Simplified Measurement of Intra-Access Pressure

ANATOLE BESARAB,* STANLEY FRINAK,* RICHARD A. SHERMAN,t

JESSE GOLDMAN,t FRANCES DUMLER,� MARIA V. DEVITA,11 TOROS KAPOIAN,�

FAHED AL�SAGHIR* and TOMMY LUBKOWSKI**Di�,ision of Nephrology and Hypertension, Department of Medicine, Henry Ford Hospital, Detroit, Michigan;

tDepartment of Medicine, Division of Nephrology, University of Medicine and Dentistry of New Jersey,

Robert Wood Johnson Medical School, New Brunswick, New Jersey; �Division of Nephrology, Department of

Medicine, Thomas Jefferson University, Philadelphia, Pennsylvania; �Division of Nephrology, Department of

Medicine, William Beaumont Hospital, Detroit, Michigan; “Division of Nephrology, Department of Medicine,

Lenox Hill Hospital. New York,

New York.

Abstract. The measurement of intra-access pressure (VIA) nor-

malized by mean arterial BP (MAP) helps detect venous outlet

stenosis and correlates with access blood flow. However, gen-

eral use of PIA/MAP is limited by time and special equipment

costs. Bernoulli’s equation relates differences between �IA

(recorded by an external transducer as �T) and the venous drip

chamber pressure, MDC; at zero flow, the difference in height

(�H) between the measuring sites and fluid density determines

the pressure � �IA � �DC#{149} Therefore, � and ‘�T

measurements were correlated at six different dialysis units,

each using one of three different dialysis delivery systems

machines. Both dynamic (i.e. , with blood flow) and static

pressures were measured. Changes in mean BP, zero calibra-

tion errors, and hydrostatic height between the transducer and

drip chamber accounted for 90% of the variance in MDC’ with

�‘�H � 1.6 + 0.74 z�H (r = 0.88, P < 0.001). The majordeterminants of static PtA/MAP were access type and venous

outflow abnormalities. In grafts, flow averaged 555 ± 45

mllrnin for PIA/MAP > 0.5 and 1229 ± 1 12 mllmin for

PtA/MAP < 0.5. � varied from 9.4 to 17.4 mmHg among

the six centers and was related to �H between the drip chamber

and the armrest of the dialysis chair. Concordance between

values of PIA/MAP calculated from �T and from �DC �

was excellent. It is concluded that static �DC measurements

corrected by an appropriate � can be used to prospectively

monitor hemodialysis access grafts for stenosis. (J Am Soc

Nephrol 9: 284-289, 1998)

Hemodialysis access failure is a major and increasing source of

morbidity and cost in chronic hemodialysis patients (I ,2). The

predominant hemodialysis access in use in the United States is

the polytetrafluoroethylene (PTFE) arteriovenous (AV) graft

(3) that commonly fails as the result of stenotic lesions that

culminate in thrombosis (4), a complication preventable by

early recognition and correction of functionally significant

stenotic lesions (5,6).

Dynamic measurement of venous line drip chamber pressure

(MDc) at pump blood flow rates of 200 mllmin is one of the

most commonly used screening tools for detecting venous

stenoses in vascular access grafts (5). Developing stenotic

lesions raise intra-access pressure (viA) proximal to the lesion

impeding blood flow through the access. This increase in �IA

is reflected as an increase in �DC#{149} However, at a blood flow of

200 ml/min, much of the pressure recorded at the venous drip

chamber is caused by flow resistance through the venous

Received April 3. 1997. Accepted July 20, 1997.Correspondence to Dr. Anatole Besarab, Division of Nephrology and Hyper-

tension, CFP-5, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit,MI.

1046-6673/0902-0284$03.OO/OJournal of the American Society of NephrologyCopyright © 1998 by the American Society of Nephrology

needle and not by ‘31A (7). Thus, dynamic measurements of

�DC are susceptible to errors created by inconsistencies inblood pump speed and variations in needle size and needle

placement (6). �1A is also influenced by mean arterial BP

(MAP). Fluctuations in MAP, which alter �IA and therefore

‘�DC, are seldom considered in protocols that use �DC to mon-itor vascular accesses.

