is the venous (PIA) pressure ratio. Strictures between the area of arterial and needle cannulation cannot be detected by measuring venous (PIA) pressure alone (1). Detection of these lesions requires the simultaneous measurement of pressures from both the arterial and venous needles. Central stenoses that have collateral circulation may have "normal pressures," but these usually present with significant ipsilateral edema. Accesses can be classified into the categories listed in the table below using the equivalent PIA ratios from the arterial or venous needles; the criteria must be met on each of two consecutive weeks to have a high likelihood of a 50% diameter lesion. The criterion in bold type is the primary criterion for the location of the stenosis, the other is supportive.
Access Type Graft Graft Native Native Normalized PIA Arterial Ratio Venous Ratio Arterial Ratio Venous Ratio Normal 0.35-0.74 0.15-0.49 0.13-0.43 0.08-0.34 Stenosis Venous outlet >0.75 >0.5 >0.43 or >0.35 Intra-access >0.75 and <0.5 >0.43 and <0.35 Arterial Inflow <0.3 NA <0.13 + clinical findings NA
Patients who develop a progressive and reproducible increase in venous or arterial segment > 0.25 above their previous baseline irrespective of access type are also likely to have a hemodynamically significant lesion. Intra-access strictures are usually characterized by the development of a difference between the arterial and venous pressure ratios > 0.5 in grafts or >0.3 in native fistula.
Table III-5. Dynamic Venous Dialysis Pressure Surveillance Protocol
Establish a baseline by initiating measurements when the access is first used.
Measure venous dialysis pressure from the hemodialysis machine at Qb 200 mL/min during the first 2 to 5 minutes of hemodialysis at every hemodialysis session.
Use 15-gauge needles (or establish own protocol for different needle size).
Assure that the venous needle is in the lumen of the vessel and not partially occluded by the vessel wall.
Pressure must exceed the threshold three times in succession to be significant.
Assess at the same level relative to hemodialysis machine for all measurements.
Interpretation of Result: Three measurements in succession above the threshold are required to eliminate the effect of variation caused by needle placement. Hemodialysis machines measure pressure with different monitors and tubing types and lengths. These variables, as well as needle size, influence venous dialysis pressure. The most important variable affecting the dynamic pressure at a blood flow of 200 mL/min is the needle gauge.9,129 It is essential to set thresholds for action based on machine manufacturer, tubing type, and needle gauge.
Using 15-gauge needles, the threshold that indicates elevated pressure (and therefore the likely presence of a hemodynamically significant venous outlet stenosis) for Cobe Centry 3 machines is a pressure of 125 mmHg, whereas the threshold for Gambro AK 10 machines is a pressure of 150 mmHg. Data for Baxter, Fresenius, Althin, and other dialysis machines are not available but are likely to be similar to those of the Cobe Centry 3 if the same gauge venous needle is used. Trial and error at each institution will determine each units threshold pressure.
Trend analysis is more important than any single measurement. Upward trends in hemodialysis pressure over time are more predictive than absolute values. Each unit should establish its own venous pressure threshold values.
Patients with progressively increasing pressures or those who exceed the threshold on three consecutive hemodialysis treatments should be referred for venography.
Rationale Physical examination can be used as a monitoring tool to exclude low flows associated with impending graft failures.106-108
Access flow determines the characteristics of pulse, thrill, and bruit. Palpable thrill at the arterial, mid, and venous segments of the graft predicts flows >450 mL/min.108 A pulse suggests lower flows. An intensification of bruit suggests a stricture or stenosis.108
In dialysis AV grafts, thrombotic events result primarily from progressive
venous outflow stenosis.9,13,66,103,109-113
Thrombotic events that cannot be resolved are the leading cause of access loss.
