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Quantitation and adequacy Troubleshooting on haemodialysis
Kt/V and URR calculations Dialysis-related critical incidents
Routine monitoring Non-adherence
Medications (local patients)  

Quantitation and adequacy

Measurements of small molecule clearance have become an accepted way of assessing the adequacy of haemodialysis, because a correlation with mortality has been shown in large studies. It is a Renal Association standard that adequacy is checked monthly on all chronic haemodialysis patients.

Kt/V for urea* is the most widely accepted of these measurements, although a simple surrogate for this, urea reduction ratio (URR), is more readily measured and is also widely used. There is a calculator for Kt/V in Proton.

* K is a dialyser-specific figure for rate of urea clearance on an individual dialyser, at a given blood flow and dialysate flow. t is duration of dialysis. V is the volume of distribution of urea in a patient. 

Urea reduction ratio method

In the RIE we check Urea Reduction Ratio (URR), calculated as:

URR = 100 x [1 - Post(Urea)/Pre(Urea)]

The Edinburgh unit participates in a multicentre audit with comparison of URR results between all units in Scotland. This is the protocol approved by the Scottish Renal Association:

  • The ‘pre’ sample is taken immediately after cannulation of the fistula with a dry needle and before the dialysis starts. For central venous cannulae, a sample can be drawn from the line after the Heparin lock has been removed. Take care not to contaminate the samples with saline.
  • The post sample is taken by the stop flow method ie. the dialysate flow is stopped for 5 minutes, leaving the blood pump running, before sampling from the port in the arterial blood line. This is to allow time for the blood in the access to equilibrate with the central circulation, but not enough time for the equilibration from the tissue pool

Only these bloods should be marked PRE (001) and POST (002) and are done by staff in the dialysis Units on a monthly basis.

Bloods taken without this stop flow will overestimate urea clearance on dialysis. Hence, while it may be appropriate to take bloods pre-dialysis on eg. in-patients to save patients venepuncure, and it is often necessary to know post dialysis potassium in out-patients who have not for whatever reason, completed their dialysis prescription, mark these bloods with the time they are taken, not pre/post or the 001/002 codes. This is very important as failure to do so causes problems with download of data to the Registry.

Problems with Kt/V & URR

  • Small molecule clearance is not the only or even always the most important factor in determining dialysis adequacy.
  • Fluid balance is important for mortality and is not measured by Kt/V or URR. It is more affected by dialysis frequency and duration.
  • Calculations that use measurements of blood urea (instead of dialysate urea) are prone to sampling errors, particularly urea rebound.
  • V (volume of distribution of urea) is low in wasted, malnourished patients – high Kt/V values may be caused by this, rather than by excellent dialysis.

Kt/V versus URR

Calculations based on URR alone underestimate Kt/V because no account is taken of the consequences of ultrafiltration.  This removes urea, but is ‘invisible’ by URR monitoring alone.

Table: correlation of URR to Kt/V corrected for ultrafiltration (NKF K/DOQI Guideline 2000). Calculated from single-pool, variable volume model with a body weight of 67.3kg, V of 35L, and NPCR of 1.0. Wt/BWt is the ultrafiltration volume/post dialysis weight x 100

 

URR Values at Kt/V

DWt/BWt

%

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0

53

57

61

65

68

71

74

76

1

52

57

61

64

68

71

73

76

2

50

55

59

63

66

69

72

75

3

49

54

58

62

65

68

71

74

4

48

53

57

61

64

67

70

73

5

47

51

56

60

63

67

70

72

6

46

50

55

59

62

66

69

71

7

44

49

53

58

61

65

68

71

8

43

48

52

57

60

64

67

70

9

42

47

51

56

59

63

66

69

10

41

46

50

54

58

62

65

68

Prescribed Dialysis:

Dialysis should be prescribed on an individual basisThe variables influencing the amount of dialysis required are

