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Kidney Support/Dialysis/Vascular Access

Study of Discrepancies Between Recorded and Actual Blood Flow in Hemodialysis Patients

Kimata, Naoki*; Wakayama, Koji; Okano, Kazuhiro*‡; Hibi, Asako*; Sawada, Anri; Tajima, Yoko; Makabe, Shiho; Tsuchiya, Ken*‡; Mineshima, Michio; Nitta, Kosaku; Akiba, Takashi*

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doi: 10.1097/MAT.0b013e3182a708b9
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Abstract

According to a Japanese nationwide statistical survey in 2008, the average (± standard deviation) blood flow (Qb) measured during hemodialysis (HD) treatment was 197 ± 31 ml/min.1 The percentage of patients treated with Qb < 199 ml/min or 200–299 ml/min are 32.7% and 65.6%, respectively. Although HD patients in Japan are generally treated with relatively low Qb during HD, low mortality rates are frequently reported in these patients (1 year survival rate = 87%, 5 years survival rate = 60%).2 According to the Kidney Disease Outcomes Quality guideline, vascular access (VA) is inadequate when a Qb of 300 ml/min is not achieved.3 Because mortality rates in HD patients improve with increases in the number for quantifying HD and peritoneal dialysis treatment adequacy (Kt/V) as well as the urea reduction ratio (URR), the need to maximize delivered Qb (dQb) has increased in importance. Higher Qb offers the benefits of increased clearances, when used in conjunction with high-efficiency and high-flux dialysis membranes. Some practitioners decrease the rate of Qb during periods of intradialytic hypotension. A report revealed that blood pressure (BP) during HD is often higher at a higher Qb rate than at a lower Qb rate.4 Thus, high Qb is beneficial for patients if VA has the potential to provide adequate Qb.

On the other hand, there is an increase in the number of HD patients with blood flow problems related to VA.2 Although the importance of actual dQb is often mentioned, it is rarely measured. Williams et al.5 reported that better Kt/V could be achieved when the prescribed Qb rate was equivalent to the actual dQb rate. When Qb defined as reported Qb (rQb) is recorded using a dialysis machine, close monitoring is required to check for large discrepancies between dQb and rQb. The purpose of the present study was to assess discrepancies between actual dQb and various factors, including rQb recorded using a dialysis machine.

Methods

Participants

We measured dQb in 100 HD patients with arteriovenous fistulas (AVFs). HD was performed in these patients as shown in Table 1. Cause of uremia were chronic glomerulonephritis (n = 53), diabetes mellitus (n = 20), renal sclerosis (n = 5), polycystic kidney disease (n = 2), and other diseases, including diseases with unknown causes (n = 20). Maintenance HD were carried out for 3–4 h every session, three times a week with dialysate flow rate of 500 ml/min containing Na+ (140 mEq/L), K+ (2.0 mEq/L), Cl (110 mEq/L), Ca2+ (3.0 mEq/L), Mg2+ (1.0 mEq/L), HCO3 (30 mEq/L), and CH3COO (10–15 mEq/L). We used three needle with different gauge from Medikit (Tokyo, Japan) and from Covidien (Tokyo, Japan). Hematological and biochemical blood samples were collected after at least 10 h of fasting just before HD session and tested at the laboratory at SRL, Inc. (Tokyo, Japan). The spKt/V was evaluated according to the procedure of Daugirdas.6 BP was measured after 5–10 min rest before every HD session for 1 month, and average BP was calculated. Mean BP was defined as follows:

Table 1
Table 1:
Characteristics of the Patients

Arterial and venous pressures were recorded throughout each HD session. Pressure at the start of each session was presented as the mean of the pressures recorded in the first 15 min of the session. Pressure at the end of each session was presented as the mean of the pressures recorded in the last 15 min of the session. Magnitude of the blood volume variation in percentage (%ΔBV) during HD was measured using a DBG-03 dialysis monitor (Nikkiso, Tokyo, Japan). All the patients gave their informed consent to the measurement.