These stenoses are caused by intimal and fibromuscular hyperplasia in the venous
outflow tract, typically at the vein graft anastomosis.9,13,66,103,
As such stenoses increase in severity, they cause an increase in intra-access
pressure with an accompanying decrease in blood flow.114
It has been shown that when access flow is measured repeatedly, trends of decreasing
flow are predictive of access stenosis.115
Grafts with access blood flows less than 600 mL/min have a higher rate of access
thrombosis than grafts with flow rates greater than 600 mL/min.116-118
Recently, several investigators have shown that a trend of decreasing access
flow is more predictive of venous stenoses than any single measurement of access
flow (see Table III-3). The measurement of access flow
has also been shown to be a valuable tool in determining the success of a therapeutic
intervention. Failure to increase access flow by at least 20% following an intervention
reflects failure of the intervention to correct the underlying problem.208,209
|Table III-6. Patient Education Basics|
All patients should be taught how to:
1. Compress a bleeding access
2. Seal the site of a central venous catheter (CVC) with ointment to keep air embolus from entering
3. Wash skin over access with soap and water daily and before dialysis
4. Recognize signs and symptoms of infection
5. Select proper methods for exercising AV fistula arm with some resistance to venous flow
6. Palpate for thrill/pulse daily and after any episodes of hypotension, dizziness, or lightheadedness
7. Listen for bruit with ear opposite access if cannot palpate for any reason
All patients should know to:
1. Avoid carrying heavy items draped over the access arm or wearing occlusive clothing
2. Avoid sleeping on the access arm
3. Insist that staff rotate cannulation sites daily
4. Insure that staff are using proper techniques in preparing skin prior to cannulation
5. Report any signs and symptoms of infection or absence of bruit/thrill to dialysis personnel immediately
Because the development and severity of stenosis evolve to varying degrees among patients over time, the likelihood of detecting a hemodynamically significant stenosis increases if the Surveillance test is repeated. Therefore, surveillance should be performed at intervals of 1 month or lessdepending on the complexity and costto detect access dysfunction early and to permit sufficient lead-time for intervention. The Work Group concluded that trend analysis may be as important as any individual value for any monitoring technique.
Intervention with percutaneous transluminal angioplasty (PTA) or surgical revision to correct stenoses dramatically reduces the rate of AV graft thrombosis and loss.9,103,109,113,119
Sequential timely repetitive measurement of access flow is the preferred method for surveillance of AV grafts. To date, Doppler flow,115- 117,120 ultrasound dilution,118,121,122 and magnetic resonance123 have been the most extensively evaluated. All require specialized devices. Although Doppler studies can be predictive of access stenosis and the likelihood for failure,120,121 frequency of measurement may be limited by expense. In addition, inter-observer variability in measurement of Doppler flow in some instances can reduces the reliability of Doppler flow measurement.124 Variation in Doppler flow measurements performed by machines produced by different manufacturers also occurs. Magnetic resonance flow is accurate but expensive. Both Doppler flow and magnetic resonance are difficult to perform during dialysis sessions. In contrast, flow measurements performed by ultrasound velocity and other techniques using blood dilution are reliable and valid118,121,122 and can be done on-line during dialysis, thereby providing rapid feedback. The Work Group expects that, as technology improves, online access flow measurements using dilution technology will become more clinically applicable, and could supplant all other techniques by permitting accurate and inexpensive repetitive measurements at monthly intervals.
The Work Group believes that the value of routine use of any technique for detecting anatomic stenosis without concomitant measurement of access flow, venous pressure, recirculation, or other physiologic parameter has not been established.
Prospective Surveillance using dynamic or static venous dialysis pressures
detects outflow stenoses. Both methods have acceptable sensitivity and specificity,
are inexpensive, and are readily available.103,119
Using a standardized protocol (eg, measurements made with blood pump flow rates
of 200 mL/min), dynamic venous pressure monitoring is easily performed with
available methodology and existing equipment. Measuring venous pressure is the
least expensive method of Surveillance for stenosis.103,119
A protocol is provided in Tables III-4 and III-5.103,119
These techniques have been validated in prospective trials9,103,109,119,125,126
and are recommended weekly. Venous pressures (dynamic) while less predictive
than flow measurements, have been validated and should continue to be used until
flow measurements are widely available. Shortcomings of dynamic venous pressure
techniques are the need to standardize for blood tubing, needle size, and hemodialysis
Table III-7. Protocol for Urea-Based Measurement of Recirculation
Perform test after approximately 30 minutes of treatment and after turning off ultrafiltration.