Sequential direct monitoring of �IA and the ratio of VIA’

MAP has a high level of sensitivity and specificity as a screen-

ing tool for venous outlet stenoses in patients with AV grafts

(6,8), but cost and time requirements preclude routine appli-

cation of this procedure. Dynamic effects on �DC are elimi-

nated at zero blood flow, i.e. , static conditions. Under static

conditions, ‘�IA �S equal to �DC plus the hydrostatic pressure

created by the column fluid between the access site and venous

drip chamber transducer. This hydrostatic pressure difference,

�‘�H �IA � ‘�DC� �S a function of fluid density and the heightdifferences between the two sites (Figure 1 ). Therefore, under

static conditions, an equivalent intra-access pressure and ratio,

EQ�IA = ‘�DC � �‘�H and EQPIA/M, can be calculated from

PDc that is equivalent to PtA/MAP.To establish EcPIA/MAP �IS an effective and practical method

for monitoring PIA/MAP, we evaluated the relationship of �IA and

PDC measurements on a variety of dialysis delivery systems indifferent centers to calculate center-specific appropriate mean

�DC

Transducer

Transducer 4”

Simplified Measurement of Intra-Access Pressure 285

values. The variation in � among different dialysis ma-chines and centers was related to a simply measured parameter,

the height difference between the drip chamber and the armrest of

the dialysis chair. To define the clinical relevance of EQPIA/MAP

in detecting hemodynamically significant vascular access steno-

ses, Doppler ultrasound studies and angiography were performed

at the initial dialysis site (hereafter referred to as the alpha site),

where the methodology was developed.

Materials and MethodsThe study included six different dialysis centers in the United

States. Dialysis delivery systems studied used digitally displayed

venous mean pressures; these included Fresenius-H (F-H), Cobe Cen-tury 3 (CC3), and Althin 2000 (ALT). At each center, a pressure

transducer (Namic USA, Glenns Falls, NY) was placed between the

venous return tubing and the venous needle and connected to a

pressure monitor (Hewlett-Packard, King of Prussia, PA), as depicted

in Figure 1. The transducer was taped to the extremity at the bevel ofthe venous needle in the access, and pressures were recorded from the

patient monitor connected to the transducer, �T’ and from the venousdrip chamber pressure display. �DC#{149} Atmospheric pressure served as

the zero reference point for the access transducer and the drip chamber

transducer. �DC and �T were recorded at the patient’s usual blood

flow (250 to 500 ml/min), at zero (static) flow, and at one or more

intermediate blood flows.

To measure �DC accurately during no-flow conditions, the drip

chamber has to be isolated from the effects of any ultrafiltration in thedialyzer to prevent dissipation of pressure through the dialyzer mem-

brane. This can be accomplished by either shutting off the ultrafiltra-tion or clamping the venous line between the dialyzer and venous drip

chamber. Preliminary studies in 12 patients revealed no differencebetween the two techniques, ��H’ averaging 17. 1 ± 1.6 mmHg versus

18.5 ± 1.3 mmHg (P = NS, paired t test). Because the clamp method

was simpler and quicker, it was used thereafter at all sites. Equilib-

rium static pressures, �DC and �T’ were recorded 20 to 30 s after the

blood pump was stopped. Simultaneously. MAP was recorded by an

automatic BP monitoring system.

At the alpha site, the height between the level of the transducer and

the top of the blood column in the venous drip chamber was measured

in 33 patients with a ruled T-square and level as �DC and �T were

measured. Pressures and height differences were remeasured 1 to 2 wk

later to assess replicability and to determine the source of variability

in P�. At the end of dialysis, calibration of the drip chamber

transducer was verified using a mercury manometer. At two centers,

measurements were conducted by dialysis nurses or technicians to

assess the practicality of the procedure.

Direct or indirect measurement of intra-access pressures is of little

clinical relevance unless the measurements reflect access flow and

predict hemodynamically significant stenosis. A 50% by diameter

stenosis is widely accepted as hemodynamically significant and as a

precursor of access failure (5,6,9,10). Flow and pressure in grafts areinversely related (I 1). Flow less than 500 to 650 mb/mm in grafts

correlates with either stenosis or impending thrombosis (9-12).

Therefore, patient enrollment at the alpha site required informed

consent that included subject willingness to undergo Doppler flow

studies and angiography. Doppler flow measurements were conducted

on all 33 patients with a permanent access. Angiography was per-

formed on all 26 prosthetic graft accesses and three of the seven

autologous fistulas. The degree of stenosis was graded by the radiol-

ogists as: absent to mild (<30% lumen reduction by diameter [%D]),

possibly hemodynamically significant (30 to 40%D), and hemody-

namically significant (>50%D). At the other sites, patients were

selected at random to represent the mix of access types in use. Neither

Doppler flow nor angiography studies were performed.