  • Weight:  Higher weight = more dialysis required. Note that in serial audits, we have previously underprescribed dialysis for large patients
  • Sex: Men require more than women of the same weight due to greater proportional body water

The variables in the prescription that will improve dialysis adequacy are:

  • Time:  this is the most important variable to achieve adequacy and the time likely to be required needs to be explained to patients BEFORE they choose haemodialysis
  • Blood flow rate (Qb): aim for high e.g., 400ml/min – monitor venous pressures – allow up to 200ml/min
  • Dialyser surface area : FX8 is smaller than FX10
  • Dialysate flow rate (Qd) :  either 500 or 800ml/min possible (NB the difeerence this makes is not quantified and likely to be small but if Qb is 400ml/min, use Qd=800ml/min

To prescribe adequate dialysis, please consult the following chart for a guide. Hours required to achieve Kt/V>1.3

  • Always round up to the higher time and assume that Qb initially at least will be less than you prescribe
  • Work on the principle that the average dialysis duration is ~4.5 hrs – start with this and then tailor to the patient on the basis of access performance & measured dialysis adequacy
  • Do not prescribe less than 3 hours three times a week except in exceptional circumstances (discuss with named consultant)
  • Maximise Qb – particularly important in larger patients
  • Do not use FX8 in large people or FX10 in small people (though FX60 may be appropriate - discuss with named consultant)
  • High flux dialysis (with FX60 dialyser) or haemodiafiltration (with FX80 and adaptation to dialysis machine) may be appropriate for anuric patients on (or expected to be on) dialysis for a prolonged period or with signs/symptoms of dialysis amyloid to aid middle molecule clearance
  • HDF may also be appropriate for larger patients with very long hours (discuss with named consultant)

MEN

FX8

 

 

 

FX10
FX60

 

 

 

 

Qb 250

Qb 300

Qb 350

Qb 400

Qb 250

Qb 300

Qb 350

Qb 400

40kg

3

3

3

3

3

3

3

3

50kg

3

3

3

3

3

3

3

3

60kg

3.75

3.25

3

3

3.5

3.25

3

3

70kg

4.25

3.75

3.5

3.25

4.25

3.75

3.25

3

80kg

4.75

4.25

4

3.5

4.75

4.25

3.75

3.5

90kg

5.5

4.75

4.5

4

5.25

4.75

4.25

4

100kg

6

5.5

5

4.5

6

5.25

4.75

4.25

110kg

6.5

6

5.25

5

6.5

5.75

5.25

4.75

120kg

7.25

6.5

5.75

5.25

7

6.25

5.75

5.25

 

 

 

 

 

 

 

 

 

WOMEN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

40kg

3

3

3

3

3

3

3

3

50kg

3

3

3

3

3

3

3

3

60kg

3.5

3

3

3

3.5

3

3

3

70kg

4

3.5

3.25

3

3.75

3.5

3

3

80kg

4.5

4

3.5

3.25

4.35

4

3.5

3.25

90kg

5

4.5

4

3.75

5

4.25

4

3.5

100kg

5.5

5

4.5

4

5.5

4.75

4.25

4

110kg

6

5.5

5

4.5

6

5.35

4.75

4.25

120kg

6.5

6

5..25

5

6.5

5.75

5.25

4.75


FX8 : K= 0.51Qb + 93 t = dialysis time (min) FX10/60 : K= 0.55Qb + 87.3

V = 0.6 x dry wt (men), (x 0.55 women) FX80 : K= 0.8Qb + 38

When prescribed vs delivered dialysis differ:

If the prescribed vs delivered dialysis is very different, consider:

  • Access recirculation, interrupted or shortened dialysis, slowed pump speeds, clotting dialysers, delayed re-equilibration of urea (eg in shock or cardiac failure), and errors in assumptions about V, will often tend to reduce actual dialysis dose. This is particularly likely in acute renal failure.

Routine monitoring of haemodialysis patients

This section describes the Edinburgh protocol.