Measurement of dQb

Delivered Qb was measured at two different time points during each HD session, that is, at the start and end of each session, using an ultrasound technique using Transonic HD monitor HD-02 (Nipro, Tokyo, Japan) as previously described.7,8 For accuracy of the measurement, those patients with more than 5% blood access recirculation (AR) were excluded. Pressure at the predialyzer and venous line was monitored at arterial and venous chamber, respectively. Furthermore, patients in whom segments of the blood line were partially collapsed were excluded. Using actual dQb and rQb recorded using a dialysis machine, the ratio between dQb and rQb (dQb/rQb) was calculated. When dQb/rQb was <1, a discrepancy was observed between actual dQb and rQb recorded using a dialysis machine.

Statistical Analysis

Clinical parameters are expressed as mean ± standard deviation. Unpaired Student’s t-test was used for comparison of group mean values for continuous variables. For categorical variables, characteristics of the subjects were compared by κ2 test. A p value of <0.05 was considered to be statistically significant.

Results

Characteristics of the study participants are shown in Table 1. The average levels of dQb/rQb at start and end of HD session were 1.01 ± 0.04 and 0.98 ± 0.05, respectively. Although both arterial and venous pressures tended to increase as the HD session progressed, no significant relationship was observed between change in dQb/rQb and the increase in arterial or venous pressures (data not shown). We showed the percentages of the dQb/rQb < 1 that were classified at rQb recorded using a dialysis machine (Table 2). The percentage of patients with dQb/rQb < 1 increased with increase in rQb or at the end of the HD session compared with that at the start of the session (Table 3). This suggests that reduced serum volume at the end of the HD session hindered adequate Qb.

Table 2
Table 2:
Characteristics of Needles Used in the Study
Table 3
Table 3:
The Number and Percentage of dQb/rQb <1 Classified With the Levels of rQb

Next, we examined whether selection of the needle gauge affects dQb/rQb. At the start of the HD session, mean levels of dQb were 272.5 ± 38.8 ml/min with a 15 gauge needle, 205.3 ± 16.4 ml/min with a 16 gauge needle, and 164.4 ± 19.2 ml/min with a 17 gauge needle (Figure 1). At the end of the HD session, the mean levels of dQb were 268.6 ± 37.9 ml/min with a 15 gauge needle, 191.1 ± 16.1 ml/min with a 16 gauge needle, and 159.3 ± 17.2 ml/min with a 17 gauge needle. In the 16 gauge group, the percentage of patients with dQb/rQb < 1 was >50% at the end of the session when rQb exceeded 161 ml/min (Table 4). Even at the start of the session, dQb/rQb < 1 was observed in approximately 40% of patients in the 16 gauge group when rQb exceeded 201 ml/min. dQb/rQb < 1 was observed in >50% of patients in the 17 gauge group when rQb exceeded 161 ml/min at the start and end of the session. In patients of the 15 gauge group, there was no tendency of dQb/rQb increasing in accordance with increase in rQb.

Table 4
Table 4:
The Number and Percentage of dQb/rQb <1 Classified With the Levels of rQb and Needle Gauge
Figure 1
Figure 1:
The mean blood flow with needles of three different size of gauge at start and at end of dialysis session.

Blood pressure is an important factor for Qb volume in patients with AVFs. dQb/rQb < 1 was therefore calculated according to classification of the mean BP at the start and end of the HD session (Table 5). Although we hypothesized that lower mean BP would increase the percentage of patients with dQb/rQb < 1, no clear relationship was observed between mean BP and the percentage of patients with dQb/rQb < 1. Finally, we examined the relationship between dQb/rQb and %ΔBV. Studies have reported %ΔBV as a good indicator of extracellular fluid volume, atrial natriuretic peptides, and inferior vena cava diameter during quiet expiration.9 This parameter was not examined in all participants because of the lack of access to the machine for calculation of %ΔBV. However, in patients in whom %ΔBV was measured, no clear relationship was observed between %ΔBV and dQb/rQb when we differentiated the patients with needle gauges (Table 6).

Table 5
Table 5:
The Number and Percentage of dQb/rQb <1 Classified With Mean Blood Pressure and Needle Gauge
Table 6
Table 6:
The Number and Percentage of dQb/rQb <1 Classified With Magnitude of the Blood Volume Variation in Percentage (%DBV) and Needle Gauge

Discussion

In this study, actual dQb was compared with rQb recorded using a dialysis machine during HD sessions in patients with AVFs. Higher Qb provides increased clearance when used in conjunction with high-efficiency and high-flux dialysis membranes.10 Although dialysis efficiency, measured by removal of small molecules or Kt/V, depends on clearance of the dialyzer and treatment time, increased clearance is closely related to actual dQb. In other words, actual dQb, not rQb, is an important determinants of dialysis efficiency.