Statistical Analyses

Data were analyzed using Systat for Windows (version 5.02. Sys-

tat, Inc., Evanston. IL). All results are presented as mean ± SEM. A

P value <0.05 was accepted as statistically significant.

L� H ( height)

Arterial

Venous

Figure 1. Schematic of circuit showing that the measurement of pressure at the level of the access by an external transducer and from the drip

chambers differs by a height difference (L�H).

XPH=-l.6+O.74�H.

(p<O.O()I)

10 20 30 40 50

y = 24+ O.34x +O.00037x2

. Venous Drip Chamber

. Transducer

static level

300 400 500 600

286 Journal of the American Society of Nephrology

ResultsOverall, we studied 185 arm prosthetic grafts, 57 native

arteriovenous fistulas, and four central venous catheters; the

number of permanent accesses studied among centers varied

from 16 to 7 1 . Inclusion of native AV fistulae along with a few

catheters allowed a wider variation in pressures and in �

than that allowed by the study of prosthetic grafts alone. At

blood flows most often used for dialysis, P� exceeded �T by

more than 40 mmHg at all sites (data not shown). Figure 2

shows the relationship between �T and �DC measured over a

range of flows at the alpha site. At a blood flow of 150 ml/min,

‘3T and �DC were virtually identical; however, at this flow �DC

exceeded ‘�T by 1 1 mmHg with Cobe and Althin dialysis

equipment (data not shown).

At zero flow, Figure 2 shows that �DC is less than �T#{149}This

observation was consistently found at all six centers (Table 1).

At zero blood flow, �T �S equal to VIA’ therefore, � =

- �DC The mean � ranged from 9.4 to 17.4 mmHg amongcenters. At the Fresenius alpha center, � was 1 7.3 ± 1.1

mmHg. The mean � in clinical practice obtained by the

nursing staff at Fresenius center B in 62 patients was 17.4 ±

0.7 mmHg. Figure 3 depicts the correlation between � and

the height difference (�H) in centimeters during static condi-tions, � � 1.6 + 0.74 i�H (r 0.88, P < 0.001). The

theoretical value for the slope is 0.76.

The influence of different factors on static P� was exam-

med at the alpha site in 33 patients during replicate measure-

ments conducted 1 to 2 wk apart. Using the first measurement

as the reference value, the second value differed by -3.3 ± 1.9

mmHg (range, -34 to + 13 mmHg; P = NS). Fluctuations in

systemic BP, differences in zero calibration among dialysis

machines, variability in �H from one session to the next, and

other measurement inaccuracies accounted for 45, 34, 1 1 , and10%, respectively, of the total variance in P�.

Although �H is the major determinant of the pressure dif-

ference between #{241}A and �DC at zero flow, individual measure-

Table 1. Pressures measured with blood flow set to zero�’

Center (,,) �T MDc: �H

Fresenius-H 66 43 ± 3 25 ± 3 1 7.3 ± 1.1

Fresenius-H 62 40 ± 2 23 ± 3 17.4 ± 0.7

Cobe Century 3 71 42 ± 2 25 ± 2 16.4 ± 0.8

Cobe Century 3 3 1 36 ± 4 24 ± 4 1 1.7 ± 0.8

Althin 33 43 ± 5 33 ± 5 10.5 ± 0.9

Althin 16 41 ± 7 31 ± 7 9.4 ± 1.2

a �T’ pressure transducer; P0�. venous drip chamber pressure;

�‘�H’ hydrostatic pressure difference.

(mm Hg)

40

30

20

10

0

-10-10

Height Difference (cm)

Figure 3. Individual measurements of the height difference between

the access site and blood level in the venous drip chamber versus the

difference in pressure, i�PH. The intercept does not differ from zero,

and the slope is equal to that predicted from the specific gravity of

mercury, 0.76.

ments of �H may be impractical. Therefore, we examined the

use of the average Z�PH obtained from all patients at each

center to calculate an equivalent VIA’ defined as EQ�IA = �DC

+ center-specific mean � Results obtained from 66 mdi-

350

300

Pressure

(mm Hg) 250

200

150

100

50

0

0 100 200

Blood Flow Rate (mL/min)

Figure 2. Relationship between the pressure measured at the drip chamber and at the transducer at a variety of blood flows. Each set of datafitted as quadratic equations; the intercepts differ by 17 mmHg.