Pre- and post-dialysis U & E, Creat monthly
LFTs, Ca, Alb, PD4 monthly
FBC monthly (fortnighly in RIE or if recent change in EPO dose
Iron Studies 2 monthly
PTH 2 monthly
Lipids annually, more often if high
HbA1c in diabetics 2 monthly
HepBsAg, HepC Ab 3 monthly
HIV Ab 12 montly
Aluminium 3 monthly
Cytotoxic antibodies (if on transplant list) monthly

 
Patients on HDF have in addition:

Trace metal & micronutrient screen, b2 microglobulin, B12 & folate
3 monthly

 
Adequacy targets:

For thrice weekly haemodialysis, the MINIMUM targets for patients with no residual renal function (SRA, RA, DOQI) are:

Kt/V >1.2 orURR >65% for all patients on chronic haemodialysis

To achieve this, the population mean needs to be Kt/V=1.3 or URR~70%

Higher values may be beneficial – and the unit average needs to be appreciably higher than these minima. Up to a Kt/V of 1.5-1.7 may be ‘good’, as long as not due to low V (weight).

Be aware that proton corrects URR to 2 significant places. Thus 65% can be 64.5-65.4% - While the target is arbitrary, it is based on survival data and 64.5 is not adequate dialysis

Other targets for monitoring (Renal Association – Guidelines March 2007)

K

3.5-6.5 mmol/l

PO4

1.1-1.8 mmol/l

Ca (corrected)

normal range

Ca x PO4 product

<4.8 mmol2/l2

PTH

130-260 (x2–x4 upper limit normal

Hb

>105-125g/dl (aim for 110g/dl)

Ferritin

100-800

Transferrin saturations

>20%

Aluminium

<2.2mmol/l

Bicarbonate

20-26mmol/l

Cholesterol

<5mmol/l

HbA1c

<7%

Pre-dialysis BP*

<140/90 mmHg

Post-dialysis BP*

<130/80 mmHg

* local not Renal Assoc target

Medications

Patients dialysing at the RIE are reviewed in a multidisciplinary team meeting approximately every 8 weeks.

  • Most patients receive their drugs from their GP
  • Erythropoietin and i.v. alfacalcidol or iron preparations are supplied by the hospital 

Troubleshooting on haemodialysis

Hypotension

Usually occurs for one of three broad reasons:

  • Patient is below dry weight (q.v.)
  • Fluid removal is faster than redistribution can occur (eg, too large weight gains, unstable circulation)
  • Some effect of dialysate/ membrane/ extracorporeal circuit on cardiac output and/or peripheral resistance

Acute hypotension is usually managed by saline infusion, reducing weight loss; if out of character consider cardiac problems (rhythm, ischaemia). Management for recurrent hypotension is something like:

  • Advise patient about fluid, salt, etc
  • Review dry weight (see 'Dry Weight')
  • Consider longer, slower dialysis (unpopular)
  • Consider serial ultrafiltration followed by isovolaemic dialysis (lengthens dialysis again; can be used for a time to get nearer to dry weight)
  • Review haemoglobin (effect of anaemia possibly via cardiac oxygenation)
  • Consider providing oxygen during dialysis
  • Tricks with dialysate: (1) cooling (causes increased peripheral resistance); (2) sodium profiling, or ramping, in which the dialysate sodium is altered during dialysis. A higher dialysate Na reduces hypotension (probably by maintaining ECF osmolality) - but reduces Na removal. Start high, lower later helps. Ultrafiltration rate can also be profiled on some machines.
  • Avoid eating and drinking before/during dialysis (reduces peripheral resistance by causing splanchnic vasodilation)
  • Omit hypotensive agents on the morning (or evening) before dialysis
  • Oral midodrine, an a1 adrenergic agonist (currently available in the UK on a named-patient basis)
  • Consider haemofiltration or haemodiafiltration (different membranes, but in the case of haemofiltration, also usually a slower treatment - and possibly with more cooling of blood)

Cramps

Muscle cramps are very common during dialysis and can be of sufficient severity that they result in termination of the procedure. Their cause is unclear but the majority occur towards theend of the procedure after a significant volume of fluid has been removed. Their etiology is postulated to involve volume depletion and tissue hypoxia. They are associated with large requiremtns for fluid removal.