1. Draw arterial (A) and venous (V) line samples.
2. Immediately reduce blood flow rate (BFR) to 120 mL/min.
3. Turn blood pump off exactly 10 seconds after reducing BFR.
4. Clamp arterial line immediately above sampling port.
5. Draw systemic arterial sample (S) from arterial line port.
6. Unclamp line and resume dialysis.
7. Measure BUN in A, V, and S samples and calculate percent recirculation (R).
Surveillance protocols that use static venous dialysis pressure (ie, venous dialysis pressure at zero blood pump flow) are even more strongly predictive of outflow stenoses than dynamic pressure measurements, but these approaches currently require specialized devices.9,114 The Work Group believes that measurement of static pressure every 2 weeks is the maximum frequency feasible with current hemodialysis staffing patterns. New techniques, however, may eliminate these shortcomings.127 If it becomes possible to adapt existing hemodialysis machines for static pressure measurement, then weekly measurement will be appropriate.
Trends in either dynamic or static venous dialysis pressure measurements are more predictive of access stenosis than any single pressure measurement.9,103,119 (See protocol provided in Tables III-4 and III-5.)
Increases in urea recirculation are also predictive of venous stenoses.113,128 However, the Work Group believes that recirculation is a relatively late predictor of access dysfunction. Urea measurement for the calculation of recirculation must be done under standardized conditions. Nonurea-based recirculation measurements are very accurate but require specialized devices (see Guideline 12: Recirculation Methodology, Limits, Evaluation, and Follow-Up).
Unexplained decreases in delivered dialysis dose, as measured by Kt/V or URR, are frequently associated with venous outflow stenoses.113 However, many other factors influence Kt/V and URR, making them less sensitive and less specific for detecting access dysfunction.
Regular assessment of physical findings (Monitoring) may supplement and enhance an organized surveillance program to detect access dysfunction.103,107,108 Specific findings predictive of venous stenoses include: edema of the access extremity, prolonged bleeding postvenipuncture (in the absence of excessive anticoagulation), and changes in the physical characteristics of the pulse or thrill in the graft.103,108 Physical examination is a useful screening tool to exclude low flow (<450 mL/min) in grafts with impending failure.106-108 A palpable thrill at the arterial, mid-graft, and venous segments is associated with flow >450 mL/min.108 Conversion of thrill to pulse indicates lower flows. Intensification of bruit (higher pitch) indicates a stenosis.106,107 Therefore, in the context of proper needle position, an elevated negative arterial pre-pump pressure that prevents increasing the blood flow rate to the prescribed level is also predictive of arterial inflow stenoses.
When a test indicates the likely presence of a stenosis, venography or fistulography are used to confirm the lesion.
In addition to monitoring conducted by the professionals on the healthcare team, the patient and the patients caregivers should be educated about simple emergency procedures and basic care of the access site (see Table III-6).
Monitoring Primary AV Fistulae for Stenosis
Primary AV fistulae should be monitored as outlined for dialysis AV grafts (see Guideline 10: Monitoring Dialysis AV Grafts for Stenosis). (Opinion)
Direct flow measurements, if available, are preferable compared to more indirect measures. (Evidence)
Methods appropriate for monitoring stenosis in grafts (eg, static and dynamic venous pressures) are not as accurate for monitoring in primary AV fistulae. (Evidence) Recirculation and Doppler analysis are of potential benefit. (Opinion)
Rationale In primary AV fistulae, inadequate flow through the access is the primary functional defect predictive of thrombosis and access failure. Thus, flow measurements should be used when available to monitor for stenosis and thrombosis in AV fistulae.121,122 Stenoses in AV fistulae tend to occur more centrallyin the outflow tract at areas of vein bifurcation, pressure points, and venous valvesrather than in close proximity to the venous outlet (as is the case with grafts).9,114 As a result, collateral veins draining an AV fistula develop, preventing a marked increase in pressures.9,114,130 Indirect measures of flow, such as dynamic and static venous dialysis pressure, are therefore less predictive of thrombosis and access failure in AV fistulae compared to AV grafts. Measurement of recirculation, on the other hand, becomes a more useful screening tool in AV fistulae compared to AV grafts because flow in AV fistulae, unlike AV grafts, can decrease to a level less than the prescribed blood pump flow (ie, less than 300 to 500 mL/min), while still maintaining access patency115,130 (see Guideline 12: Recirculation Methodology, Limits, Evaluation, and Follow-Up). Since pressure measurement and recirculation may be late predictors of access dysfunction in AV fistulae, Doppler ultrasound may be useful despite its increased cost. However, the absence of validation studies precludes Work Group recommendations at this time.