�D � 17.3

(mm Hg)

.

lOt)

90

80

70

60

50

40

30

20

10

0

I �

Simplified Measurement of Intra-Access Pressure 287

vidual measurements at the alpha site (��H = 17.3) are shown

in Figure 4. Values predicted from the regression equation are

not significantly different from the identity line (paired t test).

Results for the degree of agreement between the PIA/MAP ratio

and the EQPIA/MAP ratio calculated from adjusted P,� data are

given in Table 2. No significant differences between PIA/ MAP

and EQPIA/MAP ratios (paired t test) were noted for either

native or graft access types at any center. Concordance ofPIA/MAP and EQPIA/MAP ratios was attained in 91% of 210

prosthetic graft accesses (Table 3) when using a ratio of >0.5

as evidence of a 50% by diameter venous outflow stenosis

(vida infra). Discordance in most of the remaining cases re-

sulted from small differences in PIA/MAP compared with

EQPlA�’MAP, producing a straddling across the criterion.To make measurement of EQPIA more practical, we analyzed

the factors that determined average � at a given center. The

height from the floor to the top of the venous drip chamber for

any dialysis machine is set by the holder in which it is placed.

Most patients have an arm access whose reference point is the

armrest of the chair on which the access rests. Therefore, if the

measurements are made in such a manner that the access is

always fixed relative to the armrest of the chair, the average

� should be proportional to the difference in heights fromthe floor to armrest and from the floor to the top of the drip

chamber. Figure 5 shows that the average L�PH is a direct

function of this height difference. The positive intercept re-

flects that the access is actually above the armrest.

To determine which value of PIA/MAP is a critical value for

the likely presence of stenosis in a graft access, we analyzedour previously reported data set (6), using mean intra-access

pressures (rather than systolic pressures) and MAP pairs to

construct a receiver operating curve. In grafts, a PtA/MAP ratio

of 0.5 maximized sensitivity (8 1%) and specificity (80%) for

detecting a 50% diameter stenosis. Fourteen of 26 subjects

with grafts at the alpha site had an EQPIA/MAP >0.5. By

angiography, 12 of the 14 had at least one stenosis >50% by

0 10 20 30 40 50 60 70 80 90 100

�T (mm Hg)

Figure 4. Comparison of the value of intra-access pressure derived

from the use of an average � = I 7.3 mmHg as opposed to an

individually measured value with the actual intra-access pressure, P�.

The regression line does not differ from identity. In 53 of 66 mea-surements, the estimated and directly measured intra-access pressures

differ by less than 10 mmHg.

diameter. Intra-access flow (by Doppler) in these 12 patients

with severe stenosis averaged 555 ± 45 mb/mm (range, 395 to

903 mllmin) with flow <650 ml/min in 10 of these accesses.

The two grafts with less severe lesions between 30 and 50%

had flows >800 ml/min. By contrast, only one of 12 subjects

with a ratio <0.5 had a stenosis greater than 50% by Doppler;

mean flow of these 12 patients was I 229 ± 1 12 mi/mm (range,

783 to 1827 mI/mm), and none had a flow <650 mI/mm. For

comparative purposes, the seven native fistulas all had EQ�IA’

MAP <0.5, and flow averaged 1064 ± 325 ml/min, although

the degree ofdispersion was large (range, 376 to 2128 ml/min).

DiscussionRecognizing and treating the failing vascular access is an

issue of growing concern for those taking care of maintenance

hemodialysis patients. The increasing use of permanent in-

dwelling central catheters and the construction of exotic ter-

tiary accesses (3,13-16) attest to the increasing difficulty of

maintaining vascular access patency in an aging population. To

date, the most commonly used clinical techniques to detect

functionally significant access problems have been venous drip

chamber pressures (“venous pressure”) and recirculation mea-

surements. The goal has been to detect and treat stenotic

lesions (17-19) by surgery or angioplasty to decrease the risk

of clotting and loss of the access site (5,6,20,2 1 ). The best

access evaluation tool in the future is likely to be the direct

on-line measurement of access flow rate during dialysis (22).

Until then, venous pressure measurements in grafts act as

imperfect surrogates for flow.