  • Acute management often involves the administration of hypertonic fluid, most commonly 50% dextrose (50mls), in order to raise plasma osmolality.
  • Quinine 2-300mg before dialysis or at bedetime can be tried but is unproven
  • Oral agents such as clonazepam, vitamin E, carnitine, or anti-convulsants are sometimes used as prophylaxis but their benefits are even less certain.
  • Limitation of inter-dialytic weight gains, ensuring that post-dialysis dry weight is correct and the use of an appropriate dialysate sodium are the best means to prevent this problem. Remember that a higher dialysate sodium will reduce intra-dialytic symptoms at the expense of thirst and weight gains; the converse holds true for a lower dialysate sodium. Sodium profiling may again be of benefit.
  • Pateints who experience cramps at night may benefit from muscle-stretching for a minute or two. Heat and massage for the camping muscle can help.

Pyrexia

Pyrexia in haemodialysis patients is usually related to the use of semi-permanent tunnelled access and most commonly due to Gram positive sepsis. It is common for dialysis to precipitate pyrexia in these circumstances. The exit site and tunnel should be checked for discharge, tenderness or erythema, but is commonly not abnormal.

  • Take blood cultures from the line and from a peripheral vien, along with routine bloods and an examination for other causes.
  • Treat with intravenous antibiotics according to the Antimicrobial protocol, with removal of catheter if recurrent or if it fails to respond promptly.

All patients with ESRF are relatively immunosuppressed. 'Typical' bacterial infections of all types seem to be more common, and infection is the second most common cause of death in dialysis patients. Tuberculosis is possibly as common in haemodialysis patients as it is in patients immunosuppressed after receiving a renal transplant.

Non-functioning access

This is described in the section on Vascular Access

Dialysis-related critical incidents

The most serious acute events include air embolism, line disconnection leading to haemorrhage, acute haemolysis or toxicity related to line kinking or dialysis contamination, and acute allergic reactions to dialysers or sterilants (e.g., the 'first-use' syndrome attributed to antibody formation to ethylene oxide). If any such crisis occurs and the explanation is not entirely clear, in addition to all the necessary supportive measures:

  • Stop dialysis
  • Take samples from venous and arterial lines - look for alterations in haematology and biochemistry
  • Disconnect the patient. Record their weight and routine observations.
  • Keep a sample of dialysate
  • Keep the used dialyser
  • Take the machine out of use. Inform the dialysis technicians that it was in use when an incident involving a patient occurred (e.g., by a prominent notice on the machine), so that an investigation can be made and evidence preserved.
  • Record all the details, including the precise circumstances (patient's position, first symptoms, full history)
  • Fill in the appropriate incident report form

The MHRA page on dialysis has alerts and information on haemodialysis and peritoneal dialysis equipment. In Scotland, report incidents via this dialysis incident-reporting link for Scotland (at SHOW; only works from inside NHSnet).

Non-adherence

Non-adherence to dialysis hours, dietary and fluid restrictions, etc are associated with increased mortality. These effects are probably indirectly associated as well as direct effects, but they are important and should be pointed out to patients who struggle to comply.  Download  warning leaflet from the foot of this page Patient information leaflet about non-adherence (pdf file, 1 page).

Further info

Calculating Kt/V - www.kt-v.net is a useful online tool from Asher Schachter of the Boston Children's Hospital. 

 

Acknowledgements:   Jane Goddard, Mariana Dimova and Neil Turner were the main authors for this page. The last modified date is shown in the footer.

 

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This page last modified 06.08.2013 09:31 by Emma Farrell. edren and edrep are produced by the Renal Unit at the Royal Infirmary of Edinburgh and the University of Edinburgh. CAUTIONS and Contact us.