Our data showed that the percentage of patients with dQb/rQb < 1 increased with increase in rQb. The ratio at the end of the HD session increased compared with that at the start of the session, suggesting that reduced serum volume at the end of the HD session precluded adequate Qb. In addition, our data suggested that there is a relationship among selection of needle gauge, rQb, and the percentage of patients with dQb/rQb < 1. Thus, the selection of needle may be an important factor for actual dQb, especially when rQb exceeds 200 ml/min.

Depner et al.11 indicated that blood pump meter readings >400 ml/min are usually inaccurate because of low inflow pressure and that prepump monitoring of arterial inflow pressure can prevent hidden reductions in blood flow. To avoid this problem, we excluded patients in whom predialyzer and venous pressures recorded were between 0 and 250 mm Hg and in whom segments in the blood line had clearly collapsed. AR results in a lower efficiency procedure that adversely affects the delivered dose of dialysis. Mohan et al.12 and Tan et al.13 employed bedside monitoring using effective ionic dialysance/access blood flow ratio (EID/Qb) as a sensitive indicator for accessing AR in AVF and catheter, respectively. EID/Qb may be another strong tool to detect discrepancy between Qb and Qd. HD efficiency is worthy to be further discussed employing EID/Qb and Qb/Qd.

In the present study, rQb was not equal to dQb in many patients. The discrepancy between dQb and rQb was ±5%, and blood pump segment tubing was not observed. Thus, this discrepancy between dQb and rQb may occur in any dialysis unit even when arterial and venous pressures are within normal range and when no obvious collapse in blood line has occurred.

Many studies recommended high Qb exceeding 300 ml/min to ensure adequate HD efficiency. Approximately, 80% of patients with Qb between 250 and 300 ml/min had URR of >65%.14 The Hemodialysis, or HEMO, study showed mean Qb of 373 ± 69 ml/min in patients in whom HD was performed with high-flux membranes. High-flux membranes enable a urea clearance of >90%.15

Most previous studies acquired data from clinical records of various centers. In these studies, the Qb mentioned was the recorded one, and a discrepancy between actual dQb and rQb may have occurred. Few studies have noted the importance of the discrepancy between dQb and rQb, or the fact that differing blood lines may affect this discrepancy, resulting in variation in delivered Kt/V.16 Furthermore, no report has examined differences in needle gauge in HD treatment. Our results showed that needles of a gauge of 15 or more are required to obtain actual dQb of 241 ml/min or above. Thus, care must be taken in the choice of needle gauge in HD treatment for optimal dialysis efficiency.

In the the Dialysis Outcomes and Practice Patterns Study, or DOPPS, relatively low Qb (200 ml/min) was observed in Japan compared with that in the United States (300–400 ml/min), but more favorable mortality rates were recorded in Japan than in the United States.1,17 This phenomenon could be partially explained by differences in actual dQb being fewer than those in rQb between the two countries.

Blood pressure and cardiac function are strong mediators for actual Qb of VA. Pandeya and Lindsay18 identified a relationship between Qb and cardiac output. Our data showed a decrease in mean BP and an increase in the percentage of patients with dQb/rQb < 1 when values at the start and at end of treatment were compared. Cardiac output and BP are closely related. Therefore, BP is another important factor in achieving adequate dQb.

%ΔBV has been used as a tool to estimate adequate water removal during HD session.17 No obvious relationship between %ΔBV and dQb/rQb was found in the present study, probably because Qb of VA is affected by various other factors, including cardiac output, systemic vasoconstriction, and others.

This study had several limitations. First, dQb was measured only once for each participant. Second, the number of participants was relatively small. Very few studies have examined the relationship between dQb and rQb.

In conclusion, the percentage of patients with dQb/rQb < 1 increased in accordance with the increase in rQb recorded using a dialysis machine or as the HD session progressed. Selection of needle gauge and rQb were identified as important factors for determining actual dQb. HD efficiency must be carefully monitored, especially in patients treated with high Qb during HD sessions.

References

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Keywords:

dialysis efficiency; blood flow; vascular access; needle gauge

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