Many dialysis units use dynamic pressure monitoring of P�

at 200 ml/min, as originally described by Schwab et a!. (5), toscreen for venous outlet stenosis despite the dependence of

�DC #{176}� needle gauge (6) and blood flow (7). At the recom-mended blood flow rate of 200 mI/mm, our data indicate that

the measured venous pressure, resulting largely from needle

resistance, is two- to fourfold higher than the actual intra-access

pressure (Figure 2). Most centers that adopted dynamic P,�

monitoring never considered the effect of needle gauge on the

threshold pressure that should be used to trigger further access

evaluation. Static intra-access pressure monitoring minimizes the

complicating effects of flow, needle gauge, and needle malposi-

tion within the access. A comparative study found that static

measurement of the intra-access pressure was superior to low-

flow dynamic pressures in detecting access stenosis (23).

Our studies have attempted to advance the use of staticintra-access venous segment pressures as a screening tool in

AV grafts to detect hemodynamically significant (greater than

50% by diameter) venous outlet stenoses. For static measure-

ments to be as “user friendly” as dynamic measurements, we

have focused on obviating the need for an in-line pressure

transducer and separate pressure-monitoring system. Our data

show that the major determinant of the difference in static

pressure recorded from the venous drip chamber transducer

and the in-line access transducer is the height difference be-

tween the two sites. Therefore, equivalent intra-access pres-

sures, EQ’�IA� can be reliably estimated from the drip chamber

pressure by making the measurement at zero blood flow and

I:

.

40

288 Journal of the American Society of Nephrology

Table 2. PIA/MAP ratio and EQPIA/MAP ratio calculated from adjusted �DC�’

Access Type Center Native PIA/MAP Native EQPIA/MAP Graft PIA/MAP Graft EQPJA/MAP

Fresenius-H 0.20 ± 0.02 0.23 ± 0.03 0.48 ± 0.03 0.47 ± 0.03

Fresenius-H 0. 16 ± 0.02 0. 1 8 ± 0.03 0.42 ± 0.02 0.42 ± 0.03

Cobe Century 3 0. 1 1 ± 0.03 0.07 ± 0.02 0.49 ± 0.02 0.49 ± 0.03

Cobe Century 3 0.23 ± 0.02 0.23 ± 0.02 0.39 ± 0.01 0.39 ± 0.01

Althin 0.12 ± 0.04 0.12 ± 0.04 0.49 ± 0.04 0.48 ± 0.04

Althin 0.26 ± 0.08 0.27 ± 0.09 0.53 ± 0.09 0.52 ± 0.08

a P/MAP intra-access pressure/mean arterial BP; EQPIA. equivalent intra-access pressure.

Table 3. Concor dance of PIA/MAP and EQPI A/MAP ratiosa

CenterTrue

. .

PosltlveTrue

.

NegatlveFalse

. .

PosltlveFalse

.

NegattveTotal

Fresenius-H 26 21 2 1 50

Fresenius-H 15 18 2 3 38

Cobe Century 3 20 32 6 2 60

Cobe Century 3 6 1 8 2 0 26

Althin 11 16 0 1 28

Althin 5 3 0 0 8

Total 83 108 12 7 210

a Abbreviations as in Table 2.

( mm Hg)

L�Pu 0.35M1+ 3.4

r2 = 0.93

10 20 30

Height in Centimeters

(Drip Chamber - Arm Rest Height)

Figure 5. Mean � determined at each center versus the height differ-

ence between the armrest of the chair and top of the venous drip chamber.

taking into account the vertical distance between the measure-

ment points. This simplification permits easy, cost-free, se-

quential estimation of �lA without the requirement of any

extraneous pressure monitoring system. Although measure-

ment of an individual height difference with each measurement

of �DC would provide the most accuracy, it is clinically im-

practical and appears unnecessary.

This fortuitous outcome arises from the overriding impor-

tance of MAP on intra-access pressure. As with �lA’ the most

important influence on static �DC is the systemic BP. Almost

half of the total variation in � during replicate measurements

comes from changes in MAP. Only 1 1 % of the total variation

in P0C comes from height differences between sessions. Zero

calibration errors of the drip chamber transducer can produce

significant variation, requiring that transducers on machines be

frequently checked and maintained.

Despite the use of an average center-specific z�PH rather than a

session-specific value to calculate EQPIA� the mean EQP IA/MAP

ratios did not statistically differ from those obtained by measuring

PtA/MAP directly. Correlation between the mean value of VIA’

MAP and EQPIA/MAP was excellent. More importantly, the de-

gree of concordance for single individual measurements was quite

high (Table 3). In individual patients, a deviation in EQP IA/MAP

from PIA/MAP of up to ±0. 16 did occur in isolated measure-

ments, but such differences were not reproducible.