Recirculation Methodology, Limits, Evaluation, and Follow-Up
Recirculation should be measured using a nonurea-based dilutional method or by using the two-needle urea-based method. The three-needle peripheral vein method of measuring recirculation should not be used. (Evidence)
Any access recirculation is abnormal. Recirculation exceeding 10% using the recommended two-needle urea-based method, or 5% using a nonurea-based dilutional method, should prompt investigation of its cause. (Evidence)
If access recirculation values exceed 20%, correct placement of needles should be confirmed before conducting further studies. (Evidence/Opinion)
of access recirculation should be investigated using angiography (fistulography)
to determine whether stenotic lesions are impairing access blood flow. (Evidence)
|Fig III-2. Access and cardiopulmonary recirculation. The figure shows a simplified sketch of the dialysis circuit with an AV access depicting local access recirculation (dotted line with arrows) and cardiopulmonary recirculation. Cardiopulmonary recirculation is associated with an arteriovenous difference in BUN (100 versus 95 mg/dL) which results from dialyzed blood "short circuiting" the capillaries where blood is "refilled" with urea (95 to 99.4 mg/dL); this "short circuiting" decreases dialysis efficiency. This sketch includes regional blood flow inequalities and the resulting impact on BUN in veins draining poorly perfused (BUN >99.4 mg/dL) and well-perfused (BUN <99.4 mg/dL) areas. This venovenous disequilibrium increases late in dialysis when greater differences develop in urea concentrations among the regions of the body, a consequence of varying urea washout (regional blood flow model).134 Reprinted with permission.134|
Rationale The three-needle, peripheral vein method for measuring recirculation overestimates access recirculation in an unpredictable manner and requires unnecessary venipuncture.131-133 This is illustrated in Fig III-2,134 which shows both the effect of regional blood flow differences (venovenous disequilibrium) and the effect of the return of a fraction of dialyzed blood to the dialyzer without passage through the tissues first (arteriovenous disequilibrium or cardiopulmonary recirculation). Because of these effects, the peripheral venous blood urea nitrogen (BUN) is substantially greater than the BUN in arterial blood. The use of a peripheral venous sample in place of an arterial sample overestimates actual access recirculation.
The urea-based method described uses two needles and avoids overestimation of recirculation (see Table III-7). The recommended approach for this method is simple and is based on two considerations. The dead space of arterial lines to the sampling port is less than 12 mL. Access recirculation generally does not occur (except for reversed needles) unless access blood flow rates are less than dialyzer blood pump flow rates.130 This conclusion is supported by studies using nonurea-based methods that show that recirculation is absent (0%) in a properly cannulated, well-functioning access.135-138 A blood flow rate of 120 mL/min for 10 seconds will clear the arterial line dead space. Sampling at this time will provide arterial blood prior to onset of rebound. The results using the recommended two-needle method should average zero (−5% to +5%) in patients with unimpaired accesses. Although nonurea-based dilutional methods are more accurate and avoid problems with cardiopulmonary recirculation, they require specialized devices,138 which limit their applicability.
New vascular accesses are at particular risk for reversed needle placement due to a lack of familiarity with the access anatomy. Whenever possible, an access diagram that depicts the arterial and venous limbs should be obtained from the surgeon who constructed the access to aid in proper cannulation. If not available, the anatomy can be deduced by temporarily occluding the graft at its midportion. The portion retaining a pulse is the arterial limb.
The amount of recirculation occurring with reversed needles is usually substantial (>20%). However, even with ideal sample timing and proper cannulation, laboratory variability in urea-based measurement methods will produce variability in calculated recirculation.137 Therefore, individual recirculation values <10% using urea-based methods may be clinically unimportant. The Work Group believes they do not prompt further evaluation. Values >10% using urea-based recirculation measurement methods do require investigation. However, the Work Groups opinion is that values >5% using nonurea-based dilution techniques are a reliable indication of an abnormality and should prompt investigation into the cause.
Access recirculation in a properly cannulated access is a sign of low access blood flow130 and is a marker for the presence of vascular access stenoses. Such stenoses can be corrected, thereby decreasing the risk of access thrombosis and prolonging access longevity.128
Angiography/fistulography are used to establish the presence of the stenosis (see Guideline 10: Monitoring Dialysis AV Grafts for Stenosis, and Guideline 17: When to Intervene Dialysis AV Grafts for Venous Stenosis, Infection, Graft Degeneration, and Pseudoaneurysm Formation).
© 2001 National Kidney Foundation, Inc