Clinical use of normalized intra-access pressure ratios re-

quires the establishment of a criterion value that has adequate

sensitivity and specificity in detecting a 50% by diameter

venous outlet stenosis associated with low flow. Analysis of

our previous data (6) suggested that a ratio of 0.5 in grafts

would have sufficient discriminatory power. We validated this

criterion value, EQPIA/MAP ratio >0.5, by demonstrating its

association with hemodynamically significant (lower flow)

angiographically confirmed stenosis. At the alpha site, all 26

patients with grafts were studied with angiography and Dopp-

ler ultrasound to evaluate access flow. Twelve of 14 patients

with EQPIA/MAP ratio >0.5 had documented 50% or greater by

diameter stenosis. This was associated with decreased access

flow, I 0 of the I 2 having a flow <650 ml/min. In contrast,

only one of 12 patients with an EQPIA/MAP ratio <0.5 had a

50% diameter stenosis, and the flow averaged more than 1 L.

The sensitivity of EQPIA/MAP ratio >0.5 for detecting a 50%

diameter stenosis was 86% with a specificity of 92%.

A PTFE graft with a ratio of >0.5 is therefore likely to have

a venous outlet stenosis producing a flow rate below 650

ml/min. Many studies indicate that a flow in this range pro-

duces a substantially increased risk for thrombosis (9-12,

21 ,23,24). The likelihood of thrombosis within 6 mo increases

severalfold when access flow decreases below 600 mllmin

(I 2,24). Thrombosis rates also increase with the degree of

stenoses (9). Elective angioplasty of hemodynamically signif-

icant lesions prevents thrombosis by increasing the flow (1 1).

Our more recent studies have shown regression of EQPIA/MAP

toward the true mean value of PIA/MAP within three to five

measurements (our unpublished observations). Thus, a stable

Simplified Measurement of Intra-Access Pressure 289

baseline can be established within 1 mo when values are obtainedweekly. This interval is short relative to that over which a stenosis

develops. Sequential measurements thereafter should be valuable

in diagnosing whether a prosthetic graft is developing a hemody-

namically significant lesion and in determining the timing of

additional studies or referral for intervention. A determination of

EQPIA can be made in less than 1 mm and can potentially be

automated. A progressive and persistent increase of EQPIA/MAP

of 0.2 above baseline would indicate the need for further evalu-

ation with flow measurements, using dilution techniques or direct

referral for ultrasound/fistulography. We are currently using such

a combined approach with considerable success.

It should be recognized that the use of intra-access pressure

to indirectly evaluate access flow is imperfect. First, elevatedvenous segment pressures typically occur in grafts only if the

stenosis is distal to the graft segment used for venous needle

insertion. Mid-access intra-access strictures or stenoses proxi-

mal to the venous cannulation sites, less common than venous

outlet stenosis, cannot be detected with venous needle VIA’

MAP measurements alone. Approximately 15% of patients

with grafts will have stable EQPIA/MAP >0.5 values but he-

modynamically insignificant access stenoses. To avoid exces-

sive referrals, we recommend establishing a baseline access

flow by Doppler or by one of the new on-line dilution tech-niques followed by weekly determination of EQPIA/MAP for all

PTFE grafts. Still other patients with grafts accesses have

repeatedly measured PIA/MAP ratios <0.5 but relatively low

flow (<600 mI/mm) due to either decreased cardiac output orfrom calcific disease of the artery supplying the access. The

risk for access thrombosis in this subgroup of patients is

undefined at present.In conclusion, the results of this study establish that a simple

measurement of venous drip chamber pressure made with

blood flow set to zero can be used to calculate an equivalent

intra-access pressure. In practice, this is easiest to achieve by

clamping the tubing segment between the dialyzer and venous

drip chamber with a hemostat immediately upon stopping the

blood pump. �DC is measured at 30 s. The height difference

between the arm of the chair and the blood level in the drip

chamber (z�H) can be determined at any time and should be

constant for any particular chair. One then calculates EQPIA =

PDc2 + (0.35 X �H + 3.4). A simultaneous measurement of

MAP allows for the calculation of the EQPIA/MAP ratio. This

ratio is an effective method of routine monitoring for venous

outlet stenoses in graft accesses. This simple, essentially freemonitoring method has the potential to significantly improve

patient care and at the same time reduce the medical cost of

chronic hemodialysis.

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